Cosmological time dilation in the early Universe(advancedsciencenews.com)
advancedsciencenews.com
Cosmological time dilation in the early Universe
https://www.advancedsciencenews.com/time-ran-slowly-in-the-early-universe-just-as-einstein-predicted/
250 comments
The description in the article is seriously misleading. Your misgivings about what it appears to mean are entirely correct: it doesn't mean what it appears to mean. The effect in question is only an appearance; it does not mean that clocks 1.5 billion years after the Big Bang "actually" ran 5 times slower than clocks today (as you point out, it is impossible to even make such a comparison). It just means that, because of redshift effects, when we look at light from events that happened 1.5 billion years after the Big Bang, we see those events appear to happen 5 times slower than similar events today. But we would see the same thing if we watched events happening on a spaceship that was flying away from us with the same redshift.
Reciprocity in Minkowski space is relevant.
In the special relativistic setup in your final sentence, the spaceship occupants could in principle watch our clocks and observe that ours are ticking slowly, with the same redshift.
Observers at a high cosmological redshift cannot watch our clocks at all. They can only look at the even more distant past of what will become our clocks in billions of years, and there's no reason to think that the redshift we ascribe to them is the redshift they ascribe to our precursors.
(Indeed the gravitational redshift from the metric sourced by Earth (assuming "us" are Earthbound) breaks the reciprocity -- if only slightly -- that would be observed in Minkowski space.)
In the special relativistic setup in your final sentence, the spaceship occupants could in principle watch our clocks and observe that ours are ticking slowly, with the same redshift.
Observers at a high cosmological redshift cannot watch our clocks at all. They can only look at the even more distant past of what will become our clocks in billions of years, and there's no reason to think that the redshift we ascribe to them is the redshift they ascribe to our precursors.
(Indeed the gravitational redshift from the metric sourced by Earth (assuming "us" are Earthbound) breaks the reciprocity -- if only slightly -- that would be observed in Minkowski space.)
> Observers at a high cosmological redshift cannot watch our clocks at all.
This is true, but it doesn't change the fact that the "time dilation" described in the article is like the time dilation in special relativity, and is not like gravitational time dilation, with respect to what the article is talking about.
> there's no reason to think that the redshift we ascribe to them is the redshift they ascribe to our precursors.
Indeed, there is every reason not to think this. But that still doesn't change the fact that the "time dilation" due to cosmological redshift is like the time dilation in SR, not gravitational time dilation, with respect to what the article is talking about.
This is true, but it doesn't change the fact that the "time dilation" described in the article is like the time dilation in special relativity, and is not like gravitational time dilation, with respect to what the article is talking about.
> there's no reason to think that the redshift we ascribe to them is the redshift they ascribe to our precursors.
Indeed, there is every reason not to think this. But that still doesn't change the fact that the "time dilation" due to cosmological redshift is like the time dilation in SR, not gravitational time dilation, with respect to what the article is talking about.
[deleted]
What the article actually wants to say (and it does so down the line) is that:
> If you look at a pulsar, that is 12B light years away from us, it will pulsate significantly slower than expected.
In this case, I think, it is that the pulsar blinks slower by a factor of 5. I presume they already have corrected out the redshift related time dilation and 5 is what was left.
> If you look at a pulsar, that is 12B light years away from us, it will pulsate significantly slower than expected.
In this case, I think, it is that the pulsar blinks slower by a factor of 5. I presume they already have corrected out the redshift related time dilation and 5 is what was left.
I was made aware of one inaccuracy in my comment and I cannot edit any more so here goes:
The research was on the variations of "quasar radio intensity", not pulsar frequency. The scientists formulated a model to statistically predict intensity variation and used those projections as their "clock".
The research was on the variations of "quasar radio intensity", not pulsar frequency. The scientists formulated a model to statistically predict intensity variation and used those projections as their "clock".
How can you tell difference between effect of redshift and time dilation
Hydrogen and other emission lines, presumably.
No. If a hydrogen atom makes a transition "there", then it does so with a frequency determined by the clock "there", at the point where the transition happened. If we detect the emission "here", we see both the red shift and the time dilation between "there" and "here", and we see them in exactly the same way - as a frequency shift.
I see. Interesting question.
Does time dilation not affect frequency (and thence spectra)? The two effects seem perfectly equivalent to me.
How can you tell the difference between spin and time dilation?
That was my question. Do all pulsars have the same speed, so slower rates are attributable to this?
Your question about pulsars is a good one and I'll answer it first, but the study at the top is about quasars, and I'll address that further below.
With one exception all known pulsars are in the Milky Way; they are too intrinsically dim (and obscured by dust, much of which is local to them, produced in their progenitor star's explosive demise) to see them at extragalactic distances. The exception is PSR J0540-6919 in the Large Magellanic Cloud (the LMC, a satellite galaxy to the Milky Way), which is atypically bright in gamma rays and so is visible to the Fermi-GLAST telescope in low Earth orbit.
Pulsars have unique periods, ranging from about a millisecond to about ten seconds. The period is determined by a number of factors including the rotation rate (spin), the offset of the beam from the spin axis, precession of the beam axis and the spin axis, differential rotation within the pulsar, time-dependent collimation of the beam, and so on. Here's a quickie image: <https://www.astronomy.ohio-state.edu/ryden.1/ast162_5/pulsar...>. When we see a pulsar we are catching some of the cyan-shaded region as it's dragged around the spin axis. In the case of linked diagram above, if we are way to the below-left, we would see the strongest part of the pulse about once per full rotation.
Some pulsars have highly predictable periods of a few milliseconds; this pulse rate can be stable for decades. Other pulsars are subject to occasional and unpredictable speed-ups and speed-downs. These are probably driven by the movements of relatively large internal structures and/or the infall of material onto the crust of the neutron star (its own supernova debris, or gas from a binary-or-multi-star partner).
Since all but one discovered pulsar is in the Milky Way and the LMC is close enough, there is no cosmological redshift involved in pulsars. We may eventually have telescopes powerful enough to spot bright pulsars at the margins of other galaxies, but not soon. When we can, it will be interesting to see what spectral features have a redshift dependence.
Now, on to quasars and the study press-released at the link at the top of the page.
Variable quasars only work vague like pulsars where we substitute a few solar mass neutron star a few thousand light years away for a billions of solar mass black hole near the centre of a distant galaxy billions of light years away. Dust and gas around the distant supermassive black hole generates a magnetic field which causes a relativistic jet to form, in which charged particles from near the black hole are swept into a long tube and thrown far away from the black hole; in the process these charged particles (especially electrons) emit synchrotron radiation, which we detect. All quasars are extragalactic -- there are none very close to us -- so all quasars have some nonzero redshift distance.
The periodicity of variable quasars probably has little to do with the spin of the central black hole, which can be one rotation every few hours up to practically not rotating at all. Quasar variability is best measured in months. We tend to conclude that we only see bright quasars when the beams are always pointed in roughly our direction (and that quasar beams are very wide because they spread out with distance). This should hold true no matter how quickly or slowly the black hole is spinning. The study summarized in the link at the top is in part a check on that.
As to the light (and radio and gammas and so on) from bright quasars, astronomers mostly pay attention to are broad emission lines (BELs) that appear and disappear; these are the "changing-look quasars" (CLQ) and the optical luminosity varies enormously (> order of magnitude detected here). The CLQ BELs are predictable enough that they can reveal the redshift on the emission lines particularly the ones in the mid-infrared, giving a distance to the CLQ.
There are also quasars that brighten and dim without a change in spectral lines, the so-called changing-state quasars (CSQ); the most highly-variable quasars (HVQ) are CSQs. The mechanisms for variability for CLQs and CSQs differ, but both have to do with dynamics in the accretion disc rather than being directly driven by the black hole spin.
Unlike pulsars which have bright/dim periods of milliseconds to seconds, highly variable quasars have bright/dim periods best measured in months. Pulsars also generally run on a bright/dim/bright/dim/... cycle or (for a favourable orientation) bright/dim/VERYbright/dim/bright/dim/VERYbright/... but variable quasars can get brighter And brighter AND brighter and then suddenly quite dim and then brighter and dimmer and so forth, with practically no predictability (It's "stochastic"). The brightening and dimming can be at different wavelengths, presumably because the accretion disc (bright in most wavelengths) is hidden behind host-galaxy dust and gas (which appears as thin spectral lines, redshifted according to distance).
For near-experts wanting to know more about CLQ vs CSQ, the CRTS transient survey has a nice overview <https://arxiv.org/abs/1905.02262> (see fig 1.).
The study summarized at the top and in preprint found at <https://arxiv.org/abs/2306.04053> focuses on the differences in luminosity variability in different wavelengths in CSQs. Roughly, the absorption lines near the quasar and in gas and dust and so forth between the quasar and us imprints the spectrum in a way that allowed the authors to sort many quasars into initial groups ("buckets") with common spectral features. In this study the preserved freatures are the relative brightness at a number of wavelengths seen by different telescopes. As long as the varying brightness doesn't cause quasars to be ejected from their initial buckets, the cause of the variability is not related to a redshift (neither because of local motions in the host galaxy, or local motions of Earth, nor because of cosmological redshift). Consequently cause of the variability must be either very local to the quasar or very local to us. Since quasars in a bucket are scattered across the sky, the reason for the variability is most likely not something local to us. Also we would struggle to explain why quasars might look variable due to some near-Earth influence, but why type Ia supernovae (and your friends the galactic pulsars) do not.
If one decides the variability is caused by something local to each quasar, but not having to do with the quasar's meanderings through its host galaxy, then one compares the variability period of quasars in a bucket. It turns out that, for each bucket (with ~identical strengths in each of a set of wavelengths), the varability period (each dimming or brightening event) is longer at higher redshift. This is consistent with an expanding universe identical to the one in the standard model of cosmology.
See also https://www.astro.ucla.edu/~wright/Lyman-alpha-forest.html or https://w.astro.berkeley.edu/~jcohn/lya.html for one specific absorption line of great interest, driven by bright quasars, and affected by the redshifts both of matter very near the quasar and matter along the way from quasar to us.
With one exception all known pulsars are in the Milky Way; they are too intrinsically dim (and obscured by dust, much of which is local to them, produced in their progenitor star's explosive demise) to see them at extragalactic distances. The exception is PSR J0540-6919 in the Large Magellanic Cloud (the LMC, a satellite galaxy to the Milky Way), which is atypically bright in gamma rays and so is visible to the Fermi-GLAST telescope in low Earth orbit.
Pulsars have unique periods, ranging from about a millisecond to about ten seconds. The period is determined by a number of factors including the rotation rate (spin), the offset of the beam from the spin axis, precession of the beam axis and the spin axis, differential rotation within the pulsar, time-dependent collimation of the beam, and so on. Here's a quickie image: <https://www.astronomy.ohio-state.edu/ryden.1/ast162_5/pulsar...>. When we see a pulsar we are catching some of the cyan-shaded region as it's dragged around the spin axis. In the case of linked diagram above, if we are way to the below-left, we would see the strongest part of the pulse about once per full rotation.
Some pulsars have highly predictable periods of a few milliseconds; this pulse rate can be stable for decades. Other pulsars are subject to occasional and unpredictable speed-ups and speed-downs. These are probably driven by the movements of relatively large internal structures and/or the infall of material onto the crust of the neutron star (its own supernova debris, or gas from a binary-or-multi-star partner).
Since all but one discovered pulsar is in the Milky Way and the LMC is close enough, there is no cosmological redshift involved in pulsars. We may eventually have telescopes powerful enough to spot bright pulsars at the margins of other galaxies, but not soon. When we can, it will be interesting to see what spectral features have a redshift dependence.
Now, on to quasars and the study press-released at the link at the top of the page.
Variable quasars only work vague like pulsars where we substitute a few solar mass neutron star a few thousand light years away for a billions of solar mass black hole near the centre of a distant galaxy billions of light years away. Dust and gas around the distant supermassive black hole generates a magnetic field which causes a relativistic jet to form, in which charged particles from near the black hole are swept into a long tube and thrown far away from the black hole; in the process these charged particles (especially electrons) emit synchrotron radiation, which we detect. All quasars are extragalactic -- there are none very close to us -- so all quasars have some nonzero redshift distance.
The periodicity of variable quasars probably has little to do with the spin of the central black hole, which can be one rotation every few hours up to practically not rotating at all. Quasar variability is best measured in months. We tend to conclude that we only see bright quasars when the beams are always pointed in roughly our direction (and that quasar beams are very wide because they spread out with distance). This should hold true no matter how quickly or slowly the black hole is spinning. The study summarized in the link at the top is in part a check on that.
As to the light (and radio and gammas and so on) from bright quasars, astronomers mostly pay attention to are broad emission lines (BELs) that appear and disappear; these are the "changing-look quasars" (CLQ) and the optical luminosity varies enormously (> order of magnitude detected here). The CLQ BELs are predictable enough that they can reveal the redshift on the emission lines particularly the ones in the mid-infrared, giving a distance to the CLQ.
There are also quasars that brighten and dim without a change in spectral lines, the so-called changing-state quasars (CSQ); the most highly-variable quasars (HVQ) are CSQs. The mechanisms for variability for CLQs and CSQs differ, but both have to do with dynamics in the accretion disc rather than being directly driven by the black hole spin.
Unlike pulsars which have bright/dim periods of milliseconds to seconds, highly variable quasars have bright/dim periods best measured in months. Pulsars also generally run on a bright/dim/bright/dim/... cycle or (for a favourable orientation) bright/dim/VERYbright/dim/bright/dim/VERYbright/... but variable quasars can get brighter And brighter AND brighter and then suddenly quite dim and then brighter and dimmer and so forth, with practically no predictability (It's "stochastic"). The brightening and dimming can be at different wavelengths, presumably because the accretion disc (bright in most wavelengths) is hidden behind host-galaxy dust and gas (which appears as thin spectral lines, redshifted according to distance).
For near-experts wanting to know more about CLQ vs CSQ, the CRTS transient survey has a nice overview <https://arxiv.org/abs/1905.02262> (see fig 1.).
The study summarized at the top and in preprint found at <https://arxiv.org/abs/2306.04053> focuses on the differences in luminosity variability in different wavelengths in CSQs. Roughly, the absorption lines near the quasar and in gas and dust and so forth between the quasar and us imprints the spectrum in a way that allowed the authors to sort many quasars into initial groups ("buckets") with common spectral features. In this study the preserved freatures are the relative brightness at a number of wavelengths seen by different telescopes. As long as the varying brightness doesn't cause quasars to be ejected from their initial buckets, the cause of the variability is not related to a redshift (neither because of local motions in the host galaxy, or local motions of Earth, nor because of cosmological redshift). Consequently cause of the variability must be either very local to the quasar or very local to us. Since quasars in a bucket are scattered across the sky, the reason for the variability is most likely not something local to us. Also we would struggle to explain why quasars might look variable due to some near-Earth influence, but why type Ia supernovae (and your friends the galactic pulsars) do not.
If one decides the variability is caused by something local to each quasar, but not having to do with the quasar's meanderings through its host galaxy, then one compares the variability period of quasars in a bucket. It turns out that, for each bucket (with ~identical strengths in each of a set of wavelengths), the varability period (each dimming or brightening event) is longer at higher redshift. This is consistent with an expanding universe identical to the one in the standard model of cosmology.
See also https://www.astro.ucla.edu/~wright/Lyman-alpha-forest.html or https://w.astro.berkeley.edu/~jcohn/lya.html for one specific absorption line of great interest, driven by bright quasars, and affected by the redshifts both of matter very near the quasar and matter along the way from quasar to us.
So if I understood you and the original paper correctly, this observation is not in conflict with the Standard Model or Lambda-CDM and that this is an observational outcome of general relativity. Is that a correct statement? If so, then those models may need to incorporate why this may have happened, right?
Edit: Thank you for the detailed post. Learned a few things from it.
Edit: Thank you for the detailed post. Learned a few things from it.
It's support for \Lambda-CDM, which is the standard model of cosmology. The standard model of particle physics is only involved to the extent that it explains spectral emission and absorption lines in the first place; locally the physics of electromagnetism vs hydrogen ions (among others) works the same over-there-back-then and here-and-now.
Sorry that there's two "standard model"s. One often hears "concordance model" or "standard cosmology" or \Lambda-CDM instead, as a result.
This study's novelty is in showing that [a] the changing luminosities of both families of variable quasar can be usefully compared and [b] similar variable quasars have similar luminosity-changing rates at similar redshift; if the redshifts differ, the less-redshifted one's brightness changes more frequently and more quickly.
Sorry that there's two "standard model"s. One often hears "concordance model" or "standard cosmology" or \Lambda-CDM instead, as a result.
This study's novelty is in showing that [a] the changing luminosities of both families of variable quasar can be usefully compared and [b] similar variable quasars have similar luminosity-changing rates at similar redshift; if the redshifts differ, the less-redshifted one's brightness changes more frequently and more quickly.
You can't. They're the same thing.
Thank you. I came here after reading the article ready to post 'What is it about the quasar that gives us this information?'. Did not have to read far to get the answer.
If I'm not mistaken it is this part:
> Lewis explained that the brightening and fading of quasar light, known as their variability is like the stock mark, unpredictable on the surface but with statistical properties that allow it to be modelled over time. This variability can be used like the ticking of a cosmic clock. It was this understanding and the complex quasar firework display from 190 of these feeding supermassive black holes that allowed the team to spot time dilation when the universe was just 1.5 billion years old, less than 10% of its current age.
> Lewis explained that the brightening and fading of quasar light, known as their variability is like the stock mark, unpredictable on the surface but with statistical properties that allow it to be modelled over time. This variability can be used like the ticking of a cosmic clock. It was this understanding and the complex quasar firework display from 190 of these feeding supermassive black holes that allowed the team to spot time dilation when the universe was just 1.5 billion years old, less than 10% of its current age.
> If you're comparing two physically separated frames of reference it makes sense to talk about "this many seconds here corresponds to that many seconds over there"
Even that comparison doesn't make much sense because, as raattgift already pointed out in the comments[0],
> Reciprocity in Minkowski space is relevant.
So from observer A's point of view the clock in observer B's frame of reference will tick more slowly, but from observer B it will appear the other way around.
This is sounds contradictory but the reason for this is that observers in different frames won't even agree on what a valid time measurement is. This is called relativity of simultaneity[1].
[0]: https://news.ycombinator.com/item?id=37250897
[1]: https://en.wikipedia.org/wiki/Relativity_of_simultaneity
Even that comparison doesn't make much sense because, as raattgift already pointed out in the comments[0],
> Reciprocity in Minkowski space is relevant.
So from observer A's point of view the clock in observer B's frame of reference will tick more slowly, but from observer B it will appear the other way around.
This is sounds contradictory but the reason for this is that observers in different frames won't even agree on what a valid time measurement is. This is called relativity of simultaneity[1].
[0]: https://news.ycombinator.com/item?id=37250897
[1]: https://en.wikipedia.org/wiki/Relativity_of_simultaneity
> What does 1.5 billion years mean if time ran slower?
That's a good question and the Wikipedia has an oversimplified answer here: https://en.wikipedia.org/wiki/Cosmic_time
Basically, what it boils down to is imagine a clock traveling at the same rate of expansion as the universe (the Hubble flow). Now further imagine that clock ticks once per year. The phrase "1.5 billions years after the Big Bang" means such a clock will have ticked 1.5 billion times. It's the only way you can do time that makes any kind of sense, when you think about it, since time is literally relative.
Relativity is what makes things weird, because what you're really wondering is if we somehow had a clock on earth that also ticked once per year (standard years like we measure today), how many ticks would it have ticked after the aforementioned clock ticked 1.5 billion times? Such an "earth clock" will have ticked more times, due to the difference in time dilation. If you're still following along, then that means when we say such a clock I first described, the cosmic clock, has ticked 13.8 billion times, this supposed earth-bound clock will have ticked a near infinite amount of time - meaning, from our perspective, the Big Bang singularity is an infinite amount of our time in the past, even though our cosmic clock will have only ticked 13.8 billion times since the event!
Welcome to relativity!
Note this is the same issue with approaching the singularity of a black hole. The clock approaching the singularity will reach the singularity in the time its gravitational acceleration indicates and will proceed normally to the singularity. From outside of the black hole (you have to imagine being able to make this observation) you will never see the object reach the singularity. Such an event is always in that observer's future.
If you think this is bollocks - it's GR's time paradox and is the result of a singularity, which is a strong sign that things have gone awry. BUT - we do know time dilation exists and we know GR works well right up to the point of the actual singularity so while from our perspective the object reaching the singularity probably isn't infinitely in our future, it's very, very far in our future. Likewise, for the Big Bang singularity even though the cosmic clock ticked off 13.8 billion years, from our perspective the event was much further back in time than that.
Weird stuff!
That's a good question and the Wikipedia has an oversimplified answer here: https://en.wikipedia.org/wiki/Cosmic_time
Basically, what it boils down to is imagine a clock traveling at the same rate of expansion as the universe (the Hubble flow). Now further imagine that clock ticks once per year. The phrase "1.5 billions years after the Big Bang" means such a clock will have ticked 1.5 billion times. It's the only way you can do time that makes any kind of sense, when you think about it, since time is literally relative.
Relativity is what makes things weird, because what you're really wondering is if we somehow had a clock on earth that also ticked once per year (standard years like we measure today), how many ticks would it have ticked after the aforementioned clock ticked 1.5 billion times? Such an "earth clock" will have ticked more times, due to the difference in time dilation. If you're still following along, then that means when we say such a clock I first described, the cosmic clock, has ticked 13.8 billion times, this supposed earth-bound clock will have ticked a near infinite amount of time - meaning, from our perspective, the Big Bang singularity is an infinite amount of our time in the past, even though our cosmic clock will have only ticked 13.8 billion times since the event!
Welcome to relativity!
Note this is the same issue with approaching the singularity of a black hole. The clock approaching the singularity will reach the singularity in the time its gravitational acceleration indicates and will proceed normally to the singularity. From outside of the black hole (you have to imagine being able to make this observation) you will never see the object reach the singularity. Such an event is always in that observer's future.
If you think this is bollocks - it's GR's time paradox and is the result of a singularity, which is a strong sign that things have gone awry. BUT - we do know time dilation exists and we know GR works well right up to the point of the actual singularity so while from our perspective the object reaching the singularity probably isn't infinitely in our future, it's very, very far in our future. Likewise, for the Big Bang singularity even though the cosmic clock ticked off 13.8 billion years, from our perspective the event was much further back in time than that.
Weird stuff!
Thanks to light traveling at a fixed speed, we can compare points across time in the past with today as a reference. We just look at objects at the distance we want to observe.
const X = 299,792,458
c = X m/s
m = c * 1s / X = X/X m
s = X * 1m / c = X/X s
c = X*(X/X m)/(X/X s)
X is const, so c is const too.
c = X m/s
m = c * 1s / X = X/X m
s = X * 1m / c = X/X s
c = X*(X/X m)/(X/X s)
X is const, so c is const too.
I… don’t quite follow your point.
We observed pulsars 12B light years away. They pulsed at a frequency significantly less than what we see from equivalent pulsars today. Implication is that time passed more slowly at that time in the universe.
We observed pulsars 12B light years away. They pulsed at a frequency significantly less than what we see from equivalent pulsars today. Implication is that time passed more slowly at that time in the universe.
The frequency of a pulsar's rotation depends on factors that existed before the pulsar formed. Is it so far fetched to imagine that the universe was indeed a very different place in the distant past, and thus affected celestial bodies differently than we observe today?
Instead of assuming that "everything must always be the same", would it not be easier to assume that sometimes things are just wildly different, even when they are of the same type?
Instead of assuming that "everything must always be the same", would it not be easier to assume that sometimes things are just wildly different, even when they are of the same type?
I’m just some guy on HN distilling down an already distilled-down interpretation of the original work done by these scientists.
If you think that the Ph.D astrophysicists who concluded this totally overlooked this obvious flaw, take it up with them?
If you think that the Ph.D astrophysicists who concluded this totally overlooked this obvious flaw, take it up with them?
Ha, fair enough. I appreciate your candid response.
Yes, until you start seeing multiples of things behaving unexpectedly.
Multiple things in the distant past, where conditions were different.
To be fair, time being slower is a different condition.
Someone correct me if I'm mistaken but I don't think the implication is that time passed more slowly in some kind of absolute sense but that due to time-dilation effects we observe these rapidly receding quasars in slower time. And that if they in turn looked back at us, they would also see our portion of the universe in slower time. Of course whenever you look at anything you are looking at its past, so the effect is that the distant past everywhere appears to be moving slower.
But imagine the following: That 12 billion years ago some scientist built a briefcase-sized machine with a laser array with trillions of mirrors in a complex pattern. The laser is inside the machine and fires a regular one-second pattern into the bank of mirrors. From there the light just bounces around and around in the machine until it emerges out of an aperture. Because there are so many mirrors constructed in a complex pathway, it ends up taking 12 billion years for the light to emerge from the device. We come across the briefcase sitting in one of the craters on the Moon's south pole just as the pattern of light is emerging for the first time. (Let's pretend that whoever built the device included tech that would prevent the signal from attenuating.) The question is, would the pattern still be at a one-second interval, or will it be slowed down? If I'm understanding this news item correctly, the pattern would be still at the original speed because it will not have experienced time-dilation effects relative to us on Earth like we see in distant quasars.
But imagine the following: That 12 billion years ago some scientist built a briefcase-sized machine with a laser array with trillions of mirrors in a complex pattern. The laser is inside the machine and fires a regular one-second pattern into the bank of mirrors. From there the light just bounces around and around in the machine until it emerges out of an aperture. Because there are so many mirrors constructed in a complex pathway, it ends up taking 12 billion years for the light to emerge from the device. We come across the briefcase sitting in one of the craters on the Moon's south pole just as the pattern of light is emerging for the first time. (Let's pretend that whoever built the device included tech that would prevent the signal from attenuating.) The question is, would the pattern still be at a one-second interval, or will it be slowed down? If I'm understanding this news item correctly, the pattern would be still at the original speed because it will not have experienced time-dilation effects relative to us on Earth like we see in distant quasars.
Implication is that time passed more slowly at that time in the universe.
Or that light travelled more slowly at that time in the universe. I don't think we have conclusively proven c to be a constant, right? Or is this a case of "c and t are so inextricably linked, it doesn't matter in practice which one changes, the result is effectively the same"?
Or that light travelled more slowly at that time in the universe. I don't think we have conclusively proven c to be a constant, right? Or is this a case of "c and t are so inextricably linked, it doesn't matter in practice which one changes, the result is effectively the same"?
> I don't think we have conclusively proven c to be a constant, right?
We obviously can't directly test what the speed of light was 1.5 billion years after the Big Bang. But we can test for changes in the fine structure constant back then, which would have to have been different back then if the speed of light were different. No such changes have been found.
We obviously can't directly test what the speed of light was 1.5 billion years after the Big Bang. But we can test for changes in the fine structure constant back then, which would have to have been different back then if the speed of light were different. No such changes have been found.
I’m going to assume the credentialed astrophysicists who came to this conclusion probably have a good reason why they didn’t go with that interpretation of things.
FTL is just a matter of reflashing the universe's built-in governor circuit.
Well, except then I guess it's not technically FTL. Damn you, universe!
Well, except then I guess it's not technically FTL. Damn you, universe!
The latter, I believe. If c were 0, would time pass?
Those are quasars (supermassive black holes), not pulsars (neutron stars).
They are not pulsing, they have variations.
Anyway, meter and second are defined as fraction of c, which ruins any discussion about variability of time or distance, because we are talking about c instead.
However, when we are talking about variable time, we need to talk about variable size too. If distant quasar was 5x slower, then it must be 5x larger.
Anyway, meter and second are defined as fraction of c, which ruins any discussion about variability of time or distance, because we are talking about c instead.
However, when we are talking about variable time, we need to talk about variable size too. If distant quasar was 5x slower, then it must be 5x larger.
The frame of reference is us, now. And "slow" means the maximum speed of a photon was slower back then. The "back then" that we are now just observing from our frame of reference. At least, that's my takeaway.
Photons travel, and, as far as we can tell, have always traveled, at the same constant speed in a vacuum. It's "so constant" that even if you try to fly away from a photon at nearly c, it will still hit you with velocity c. Even if you try to fly into it at nearly c, it will still hit you with velocity c.
No time dilation, not even from black holes, can slow down (or speed up) light.
Instead, the light's frequency from your perspective is what changes. This is where redshift/blueshift comes from. (Imagine the waveform being compressed/stretched out.)
This is one of the crucial insights of special relativity.
No time dilation, not even from black holes, can slow down (or speed up) light.
Instead, the light's frequency from your perspective is what changes. This is where redshift/blueshift comes from. (Imagine the waveform being compressed/stretched out.)
This is one of the crucial insights of special relativity.
>The frame of reference is us, now.
I was about to write that myself. It's our frame of reference vs distant massive frame of reference. It is a quite natural explanation.
I was about to write that myself. It's our frame of reference vs distant massive frame of reference. It is a quite natural explanation.
> slow" means the maximum speed of a photon was slower back then
No, it doesn't. The laws of physics, including the speed of light, were the same then as now. The article's description is seriously misleading. The effect it is talking about is only an appearance due to the redshift of light from back then. It does not mean that anything was "really" happening slower.
No, it doesn't. The laws of physics, including the speed of light, were the same then as now. The article's description is seriously misleading. The effect it is talking about is only an appearance due to the redshift of light from back then. It does not mean that anything was "really" happening slower.
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"You see, Vergon 6 was once filled with a super-dense substance known as dark matter, each pound of which weighs over ten thousand pounds."
To be specific it's the "average time across all space" ran more slowly, which is sort of a weird thing to say, but I'll go with it.
Time runs more slower near massive objects like black holes. Since the average density of the universe when it was young was way higher, then yes on average time ran more slowly.
But it's also a little weird to say because you go back far enough there's no "low density" patch so you're actually comparing it to something that doesn't exist quite yet, hence a bit weird?
Time runs more slower near massive objects like black holes. Since the average density of the universe when it was young was way higher, then yes on average time ran more slowly.
But it's also a little weird to say because you go back far enough there's no "low density" patch so you're actually comparing it to something that doesn't exist quite yet, hence a bit weird?
> To be specific it's the "average time across all space" ran more slowly,
No, that is not what happened. The article's description is seriously misleading. The effect it is describing is only an appearance due to the redshift. it does not mean time "actually ran slower" back then.
> here's no "low density" patch so you're actually comparing it to something that doesn't exist quite yet, hence a bit weird?
Not just "a bit weird"--wrong. What you are saying here is a crucial difference between the universe as a whle and an isolated gravitating mass like a black hole, and that difference means that the notion of "gravitational time dilation" that works for an isolated gravitating mass is not valid for the universe as a whole. The "time dilation" described in the article is not like the gravitational time dilation around a massive object. It is like the "time dilation" due to relative motion in special relativity, which is only apparent and does not reflect any actual difference in clock rates.
No, that is not what happened. The article's description is seriously misleading. The effect it is describing is only an appearance due to the redshift. it does not mean time "actually ran slower" back then.
> here's no "low density" patch so you're actually comparing it to something that doesn't exist quite yet, hence a bit weird?
Not just "a bit weird"--wrong. What you are saying here is a crucial difference between the universe as a whle and an isolated gravitating mass like a black hole, and that difference means that the notion of "gravitational time dilation" that works for an isolated gravitating mass is not valid for the universe as a whole. The "time dilation" described in the article is not like the gravitational time dilation around a massive object. It is like the "time dilation" due to relative motion in special relativity, which is only apparent and does not reflect any actual difference in clock rates.
Why would there not be gravitational time dilation in a denser early universe as the article describes?
The reason the GP gives is a step in the right direction: the density is constant everywhere in space at a given point in time, and variations in "gravitational potential" (which are necessary for gravitational time dilation) can only be present if there are differences in density.
A more technically correct reason is that the concept of "gravitational potential" (and hence gravitational time dilation) is not even well-defined in an expanding universe. "Gravitational potential" is only well-defined in a stationary spacetime, which means one in which the geometry does not change with time. An expanding universe is not a stationary spacetime. (But even in a stationary spacetime, variations in gravitational potential, which require variations in density, are necessary for gravitational time dilation to exist. That is why the GP's answer described above is a step in the right direction.)
A more technically correct reason is that the concept of "gravitational potential" (and hence gravitational time dilation) is not even well-defined in an expanding universe. "Gravitational potential" is only well-defined in a stationary spacetime, which means one in which the geometry does not change with time. An expanding universe is not a stationary spacetime. (But even in a stationary spacetime, variations in gravitational potential, which require variations in density, are necessary for gravitational time dilation to exist. That is why the GP's answer described above is a step in the right direction.)
Could it be said that entanglement was originally near infinite, as it might now be in black holes? Perhaps now that things are spread out, time appears to move faster because there is less stickiness of entangled particles depending on others in order to proceed, whereas mass entanglement acts like anti-time due to the need of so many webs of chains of entanglement to be notified back to the original particle so they can act in reverse order (as entanglement appears to work). Could a massive density of anti-time entanglement help explain event horizons more completely than gravitational effects alone?
Could the big bang-like event have been entanglement flipping like a magnetic pole and generating a diaspora where things began happening causally -- which is slowly re-condensing when things group densely with the help of gravity as it catches up?
I get the feeling that there is a lot more entanglement than we give credit to, with time-altering effects as complexity grows, maybe leading to brains with consciousness rather than classically predictable chemical networks, and...perhaps mild cell division-sourced innate entanglement? Look forward to finding out.
Could the big bang-like event have been entanglement flipping like a magnetic pole and generating a diaspora where things began happening causally -- which is slowly re-condensing when things group densely with the help of gravity as it catches up?
I get the feeling that there is a lot more entanglement than we give credit to, with time-altering effects as complexity grows, maybe leading to brains with consciousness rather than classically predictable chemical networks, and...perhaps mild cell division-sourced innate entanglement? Look forward to finding out.
> Could the big bang-like event have been entanglement flipping like a magnetic pole
No. Nothing like this appears in any of our scientific models of the Big Bang.
No. Nothing like this appears in any of our scientific models of the Big Bang.
And how is density measured? By the amount of energy per cubic meter, yeah? So how is a meter defined? By the distance light travels in 1/299792458 of a second. I get the feeling that it's all homomorphic.
> Time runs more slower near massive objects like black holes.
Do you mean proper time or time as seen by an outside observer?
Do you mean proper time or time as seen by an outside observer?
Originally, I meant it as - as seen from an outside observer.
I'm talking out of my pay grade now, but it also could be for proper time if you started away from the black hole and fell towards it then compared it from your start and end? Probably unnecessarily complex for this discussion though.
I'm talking out of my pay grade now, but it also could be for proper time if you started away from the black hole and fell towards it then compared it from your start and end? Probably unnecessarily complex for this discussion though.
Also punching above my weight class here: isn't observation a function of time as well though, so a local observer would notice no difference in their perception of the passage of time? It's only across distance that the perception of time would change per relativity, so obseevation of self would remain the same while observation of distant objects would appear to experience time faster, while an outside observer would perceive their own time constant and that of the infalling observer to slow through time.
Why does density affect time? How are the two related?
The stress-energy (of which matter density is a component) of a region of spacetime determines its curvature.
In empty space the metric, which tells us how to measure distances between events, is `ds^2 = dt^2 - (dx^2 + dy^2 + dz^2)`. Integrating this along a trajectory tells you the (square of the) proper time experienced by an observer traveling along it. As you can see, the further you move in space in a given amount of coordinate-time, the less proper time you experience: this is the source of time dilation in special relativity.
Around a spherical body of mass M, the metric is (in spherical coordinates, and ignoring some scale factors for convenience):
`ds^2 = (1-M/r)dt^2 - (1-M/r)^-1 (dr^2) - r^2 (dtheta^2 + sin(theta)^2 dphi^2)`
Compare to the flat space metric from before, now in spherical coordinates as well:
`ds^2 = dt^2 - dr^2 - r^2 (dtheta^2 + sin(theta)^2 dphi^2)`
So we pick up some additional time dilation as long as `M < r` (and it always is, unless you're in the interior of a black hole)
In empty space the metric, which tells us how to measure distances between events, is `ds^2 = dt^2 - (dx^2 + dy^2 + dz^2)`. Integrating this along a trajectory tells you the (square of the) proper time experienced by an observer traveling along it. As you can see, the further you move in space in a given amount of coordinate-time, the less proper time you experience: this is the source of time dilation in special relativity.
Around a spherical body of mass M, the metric is (in spherical coordinates, and ignoring some scale factors for convenience):
`ds^2 = (1-M/r)dt^2 - (1-M/r)^-1 (dr^2) - r^2 (dtheta^2 + sin(theta)^2 dphi^2)`
Compare to the flat space metric from before, now in spherical coordinates as well:
`ds^2 = dt^2 - dr^2 - r^2 (dtheta^2 + sin(theta)^2 dphi^2)`
So we pick up some additional time dilation as long as `M < r` (and it always is, unless you're in the interior of a black hole)
Could someone help settle a debate?
The universe is infinite, or at least appears so. The cosmic background radiation was because the early universe was hot everywhere. However, I’ve been telling my wife that the universe didn’t start from a single point; it has always been infinite. The concept of “smaller” is only because the matter was less dispersed than it is today. Veritasium’s video seems to support this argument.
She’s pointed towards a few different articles saying that the most accepted theory of physics is that the universe started from a single point, and that there is language such as “it grew to the size of an apple” within a small amount of time after the Big Bang.
How can both views be correct? Either the universe isn’t infinite, and therefore has some kind of edge, or it is, and has always had infinite size.
The universe is infinite, or at least appears so. The cosmic background radiation was because the early universe was hot everywhere. However, I’ve been telling my wife that the universe didn’t start from a single point; it has always been infinite. The concept of “smaller” is only because the matter was less dispersed than it is today. Veritasium’s video seems to support this argument.
She’s pointed towards a few different articles saying that the most accepted theory of physics is that the universe started from a single point, and that there is language such as “it grew to the size of an apple” within a small amount of time after the Big Bang.
How can both views be correct? Either the universe isn’t infinite, and therefore has some kind of edge, or it is, and has always had infinite size.
The correct answer is, we don't know. We can see back until about 380,000 years when the universe became transparent enough for light to reach us, the cosmic microwave background. Everything before that is theoretical and depends on the model used, for example cosmic inflation, which has experimental support in so far as it correctly predicts features of the cosmic microwave background. Did inflation actually happen? Maybe. But even if inflation is the correct theory, go back far enough towards time zero and quantum effects become relevant to avoid the classical big bang singularity but we don't have an accepted theory of quantum gravity. So who knows what was actually going on. Observing the cosmic neutrino background could allow us the see past the cosmic microwave background but for the very first moments - if there was such a thing - we will probably have to rely on theory - which we don't have - for the foreseeable future.
We can "see" a little bit further than that. The observed helium abundance for example shows that our understanding of the first three minutes appears to be quite robust.
Inflation covers a much different era, 10^-32s after the big bang.
Inflation covers a much different era, 10^-32s after the big bang.
"most accepted theory of physics is that the universe started from a single point": This is not true. The most accepted theory is that the universe was extremely dense, but it is not accepted that it was literally a "point" of zero volume. Our laws of physics are thought to break down beyond a certain density of matter, so we don't have any reliable predictions about it. The "dense" universe could still have been infinite even back then. Just super dense and infinite in all directions - and now all of it has expanded and it's still infinite (a bigger infinity?).
Not only at very high temperatures and densities we cannot be certain about the properties of matter, but there exists absolutely no evidence that can justify the extrapolation of the evolution of the universe towards such temperatures and densities.
The properties of the observable universe are consistent with an initial state where the temperature was so high that the protons and neutrons were free, not bound in atomic nuclei, like today.
This means that the temperature corresponded to a kinetic energy of several tens of MeV per particle. At such a temperature, the state of the matter is a plasma composed only of protons, neutrons, electrons, positrons, photons and neutrinos.
At this temperature, the matter has the simplest possible composition. At lower temperatures the particles become bound in nuclei, atoms and molecules, which become more and more complex with decreasing temperatures. At increasing temperatures, the higher the temperature is, more and more mesons, baryons and heavy leptons are generated and the plasma composition becomes more and more complex.
However, at that temperature where the matter has the simplest composition, any traces of its former evolution have been practically erased and we do not have any rational reason for making suppositions about it. Perhaps the matter was indeed even denser and even hotter, but there is no basis for this extrapolation except some philosophical belief that is not based on any scientific observation.
An extrapolation towards greater temperatures and densities could be justified only if we knew boundary conditions around the universe, which we do not know.
In conclusion, what can be known with reasonable certainty about the Big Bang starts from a moment in time when the temperature corresponded to a kinetic energy of several tens of MeV per particle, and all extrapolations to times before that moment are hypotheses that are not based on any experimental data, so they can be neither verified nor falsified.
The supposition that the Universe may have started from a single point is something that has nothing to do with science, but it may be a valid religious belief.
The properties of the observable universe are consistent with an initial state where the temperature was so high that the protons and neutrons were free, not bound in atomic nuclei, like today.
This means that the temperature corresponded to a kinetic energy of several tens of MeV per particle. At such a temperature, the state of the matter is a plasma composed only of protons, neutrons, electrons, positrons, photons and neutrinos.
At this temperature, the matter has the simplest possible composition. At lower temperatures the particles become bound in nuclei, atoms and molecules, which become more and more complex with decreasing temperatures. At increasing temperatures, the higher the temperature is, more and more mesons, baryons and heavy leptons are generated and the plasma composition becomes more and more complex.
However, at that temperature where the matter has the simplest composition, any traces of its former evolution have been practically erased and we do not have any rational reason for making suppositions about it. Perhaps the matter was indeed even denser and even hotter, but there is no basis for this extrapolation except some philosophical belief that is not based on any scientific observation.
An extrapolation towards greater temperatures and densities could be justified only if we knew boundary conditions around the universe, which we do not know.
In conclusion, what can be known with reasonable certainty about the Big Bang starts from a moment in time when the temperature corresponded to a kinetic energy of several tens of MeV per particle, and all extrapolations to times before that moment are hypotheses that are not based on any experimental data, so they can be neither verified nor falsified.
The supposition that the Universe may have started from a single point is something that has nothing to do with science, but it may be a valid religious belief.
But if it was dense and infinite in all directions, why do so many people use language like “it was the size of an apple”? This is the interesting contradiction. An apple is only a certain number of atoms, far fewer than the 10^80 we have today.
I think that's supposed to refer to the currently observable bit being the size of an apple; though given how bad newspaper analogies are, this may all derive from misunderstandings one hungry physicist 60 years ago and subsequently all quoting each other.
Here’s my take:
- today’s observable universe is roughly 13.8B years. By the time you travel to the edge of that universe, others stuff has travel from that edge to even further position at various speed including faster than you do. Therefore you never get to that edge, making it infinite.
- before/at big bang universe is a single point. You are within this single point but can’t go outside because time does not exist and traveling need time.
- when the big band happened, it grew in size very, very fast, close to speed of light. During that enlargement phase it grew to the size of an Apple and if you were within it and try to look in any direction, the further think you can see is the Apple skin. That’s the observable universe. But to see the skin you need to go there or wait for particule to come to you (eg photon). By that time the universe has already dilated and grown a lot wider. Some particules from the skin are already far away. You’ll never be able to see them : by the time needed for information to come to you, other information goes in the opposite direction.
- today’s observable universe is roughly 13.8B years. By the time you travel to the edge of that universe, others stuff has travel from that edge to even further position at various speed including faster than you do. Therefore you never get to that edge, making it infinite.
- before/at big bang universe is a single point. You are within this single point but can’t go outside because time does not exist and traveling need time.
- when the big band happened, it grew in size very, very fast, close to speed of light. During that enlargement phase it grew to the size of an Apple and if you were within it and try to look in any direction, the further think you can see is the Apple skin. That’s the observable universe. But to see the skin you need to go there or wait for particule to come to you (eg photon). By that time the universe has already dilated and grown a lot wider. Some particules from the skin are already far away. You’ll never be able to see them : by the time needed for information to come to you, other information goes in the opposite direction.
> today’s observable universe is roughly 13.8B years. By the time you travel to the edge of that universe
Careful. That's the age of the observable universe, not the size. The radius of the observable universe is 47B light years, which is larger because of inflation.
> Therefore you never get to that edge, making it infinite.
FWIW, I disagree with the conclusion. Finite but expanding fast is not infinite (disclaimer: mathematician not a physicist).
Careful. That's the age of the observable universe, not the size. The radius of the observable universe is 47B light years, which is larger because of inflation.
> Therefore you never get to that edge, making it infinite.
FWIW, I disagree with the conclusion. Finite but expanding fast is not infinite (disclaimer: mathematician not a physicist).
> today’s observable universe is roughly 13.8B years. By the time you travel to the edge of that universe, others stuff has travel from that edge to even further position at various speed including faster than you do.
This is true but it's not what astronomers mean when they say the universe may be infinite. They're talking about the whole universe, not the just the observable part, and they really do mean actually infinite in extent.
> before/at big bang universe is a single point.
Unlikely. We know that what is now the observable universe was very very very very very dense. Predictions of a literal singular point are the result of extrapolating past the point where we know for a fact that our best physical theories break down.
This is true but it's not what astronomers mean when they say the universe may be infinite. They're talking about the whole universe, not the just the observable part, and they really do mean actually infinite in extent.
> before/at big bang universe is a single point.
Unlikely. We know that what is now the observable universe was very very very very very dense. Predictions of a literal singular point are the result of extrapolating past the point where we know for a fact that our best physical theories break down.
You are correct.
> She’s pointed towards a few different articles saying that the most accepted theory of physics is that the universe started from a single point, and that there is language such as “it grew to the size of an apple” within a small amount of time after the Big Bang.
These articles implicitly talk about the observable universe. Or they're just wrong. It definitely didn't start from a single point, but there was a time where the observable universe was the size of an apple (or anything else that is larger than a point).
> She’s pointed towards a few different articles saying that the most accepted theory of physics is that the universe started from a single point, and that there is language such as “it grew to the size of an apple” within a small amount of time after the Big Bang.
These articles implicitly talk about the observable universe. Or they're just wrong. It definitely didn't start from a single point, but there was a time where the observable universe was the size of an apple (or anything else that is larger than a point).
Isn’t it possible that there was a period before the Big Bang, and then for some reason the universe became hot everywhere?
In other words, the idea that the observable universe could be the size of an apple has its own contradictions. An apple is a definite size, and can fit only a certain number of atoms. However the expansion of the universe doesn’t seem to be creating more atoms; it’s only moving them around. Therefore all apples would’ve been the same size they were today, just packed more closely together. Hence “the size of an apple” seems to lose all meaning.
This contradiction seems to clear itself up if one allows for a “before the big bang”, since there’s no need to say that everything was compressed to a single small volume.
In other words, the idea that the observable universe could be the size of an apple has its own contradictions. An apple is a definite size, and can fit only a certain number of atoms. However the expansion of the universe doesn’t seem to be creating more atoms; it’s only moving them around. Therefore all apples would’ve been the same size they were today, just packed more closely together. Hence “the size of an apple” seems to lose all meaning.
This contradiction seems to clear itself up if one allows for a “before the big bang”, since there’s no need to say that everything was compressed to a single small volume.
> Isn’t it possible that there was a period before the Big Bang, and then for some reason the universe became hot everywhere?
Well, sure, it's possible. I was assuming the current cosmological standard model is correct, which might in fact be wrong, or at least incomplete. Actually, I'm certain it's incomplete.
Today we can trace back the universe to when it was a tiny fraction of a second old using standard physics. Everything is well understood. Before that: who knows?
> An apple is a definite size, and can fit only a certain number of atoms.
There were no atoms when the current observable universe was the size of an apple. It was too hot for atoms. It was more like a quark-gluon plasma.
> Therefore all apples would’ve been the same size they were today, just packed more closely together. Hence “the size of an apple” seems to lose all meaning.
Obviously "the size of an apple" is a phrase for laymen. We could say "10cm in diameter" using the standard definitions of the meter and second.
>This contradiction seems to clear itself up if one allows for a “before the big bang”, since there’s no need to say that everything was compressed to a single small volume.
There is no contradiction.
Well, sure, it's possible. I was assuming the current cosmological standard model is correct, which might in fact be wrong, or at least incomplete. Actually, I'm certain it's incomplete.
Today we can trace back the universe to when it was a tiny fraction of a second old using standard physics. Everything is well understood. Before that: who knows?
> An apple is a definite size, and can fit only a certain number of atoms.
There were no atoms when the current observable universe was the size of an apple. It was too hot for atoms. It was more like a quark-gluon plasma.
> Therefore all apples would’ve been the same size they were today, just packed more closely together. Hence “the size of an apple” seems to lose all meaning.
Obviously "the size of an apple" is a phrase for laymen. We could say "10cm in diameter" using the standard definitions of the meter and second.
>This contradiction seems to clear itself up if one allows for a “before the big bang”, since there’s no need to say that everything was compressed to a single small volume.
There is no contradiction.
Okay, but 10cm can only fit a certain number of quarks and gluons, far fewer than today’s observable universe. Or is it true that there were always 10^80 atoms worth of quarks and gluons in the observable universe, and always will be?
Galaxies recede faster than light, so atoms are shedded by that process. If anything it seems like there must have been more matter in the early observable universe than today. But “10cm in diameter” seems to imply otherwise.
Galaxies recede faster than light, so atoms are shedded by that process. If anything it seems like there must have been more matter in the early observable universe than today. But “10cm in diameter” seems to imply otherwise.
> Or is it true that there were always 10^80 atoms worth of quarks and gluons in the observable universe, and always will be?
Pretty much, yes. (Let's ignore the possibility of proton decay for now.)
> Galaxies recede faster than light, so atoms are shedded by that process. If anything it seems like there must have been more matter in the early observable universe than today. But “10cm in diameter” seems to imply otherwise.
Not sure what you mean by "shedding atoms", but I'm pretty sure I can say that this is not the case.
Pretty much, yes. (Let's ignore the possibility of proton decay for now.)
> Galaxies recede faster than light, so atoms are shedded by that process. If anything it seems like there must have been more matter in the early observable universe than today. But “10cm in diameter” seems to imply otherwise.
Not sure what you mean by "shedding atoms", but I'm pretty sure I can say that this is not the case.
well we loose access to them, so for all intents and purposes observable universe looses atoms. Very anthropo-centric, but not wrong.
We've lost access to many which are still visible, thanks to the continuing expansion of the universe; eventually the light from them may be red-shifted to a wavelength larger than the visible universe, making them undetectable in any meaningful sense.
Physics is always anthropocentric: if we can't measure it, we can't say anything about if it exists
Alright. Thank you.
Language such as “it grew to the size of an apple” always refers to the currently observable universe. That portion of the universe was the size of an apple at some point in time.
You put into words a discrepancy that has annoyed the back of my mind for some time now.
I had actually always learned that it went from single point to infinite instantly but grows more dispersed. But recently have been seeing a lot things in a similar vein, Kurzgesagt put out a video and even a special pin about the size of the universe at different points of time. It would seem if we know that to be the case and we know the growth rate we could then estimate the size of the universe now, but nobody suggests a finite universe with a certain size as the present universe, just the past.
That would additionally suggest that there is a ‘center’ of the universe which I again have always learned as N/A since it is infinite. Really confuses me.
I had actually always learned that it went from single point to infinite instantly but grows more dispersed. But recently have been seeing a lot things in a similar vein, Kurzgesagt put out a video and even a special pin about the size of the universe at different points of time. It would seem if we know that to be the case and we know the growth rate we could then estimate the size of the universe now, but nobody suggests a finite universe with a certain size as the present universe, just the past.
That would additionally suggest that there is a ‘center’ of the universe which I again have always learned as N/A since it is infinite. Really confuses me.
Key word is “Observable”.
The observable universe has a definite size. Shortly after the Big Bang, what is now the observable universe was the size of an apple etc. Now it is many tens of billions of light years across.
The observable universe does have a center - you, the observer.
The observable universe has a definite size. Shortly after the Big Bang, what is now the observable universe was the size of an apple etc. Now it is many tens of billions of light years across.
The observable universe does have a center - you, the observer.
Okay that distinction would make sense to me - we know everything was closer together - so what is currently the observable universe was once different sizes but we can’t speak to the grander universe as a whole as having a size in any meaningful terms. That sound correct?
We’re reasonably sure the universe as a whole is infinite. Whether or not you want to accept that as a valid size is up to you, but I’d recommend defining “size” to include ordinals.
> We’re reasonably sure the universe as a whole is infinite.
I don't know what that means. As a kid, my notion was that spacetime was "infinite", that galaxies floated in spacetime, and were moving apart. Gradually I became (a bit) more comfortable with the notion that spacetime is somehow created by "stuff", and that a location in spacetime that has never seen a photon isn't a location in spacetime at all.
A lot of commenters here seem to be saying that spacetime was always infinite; they claim that the universe that used to be the size of an apple is now the whole observable universe. The implication is that there was plenty of stuff beyond the "apple-peel", but that all that external stuff has never had a chance to interact in any way with the interior of the apple.
I don't know how to grok that. There's plenty of stuff that used to be part of the apple that isn't observable; presumably there was some location within the apple that later became the Milky Way, and any photon departing that location 13.8B years ago will never be observable. Arguably, more than half of what used to be the apple will never be observable.
The story I've been sold is that it's not that stuff is moving apart, but that spacetime is expanding. I can visualize 3D space expanding; one moment it's the size of an apple, a few moments later it's the size of a grapefruit. But what does it mean for time to expand? The very idea of expansion is of a thing that grows relative to time. Spacetime is the idea of everything, everywhere, all at once. It's not an anthropocentric notion; it doesn't depend on observers, and I fail to see how spacetime can be considered to be evolving in time.
I don't know what that means. As a kid, my notion was that spacetime was "infinite", that galaxies floated in spacetime, and were moving apart. Gradually I became (a bit) more comfortable with the notion that spacetime is somehow created by "stuff", and that a location in spacetime that has never seen a photon isn't a location in spacetime at all.
A lot of commenters here seem to be saying that spacetime was always infinite; they claim that the universe that used to be the size of an apple is now the whole observable universe. The implication is that there was plenty of stuff beyond the "apple-peel", but that all that external stuff has never had a chance to interact in any way with the interior of the apple.
I don't know how to grok that. There's plenty of stuff that used to be part of the apple that isn't observable; presumably there was some location within the apple that later became the Milky Way, and any photon departing that location 13.8B years ago will never be observable. Arguably, more than half of what used to be the apple will never be observable.
The story I've been sold is that it's not that stuff is moving apart, but that spacetime is expanding. I can visualize 3D space expanding; one moment it's the size of an apple, a few moments later it's the size of a grapefruit. But what does it mean for time to expand? The very idea of expansion is of a thing that grows relative to time. Spacetime is the idea of everything, everywhere, all at once. It's not an anthropocentric notion; it doesn't depend on observers, and I fail to see how spacetime can be considered to be evolving in time.
> As a kid, my notion was that spacetime was "infinite", that galaxies floated in spacetime, and were moving apart. Gradually I became (a bit) more comfortable with the notion that spacetime is somehow created by "stuff", and that a location in spacetime that has never seen a photon isn't a location in spacetime at all.
Neither is especially accurate. Spacetime is not a substance, and it's not created or destroyed. It's the thing physics happens on, for lack of a better non-mathematical description.
> But what does it mean for time to expand? The very idea of expansion is of a thing that grows relative to time.
Same thing as for space: that component of the metric - the shape of spacetime - is an increasing function of coordinate time. But you can always pick a coordinate system where only the spatial components are expanding. It's not physically meaningful either way. The metric is the real object.
Neither is especially accurate. Spacetime is not a substance, and it's not created or destroyed. It's the thing physics happens on, for lack of a better non-mathematical description.
> But what does it mean for time to expand? The very idea of expansion is of a thing that grows relative to time.
Same thing as for space: that component of the metric - the shape of spacetime - is an increasing function of coordinate time. But you can always pick a coordinate system where only the spatial components are expanding. It's not physically meaningful either way. The metric is the real object.
> How can both views be correct?
They aren't. Your view is correct, at least according to our best current model of the universe.
I suspect that your wife is misinterpreting the articles she is reading; I suspect they are saying only that our current observable universe, which is finite in size, started out as a single point. Which is sort of correct, but is not the same as saying the entire spatially infinite universe started out as a single point.
Part of the problem here is that the "initial singularity" in the models being described (which is where the "single point" language comes from) is not actually part of the universe; it's a mathematical limit that does not actually correspond to anything physical.
> Either the universe isn’t infinite, and therefore has some kind of edge
The "edge" part is not correct. A spatially infinite universe would have the spatial topology of a 3-sphere, which has a finite volume bot no edge (just as the 2-sphere surface of our Earth has a finite area but no edge).
They aren't. Your view is correct, at least according to our best current model of the universe.
I suspect that your wife is misinterpreting the articles she is reading; I suspect they are saying only that our current observable universe, which is finite in size, started out as a single point. Which is sort of correct, but is not the same as saying the entire spatially infinite universe started out as a single point.
Part of the problem here is that the "initial singularity" in the models being described (which is where the "single point" language comes from) is not actually part of the universe; it's a mathematical limit that does not actually correspond to anything physical.
> Either the universe isn’t infinite, and therefore has some kind of edge
The "edge" part is not correct. A spatially infinite universe would have the spatial topology of a 3-sphere, which has a finite volume bot no edge (just as the 2-sphere surface of our Earth has a finite area but no edge).
How does something infinite "expand" though?
It's not expanding into anything, because it's infinite.
I always hear "space itself is expanding" but how does that work with topology?
When you travel in a given direction you will arrive back where you started?
Surely that "shape" or "object" or topology or whatever you want to call it has a volume, relative to something else, or it wouldn't be able to be calculated?
As you can tell, I have no idea what I'm talking about but endlessly curious on this subject.
It's not expanding into anything, because it's infinite.
I always hear "space itself is expanding" but how does that work with topology?
When you travel in a given direction you will arrive back where you started?
Surely that "shape" or "object" or topology or whatever you want to call it has a volume, relative to something else, or it wouldn't be able to be calculated?
As you can tell, I have no idea what I'm talking about but endlessly curious on this subject.
>How does something infinite "expand" though?
It's not expanding into anything, because it's infinite.
I’ve found it easiest to think of this in one dimension first.
Imagine a number line. It’s infinite in both directions, in the sense that you can move along it and never reach an end. Suppose we’re sitting at X=0 and look in the positive direction at X=1. That point is distance 1 away from us. Look the other way at X=-1, and that point is distance 1 away, too.
Now let’s multiply the number line by 2. The point that used to be 1 is now 2, and 2 is now 4. 0.5 is now 1. Look in both directions, and it’s still infinite by the same definition; you can travel in either direction and never reach an end. We’re still at X=0 and look in the positive direction. The point that used to be 1 is now distance 2 away. Same with what was previously -1.
There’s nothing special about the location at the origin either. If we had originally sat at X=10 and looked at X=11, we’d now be at X=20 and looking at X=22: a distance of 2 away. In this way, every point has an equal claim to being the “center of expansion.”
In one dimension, this is equivalent to saying that “space expanded,” it was infinite before, and it’s infinite after expansion.
I’ve found it easiest to think of this in one dimension first.
Imagine a number line. It’s infinite in both directions, in the sense that you can move along it and never reach an end. Suppose we’re sitting at X=0 and look in the positive direction at X=1. That point is distance 1 away from us. Look the other way at X=-1, and that point is distance 1 away, too.
Now let’s multiply the number line by 2. The point that used to be 1 is now 2, and 2 is now 4. 0.5 is now 1. Look in both directions, and it’s still infinite by the same definition; you can travel in either direction and never reach an end. We’re still at X=0 and look in the positive direction. The point that used to be 1 is now distance 2 away. Same with what was previously -1.
There’s nothing special about the location at the origin either. If we had originally sat at X=10 and looked at X=11, we’d now be at X=20 and looking at X=22: a distance of 2 away. In this way, every point has an equal claim to being the “center of expansion.”
In one dimension, this is equivalent to saying that “space expanded,” it was infinite before, and it’s infinite after expansion.
> How does something infinite "expand" though?
Infinite things don't work like our ordinary intuitions expect.
> I always hear "space itself is expanding" but how does that work with topology?
The topology doesn't change as the universe expands. "Expanding universe" just means that a particular metric (geometry) is imposed on the topology. If you think of the spacetime of the universe as a stack of 3-dimensional spatial slices, "expansion" just means that a given pair of points in space gets further and further apart as you advance from slice to slice into the future. That is perfectly well-defined for infinite slices, even though it's hard to imagine intuitively. The math works just fine.
> When you travel in a given direction you will arrive back where you started?
No. That would be the case for a spatially finite universe with the topology of a 3-sphere. But our spatially infinite universe has the topology of Euclidean 3-space.
> Surely that "shape" or "object" or topology or whatever you want to call it has a volume
The spatial 3-volume of the universe is infinite.
> relative to something else, or it wouldn't be able to be calculated?
Not at all. The universe doesn't need to be embedded in anything else to have a well-defined volume.
Infinite things don't work like our ordinary intuitions expect.
> I always hear "space itself is expanding" but how does that work with topology?
The topology doesn't change as the universe expands. "Expanding universe" just means that a particular metric (geometry) is imposed on the topology. If you think of the spacetime of the universe as a stack of 3-dimensional spatial slices, "expansion" just means that a given pair of points in space gets further and further apart as you advance from slice to slice into the future. That is perfectly well-defined for infinite slices, even though it's hard to imagine intuitively. The math works just fine.
> When you travel in a given direction you will arrive back where you started?
No. That would be the case for a spatially finite universe with the topology of a 3-sphere. But our spatially infinite universe has the topology of Euclidean 3-space.
> Surely that "shape" or "object" or topology or whatever you want to call it has a volume
The spatial 3-volume of the universe is infinite.
> relative to something else, or it wouldn't be able to be calculated?
Not at all. The universe doesn't need to be embedded in anything else to have a well-defined volume.
Infinity is a tricky concept - a 2D creature on the surface of a 3D inflating balloon would view the surface they were on as infinite, particularly if the balloon was inflating faster than the bug could travel, such that no circumnavigation of the sphere was possible. Infinite space implies that one could travel forever in a straight line without encountering a boundary, but if we move to higher dimensions then it's possible that the universe is something like a 4D sphere of finite 4D volume bounded by a 3D 'surface' which is apparently infinite to its inhabitants.
Moving to higher dimensions it might be the case that there are many universes, i.e. perhaps from a 5D perspective the multiverse is an infinite series of nested 4D universe hyperspheres, and one could go on and on like this into a infinite series of dimensions, but it's all just mathamatical metaphysics at that point as there's no observational evidence for any of that.
Moving to higher dimensions it might be the case that there are many universes, i.e. perhaps from a 5D perspective the multiverse is an infinite series of nested 4D universe hyperspheres, and one could go on and on like this into a infinite series of dimensions, but it's all just mathamatical metaphysics at that point as there's no observational evidence for any of that.
You can use Occam's razor[0]: simpler solution with smaller set of elements has better probability to be true.
a) Bing Bang model of evolution of Universe: single point, big bang, expansion of Universe, acceleration of expansion, time dilation, space-time warping. (6 extra elements)
b) Steady Universe model of evolution of Universe: tired light. (1 extra element)
[0]: https://en.wikipedia.org/wiki/Occam%27s_razor
a) Bing Bang model of evolution of Universe: single point, big bang, expansion of Universe, acceleration of expansion, time dilation, space-time warping. (6 extra elements)
b) Steady Universe model of evolution of Universe: tired light. (1 extra element)
[0]: https://en.wikipedia.org/wiki/Occam%27s_razor
My favorite cyclic theory so far is probably Roger Penrose's conformal cyclic cosmology. In essence the universe keeps expanding exponentially which in the end becomes equivalent to a new big bang since the universe at that point will loose all notions of scale due to there being no more particles to drive scale. A much better explanation is given here https://youtu.be/FVDJJVoTx7s
I'm a Penrose fan too. But I don't think CCC envisages any Big Bangs; CCC envisages a future heat death, where everything has turned into solitary photons. Photons have no clock, and don't experience the passing of time; distance is nothing to a photon, the moment it starts travelling, it arrives at its destination.
So a universe containing only photons has no definite scale - it can re-imagine itself as being the size of a grapefruit, or any size you like. Penrose uses this idea to wish away the awkward and evidence-free idea of cosmic inflation.
So a universe containing only photons has no definite scale - it can re-imagine itself as being the size of a grapefruit, or any size you like. Penrose uses this idea to wish away the awkward and evidence-free idea of cosmic inflation.
To add to your question and a weak attempt at answering (as a Layman)- when the universe was the size of an apple, how long would it take for a particle to travel from one side to the other? I think the answer would be forever which in a sense makes it infinite, regardless of the universe's finite size.
I guess we have the luxury of removing ourselves from the universe while thinking about it, if Platonism is a thing.
Reminds me of the halting problem.
I guess we have the luxury of removing ourselves from the universe while thinking about it, if Platonism is a thing.
Reminds me of the halting problem.
I find it easier to picture the universe as more of a growing network of connected things, rather than an object with strict internal or external dimensions. I don’t think it (or anything) is infinite, but it might as well be because it cleverly prevents all parts from interacting with each other at all times (due to limits on information processing capacity, if you want to get quasi-religious about it).
Greetings fellow Monad.
I’m not 100% on this but the way I understood this is by analogy to the 2D surface of the Earth. The surface of the Earth is finite yet has no reachable edge. The universe is the same in 3D. Like if the Earth grew then there would be more land but still no more or less “edge” of the world, and the same could hold for the universe. That said I think this is one theory rather than accepted fact?
> The universe is the same in 3D.
This would mean the universe has positive curvature. Experimental evidence points towards the universe being flat (zero curvature), though there is some margin for error that could go either way (positive or negative curvature).
This would mean the universe has positive curvature. Experimental evidence points towards the universe being flat (zero curvature), though there is some margin for error that could go either way (positive or negative curvature).
> This would mean the universe has positive curvature.
There are several flat compact manifolds, they're just not simply connected.
There are several flat compact manifolds, they're just not simply connected.
In particular, almost none of them are isotropic. The assumption that the universe is isotropic is part of the cosmological principle and the foundation of modern cosmology. It's a very natural assumption to make, so the vast majority of cosmologists are quite comfortable with it, but at the end of the day it's just an assumption that could be wrong.
In the case of a universe of finite size, this analogy explains how there can be such a thing as a finite space without there being boundaries provided the space is (slightly) curved.
So this has less to do with an infinitely sized universe and more with the question of “What exists beyond the edge of the universe if it would be finite in size?”
So this has less to do with an infinitely sized universe and more with the question of “What exists beyond the edge of the universe if it would be finite in size?”
Our universe is probably a black hole in another, larger universe we cannot reach. And then turtles all the way down.
> The universe is infinite, or at least appears so.
The universe is certainly very big but whether it's finite or infinite is not known with certainty, see https://en.wikipedia.org/wiki/Shape_of_the_universe#Infinite...
The universe is certainly very big but whether it's finite or infinite is not known with certainty, see https://en.wikipedia.org/wiki/Shape_of_the_universe#Infinite...
I believe it's an open question, the curvature of the universe, and hence whether the universe is infinite.
Others have already clarified that the big bang tells us the observable universe used to be tiny. In other words, the universe was much denser. It does not tell us the absolute size of the entire universe.
Others have already clarified that the big bang tells us the observable universe used to be tiny. In other words, the universe was much denser. It does not tell us the absolute size of the entire universe.
"Denser Universe" means that we were (or are) in a black hole. There is no other way around a huge mass in a small space.
Imagine you're inflating an air balloon, first to a size of an apple then size of your head then to enormous like a weather balloon. Now consider that the surface of balloon is the space: points on the surface get further and further apart but there is no edge or boundary.
How is it possible to say with any certainty that the universe was the size of an apple?
That analogy also implies that space wraps around, which no experiment has ever shown to be true. This contradiction is why I have trouble.
It’s a good analogy otherwise, but the details are the interesting part.
That analogy also implies that space wraps around, which no experiment has ever shown to be true. This contradiction is why I have trouble.
It’s a good analogy otherwise, but the details are the interesting part.
> How is it possible to say with any certainty that the universe was the size of an apple?
Haven't read the paper but one likely could tell from the density.
> That analogy also implies that space wraps around, which no experiment has ever shown to be true.
It's an analogy meant to highlight one particular aspect, not be a perfect model of spacetime. In mathematics, there is no shortage of topological objects that are infinite but not closed otherwise.
Haven't read the paper but one likely could tell from the density.
> That analogy also implies that space wraps around, which no experiment has ever shown to be true.
It's an analogy meant to highlight one particular aspect, not be a perfect model of spacetime. In mathematics, there is no shortage of topological objects that are infinite but not closed otherwise.
Wouldn’t that be more applicable in a universe that wasn’t flat (unlike ours as best as we can measure).
There is also no air inside and nobody's actually blowing. It's an analogy.
It is an analogy that is used to describe spacetime curvature though… its the wrong analogy.
No it's an analogy used to describe inflation with no boundary.
Air balloon has clear boundary.
The volume enclosed by the balloon's surface has a boundary, the surface itself does not.
The universe isn’t thought to be infinite in that sense- it’s finite but unbounded. There is a finite amount of space that did, we think, originate from an infinitesimal point.
Imagine getting in a spaceship and setting off in a direction, close to the speed of light- if you travelled for long enough you’d arrive back where you started. Like someone walking on the surface of the earth.
Any current cosmologists can correct me, but I believe the consensus is that the shape of the universe is a hyper-spheroid, I.e a 4D sphere.
Imagine getting in a spaceship and setting off in a direction, close to the speed of light- if you travelled for long enough you’d arrive back where you started. Like someone walking on the surface of the earth.
Any current cosmologists can correct me, but I believe the consensus is that the shape of the universe is a hyper-spheroid, I.e a 4D sphere.
This is a very compelling theory - but it has no evidence for it. Searches for "repeats" in the CMB (where light traveled all the way around and repeated) have not found anything.
As cool as that would be, most experiments still believe the curvature of space to be flat, which means if you travel endlessly in one direction relative to a starting point, you will never return to that starting point.
https://en.m.wikipedia.org/wiki/Shape_of_the_universe
https://en.m.wikipedia.org/wiki/Shape_of_the_universe
Not returning to your start means not connected, rather than not flat. Pac Man and Asteroids are flat but connected, a helix is curved everywhere but not connected.
That's true but when talking about space being flat vs having a curve, 2D and 3D visualizations are just to help explain the concept.
All current evidence points to the shape of space and the universe being relatively "boring" in that it behaves roughly like people would expect unless in the presence of a strong gravitational field.
All current evidence points to the shape of space and the universe being relatively "boring" in that it behaves roughly like people would expect unless in the presence of a strong gravitational field.
From your own link:
> Although it is usually assumed in the literature that a flat or negatively curved universe is infinite, this need not be the case if the topology is not the trivial one. For example, a multiply connected space may be flat and finite, as illustrated by the three-torus.
> Although it is usually assumed in the literature that a flat or negatively curved universe is infinite, this need not be the case if the topology is not the trivial one. For example, a multiply connected space may be flat and finite, as illustrated by the three-torus.
Exactly. Which leads straight to the contradiction when saying that the universe was only 10cm in diameter at a certain early time. If there’s no end, how does it have a diameter?
The portion of the universe we can observe was 10cm in diameter. If there's more it's outside our light cone and we can never reach it or be affected by it, so it may as well not exist. The observable universe seems to be fininte, but seems to act like a finite portion of an infinite universe.
The apple explanation is wrong and needs to be qualified with "observable". Your understanding is correct.
Is there an experiment which leads us to believe that the universe wraps around? I’d be very interested in any details supporting this idea.
If there were certain patterns in two different (opposite?) parts of the CMB, that would suggest it wraps. None have been found, so if it does wrap it's bigger than that.
I mean, there’s plenty of evidence for the Big Bang, and if that’s the case then the most likely shape that falls out of the maths is a hypersphere, like inflating a balloon as others have said.
No, the big bang model allows for three topologies: flat, spherical or hyperbolical.
A picture like this is in every cosmology textbook: https://upload.wikimedia.org/wikipedia/commons/9/98/End_of_u...
Which one it is depends on the density. There is a critical density at which the universe is flat, and our current best measurements basically allow for all three possibilities. It's just hard to say if something is flat or actually spherical but really, really huge.
A picture like this is in every cosmology textbook: https://upload.wikimedia.org/wikipedia/commons/9/98/End_of_u...
Which one it is depends on the density. There is a critical density at which the universe is flat, and our current best measurements basically allow for all three possibilities. It's just hard to say if something is flat or actually spherical but really, really huge.
Its also possible that the entire night sky lights up one day as the light from many big bangs and their universes beyond the edge of our “universe” finally reaches us
so we just don’t know. we have no evidence of that but all theories would simply change with the new information and that would become standard physics.
so we just don’t know. we have no evidence of that but all theories would simply change with the new information and that would become standard physics.
Right now as a pulse of light hits your retina (or a radiotelescope nearby) the source of that pulse must be some event at a finite time in the past. That is the Hubble time, which is non-identical to the age of the universe because the Hubble time captures the history of the expansion of space, which is not linear. That nonlinearity makes the present Hubble time greater than the present age of the universe. In general, however, the Hubble time grows in the future and shrinks in the past. One can also think of the Hubble time at any moment as the amount of time it would take the universe to expand to its size at that moment, if the expansion were linear; this is typically how it's taught [*].
If we multiply the Hubble time by c, we have the Hubble length, which is what is being talked about with expressions like "the size of an apple". That is, if the universe was "apple-sized" the Hubble length was on the order of an attoparsec rather than the ~4.4 gigaparsecs today. Extending the Hubble length in all directions gives us a spatial surface at the Hubble time, which means that on (or inside) that surface is everything that generates something which can in principle be detected here-and-now. That's the "size" which is being talked about. Because the Hubble time is shorter in the past, so too is the "size" of the Hubble volume in the past.
It is somewhat easier to think about your question in respect to the universe a bit later, like during Big Bang nucleosynthesis (BBNS) when the Hubble length was on the order of 100 parsecs and adiabatic expansion had cooled the contents of the Hubble volume (Hubble length in every direction) enough that hydrogen, helium, and lithium nuclei could form. Going back to the Hubble time we turn ~100 parsecs into ~300 light years, divde that by c and have a then-Hubble-time of 10 billionths of a second. So something ~speed-of-light (actual light, gravitational radiation, neutrinos, ultrarelativistic charged particles) emitted billionths of a second earlier and at maximum distance (~100 parsecs) would be reaching our galaxy now, billions of years later.
Anything emitted then towards us but from further than that ~100 parsec boundary would not reach us at all. Anything emitted from then but travelling on longer paths (gravitational lensing, interaction with gas or dust, or simply having intrinsic mass like a neutrino) will arrive later. And of course anything emitted then might not be aimed at us, so might never get here at all. So, and I'll touch on this again below, what happens to a BBNS proton that formed about 100 parsecs from protons that ended up in the Milky Way, if it happens to get a kick in a direction away from those Milky-Way-BBNS protons? It becomes unobservable, leaving the observable universe. But what do we call wherever it ends up? And in that region, whatever we all it (the unobservable universe?), are physics the same as in the parts of the observable universe we've studied?
Back to your apple-sized observable universe. We can continue a regression before BBNS, which takes us through epochs where electrons and photons and weak force counterparts are so hot and compressed they become something else (the fields of the electroweak unification), and so on. As we do so the Hubble time is shortened, and so too is the Hubble length. Eventually the Hubble time is so short that gravitational time dilation (from differing concentrations of energy and momentum) becomes a significant fraction. Thus it, and c*Hubble_time -> Hubble_length is subject to fluctuations of significant and growing fractions of the the average (mean or mode). [Ultimately this is where there is an early-universe conflict in the predictions of General Relativity (which is fine with this; we just stop splitting spacetime into spaces indexed by an averaged Hubble time {technically, the derivatives of the Robertson-Walker scale factor \a becomes un-useful}, and index on some other notion of time (or treat the whole region of spacetime as a block) and quantum field theory (which needs a clearer and quite linear notion of time to plug into its equations).]
Regressing further we will calculate a lower-bound Hubble volume which is for all practical purposes infinitesimal, and thus "a single point" is a reasonable description for the observable universe at about that early age. Anything outside the then-infinitesimal Hubble volume will never reach us. Anything inside could in principle, but again lots of that wasn't aimed in our direction and/or ended up becoming "frozen" into non-massless matter (some of which will eventually radiate, like galaxies full of stars, with some of that radiation aimed at us).
> didn't start from a single point; it has always been infinite
We don't know what if anything is outside the Hubble volume at any time, including in the most distant past. It's strange to think that there is nothing at all one parsec beyond the present Hubble length of ~ 4.4 billion parsecs. Where does the light and cosmic rays from distant galaxies radiated radially away from us go? Does each distant galaxy have its own Hubble volume[*], just like each of us locally have? We can only decide to adopt the Copernican principle and say that some distant barred-spiral galaxy has the same view as the Milky Way does -- galaxies scattered similarly in every direction. Adopting that view motivated Guth, one of the originators of studies of cosmic inflation, to calculate that there may be something like 10^23 Hubble volumes "outside" our own.
These would be 10^23 100-parsec volumes adjacent to each other all arranged around our 100-parsec BBNS volume. Each such volume may have the same early expansion history as ours, so (regressing) this would be a lot of apple-sized Hubble volumes, and ultimately a lot of effectively point-sized Hubble volumes. 10^23 is finite, of course, but I don't think we can preclude a much much larger finite number, and we can go so far as to ask questions about a literal infinite number. There will be some lower bound of Hubble volumes adjacent to, outside of, and causally disconnected from our own; that lowest number is much greater than one, however, or cosmic inflation fails to solve certain geometry problems. Additionally, zero ("our Hubble volume is it") raises the hard question of what happens even just a metre beyond the Hubble length. Is it new physics? Would there be some evidence close to but inside the Hubble length? Maybe even in galaxy clusters only tens of millions of parsecs away their radiation or morphology may show proof that they're close to an observer-independent sharp Hubble edge? JWST is already generating relevant data.
Finally, from a comment of yours downthread,
> An apple is only a certain number of atoms, far fewer than the 10^80 that we have today
When the Hubble length was about half the length of an apple, there weren't any atoms at all -- it was too hot and dense for any of the particles you'd recognize in the Standard Model. That's why I chose the BBNS epoch (~100 parsec) instead of the deep elecroweak or GUT epoch at "apple-size" (~ 1 attoparsec). (And additionally atoms can be wholly ionized at much lower temperatures, so I write here about nuclei instead).
At BBNS only hydrogen and some helium and trace lithium atomic nuclei were formed. All the carbon and so forth in an apple today was formed through stellar nucleosynthesis.
There are fewer atomic nuclei today than at the end of BBNS because stellar processes fuse multiple hydrogens into carbon, oxygen and other "non-metal" (in the astronomy sense) nuclei. Also, stellar black holes have taken lots of atomic nuclei out of the picture. And also some nuclei will have taken trajectories away from us and are now further than the present day Hubble length.
In cosmology we talk about the average density of baryons, compared to the averaged density of other things (among them radiation (mostly photons), relativistic and cold neutrinos, dark matter, dark energy). That is we are interested in the fraction of energy at a point that is baryonic (atomic nuclei, and more particularly protons, being the bulk of that). At earlier times, when the Hubble length is shorter, the (averaged, expected, typical) contribution of baryons at a (typical, average) point is higher. Baryonic matter has diluted with the expansion of the universe.
Your 10^80 figure will come from taking this pointwise baryon density and applying it to all the points in space at the present Hubble time, cross-checking it with estimates of the masses of typical galaxies and an estimate of the galaxy count. The baryon density today can also be arrived at by comparing the baryon density at the time the cosmic microwave background radiation formed (that density left imprints in it) and the expansion history since then (which we get by various means).
- --
[*] Unlike the Hubble time as the reciprocal of the Hubble constant, it is useful to think of the Hubble time at a point (in spacetime) as capturing the expansion history at that point. Since gravitation retards the expansion, every point (in spacetime) has its own Hubble time. Importantly, the Hubble time of a hydrogen nucleus peacefully floating in deep inter-galactic-cluster space will be longer than that of a hydrogen nucleus within our sun, even if they were both formed primordially. The gravitation of the sun and the Milky way and other nearby masses means the solar hydrogen has experienced less expansion. This takes us a tiny step closer to understanding the expanding universe in systems of coordinates other than the standard comoving coordinates, which is more relativistic in spirit. It reminds us that in comoving coordinates we are working with uniform densities of matter thanks to averaging, and are explicitly ignoring significant overdensities of matter (which slow down the ticking of clocks nearby).
If we multiply the Hubble time by c, we have the Hubble length, which is what is being talked about with expressions like "the size of an apple". That is, if the universe was "apple-sized" the Hubble length was on the order of an attoparsec rather than the ~4.4 gigaparsecs today. Extending the Hubble length in all directions gives us a spatial surface at the Hubble time, which means that on (or inside) that surface is everything that generates something which can in principle be detected here-and-now. That's the "size" which is being talked about. Because the Hubble time is shorter in the past, so too is the "size" of the Hubble volume in the past.
It is somewhat easier to think about your question in respect to the universe a bit later, like during Big Bang nucleosynthesis (BBNS) when the Hubble length was on the order of 100 parsecs and adiabatic expansion had cooled the contents of the Hubble volume (Hubble length in every direction) enough that hydrogen, helium, and lithium nuclei could form. Going back to the Hubble time we turn ~100 parsecs into ~300 light years, divde that by c and have a then-Hubble-time of 10 billionths of a second. So something ~speed-of-light (actual light, gravitational radiation, neutrinos, ultrarelativistic charged particles) emitted billionths of a second earlier and at maximum distance (~100 parsecs) would be reaching our galaxy now, billions of years later.
Anything emitted then towards us but from further than that ~100 parsec boundary would not reach us at all. Anything emitted from then but travelling on longer paths (gravitational lensing, interaction with gas or dust, or simply having intrinsic mass like a neutrino) will arrive later. And of course anything emitted then might not be aimed at us, so might never get here at all. So, and I'll touch on this again below, what happens to a BBNS proton that formed about 100 parsecs from protons that ended up in the Milky Way, if it happens to get a kick in a direction away from those Milky-Way-BBNS protons? It becomes unobservable, leaving the observable universe. But what do we call wherever it ends up? And in that region, whatever we all it (the unobservable universe?), are physics the same as in the parts of the observable universe we've studied?
Back to your apple-sized observable universe. We can continue a regression before BBNS, which takes us through epochs where electrons and photons and weak force counterparts are so hot and compressed they become something else (the fields of the electroweak unification), and so on. As we do so the Hubble time is shortened, and so too is the Hubble length. Eventually the Hubble time is so short that gravitational time dilation (from differing concentrations of energy and momentum) becomes a significant fraction. Thus it, and c*Hubble_time -> Hubble_length is subject to fluctuations of significant and growing fractions of the the average (mean or mode). [Ultimately this is where there is an early-universe conflict in the predictions of General Relativity (which is fine with this; we just stop splitting spacetime into spaces indexed by an averaged Hubble time {technically, the derivatives of the Robertson-Walker scale factor \a becomes un-useful}, and index on some other notion of time (or treat the whole region of spacetime as a block) and quantum field theory (which needs a clearer and quite linear notion of time to plug into its equations).]
Regressing further we will calculate a lower-bound Hubble volume which is for all practical purposes infinitesimal, and thus "a single point" is a reasonable description for the observable universe at about that early age. Anything outside the then-infinitesimal Hubble volume will never reach us. Anything inside could in principle, but again lots of that wasn't aimed in our direction and/or ended up becoming "frozen" into non-massless matter (some of which will eventually radiate, like galaxies full of stars, with some of that radiation aimed at us).
> didn't start from a single point; it has always been infinite
We don't know what if anything is outside the Hubble volume at any time, including in the most distant past. It's strange to think that there is nothing at all one parsec beyond the present Hubble length of ~ 4.4 billion parsecs. Where does the light and cosmic rays from distant galaxies radiated radially away from us go? Does each distant galaxy have its own Hubble volume[*], just like each of us locally have? We can only decide to adopt the Copernican principle and say that some distant barred-spiral galaxy has the same view as the Milky Way does -- galaxies scattered similarly in every direction. Adopting that view motivated Guth, one of the originators of studies of cosmic inflation, to calculate that there may be something like 10^23 Hubble volumes "outside" our own.
These would be 10^23 100-parsec volumes adjacent to each other all arranged around our 100-parsec BBNS volume. Each such volume may have the same early expansion history as ours, so (regressing) this would be a lot of apple-sized Hubble volumes, and ultimately a lot of effectively point-sized Hubble volumes. 10^23 is finite, of course, but I don't think we can preclude a much much larger finite number, and we can go so far as to ask questions about a literal infinite number. There will be some lower bound of Hubble volumes adjacent to, outside of, and causally disconnected from our own; that lowest number is much greater than one, however, or cosmic inflation fails to solve certain geometry problems. Additionally, zero ("our Hubble volume is it") raises the hard question of what happens even just a metre beyond the Hubble length. Is it new physics? Would there be some evidence close to but inside the Hubble length? Maybe even in galaxy clusters only tens of millions of parsecs away their radiation or morphology may show proof that they're close to an observer-independent sharp Hubble edge? JWST is already generating relevant data.
Finally, from a comment of yours downthread,
> An apple is only a certain number of atoms, far fewer than the 10^80 that we have today
When the Hubble length was about half the length of an apple, there weren't any atoms at all -- it was too hot and dense for any of the particles you'd recognize in the Standard Model. That's why I chose the BBNS epoch (~100 parsec) instead of the deep elecroweak or GUT epoch at "apple-size" (~ 1 attoparsec). (And additionally atoms can be wholly ionized at much lower temperatures, so I write here about nuclei instead).
At BBNS only hydrogen and some helium and trace lithium atomic nuclei were formed. All the carbon and so forth in an apple today was formed through stellar nucleosynthesis.
There are fewer atomic nuclei today than at the end of BBNS because stellar processes fuse multiple hydrogens into carbon, oxygen and other "non-metal" (in the astronomy sense) nuclei. Also, stellar black holes have taken lots of atomic nuclei out of the picture. And also some nuclei will have taken trajectories away from us and are now further than the present day Hubble length.
In cosmology we talk about the average density of baryons, compared to the averaged density of other things (among them radiation (mostly photons), relativistic and cold neutrinos, dark matter, dark energy). That is we are interested in the fraction of energy at a point that is baryonic (atomic nuclei, and more particularly protons, being the bulk of that). At earlier times, when the Hubble length is shorter, the (averaged, expected, typical) contribution of baryons at a (typical, average) point is higher. Baryonic matter has diluted with the expansion of the universe.
Your 10^80 figure will come from taking this pointwise baryon density and applying it to all the points in space at the present Hubble time, cross-checking it with estimates of the masses of typical galaxies and an estimate of the galaxy count. The baryon density today can also be arrived at by comparing the baryon density at the time the cosmic microwave background radiation formed (that density left imprints in it) and the expansion history since then (which we get by various means).
- --
[*] Unlike the Hubble time as the reciprocal of the Hubble constant, it is useful to think of the Hubble time at a point (in spacetime) as capturing the expansion history at that point. Since gravitation retards the expansion, every point (in spacetime) has its own Hubble time. Importantly, the Hubble time of a hydrogen nucleus peacefully floating in deep inter-galactic-cluster space will be longer than that of a hydrogen nucleus within our sun, even if they were both formed primordially. The gravitation of the sun and the Milky way and other nearby masses means the solar hydrogen has experienced less expansion. This takes us a tiny step closer to understanding the expanding universe in systems of coordinates other than the standard comoving coordinates, which is more relativistic in spirit. It reminds us that in comoving coordinates we are working with uniform densities of matter thanks to averaging, and are explicitly ignoring significant overdensities of matter (which slow down the ticking of clocks nearby).
How can one measure the speed of things through time relative to things in their past or future? Can't we only measure the speed of things through time relative to other things that coexist at the same time? I read the article but couldn't make sense of it.
> Can't we only measure the speed of things through time relative to other things that coexist at the same time?
They weren't using speed. They were watching how fast a clock ticked back then, the clock being a quasar. The article was light on the details, but the difficulty seemed to be that quasar's aren't reliable clocks, so they needed to develop a statistical model to measure the ticking accurately. Developing the model was actual science being reported on. It happened to agree with Einstein's predictions.
But even if they were using speed, gravity doesn't slow down light. It redshift's it. So there is something you could compare your speed to back then.
Finally, we are comparing how fast time ticks for us now versus how fast a quasar ticked back then. I don't know if the quasar it still around, but if it is it's probably a very different beast now. In any case we are comparing two things that don't exist at the same time. In fact, my guess is it's downright difficult compare yourself to something that exists at the same time, because the speed of light is finite. That means by the time you receive the measurement, whatever you've measured has moved on.
They weren't using speed. They were watching how fast a clock ticked back then, the clock being a quasar. The article was light on the details, but the difficulty seemed to be that quasar's aren't reliable clocks, so they needed to develop a statistical model to measure the ticking accurately. Developing the model was actual science being reported on. It happened to agree with Einstein's predictions.
But even if they were using speed, gravity doesn't slow down light. It redshift's it. So there is something you could compare your speed to back then.
Finally, we are comparing how fast time ticks for us now versus how fast a quasar ticked back then. I don't know if the quasar it still around, but if it is it's probably a very different beast now. In any case we are comparing two things that don't exist at the same time. In fact, my guess is it's downright difficult compare yourself to something that exists at the same time, because the speed of light is finite. That means by the time you receive the measurement, whatever you've measured has moved on.
> Lewis explained that the brightening and fading of quasar light, known as their variability is like the stock mark, unpredictable on the surface but with statistical properties that allow it to be modelled over time. This variability can be used like the ticking of a cosmic clock. It was this understanding and the complex quasar firework display from 190 of these feeding supermassive black holes that allowed the team to spot time dilation when the universe was just 1.5 billion years old, less than 10% of its current age.
> when the universe was just 1.5 billion years old, less than 10% of its current age.
Which universe are they talking about? Most sources I know of cite 13.78 billion years for our universe, and the quoted 1.5 billion years is definitely over 10% of that.
Which universe are they talking about? Most sources I know of cite 13.78 billion years for our universe, and the quoted 1.5 billion years is definitely over 10% of that.
Based on the article and my guesses, here's how they measured time on the past.
When we look at things far away from us, we're seeing their state in the past. Quasars that are 10 billion light years away took 10 billion years for their light to reach us, so right now we are seeing their images 10 billion years ago.
Quasar is bright light powered by gas spiraling at a high velocity into a black hole. When things revolving around a black hole, the light given out varies. The light of the gas spinning behind the black hole is blocked and the light is shown again when the gas spinning on the side of the black hole facing us. When we're looking at it, it kind of blinks periodically in constant rate. The blinking rate can serve as clock ticks.
The people of the paper figure out a way to compute the blinking rate of the quasars statistically. They look at hundreds of quasars at 12 billion light years away, i.e. looking at their state 12 billion years ago. Compute their blinking rate from twenty years of telescope observation. Compare that to the quasars close to us. They found that the far away quasars blink 5 times slower, i.e. they're blinking 5 times slower 12 billion years ago. The universe is about 13.7 billion years old, so the conclusion is that time ran slower at the early age. It actually fits the prediction from Einstein's general relativity.
When we look at things far away from us, we're seeing their state in the past. Quasars that are 10 billion light years away took 10 billion years for their light to reach us, so right now we are seeing their images 10 billion years ago.
Quasar is bright light powered by gas spiraling at a high velocity into a black hole. When things revolving around a black hole, the light given out varies. The light of the gas spinning behind the black hole is blocked and the light is shown again when the gas spinning on the side of the black hole facing us. When we're looking at it, it kind of blinks periodically in constant rate. The blinking rate can serve as clock ticks.
The people of the paper figure out a way to compute the blinking rate of the quasars statistically. They look at hundreds of quasars at 12 billion light years away, i.e. looking at their state 12 billion years ago. Compute their blinking rate from twenty years of telescope observation. Compare that to the quasars close to us. They found that the far away quasars blink 5 times slower, i.e. they're blinking 5 times slower 12 billion years ago. The universe is about 13.7 billion years old, so the conclusion is that time ran slower at the early age. It actually fits the prediction from Einstein's general relativity.
Think of looking at someone running across the street and then looking through a telescope at someone running far away. The person far away would be in the past because of whatever amount of distance light had to travel. The person far away would seem to be running in slow motion.
Instead of whatever tiny fraction of a second your telescope would enable you to look into the past, the telescopes looking at whole-ass galaxies in the early universe that appear to be "running slower".
Instead of whatever tiny fraction of a second your telescope would enable you to look into the past, the telescopes looking at whole-ass galaxies in the early universe that appear to be "running slower".
I think they're looking at parts of the universe that are very old, and so they can watch it move more slowly.
What they are measuring is the apparent "speed" of processes back then. But that apparent speed is an effect of the redshift. It does not mean that clocks were actually running slower back then. The article's description is seriously misleading.
My main take away from the article was that we, apparently, need a time traveler to confirm/disprove Einstein's theory.
Without being an astrophysicist I think critique of this paper is impossible. So I ask any astrophysicists here, to explain why this is not an observer-effect, or a consequence of the extreme time and distance the signal has gone through, if you like a time effect analogous to red shift.
Anything about observations made at this distance in both space and time has me think it should always be "in rats"
Anything about observations made at this distance in both space and time has me think it should always be "in rats"
Also not an astrophysicist, but wouldn't we be able to work the consequence of the distance traveled into the prediction made before the experiment?
There is a possibility that our understanding of what effect the distance would have on the readings, but as long as you don't print things like it "confirms time ran slower" there's no real problem...
There is a possibility that our understanding of what effect the distance would have on the readings, but as long as you don't print things like it "confirms time ran slower" there's no real problem...
Yes. I am sure the paper makes adjustments, error corrections. They're good scientists. But I'm a layperson. So, I guess the problems are headlines. Because the headline didn't say confirm, but it states positively Time ran slowly
Outside of a dog, a cigar is a mans best friend. Inside of a dog, it's too dark to see.
Inside of a slowly running time period, a second is still a second, surely? So I ask myself how inside the universe, we can make assertions about the relative speed a second "is" but remain inside the same system. The implications some other attribute of observed reality can remain unchanged, and confirm the "duration" of a second was different then, to now, based on observations made solely within this universe.. I'm struggling.
Outside of a dog, a cigar is a mans best friend. Inside of a dog, it's too dark to see.
Inside of a slowly running time period, a second is still a second, surely? So I ask myself how inside the universe, we can make assertions about the relative speed a second "is" but remain inside the same system. The implications some other attribute of observed reality can remain unchanged, and confirm the "duration" of a second was different then, to now, based on observations made solely within this universe.. I'm struggling.
Redshift isn't mentioned anywhere in the article except in the citation of the original paper
https://www.nature.com/articles/s41550-023-02029-2
but redshift here would be the key to making this determination.. I don't have access to the paper and it would probably be beyond me, but presumably you can make observations in redshifted light from quasars of different ages (or distances from the centre of the universe) and because we're fairly sure the properties of light haven't changed since the universe started, we use these subtle differences in redshifts like a sort of "cosmological GPS" and figure out that time must've been slower in the past
https://www.nature.com/articles/s41550-023-02029-2
but redshift here would be the key to making this determination.. I don't have access to the paper and it would probably be beyond me, but presumably you can make observations in redshifted light from quasars of different ages (or distances from the centre of the universe) and because we're fairly sure the properties of light haven't changed since the universe started, we use these subtle differences in redshifts like a sort of "cosmological GPS" and figure out that time must've been slower in the past
So I don't think this is an answer, certainly not THE answer, but I worry that aspects of the early universe like the local density of matter or electric fields were radically different to what we have now. If you imagine a beacon of light which is a signal from time "back then" which has to pass through both the early states of the universe with different densities of things between then and now, and us and them, AND has to be observed, I can think of ways both matter and energy could interfere with that signal differently across time. Consequently, the amount of red shift we see in modern times for more modern (ie closer) things reflects more modern states of the universe. As a correction factor, we'd be applying what we know, to what we infer, about the density in those times.
This is why I express skepticism: It's very hard to believe there are no confounding factors in the states of being then, compared to now, which might lie inside the error/noise thresholds of their calculations.
"it had to be time was different then" feels like "because I want it to" against all the sources of noise and conjecture, between then and now.
I hasten to add I am not an astrophysicist. So, I can be put firmly back in my box and told to accept good science says otherwise.
This is why I express skepticism: It's very hard to believe there are no confounding factors in the states of being then, compared to now, which might lie inside the error/noise thresholds of their calculations.
"it had to be time was different then" feels like "because I want it to" against all the sources of noise and conjecture, between then and now.
I hasten to add I am not an astrophysicist. So, I can be put firmly back in my box and told to accept good science says otherwise.
I would say perhaps the key difference here is we're talking about cosmological scale observations. Yes, a few quasars might've had to pass through some gas clouds or an asteroid belt or space dust etc. but at the scale we're talking about, that's probably not going to skew the numbers too much. There's 190 quasars being measured here. Presumably more observations can be made over time and with other quasars involved but I don't think these scientists need more data to be confident in what they're seeing.
If you think about a pebble getting in the way of old faithful, it's not going to make much difference if you're measuring how much water sprays out over a hundred years. Now you have 190 old faithfuls and they're all spraying out water in pretty much the same way, you don't really need to account for that pebble too much.
If you think about a pebble getting in the way of old faithful, it's not going to make much difference if you're measuring how much water sprays out over a hundred years. Now you have 190 old faithfuls and they're all spraying out water in pretty much the same way, you don't really need to account for that pebble too much.
The general theory of relativity predicts that time would run slower, and if that was the case these ancient quasars would demonstrate certain properties when looked at by our telescopes. We then looked at ancient quasars, and the readings we got matched the predictions, with some amount of error.
This can then be used as another piece of experimental data supporting the general theory of relativity, even though you are right that these readings could be achieved through some other bizarre set of circumstances.
This can then be used as another piece of experimental data supporting the general theory of relativity, even though you are right that these readings could be achieved through some other bizarre set of circumstances.
> I don't have access to the paper
https://arxiv.org/abs/2306.04053
Someone usually posts up a preprint in discussions where the original link is a summary put out by one of the authors' institutions' public relations department and published unchanged or made even worse by some aggregator site's editing.
You can also install the Unpaywall extension which gives you a one-click button to (usually preprints of) a large proportion of freshly published papers (it works on more than half of astrophysics-related papers). http://unpaywall.org/ is one of the best things on the Internet.
> it would probably be beyond me
That's OK, it's also clearly totally beyond the author of the article reproduced at "advancedsciencenews".
You are correct to be puzzled about the lack of "redshift" appearing in the article; "high-redshift" is literally the eighth word in the title at Nature, and "redshift-dependent time dilation" appears in the first sentence of the abstract.
The first sentence of the Introduction (in preprint) reads, "A fundamental consequence of the relativistic picture of expanding space is cosmological time dilation, where events in the distant universe appear to move slowly compared to those in the local cosmos." Emphasis mine.
That's correct and totally conventional.
The first sentence of the "advancedsciencenews" article reads, "Scientists have confirmed that just 1.5 billion years after the Big Bang, time ran five times slower than it does today, 13.8 billion years later". I count several errors there, and struggle to see how a mere broken-telephone effect arrived at that wording.
1.5 billion years after the big bang, atomic electron transitions (like the n = 2 orbital to n = 1 ground state transition in atomic hydrogen) happened at the same ~nanosecond speed as 13.8 billion years after the big bang, emitting the same frequency photon (2.47 PHz) at both times. That transition speed and photon-frequency-at-emission each forms a sort of a local clock (or really frequency standard) which has travelled along from well before 1.5 billion years, through that 1.5 billion year mark, to the current age of the universe. We can test them tomorrow. We can test them next week. We can test them on the Moon or in more distant spacecraft. The ~nanoseconds transition and more precisely the emitted-photon frequency are always and everywhere the same when measuring locally, i.e. very near the emission and not moving quickly with respect to it.
General relativity tells us that, when there is significant gravitation or expansion of space, when we look at distant clocks (or really frequency standards) they appear to tick at different rates than clocks local to us. In particular, clocks in the distant past of an expanding universe appear to tick slower than clocks in the less distant past (and those tick slower than clocks here-and-now). (Special relativity tells us that when we look at clocks that are in motion relative to a clock we have locally, the "moving" clock appears to tick slower.)
Consequently, the photon of an n=2 -> n=1 atomic hydrogen electron transition in the early universe will in the present universe have a frequency lower than 2.47 PHz. If we get to watch a transition happen (we can't, they're too small to see at great distances, but we can see the speed reflected in the bulk behaviour of atomic hydrogen gas lit by bright sources like ... quasars!) we'd see it happening slower. And from this we have the first line of the actual paper.
Additionally, hopefully shedding further light, we could drop in someone who is half way between 1.5 billion years and the present age of the universe, and locally they'd measure the same local ~nanosecond transition and the same 2.47 PHz photon frequency. Looking at the same 1.5 billion year universe source, the intermediate observer will also see a redshifted photon-frequency and a slower transition, but not as slow as we see even more billions of years later. In general, observers of the same source who are ever closer in time to that source will see a photon frequency and atomic electron transition speed ever closer to what they measure locally; whereas later observers will see an ever-greater difference (the later the observer the redder (slower transition, lower frequency) the ultimate observation).
The "appear" in the Nature article's first sentence deserves my italicization. It's important. And it seems to have not been noticed by the author of the advancedsciencenews summary. And it's clearly confused many people here. Hopefully with this comment I can unconfuse at least one person, at least a little.
Finally, the paper is about showing that variable quasars have an intrinsic set of luminosity-variation statistics, and intrinsic means that those statistics are local to the quasar. We should then see those statistics change when redshift changes. And that's what the authors show: the "intrinsic brightness-change-rate statistics" appear to generate slower brighness-variations for quasars that are at higher redshifts (measured by other means including the redshifting of absorption and emission spectral lines caused by the quasar's light passing through atomic hydrogen for example, and including the apparent area on the sky and brightness of the quasar's host galaxy).
https://arxiv.org/abs/2306.04053
Someone usually posts up a preprint in discussions where the original link is a summary put out by one of the authors' institutions' public relations department and published unchanged or made even worse by some aggregator site's editing.
You can also install the Unpaywall extension which gives you a one-click button to (usually preprints of) a large proportion of freshly published papers (it works on more than half of astrophysics-related papers). http://unpaywall.org/ is one of the best things on the Internet.
> it would probably be beyond me
That's OK, it's also clearly totally beyond the author of the article reproduced at "advancedsciencenews".
You are correct to be puzzled about the lack of "redshift" appearing in the article; "high-redshift" is literally the eighth word in the title at Nature, and "redshift-dependent time dilation" appears in the first sentence of the abstract.
The first sentence of the Introduction (in preprint) reads, "A fundamental consequence of the relativistic picture of expanding space is cosmological time dilation, where events in the distant universe appear to move slowly compared to those in the local cosmos." Emphasis mine.
That's correct and totally conventional.
The first sentence of the "advancedsciencenews" article reads, "Scientists have confirmed that just 1.5 billion years after the Big Bang, time ran five times slower than it does today, 13.8 billion years later". I count several errors there, and struggle to see how a mere broken-telephone effect arrived at that wording.
1.5 billion years after the big bang, atomic electron transitions (like the n = 2 orbital to n = 1 ground state transition in atomic hydrogen) happened at the same ~nanosecond speed as 13.8 billion years after the big bang, emitting the same frequency photon (2.47 PHz) at both times. That transition speed and photon-frequency-at-emission each forms a sort of a local clock (or really frequency standard) which has travelled along from well before 1.5 billion years, through that 1.5 billion year mark, to the current age of the universe. We can test them tomorrow. We can test them next week. We can test them on the Moon or in more distant spacecraft. The ~nanoseconds transition and more precisely the emitted-photon frequency are always and everywhere the same when measuring locally, i.e. very near the emission and not moving quickly with respect to it.
General relativity tells us that, when there is significant gravitation or expansion of space, when we look at distant clocks (or really frequency standards) they appear to tick at different rates than clocks local to us. In particular, clocks in the distant past of an expanding universe appear to tick slower than clocks in the less distant past (and those tick slower than clocks here-and-now). (Special relativity tells us that when we look at clocks that are in motion relative to a clock we have locally, the "moving" clock appears to tick slower.)
Consequently, the photon of an n=2 -> n=1 atomic hydrogen electron transition in the early universe will in the present universe have a frequency lower than 2.47 PHz. If we get to watch a transition happen (we can't, they're too small to see at great distances, but we can see the speed reflected in the bulk behaviour of atomic hydrogen gas lit by bright sources like ... quasars!) we'd see it happening slower. And from this we have the first line of the actual paper.
Additionally, hopefully shedding further light, we could drop in someone who is half way between 1.5 billion years and the present age of the universe, and locally they'd measure the same local ~nanosecond transition and the same 2.47 PHz photon frequency. Looking at the same 1.5 billion year universe source, the intermediate observer will also see a redshifted photon-frequency and a slower transition, but not as slow as we see even more billions of years later. In general, observers of the same source who are ever closer in time to that source will see a photon frequency and atomic electron transition speed ever closer to what they measure locally; whereas later observers will see an ever-greater difference (the later the observer the redder (slower transition, lower frequency) the ultimate observation).
The "appear" in the Nature article's first sentence deserves my italicization. It's important. And it seems to have not been noticed by the author of the advancedsciencenews summary. And it's clearly confused many people here. Hopefully with this comment I can unconfuse at least one person, at least a little.
Finally, the paper is about showing that variable quasars have an intrinsic set of luminosity-variation statistics, and intrinsic means that those statistics are local to the quasar. We should then see those statistics change when redshift changes. And that's what the authors show: the "intrinsic brightness-change-rate statistics" appear to generate slower brighness-variations for quasars that are at higher redshifts (measured by other means including the redshifting of absorption and emission spectral lines caused by the quasar's light passing through atomic hydrogen for example, and including the apparent area on the sky and brightness of the quasar's host galaxy).
I was thinking the same thing. This explanation if
right makes some sense: https://news.ycombinator.com/item?id=37245317
Basically is the same “time goes more slowly” as a atomic clock that is not on a jet plane
Basically is the same “time goes more slowly” as a atomic clock that is not on a jet plane
Also in terms of a second being a second, yes, presumably if you took a pocket watch back to the beginning of the universe, it would not operate observably slower.
Imagine you had an inter-dimensional being that could traverse spacetime at will. They have 2 stopwatches, one is in their left hand at the center of the universe 12bn years ago, the other is in their right hand at the edge of the universe today.
If they started both stopwatches at the same time, the one in their left hand would take longer to reach 5 seconds than the one in their right hand.
Imagine you had an inter-dimensional being that could traverse spacetime at will. They have 2 stopwatches, one is in their left hand at the center of the universe 12bn years ago, the other is in their right hand at the edge of the universe today.
If they started both stopwatches at the same time, the one in their left hand would take longer to reach 5 seconds than the one in their right hand.
I think you understood it just fine without being an astrophysicist?
The fact that matter warps spacetime still blows my mind. We can basically neglect the effect on light near Earth, although it's still present. But that "gravity" is merely what we call the minimizing the action of matter near a warped spacetime is an astonishing claim. If I'm being honest, I really only believe it because people I believe are smarter say it is so; it's so strange and alien that I find it almost impossible to accept. Luckily, apart from a few minor effects here and there, I don't have to think about it and can just accept gravity as a simple force with some unknown mechanism behind it, like Newton did, and I have to say that makes me happier. This talk of time slowing down gives me the creeps.
Is it creepy when you throw a ball up and it magically falls back into your hand? It moves back into your hand specifically because time on the ground is moving slower. The ball, like everything else, moves towards the slower moving timeline (a lesser inertial state). That's actually not creepy at all. It would be creepy if the ball went the other way for no apparent reason like a poltergeist.
The fact that there exist consistent structured phenomena, instead of everything being pure noise, is actually creepy.
Yes, it is creepy. And now that you mention it, Fermat's principle (the principle of least time) is creepy too. Why is physics so weird? It's like everything is choosing things all the time! And it gets worse with quantum effects, since apparently there is hidden computational machinery that happily computes superposition of essentially infinite wave forms - and that scales exponentially - without slow-down or effort.
The comprehensibility of the world is an illusion. Humans don't explain, we describe, and we give ourselves credit for undoing the confusion of some superficial effects that turned out to have a simple description. The mystery and awe of what the universe is doing and how remains poignant.
The comprehensibility of the world is an illusion. Humans don't explain, we describe, and we give ourselves credit for undoing the confusion of some superficial effects that turned out to have a simple description. The mystery and awe of what the universe is doing and how remains poignant.
Sure, that's true. My comment is expanding on your claim that you accept gravity as a simple force in contrast to the mystery of creepy spacetime concepts of the universe. You accept gravity which is the same force as time dilation. It sounds like if I change the name it's less creepy for you.
Upon further reflection the idea of inanimate objects "wanting things" is what I find most creepy. It's enough to make a person consider monadism seriously. The notion of inanimate objects modifying spacetime just by existing is secondarily creepy - it's destructive to the notion of "introduce a coordinate system" because now the phenomena you're describing messes with the coordinate system you picked to describe it! And yes renaming things does help. I don't like thunderstorms, I find them legitimately scary, and understanding their physics didn't help but calling them "unicorn farts" did.
> it's destructive to the notion of "introduce a coordinate system" because now the phenomena you're describing messes with the coordinate system you picked to describe it!
That's the whole point of GR - coordinate systems aren't physically meaningful.
That's the whole point of GR - coordinate systems aren't physically meaningful.
Its not that the ball wants things. Its own gravity and its behavior toward the gravity of other objects is just a result of what it is: a cluster of protons and neutrons and electrons in a given radius.
I'm being a bit cheeky; I have a physics degree. And I love Feynman's take on magnetism if you look up that video. He makes a strong distinction between "why" and "how" noting that physics doesn't really know why. It's a needed dose of humility for those who need to be reminded that humans are NOT the masters of the universe, and our understanding of physics has profound limits (and is poorly distributed throughout the human population). Why is the principle of least action a thing? Why does entropy only ever increase? What is actually doing all that computation behind quantum effects? No-one knows.
About Hamilton's stationary action (which you refer to as 'least action').
I have created an educational resource in which I address the question of how it comes about that F=ma can be recovered from Hamilton's stationary action. This resource gives a two-pronged approach: the concepts are illustrated with interactive diagrams, and parallel to that a full presentation of the mathematics.
I start with a discussion of the nature of Calculus of Variations. I use the problem of a soap film stretching between two parallel concentric rings as motivating example. This leads to a derivation of the Euler-Lagrange equation.
Then I move to the Catenary problem. Interestingly, with the catenary problem both approaches are possible; you can solve for the catenary with differential calculus (as Leibniz did) or you can apply calculus of variations. What that means is that the catenary problem can serve as a Rosetta stone, offering a bridge between differential calculus and calculus of variations.
http://cleonis.nl/physics/phys256/calculus_variations.php
The discussion specifically for Hamilton's stationary action is in an article of its own:
http://cleonis.nl/physics/phys256/energy_position_equation.p...
I have created an educational resource in which I address the question of how it comes about that F=ma can be recovered from Hamilton's stationary action. This resource gives a two-pronged approach: the concepts are illustrated with interactive diagrams, and parallel to that a full presentation of the mathematics.
I start with a discussion of the nature of Calculus of Variations. I use the problem of a soap film stretching between two parallel concentric rings as motivating example. This leads to a derivation of the Euler-Lagrange equation.
Then I move to the Catenary problem. Interestingly, with the catenary problem both approaches are possible; you can solve for the catenary with differential calculus (as Leibniz did) or you can apply calculus of variations. What that means is that the catenary problem can serve as a Rosetta stone, offering a bridge between differential calculus and calculus of variations.
http://cleonis.nl/physics/phys256/calculus_variations.php
The discussion specifically for Hamilton's stationary action is in an article of its own:
http://cleonis.nl/physics/phys256/energy_position_equation.p...
It's not just gravity. All the other forces are pretty weird as well, if you think about them on a deep enough level.
For example, rubber bands resisting elongation isn't even a 'real' force: https://en.wikipedia.org/wiki/Entropic_force
For example, rubber bands resisting elongation isn't even a 'real' force: https://en.wikipedia.org/wiki/Entropic_force
As a physicist who learnt it years ago it’s actually not really that crazy to me now and is far more intelligible than quantum field theory
Can I get an ELI5 for how we can use words like "slower" about time? In my mind "slower" and "faster" are comparisons within a timeframe. How can something be slower than something else unless they both exist in the same time?
(Not a physicist)
Yes, you are correct - its not within the same frame of reference but observing from one frame to another.
Lets say you're watching a DVD; the main characters are walking having a conversation. Now fast seek - they speed up and talk faster. If the characters were conscious entities then within their world nothing has changed, but from your viewpoint everything is going faster.
One difference with the physical world is the effect is symmetrical - so if you were observing the movement of crew on a fast moving spaceship from far away you would see them seeming to move very slowly; conversely they would see the same slowing down when observing you.
But in each case you are correct that "slower" and "faster" are not in operation within the "same time" but only when viewing into one time "bubble" (DVD) from another.
The "bubble-to-bubble slowing down" itself is not some marking off against a universal time axis either (which is at the root of the difficulty in understanding I think), but - if we remember that time is subordinate to the constancy of speed of light then to maintain this constancy, the measurements of time and space in different frames must adjust relative to each other as the frames speed up and slow down.
It is this latter waxing and waning that produces the effect "slower" reflected between two different frames of reference.
Yes, you are correct - its not within the same frame of reference but observing from one frame to another.
Lets say you're watching a DVD; the main characters are walking having a conversation. Now fast seek - they speed up and talk faster. If the characters were conscious entities then within their world nothing has changed, but from your viewpoint everything is going faster.
One difference with the physical world is the effect is symmetrical - so if you were observing the movement of crew on a fast moving spaceship from far away you would see them seeming to move very slowly; conversely they would see the same slowing down when observing you.
But in each case you are correct that "slower" and "faster" are not in operation within the "same time" but only when viewing into one time "bubble" (DVD) from another.
The "bubble-to-bubble slowing down" itself is not some marking off against a universal time axis either (which is at the root of the difficulty in understanding I think), but - if we remember that time is subordinate to the constancy of speed of light then to maintain this constancy, the measurements of time and space in different frames must adjust relative to each other as the frames speed up and slow down.
It is this latter waxing and waning that produces the effect "slower" reflected between two different frames of reference.
My understanding is - It's slower or faster with respect to our current frame of reference.
Time is warped by gravity. Gravity is produced by matter with mass. The early universe had a higher mass-density resulting in this time-warping effect.
Dumb question perhaps but if the "1.5 billion years" is in today's time ticks what's the equivalent age of the universe?
The singer Katie Melua had to update her song with the correct answer after being challenged by the scientifically minded [1]. Given this new insight, she might need a new version once again.
[1] https://youtu.be/iXhxDDY1GYI
[1] https://youtu.be/iXhxDDY1GYI
I guess that really means the age of the universe is some sort of unknowable number.
Or the age of the universe is different for all observers.
How do you describe a story without a beginning or end?
It’s simultaneously relative to any unit of 1
It's hard to wrap your mind around the fact that time and space everywhere are whimsical and warped... there are no straight lines in nature and every point in space and time are locally doing their own thing...
the universe just calculates everything everywhere all at once and there's no way to fit it into human comprehension.
the universe just calculates everything everywhere all at once and there's no way to fit it into human comprehension.
> The universe just calculates everything everywhere all at once and there's no way to fit it into human comprehension.
Because the speed of light is finite, the universe wouldn't have to calculate everything everywhere all at once.
If the calculations around Alpha Centauri stopped updating for a while for a few months, and then caught up quickly, we wouldn't notice, because it's a few light years away.
See also https://en.wikipedia.org/wiki/Cosmological_horizon
Because the speed of light is finite, the universe wouldn't have to calculate everything everywhere all at once.
If the calculations around Alpha Centauri stopped updating for a while for a few months, and then caught up quickly, we wouldn't notice, because it's a few light years away.
See also https://en.wikipedia.org/wiki/Cosmological_horizon
It's the idea that the wave function of reality doesn't collapse until it is observed ?
au contraire. nature loves 'straight lines' (just on non-euclidean manifolds).
From the article it sounds like this is just a normally expected thing, that time runs more slowly near a massive object, and shortly after the big bang everything was relatively close to a lot of other mass, so from our point of view it looks like time was slower then.
I wonder... if the Big Bang started with an explosion of matter from a specific point, and the closer we get to that moment the slower time goes, then might it be correct in some sense to say that the big bang happened an infinite amount of time ago? Maybe it's a meaningless thing to say, without a suitable frame of reference from which one could observe the big bang from "outside"?
Thinking about this made me wonder why the big bang didn't just immediately collapse into a black hole. One answer to that, apparently, is that the universe was too uniform. It was very dense, but with ever spec of matter (or whatever it was the early universe was made of) being pulled equally in all directions at once, there wasn't an opportunity for a black hole to form.
I wonder if it's like that for time, too? If you're being pulled strongly in all directions equally, does it cancel out and cause time to run at a "normal" rate, or does time run super slow because all the gravitational forces add together?
In the former case, I guess it's possible that time ran normally at the very beginning and then slowed down as the universe differentiated itself.
I wonder... if the Big Bang started with an explosion of matter from a specific point, and the closer we get to that moment the slower time goes, then might it be correct in some sense to say that the big bang happened an infinite amount of time ago? Maybe it's a meaningless thing to say, without a suitable frame of reference from which one could observe the big bang from "outside"?
Thinking about this made me wonder why the big bang didn't just immediately collapse into a black hole. One answer to that, apparently, is that the universe was too uniform. It was very dense, but with ever spec of matter (or whatever it was the early universe was made of) being pulled equally in all directions at once, there wasn't an opportunity for a black hole to form.
I wonder if it's like that for time, too? If you're being pulled strongly in all directions equally, does it cancel out and cause time to run at a "normal" rate, or does time run super slow because all the gravitational forces add together?
In the former case, I guess it's possible that time ran normally at the very beginning and then slowed down as the universe differentiated itself.
> big bang happened an infinite amount of time ago
The big bang might have created time itself, whatever that means.
The big bang might have created time itself, whatever that means.
I had no idea that this effect was so noticeable: "the global positioning system (GPS) technology we rely on for navigation wouldn’t work if the satellites they use didn’t have clocks that account for the fact that time runs more slowly at the surface of Earth than it does at their position in orbit"
I wonder if you could measure this even from sea level vs somewhere elevated a few thousand feet.
Free research paper:
https://arxiv.org/abs/2306.04053
https://arxiv.org/abs/2306.04053
Surely this is an observer dependent effect due to the expansion of the universe?
For example, if a light ray was emitted each second billions of years ago, by now we observe that a redder light is emitted every 5 seconds. Because while it was travelling, the space through which it travelled expanded.
When the article says things like "time was slower in the past" it makes my head hurt. Don't they mean "events in the past appear to run 5 times slower when observed today?"
For example, if a light ray was emitted each second billions of years ago, by now we observe that a redder light is emitted every 5 seconds. Because while it was travelling, the space through which it travelled expanded.
When the article says things like "time was slower in the past" it makes my head hurt. Don't they mean "events in the past appear to run 5 times slower when observed today?"
I've actually had this question for a while regarding the JWST and newly discovered distant galaxies being stated as 13+ billion years old — isn't our estimation of their age based on the distance the light has traveled (red shifted) from emission? If so, wouldn't that mean that the 13+ billion year old galaxy's would actually be much younger. e.g. the light could've been emitted 10 billion years ago, but 3 billion light years of additional space has grown between us since then.
My understanding is that this is dependent on hubble's law, and we do not know the exact rate of expansion now or in the past, but stating an object is 13+ billion years old without that knowledge seems... Unscientific.
My understanding is that this is dependent on hubble's law, and we do not know the exact rate of expansion now or in the past, but stating an object is 13+ billion years old without that knowledge seems... Unscientific.
I believe cosmologists account for all of this. It takes about 13 billion years for space to expand enough to give the observed red shift. The distance between us is now greater than 13 billion light years.
Yes, there is uncertainty about the rate of expansion, with different techniques disagreeing. So none of this is fully settled. Which is very scientific!
Yes, there is uncertainty about the rate of expansion, with different techniques disagreeing. So none of this is fully settled. Which is very scientific!
Time is relative. Events near heavy objects appear slower to observers not near heavy objects. An observer in a similar gravitational field as the event (which is implied if the observer and event are in the same place) will see the events at "normal" speed.
The separation of event and observer can be mostly in space when observing a relatively close object falling into a black hole, but can also be in both space and time when looking far into the universe (and thus back in time).
The separation of event and observer can be mostly in space when observing a relatively close object falling into a black hole, but can also be in both space and time when looking far into the universe (and thus back in time).
Isn't cosmological time dilation a different thing though, entirely due to the expansion of space?
It is not due to differences in gravitational fields or relative speeds approaching the speed of light.
It is not due to differences in gravitational fields or relative speeds approaching the speed of light.
Slower relative to what?
What kind of "clocks" existed in the early universe? Can we for example compare the half life of some isotopes between then and now and say something about how they differed?
This is pretty insane! I mentally run the universe backwards in time to relate to current very early JWST discoveries and it's so easy to think "similar to now but earlier in terms of star evolution etc", but this hammers in how we're truly looking back at something very alien and weird when we see its recent early age photos. JWST even peers farther back than 1.5 billion years since Big Bang.
The article feels dated, like the study began long before the latest revelations about our cosmos have been announced. E.g., we believe the current universe is > 23 billion years old, and that the 13.x bn year timeline is now out of date. Secondly, there is a growing agreement in the academic community that the big bang is just teleological fantasy at its best, and that there is no way to know what happened before inflation, just that inflation happened. So accurately we now know the universe is > 23bn years since inflation, and we have no idea what happened beyond that border, just that it "appears" to be expanding from a point.
Its also very dated to perceive time as a 4th dimension. This is not academic, this is now common knowledge. 4th dimensional reality is the superposition of all objects in 3 dimensions, see the term "tesseract." Its the stuff of kindergartners to believe time is the 4th dimension.
Also agree with the other comments that it sounds naive to measure things in "seconds" when there is nothing relative from which to measure time in the earliest states of the universe.
The article basically sounds like it came from 1970s WASP academia, and is a few decades behind in the current academic perspectives of astrophysics and cosmology.
Its also very dated to perceive time as a 4th dimension. This is not academic, this is now common knowledge. 4th dimensional reality is the superposition of all objects in 3 dimensions, see the term "tesseract." Its the stuff of kindergartners to believe time is the 4th dimension.
Also agree with the other comments that it sounds naive to measure things in "seconds" when there is nothing relative from which to measure time in the earliest states of the universe.
The article basically sounds like it came from 1970s WASP academia, and is a few decades behind in the current academic perspectives of astrophysics and cosmology.
> time progressed five times slower just after the Big Bang.
How do you interpret that: five times slower than what, where?
Time runs more slowly for someone on a fast rocket, and what that means is that when they come home, they are younger than their twin brother who stayed home.
Without such a comparison, it is meaningless.
In particularly, how can you say time ran slower before such and such a time, compared to after, because those periods do not overlap; no meaningful comparison can be made.
If the 13.77 billion year estimate of the age of the universe is measured in relative time within the universe then that's basically what it is: the speed of time is 1: 13.77 billion years have elapsed in a span of 13.77 billion years. Some imaginary frame of reference outside of the universe might have a different opinion about long things "actually" took, but that's either irrelevant or impossible or both.
In other news, five times as many angels fit on the head of a pin in the first 300 femtoseconds of the Big Bang, compared to afterwards ...
How do you interpret that: five times slower than what, where?
Time runs more slowly for someone on a fast rocket, and what that means is that when they come home, they are younger than their twin brother who stayed home.
Without such a comparison, it is meaningless.
In particularly, how can you say time ran slower before such and such a time, compared to after, because those periods do not overlap; no meaningful comparison can be made.
If the 13.77 billion year estimate of the age of the universe is measured in relative time within the universe then that's basically what it is: the speed of time is 1: 13.77 billion years have elapsed in a span of 13.77 billion years. Some imaginary frame of reference outside of the universe might have a different opinion about long things "actually" took, but that's either irrelevant or impossible or both.
In other news, five times as many angels fit on the head of a pin in the first 300 femtoseconds of the Big Bang, compared to afterwards ...
> five times slower than what, where
Slower than the time we measure here on Earth, or outside Earth/solar system/galaxy. They are very minor time dilation differences between those cases.
> In particularly, how can you say time ran slower before such and such a time, compared to after, because those periods do not overlap; no meaningful comparison can be made.
Of course you can compare different time periods. When the twin from twin paradox returns younger to Earth, you can say time ran slower for him in the past.
We are actually seeing the past (of those quasars) unfold in slower time right now.
Slower than the time we measure here on Earth, or outside Earth/solar system/galaxy. They are very minor time dilation differences between those cases.
> In particularly, how can you say time ran slower before such and such a time, compared to after, because those periods do not overlap; no meaningful comparison can be made.
Of course you can compare different time periods. When the twin from twin paradox returns younger to Earth, you can say time ran slower for him in the past.
We are actually seeing the past (of those quasars) unfold in slower time right now.
OK, so the quasars are the observable phenomenon. So, question: any of this mean we have to adjust our estimate of the age of the universe?
It would be interesting to speculate a future where people doing deep work on big projects are on some near light speed craft, to buy an expert a longer working life on some essential project with a sound mind due to time dilation, versus if they did that work in a lab on earth.
So, will time speed up the closer we get to the heat death of the universe?
Or is it just less slowed down?
Logically based on this evidence you’d have to assume so, yes?
Is it due to gravity caused by higher density, near light speed of relative motion or space expansion?
Isn't all of this explained by the fact that we have a supermassive black hole at the middle of the galaxy?
We perceive distant time to run slow because we're falling into a black hole.
This also explains a lot of the expansion observations . . .they're not moving away from each other . . .our local perspective is shifting so that the ENTIRE outside universe seems to be doing that.
We perceive distant time to run slow because we're falling into a black hole.
This also explains a lot of the expansion observations . . .they're not moving away from each other . . .our local perspective is shifting so that the ENTIRE outside universe seems to be doing that.
> Isn't all of this explained by the fact that we have a supermassive black hole at the middle of the galaxy?
No.
> We perceive distant time to run slow because we're falling into a black hole.
We aren't falling into a black hole.
> This also explains a lot of the expansion observations . . .they're not moving away from each other . . .our local perspective is shifting so that the ENTIRE outside universe seems to be doing that.
The way our local perspective would need to be shifting for that to happen is much more complicated and unrelated to anything else, than the expansion hypothesis.
We can check the predictions of the accepted theories of gravity with things like gravitational lensing and flying very precise clocks very fast under different strengths of gravity etc. That accepted theory does not predict what you are suggesting for our local perspective.
Of course, we can categorically exclude a new effect like what you describe, just like we can't exclude Russell's teapot. But there's no evidence for it, and it doesn't allow us to make new predictions.
No.
> We perceive distant time to run slow because we're falling into a black hole.
We aren't falling into a black hole.
> This also explains a lot of the expansion observations . . .they're not moving away from each other . . .our local perspective is shifting so that the ENTIRE outside universe seems to be doing that.
The way our local perspective would need to be shifting for that to happen is much more complicated and unrelated to anything else, than the expansion hypothesis.
We can check the predictions of the accepted theories of gravity with things like gravitational lensing and flying very precise clocks very fast under different strengths of gravity etc. That accepted theory does not predict what you are suggesting for our local perspective.
Of course, we can categorically exclude a new effect like what you describe, just like we can't exclude Russell's teapot. But there's no evidence for it, and it doesn't allow us to make new predictions.
> We aren't falling into a black hole.
I'm going to nitpick here a bit but… we actually don't know. For all we know we could already be inside a gigantic black hole (one that encompasses our entire observable universe). Or our (Earth's) worldline might cross some black hole's event horizon at some point in the far future.
The thing is, the event horizon is not a place in space but a three-dimensional hypersurface that also extends across time. In order to recognize an event horizon you would need to now the entire future evolution of spacetime[0].
[0]: https://en.wikipedia.org/wiki/Absolute_horizon#Nature_of_the...
I'm going to nitpick here a bit but… we actually don't know. For all we know we could already be inside a gigantic black hole (one that encompasses our entire observable universe). Or our (Earth's) worldline might cross some black hole's event horizon at some point in the far future.
The thing is, the event horizon is not a place in space but a three-dimensional hypersurface that also extends across time. In order to recognize an event horizon you would need to now the entire future evolution of spacetime[0].
[0]: https://en.wikipedia.org/wiki/Absolute_horizon#Nature_of_the...
Yes, that might be a valid discussion to be had.
If I wanted to be more precise, I could have said something like 'to a first approximation, we are not currently falling into the black hole in the centre of our galaxy.'
If I wanted to be more precise, I could have said something like 'to a first approximation, we are not currently falling into the black hole in the centre of our galaxy.'
Yay, I love Big Crunch. BC is such a tasty sugary addictive treat. It generates lots of black holes in the (re-)collapsing universe just like Chocolate Frosted Sugar Bombs generate lots of black holes in Calvin's tooth enamel.
I think I mostly agree with you, loosely, that we could be in a BC cosmos but I also think it's not great science communication to say so because "could" is doing a lot of work there. :-)
Let's start by ignoring the fact that all the new z > 11 (!!!) galaxies JWST is spotting lately do not seem to be rushing inwards from the horizon[*], because I think those observations will kill (inertial) BC dead real soon now.
Expanding R-W gives rise to congruences that are incompatible with collapsing spacetimes, and those congruences are observed (e.g. the timelike geodesic congruence of COMs of galaxy clusters; the CMB's null geodesic congruences; this motivates the use of FRW dusts).
Collapsing R-W is plausible, since the Friedmann equations still work there; collapsing spatially flat FLRW is enough like Oppenheimer-Snyder that it's not worth calling "not a black hole" outside an academic context. I think at best we can say that maybe the expansion reverses at some point, and then try to work out whether evidence strongly disfavours that point being in the past. One would probably start conventionally, trying to measure the critical density by non-geometrical means (since the geometrical approach is pretty suggestive that recently (z < 0.1) parallel lines will stay parallel or diverge). That leaves coming up with some notion of quasilocal mass and counting it, but then I think you're not going to make much progress without a better understanding of the dark sector. Big project, many previous attempts (some high-profile), none especially satisfying.
Now you have me wondering how one deals with the Raychaudhuri vorticity tensor and other terms that would oppose (re-)collapse. Where does all the angular momentum within large galaxy clusters go? Hierarchical BH mergers probably can't be the whole story. I'm not sure we'd worry about that until well into the (re-)collapse, but I think we'd already have to look beyond the FRW dusts. (One would also have to overcome opposition to (re-)collapse from a negative trace of the electrogravitic tensor thanks to a CC-compatible DE. "DE undergoes phase change" is something I'd buy as an idea. Otherwise I think observations from Chandra/ROSAT (data in Vikhlinin et al., 2008) were a fatal problem for BC and big rip).
Ultimately I think we end up in the land of "something about the accelerated expansion would have to give" and speculate wildly against the trend. Taking that path seriously seems like a recipe for frustrating tension headaches.
- --
[*] the premise here is that BC should look like the actual expansion history under time reversal, and thus we'd expect to find the most distant galaxies brighter, smaller (because we reverse the angular diameter turnaround), and bluer than they are turning out to be.
P.S.: "In order to recognize an event horizon you..." should read Visser 2014 https://arxiv.org/abs/1407.7295
P.P.S.: Yes, "someone" could already have focused shells of radiation on our solar system such that we are already inside an event horizon and don't know it. Somewhere someone diagrammed a Kugelblitz spacetime with that in mind, but I don't seem to be able to find it, and am not sure I trust my memory that it was fairly rigorous (ignoring implausible initial conditions). It wasn't video so it's not the PBS Spacetime "Escape the Kugelblitz Challenge". That's not only very far from rigorous, it's pretty consciously silly. (The challenge should ramp up the mass so that the settled horizon would be trans-Neptunian, and ask whether humanity can engineer anything at all which could escape).
P.P.P.S.: None of this is about falling into an astrophysical black hole like Sag A* as opposed to already unknowingly being in a "black hole".
I think I mostly agree with you, loosely, that we could be in a BC cosmos but I also think it's not great science communication to say so because "could" is doing a lot of work there. :-)
Let's start by ignoring the fact that all the new z > 11 (!!!) galaxies JWST is spotting lately do not seem to be rushing inwards from the horizon[*], because I think those observations will kill (inertial) BC dead real soon now.
Expanding R-W gives rise to congruences that are incompatible with collapsing spacetimes, and those congruences are observed (e.g. the timelike geodesic congruence of COMs of galaxy clusters; the CMB's null geodesic congruences; this motivates the use of FRW dusts).
Collapsing R-W is plausible, since the Friedmann equations still work there; collapsing spatially flat FLRW is enough like Oppenheimer-Snyder that it's not worth calling "not a black hole" outside an academic context. I think at best we can say that maybe the expansion reverses at some point, and then try to work out whether evidence strongly disfavours that point being in the past. One would probably start conventionally, trying to measure the critical density by non-geometrical means (since the geometrical approach is pretty suggestive that recently (z < 0.1) parallel lines will stay parallel or diverge). That leaves coming up with some notion of quasilocal mass and counting it, but then I think you're not going to make much progress without a better understanding of the dark sector. Big project, many previous attempts (some high-profile), none especially satisfying.
Now you have me wondering how one deals with the Raychaudhuri vorticity tensor and other terms that would oppose (re-)collapse. Where does all the angular momentum within large galaxy clusters go? Hierarchical BH mergers probably can't be the whole story. I'm not sure we'd worry about that until well into the (re-)collapse, but I think we'd already have to look beyond the FRW dusts. (One would also have to overcome opposition to (re-)collapse from a negative trace of the electrogravitic tensor thanks to a CC-compatible DE. "DE undergoes phase change" is something I'd buy as an idea. Otherwise I think observations from Chandra/ROSAT (data in Vikhlinin et al., 2008) were a fatal problem for BC and big rip).
Ultimately I think we end up in the land of "something about the accelerated expansion would have to give" and speculate wildly against the trend. Taking that path seriously seems like a recipe for frustrating tension headaches.
- --
[*] the premise here is that BC should look like the actual expansion history under time reversal, and thus we'd expect to find the most distant galaxies brighter, smaller (because we reverse the angular diameter turnaround), and bluer than they are turning out to be.
P.S.: "In order to recognize an event horizon you..." should read Visser 2014 https://arxiv.org/abs/1407.7295
P.P.S.: Yes, "someone" could already have focused shells of radiation on our solar system such that we are already inside an event horizon and don't know it. Somewhere someone diagrammed a Kugelblitz spacetime with that in mind, but I don't seem to be able to find it, and am not sure I trust my memory that it was fairly rigorous (ignoring implausible initial conditions). It wasn't video so it's not the PBS Spacetime "Escape the Kugelblitz Challenge". That's not only very far from rigorous, it's pretty consciously silly. (The challenge should ramp up the mass so that the settled horizon would be trans-Neptunian, and ask whether humanity can engineer anything at all which could escape).
P.P.P.S.: None of this is about falling into an astrophysical black hole like Sag A* as opposed to already unknowingly being in a "black hole".
Thanks for your enlightening comment, raattgift! As always, you know much more about these things than I do, so I won't be able to contribute much to the BC discussion, but I always enjoy learning something new!
The Visser paper I already knew from one of your previous comments. :-)
The Visser paper I already knew from one of your previous comments. :-)
Thank you for the kind words.
Nobody's wanted to talk seriously about "terminal" Big Crunch for many years -- the 1998 type Ia SN light curve redshift work that earned the 2011 Nobel prize in physics pretty much killed off the idea of an inertially-expanding universe, rather than an accelerated expansion. It's much easier to recollapse a inertial expansion: "inertial" means there was an effective single impulse that started the bits flying apart from one another, and the bits continue to fly apart along the lines of Newton's First Law. However, as far as anyone can tell the driver of the acceleration is a small positive cosmological constant (constant everywhere in time and space), and to recollapse you have to either turn off (and even reverse) the cosmological constant or you have to overwhelm it with a long-range "fifth force".
This line of discussion surfaces a Big Crunch paper with a wonderful title. In 1982 Andrei Linde proposed his "new inflation"[1] which introduced a scalar field as the driver of inflation. The scalar field starts with high values and slowly rolls down a potential "hill" taking on lower values as it rolls. Part of the hill is very shallow, "sufficiently flat", and so the potential rolls very slowly there; it steepens later. When the evolution of the scalar field is slow compared to the expansion, inflation occurs. When the scalar field is on the steep part, inflation ends. In 2004, Linde and co-authors wrote a paper that used a slow-rolling scalar field to drive the metric expansion (rather than inflation), and in that it is a "fifth force". Rolling slowly near the top of the potential "hill" the scalar field drives the accelerated expansion of space; rolling quickly in the steep part further down the "hill" the accelerated expansion stops and can even reverse leading to an accelerating contraction of space as the scalar field rolls further down an ever-steepening hill and as formerly-separated galaxy clusters become gravitationally bound to one another.
This leads to a relatively quick accelerated shrinking of space, much quicker than the expansion, motivating the poetic title of
Wang, Kratochvil, Linde & Shmakova, "Current observational constraints on cosmic doomsday" (2004, JCAP vol 12).
https://iopscience.iop.org/article/10.1088/1475-7516/2004/12... (aka <https://arxiv.org/abs/astro-ph/0409264>).
Warning, though, that is really a Bayesian reasoning paper dressed up as cosmology. :-) tl;dr: their scheme would destroy the universe in somewhere between 20 billion and 4 trillion years.
There is an assortment papers exploring "big bounce" where there is a partial contraction of the universe; this is often to try to abolish either the singularity at the early boundary or the extremely low entropy at the early boundary. I prefer a Big Crunch that stays crunched, rather than going all soggy and spongey and threatening to cause me to repeat all my mistakes in life over again. Cyclical expansion and contraction also appears with surprising frequency when diverse types of modifications to General Relativity are made, thus it pops up in lots of quantum gravity approaches. These all seem to struggle with observational support for the cosmological constant (and at the extreme anti-de Sitter space has a cosmological constant, but with the wrong sign, which does weeeiiird things, like Hawking radiation reflects off the boundary -- for a single central black hole that means it never evaporates; the same reflecting boundary conditions in an AdS universe with lots of black holes makes the whole universe unstable; and (very) weak vacuum perturbations lead to black holes after sufficiently long times).
- --
[1] https://www.sciencedirect.com/science/article/abs/pii/037026... (which is also on sci-hub)
Nobody's wanted to talk seriously about "terminal" Big Crunch for many years -- the 1998 type Ia SN light curve redshift work that earned the 2011 Nobel prize in physics pretty much killed off the idea of an inertially-expanding universe, rather than an accelerated expansion. It's much easier to recollapse a inertial expansion: "inertial" means there was an effective single impulse that started the bits flying apart from one another, and the bits continue to fly apart along the lines of Newton's First Law. However, as far as anyone can tell the driver of the acceleration is a small positive cosmological constant (constant everywhere in time and space), and to recollapse you have to either turn off (and even reverse) the cosmological constant or you have to overwhelm it with a long-range "fifth force".
This line of discussion surfaces a Big Crunch paper with a wonderful title. In 1982 Andrei Linde proposed his "new inflation"[1] which introduced a scalar field as the driver of inflation. The scalar field starts with high values and slowly rolls down a potential "hill" taking on lower values as it rolls. Part of the hill is very shallow, "sufficiently flat", and so the potential rolls very slowly there; it steepens later. When the evolution of the scalar field is slow compared to the expansion, inflation occurs. When the scalar field is on the steep part, inflation ends. In 2004, Linde and co-authors wrote a paper that used a slow-rolling scalar field to drive the metric expansion (rather than inflation), and in that it is a "fifth force". Rolling slowly near the top of the potential "hill" the scalar field drives the accelerated expansion of space; rolling quickly in the steep part further down the "hill" the accelerated expansion stops and can even reverse leading to an accelerating contraction of space as the scalar field rolls further down an ever-steepening hill and as formerly-separated galaxy clusters become gravitationally bound to one another.
This leads to a relatively quick accelerated shrinking of space, much quicker than the expansion, motivating the poetic title of
Wang, Kratochvil, Linde & Shmakova, "Current observational constraints on cosmic doomsday" (2004, JCAP vol 12).
https://iopscience.iop.org/article/10.1088/1475-7516/2004/12... (aka <https://arxiv.org/abs/astro-ph/0409264>).
Warning, though, that is really a Bayesian reasoning paper dressed up as cosmology. :-) tl;dr: their scheme would destroy the universe in somewhere between 20 billion and 4 trillion years.
There is an assortment papers exploring "big bounce" where there is a partial contraction of the universe; this is often to try to abolish either the singularity at the early boundary or the extremely low entropy at the early boundary. I prefer a Big Crunch that stays crunched, rather than going all soggy and spongey and threatening to cause me to repeat all my mistakes in life over again. Cyclical expansion and contraction also appears with surprising frequency when diverse types of modifications to General Relativity are made, thus it pops up in lots of quantum gravity approaches. These all seem to struggle with observational support for the cosmological constant (and at the extreme anti-de Sitter space has a cosmological constant, but with the wrong sign, which does weeeiiird things, like Hawking radiation reflects off the boundary -- for a single central black hole that means it never evaporates; the same reflecting boundary conditions in an AdS universe with lots of black holes makes the whole universe unstable; and (very) weak vacuum perturbations lead to black holes after sufficiently long times).
- --
[1] https://www.sciencedirect.com/science/article/abs/pii/037026... (which is also on sci-hub)
Very interesting, thanks so much for elaborating! I was aware of the infamous instability of AdS, but that was about it.
If you don't mind me asking, what is your field of research? (If you like, feel free to share your website or papers or something!) I've read so many interesting comments from you over the past year or two that I've been getting the feeling your interests are quite eclectic. :)
If you don't mind me asking, what is your field of research? (If you like, feel free to share your website or papers or something!) I've read so many interesting comments from you over the past year or two that I've been getting the feeling your interests are quite eclectic. :)
This is the attitude that just makes the whole field a piece of shit to engage with.
"No" - Try writing an explanation. Words are free. If you've chosen to engage with negative cynicism and consider yourself an expert in the field, you have a duty to explain your damned position instead of a "no"
"We aren't falling into a black hole" - You say this with what level of certainty? Are gravitational effects of the galactic center so insignificant at our location? The images of galaxies would suggest otherwise.
"Accepted theory" can kiss my ass for all its worth. If we went with accepted theory through history, i don't have to tell you the kind of seemingly obvious mistaken beliefs we've held.
"No" - Try writing an explanation. Words are free. If you've chosen to engage with negative cynicism and consider yourself an expert in the field, you have a duty to explain your damned position instead of a "no"
"We aren't falling into a black hole" - You say this with what level of certainty? Are gravitational effects of the galactic center so insignificant at our location? The images of galaxies would suggest otherwise.
"Accepted theory" can kiss my ass for all its worth. If we went with accepted theory through history, i don't have to tell you the kind of seemingly obvious mistaken beliefs we've held.
> "No" - Try writing an explanation.
The explanation is in the rest of the comment.
I am not an expert in the field.
> "We aren't falling into a black hole" - You say this with what level of certainty? Are gravitational effects of the galactic center so insignificant at our location? The images of galaxies would suggest otherwise.
For the same reason that we aren't falling into the sun, or the moon isn't falling into earth. Orbits are a thing. (The orbits in a galaxy are more complicated than around the sun, but the same principles apply. And galaxies have been pretty stable.)
Similarly, if you were to suddenly replace our own sun with a black hole of the same mass, none of the solar system's orbits would change. Earth would still take one year to peacefully complete its circle around the black hole sun.
> "Accepted theory" can kiss my ass for all its worth. If we went with accepted theory through history, i don't have to tell you the kind of seemingly obvious mistaken beliefs we've held.
Who is 'we'? Accepted theory isn't always right, but it's right more often than stuff we just made up five minutes ago in our armchairs. Especially these days.
The explanation is in the rest of the comment.
I am not an expert in the field.
> "We aren't falling into a black hole" - You say this with what level of certainty? Are gravitational effects of the galactic center so insignificant at our location? The images of galaxies would suggest otherwise.
For the same reason that we aren't falling into the sun, or the moon isn't falling into earth. Orbits are a thing. (The orbits in a galaxy are more complicated than around the sun, but the same principles apply. And galaxies have been pretty stable.)
Similarly, if you were to suddenly replace our own sun with a black hole of the same mass, none of the solar system's orbits would change. Earth would still take one year to peacefully complete its circle around the black hole sun.
> "Accepted theory" can kiss my ass for all its worth. If we went with accepted theory through history, i don't have to tell you the kind of seemingly obvious mistaken beliefs we've held.
Who is 'we'? Accepted theory isn't always right, but it's right more often than stuff we just made up five minutes ago in our armchairs. Especially these days.
Good to know you're atleast self aware that you're not an expert in the field.
oRbItS R a tHiNg - and we all just fly off in perpetuity.
oRbItS R a tHiNg - and we all just fly off in perpetuity.
Why are you so hostile? I’ve encountered responses like yours before as a physicist myself. Is it a fear of the unknown response?
> Isn't all of this explained by the fact that we have a supermassive black hole at the middle of the galaxy?
> We perceive distant time to run slow because we're falling into a black hole.
We’re not falling into it
The mass of Sagittarius A* is 4.1 million solar masses while the mass of the galaxy is 1.5 trillion solar masses.
Our orbital velocity easily exceeds the necessary velocity to orbit Sagittarius A* at our current distance.
You feel more gravity from a paperclip a meter away than you do from Sag A*
> We perceive distant time to run slow because we're falling into a black hole.
We’re not falling into it
The mass of Sagittarius A* is 4.1 million solar masses while the mass of the galaxy is 1.5 trillion solar masses.
Our orbital velocity easily exceeds the necessary velocity to orbit Sagittarius A* at our current distance.
You feel more gravity from a paperclip a meter away than you do from Sag A*
We're in the gravity well of not just Sag A but all the mass that orbits Sag A.
That is a scary thought ..
...if you believe in an early, non-eternal universe. Tired Light will be vindicated! Repent!
Time used to run slowly in my early life too.
Now every year is getting shorter.
I never seem to find the time.
One day I found 10 years had got me behind me.
And no one had told me when to run.
There is some theory that say by age of 35-40 years one has lived about 80% of their lives in term of experiences. This seems intuitively true to me considering increasingly monotonous routine nowadays.
There could be just ten things that I remember from last 10 years.
There could be just ten things that I remember from last 10 years.
It doesn’t have to be this way! Any year can be the most memorable of your life. Think about what you want, then go get it.
Just as soon as my kids grow up and I pay off this immense mortgage that is smothering us.
Not to be a huge downer, I generally agree with you. There is a choice to be made. It's difficult though, and for some, maybe practically impossible. The demands of life can be extreme.
For me, getting in the ocean is a pretty good way to make time stretch out. I try to do it as much as I can. Otherwise, quality time with my kids is great, though harder to fit in as they socialize more and more. There are ways to slow down time though, for sure. I like to think those things are a real gift.
Also, this is hard I think, but I read something recently in the context of mindfulness and meditation. It went something like "The next thing you'll need to do is likely something you've done countless times before. Think about it closely this time". And that's a good strategy too. We forget to pay attention, as if run by a motor. I guess that's a choice as well.
Not to be a huge downer, I generally agree with you. There is a choice to be made. It's difficult though, and for some, maybe practically impossible. The demands of life can be extreme.
For me, getting in the ocean is a pretty good way to make time stretch out. I try to do it as much as I can. Otherwise, quality time with my kids is great, though harder to fit in as they socialize more and more. There are ways to slow down time though, for sure. I like to think those things are a real gift.
Also, this is hard I think, but I read something recently in the context of mindfulness and meditation. It went something like "The next thing you'll need to do is likely something you've done countless times before. Think about it closely this time". And that's a good strategy too. We forget to pay attention, as if run by a motor. I guess that's a choice as well.
Your spouse will take care of the children
Also the more you experience the more your brain is running on auto pilot. The key is to change things up and not allow the brain to fall into familiar patterns.
You missed the starting gun!
And you run and you run to catch up with the sun, but its sinking...racing around to come up behind you again. The sun is the same in a relative way, but you're older. Shorter of breath and one day closer to death.
It’s nice others got the idea! I love that song, there’s something special about it.
Though I still feel that life is long, but years have started feeling significantly shorter. I hope that compounding effect of life long investments kicks in soon to compensate.
Can anyone who knows anything about this explain what it means for time to run slower? How do you even measure that?
It's not like you can say "in one second, only half a second of time passed," because that makes no sense surely...? How can you measure how fast time is passing?
It's not like you can say "in one second, only half a second of time passed," because that makes no sense surely...? How can you measure how fast time is passing?
You have to compare two clocks.
Another example of time dilation is this: take two really precise clocks, carry one up a mountain and back, and compare it to the clock that stayed at sea level. The one at sea level will have run slower and be behind compared to the other clock because it spend some time in a place where gravity was less.
Here is a guy that did exactly that in 2005 on a tour using his van. He even took his kids: http://www.leapsecond.com/great2005/tour/
Another example of time dilation is this: take two really precise clocks, carry one up a mountain and back, and compare it to the clock that stayed at sea level. The one at sea level will have run slower and be behind compared to the other clock because it spend some time in a place where gravity was less.
Here is a guy that did exactly that in 2005 on a tour using his van. He even took his kids: http://www.leapsecond.com/great2005/tour/
You just described it. Time passes slower at high velocities for example.
after 6 months on the International Space Station (ISS), orbiting Earth at a speed of about 7,700 m/s, an astronaut would have aged about 0.005 seconds less than those on Earth.
1. https://en.m.wikipedia.org/wiki/Time_dilation
after 6 months on the International Space Station (ISS), orbiting Earth at a speed of about 7,700 m/s, an astronaut would have aged about 0.005 seconds less than those on Earth.
1. https://en.m.wikipedia.org/wiki/Time_dilation
Time doesn’t pass slower at higher velocities unless you compare it to a clock that is moving relative to you at that velocity
So then....what is absolute time? This would imply our own clock on earth is moving much slower relative to a static inertial frame of reference, right? given the speed with which the milky way is hurtling through space? doesn't that mean we're experiencing a dramatically slowed down version of "true time" or "base time"?
And then also, does the velocity have to be constant in some direction? i mean what if you're moving back and forth one inch at the speed of light, would time still be slowed for you? what if you're moving back and forth one micron?
And then also, does the velocity have to be constant in some direction? i mean what if you're moving back and forth one inch at the speed of light, would time still be slowed for you? what if you're moving back and forth one micron?
There is no base time. In the point of view of the thing moving quickly away from us, OUR clocks are slower. For both of us, the other’s clock seems to tick slower. The situation is symmetric. This is why it’s called relativity, there is no objective static frame of reference
And yes even if you were moving back and forth by one micron at close to the speed of light then an observer stationary relative to you would see your clock tick slower
And yes even if you were moving back and forth by one micron at close to the speed of light then an observer stationary relative to you would see your clock tick slower
Easy: there is no absolute time.
do you feel this is a useful nitpick. You are correct, but I feel like "to our time (humans on earth)" is kind of implied.
The user wrongly thought there was an absolute time, a notion that I have now corrected. I don't think that's a "nitpick"
If you're comparing two physically separated frames of reference it makes sense to talk about "this many seconds here corresponds to that many seconds over there". But when you're talking about the speed of time for the entire universe, how do you form such a ratio?