Black holes are black because you can’t go faster than the speed of light. So what about the speed of sound?
Of course there is no problem in having something go faster than sound, but sound waves themselves are stuck with that speed limit. That fairly elementary fact inspired Bill Unruh years back to propose a clever idea: a black hole that you could make in the laboratory, but using sound rather than light. He called them dumb holes, although I’m not sure people get the right idea when they hear that name.
A sonic black hole in a density-inverted Bose-Einstein condensate
Authors: O. Lahav, A. Itah, A. Blumkin, C. Gordon, J. Steinhauer
Abstract: We have created the analogue of a black hole in a Bose-Einstein condensate. In this sonic black hole, sound waves, rather than light waves, cannot escape the event horizon. The black hole is realized via a counterintuitive density inversion, in which an attractive potential repels the atoms. This allows for measured flow speeds which cross and exceed the speed of sound by an order of magnitude. The Landau critical velocity is therefore surpassed. The point where the flow speed equals the speed of sound is the event horizon. The effective gravity is determined from the profiles of the velocity and speed of sound.
The idea is simply that you get a fluid flowing faster than its speed of sound in some region, so that the sound waves cannot escape the “horizon” bounding that region. (The flow speed has to change within the material; taking a balloon full of air and putting it on a supersonic jet doesn’t count.)
But the reason this could some day be very exciting is when quantum mechanics gets into the game. Just like black holes, dumb holes should have “Hawking radiation” — but instead of particles, the holes should emit quantized sound waves (conventionally known as “phonons”). That would be very interesting to observe, although the experimental state of the art isn’t there yet.
To be clear, we wouldn’t be learning much about quantum gravity if we observed Hawking phonons from dumb holes. The underlying physics is still that of atoms (and, in this case, a Bose-Einstein condensate), not that of general relativity. Indeed, one of Unruh’s original motivations was to show that the physics on small scales didn’t affect the prediction of Hawking radiation. So the prediction of Hawking phonons should be rock-solid, no matter how little we know about quantum gravity. Still, it would be very cool.