Black Holes in the Bathtub

If you stuck your hand inside a black hole recently created in a Canadian laboratory, you wouldn’t get sucked in like a string of spaghetti. You’d just get wet.

This black hole exists in a water tank, its forces afflicting water waves rather than unsuspecting space travelers. Technically this bathtub version is a white hole, an inverted black hole that keeps waves out rather than sucking them in. But the white hole can serve as an analog because it shares an important feature with astrophysical black holes — an imaginary boundary that emits an unusual kind of radiation.

Black holes are notorious for sucking matter in, but physicist Stephen Hawking proposed in 1974 that a signal of their existence, now called Hawking radiation, would also leak out. In the bubbling quantum vacuum surrounding a black hole, particle-antiparticle pairs pop into existence. An electron, for example, and its partner, the positron, would emerge from the vacuum and then burst into a flash of energy after colliding an instant later. But if one particle slipped inside the black hole, forever trapped, the other particle would whiz away.

Of even more interest to scientists would be particles of light, or photons, since they would provide an optical signature. As their own antiparticles, photons should pop up in twos at a black hole’s edge. Spotting photons whizzing away from a black hole would confirm the existence of Hawking radiation, which might provide hints in the search for a theory to unite the physics of the really big with the bizarre behavior of the really small.

Unfortunately, though, radiation from real black holes is too weak to see. So scientists have taken another approach to observing Hawking radiation, building black hole analogs in Earth-bound labs.

The Canadian team with the water-based black hole analog now sees the radiation in the form of water waves. Another team observes photons emitted from a black hole analog in glass. Yet another has created a black hole in ultracold gas that could be probed for the signal in the form of sound. These lab-made emitters of Hawking radiation share one required feature with their astrophysical counterparts — a point of no return, analogous to the black hole’s outer boundary, or event horizon.

“Once there is a horizon, in spite of the many approximations underlying the theory, Hawking radiation always survives,” says physicist Iacopo Carusotto of the University of Trento in Italy. “It seems to be a robust phenomenon in nature.”

Sister horizons
In an astrophysical black hole, the event horizon exists because of the pull of gravity created by the black hole’s mass, which can be millions of times that of the sun. The horizon is the point at which nothing can escape. Even sound and light are trapped.

But black holes don’t have to be astrophysical. Take, as an example, water rushing over Niagara Falls, sweeping along a tiny fish. If the fish could yelp as it began descending to the rocks below, the sound waves carrying its cries would propagate in all directions and the screams could be heard at the top of the falls.

But if the plunging water’s velocity exceeds the speed of sound, an event horizon is created around the yelping fish. Sound waves traveling toward the top of the falls get pushed back by the plunging water and stuck behind the invisible horizon. The sound is trapped in a watery black hole.

It’s like Alice’s frustration in Through the Looking-Glass when sprinting to keep up with a certain monarch. “It becomes like the Red Queen running as fast as she can and never getting anywhere,” says William Unruh, a physicist at the University of British Columbia in Vancouver.

The flailing fish was a cute opener for a lecture Unruh gave in 1972 on black holes, then unfamiliar objects to many scientists. It wasn’t until 1980, while teaching a course on fluid mechanics, that he noticed the equations governing fluids looked suspiciously similar to the math for gravitational fields around a black hole. Unruh realized that if he could create an event horizon with water in the lab, he should be able to create Hawking radiation, too.

In August, Unruh and colleagues announced that they had made such a horizon in a water channel. Shielding the setup under a tent of black plastic, they sent a steady flow of water in one direction. As it passed over a piece of wood whittled in the shape of an airplane wing, the water traveled faster. In the opposite direction, the group created water waves. When these waves approached the wing, where water was flowing faster, they slowed to a stop. This created the inverted version of a black hole, or a white hole. Both types have horizons, so both ought to emit Hawking radiation.

In fact, pairs of short-wavelength waves were created at the horizon and swept away, Unruh’s team reports in an upcoming Physical Review Letters. And the energy of these emitted waves matches what would be predicted from Hawking radiation around a real black hole.

“At first, we would have been happy to see any evidence whatsoever,” says Unruh. “What was amazing was our results were far, far better than anything we expected.”

Unruh’s original idea for the experiment has spurred researchers around the world to create other analogs. A team of researchers led by Daniele Faccio at the University of Insubria in Italy has made an event horizon by pulsing laser light through glass. The laser pulse creates a small increase in the density of the glass that propagates like a wave at the speed of light. But because light slows when it passes from a less dense to a more dense medium, any light jogging to catch up won’t make it past the region of increased density — trapped like the Red Queen. Photons popped into existence at the event horizon, the researchers report in the Nov. 12 Physical Review Letters. If they can tweak the experiment to show that the photons were emitted in opposite directions — a tougher task — the researchers will be more certain they’ve seen Hawking radiation.

Unruh says he is not sure the team’s signal will turn out to be the sought-after radiation. “I still have questions about it,” he says.

Another research group, led by Jeff Steinhauer of the Technion-Israel Institute of Technology in Haifa, has made a black hole in an ultracold form of matter that should be emitting Hawking radiation, the team reports in an upcoming Physical Review Letters. In this material, called a Bose-Einstein condensate, supercooled atoms behave as one atom and flow with little resistance. Though the researchers haven’t looked for Hawking radiation yet, computerized simulations done by a separate team suggest that the Bose-Einstein black hole sends off pairs of phonons, the particle-like carriers of sound vibrations.

Beyond black holes

These and other analog systems may help solve a lingering problem in Hawking’s original proposal. His model suggested that radiating light could have wavelengths shorter than the Planck length, supposedly the shortest length allowed by quantum mechanics. If light could have such short wavelengths, and thus really high frequencies, one photon could carry more energy than contained in the entire universe — clearly fishy. Hawking had wanted to eliminate this possibility, but he couldn’t make his equations work without it.

But now, Unruh says, black hole analogs — particularly his, which uses the math of water waves — exhibit Hawking radiation without running into the troubling energy problem. “This experiment gives one a lot more faith that Hawking radiation doesn’t depend on the absurdly high frequencies,” says Unruh. “It’s basically not a problem.”

The analogs may also offer insights into one of the most challenging open problems in the field: uniting quantum mechanics, the math describing the intricate dance of tiny particles, with the physics of gravity, the force that holds together galaxies, stars and planets. 

Black holes are the ideal systems for seeking a theory of quantum gravity because Hawking radiation would be a quantum effect, yet black holes have strong gravitational fields. Just as measuring the energy levels of hydrogen was a crucial insight for deriving quantum mechanics, so is the classical understanding of black holes necessary for a theory of quantum gravity, says Gonzalo Olmo, a theoretical physicist at the University of Valencia in Spain. “The black hole is like the tip of the iceberg for quantum gravity,” Olmo says.

Not all of the laboratory event horizons will necessarily bear theoretical fruit — some of the contraptions may get sucked into an academic black hole of sorts. But scientists are just getting their feet wet.