I saw a nice example in a fountain in Vienna once. Water flows smoothly down a channel, faster and faster, until it pours off the edge in a small waterfall. If you tap the water’s smooth surface with your finger, a wave goes out. In the region where the water flows more slowly than the speed of this wave, it goes out in all directions — but faster downstream than upstream. In the region where the water flows faster than the speed of this wave, the whole wave is carried downstream: no information propagates upstream. Between these regions there is a boundary: an ‘event horizon’. No information from water waves can get out of this event horizon. (Well, at least that’s true for waves of small amplitude and certain frequencies.) So it acts a bit like a black hole.
And the really interesting thing is that if you quantize this problem, you’ll get Hawking radiation! Roughly speaking, wave-antiwave pairs will form right near the event horizon, thanks to quantum fluctuations, and some of the waves will escape upstream!
People sometimes call the sound analogue of a black hole a dumb hole, not because the analogy is stupid, but because ‘dumb’ also means ‘unable to speak’. (I guess ‘mute hole’ would be a politically correct substitute.)
— Liquid Light
— John Baez
2011.12.25 Sunday ACHK
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