Researchers find experimental evidence for a graviton-like particle

[laymanhobbist]

Researchers find first experimental evidence for a graviton like particle in a quantum material

https://phys.org/news/2024-03-experimental-evidence-graviton-particle-quantum.html

A team of scientists from Columbia, Nanjing University, Princeton, and the University of Munster, writing in the journal Nature, have presented the first experimental evidence of collective excitations with spin called chiral graviton modes (CGMs) in a semiconducting material

A CGM appears to be similar to a graviton, a yet-to-be-discovered elementary particle better known in high-energy quantum physics for hypothetically giving rise to gravity, one of the fundamental forces in the universe, whose ultimate cause remains mysterious.

:)

 

[Gary Venter]

I thought our best models no longer postulated a force of gravity. Mass makes space-time curve, which produces measurements that seem like a pull of gravity, but actually agree better with experimental data. When we feel a downward pull when walking up or down a hill, that is due to acceleration of the Earth's surface, which does not move because it is in a non-inertial frame of reference. So what would a graviton do anyway?

 

[renormalize]

I thought our best models no longer postulated a force of gravity. Mass makes space-time curve, which produces measurements that seem like a pull of gravity, but actually agree better with experimental data. When we feel a downward pull when walking up or down a hill, that is due to acceleration of the Earth's surface, which does not move because it is in a non-inertial frame of reference. So what would a graviton do anyway?

A graviton would bear the same relation to a classical gravitational wave as the photon does to a classical electromagnetic wave.

 

[Ibix]

Note that this is condensed matter physics. These things are not gravitons, but phenomena in strange matter states that are formally described by theories that are mathematically similar to theories of gravity.

That's not to say there's not interesting physics here, but this isn't an actual graviton detection.

 

[Ibix]

So what would a graviton do anyway?

In versions of quantum gravity that have gravitons, they propagate changes in the gravitational field. As @renormalize says, this is analogous to what photons do in the quantum theory of electromagnetism.

Such theories don't, as far as I'm aware, model gravity as spacetime curvature. They do predict that classical experiments would produce results indistinguishable from a spacetime curvature model.

 

[Gary Venter]

@renormalize and @Ibix answer this in terms of gravitational waves, which in quantum gravity are waves in the gravitational field. But in General Relativity, I assume that these waves are ripples in the curvature of space-time, not propagated by any force. Is that right? Is cause and effect still working?

 

[Ibix]

General relativity models gravity as spacetime curvature. Some theories of quantum gravity model gravity in broadly similar ways to other fundamental interactions, as a field on a flat background and these require gravitons. I don't see where you think cause and effect come into this.

 

[PeterDonis]

in General Relativity, I assume that these waves are ripples in the curvature of space-time

Yes.

not propagated by any force

Not by any non-gravitational force, no. But since it is known that the classical limit of the quantum field theory of a massless spin-2 field (i.e., the "graviton") is classical GR, it is perfectly consistent to view the classical model of gravitational waves in GR as an approximation to an underlying QFT interaction.

 

[Gary Venter]

Forces acting on things is standard causation, so apparently that is still operative in quantum gravity. I'm not so sure it is in general relativity. Matter does not curve space-time through the action of forces and apparently gravitational waves in GR do not require forces either. In force fields, the laws are carried out by forces, but without force fields it's hard to understand how the laws are operating. Maybe causation is a term that should be avoided, but without a mechanism, GR sounds like a more radical change than quantum gravity.

 

[PeterDonis]

Forces acting on things is standard causation, so apparently that is still operative in quantum gravity. I'm not so sure it is in general relativity.

"Forces acting on things" might be one form of "standard causation", but it is by no means the only one. In GR, the source of spacetime curvature is the distribution of stress-energy, through the Einstein Field Equation. This is perfectly consistent with "standard causation".

I strongly urge you to learn more about how the physical theories under discussion actually work. You appear to have a personal viewpoint that is not only highly idiosyncratic but does not seem well informed. Trying to push such a viewpoint here will not work out well for you.

 

[PeterDonis]

Moderator's note: Thread moved to Other Physics Topics.

 

[Ibix]

I'm not so sure it is in general relativity.

What's the difference between "forces did it" and "spacetime curvature did it"? The only reason to take either one seriously is the maths underlying them and the predictions we can make with it. And they're equals in that regard.

Maybe causation is a term that should be avoided,

In this context I would say so. Causal structure is a topic in general relativity to do with what regions of spacetime can signal to others even in principle, but here you are talking about how physical effects propagate.

GR sounds like a more radical change than quantum gravity.

I would tend to agree. And since all quantum theories rely on a background spacetime, a theory of the behaviour of spacetime is a necessity. So I'm (personally) not sure making gravity into just another quantum field can be the final answer.

 

[PeterDonis]

all quantum theories rely on a background spacetime

Not all quantum theories, no. Standard quantum field theories rely on a background spacetime (which can be curved). But not all quantum theories are standard quantum field theories. String theory is a sort of quantum field theory but there is no fixed background spacetime. Loop quantum gravity is a quantum theory, but not a QFT, and any concept of "spacetime" is emergent in that framework.

I'm (personally) not sure making gravity into just another quantum field can be the final answer.

The original issue with the massless spin-2 QFT I mentioned was that it is not renormalizable. Now, however, that is taken to merely be a sign that that theory must be an effective theory that emerges from something else, and all known QFTs, even the renormalizable ones that go into the Standard Model of particle physics, are viewed the same way. So I don't think anybody views "making gravity into just another quantum field" as a final answer; physicists agree that there has to be something else underneath, there is just no agreement on what that is. And GR, on this view, is also emergent and is not believed to be a final answer either, so it is not any "more radical" than quantum gravity.

 

[Gary Venter]

What's the difference between "forces did it" and "spacetime curvature did it"? The only reason to take either one seriously is the maths underlying them and the predictions we can make with it. And they're equals in that regard.

Good point. I was thinking too narrowly. Both are mechanisms that can have effects. For purely predictive purposes just the math is enough but a lot of people like to be able to have a mechanism for it. That's the problem some people have with non-locality. The math works fine but there is no explanation of what is going on. Einstein called the mechanism (rejected it actually) "spooky action" but we still have no spooky mechanism for how it happens. Spacetime curvature works fine, but what's missing is a mechanism for how matter can make space curve. Einstein's equations are the math but not a mechanism for what makes it curve.

People are working on models of that, e.g., see https://web.stanford.edu/~oas/SI/QM/papers/SpaceFromQMCarroll.pdf. Sean Carroll says "rather than quantizing gravity, maybe we should try to gravitize quantum mechanics." It's a model where spacetime emerges from the quantum field through mutual information of entangled particles.

That was 8 years ago and is mostly speculative but is an attempt to show an underlying reality to Einstein's equations. Quantum gravity at least has a force working, so already has a mechanism.

 

[PeterDonis]

Spacetime curvature works fine, but what's missing is a mechanism for how matter can make space curve.

That's only missing if you believe a "mechanism" is required, and if you believe that QFT interactions count as a "mechanism" but somehow other things don't. This is, to say the least, an idiosyncratic viewpoint. Past physicists who looked for "mechanisms", such as Maxwell conceptualizing electromagnetism in terms of vortices in the ether, would be flabbergasted to find something as abstract as a quantum field being called a "mechanism".

Sean Carroll says "rather than quantizing gravity, maybe we should try to gravitize quantum mechanics."

And this is fine as a research program, but to characterize it as "well, we're missing a mechanism for gravity so we need to find one" strikes me as a highly dubious way to look at it. For example, Einstein would have protested loudly at the idea that QM, with its nonlocality (which Einstein, as you correctly note, singled out as something highly counterintuitive about QM that we don't really understand), is a better candidate for a "mechanism" than classical GR.

It's a model where spacetime emerges from the quantum field through mutual information of entangled particles.

Which involves the very non-locality that, as you yourself acknowledge, we do not have a "mechanism" for. So again it seems highly dubious to me to view this as a "solution" to the "problem" of not having a mechanism for gravity.

Note also that, to the extent Carroll's program is a search for a "mechanism" for anything, it's for a "mechanism" of how spacetime curvature works, not a "mechanism" for how stress-energy produces spacetime curvature. So it doesn't even solve the "problem" you claimed was there with classical GR.

 

[ohwilleke]

Note that this is condensed matter physics. These things are not gravitons, but phenomena in strange matter states that are formally described by theories that are mathematically similar to theories of gravity.

That's not to say there's not interesting physics here, but this isn't an actual graviton detection.

This is the real important point.

An actual fundamental particle that is a graviton would be profoundly more difficult to detect.

 

[Gary Venter]

For example, Einstein would have protested loudly at the idea that QM, with its nonlocality (which Einstein, as you correctly note, singled out as something highly counterintuitive about QM that we don't really understand), is a better candidate for a "mechanism" than classical GR.

Yes and sorry if something I said implied that QM was such. Both QM and GR have aspects that work as math but do not have any worked-out mechanisms for how the results get carried out. That's something they have in common.

 

[PeterDonis]

Both QM and GR have aspects that work as math but do not have any worked-out mechanisms for how the results get carried out.

Again, this is your personal opinion, not physics. There is no law of physics that says everything has to have a "mechanism" by your definition in order to be fully explained. Such talk is off topic here. Please do not post further about this. You are hijacking someone else's thread.