Does light have an infinite lifetime?(bigthink.com)
bigthink.com
Does light have an infinite lifetime?
https://bigthink.com/starts-with-a-bang/light-infinite-lifetime/
105 comments
Feynmann is very clear that the photons bouncing off a mirror are not the same ones that hit it. They are absorbed by electric oscillators (free electrons in the material) and destroyed; then new ones are created and emitted.
He's also careful enough to point out that electrons don't really have an identity that would allow you to meaningfully define "not the same". :)
He's also careful enough to point out that electrons don't really have an identity that would allow you to meaningfully define "not the same". :)
Electrons aren't unique in not having identities. Photons of the same wavelength are similarly fungible if I remember right, though it's a little more defensible to call the reflected light not the same photons since the original photons are transformed to something we don't call photons before photons are produced again.
But at a reflection event you aren't producing a new photon, you are only reflecting a portion of the original determined by the refractive indices of the reflection interface. At that interface, part of the original photon is reflected and the rest is transmitted across the interface as a new photon and both have a modified bandwidth relative to the parent photon. So the one you see reflected is the same photon after modification by the medium it traversed and the transmitted components escape your view unless you are monitoring the other side of the medium.
A photon is indivisible that’s the quantum nature of light. You aren’t reflecting a part of it. To do so is a classical wave interpretation of light. The photon after reflection is not the same photon.
That would imply a frequency shift, which clearly doesn't happen (mirrors don't redshift the light that hits them).
I balked at that too, because infinite mirrors and mirror houses also don't show any wavelenght shift no matter how many reflections they do. They do change perceived amplitude, as no mirror is perfectly reflective.
Blue, but it's more to do with manufacturing defects and the minerals used for the mirrors.
Yeah, I meant to say "photon" in my last sentence; but the point is equally true for electrons.
Electrons? We were just told that there is only one single electron in yesterday's posts. It's almost like they don't know what an electron is
I believe they're right about there being only one single electron. I tried to start my Pathfinder yesterday and it was dead as a doornail. None of the dash lights lit and there was not even a click from the starter. Someone else must've been using that electron though since I tried again a few minutes later and it cranked right up. I had my turn with it so I'm not mad at all.
Um maybe check your battery terminals for corrosion :)
I covered all those bases. I'm not 100% sure what happened but when I initially tried to start the vehicle I had it connected to a solar battery maintainer in full sunlight. It should've had bazillions of photons sacrificing part of themselves to be stored electrons but instead I got nothing. After disconnecting that from the battery and testing all the fuses I made another attempt to start it up and it worked fine. It's possible that the voltage output of the panel swamped the voltage expected by the ECU and it refused to energize to save the system from electrical damage by overvoltage.
Or maybe that single electron hadn't quite made it to my battery on my first attempt. That's one busy e.
Or maybe that single electron hadn't quite made it to my battery on my first attempt. That's one busy e.
I find pouring electrons into a funnel usually helps by avoiding an electron spill all over your battery. Glad it works tho!
So sorry for the delayed reply. I had to hit several O'Reilly stores to find someone who had any idea what I was looking for. I finally found one with a college-looking kid behind the counter instead of all those old farts like me and after I explained what I wanted he knew just which aisle it was on.
Turns out that an electron funnel isn't a fancy new tool. Instead, I already have several of them in my shop but just didn't make the connection. It became clear though when he handed me the plastic funnel after rubbing it through his hair vigorously for a few seconds and I got shocked by the crackling electrons streaming from the funnel to my fingers.
If this power thing ever happens to my battery again I'm just gonna get the big funnel from the set, rub it through the remnants of my hair and pour some electrons at it till it charges enough to start the engine.
Thanks for this suggestion about a new way to use an old tool.
Turns out that an electron funnel isn't a fancy new tool. Instead, I already have several of them in my shop but just didn't make the connection. It became clear though when he handed me the plastic funnel after rubbing it through his hair vigorously for a few seconds and I got shocked by the crackling electrons streaming from the funnel to my fingers.
If this power thing ever happens to my battery again I'm just gonna get the big funnel from the set, rub it through the remnants of my hair and pour some electrons at it till it charges enough to start the engine.
Thanks for this suggestion about a new way to use an old tool.
I wonder if that's how they get electrons into the particle accelerators too?
They gotta get'em somewhere. Seems easy enough to do it like that.
Yeah but they just don't make 'em like they used to.
MBA's took over and cut corners everywhere - sourcing production from subpar isotopes, skimping down how much charge you get in each one, shrinkflation schemes packing less in every bag of amps, particle dilution with off-brand fake leptons... QA was downsized 'till all you got was beta junk that decays in no time. Now they just rent seek that single universal virtual electron, unlocked by keys you're forced to tie to the cloud, and it can be glitchy and pop in and out of existence from time to time especially driving through tunnels. Sorry to hear about your car, try upgrading to the new positron model on the roadmap for next year, they launched the presale NFT last week.
MBA's took over and cut corners everywhere - sourcing production from subpar isotopes, skimping down how much charge you get in each one, shrinkflation schemes packing less in every bag of amps, particle dilution with off-brand fake leptons... QA was downsized 'till all you got was beta junk that decays in no time. Now they just rent seek that single universal virtual electron, unlocked by keys you're forced to tie to the cloud, and it can be glitchy and pop in and out of existence from time to time especially driving through tunnels. Sorry to hear about your car, try upgrading to the new positron model on the roadmap for next year, they launched the presale NFT last week.
It’s a true statement that they don’t know what the electron is. In the standard model the electron is a point particle with no volume.
I feel like this applies: https://www.youtube.com/watch?v=MO0r930Sn_8 (Feynman on how certain questions can be very hard to answer)
> only one single electron
> an electron
THE electron? /s
> an electron
THE electron? /s
I wonder about the all the photons lost in fiber optic installations. What happens to them in their short lives? There must be a creation and an extinction event.
A photon jumps into the glass fiber and travels until it encounters an opto-electric coupler where the photon craps out and is converted to an energized stream of electrons, or maybe it borrows the only real electron in the universe for an instant as it flips across the coupler to the next glass fiber where a new photon is born, only to flare out at the next junction.
A photon jumps into the glass fiber and travels until it encounters an opto-electric coupler where the photon craps out and is converted to an energized stream of electrons, or maybe it borrows the only real electron in the universe for an instant as it flips across the coupler to the next glass fiber where a new photon is born, only to flare out at the next junction.
My understanding is that photons don't have a life the way we do. They move at the speed of light and thus time does not advance for them. They cannot change between emission and absorption, no matter the distance. Always bends my mind to think about it.
Almost everything you said is correct. Photons do not have a reference frame, so time does not advance because for a photon there is no time coordinate system in the first place. It's not simply that photons don't experience time, it's that time and space don't exist for photons.
>They cannot change between emission and absorption, no matter the distance.
From the point of view of a photon, neither time or space exist. They have no reference frame at all. However, from an outside frame of reference that is travelling less than the speed of light, photons do change for example they get red shifted as they move through stronger gravitational fields.
>They cannot change between emission and absorption, no matter the distance.
From the point of view of a photon, neither time or space exist. They have no reference frame at all. However, from an outside frame of reference that is travelling less than the speed of light, photons do change for example they get red shifted as they move through stronger gravitational fields.
Not to quibble about definitions of what "change" means, but I thought the red shift depended entirely on the relative speed of the emitting and receiving body? A laser beam for example could not be analyzed for its red shift, unless we knew the original frequency, because there would be no spectral absorption pattern to determine the shift. So we cannot tell how far laser light traveled before it reached our sensor.
Not a quibble at all. The redshift I'm referring to is the kind of redshift due to gravity, as opposed what you're describing which is redshift due to the Doppler effect.
I'm trying to come up with a scenario to understand the difference: Say I have a triangle of emitter, receiver, and a large body, all at a fixed distance. And the receiver would see emitted light both directly and bent around the body. The bent light would be red shifted?
> They move at the speed of light and thus time does not advance for them.
Isn't it more accurate to say that photons move at the speed of causality, when the medium is a pure vacuum? Because in some other medium like glass, the speed of light is slower than the speed of causality.
So my follow-up question is: do slower photons (such as those propagating through a fiber-optic strand, or water) then experience the advancement of time?
Isn't it more accurate to say that photons move at the speed of causality, when the medium is a pure vacuum? Because in some other medium like glass, the speed of light is slower than the speed of causality.
So my follow-up question is: do slower photons (such as those propagating through a fiber-optic strand, or water) then experience the advancement of time?
There is no such thing as slow photons, photons always travel at the speed of light.
When light enters a medium there are two mostly (but not entirely) equal ways to think about what happens, one is to view light as a purely electromagnetic wave that interacts with atoms and causes the atoms to oscillate. This oscillation produces its own electromagnetic wave that interferes with the original wave. The result of this interference will be an electromagnetic wave with the same frequency, same amplitude, and travelling in the same direction as the incoming light but shifted backwards and it's that shift backwards that gives the appearance of light slowing down.
That explanation is pretty good and accounts for almost everything except for the latency of light through a medium.
If that's what you want to model, then it's better to think of light as made up of photons instead of being a wave, and then when photons enter a material they no longer exist as independent particles but through a process of absorption and reemission by electrons in the material become particles called polaritons. Polaritons do have mass and hence travel slower than the speed of light.
Neither of these explanations are perfect, but the full explanation is ridiculously complicated and there's no suitable metaphor for it. If you are interested in knowing the edge latency of light through a medium, then the polariton explanation is appropriate. If you want to know the "bandwidth" explanation of light through a medium, then the wave explanation is appropriate.
When light enters a medium there are two mostly (but not entirely) equal ways to think about what happens, one is to view light as a purely electromagnetic wave that interacts with atoms and causes the atoms to oscillate. This oscillation produces its own electromagnetic wave that interferes with the original wave. The result of this interference will be an electromagnetic wave with the same frequency, same amplitude, and travelling in the same direction as the incoming light but shifted backwards and it's that shift backwards that gives the appearance of light slowing down.
That explanation is pretty good and accounts for almost everything except for the latency of light through a medium.
If that's what you want to model, then it's better to think of light as made up of photons instead of being a wave, and then when photons enter a material they no longer exist as independent particles but through a process of absorption and reemission by electrons in the material become particles called polaritons. Polaritons do have mass and hence travel slower than the speed of light.
Neither of these explanations are perfect, but the full explanation is ridiculously complicated and there's no suitable metaphor for it. If you are interested in knowing the edge latency of light through a medium, then the polariton explanation is appropriate. If you want to know the "bandwidth" explanation of light through a medium, then the wave explanation is appropriate.
It could be the same electron after all. Although probably not.
One electron theory. https://youtu.be/9dqtW9MslFk?si=qdWGUyJnDRCOns9F
One electron theory. https://youtu.be/9dqtW9MslFk?si=qdWGUyJnDRCOns9F
Does Feynman say that? If an electron absorbs a photon, why would it create another one of the same frequency, and with polarization perfectly rotated by 90°?
Because of the layout of the electron orbitals.
Feynman certainly didn’t say anything like that, or only the photons with certain frequencies would reflect, those corresponding to the energy differences between various orbitals.
If you haven't yet, you should definitely read the famous little scifi story Light of Other Days.
Full text: https://www.physics.utoronto.ca/~jharlow/slowglass.htm
Full text: https://www.physics.utoronto.ca/~jharlow/slowglass.htm
Wow. Now that's nice. Someone thought outside our box for that story.
Part of the reason that I initially posted on this thread was to ponder the question about a world where mirrors have a memory function. A mirror with a memory would have perfect fidelity though the image would be a mirror image of reality, like looking into a replay from a mirror image universe. I wrote a poem on that subject a couple years ago and the whole question about photons and their potential infinite existence triggered the memory.
Part of the reason that I initially posted on this thread was to ponder the question about a world where mirrors have a memory function. A mirror with a memory would have perfect fidelity though the image would be a mirror image of reality, like looking into a replay from a mirror image universe. I wrote a poem on that subject a couple years ago and the whole question about photons and their potential infinite existence triggered the memory.
Photons don’t experience time, from their relativistic frame they are created and destroyed instantly. This is because they are massless and therefore always travel at the speed of light. Due to relativity they experience a different simultaneity - in our reference frame we can observe their emission and absorption as energy and existence and measure them etc but, even given they can’t experience anything anyways, time doesn’t pass for them. This is one of the weirdnesses of relativity.
Right, this would be true if it were possible for a photon to"experience", but at i understand it we currently believe it's not.
What makes you so sure the photon didn’t originate from your eye and finish where you saw them?
I assume you’re referring to Feynmans demonstration that since photons are neutral with no antiparticle in QED a photon going forward in time and backwards in time are the same photon. But from our frame of reference the photon always originates at the origin and hits our eye. From the photons point of reference it’s not meaningful tho to say they go backwards or forwards because time actually doesn’t elapse for them.
From the perspective of a photon, there is no such thing as time. It's emitted, and might exist for hundreds of trillions of years, but for the photon, there's zero time elapsed between when it's emitted and when it's absorbed again. It doesn't experience distance either.
Something about this doesn't make sense. This means that, from its own perspective, every photon is emitted at time T at location x, then absorbed at time T at location y, meaning there is no time "in between" those... yet somehow, at a distinct point in between those events, an object can get in the way and block the photon from reaching point y. Meaning you have two points in time that have the same value but aren't identical? How does that work, mathematically? Do you need nonstandard calculus or something to make sense of it?
This is because a photon has no mass and always travels at the speed of light in all reference frames. A photon never observes time because the effects of time dilation at the speed of light is infinite and due to relativity they don’t experience the events between their emission and absorption as they don’t decelerate when they’re absorbed. In a very real sense from the photons perspective they never exist and don’t experience a temporal dimension. However from a frame of reference less than the speed of light - which all objects of mass experience - we can observe their creation, propagation, and absorption.
Reality is stranger than fiction!!
Reality is stranger than fiction!!
Think of time as defined by how many times a photon bounces between two mirrors. If the two mirrors move at the speed of light with the photon, the photon will never reach the mirror to bounce. Therefore, the photon experiences no time.
There is no universal time T in space time. Time is relative to the observer.
I suggest reading https://en.wikipedia.org/wiki/Minkowski_space for the mathematics.
There is no universal time T in space time. Time is relative to the observer.
I suggest reading https://en.wikipedia.org/wiki/Minkowski_space for the mathematics.
I am not a Physicist, so please don't rely on this, but my understanding is that this comes down to the frames of reference in special relativity.
If you're in a car on the highway, an observer standing still outside the car sees you moving at 50mph, but an observer inside the car with you, sees you not moving at all, but the world rushing past at 50mph. These are two different reference frames, and the second one is called a rest frame - any object can be considered to have a rest frame of reference where it isn't moving at all. Any object that is, which has mass.
Massless objects are always observed to be moving at the speed of light by any observer in any frame of reference. This is one of the most mind-bending parts of relativity, but it ultimately connects to the part of special relativity where speed affects time - the faster you go, the slower your clock runs compared to things moving slower than you, and the closer you get to the speed of light, the closer your clock gets to zero. Massless particles only travel at the speed of light, and thus have no clock, therefore they experience no time.
I hope I got that right, but even if I did, I still don't feel like I actually understand it at all!
If you're in a car on the highway, an observer standing still outside the car sees you moving at 50mph, but an observer inside the car with you, sees you not moving at all, but the world rushing past at 50mph. These are two different reference frames, and the second one is called a rest frame - any object can be considered to have a rest frame of reference where it isn't moving at all. Any object that is, which has mass.
Massless objects are always observed to be moving at the speed of light by any observer in any frame of reference. This is one of the most mind-bending parts of relativity, but it ultimately connects to the part of special relativity where speed affects time - the faster you go, the slower your clock runs compared to things moving slower than you, and the closer you get to the speed of light, the closer your clock gets to zero. Massless particles only travel at the speed of light, and thus have no clock, therefore they experience no time.
I hope I got that right, but even if I did, I still don't feel like I actually understand it at all!
If something blocks the photon, that was "agreed" upon before the photon was created.
A photon travels between an electron that emits it and an electron that absorbs it. These two electrons must agree on the photon transfer beforehand. This has some strange implications:
- When you see a star at night, it's because photons from the distant star hit your eyes.
- These photons started traveling billions of years ago, when neither you nor your eyes existed yet.
- Yet the electrons in your eyes and the electrons in the star (which may no longer even exist) must have agreed to transfer these photons.
- From the photon's perspective, it traveled instantaneously from the star to your eye; it traveled through billions of years of time!
Source: https://www.youtube.com/watch?v=bAedYtUredI&t=2630s
A photon travels between an electron that emits it and an electron that absorbs it. These two electrons must agree on the photon transfer beforehand. This has some strange implications:
- When you see a star at night, it's because photons from the distant star hit your eyes.
- These photons started traveling billions of years ago, when neither you nor your eyes existed yet.
- Yet the electrons in your eyes and the electrons in the star (which may no longer even exist) must have agreed to transfer these photons.
- From the photon's perspective, it traveled instantaneously from the star to your eye; it traveled through billions of years of time!
Source: https://www.youtube.com/watch?v=bAedYtUredI&t=2630s
While I think you can conceive of it this way I don’t think it’s precisely right. Photons don’t experience time at all and never exist from their own perspective as they have no mass and always travel at the speed of light in all reference frames. However causality at speeds less than the speed of light is a real thing and due to special relativity we can interact with photons with a different simultaneity. This is partially because they can’t experience the events of the universe going less than the speed of light due to the fact they absorb rather than decelerate.
This doesn’t mean there’s some cross time agreement between particles mediated by the photon because the reference frame of those particles are different than the photons and are necessarily slower than the speed of light.
This doesn’t mean there’s some cross time agreement between particles mediated by the photon because the reference frame of those particles are different than the photons and are necessarily slower than the speed of light.
If time doesn't exist objectively, then there's no puzzle. What if photons are thin threads connecting distant things. The entire web of these light threads remains still, like an art sculpture. We may explore thin slices of this sculpture, one after another, and this slideshow will create an illusion that a few points, where the light threads cross the slices, move. We may try to find such a slice of this sculpture that the light threads would be seen in their entirety. Such a slice exists, but the light thread on it appears so densely packed that it fills the entire slice. It's truly everywhere on the slice. It's quite possible that in this sculpture all the light threads are really one thread.
Interesting. I had no idea that the universe would eventually be basically just photons. I wonder if that is why Stephen Baxter had his 'photino birds' as the final form of life in his Xeeleverse?
The photino birds want the universe to be populated with white dwarfs, because they can feed off their gravity wells without the risk of deadly supernovas and black holes, which can kill them.
After the xelee and humans leave, the universe becomes a cold place dominated by photino birds living in the cold pinpricks of white dwarfs. Eventually matter evaporates into photons, and the photino birds die.
However, it turns out photino birds can always just time travel to a time when the universe still had matter. So they're more or less indifferent to the eventual heat death .
See: https://xeelee.fandom.com/wiki/Photino_Birds
I hypothesize that there is may be only one photino bird. When it appears to die, it is just traveling to another time. When we see multiple photino birds, we're just looking at different segments of the same bird's world line. These are my own speculations, inspired by Wheeler's idea that the universe has only one electron, which travels back in time as a positron, and interacts with itself so many times that it creates the observable universe of matter:
https://en.m.wikipedia.org/wiki/One-electron_universe
After the xelee and humans leave, the universe becomes a cold place dominated by photino birds living in the cold pinpricks of white dwarfs. Eventually matter evaporates into photons, and the photino birds die.
However, it turns out photino birds can always just time travel to a time when the universe still had matter. So they're more or less indifferent to the eventual heat death .
See: https://xeelee.fandom.com/wiki/Photino_Birds
I hypothesize that there is may be only one photino bird. When it appears to die, it is just traveling to another time. When we see multiple photino birds, we're just looking at different segments of the same bird's world line. These are my own speculations, inspired by Wheeler's idea that the universe has only one electron, which travels back in time as a positron, and interacts with itself so many times that it creates the observable universe of matter:
https://en.m.wikipedia.org/wiki/One-electron_universe
One potential hiccup with your one-photino-bird-universe theory (which is quite fun!): I believe I remember a scene in which the ‘birth’ of a photino bird was described. If I remember correctly, it was indeed described as a clone of its parent.
As long as we only see one birth, that's probably fine. It just implies a stable time loop.
> However, Photino Birds carry with them one unassailable advantage the Xeelee could not overcome; they are not bound by time. They can freely exist in and travel to any point in time. The creatures do not age nor die naturally, not even by the end of the universe; they just travel back to the beginning of time, including to points so soon after the Big Bang that baryonic matter (including the Xeelee) could not yet exist.
Where is this described in the books? I don't remember the Photino Birds having this ability.
Where is this described in the books? I don't remember the Photino Birds having this ability.
Roger Penrose has a theory about a cyclical conformal universe, which requires that the universe eventually only contains photons. As photons travel at the speed of light, they do not experience the passage of time, so they also do not experience distance (space). So in the deep remote future, when only photons are left, the difference between the very large and miniscule disappears, and you get all photons in the universe existing at the same point at the same time, which causes a new big bang. At least, that is Penrose's theory. I believe they found some evidence in the CMB supporting it.
https://en.m.wikipedia.org/wiki/Conformal_cyclic_cosmology
It seems the paper made mistakes, using a non-standard model of the CMB that failed upon replication. It's an interesting theory thoughb.
It seems the paper made mistakes, using a non-standard model of the CMB that failed upon replication. It's an interesting theory thoughb.
As a non-Physicist, I absolutely love the CCC hypothesis and I really hope it turns out to be true. It seems like such a beautiful alternative to the idea of the heat death of the universe just lasting forever and nothing ever happening again.
This is some absolutely crazy idea. I mean I'm in no position to analyze it critically but that is pretty mind blowing.
I believe proton decay is still considered hypothetical, and the electron is pretty confidently believed to be stable; https://profmattstrassler.com/articles-and-posts/particle-ph... is a good overview.
If protons do decay, their decay product would include a positron that would in turn annihilate with the electron. Leaving a universe with just photons and neutrinos.
If neutrinos are their own antiparticle then they could react with each other and produce ??? Not sure, because you would have to find out of the lepton number is conserved.
If neutrinos are their own antiparticle then they could react with each other and produce ??? Not sure, because you would have to find out of the lepton number is conserved.
> I believe proton decay is still considered hypothetical
Definitely.
> [link] is a good overview
That overview seems to overstate the likelihood of proton decay. In fact, it has it backwards. The default position is that protons are stable, per the standard model, not that they're susceptible to decay.
Definitely.
> [link] is a good overview
That overview seems to overstate the likelihood of proton decay. In fact, it has it backwards. The default position is that protons are stable, per the standard model, not that they're susceptible to decay.
> The default position is that protons are stable, per the standard model, not that they're susceptible to decay.
It's only the "default" in the sense that the simplest model explaining data gathered to date (the standard model) predicts no decay. However, most physicist do not believe the standard model is the last word (and surely it cannot be when you go to the Planck scale), and many models post-SM models predict proton decay. I would guess if you surveyed high energy physicists, you'd find the majority expect the proton does in fact decay, so it's the "default" in that sense.
It's only the "default" in the sense that the simplest model explaining data gathered to date (the standard model) predicts no decay. However, most physicist do not believe the standard model is the last word (and surely it cannot be when you go to the Planck scale), and many models post-SM models predict proton decay. I would guess if you surveyed high energy physicists, you'd find the majority expect the proton does in fact decay, so it's the "default" in that sense.
Apologies for the late reply -- I just saw your comment now.
I don't think it's the case that many post-SM models predict proton decay. A few do, but even those are now tightly constrained by observations that place the proton's half life at more than 10^34 years. A lot of models that previously predicted proton decay have been empirically falsified on such grounds, and those that remain are looking pretty shaky. So I don't know. If you run that survey, I believe that the average physicist would come out against proton decay. It would be an interesting survey, in any case!!
I don't think it's the case that many post-SM models predict proton decay. A few do, but even those are now tightly constrained by observations that place the proton's half life at more than 10^34 years. A lot of models that previously predicted proton decay have been empirically falsified on such grounds, and those that remain are looking pretty shaky. So I don't know. If you run that survey, I believe that the average physicist would come out against proton decay. It would be an interesting survey, in any case!!
There is some debate whether this is knowable, some particles are not believed to decay into light at a rate faster than their spontaneous creation from light. (And humanity wont be around to check).
a truly beautiful exploration of that idea is landis's "the melancholy of infinite space": http://www.geoffreylandis.com/infinite.htp
Isn't it a meaningless question? Photons aren't things, but loosely localized areas of motion energy that's temporarily assumed the shape of a photon. Upon collision with other similar waves, it may change shape and become another partickey like an electron. But the light itself is a motion itself, which is a pure abstraction. At the end of the universe, photons probably will keep spreading over larger and larger areas, slowly turning into a uniform sea of that pure motion.
Yeah, this is like asking if energy has an infinite lifetime.
My suspicion that everything is made of pure abstract motion can be backed with a thought experiment. A spaceship with a light sail uses kinetic energy of photons that hit the sail. We know that anything can be dissolved into photons, we even know the exact amount of those photons from the e=mc2 formula, and when all those photons hit the light sail, they vanish from existence and become the kinetic energy of the spaceship. And what is kinetic energy? It's not even real. And as we know from the relativity theory, even as an abstraction, kinetic energy can't be measured with an absolute number, it has to be measured with respect to some imaginary reference frame.
The light also doesn't have an absolute energy though; like the ship's kinetic energy, the light’s frequency (hence energy) also depends on your relative speed.
None of this stuff is “real”, but boy does the bookkeeping seem to work out...
None of this stuff is “real”, but boy does the bookkeeping seem to work out...
Here, a second time, I point to this measured exchange of gravitational waves for photonic waves. If photons come from gravity, can photons end as gravity; and then do they have an infinite lifetime?
https://news.ycombinator.com/item?id=40090332
https://news.ycombinator.com/item?id=40090332
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Huh? Light has zero lifetime, along with all massless particles. When you travel at c your clock doesn’t tick. From the viewpoint of a photon it is emitted and absorbed at the same instant. You cannot decay if you don’t experience time. I’m not sure exactly how index of refraction works with this.
Hoping a physicist can correct me here, but... I believe index of refraction is a function of the photon being absorbed and reemitted by the electrons in the dielectric material, so it's no longer correct to think of a photon moving at a fraction of the speed of light inside the material, it's more like a churning series of them being created (always moving at c) but constantly being absorbed and canceling each other out.
I also seem to recall that the speed of light below c is actually the group velocity, and each individual photon still would move at c. I'm also not entirely sure if photons can be said to exist except at creation and absorption; isn't a photon a phenomenon best described by particle interactions, and moving through free space it's more correctly described as a field? Genuine question, though I somehow doubt I'd understand any good elaboration.
IANAPhysicist, though. I just play with light recreationally.
I also seem to recall that the speed of light below c is actually the group velocity, and each individual photon still would move at c. I'm also not entirely sure if photons can be said to exist except at creation and absorption; isn't a photon a phenomenon best described by particle interactions, and moving through free space it's more correctly described as a field? Genuine question, though I somehow doubt I'd understand any good elaboration.
IANAPhysicist, though. I just play with light recreationally.
For those curious about what the above poster is talking about, here’s a well done video explaining the topic of the apparent changes of light’s “speed” through materials
3Blue1Brown - But why would light "slow down"? | Optics puzzles 3: https://m.youtube.com/watch?v=KTzGBJPuJwM
3Blue1Brown - But why would light "slow down"? | Optics puzzles 3: https://m.youtube.com/watch?v=KTzGBJPuJwM
From the point of view of any observer, a photon has a definite lifetime, between the moments of its emission and its absorption.
The Lorentz transformations are defined only between reference systems where the relative speed between them is less than the speed of light.
It is not possible to attach a reference system to a photon or to any other particle that moves with the speed of light, because there are no conversion rules between the coordinates in such a system and those in a normal reference system.
Therefore it is not correct to say that the lifetime of a photon in a reference system attached to it is zero or infinite or it has any other value.
This lifetime is just undefined, while the lifetimes in any other reference systems are well defined.
The photon does not decay in the absence of interactions with other particles because that would violate several conservation laws. However, when the photons have energies that are high enough, the interaction between themselves can generate other particles, in particle-antiparticle pairs, in order to satisfy all conservation laws.
The Lorentz transformations are defined only between reference systems where the relative speed between them is less than the speed of light.
It is not possible to attach a reference system to a photon or to any other particle that moves with the speed of light, because there are no conversion rules between the coordinates in such a system and those in a normal reference system.
Therefore it is not correct to say that the lifetime of a photon in a reference system attached to it is zero or infinite or it has any other value.
This lifetime is just undefined, while the lifetimes in any other reference systems are well defined.
The photon does not decay in the absence of interactions with other particles because that would violate several conservation laws. However, when the photons have energies that are high enough, the interaction between themselves can generate other particles, in particle-antiparticle pairs, in order to satisfy all conservation laws.
If it's travelling at c then isn't the length contraction infinite? Or is that dependent on Lorentz transformations as well?
Length contraction and time dilation are words that describe changes that are the consequence of a Lorentz transformation.
Like I have said, the formulae of a Lorentz transformation are defined only when the relative velocity between the two systems is less than the speed of light.
Attempting to pass to a limit when the relative speed approaches the speed of light does not produce any useful result, because at the limit you no longer obtain a reference system, so you no longer get a transformation between reference systems.
Without a reference system, there is no meaning for the concepts of distance and time.
Any reference system for the 4-dimensional space-time must be attached to normal matter made of leptons and quarks, it cannot be attached to photons. In any reference system for the 4-dimensional space-time, the photons are particles that move with equal speeds in space and in time, while the normal matter moves faster in time than in space. The notion of proper time (i.e. the time measured for an object that moves only in time, without moving in space) is not defined for photons, because they always also move in space, not only in time.
This should be obvious from the rule introduced by Einstein that the speed of light is the same in all possible reference systems, from which the Lorentz transformations can be deduced. If a reference system were attached to a photon, in that reference system the speed of light could not have the same value as in the normal reference systems, so within Einstein's theory such a reference system cannot exist.
Like I have said, the formulae of a Lorentz transformation are defined only when the relative velocity between the two systems is less than the speed of light.
Attempting to pass to a limit when the relative speed approaches the speed of light does not produce any useful result, because at the limit you no longer obtain a reference system, so you no longer get a transformation between reference systems.
Without a reference system, there is no meaning for the concepts of distance and time.
Any reference system for the 4-dimensional space-time must be attached to normal matter made of leptons and quarks, it cannot be attached to photons. In any reference system for the 4-dimensional space-time, the photons are particles that move with equal speeds in space and in time, while the normal matter moves faster in time than in space. The notion of proper time (i.e. the time measured for an object that moves only in time, without moving in space) is not defined for photons, because they always also move in space, not only in time.
This should be obvious from the rule introduced by Einstein that the speed of light is the same in all possible reference systems, from which the Lorentz transformations can be deduced. If a reference system were attached to a photon, in that reference system the speed of light could not have the same value as in the normal reference systems, so within Einstein's theory such a reference system cannot exist.
Consider a simpler example from basic math. Is 1/x infinite when x==0? The answer is that 1/x is undefined when x==0. In calculus one can take limits as x "approaches" 0 but x==0 is still undefined. Likewise, the Lorentz length contraction is undefined when traveling at c.
> You cannot decay if you don’t experience time.
Interesting take! But if photons couldn't decay due to not experiencing time, they couldn't do anything else either.
The reality is that a photons creation and destruction are not prohibited, but simply "experienced" as two events at different locations at the same time, with the photon being the "thing" that connects those events.
Given that interpretation, it might be reasonable to assume that all photons have beginnings and ends, regardless of the duration we perceive between them, or they wouldn't exist.
Time being no barrier at all for photons.
Interesting take! But if photons couldn't decay due to not experiencing time, they couldn't do anything else either.
The reality is that a photons creation and destruction are not prohibited, but simply "experienced" as two events at different locations at the same time, with the photon being the "thing" that connects those events.
Given that interpretation, it might be reasonable to assume that all photons have beginnings and ends, regardless of the duration we perceive between them, or they wouldn't exist.
Time being no barrier at all for photons.
From a photons perspective, emition and absorption are simultaneous.
While from our perspective it is a form of causal connection, that is mearly due to the frame of reference.
While we can infer the connection between each, it is possibly better to consider the speed of light as the speed of causality.
But as there are no privileged reference frames under GR the choice is yours.
But from the photons perspective, it doesn't experience time at all so it can't be a barrier.
But don't confuse the map for the territory. GR is a model, not the system itself.
The fact that almost every test we can figure out has only confirmed it doesn't change that.
Under the 'all models are wrong but some are useful' idea, in GR photons not experiencing time is important to that model.
While from our perspective it is a form of causal connection, that is mearly due to the frame of reference.
While we can infer the connection between each, it is possibly better to consider the speed of light as the speed of causality.
But as there are no privileged reference frames under GR the choice is yours.
But from the photons perspective, it doesn't experience time at all so it can't be a barrier.
But don't confuse the map for the territory. GR is a model, not the system itself.
The fact that almost every test we can figure out has only confirmed it doesn't change that.
Under the 'all models are wrong but some are useful' idea, in GR photons not experiencing time is important to that model.
>But if photons couldn't decay due to not experiencing time, they couldn't do anything else either.
I mean we are jumping way out of the classical behavior that objects like you and I exist in. To the photon itself is a timeless object. It 'moves' in a null geodesic where t=0. Attempting to apply any classical behavior that occurs in time-like objects just isn't going to work when applying them to massless light-like objects.
I mean we are jumping way out of the classical behavior that objects like you and I exist in. To the photon itself is a timeless object. It 'moves' in a null geodesic where t=0. Attempting to apply any classical behavior that occurs in time-like objects just isn't going to work when applying them to massless light-like objects.
Would that seem like a fold in spacetime to the photon?
I can't top the sibling comment about a summer breeze! But it is an interesting question.
Not only does the photon not experience any delay between its two end points, but it experiences its path between them as a simple shortest-distance straight line segment, even if the same path looks like a curve through gravitationally warped space-time to us.
The photon does experience a form of distance, i.e. the number of wave lengths between its ends. But just the number of cycles, not the actual wave lengths which we would see varying as we experienced dark energy and space stretching the photon's wavelength from our viewpoint.
So a photon "experiences" two spacially separated ends, and a number of wave cycles between them, and that's it? Perhaps.
Not only does the photon not experience any delay between its two end points, but it experiences its path between them as a simple shortest-distance straight line segment, even if the same path looks like a curve through gravitationally warped space-time to us.
The photon does experience a form of distance, i.e. the number of wave lengths between its ends. But just the number of cycles, not the actual wave lengths which we would see varying as we experienced dark energy and space stretching the photon's wavelength from our viewpoint.
So a photon "experiences" two spacially separated ends, and a number of wave cycles between them, and that's it? Perhaps.
Probably more like a summer breeze
But if your clock doesn't tick, then infinite lifetime means the same as zero lifetime.
From the point of view of the photon, time doesn't even exist. So it is pointless to ask the question from the point of view of the photon.
From the point of view of the photon, time doesn't even exist. So it is pointless to ask the question from the point of view of the photon.
> You cannot decay if you don’t experience time
That's a common misconception, there's no a priori reason a particle without a restframe can't decay. For all known particles with a finite lifetime we give this lifetime as measured in its restframe (i.e. with the particle standing still), but in principle it is an observer-dependent quantity, faster moving particles will take longer to decay. If we, for example, assume the lifetime of a massless particle is proportional to its energy, we retain the same expected Lorentz covariance.
Of course, if you actually go through the math, the known massless particles in our universe, photons and gluons, turn out to be stable.[1]
1: https://arxiv.org/abs/hep-th/9508018
That's a common misconception, there's no a priori reason a particle without a restframe can't decay. For all known particles with a finite lifetime we give this lifetime as measured in its restframe (i.e. with the particle standing still), but in principle it is an observer-dependent quantity, faster moving particles will take longer to decay. If we, for example, assume the lifetime of a massless particle is proportional to its energy, we retain the same expected Lorentz covariance.
Of course, if you actually go through the math, the known massless particles in our universe, photons and gluons, turn out to be stable.[1]
1: https://arxiv.org/abs/hep-th/9508018
I wish people would stop repeating this.
If light is emitted and absorbed at the same instant, how does it know when/where in spacetime it's been emitted and absorbed?
You cannot apply SR to light in this naive way. SR works just fine for anything with mass, but without a theory of quantum gravity no one has the first clue how massless particles operate in spacetime, or what they do or don't "experience."
If light is emitted and absorbed at the same instant, how does it know when/where in spacetime it's been emitted and absorbed?
You cannot apply SR to light in this naive way. SR works just fine for anything with mass, but without a theory of quantum gravity no one has the first clue how massless particles operate in spacetime, or what they do or don't "experience."
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Right, but doesn’t light travel at less than c in some situations (passing through glass, etc)? Would we say they experience “time” in those cases?
In these situation photons are either bouncing off matter or are getting absorbed and emitted by matter. In the former case they don't travel a straight path, in the latter case there is a short time lag between absorption and emission.
I think this is sort of besides the point. If you build a box, paint the walls black, and put a flashlight in the box, then the photons coming from the flashlight are shorter lived than if you shine the flashlight into the sky on a cloudless day or night. Not shorter lived from their own perspective — shorter lived from an outside observer’s perspective. Sure, one could quibble about the choice of observer, but you would he hard-pressed to put an observer in the box who thinks the photons last very long.
Why is the speed of causality beside the point?
lets take two magical particles that have clocks on it. One is a photon and the other is a neutrino. I send these off towards you in a perfect vacuum. When you receive these particles the clock on the photon will be 0. It will be be the exact same photon that left my emitter, it will not have changed in any way as it did not interact with anything along the way. And as long as you are not moving relative to me, you'll perceive the photon as the same color/wavelength I emitted it at.
Meanwhile that neutrino will arrive billions of a second later (well depending on our distance) and will have oscallated at least trillions of times if not far more. The clock on the neutrino will have ticked the difference between the photon arrival to the neutrino arrival.
Don't apply classical behavior to light-like objects. They play be different sets of rules.
lets take two magical particles that have clocks on it. One is a photon and the other is a neutrino. I send these off towards you in a perfect vacuum. When you receive these particles the clock on the photon will be 0. It will be be the exact same photon that left my emitter, it will not have changed in any way as it did not interact with anything along the way. And as long as you are not moving relative to me, you'll perceive the photon as the same color/wavelength I emitted it at.
Meanwhile that neutrino will arrive billions of a second later (well depending on our distance) and will have oscallated at least trillions of times if not far more. The clock on the neutrino will have ticked the difference between the photon arrival to the neutrino arrival.
Don't apply classical behavior to light-like objects. They play be different sets of rules.
This is all true, but the article isn’t about how long a photon thinks it lives or how much it experiences the passage of tone. It’s about whether the photon keeps going forever from the perspective of someone approximately at rest [0] in the universe (like astronomers on Earth!).
[0] General relativity has no preferred “at rest” frame, but the generally accepted FLRW model of the universe does. You can be at rest with respect to the universe, or you can be moving. If you are moving, distant objects in front of you will appear blue-shifted on average as compared to distant objects behind you.
[0] General relativity has no preferred “at rest” frame, but the generally accepted FLRW model of the universe does. You can be at rest with respect to the universe, or you can be moving. If you are moving, distant objects in front of you will appear blue-shifted on average as compared to distant objects behind you.
Infinite in both directions of time?
Anybody care to attempt a TL;DR of this?
Kagi Summary:
The passage discusses whether photons, the quantum particles of light, have a finite or infinite lifetime. It explains that while photons can be created and destroyed through various interactions, they cannot truly be "killed off" entirely. Even as the universe expands and photons redshift to lower and lower energies, new photons will continue to be produced, ensuring that photons will always exist in some form. The passage also dismisses the "tired light" hypothesis as a way for photons to lose energy, as it is not supported by observational evidence. Overall, the passage concludes that based on our current understanding of physics, photons appear to have an infinite lifetime and will continue to exist in the universe indefinitely.
The passage discusses whether photons, the quantum particles of light, have a finite or infinite lifetime. It explains that while photons can be created and destroyed through various interactions, they cannot truly be "killed off" entirely. Even as the universe expands and photons redshift to lower and lower energies, new photons will continue to be produced, ensuring that photons will always exist in some form. The passage also dismisses the "tired light" hypothesis as a way for photons to lose energy, as it is not supported by observational evidence. Overall, the passage concludes that based on our current understanding of physics, photons appear to have an infinite lifetime and will continue to exist in the universe indefinitely.
Since there is no time from the perspective of a photon, there is only one photon everywhere at anytime. Our perception of multiple photons is incorrect. Its just multiple timelines with the same photon.
I feel like this article could be condensed into a simple answer. I got tto annoyed looking for it to find more than the answer, "yes, if the standard model holds"
So at the risk of venting unconstructively, I wish I had a way of screening physics writing that is not a physics paper. Articles like this are frustrating because they only have one or two interesting tidbits for me, but they hide them in a whole lot of highschool level hand waving.
Honestly HN would be perfect if it only allowed physics papers - no pop-physics - and if it banned any blogs or news sites with paywalls or newsletter nagware (looking at you, medium, new york times).
Alas. I'm wishing for something I will have to build if I really want it.
So at the risk of venting unconstructively, I wish I had a way of screening physics writing that is not a physics paper. Articles like this are frustrating because they only have one or two interesting tidbits for me, but they hide them in a whole lot of highschool level hand waving.
Honestly HN would be perfect if it only allowed physics papers - no pop-physics - and if it banned any blogs or news sites with paywalls or newsletter nagware (looking at you, medium, new york times).
Alas. I'm wishing for something I will have to build if I really want it.
"God is light." (1John 1:5)
Genesis 1:3 has: "And G-d said, Let there be light, and there was light[0]." This says on the literal level that light is a created thing distinct from G-d, and thus finite (though created very close to the creation of the universe itself, on day 1).
To be clear, light is a very common metaphor for G-dliness in Judaism and the quote from John resonates as a perfectly fine metaphor, rather than a literal assertion of equivalence.
[0] "וַיֹּ֥אמֶר אֱלֹהִ֖ים יְהִי־א֑וֹר וַֽיְהִי־אֽוֹר"
To be clear, light is a very common metaphor for G-dliness in Judaism and the quote from John resonates as a perfectly fine metaphor, rather than a literal assertion of equivalence.
[0] "וַיֹּ֥אמֶר אֱלֹהִ֖ים יְהִי־א֑וֹר וַֽיְהִי־אֽוֹר"
Do you have something constructive to add related to the content of the article or are we just quoting random bible verses on HN for no reason?
It wasn't exactly random. The topic of the article is that light is eternal, and the commenter shared a quote from over a thousand years ago stating the same thing. So, the idea of light having an infinite lifetime is apparently not new... even if the mechanics of light are better understood these days.
This is in line for me with the moment of "creation." For most of the history of science as a thing, the scientific view held that the universe was infinitely old cosmos without a beginning or end. The greek model. It was only in 1900s that big bang was theorized (by a catholic priest) that science now views that there was a moment before which the universe didn't exist and after which it did.
The fact that someone reading Genesis would have had a more accurate conception of the origin of the universe, prior to big bang becoming popularized very recently in the grand scheme of things is noteworthy.
The fact that someone reading Genesis would have had a more accurate conception of the origin of the universe, prior to big bang becoming popularized very recently in the grand scheme of things is noteworthy.
Broken clocks...
"The light shines in the darkness,
and the darkness can never extinguish it.
God sent a man, John the Baptist, to tell about the light so that everyone might believe because of his testimony. John himself was not the light; he was simply a witness to tell about the light. The one who is the true light, who gives light to everyone, was coming into the world.
He came into the very world he created, but the world didn’t recognize him. He came to his own people, and even they rejected him. But to all who believed him and accepted him, he gave the right to become children of God."(John 1:5-12)
God sent a man, John the Baptist, to tell about the light so that everyone might believe because of his testimony. John himself was not the light; he was simply a witness to tell about the light. The one who is the true light, who gives light to everyone, was coming into the world.
He came into the very world he created, but the world didn’t recognize him. He came to his own people, and even they rejected him. But to all who believed him and accepted him, he gave the right to become children of God."(John 1:5-12)
cool story, bro.
If they have infinite lifetimes then does each one carry a memory of it's creation event? When I burn a log in my fireplace and a cinder flares and pops creating a spark, does the photon exist after the spark energy flares out and the cinder is no longer illuminated enough to be detected by my eyes?
If photons are infinite then harvesting light would be the first step in deciphering the complete history of the universe. All that would be left for us to do would be to derive the algorithms to unravel and categorize each photon into discrete groups based on their historical particle paths.
If we wanted to harvest light to test whether photons have infinite lifetimes we would need to design a structure that forms a light trap using materials with different refractive indices so that photons entering are forced onto paths from which they perfectly reflect in a lossless manner.
If photons are infinite then mirrors may have a memory if we can trap and monitor the photons that pass through them and force them to unravel their travel paths. Why can't I step in front of the mirror and have it replay every event that the mirror has seen? It would be a better replacement for photo or video mementos of lost loved ones if we could simply take the mirrors from their homes and spin back to watch their happiest moments forever by reconstructing the photon impingement history of the glass and mirror substrate.