Turn on your kitchen tap and the steady stream of water will spread out into a thin circular disc when it hits the sink. This disc has an unusual property: it is surrounded by a circular “lip”, where the height of the water changes suddenly.
This so-called hydraulic jump has puzzled physicists for at least a hundred years (John Strutt, otherwise known as Lord Rayleigh, published the first mathematical description of the phenomenon in 1914). These kinds of hydrodynamic problems are notoriously difficult to tackle.
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In recent years, the study of hydraulic jumps has intensified. That’s because various physicists have pointed out that hydraulic jumps are examples of much more exotic objects: white holes, the time-reversed equivalent of black holes. (A white hole is a region that can emit waves and particles but which waves and particles cannot enter.) While that’s an interesting conjecture, nobody has come up with experimental proof.
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Until now. Today, Gil Jannes at the Universite de Nice Sophia Antipolis in France et amis provide exactly the proof that everyone has been looking for. A white hole exists if the water flow across the hydraulic jump is faster than waves can travel. This means the waves can escape from the hole but cannot enter it against the flow.
But proving this is tricky. The problem boils down to measuring these two characteristic properties of the white hole: the flow of water in the circular disc and the speed at which waves travel on the surface of this water.
The trouble is that the water layer is wafer thin, making it hard to measure with any accuracy.
Jannes and co have a way round this problem, however. Instead of measuring these two properties directly, it is possible to find their ratio by measuring “the Mach cone associated with the supercritical flow in the inner region of the jump.”
That sounds complex but is actually straightforward. Stick your finger into the flat disc of water and the v-shaped flow around it is the Mach cone. The angle of this v-shape is the critical value you need to measure (see picture).
Jannes et amis say their measurements prove for the first time that the circular jump is indeed as exotic as everyone supposed. “We provide an experimental demonstration that the circular hydraulic jump represents a hydrodynamic white hole,” they say.
That’s an amazing result. It means that it is possible to reproduce the characteristics of the most exotic objects in the universe in your kitchen sink. Cool!
