The phenomenon, predicted in 1976 by William Unruh, that an accelerating body would seem to be surrounded by particles at a nonzero temperature, with this temperature being proportional to the acceleration. The vacuum state of a nonaccelerating observer is different from that of an accelerating observer because of distortion of the zero-point fluctuations. Unruh radiation is associated with this effect. The effect itself is very small and has not been verified experimentally. It was discovered by the Canadian physicist William Unruh (1945– ).