Here’s something interesting that came up on Slashdot today: scientists at the Israel Institute of Technology report having created an “acoustic black hole”, a region from which no sound waves can escape, just as a normal black hole is a region from which no light waves can escape.

How did they do it? Well, whenever sound travels through a medium, it does so at a characteristic speed — about \(\unit{343}{\frac{\meter}{\second}}\) in air, for example. That speed is relative to the medium, though, so if you can get the medium to move through your lab at a faster speed, the sound waves won’t be able to propagate fast enough to move against it (relative to the lab). If you had a wind tunnel blowing air to the right at \(\unit{400}{\frac{\meter}{\second}}\), the air would carry along all sound waves traveling through it, even those emitted in the leftward direction. Any sound waves produced at the right end of the tunnel would be stuck there — in effect, it’s a one-dimensional acoustic black hole, with an event horizon at the point (surface, really) where the air accelerates past the speed of sound as it …