THE creation of particles out of the vacuum will occur in regions of space-time where the metric is changing rapidly. Theoretical discussions of this process encounter some interpretational difficulties, however, because the concept of 'particle' is only well understood in Minkowski space. Nevertheless, in some simple cases, for example with the metric of homogeneous cosmologies, or of black holes of the Kerr and Schwarzschild type, the existence of a global timelike Killing vector allows a very plausible extension of the Minkowski space definition of particle. A number of exact results may then be proved. One of these results (ref. 1, and C. J. Isham and J. G. Taylor, personal communication) states that there is no creation of massless particles in the exterior region of a Schwarzschild black hole, which is the static end state reached as a result of spherically symmetric gravitational collapse. This result is not valid for a Kerr (rotating) black hole for essentially classical reasons2.
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