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Question:
The gravity of a black hole bends the path of light, and also "sucks in" light. Would not the "ingestation" of the light make the hole directly invisible, since it dose not refract light, and detection would be based solely on its effects, like detection of wind?



Replies:
Surely. If you had a big piece of white paper behind the black hole, and shined your big flashlight on it, you would see a big black...um...hole right where the black hole was. The problem of seeing black holes against the background of space is basically that space, as you have noticed, is unfortunately black itself. But you are right on the money about detecting the little beasts indirectly. The idea is that they consume matter at a ferocious rate, and as all that matter falls into the hole it gets very compressed and hot and radiates fantastic amounts of energy in the X-ray region of the spectrum. The most convincing evidence for the existence of black holes is the fact that no one can think of anything other than matter falling into a black hole that can generate so much energy of this type in such a small space. Sooooo, the black holes we think we see are not really black but actually very very bright. Incidentally, Stephen Hawking's big contribution to this field is that black holes that are not sucking up matter eventually evaporate, by means of what I would call "rectifying fluctuations of the vacuum."

christopher grayce



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