Quantum field theory in curved space-time predicts that black holes have a finite lifetime, proportional to the mass of the black hole. The larger the black hole, the longer it takes to evaporate. This prediction is widely believed because of the theoretical plausibility of the connection it makes between thermodynamics and curved space-time, but it is, nevertheless, an unverified speculation. However, a team of astronomers analysing archive data from the Parkes radio telescope in Australia, have discovered a very short, but very powerful burst of radio-wavelength radiation, estimated to have originated from a source 3 billion light years away. The signature of this radio burst has never previously been seen, and, indeed, the sensitivity of many radio telescopes is insufficient to detect such short bursts. The astronomers speculate that the radiation may be the 'last gasp' of an evaporating black hole. Personally, I wonder if it could just be a statistical fluctuation in the data...
Wouldn't that mean you can never fall in to a black hole, because it evaporates before you cross the event horizon?
ReplyDeleteIt takes a finite 'proper time' for an observer to fall into a black hole, across the event horizon. (Proper time is the time which elapses along the 'worldline' of an observer; it's coordinate-independent.) For a stellar-mass black hole, this would be a shorter time than the lifetime of the black hole.
ReplyDeleteYou may be thinking of the claims that it takes an infinite time for an observer to cross the horizon, but this is only an artifact of choosing a coordinate system which is ill-defined on the event horizon.
From what frame of reference are you measuring the evaporation of the black hole? What is the length of time it takes a particle to cross the event horizon in that frame? How does that compare to the amount of time it takes the black hole to evaporate in that same frame?
ReplyDeleteIn any coordinate system in which the geometry is well-defined on the event horizon, the proper time which elapses along the worldline of an observer falling into the black hole, is finite, and much smaller than the proper time which elapses along the worldlines of external observers before their past light-cones intersect the black hole evaporation event.
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