Relativistic quantum measurements, the Unruh effect, and black holes

The restricted path integral (or quantum corridor) technique can be used to analyze relativistic measurements. This technique clarifies the physical nature of the thermal effects observed by an accelerated observer in Minkowski space-time (the Unruh effect) and by a distant observer in the field of a black hole (the Hawking effect). The physical nature of the “thermal atmosphere” around the observer is analyzed for three cases: (a) the Unruh effect, (b) an eternal black hole, and (c) a black hole forming in collapse. The thermal particles are real only in case “c”. In case “b”, they are indistinguishable from real particles but do not carry away the mass of the black hole until absorbed by the distant observer. In case “a”, the thermal particles are virtual.