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. I used to think that this was an amusing thought experiment, but was believed to be unrealistic to actually attempt. But now Lahav et al. have apparently done it! (Via Swans on Tea and arXiv blog.)
A sonic black hole in a density-inverted Bose-Einstein condensate Authors: O. Lahav, A. Itah, A. Blumkin, C. Gordon, J. Steinhauer Abstract: ...
