Updated: Feb 21, 2021
Black holes are regions in space where gravity is very powerful, so strong that nothing, even light, that enters them can escape. Theoretical models say there is a radius defined as the event horizon circling black holes. It can no longer avoid a black hole until anything reaches the event horizon, as gravity becomes stronger as it approaches its core.
Theoretical physicist Stephen Hawking predicted that black holes spontaneously emit a finite amount of light, which is known as Hawking radiation, although nothing can escape from within them. This radiation is random (i.e., it emerges from nothing) and stationary, according to his predictions (i.e., its intensity does not change much over time).
Researchers at the Technion-Israel Institute of Technology have recently performed a study to test the theoretical predictions of Hawking. More precisely, they investigated whether the Hawking radiation equivalent was stationary in an "artificial black hole" produced in a laboratory setting.
"If you go inside the event horizon, there's no way to get out, even for light, "If you go inside the event horizon, there's no way out, except for the light. "Hawking radiation starts just outside the event horizon, where light can barely escape. That is really weird because there's nothing there; it's empty space. Yet this radiation starts from nothing, comes out, and goes towards Earth."
The artificial black hole that Steinhauer and his colleagues developed was about 0.1 millimeters long and consisted of a gas consisting of 8000 atoms of rubidium, which is a relatively small number of atoms. The black hole was killed each time the researchers took a photo of it. They then had to generate the black hole to track its evolution over time, take a picture of it and then create another one. Many times, for months, this process has been replicated.
Rather than light waves, the Hawking radiation produced by this analog black hole is made of sound waves. The atoms of rubidium flow faster than the speed of sound, but sound waves are unable to reach the horizon of the occurrence and escape from the black hole. However, the gas moves slowly outside of the event horizon, so sound waves can travel freely.
"The rubidium is flowing fast, faster than the speed of sound, and that means that sound cannot go against the flow,"The rubidium is flowing fast, faster than the speed of the sound, and that means the sound does not go against the flow. "Let's say you were trying to swim against the current. If this current is going faster than you can swim, then you can't move forward, you are pushed back because the flow is moving too fast and in the opposite direction, so you're stuck. That's what being stuck in a black hole and trying to reach the event horizon from inside would be like."
The radiation released by black holes is, according to Hawking's predictions, random. In one of their previous experiments, in their simulated black hole, Steinhauer and his peers were able to confirm this prediction. They set out to investigate in their latest research whether the radiation produced by their black hole is indeed stationary (i.e., if it remains constant over time).
According to Hawking's predictions, the radiation emitted by black holes is random. Steinhauer and his peers were able to test this prediction in one of their previous experiments, in their simulated black hole. In their most recent study, they set out to investigate whether the radiation emitted by their black hole is still stationary (i.e., if it remains constant over time).
Hawking radiation consists of pairs (i.e., light particles) of photons: one arising from a black hole and one dropping back into it. Steinhauer and his colleagues thus looked for similar pairs of sound waves while trying to identify the Hawking radiation emitted by the analog black hole they produced, one coming out of the black hole and one going into it. The researchers tried to decide whether there were so-called associations between them once they identified these pairs of sound waves.
"We had to collect a lot of data to see these correlations," Steinhauer said. "We thus took 97,000 repetitions of the experiment; a total of 124 days of continuous measurement."
Overall, the results seem to indicate that, as expected by Hawking, the radiation released by black holes is stationary. Although these results mainly refer to the analog black hole they have developed, theoretical studies may help to validate whether they can be applied to actual black holes as well.
"Our study also raises important questions, because we observed the entire lifetime of the analog black hole, which means that we also saw how the Hawking radiation started, "Our thesis also raises important questions because we observed the entire life of the analog black hole, which means that we also saw how the Hawking radiation began. "In future studies, one could try to compare our results with predictions of what would happen in a real black hole, to see if 'real' Hawking radiation starts from nothing and then builds up, as we observed."
The radiation around their analog black hole became very intense at some stage during the researchers' experiments, as the black hole developed what is known as an 'inner horizon.' In addition to the event horizon, Einstein's theory of general relativity predicts the presence of an inner horizon, a radius inside black holes that delineates another area closer to its center.
The gravitational force is far weaker in the area beyond the inner horizon, so objects can travel about easily and are no longer drawn into the middle of the black hole. Yet, since they do not pass through the inner horizon in the opposite direction, they are now unable to leave the black hole (i.e., heading toward the event horizon).
"Essentially, the event horizon is a black hole's outer sphere, and inside it, there's a small sphere called the inner horizon," Steinhauer said. "If you fall through the inner horizon, then you're still stuck in the black hole, but at least you don't feel the weird physics of being in a black hole. You'd be in a more 'normal' environment, as the pull of gravity would be lower, so you wouldn't feel it anymore."
Some scientists have predicted that the radiation that it produces becomes stronger when an analog black hole forms an internal horizon. It is important to note that this is exactly what happened in the analog black hole created by Technion's researchers. This research could thus encourage other physicists to study the influence of the creation of an inner horizon on the intensity of the Hawking radiation of a black hole.