4 ms·

Probably a dumb question, but... is it basically proven then that gravity doesn't exist (it's effects are just a result of spacetime's geometry?). Because it se

by umvi 2y ago

Probably a dumb question, but... is it basically proven then that gravity doesn't exist (it's effects are just a result of spacetime's geometry?). Because it seems like these gravitational waves experiments show that spacetime exists and has measurable geometry. Yet every time quantum mechanics comes up everyone talks about how we haven't found the gravity force carrier yet which doesn't make sense to me if gravity doesn't exist and is a consequence of the geometry of spacetime.

mati365 2y ago
Isn't space gravity carrier itself? Something like the water that has waves?
- simcop2387 2y ago
  I believe they're referring to the idea of a gravitational force carrying particle, like photons for electromagnetism, W/Z bosons for the weak force, and gluons for the strong force. Typically this gets called a graviton but they've never been observed and the theories predicting them as far as I know don't predict that we can detect them in any meaningful/practical way right now which is also one of the problems for issues with a quantum theory of gravity. I've always wondered (but not done the research/reading) on how that would mesh with black holes since you'd need gravitons to escape to mediate the curvature of space-time but that'd seemingly (to me) require them to be able to either ignore the curvature of space-time or travel faster than the speed of light in order to do so. And I believe that those two options there are actually mathematically equivalent as far as the consequences of things go.
  - superposeur 2y ago
    This question isn’t actually anything to do with quantum mechanics. One can already ask, for GR, can a gravity wave generated by a wiggling object that has already passed the event horizon propagate back out through the event horizon? The answer from gr is unambiguously no. Effectively, all complicated goings-on inside the black hole are washed out into just a couple parameters for an outside observer: total mass, total charge, total angular momentum. All the matter contained by the black hole does still make its gravitational effect felt on outside objects through these three parameters. This observation in fact is what inspired wheeler to ask his grad student bekenstein what then happens to the entropy of a cup of tea thrown into the black hole — how to reconcile with 2nd law of thermo. Which in turn was the start of the very long story of black hole thermodynamics.
- atombender 2y ago
  That would suggest space is "made of" something, so what would that be? If it's made of something, how is that "carrying" the wave, causing it to distort in the presence of mass?
  - adolph 2y ago
    Or that gravity is made of something traveling through the
simcop2387 2y ago
So I want to try to answer what I can, despite being a layman on this. Gravity exists, it manifests as the warping/geometry of space. This is in contrast to the other fundamental forces which get explained via Quantum Field Theory. That's the very high level difference of the two, our current understanding of gravity does not work the same way as the way everything else does, and so far we can't find a provable theory (yet) that makes the two work together at all scales. String theory purported as a way to create a quantum theory of gravity and explain everything else, but my understanding is that it's fallen out of favor because it mostly turned into a tunable mathematical framework that could just change to fit any observations that were made, so it doesn't have the same kind of predictive power that people want (i.e. too much freedom so it can be used to explain anything, not just everything). I believe this is where predictions about a possible gravitational force carrier generally come from, aka the graviton. Then there's theories like Loop Quantum Gravity, where the way it works is that space-time itself is quantized and that's how you get things to mesh because you can now use the same wave-function style of things that all other quantum theories use. Though I think this doesn't predict much about a quantum field for gravity on it's own. I believe one of the other things that runs into everything being difficult is that with relativity you end up with a lot of infinities in the equations and results and so there's a "new" kind of math for it that gets called "renormalization" that prevents them from coming out but it also has issues when translating between quantum theories and relativity.
- nyssos 2y ago
  > it doesn't have the same kind of predictive power that people want (i.e. too much freedom so it can be used to explain anything, not just everything). That's the popsci version that's been disseminated, yes. It's not exactly wrong, but it's misleading. First a bit of background. Quantum field theories like the standard model are effective theories, not fundamental ones. We know we don't know the real high-energy physics, so we treat it as a black box and loosely speaking "average it out" as a new free parameter. This is analogous to how an engineer designing a bridge can ignore the fact that iron has a crystal structure and treat it as a continuum with bulk properties like tensile strength. In reality this having a particular tensile strength is a state, not an intrinsic property, and you could end up with a different tensile strength if you melted the iron and let it resolidify (I'm not a metallurgist, substitute some other material if that's not true for iron), but we can build bridges without knowing that. In the same way, Standard Model is a particular form of "solidified" string theory. It's true that there are many, many, many others, but they're not free parameters in the same way. You can write down perfectly reasonable looking quantum field theories that string theory can't produce, and if our best effective theory was one of them then we would have good reason to reject string theory. But it's not. So the situation we're in is that we have some solid material, and we want to know what a single molecule of it looks like, but we can't see anything other than the bulk properties. What the "string theory is unfalsifiable" crowd is demanding is that whatever molecule we predict have only a few possible crystal structures. And maybe it does. That would be convenient. But sometimes nature inconveniences us: it might be some crazy carbon allotrope. It might be glass.
superposeur 2y ago
These experiments confirm the classical theory of gravity, which is Einstein’s general relativity, just as Maxwell’s equations are the classical theory of the electromagnetic field. Just as Maxwell’s equations are perfectly adequate for waves of macroscopic intensity, GR is perfectly adequate for astrophysical gravity waves. A whole separate question is what is the quantum mechanical theory that has general relativity as its classical limit? For electromagnetism, quantum electrodynamics (understood in the 40’s) is the quantized version of Maxwell’s and predicts that electromagnetic energy measurement outcomes come in “chunks” (photons). But, although much is known about features of “quantum gravity” (like that gravitons will be massless, spin 2), there is famously no consensus yet about the precise theory. As to how to reconcile the force carrier picture with spacetime picture — even classically one can consider an “overall” spacetime background geometry such as that created by the whole earth. Then consider little ripples perturbing this background. Gravitons are these little ripples turned on a quantized amount (heuristically). How the overall background itself gets formed as an “enormous pile of gravitons” will depend on the precise theory of quantum gravity. String theory does have a partial answer to this so can model such things as black holes quantumly.
akira2501 2y ago
We have two systems of physics. We can make predictions using either system. We cannot fully unify the two systems.

by umvi 2y ago

mati365 2y ago

Isn't space gravity carrier itself? Something like the water that has waves?

simcop2387 2y ago
I believe they're referring to the idea of a gravitational force carrying particle, like photons for electromagnetism, W/Z bosons for the weak force, and gluons for the strong force. Typically this gets called a graviton but they've never been observed and the theories predicting them as far as I know don't predict that we can detect them in any meaningful/practical way right now which is also one of the problems for issues with a quantum theory of gravity. I've always wondered (but not done the research/reading) on how that would mesh with black holes since you'd need gravitons to escape to mediate the curvature of space-time but that'd seemingly (to me) require them to be able to either ignore the curvature of space-time or travel faster than the speed of light in order to do so. And I believe that those two options there are actually mathematically equivalent as far as the consequences of things go.
- superposeur 2y ago
  This question isn’t actually anything to do with quantum mechanics. One can already ask, for GR, can a gravity wave generated by a wiggling object that has already passed the event horizon propagate back out through the event horizon? The answer from gr is unambiguously no. Effectively, all complicated goings-on inside the black hole are washed out into just a couple parameters for an outside observer: total mass, total charge, total angular momentum. All the matter contained by the black hole does still make its gravitational effect felt on outside objects through these three parameters. This observation in fact is what inspired wheeler to ask his grad student bekenstein what then happens to the entropy of a cup of tea thrown into the black hole — how to reconcile with 2nd law of thermo. Which in turn was the start of the very long story of black hole thermodynamics.
atombender 2y ago
That would suggest space is "made of" something, so what would that be? If it's made of something, how is that "carrying" the wave, causing it to distort in the presence of mass?
- adolph 2y ago
  Or that gravity is made of something traveling through the

simcop2387 2y ago

So I want to try to answer what I can, despite being a layman on this. Gravity exists, it manifests as the warping/geometry of space. This is in contrast to the other fundamental forces which get explained via Quantum Field Theory. That's the very high level difference of the two, our current understanding of gravity does not work the same way as the way everything else does, and so far we can't find a provable theory (yet) that makes the two work together at all scales. String theory purported as a way to create a quantum theory of gravity and explain everything else, but my understanding is that it's fallen out of favor because it mostly turned into a tunable mathematical framework that could just change to fit any observations that were made, so it doesn't have the same kind of predictive power that people want (i.e. too much freedom so it can be used to explain anything, not just everything). I believe this is where predictions about a possible gravitational force carrier generally come from, aka the graviton. Then there's theories like Loop Quantum Gravity, where the way it works is that space-time itself is quantized and that's how you get things to mesh because you can now use the same wave-function style of things that all other quantum theories use. Though I think this doesn't predict much about a quantum field for gravity on it's own. I believe one of the other things that runs into everything being difficult is that with relativity you end up with a lot of infinities in the equations and results and so there's a "new" kind of math for it that gets called "renormalization" that prevents them from coming out but it also has issues when translating between quantum theories and relativity.

nyssos 2y ago
> it doesn't have the same kind of predictive power that people want (i.e. too much freedom so it can be used to explain anything, not just everything). That's the popsci version that's been disseminated, yes. It's not exactly wrong, but it's misleading. First a bit of background. Quantum field theories like the standard model are effective theories, not fundamental ones. We know we don't know the real high-energy physics, so we treat it as a black box and loosely speaking "average it out" as a new free parameter. This is analogous to how an engineer designing a bridge can ignore the fact that iron has a crystal structure and treat it as a continuum with bulk properties like tensile strength. In reality this having a particular tensile strength is a state, not an intrinsic property, and you could end up with a different tensile strength if you melted the iron and let it resolidify (I'm not a metallurgist, substitute some other material if that's not true for iron), but we can build bridges without knowing that. In the same way, Standard Model is a particular form of "solidified" string theory. It's true that there are many, many, many others, but they're not free parameters in the same way. You can write down perfectly reasonable looking quantum field theories that string theory can't produce, and if our best effective theory was one of them then we would have good reason to reject string theory. But it's not. So the situation we're in is that we have some solid material, and we want to know what a single molecule of it looks like, but we can't see anything other than the bulk properties. What the "string theory is unfalsifiable" crowd is demanding is that whatever molecule we predict have only a few possible crystal structures. And maybe it does. That would be convenient. But sometimes nature inconveniences us: it might be some crazy carbon allotrope. It might be glass.

superposeur 2y ago

These experiments confirm the classical theory of gravity, which is Einstein’s general relativity, just as Maxwell’s equations are the classical theory of the electromagnetic field. Just as Maxwell’s equations are perfectly adequate for waves of macroscopic intensity, GR is perfectly adequate for astrophysical gravity waves. A whole separate question is what is the quantum mechanical theory that has general relativity as its classical limit? For electromagnetism, quantum electrodynamics (understood in the 40’s) is the quantized version of Maxwell’s and predicts that electromagnetic energy measurement outcomes come in “chunks” (photons). But, although much is known about features of “quantum gravity” (like that gravitons will be massless, spin 2), there is famously no consensus yet about the precise theory. As to how to reconcile the force carrier picture with spacetime picture — even classically one can consider an “overall” spacetime background geometry such as that created by the whole earth. Then consider little ripples perturbing this background. Gravitons are these little ripples turned on a quantized amount (heuristically). How the overall background itself gets formed as an “enormous pile of gravitons” will depend on the precise theory of quantum gravity. String theory does have a partial answer to this so can model such things as black holes quantumly.

akira2501 2y ago

We have two systems of physics. We can make predictions using either system. We cannot fully unify the two systems.