This tag is for questions regarding to the Unruh effect (also known as the Fulling–Davies–Unruh effect), the hypothetical prediction that an accelerating observer will observe a thermal bath, like blackbody radiation, whereas an inertial observer would observe none. It was described by Stephen Fulling in 1973, Paul Davies in 1975, and William Unruh in 1976.
The Fulling–Davies–Unruh effect or, simply Unruh effect is the prediction that if you fly through a quantum vacuum with constant acceleration, the vacuum no longer looks like a vacuum: rather, it looks like a warm bath full of particles, with temperature proportional to the acceleration.
So far it has not been possible to measure or observe it (because the linear acceleration needed to reach a temperature $1$ K is of order $10^{20}$ m/s$^2$). Now physicists say that instead of studying the empty space in which particles suddenly become visible when accelerating, they can create a Bose-Einstein condensate in which sound particles, or phonons, become audible to an accelerated observer in the silent phonon vacuum; the sound is not created by the detector, rather it is hearing what is there just because of the acceleration.
The Unruh theory has had a major influence on our understanding of the proper relationship between mathematical formalism and (potentially) observable physics in the presence of gravitational fields, especially those near black holes.
For more about it, you may visit the following references:
$(1)~~$Physicists Propose New Method for Observing Hypothetical Unruh Effect
$(2)~~$B. Sokolov, J. Louko, S. Maniscalco and I, Vilja ,Unruh effect and information flow, Phys. Rev. D 101, 024047 (2020)
$(3)~~$Unruh effect
and the references therein.
