Macroscopic quantum gravity phenomena

Question

A theory of quantum gravity is said to be needed when quantum and gravitational effects are strong at the same time i.e. at black hole singularities and at the big bang. This also makes it difficult to test quantum gravity.

But what about testing macroscopic quantum phenomena in different gravity regimes like flying a superconductor or liquid helium into Earth orbit and back again - would you expect gravitational time dilation or high-g accelerations to alter macroscopic quantum behaviour in a way that could test quantum gravity theories ?

Answer 1

No, quantum gravitational effects operate at too small a length scale to affect the sort of phenomena mentioned in the Wikipedia article you cite.

One possible effect is that spacetime may not be even on the Planck scale and this may very slightly change the propagation of light. This is only measurable over enormous distances like the radius of the observable universe, but it's been suggested that it might be measurable in phenomena like gamma ray bursts. See for example this paper, or Google for lots of related articles.

However, at the moment there is no unambiguous evidence for such effects, and not all theorists are convinced they exist anyway.