Experiment simulates exotic white holes

Expensive particle colliders are not the only way to explore extreme physics. It seems that water gushing from a tap and hitting a sink behaves like a white hole;  the theoretical opposite of a black hole.

A black hole is a dense concentration of mass surrounded by an extremely powerful gravitational field. Nothing that falls within a certain radius surrounding it, known as the event horizon, escapes.

A white hole is the opposite: its event horizon allows things to escape but prevents anything from entering. However, so far white holes only exist in theory, so cannot be studied observationally.

When water hits the bottom of a sink, it flows outwards in all directions. At a certain distance from the point where the water hits the sink, the outgoing liquid rapidly decelerates and piles up before continuing its outward flow, creating a ring-like ridge.

Physicists have previously suspected that any ripples that might arise beyond the ridge and travel towards it should not be able to get past the ridge.

This is because at the ridge the water flows outwards at the maximum speed that ripples could travel inwards, so the ripples would make no forward progress, like a runner on a treadmill. This makes the ridge behave like a white hole event horizon.

Now this has been experimentally confirmed by Germain Rousseaux of the University of Nice in France and colleagues.

Opening Angle
Instead of looking at water hitting a sink, the team examined what happens when a stream of viscous oil hits an empty aquarium. When they placed the tip of a needle in the path of the oil as it spread out from the collision point, it generated a v-shaped disturbance (see image).

The angle of the v depended on the relative speeds of the fluid and any ripples on its surface. When the team measured it, they found that the two speeds are indeed equal, preventing ripples flowing in and creating something akin to a white hole event horizon.

They also found that between the collision point and the ridge, the oil flowed faster than the ripple speed, causing any ripples arising there to be swiftly carried outwards – just as things inside a white hole should get spat out.

Summary

"The experiment is based on a simple idea everyone can understand and try at home," says Ulf Leonhardt of the University of St Andrews, UK.

Daniele Faccio of Heriot-Watt University in Edinburgh, UK, who recently used lasers to simulate an event horizon, says studying black and white hole analogues could provide insights into the physics of these exotic objects. For example, in 1974, Stephen Hawking showed mathematically that event horizons should radiate light.

Our telescopes are not sensitive enough to confirm this, but analogue experiments like Rousseaux's could help reveal the physical mechanism for the radiation, which remains unclear.

Journal reference: arxiv.org/abs/1010.1701