The Unruh effect (or sometimes Fulling–Davies–Unruh effect), was first described by Stephen Fulling in 1973, Paul Davies in 1975 and W. G. Unruh in 1976. It is the prediction that an accelerating observer will observe black-body radiation where an inertial observer would observe none. In other words, the background appears to be warm from an accelerating reference frame; in layman's terms, a thermometer waved around in empty space will record a non-zero temperature. The ground state for an inertial observer is seen as in thermodynamic equilibrium with a non-zero temperature by the uniformly accelerated observer.
It is currently not clear whether the Unruh effect has actually been observed, since the claimed observations are under dispute. There is also some doubt about whether the Unruh effect implies the existence of Unruh radiation.
Read more about Unruh Effect: The Equation, Explanation, Calculations, Other Implications, Unruh Radiation, Experimental Observation of The Unruh Effect
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