Why does an accelerating observer find empty space warm? (the Unruh effect)

Empty space reads warm to anyone accelerating because, measured against the motion that observer rides, its state meets the exact condition that defines a heat bath. Keep accelerating and light from far enough behind never catches you, so a boundary forms past which you can never ask anything. Every state determines an internal flow, its own clock. For empty space on your side of that boundary, Joseph Bisognano and Eyvind Wichmann proved in 1975 and 1976 that the flow is the very motion carrying you along, in step with your wristwatch. Bill Unruh calculated in 1976 what a detector would read. At rest in empty space it never fires. Accelerated, it fires as if sitting in radiation at a temperature fixed by the acceleration alone. One degree takes about twenty-five billion billion times Earth's gravity, sustained.

Also in Chapter 17: Why Is There a Speed Limit?.