When a stream of tap water hits the flat surface of the sink, it spreads out into a thin disc bounded by a raised lip, called the hydraulic jump. Physicists’ puzzlement with this jump dates back to Lord Rayleigh in 1914. More recently, physicists have suggested that, if the water waves inside the disc move faster than the waves outside, the jump could serve as an analogue event horizon. Water can approach the ring from outside, but it can’t get in.

“The jump would therefore constitute a one-directional membrane or white hole,” wrote physicist Gil Jannes and Germain Rousseaux of the University of Nice Sophia Antipolis in France and colleagues in a study on ArXiv Oct. 8. “Surface waves outside the jump cannot penetrate in the inner region; they are trapped outside in precisely the same sense as light is trapped inside a black hole.”

The analogy is not just surface-deep. The math describing both situations is exactly equivalent. But so far, no one had been able to prove experimentally that what’s going on in the kitchen sink really represents a white hole.

There are two ways to tackle the question experimentally. The most obvious strategy is to directly measure the speeds of the surface waves inside and outside the hydraulic jump, and see if the waves inside are indeed faster than the waves outside.

But these wave speeds are notoriously difficult to measure. A popular method for visualizing and measuring fluid flow, called Particle Image Velocimetry, is impractical because the fluid film is thinner than the materials used to image them. The way the wave itself interacts with the jump and the fact that waves of different wavelengths travel at different speeds can also complicate measurements.

“So from the point of view of the white hole analogy, direct measurements… are probably not the best strategy,” the team wrote.

Rather than measuring each wave speed individually, Jannes and colleagues measured their ratio by making a Mach cone. Mach cones are best known as the cone-shaped envelope of waves emitted when an object breaks the sound barrier. Interrupting the fast flow at the edge of the hydraulic jump makes a smaller but geometrically identical cone.

When the cone opens at an angle of exactly 90 degrees, it means the speed of the incoming waves is equal to the speed of the outgoing waves, which is exactly what is expected at the event horizon of a black or white hole.

To create the white hole, the physicists pumped silicon oil through a steel nozzle onto a square PVC plate about a foot across. Using silicon oil made the flow smoother and more predictable, and guaranteed the hydraulic jump was a circle, rather than a polygon or some other complicated shape.

Then they stuck a needle in the oil to make the Mach cone. Just outside the spot where the jet of oil hit the plate, the water parted around the needle at an angle of about 18 degrees. As the physicists move the needle outward, the angle smoothly increased to about 45 degrees, then rapidly opened up to reach 90 degrees near the ridge of the jump.

That implies that the speed of the waves inside the ring is equal to the speed of the waves outside the ring, “and hence constitutes a clear proof that the jump indeed represents a white hole horizon for surface waves,” the team wrote. “The fact that the circular jump represents a white hole horizon illustrates that the concept of horizons is not limited to relativity.”

“This is a brilliant experiment: Kitchen-sink physics is turned into a black-hole analogue,” commented Ulf Leonhardt, a physicist at the University of St Andrews in Scotland who works on making analogue black holes in fiber-optic cables. “Germain Rousseaux and his team used sophisticated equipment and did very careful measurements, but at its heart, the experiment is based on a simple idea everyone can understand and try at home.”

Images: 1) Wikimedia Commons. 2) G. Jannes et al.

See Also:

• Ultrafast Laser Pulse Makes Desktop Black Hole Glow
• Scientists Make Fake Black Hole in a Phone Line
• Rogue Black Holes Could Careen Across Milky Way
• Large Hadron Collider: Best- and Worst-Case Scenarios

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Authors: Lisa Grossman

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