Physicists warn that quantum gravity experiments might be fooled by perfectly ordinary spacetime

[Badger27]

A new study published in npj Quantum Information by Joshua Foo and colleagues warns that a class of experiments designed to test whether gravity itself behaves quantum mechanically could be misled by a much more mundane explanation, ordinary decoherence in curved spacetime, rather than any genuine sign of quantum gravity

The experiments in question aim to settle one of physics' oldest open questions, does gravity ultimately behave according to the rules of quantum mechanics, the way every other known force does, or does it remain fundamentally classical even at the smallest scales? The general approach involves placing a massive object in a superposition, existing in two locations at once, and checking whether gravity itself can become entangled with that superposition, which would be a genuine signature of quantum gravity if detected cleanly

Foo's team modeled what happens to this kind of spacetime superposition once general relativity's own effects are properly accounted for, and found that the curvature of spacetime, entirely mundane, already well understood physics with nothing exotic or quantum about it, can produce decoherence patterns that look deceptively similar to what a genuine quantum gravity signal would produce. In other words, an experiment could register exactly the kind of result researchers are hoping to interpret as evidence of quantum gravity, when the actual cause is just ordinary relativistic physics behaving the way Einstein's century old theory already predicts

This doesn't mean quantum gravity experiments are pointless, but it does mean researchers need considerably more careful theoretical modeling to distinguish a genuine quantum gravity signature from an ordinary relativistic mimic before they can credibly claim a detection. As increasingly precise nanoparticle interferometry and levitated mechanical systems push these experiments from thought experiment toward real, buildable hardware over the next decade, papers like this matter because they narrow the gap between an exciting headline claiming quantum gravity detected and an experiment that's actually rigorous enough to support that claim

Quantum Gravity Ordinary Spacetime Superpositionphys.org