Observation of the quantum phase of free fall and the consistency with the equivalence principle

[Filip Larsen]

Interesting experiment design, with some promise for a future QM/GR "interface" result.

https://www.science.org/doi/10.1126/sciadv.aec8045 (this page is acting weird for me)

The unification of quantum theory and the general theory of relativity, describing gravity, is one of the most important challenges in science. Einstein’s general theory of relativity is based on the principle of equivalence and has been confirmed to great accuracy for large bodies. However, in the quantum domain, the equivalence principle has been predicted to take a unique form involving a gauge phase, which is equal, in the context of a measurement on Earth, to the quantum phase of a free-falling wave packet relative to its counterpart wave packet which is static in Earth’s frame. To measure this phase, we realize a novel cold-atom interferometer in which one wave packet stays static in the laboratory frame while the other is in free fall. The observed relative phase of the wave packets confirms the predicted phase and shows that, in our low energy regime, the equivalence principle may be applied to the quantum domain. Our observation constitutes a fundamental test of the interface between quantum theory and gravity. The new interferometer also opens the door for further probing of the latter interface, as well as to searches for new physics.

Arstechnica also has a piece on this result, perhaps more suitable for non-QM people like me.

 

[PeterDonis]

with some promise for a future QM/GR "interface" result

I'm not sure how much this experiment actually "promises" in this regard. It does not in any way probe quantum properties of gravity itself. And if you don't quantize gravity, we already have an "interface" between QM and GR: quantum field theory in curved spacetime, which is well developed at this point. The experiment is a useful confirmation of what we already expect, that quantum experiments obey the equivalence principle (and note that this in itself does not involved spacetime curvature at all--quantum mechanics in flat spacetime is enough to analyze the experiment); but I think claims about this leading to any significant progress on something like a quantum gravity theory are too optimistic.

 

[flippiefanus]

I'm not sure how much this experiment actually "promises" in this regard. It does not in any way probe quantum properties of gravity itself. And if you don't quantize gravity, we already have an "interface" between QM and GR: quantum field theory in curved spacetime, which is well developed at this point. The experiment is a useful confirmation of what we already expect, that quantum experiments obey the equivalence principle (and note that this in itself does not involved spacetime curvature at all--quantum mechanics in flat spacetime is enough to analyze the experiment); but I think claims about this leading to any significant progress on something like a quantum gravity theory are too optimistic.

Why do you say this experiment can be analysed in flat spacetime? Earth's gravity does not represent flat spacetime.

 

[renormalize]

Why do you say this experiment can be analysed in flat spacetime? Earth's gravity does not represent flat spacetime.

This experiment simply measures the time-dependence of the phase $\phi\left(t\right)$ of a wave packet free-falling at the Earth's surface; i.e., a packet subject to the Hamiltonian $H=p^2/2m-mgz\,$, where $g$ is the local acceleration due to Newtonian gravity. No general relativity required.

 

[PeterDonis]

Why do you say this experiment can be analysed in flat spacetime?

Because the patch of spacetime in which it takes place is small enough that the effects of spacetime curvature are not measurable.

Indeed, this is necessary for any experiment that claims to be testing the equivalence principle, since the whole point of the equivalence principle is that a small enough patch of curved spacetime is indistinguishable from the same sized patch of flat spacetime.

Earth's gravity does not represent flat spacetime.

Earth's tidal gravity does not represent flat spacetime. But tidal gravity was irrelevant in this particular experiment. See above.