Ultra-weak gravitational field detected(nature.com)
nature.com
Ultra-weak gravitational field detected
https://www.nature.com/articles/d41586-021-00591-1
125 comments
All good questions. Fast answer: we have no clue.
Until we have a good quantum theory of gravity, we can't explain how it affects the curvature of spacetime at the quantum level.
All the current quantum gravity theories, i.e. loop quantum gravity, string theory, etc... work if you squint just right, otherwise they fall apart really fast. Spacetime has a bunch of episodes on going theories. A very large elephant in the room is, a lot of these ideas don't have a method of testing. And sure, a lot of theories start out without a practical method until decades later where technology can catch up, but we don't even have a theoretical test to start with!
Spacetime recently did an episode touching on some of your thoughts at the classical level and even then it's... relative[0].
[0] https://www.youtube.com/watch?v=OHdV9aO6jaE
Until we have a good quantum theory of gravity, we can't explain how it affects the curvature of spacetime at the quantum level.
All the current quantum gravity theories, i.e. loop quantum gravity, string theory, etc... work if you squint just right, otherwise they fall apart really fast. Spacetime has a bunch of episodes on going theories. A very large elephant in the room is, a lot of these ideas don't have a method of testing. And sure, a lot of theories start out without a practical method until decades later where technology can catch up, but we don't even have a theoretical test to start with!
Spacetime recently did an episode touching on some of your thoughts at the classical level and even then it's... relative[0].
[0] https://www.youtube.com/watch?v=OHdV9aO6jaE
>>> we don't even have a theoretical test to start with
Every quantum computing calculation, is a desktop quantum experiment ;)
Non-Gaussianity as a signature of a quantum theory of gravity
https://arxiv.org/abs/2004.01189
Another cool aspect of the optomechanical tabletop device research was that it wasn't entirely fruitless at all. Although the original mission of finding gravity in quantum space may have failed. The behavior exhibited was so extremely non-classical that an explosion of waveguide manipulating metamaterials was born!
Every quantum computing calculation, is a desktop quantum experiment ;)
Non-Gaussianity as a signature of a quantum theory of gravity
https://arxiv.org/abs/2004.01189
Another cool aspect of the optomechanical tabletop device research was that it wasn't entirely fruitless at all. Although the original mission of finding gravity in quantum space may have failed. The behavior exhibited was so extremely non-classical that an explosion of waveguide manipulating metamaterials was born!
A quantum theory of gravity would by definition explain the curvature of spacetime, since said curvature (and its dynamics) is how gravity is expressed.
Asking how a quantum theory of gravity would explain curvature is then asking someone to give you a completed quantum theory of gravity, and it is not at all clear that the first successful theory will describe the world terms of particles.
However, it can be instructive to consider electromagnetism as as classical theory and then as a quantum theory: waves in fields become enormous showers of particles, and these photons then carry the force that attracts or repels and can sometimes interact with each other.
Asking how a quantum theory of gravity would explain curvature is then asking someone to give you a completed quantum theory of gravity, and it is not at all clear that the first successful theory will describe the world terms of particles.
However, it can be instructive to consider electromagnetism as as classical theory and then as a quantum theory: waves in fields become enormous showers of particles, and these photons then carry the force that attracts or repels and can sometimes interact with each other.
A quantum theory of gravity doesn't have to explain curvature of spacetime.
It needs to produce better experimental results than modeling gravity as the curvature of spacetime. Whether that still describes it in a way that can be mapped to a curvature based explanation is irrelevant. It should for some cases give the same results?
It needs to produce better experimental results than modeling gravity as the curvature of spacetime. Whether that still describes it in a way that can be mapped to a curvature based explanation is irrelevant. It should for some cases give the same results?
The ‘funniest’ part of all the talk of curvature et al as if they were fundamental aspects of nature is that they’re nothing but a gradient across space, and are (currently, at least) best described as curvature in spacetime, but that’s just a physical interpretaton.
A quantum gravity theory might not use the vocabulary of curvature or the tensor fields and scalar objects found in the Einstein Field Equations of General Relativity, but that's not quite the same as "doesn't have to explain curvature".
For the theory to be viable it would have to explain why General Relativity is such a good effective theory at solar system scales. One can do this by treating General Relativity as a theory which emerges in some limit of the more fundamental theory of gravitation (or everything). One can compare that with how Newtonian gravitation emerges from General Relativity as one takes c to infinity, or as one takes v^2/c^2 (where c is usually set to one; one can likewise expand in other small parameters) to zero. The first is the basis for the first section of https://en.wikipedia.org/wiki/Post-Newtonian_expansion The second section has more explanatory power: Newtonian gravitation emerges from General Relativistic systems when multipole moments are small and the distance to the sources is large.
The key to the success of General Relativity compared to many of its competitors is that it very straightforwardly produces the Newtonian solution for the well-studied orbits of the Jovian system, while explaining the anomaly in the precession of Mercury's orbit.
One would have to be able to show conditions in the new theory in which General Relativity emerges (and for which General Relativity has observational support), or the theory is not going to get much practical use in astrophysics or GPS-style engineering, even if it appears to get the microscopic details right. Practically[1] nobody -- there are several serious contenders like Loop Quantum Gravity -- who has developed an alternative theory of gravitation (or everything) has been able to show at all that General Relativity emerges from it.
Lastly, because we don't have the technology to probe the quantum gravitation sector directly, but we do have the technology to probe General Relativity very finely (such is the topic of the article), I can't see how a failure to explain General Relativity's results can do anything but undermine quantum gravity candidates.
- --
[1] People have proposed turning various constants into spacetime-dependent functions, and find that such adjustments can solve some things but generally have to vanish practically everywhere, so such theories are not viable. Among these are Palatini and others https://en.wikipedia.org/wiki/F(R)_gravity and Brans-Dicke https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory. These families of theories usually show how General Relativity could emerge, but further investigation almost always shows that the parameters or features that distinguish these theories from General Relativity are incompatible with observation and so must vanish.
For the theory to be viable it would have to explain why General Relativity is such a good effective theory at solar system scales. One can do this by treating General Relativity as a theory which emerges in some limit of the more fundamental theory of gravitation (or everything). One can compare that with how Newtonian gravitation emerges from General Relativity as one takes c to infinity, or as one takes v^2/c^2 (where c is usually set to one; one can likewise expand in other small parameters) to zero. The first is the basis for the first section of https://en.wikipedia.org/wiki/Post-Newtonian_expansion The second section has more explanatory power: Newtonian gravitation emerges from General Relativistic systems when multipole moments are small and the distance to the sources is large.
The key to the success of General Relativity compared to many of its competitors is that it very straightforwardly produces the Newtonian solution for the well-studied orbits of the Jovian system, while explaining the anomaly in the precession of Mercury's orbit.
One would have to be able to show conditions in the new theory in which General Relativity emerges (and for which General Relativity has observational support), or the theory is not going to get much practical use in astrophysics or GPS-style engineering, even if it appears to get the microscopic details right. Practically[1] nobody -- there are several serious contenders like Loop Quantum Gravity -- who has developed an alternative theory of gravitation (or everything) has been able to show at all that General Relativity emerges from it.
Lastly, because we don't have the technology to probe the quantum gravitation sector directly, but we do have the technology to probe General Relativity very finely (such is the topic of the article), I can't see how a failure to explain General Relativity's results can do anything but undermine quantum gravity candidates.
- --
[1] People have proposed turning various constants into spacetime-dependent functions, and find that such adjustments can solve some things but generally have to vanish practically everywhere, so such theories are not viable. Among these are Palatini and others https://en.wikipedia.org/wiki/F(R)_gravity and Brans-Dicke https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory. These families of theories usually show how General Relativity could emerge, but further investigation almost always shows that the parameters or features that distinguish these theories from General Relativity are incompatible with observation and so must vanish.
Ah, you caught me there ;). I was hoping someone would spill their secret quantum gravity theory on Hackernews.
> But how would a theoretical graviton explain this effect?
there would be more gravitons if the field is stronger. No gravitons in the empty space.
Gravitons are quantas of the always attractive forces. So traveling in the gravitation field, photon constantly absorbs and emits gravitons, more gravitons - more interactions, thus "bending" photon trajectory in the presence of the gravitation.
If you a bit math inclined, you could look at Feynman's lectures on gravitation (Caltech, fall semester of 1962).
They are quite outdated, a lot of mistakes - he was building it as he is preparing lectures, but approach itself is very interesting
He starts with MICROSCOPIC description - lets introduce gravitons, we want bosons which always attract, so no spin 1, only spin 0 or 2. Dropped spin 0 (don't remember why) and start building interaction diagrams form spin 2 bosons like in any standard quantum field theory.
Discussed exactly the topic of "trajectory bending", got to quasi-classic limit and showed that this is Einstein field eqs, got to weak field limit and Newton, ...
And all this (even his mistakes) in beautiful style and logic.
there would be more gravitons if the field is stronger. No gravitons in the empty space.
Gravitons are quantas of the always attractive forces. So traveling in the gravitation field, photon constantly absorbs and emits gravitons, more gravitons - more interactions, thus "bending" photon trajectory in the presence of the gravitation.
If you a bit math inclined, you could look at Feynman's lectures on gravitation (Caltech, fall semester of 1962).
They are quite outdated, a lot of mistakes - he was building it as he is preparing lectures, but approach itself is very interesting
He starts with MICROSCOPIC description - lets introduce gravitons, we want bosons which always attract, so no spin 1, only spin 0 or 2. Dropped spin 0 (don't remember why) and start building interaction diagrams form spin 2 bosons like in any standard quantum field theory.
Discussed exactly the topic of "trajectory bending", got to quasi-classic limit and showed that this is Einstein field eqs, got to weak field limit and Newton, ...
And all this (even his mistakes) in beautiful style and logic.
My very light understanding of quantum physics is that it does not really translate in words. It s like trying to explain what’s apple times i in English.
That being said the standard model is very good at predicting outcomes at a small scale. The issue with gravity is things where General relativity applies need to be huge to be observe. If we want to find a quantum model of gravity we need to observe what gravity does at a small scale and try to fit a model within these observations. That’s why this experiment is a step in the right direction. Maybe gravity does “weird” things at a smaller scale… Back to your question we don’t know we just observe :)
That being said the standard model is very good at predicting outcomes at a small scale. The issue with gravity is things where General relativity applies need to be huge to be observe. If we want to find a quantum model of gravity we need to observe what gravity does at a small scale and try to fit a model within these observations. That’s why this experiment is a step in the right direction. Maybe gravity does “weird” things at a smaller scale… Back to your question we don’t know we just observe :)
> apple times i
To be fair, Apple has been doing this (multiplying things by i) for a while now. Not that they have given any explanation why.
To be fair, Apple has been doing this (multiplying things by i) for a while now. Not that they have given any explanation why.
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Steve Jobs said a long time ago, and I am paraphrasing, that a company that makes a product that has reached its maturity will eventually be taken over by advertising and data-mining. Something like that.
There's not much innovation right now in terms of personal computing devices. It's a solved problem. To show 'progress' on a year to year basis, something has to change. It used to be eSomething. Then iSomething. Now I suppose it's airSomething.
I'm not sure what comes after 'air', ordinalitly speaking.
There's not much innovation right now in terms of personal computing devices. It's a solved problem. To show 'progress' on a year to year basis, something has to change. It used to be eSomething. Then iSomething. Now I suppose it's airSomething.
I'm not sure what comes after 'air', ordinalitly speaking.
The next progression is Something+
> My very light understanding of quantum physics is that it does not really translate in words. It s like trying to explain what’s apple times i in English.
This is really one of the best comments I have read on an article like this ever. Having had a cursory introduction to quantum physics in math terms and not understanding much of it, I find it impossible to read any sort of popular science literature on the subject. It's always going to be a very vague description with analogies that raise more questions than give answers. That's not to say I don't applaud the effort of quantum physicists to explain it to a broader public- I just think that most of the time they come away thinking it's some sort of black magic rather than solid science.
This is really one of the best comments I have read on an article like this ever. Having had a cursory introduction to quantum physics in math terms and not understanding much of it, I find it impossible to read any sort of popular science literature on the subject. It's always going to be a very vague description with analogies that raise more questions than give answers. That's not to say I don't applaud the effort of quantum physicists to explain it to a broader public- I just think that most of the time they come away thinking it's some sort of black magic rather than solid science.
A book about QM by Zagoskin is one of the more recent attempts to be open and honest in this regard. It is packed with math.
The idea is that it would be similar to the way that photons mediate the effect of the electromagnetic field. Though I've personally always thought that this description wasn't too good at explaining large gradual changes in the electromagnetic field, similar to how gravitons are a bit of weird model for the large gradual curvature of spacetime (it's not that it can be done but it is a bit awkward).
Anyway the most straightforward way to implement gravitons in this sense doesn't pan out mathematically so we're not even sure if this model is at all accurate.
Anyway the most straightforward way to implement gravitons in this sense doesn't pan out mathematically so we're not even sure if this model is at all accurate.
It's generally expected that the spacetime geometry is emergent as some coherent state of gravitons
Unpacking that statement a bit:
- In quantum mechanics there is a procedure called "quantization" that takes in a classical theory and gives you a quantum theory.
- The things (excitations) that follow your classical intuition are waves [*] and when you quantize them, you get excitations that carry discrete quanta of energy (ex. photons / gravitons / <particle name of choice>)
- It's convenient to talk about states with a definite number of particles because of the way the math works out.
- To get back to something that follows your classical intuition (waves) you have to consider superpositions of the states with definite particle number. These particular states are known as "coherent states" and very much look like your normal notion of a classical wave which is something with a definite phase and amplitude
As an example: you can recover something that looks like a classical radio wave in quantum electrodynamics by taking a particular superposition of states with a definite number of photons. Since it's in superposition, the number of photons in the state isn't well defined. By well defined here, I mean if you measure the number of particles, you can get different answers due to Heisenberg uncertainty.
You can even get standing waves in a cavity (like a microwave oven) which is an oscillator and the state has a definite phase and amplitude that you can measure. Everything looks nice and classical but if you look hard enough you find that actually you can't put in an infinitesimally small amount of energy, you can only add it in multiples of some constant (h-bar).
Now replace the electromagnetic field with a field that represents the curvature of spacetime and away you go, your theory works with small excitation quanta (gravitons) but the actual things in everyday life that you see are complicated superpositions of these things [**].
So the geometry (spacetime) is some superposition of states with definite graviton numbers that have no classical description. Much like how the electric field around an electron or an oscillating electromagnetic field inside of a microwave oven is a superposition of states with definite photon number. Sidenote: it’s also easy to talk about a single graviton in isolation sitting on top of that emergent geometry
[*] This is "backwards" from pop-sci / intro physics because we are talking about electromagnetism which shares some superficial similarities with gravity
[**] For some technical reasons this viewpoint doesn't apply to particles which aren't "force-carriers" ex. electrons, protons, etc
Unpacking that statement a bit:
- In quantum mechanics there is a procedure called "quantization" that takes in a classical theory and gives you a quantum theory.
- The things (excitations) that follow your classical intuition are waves [*] and when you quantize them, you get excitations that carry discrete quanta of energy (ex. photons / gravitons / <particle name of choice>)
- It's convenient to talk about states with a definite number of particles because of the way the math works out.
- To get back to something that follows your classical intuition (waves) you have to consider superpositions of the states with definite particle number. These particular states are known as "coherent states" and very much look like your normal notion of a classical wave which is something with a definite phase and amplitude
As an example: you can recover something that looks like a classical radio wave in quantum electrodynamics by taking a particular superposition of states with a definite number of photons. Since it's in superposition, the number of photons in the state isn't well defined. By well defined here, I mean if you measure the number of particles, you can get different answers due to Heisenberg uncertainty.
You can even get standing waves in a cavity (like a microwave oven) which is an oscillator and the state has a definite phase and amplitude that you can measure. Everything looks nice and classical but if you look hard enough you find that actually you can't put in an infinitesimally small amount of energy, you can only add it in multiples of some constant (h-bar).
Now replace the electromagnetic field with a field that represents the curvature of spacetime and away you go, your theory works with small excitation quanta (gravitons) but the actual things in everyday life that you see are complicated superpositions of these things [**].
So the geometry (spacetime) is some superposition of states with definite graviton numbers that have no classical description. Much like how the electric field around an electron or an oscillating electromagnetic field inside of a microwave oven is a superposition of states with definite photon number. Sidenote: it’s also easy to talk about a single graviton in isolation sitting on top of that emergent geometry
[*] This is "backwards" from pop-sci / intro physics because we are talking about electromagnetism which shares some superficial similarities with gravity
[**] For some technical reasons this viewpoint doesn't apply to particles which aren't "force-carriers" ex. electrons, protons, etc
The idea of (the first) quantization has always looked suspicious to me. It feels like a theoretical hack, a leap of faith, as it were. No wonder it's not working as well in gravity's case.
I think static forces in quantum mechanics have a little bit different part of the framework:
https://en.wikipedia.org/wiki/Static_forces_and_virtual-part...
Even though nothing is fully static, it is a way of approximating.
So the theories would presumably have something similar for the static gravity case you are describing.
https://en.wikipedia.org/wiki/Static_forces_and_virtual-part...
Even though nothing is fully static, it is a way of approximating.
So the theories would presumably have something similar for the static gravity case you are describing.
Indeed, we have no idea what is the connection between time, space, mass and other forces.
We have some findings (hey! mass is intimately connected to curvature of spacetime!) and approximations in forms formulae we know, but that's just it.
I think our physics is akin to epicycles before we found actual place of Earth in solar system and shape of planet orbits.
We have some findings (hey! mass is intimately connected to curvature of spacetime!) and approximations in forms formulae we know, but that's just it.
I think our physics is akin to epicycles before we found actual place of Earth in solar system and shape of planet orbits.
Well, if you take into account also the fact that the "monochromatic" photon (one that has a definite wavelength, or momentum) does not even have a certain position in space, the idea of its interaction with an actual (non-virtual) graviton near a mass located at a certain point in space will start looking problematic.
How is it more problematic than a photon interacting with a normal matter particle?
[deleted]
PBS Space Time on Youtube just did an episode on light interacting with gravity. You should check it out.
Excellent question. And in this lies one of the biggest reasons successfully quantizing gravity would be huge. The two theories fundamentally don't mesh. You quickly end up with infinities all over the place.
There's a variety of ideas on how to do this but none have really played out.
There's a variety of ideas on how to do this but none have really played out.
> If a photon follows a curve around a large mass, it does so because that curve is the shortest path through spacetime, if I am not mistaken.
Correct!
> Maybe I am missing something, but how could a quantum theory of gravity explain curvature of spacetime? […] how would a theoretical graviton explain this effect?
Note that a quantum theory of gravity need not involve gravitons. The latter only appear in approaches that assume that gravity is a more or less ordinary [string] quantum field theory (QFT). Personally, I've never found this very convincing. Neither did Hawking by the way, who once wrote
> It would be rather boring if this were the case. Gravity would be just like any other field. But I believe it is distinctively different, because it shapes the arena in which it acts, unlike other fields which act in a fixed spacetime background.[0]
Anyway, assuming we're indeed looking at a quantum theory of gravity that involves gravitons, let's tackle your other questions:
> Would it interact with the photon to carry over a force?
Yes, the graviton would interact (very weakly) with all particles we know, including itself.
> And if so, how would it "know" the direction of said force (wouldn't it be itself subject to gravity?).
My first, rather theoretical answer would be: This is not a well-defined question. After all, gravity as a force is a Newtonian concept which does not exist in General Relativity. As you already said, in Relativity matter is "guided" by the curvature of spacetime, not by a force. Moreover, in quantum mechanics Newtonian forces don't (really) exist, either, and while people often speak of "forces" as if they were Newtonian, these forces are really mediated by bosons.
My second, more direct answer would be: In two-particle interactions in quantum field theory, the outgoing particles usually leave the scenery at a different angle (and momentum) than their initial one. So the photon might get deflected by interacting with the graviton.
> Or would [the graviton] interact with space itself?
As already mentioned, the graviton would interact with itself in the same way as gravitational waves can interact with one another. As for space(-time) itself, however, note that the aforementioned "theories involving gravitons" usually assume that spacetime itself is flat (i.e., it is your run-of-the-mill Minkowski spacetime) and that gravitons are just another quantum field on that (so-called) background spacetime. As I personally don't like this idea a lot (see above), I can only guess as to whether its proponents really think spacetime is flat or whether they only assume this to simplify things. After all, we have no idea how to do QFT on curved spacetime in a mathematically precise way. (In fact, we don't know how to do QFT on a flat Minkowski background in a mathematically precise way, either.)
[0]: Hawking: The Nature of Space and Time, https://arxiv.org/abs/hep-th/9409195v1
Correct!
> Maybe I am missing something, but how could a quantum theory of gravity explain curvature of spacetime? […] how would a theoretical graviton explain this effect?
Note that a quantum theory of gravity need not involve gravitons. The latter only appear in approaches that assume that gravity is a more or less ordinary [string] quantum field theory (QFT). Personally, I've never found this very convincing. Neither did Hawking by the way, who once wrote
> It would be rather boring if this were the case. Gravity would be just like any other field. But I believe it is distinctively different, because it shapes the arena in which it acts, unlike other fields which act in a fixed spacetime background.[0]
Anyway, assuming we're indeed looking at a quantum theory of gravity that involves gravitons, let's tackle your other questions:
> Would it interact with the photon to carry over a force?
Yes, the graviton would interact (very weakly) with all particles we know, including itself.
> And if so, how would it "know" the direction of said force (wouldn't it be itself subject to gravity?).
My first, rather theoretical answer would be: This is not a well-defined question. After all, gravity as a force is a Newtonian concept which does not exist in General Relativity. As you already said, in Relativity matter is "guided" by the curvature of spacetime, not by a force. Moreover, in quantum mechanics Newtonian forces don't (really) exist, either, and while people often speak of "forces" as if they were Newtonian, these forces are really mediated by bosons.
My second, more direct answer would be: In two-particle interactions in quantum field theory, the outgoing particles usually leave the scenery at a different angle (and momentum) than their initial one. So the photon might get deflected by interacting with the graviton.
> Or would [the graviton] interact with space itself?
As already mentioned, the graviton would interact with itself in the same way as gravitational waves can interact with one another. As for space(-time) itself, however, note that the aforementioned "theories involving gravitons" usually assume that spacetime itself is flat (i.e., it is your run-of-the-mill Minkowski spacetime) and that gravitons are just another quantum field on that (so-called) background spacetime. As I personally don't like this idea a lot (see above), I can only guess as to whether its proponents really think spacetime is flat or whether they only assume this to simplify things. After all, we have no idea how to do QFT on curved spacetime in a mathematically precise way. (In fact, we don't know how to do QFT on a flat Minkowski background in a mathematically precise way, either.)
[0]: Hawking: The Nature of Space and Time, https://arxiv.org/abs/hep-th/9409195v1
Spacetime is mathematical abstraction, not a real thing. Photon changes trajectory because of differences in conductivity: light propagates a bit slower in stronger gravitational force.
Ok, we don’t know G all that well, but saying
> Their estimate deviates from the internationally agreed value (see go.nature.com/2bwkrqz) by about 9% — a small amount, given that the experimental uncertainties of their system have not yet not been optimized for precise measurements of G.
is just misleading. We’re pretty sure of the value of the gravitational constant to at least 4-5 significant figures. The experiment is cool but it’s not like it’s going to get us a more accurate value.
> Their estimate deviates from the internationally agreed value (see go.nature.com/2bwkrqz) by about 9% — a small amount, given that the experimental uncertainties of their system have not yet not been optimized for precise measurements of G.
is just misleading. We’re pretty sure of the value of the gravitational constant to at least 4-5 significant figures. The experiment is cool but it’s not like it’s going to get us a more accurate value.
Getting a more accurate value was not the point. Measuring a gravitational force between those tiny spheres, distinguishing it from experimental noise was the point. Impressive experiment. Deducing a value for g that’s not wildly wrong was the icing on the cake.
Yep, I misread it.
If they can improve precision, a somewhat different G might be very interesting for, eg, MOND vs Dark Matter question
https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
Unless I'm mistaken, the difference between MOND and classical Newtonian gravity is not in the value of G, but in making the force proportional to distance (rather than distance squared) at very large distances.
[deleted]
The point is that if you put the experimentally measured distance, masses, and force into Newton's inverse square law in a regime where it does not actually hold, you will get a different G and this is how you will know that modifications are needed. G will be variable in that regime.
Does something moving fast weight less? Would it get less G if it would spin really fast ? Or does speed only affect "speed of time" ?
As objects move faster their (relativistic) mass increases*, the invariant mass however doesn’t change.
*Relative to an external observer.
*Relative to an external observer.
Are there any experimental tests of relativistic mass?
Yes, any particle accelerator has to account for the relativistic mass in order to function.
Yes, and pretty old ones at that https://en.wikipedia.org/wiki/Kaufmann%E2%80%93Bucherer%E2%8...
Special relativity is weird, but it's very much proven by countless experiments.
https://en.wikipedia.org/wiki/Tests_of_special_relativity
Special relativity is weird, but it's very much proven by countless experiments.
https://en.wikipedia.org/wiki/Tests_of_special_relativity
My understanding which could be wrong is that the kinetic energy of an object can itself cause (tiny amounts) of gravity. So, for instance, a spinning object would weigh slightly more.
Awesome experiment, and really cool findings, but I feel that...
> The findings take us a step nearer to measuring gravitational fields that are so weak that they could enter the quantum regime.
...is overstated. There's a long way between 90 milligrams and quantum gravity.
> The findings take us a step nearer to measuring gravitational fields that are so weak that they could enter the quantum regime.
...is overstated. There's a long way between 90 milligrams and quantum gravity.
In terms of mass it takes us halfway there already, from 10^30kg to 10^-3kg, with quantum effects arriving around 10^-15kg.
And in terms of force, the article states that the force involved in that experiment is equivalent to the force of earth gravity on a mass of 10^–15kg, which is probably even closer to the energy of a graviton.
And in terms of force, the article states that the force involved in that experiment is equivalent to the force of earth gravity on a mass of 10^–15kg, which is probably even closer to the energy of a graviton.
Depending on how you're cutting it, you could also call it 1/1000000000000 of the way there.
That's half (within some tolerance).
Why do you say that quantum effects arise at 10^-15kg? The carbon atom is about 2x10^-26 kg. And I don't think we can put objects much larger than a few atom molecules into a superposition of two locations.
It depends on the nature of the quantization of the gravitational field. If we expect the gravitational field itself to be quantized in a manner somewhat similar to the electromagnetic field, we may be able to detect quantum effects without the objects being particularly small. A good analogue is the photoelectric effect, which involves no superposition at all and can be observed even with macro amounts of energy involved.
Not sure if this has held up, but Google points at this 2010 paper that put a microscopic object into a superposition: https://www.researchgate.net/publication/232755709_Quantum_g...
It's science-journalism. They needed a tag to wrap up the article.
For a lay reader, it would give them an overall concept they can tie the just-read article to, increasing the likelihood of retention.
For an astute reader, yeah, it smells like bullshit.
For a lay reader, it would give them an overall concept they can tie the just-read article to, increasing the likelihood of retention.
For an astute reader, yeah, it smells like bullshit.
Wouldn't putting this device in a spherical container on Earth's orbit remove a lot of noise?
Yes it would, but launching stuff into orbit is expensive and making ultrasensitive instruments launch-proof is trickier (and more expensive) than you'd think.
Once we have a proper suite of in-orbit manufacturing (not just 3d printing a plastic part) it's going to be a straight up Cambrian explosion event for what technology we can design for a space environment by letting go of vibration tolerances, G force tolerances and rocket dimension constraints. The new ways to manufacture are cool, but the dropping of existing constraints is a dream for a potential new wave of creativity for both new problems to fix in a new environment, with orders of magnitude operational scale (Everything is still basically bespoke one offs, bar a number of companies I can count on my fingers) and how to create solutions for all that new scale entails.
On a tangent here, but "not just 3d printing a plastic part" got me thinking.
Scientists, instead of requesting time on Hubble, ask time for the 3DBuilderAndExperimenter, give it a blueprint and inputs/outputs, it runs, returns any results, and is recycled.
Scientists, instead of requesting time on Hubble, ask time for the 3DBuilderAndExperimenter, give it a blueprint and inputs/outputs, it runs, returns any results, and is recycled.
The first scientist should ask the genie for more wishes, er, I mean, for the 3DbuilderAndExperimenter to build another 3DBuilderAndExperimenter
Certainly, professor Von Neumann: https://en.m.wikipedia.org/wiki/Self-replicating_machine
We are such machines, so we know they’re possible to make, even though we don’t yet know how to make one on Earth let alone in space.
We are such machines, so we know they’re possible to make, even though we don’t yet know how to make one on Earth let alone in space.
But we only need to make one of them!
The “and is recycled” part puts a crimp in that plan. :-)
There a bunch of things that could be perhaps easier manufactured in space. For example, being in orbit provides a few conditions ideal for semiconductor fabrication - no terrestrial vibrations, high vacuum, microgravity and high cleanliness. Of course we still need to develop process that uses a minimal amount of water and power to make it viable in orbit.
I wonder if people on the ISS could make this device from components. Then you don't need to launch it.
The ISS is actually a pretty bad microgravity environment. Human occupants and their life-support systems lead to rather strong system vibrations. Also the low orbit required for efficient supply flights causes a significant amount of atmospheric drag. Space stations are mainly there for show, the actual science is better done somewhere else.
Surely how good it is for an experiment depends on the experiment? Microgravity effects on biology don’t need anything close to the same precision as the test in the link.
I meant:
- send components to ISS
- assemble
- subtly nudge the container to a different orbit
- wait for it to do the experiment and catch when the orbits meet again. Or if that's too hard just wait for results to be transmitted
- send components to ISS
- assemble
- subtly nudge the container to a different orbit
- wait for it to do the experiment and catch when the orbits meet again. Or if that's too hard just wait for results to be transmitted
I'm not sure that it would. The Earth isn't perfectly spherical so the gravitational field will be different at different times in the orbit. Then because it is orbiting you'll also have to deal with tidal forces (bits slightly closer to the Earth will want to move slightly faster), not to mention possible acceleration from solar radiation or small amounts of atmospheric drag.
Question: Since mass exerts gravitation force, wouldn’t it mean that any mass of any size shape would do that? Just because we don’t have a scientific experiment to measure gravitational forces at small scales doesn’t mean they don’t exist mathematically right? What am I missing when they say quantum particles do not fit with gravity.
Highly over simplified, but the idea is that the curvature of space-time is smooth but quantum mechanics is discrete.
Quantum mechanics is not discrete in regards to space and time. The problem has much more to do with the fact that particles can be in multiple places at once, and what that means for their gravitational pull / space-time curvature.
Try this episode of PBS SpaceTime which goes into the dichotomy between GR and QM: https://www.youtube.com/watch?v=YNEBhwimJWs.
Yep any size particle should. But gravity is described by classical physics through general relativity. When you have situations of very small quantum scales and large gravitational fields both general relativity and quantum field theory don’t work well together. This is a big problem in physics. See here for more details
https://en.m.wikipedia.org/wiki/Quantum_gravity
https://en.m.wikipedia.org/wiki/Quantum_gravity
>Just because we don’t have a scientific experiment to measure gravitational forces at small scales doesn’t mean they don’t exist mathematically right?
That is correct--it has been expected (since, you know, Newton...) but not yet measured at this scale. That is why the headline says "detected" as opposed to theorized.
>What am I missing when they say quantum particles do not fit with gravity.
Quantum mechanics predicts that Newtons description of gravity will break down under certain conditions. We currently do not have the ability to measure this effect, so it is all theoretical at this point.
That is correct--it has been expected (since, you know, Newton...) but not yet measured at this scale. That is why the headline says "detected" as opposed to theorized.
>What am I missing when they say quantum particles do not fit with gravity.
Quantum mechanics predicts that Newtons description of gravity will break down under certain conditions. We currently do not have the ability to measure this effect, so it is all theoretical at this point.
I guess kind of a similar expecation was in place for light as well - if you dim your light source then it emits less light waves.
Well, turns out if you go small enough then you see it's actually individual packets of light with certain amounts of energy where the amount of packets (photons) sent out decreases as you turn down the intensity. So, the total amount of energy outputted still matches the input, but the output is in discrete packets, where each packet can exceed the level of input energy at a specific time. So, on a large scale everything seems fine but as you go to very small energy levels it starts to look different.
I guess the expectation with gravity is that going to very small masses, some different underlying mechanism will appear. Or it may not. So they're trying to find out.
Well, turns out if you go small enough then you see it's actually individual packets of light with certain amounts of energy where the amount of packets (photons) sent out decreases as you turn down the intensity. So, the total amount of energy outputted still matches the input, but the output is in discrete packets, where each packet can exceed the level of input energy at a specific time. So, on a large scale everything seems fine but as you go to very small energy levels it starts to look different.
I guess the expectation with gravity is that going to very small masses, some different underlying mechanism will appear. Or it may not. So they're trying to find out.
The problem is that QM doesn't account for gravity so far - in QM as it exists today, particles have mass, but do not interact through gravity. This fits experiments, which is not surprising given that any theory of gravity we have today predicts that the gravitational attraction between two masses that low would be much smaller than what we can measure (remember for example that a kitchen magnet's electromagnetic force can oppose the gravitational pull of the entire earth, that's how weak gravity is compared to other forces).
Still, obviously large masses have gravity, so everyone expects that, even if we can't measure it yet, and don't know how to compute it yet, there must be some attraction between particles. There are some theories about it (for example string theory has something that looks like gravity), but since they all predict values much lower than we could hope to measure for now, the problem remains wide open.
However, simply applying Newton's or Einstein's theories of gravity to QM doesn't work. In QM, particles don't have a single position in space and time, each particle is more like a wave with peaks at different points in space at the same time. If you try to compute the gravitational field generated by all these peaks you get nonsense results. So, while we generally believe that there must be some gravitational attraction between particles with mass, we don't know how it would look like.
I think there are also some theories that predict that elementary particles do NOT interact through gravity, that gravity is somehow an emergent phenomenon of a collection of many particles (just like an elementary particle doesn't have a temperature, but a collection of many many particles does have one). I believe this is a pretty fringe theory, but not quite "flat earth" land. Just including it for completeness.
Still, obviously large masses have gravity, so everyone expects that, even if we can't measure it yet, and don't know how to compute it yet, there must be some attraction between particles. There are some theories about it (for example string theory has something that looks like gravity), but since they all predict values much lower than we could hope to measure for now, the problem remains wide open.
However, simply applying Newton's or Einstein's theories of gravity to QM doesn't work. In QM, particles don't have a single position in space and time, each particle is more like a wave with peaks at different points in space at the same time. If you try to compute the gravitational field generated by all these peaks you get nonsense results. So, while we generally believe that there must be some gravitational attraction between particles with mass, we don't know how it would look like.
I think there are also some theories that predict that elementary particles do NOT interact through gravity, that gravity is somehow an emergent phenomenon of a collection of many particles (just like an elementary particle doesn't have a temperature, but a collection of many many particles does have one). I believe this is a pretty fringe theory, but not quite "flat earth" land. Just including it for completeness.
This is what I find interesting. Assuming two quantum particles exert forces on each other. Now they both have mass but the forces they exert on each other are so much more powerful that the gravitational force would be impossible to experiment. Sort of like a man at a distance exerting gravitational force on the Milky Way with instruments the size of galaxy clusters to measure that man or force. Beyond mathematics there would be no experimental way. But my question is if mathematics itself says quantum particles do not have gravity or is that just a limitation of current understanding.
> But my question is if mathematics itself says quantum particles do not have gravity or is that just a limitation of current understanding.
The maths of QM (Schrodinger equation, standard model) currently say that particles don't have gravity. But, they also imply that nothing has gravity - gravity doesn't exist at all according to the standard model. Obviously this is wrong, so we are searching for a theory of Quantum Gravity.
There are various pieces of math that do have gravity and are consistent with standard QM, but each of them has various other problems, so none is universally accepted, and they vary significantly in how they add gravity to the standard model.
The maths of QM (Schrodinger equation, standard model) currently say that particles don't have gravity. But, they also imply that nothing has gravity - gravity doesn't exist at all according to the standard model. Obviously this is wrong, so we are searching for a theory of Quantum Gravity.
There are various pieces of math that do have gravity and are consistent with standard QM, but each of them has various other problems, so none is universally accepted, and they vary significantly in how they add gravity to the standard model.
Limit of current understanding. everything at this level is a theory, with math to boot.. most theories say particles have mass and therefore interact via gravity at insanely small levels (which we cannot currently measure), and other fringe theories postulate particles don’t interact via gravity at all... we won’t be able to experiment with these theories until sensor tech / approaches like the OP are refined enough to be in the ballpark of detecting these small levels of interaction.
Update : few other answers clarify this.
> Question: Since mass exerts gravitation force, wouldn’t it mean that any mass of any size shape would do that?
Yes. Absolutely everything should have a gravitational field.
> What am I missing when they say quantum particles do not fit with gravity.
Under Relativity, mass and energy are equivalent, therefore energy has a gravitational field. If you try to mathematically quantise the gravitational field the way the electromagnetic field was quantised (the latter giving you a photon, the former would be a graviton), then something goes wrong.
This is where my grasp of the physics gets a bit hazy; I think you find any amount of gravity should produce more gravity, and this self-generation blows up to infinity rather than summing to a finite value.
Or it might have something to do with the expected value of a corresponding zero-point gravity field? Or both? I’m not sure, I only do physics for fun.
I keep thinking it’s a gravitational equivalent to the Ultraviolet Catastrophe, but if it was that simple someone would’ve already solved it.
Yes. Absolutely everything should have a gravitational field.
> What am I missing when they say quantum particles do not fit with gravity.
Under Relativity, mass and energy are equivalent, therefore energy has a gravitational field. If you try to mathematically quantise the gravitational field the way the electromagnetic field was quantised (the latter giving you a photon, the former would be a graviton), then something goes wrong.
This is where my grasp of the physics gets a bit hazy; I think you find any amount of gravity should produce more gravity, and this self-generation blows up to infinity rather than summing to a finite value.
Or it might have something to do with the expected value of a corresponding zero-point gravity field? Or both? I’m not sure, I only do physics for fun.
I keep thinking it’s a gravitational equivalent to the Ultraviolet Catastrophe, but if it was that simple someone would’ve already solved it.
I heard the theory (not from a mainstream source) that gravity creating more gravity is the reason for the universe's expansion. Does anyone know what the standard argument against this theory is? I haven't really seen it addressed before, but on the surface it seems compellingly simple.
The Ultraviolet Catastrophe was solved by a complete rethink of the foundations (a shift from continuous to quantised energy). Perhaps the standard model needs a similar fundamental shift? (Though I have no idea what that would look like)
Anecdotally, I agree. When you look at the history of major breakthroughs in physics, mainstream understanding narrows down on some complex analytical theories (rotating shells above the earth controlling the movement of the heavenly bodies, crazy complex math) when in reality there was a fork in the road decades prior where mainstream physics took a wrong turn (heliocentric! orbits!).
Things like this are obvious in retrospect, but radical for their time.. most of the physics I learned was elegant and well-understood, then particle physics just... didn’t really feel the same way. Tons of gotchas and edge cases, difficulty generalizing problems, and the obvious disconnect between the physics of the small and that of the large...
In time we’ll look back at current iterations of the standard model and our understanding of the physics of the small and wonder how those Neanderthals didn’t just realize that xyz was the key to make everything much simpler and work in harmony... that’ll be a fun day!
Things like this are obvious in retrospect, but radical for their time.. most of the physics I learned was elegant and well-understood, then particle physics just... didn’t really feel the same way. Tons of gotchas and edge cases, difficulty generalizing problems, and the obvious disconnect between the physics of the small and that of the large...
In time we’ll look back at current iterations of the standard model and our understanding of the physics of the small and wonder how those Neanderthals didn’t just realize that xyz was the key to make everything much simpler and work in harmony... that’ll be a fun day!
> In time we’ll look back at current iterations of the standard model and our understanding of the physics of the small and wonder...
I’m not sure about that.
Everything we have to analogize to and obtain an intuition about relates to the macro world. There’s no reason the quantum phenomena have any analog to what we experience on a daily basis.
That is, all the cosmological models from Ptolemy, epicycles, heliocentric, and even Newton’s derivations were arrived at by analogy to things we already experience. To be sure, Principia Mathematica is dense, but Newton’s derivation of the effects of an inverse-squared force can be followed by, ultimately, grasping Euclid’s Elements (which in turn can be grasped through familiarity with a compass and straightedge.)
I recall a Feynman interview where he was asked why magnets repel. And, he explained there is no analogy that can help ones understanding, as the behavior of anything you’d use in your analogy is ultimately governed by the same principles that make magnets repel.
I’m not sure about that.
Everything we have to analogize to and obtain an intuition about relates to the macro world. There’s no reason the quantum phenomena have any analog to what we experience on a daily basis.
That is, all the cosmological models from Ptolemy, epicycles, heliocentric, and even Newton’s derivations were arrived at by analogy to things we already experience. To be sure, Principia Mathematica is dense, but Newton’s derivation of the effects of an inverse-squared force can be followed by, ultimately, grasping Euclid’s Elements (which in turn can be grasped through familiarity with a compass and straightedge.)
I recall a Feynman interview where he was asked why magnets repel. And, he explained there is no analogy that can help ones understanding, as the behavior of anything you’d use in your analogy is ultimately governed by the same principles that make magnets repel.
Any mass, as currently defined, distorts space-time. Interestingly enough, velocity contributes to mass. That is to say, if we imagined two toy tops, one at rest and one spinning, the spinning top would have more mass due to its additional energy.
Gravity is something like 10^32 power smaller than the next meaningful force. The problem with gravity is this-- At that resolution, we can't distinguish between two theories-- that mass warps space-time, or that there is a particle that mediates the force of gravity (a "graviton"). That is to say, is gravity analog or is it digital? If gravity is digital, there is a particle that is extremely small that transmits its force. If gravity is analog, then it changes the space around us itself and acts exactly the same way.
So we have a question for the ages-- we have 17 to 25 quantum fields (depending on how you count) and, one, analog field, gravity. So, is gravity analog, or is it impossibly small digital field?
For gods sakes is there a physicist out there, please correct me if I'm wrong. :-)
Gravity is something like 10^32 power smaller than the next meaningful force. The problem with gravity is this-- At that resolution, we can't distinguish between two theories-- that mass warps space-time, or that there is a particle that mediates the force of gravity (a "graviton"). That is to say, is gravity analog or is it digital? If gravity is digital, there is a particle that is extremely small that transmits its force. If gravity is analog, then it changes the space around us itself and acts exactly the same way.
So we have a question for the ages-- we have 17 to 25 quantum fields (depending on how you count) and, one, analog field, gravity. So, is gravity analog, or is it impossibly small digital field?
For gods sakes is there a physicist out there, please correct me if I'm wrong. :-)
I'm not really a physicist, but I can suggest better terms than "analog" vs "digital." The words you're actually looking for are "continuous" vs "quantized."
So how is mass defined? Matter, as I understand it, is excitations of a quantum field. Why should they attract each other?
This is somewhat like saying 'mountains are just the greyish areas on a map, why should they be hard to climb?'
We know that matter attracts other matter, at the macro level. As this article shows, we've been able to detect the gravitational pull of tiny masses (tiny by macro standards). Since the macro world is made out of the quantum world, QM must somehow explain how gravity arises for particles.
Until it does, there is a fundamental piece missing from our understanding of the universe, so any conclusion we draw from QM or GR must be taken with a large grain of salt. You can't say 'matter is excitation in a quantum field, why should it attract', that is exactly the problem: we know it attracts, so if e citations in a quantum field don't attract, it must mean that matter is something else.
Note: mass is generally defined by E=mc^2, where energy is defined by conservation laws.
We know that matter attracts other matter, at the macro level. As this article shows, we've been able to detect the gravitational pull of tiny masses (tiny by macro standards). Since the macro world is made out of the quantum world, QM must somehow explain how gravity arises for particles.
Until it does, there is a fundamental piece missing from our understanding of the universe, so any conclusion we draw from QM or GR must be taken with a large grain of salt. You can't say 'matter is excitation in a quantum field, why should it attract', that is exactly the problem: we know it attracts, so if e citations in a quantum field don't attract, it must mean that matter is something else.
Note: mass is generally defined by E=mc^2, where energy is defined by conservation laws.
Thanks for taking my question seriously.
I guess I get hung up on the quantum field part because I don't understand what exactly a quantum field is in a physical sense. I get that there's a mathematical concept of a quantum field, but does that map to something concrete? Is everything essentially made of nothing?
I guess I get hung up on the quantum field part because I don't understand what exactly a quantum field is in a physical sense. I get that there's a mathematical concept of a quantum field, but does that map to something concrete? Is everything essentially made of nothing?
I'm pretty sure we are not yet equipped to answer this question. In the end, QFT and QM are extraordinarily successful mathematical models that predict to extreme precision the behaviors of small numbers of particles interacting.
Whether they are directly describing reality or some kind of approximation remains an open question. I believe that some progress on the measurement problem, when it happens, could help point in this direction.
Whether they are directly describing reality or some kind of approximation remains an open question. I believe that some progress on the measurement problem, when it happens, could help point in this direction.
Note: when I'm comparing "describing reality" vs "approximation", I'm thinking of the following examples:
1. An account for how the world actually works are Newton's laws of motion - they describe how exactly objects interact and how it affects their movement. It is of course an approximation, and only works at certain scales and speeds, but it describes mathematically the phenomena that we can see happen.
2. An alternative account that gives the same values is Hamiltonian mechanics, which describes the system in terms of how its energy changes over time in certain coordinates. They both give the same predictions, and Hamiltonian mechanics is more suited for actually computing the motion of complex systems, but it is much more abstract and doesn't give a clear account of how the system actually works.
1. An account for how the world actually works are Newton's laws of motion - they describe how exactly objects interact and how it affects their movement. It is of course an approximation, and only works at certain scales and speeds, but it describes mathematically the phenomena that we can see happen.
2. An alternative account that gives the same values is Hamiltonian mechanics, which describes the system in terms of how its energy changes over time in certain coordinates. They both give the same predictions, and Hamiltonian mechanics is more suited for actually computing the motion of complex systems, but it is much more abstract and doesn't give a clear account of how the system actually works.
Maybe it's centered around the particle's quantum density field aka the average position of the possible quantum positions?
How many more decades until scientists realize chemical bonding is molecular scale gravity? I've claimed for years galaxies and molecules operate with the same logic only with a different scale. The laws of physics are dependent on scale, it's stupid to try and find rules anchored in answers figured out with absolute math because it's all fucking relative.
Is this... serious? Your theory is on the surface impossible because electromagnetic forced would dominate your “gravity”, forcing protons apart among other things. If what you’re suggesting is that at a certain scale, gravity just acts differently all of the sudden, how is this any different than modeling the interaction with a different type of force, aside from a naming convention?
I think my point, even if I'm not well educated to formulate it in scientist language, is that measuring everything in absolutes is fucking up with science's accuracy. Speed is a human construct to triangulate distance over time. Light doesn't have speed. Light is a constant that defines speed.
A lot of things we think we know and take for granted is actually slightly inaccurate when the scale shifts drastically. That's because we focus on absolutes instead of relative values. Numbers are a human construct, scientists have forgotten that. Because our brain can easily add 2+2 doesn't mean it's a technique that scales infinitely and remains true. Do we have proof of numbers stability through scale or do we just say there's an "infinite" amount of them to make it easier on the explaining?
ps: pi is another of these constants that rule the laws of the universe. We can't really explain why but it does. It's the same principle as for light. Constants are true not matter the scope, because everything is relative. Our numbers can't truly represent that because additions are inherently flawed when the scale shifts. Everything that adds recursively is actually a multiplication array, and thinking of it as an addition is inaccurate.
tldr: our math needs to evolve to follow an OoP logic instead of being stuck at a human scale perspective.
A lot of things we think we know and take for granted is actually slightly inaccurate when the scale shifts drastically. That's because we focus on absolutes instead of relative values. Numbers are a human construct, scientists have forgotten that. Because our brain can easily add 2+2 doesn't mean it's a technique that scales infinitely and remains true. Do we have proof of numbers stability through scale or do we just say there's an "infinite" amount of them to make it easier on the explaining?
ps: pi is another of these constants that rule the laws of the universe. We can't really explain why but it does. It's the same principle as for light. Constants are true not matter the scope, because everything is relative. Our numbers can't truly represent that because additions are inherently flawed when the scale shifts. Everything that adds recursively is actually a multiplication array, and thinking of it as an addition is inaccurate.
tldr: our math needs to evolve to follow an OoP logic instead of being stuck at a human scale perspective.
I would love to read the more fleshed out version of this claim.
I'm impressed about how much scientific rigor you have to ignore to claim what you're talking about.
We know the force-distance (field strength) curves of the "chemical bonding" forces[0].
[0] https://en.wikipedia.org/wiki/Nuclear_force#Field_strength
We know the force-distance (field strength) curves of the "chemical bonding" forces[0].
[0] https://en.wikipedia.org/wiki/Nuclear_force#Field_strength
We think we know a lot, but we ignore a lot to get those certainties ;)
This is a selfawarewolves comment if I’ve ever seen one (as someone who used to study physics but would consider myself a physics novice nowadays)
I’ve often wondered if it would be possible to make a type of gravitational radar. Imagine a large array of this type of detector spread over an area of a few square km. Combine the force vectors of each toward any specific point and it seems you should be able to monitor the masses at that point.
Edit: measuring the force of the ISS passing over head would require femtonewton precision for a mass of 100kg. Might be a bit tricky lol
Edit: measuring the force of the ISS passing over head would require femtonewton precision for a mass of 100kg. Might be a bit tricky lol
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This is an amazing idea for sci-fi!
Spread the masses out over orbit, measure net gravity on those (by orbit deviations?), and you should be able to detect changes in gravity from distant objects. With enough spread maaaaybe you could do something with parallax to detect distance too.
Imagine a system like this in 'the expanse' to detect the stealth asteroids hurled towards earth. Or, in general, to detect massive ships that are 'on the float'.
Spread the masses out over orbit, measure net gravity on those (by orbit deviations?), and you should be able to detect changes in gravity from distant objects. With enough spread maaaaybe you could do something with parallax to detect distance too.
Imagine a system like this in 'the expanse' to detect the stealth asteroids hurled towards earth. Or, in general, to detect massive ships that are 'on the float'.
Please correct me if I'm wrong, but the second sentence of the first paragraph already seems weird:
> The gravitational force is the weakest of these four.
Isn't that distance dependent? At macroscopic distances, the weak and strong interactions are even weaker, no?
> The gravitational force is the weakest of these four.
Isn't that distance dependent? At macroscopic distances, the weak and strong interactions are even weaker, no?
Technically yes, but usually this statement is assumed at the quantum level/distances.
Yeah, there're qualifications behind it.
[This PDF](http://web.mit.edu/sahughes/www/8.022/lec01.pdf) starts with some potentially helpful context.
Usually this claim is discussed in connection to the so-called ["hierarchy problem"](https://en.wikipedia.org/wiki/Hierarchy_problem).
[This PDF](http://web.mit.edu/sahughes/www/8.022/lec01.pdf) starts with some potentially helpful context.
Usually this claim is discussed in connection to the so-called ["hierarchy problem"](https://en.wikipedia.org/wiki/Hierarchy_problem).
Not exactly: if you had a kg of free protons and a kg of free electrons, the electromagnetic forces between them would dwarf the gravitational forces between them even at macroscopic distances. By a lot, as Feynman liked to note in his Lectures [0,1]. It’s just hard to get a kg of free protons exactly because the forces involved are so large. So charges tend to pair up and screen [2], which just camouflages the true strength of the force.
For the strong force the situation is the same in a sense, if you had a kg each of quarks of different colors — except you literally can’t have that, because the energy involved in separating a pair of quarks is so large that it actually causes two new quarks to snap into existence and pair off with the two you were trying to separate before you can separate them on anything approaching macroscopic scales [3]. So the strong force is obligatorily screened (camouflaged) at long distances, but only because it is so strong.
The kg-of-protons experiment on the other hand you could do in principle, it would just probably be better if you didn’t.
[0] https://www.feynmanlectures.caltech.edu/II_01.html
[1] https://sieste.wordpress.com/2012/04/22/feynmans-electric-fo...
[2] https://en.wikipedia.org/wiki/Electric-field_screening
[3] https://en.wikipedia.org/wiki/Color_confinement
For the strong force the situation is the same in a sense, if you had a kg each of quarks of different colors — except you literally can’t have that, because the energy involved in separating a pair of quarks is so large that it actually causes two new quarks to snap into existence and pair off with the two you were trying to separate before you can separate them on anything approaching macroscopic scales [3]. So the strong force is obligatorily screened (camouflaged) at long distances, but only because it is so strong.
The kg-of-protons experiment on the other hand you could do in principle, it would just probably be better if you didn’t.
[0] https://www.feynmanlectures.caltech.edu/II_01.html
[1] https://sieste.wordpress.com/2012/04/22/feynmans-electric-fo...
[2] https://en.wikipedia.org/wiki/Electric-field_screening
[3] https://en.wikipedia.org/wiki/Color_confinement
To nitpick here: You need less than kilogram of electrons to screen a kilogram of protons (roughly 2 grams it seems)
Quite true! I thought about saying “kg of positrons and kg of electrons” or such so that the mass/charge ratio would be more equivalent.
You're actually looking for a mol of electrons and a mol of protons.
> The kg-of-protons experiment on the other hand you could do in principle, it would just probably be better if you didn’t.
What if you want to build a fusion reactor? For now neutrons have to be along for the ride but that's only because we can't squish and heat plain protons hard enough.
What if you want to build a fusion reactor? For now neutrons have to be along for the ride but that's only because we can't squish and heat plain protons hard enough.
In that case, go for it ;)
The weak and strong interactions have other properties that screen their influence at longer ranges (just like with the EM force due to objects being mostly neutral in case of the strong interaction for example), but the strong force is indeed much stronger.
The EM force between a proton here and at the edge of the visible universe is the same as the gravitational force between two protons at 1000 meters distance. The scale difference in the "hierarchy problem" is truly not graspable by human intuition.
The EM force between a proton here and at the edge of the visible universe is the same as the gravitational force between two protons at 1000 meters distance. The scale difference in the "hierarchy problem" is truly not graspable by human intuition.
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It appears they used Frequency Modulation to prove the connection.
Radio over Gravity !
Radio over Gravity !
Interesting statement in the article that measurements of the gravitational constant, G, haven't converged on a number like other constants have. Anyone here know anything about this?
The article ends with a stated goal of measuring the gravity of masses of the order of 10^-8 kg or about 10 micrograms. This reminded me of a calculation involving known fundamental constants - h, G, c - to derive dimensionless units for mass, length and time by setting these constants to unity. Setting c=1 is common among general relativists. When you do that for all three constants, you get (iirc) a unit of mass to be 10^-8kg = 1 and of time to be 10^-43secs = 1 and 10^-35m = 1 (only stating orders of magnitude). The latter two are Planck time and Planck length and the first is Planck mass ... and we may indeed expect quantum effects to manifest at that scale. I've also read on the occasion that 10micrograms is around the scale at which quantum superpositions may be sustainable before decoherence becomes unstoppable.
>you get (iirc) a unit of mass to be 10^-8kg = 1
Sure, this is Plank mass, 2 \times 10^-8 kg.
Length unit would be obviously PLank length, ditto for Plank time
> 10micrograms is around the scale at which quantum superpositions may be sustainable before decoherence becomes unstoppable
that doesn't make sense
Sure, this is Plank mass, 2 \times 10^-8 kg.
Length unit would be obviously PLank length, ditto for Plank time
> 10micrograms is around the scale at which quantum superpositions may be sustainable before decoherence becomes unstoppable
that doesn't make sense
If they can actually measure such tiny blips then they can build particle detectors that might answer all sorts of fundamental problems. Such detectors would be exponentially smaller (read cheaper) than those in LHC-class colliders. There are no double many Nobel prizes up for grabs to anyone who can measure the gravity of particle flux.
> There are no double many Noble prizes
There are no doubt many Noble prizes?
There are no doubt many Noble prizes?
If we can get this sensitive I think it would be cool to measure under all sorts of conditions. High magnetic field, high electric field, just to see if there are any tiny effects.
I think maybe we are looking at it backwards. In order to understand what gravity is first we may need to understand what distance is. Gravity is essentially the effect of information at different distances. Matter or energy is localised information. Something here, nothing there. Gravity acts at a great distance compared to the other fundamental forces so perhaps it is not like them, not amenable to a quantum theory. Perhaps gravity is meta information about space. Space contains certain clusters or concentrations of information at different distances.The ensemble of these clusters is also information. A certain level information density may extert a pull on matter that we call the gravitational force.
But how would a theoretical graviton explain this effect? Would it interact with the photon to carry over a force? And if so, how would it "know" the direction of said force (wouldn't it be itself subject to gravity?). Or would it interact with space itself?