In 1974, Stephen Hawking theorized that the darkest weights of the universe, black holes, were not the pitch-black star swallows that astronomers suggested, but that they spontaneously emit light – a phenomenon now called Hawking radiation.
The problem is that no astronomer has ever observed Hawking’s mysterious radiation, and because it is predicted to be very faint, they will never be able to. That is why scientists today create their own black holes.
Researchers at the Technion-Israel Institute of Technology did just that. They created a black hole analog out of a few thousand atoms. They tried to confirm two of Hawking’s most important predictions, that Hawking radiation does not emerge from nothing and that over time it does not change in intensity, which means that it is stationary.
“A black hole is supposed to radiate like a black body, which is essentially a hot object that radiates a constant infrared radiation, “co-author Jeff Steinhauer, associate professor of physics at the Technion-Israel Institute of Technology, said in a statement. “Hawking suggested that black holes are just like ordinary stars, which constantly emit a certain kind of radiation. That’s what we wanted to confirm in our study, and we did.”
Related: The 12 strangest objects in the universe
The opportunity horizon
The gravity of a black hole is so powerful that not even light can escape from its grip as soon as a photon or a light particle goes beyond the point of no return, which event horizon. To escape from this boundary, a particle must break the laws of physics and move faster than the speed of light.
Hawking has shown that while nothing that crosses the horizon can escape, black holes can still spontaneously radiate light from the boundary, thanks to quantum mechanics and something called ‘virtual particles’.
As explained by Heisenberg’s uncertainty principle, even the complete vacuum of space is teeming with pairs of ‘virtual’ particles that exist in and out. These fleeting particles with opposite energies usually destroy each other almost immediately. But due to the extreme gravity at an event horizon, Hawking suggested that pairs of photons could be separated, with one particle being absorbed through the black hole and the other escaping into space. The absorbed photon has negative energy and draws energy in the form of mass from the black hole, while the escaped photon becomes Hawking radiation. From this alone, given enough time (much longer than the age of the universe), a black hole can completely evaporate.
Hawking’s theory was revolutionary because he combined the physics of quantum field theory with the general relativity, “Einstein’s theory that describes how matter is curved space-time, Steinhauer told WordsSideKick. “It still helps people to look for new physics laws by studying the combination of these two theories in a physical example. People want to verify this quantum radiation, but it’s very difficult with a real black hole because Hawking radiation is so is weak compared to the background radiation of space. ‘
This problem inspired Steinhauer and his colleagues to create their own black hole – a safer and much smaller one than the real thing.
DIY black hole
The researchers’ laboratory-grown black hole was made of a liquefied gas of about 8,000 rubidium atoms cooled to almost absolute zero and held by a laser beam. They created a mysterious state of matter known as a Bose-Einstein condensate (BEC), which allows thousands of atoms to act together as if they were a single atom.
Using a second laser beam, the team has a wreath of potential energy, which caused the gas to flow like water flowing down a waterfall and thus creating an event horizon where half of the gas is faster than the speed of sound, the other half slower. In this experiment, the team was looking for pairs of phonons, or quantum sounds, instead of pairs of photons that formed spontaneously in the gas.
A telephone on the slower half can move against the flow of gas away from the cliff, while the phonon on the faster half is trapped by the speed of the supersonic flowing gas, Steinhauer explained. ‘It’s like trying to swim against a current that is faster than you can swim. [That’s] just like you are in a black hole, once you are inside, it is impossible to reach the horizon. ‘
After finding these phonon pairs, the researchers had to confirm whether they correlated and whether the Hawking radiation remained constant over time (if stationary). The process was tricky because every time they took a photo of their black hole, it was destroyed by the heat generated in the process. The team thus repeated their experiment 97,000 times and took more than 124 consecutive days of measurements to find the correlations. Eventually, their patience paid off.
“We have shown that Hawking radiation is stagnant, which means it has not changed over time, and that is exactly what Hawking predicted,” Steinhauer said.
The researchers set out their findings in the January 4 issue of the journal Natural Physics.
Originally published on Live Science.