Vacuum Structure and Ether-Drift Experiments

In principle, ether-drift experiments could distinguish phenomenologically emergentgravity approaches, based on the hydrodynamic distortions of the same physical flatspace vacuum, from the more conventional scenario where curvature is a fundamental property of space-time down to extremely small length scales and the speed of light represents a universal constant. However, when comparing with experiments, one should take into account the possibility of random fluctuations of the signal. These fluctuations might reflect the stochastic nature of the underlying "quantum ether" (think of a superfluid in a turbulent state of motion) and produce vanishing global averages for all vectorial quantities obtained from a straightforward Fourier analysis of the data. Therefore, for a definitive check, one should also extract a positive-definite amplitude A(t) from the beat signal and compare its average value with the intrinsic stability of the individual optical resonators. An independent analysis of experiments with different systematics (both cryogenic and at room-temperature) gives an experimental value <A> = 𝒪(10^-15) which has precisely the order of magnitude expected in an emergent-gravity approach, for an apparatus placed on the Earth's surface. Since physical implications could be substantial, it would be important to check with more recent data and, possibly, with experimental set-ups operating in gravity-free environments.