In 1974, Stephen Hawking speculated that the universe’s darkest gravitational behemoths, black holes, were not the pitch-black star swallowers astronomers conceptualized, but they automatically transmitted light, a miracle now named Hawking radiation.
The issue is, no astronomer has ever witnessed Hawking’s mysterious radiation and as it is predicted to be very unclear, they may never will. This is why scientists today are producing their own black holes.
Researchers at the Technion-Israel Institute of Technology did just that. They formed a black hole analogue out of a few thousand atoms. They were attempting to confirm two of Hawking’s most significant predictions, that Hawking radiation arises from nothing and that it does not change in intensity over time, meaning it’s stationary.
The event horizon
The gravity of a black hole is so strong that not even light can escape its grasp, once a photon, or light particle, crosses past its point-of-no-return, called the event horizon. To skip this boundary, a particle must break the laws of physics and travel faster than the speed of light.
Hawking demonstrated that although nothing that crosses the event horizon can escape, black holes can still emit light from the boundary voluntarily, due to quantum mechanics and something called “virtual particles.”
As described by Heisenberg’s uncertainty principle, even the total vacuum of space is filling with pairs of ‘virtual’ particles that pop in and out of existence. These fleeting particles with contrasting energies normally annihilate each other almost immediately. However, due to the high gravitational pull at an event horizon, Hawking recommended pairs of photons to be separated, with one particle getting absorbed by the black hole and the other fleeing into space. The absorbed photon has negative energy and subtracts energy in the form of mass from the black hole, while the escaped photon becomes Hawking radiation. From this only, given enough time, much more than the age of the universe, a black hole could totally evaporate away.
This problem inspired Steinhauer and his colleagues to develop their own black hole, a safer and much smaller one than the original deal.
DIY black hole
The researchers’ lab-grown black hole was formed from a flowing gas of around 8,000 rubidium atoms cooled to almost total zero and held in place by a laser beam. They formed a weird state of matter, known as a Bose-Einstein Condensate (BEC), which lets thousands of atoms to act together in unison as though they were a single atom.
Employing a second laser beam, the team developed a cliff of potential energy, which enabled the gas to flow like water rushing down a waterfall, thereby forming an event horizon where one half of the gas was flowing faster than the speed of sound, the other half slower. In this experiment, the team was seeking pairs of phonons, or quantum sounds waves, rather than pairs of photons, naturally forming in the gas.
A phonon on the slower half could travel against the flow of gas, away from the cliff, while the phonon on the faster half became confined by the speed of the supersonic flowing gas. It’s like trying to swim against a current that’s faster than you can swim. That’s just like being in a black hole, once you’re inside, it’s illogical to reach the horizon.
Once they discovered these phonon pairs, the researchers had to establish if they were correlated and if the Hawking radiation remained constant over time, had it been stationary. That process was difficult because every time they took a picture of their black hole, it was demolished by the heat formed in the process. So the team repeated their experiment 97,000 times, taking above 124 days of continuous measurements to find the correlations. In the end, their patience paid off.
It has been established that the Hawking radiation was stationary, meaning it didn’t alter with time, which is exactly what Hawking predicted.