The most recent example of misleading hype comes, depressingly, from an otherwise reliable news outlet, the Physical Review A synopsis where Michael Shirber explains that
“A proposed interferometry experiment could test quantum gravity theories by entangling two mirrors weighing as much as apples.”
A slight shortcoming of this synopsis is that it does not explain which theories the experiment is supposed to test. Neither, for that matter does the author of the paper, Roman Schnabel, mention any theory of quantum gravity that his proposed experiment would test. He merely uses the words “quantum” and “gravity” in the introduction and the conclusion. One cannot even blame him for overstating his case, because he makes no case.Leaving aside the introduction and conclusion, the body of Schnabel’s paper (which doesn’t seem to be on the arxiv, sorry) is a detailed description of an experiment to entangle two massive mirrors using photon pressure, which means that one creates macroscopically heavy objects displaying true quantum effects. The idea is that you first let a laser beam go through a splitter to produce an entangled photon state, and then transfer momentum from the photons on the mirrors by allowing the mirrors to bounce. In this way you entangle the mirrors, or at least some of the electrons in some of their atomic orbits.
Similar experiments have been done before (and have previously been falsely claimed to test quantum gravity), but Schnabel proposes an improved experiment that he estimates to be less noise-sensitive which could thus increase the time for which the entanglement can be maintained and the precision by which it can be measured.
Now as I’ve preached numerous times, just because an experiment has something to do with quantum and something to do with gravity doesn’t mean it’s got something to do with quantum gravity. Quantum gravity is about the quantization of the gravitational field. It’s about quantum properties of space and time themselves, not about quantum properties of stuff in space-time. Schnabel’s paper is remarkable in that it doesn’t even bring in unquantized gravity. It merely discusses mass and quantization – no quantum gravity in sight anywhere.
The closest that the paper gets to having something to do with quantum gravity is mentioning the Schrödinger-Newton equation, which is basically the exact opposite of quantum gravity – it’s based on the idea that gravity remains unquantized. And then there are two sentences about Penrose’s hypothesis of gravitationally induced decoherence. Penrose’s idea is that gravity is the reason why we never observe quantum effects on large objects – such as cats that can’t decide whether to die or to live – and his model predicts that this influence of gravity should prevent us from endowing massive objects with quantum properties. It would be nice to rule out this hypothesis, but there is no discussion in Schnabel’s paper about how the proposed experiment would compare to existing bounds on gravitationally induced decoherence or the Schrödinger Newton equation. (Contacted to author and asked for details, but no reply.)
Don’t get me wrong, I think this is worthwhile experiment and one should definitely push tests of quantum mechanics on macroscopically large and heavy objects, but this is interesting for the purpose of testing the foundations of quantum mechanics, not for the purpose of testing quantum gravity.
And the weight of the mirrors btw is about 100g. If the American Physical Society can’t trust its readers to know how much that is, so that one has to paraphrase it with “weighting as much as apples,” then maybe they shouldn’t be discussing quantum gravity.
All together, the paper is a sad demonstration of the utter disconnect between experiment and theory in quantum gravity research.