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Dumb Holes and the Eects of High Frequencies on Black Hole Evaporation W. G. Unruh CIAR Cosmology Program Dept. of Physics University of B. C. Vancouver, Canada V6T 1Z1 email: unruh@physics.ubc.ca Abstract The naive calculation of black hole evaporation makes the thermal emis- sion depend on the arbitrary high frequency behaviour of the theory where the theory is certainly wrong. Using the sonic analog to black holes{ dumb holes{ I show numerically that a change in the dispersion relation at high frequencies does not seem to alter the evaporation process, lending weight to the reality of the black hole evaporation process. I also suggest a reason for the insensitivity of the process to high frequency regime. I. DUMB HOLES Black hole evaporation [1] was one of the most surprising predictions of the eld of quantum eld theory in curved backgrounds. Since the phenomenon has not been observed experimentally, it is of crucial importance that the assumptions underlying the prediction be examined especially carefully so as to try to understand the process as deeply as possible. One of the most unsettling features of the derivation is the dependence of the derivation on the behaviour of the elds at arbitrarily high frequencies. In fact, if we trace, in the usual derivation, the origin of the thermal emission from a solar mass sized black hole a second after the formation of the black hole, that thermal radiation has its origin in frequencies in the incoming (vacuum) radiation of the order of e104 (where any units make only trivial changes in the exponent). Since h! for such frequencies correspond to masses vastly larger that the mass of the universe, it seems certain that the quantum gravitational eects of such frequencies would completely alter the behaviour of the eld at such scales. Would such quantum gravitational eects also destroy the thermal emission from the black hole? Since we have no quantum theory of gravity, this question is dicult to answer. However, t