A Black Hole in Nick's Sink

William Unruh

The fastest velocity that any object in the universe can attain is the speed of light. But in certain extreme situations even the speed of light is not fast enough.

For instance, at the event horizon of a black hole, gravity warps spacetime so strongly that nothing can escape, not even a light beam. Also in a universe which is expanding at an ever-increasing rate (which was recently discovered to be the case for our own universe), there will be times and places for which the rate of expansion will be faster than light speed. When space is expanding faster than light, light traveling in such a space will always "fall behind" and there will be places ahead of it that the light can never reach--even in infinite time. Such an cosmic expansion-induced barrier to light travel is called a "future horizon" and resembles in many ways the event horizon of a black hole.

Physics thrives on experiment but we cannot build black holes nor future horizons in the laboratory. However William Wooters and his colleagues at the University of British Columbia are investigating situations in which the speed of sound in a liquid behaves in analogous ways to the speed of light in spacetime. Despite many differences between sound and light propagation there is much to be learned from these sonic analogs of black holes and future horizons.

In a recent paper "Dumb Holes: Analogues for Black Holes" Unruh imagines a fish has just fallen over a waterfall whose rate of fall at some distance is greater than the speed of sound. Once the fish falls past this point, its screams for help can never reach its fellows on top of the falls. The fish has fallen into what Unruh calls a "dumb hole" in the sense of "deaf and dumb". The fish has fallen into the sonic equivalent of a black hole. Just as light cannot escape from a black hole, sound cannot escape from a Unruhian dumb hole.

It is easy in the laboratory to produce streams of water that travel faster than sound. In fact you can do this in your kitchen sink. A stream of water falling into the sink from the faucet and hitting a flat surface will usually be traveling faster than sound and its speed will decrease as it spreads out into a circular shape. At a certain distance from the faucet this speed will have slowed to the point where it is close to the velocity of sound. At this point an unusual phase change called a "hydraulic jump" takes place in which the water abruptly slows by using some of its kinetic energy to raise a bump in the flow.

The hydraulic jump phenomenon is robust and easily reproducible. And the math describing this peculiar sonic transition is similar enough to the situations at future horizons and black hole event horizons that these simple kitchen sink experiments can stimulate and inspire better descriptions of powerful cosmic processes out near the edges of the universe.

"Black Hole" in Nick's kitchen sink