Scientists Built a ‘Mini Black Hole’ Out of Sound Waves
A tabletop black hole emits what looks like Hawking radiation.
Image: Nitzan Zohar, Technion - Israel Institute of Technology
Black holes are some of the most massive, mind-bending objects in the universe. They contain immense, almost unimaginable amounts of energy: Supermassive black holes help regulate entire galaxies, past research suggests.
Now, scientists have managed to build a smaller, tamer version in their lab—and in doing so, they've taken a big step closer to figuring out an elusive property of these super-strange objects, one that's been a big question in physics for the past four decades. In fact, black holes aren't completely black: They seem to emit some kind of radiation. An experiment using an ultra-cold gas of about two thousand atoms has produced an acoustic model of a black hole in the lab, as described in a paper published this week in Nature Physics.
First, a primer. Black holes are regions of spacetime with so much mass compressed into so little volume they create a zone from which nothing can escape—not even photons, the little particles that make up light. This means they are, well, very black. But not totally and completely so, at least in theory. In 1974, Stephen Hawking proposed that at the event horizon of a black hole—the rim that marks the point of no return—the strange effects of quantum mechanics mean that particles can pop into existence and be radiated away.
This Hawking radiation, while very weak, would ultimately mean that black holes very slowly wither and die, losing energy to this process.
"The sound waves are trying to go forward, but they can't. They're falling in"
No one has managed to detect Hawking radiation at an actual, astrophysical black hole, one that's lurking out in space. Now, a research group in Israel has built a mini-black hole in a lab and has managed to detect what they say is an analog for Hawking radiation, using an exotic material called a Bose-Einstein condensate.
As the name implies, Einstein had a hand in predicting this weird state of matter. It occurs when a dilute gas is cooled to within a whisker of absolute zero. Since there is so little thermal motion in the atoms that make up the gas, their quantum states overlap, producing some truly odd phenomena, including superfluidity, superconductivity, and the ability to simulate a black hole, in this case using acoustic waves.
"The problem with seeing real Hawking radiation is the background is more than the radiation. The fact that [the Bose-Einstein condensate] is so cold means there are very few waves in the condensate," said lead author Jeff Steinhauer, a scientist in the physics department of the Technion - Israel Institute of Technology, in an interview.
Steinhauer is applying quantum mechanics to the classical realm of gravity
With his quiescent gas of a few thousand atoms, Steinhauer put them in a long, thin tube and made them move with a laser. He was all set up to replicate a black hole with sound waves in the gas. It's not the first time scientists have made a "sonic" black hole, but using a Bose-Einstein condensate is ideal for hunting for Hawking-type radiation.
The tube of atoms has two regions: one where the atoms move slowly, and another where they move quickly. The velocity of the atoms in the fast-moving region is greater than the speed of sound in those atoms, and the transition point between these two regions is analogous to water going over a waterfall: a very sudden acceleration. It also corresponds handily to the event horizon of a black hole.
"Once the atoms are going fast, sound waves can't go forward across the flow," said Steinhauer. "It's like trying to swim against the current. That's like [the region] inside the black hole. The sound waves are trying to go forward, but they can't. They're falling in. That's like a photon trying to escape a black hole."
When Steinhauer probed outside of the event horizon of his sonic black hole, he saw sound waves being emitted, which corresponded to what he expects Hawking particles to look like.
The effect is so subtle Steinhauer had to repeat the experiment 4,600 times, "which corresponds to six days of continuous measurement," he said.
This discovery has a deeper meaning than just understanding more about what happens in weird parts of spacetime. By proving the existence of Hawking radiation, at least at a little black hole made in the laboratory, Steinhauer is applying quantum mechanics to the classical realm of gravity. Physicists are chomping at the bit to unite the two fields, and confirming Hawking radiation at an astrophysical one will be a big step on that journey.
"Black holes are really a testing ground for the new laws of physics," said Steinhauer. Brave new frontiers in a tiny cloud of atoms.
Image above: Scientist Jeff Steinhauer. The blue light emanates from the location of the sonic black hole. This light creates the sonic horizon.