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Silke Weinfurtner on understanding the mysteries of black holes: build a big bathtub

Silke Weinfurtner Related
Physics 06/12/2015

Don’t be alarmed, but there is a black hole in your bathtub. When you drain the tub, water converges on the plughole and speeds up, eventually flowing too fast for surface waves to propagate outwards. Those waves get swept down the drain like hapless astronauts falling into a black hole. For even more fun, there’s a time-reversed black hole, known as a white hole, in your sink. When a stream of water from your faucet strikes the sink and splays out, it initially moves too fast for ripples to propagate inwards. As it diverges, it slows. Within an inch or two, waves become able to propagate in every direction—a transition that is clearly visible as a discontinuity, or “hydraulic jump,” marking the perimeter of the white hole.
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