Physicists in Israel have drawn up an analogue for black holes. Instead of being a force of gravity that does not let the light escape, it is a sonic black hole that does not allow sound waves to escape. In the process, the researchers measured the long-theoretical Hawking radiation, with significant implications for physics.
A black hole is an area of space-time where gravity pulls so strongly that even light cannot emerge once it crosses a point of return, known as the event horizon – but you probably already knew that. What may be new to you is that Stephen Hawking proposed a more nuanced view almost 50 years ago, whereby black holes can also generate light.
According to Hawking, black holes can spontaneously emit photons at the event horizon thanks to transient quantum changes known as virtual particles. Despite their name, virtual particles are indeed real particles – it’s just that they exist in and out for a fleeting time.
Virtual particles occur in pairs and in the vast majority of cases they destroy each other almost immediately. However, if they appear in the vicinity of a black hole, Hawking suggested that it is possible for one particle in a pair to be absorbed through the black hole while the other escapes into space.
This stream of particles is known as stationary Hawking radiation, but since this phenomenon is so subtle, it is virtually impossible for our instruments to detect it. But if you think outside the box, it is possible to gain insight into this elusive cosmic phenomenon.
To study Hawking radiation, scientists from the Technion-Israel Institute of Technology designed an abbreviated version of a black hole or analog in the laboratory. An example of a black hole analog is in your own home: a vortex. The water that flows into the drain can be compared to the growth of black holes in matter – but that is not what the physicists in Israel used.
Instead, the team cooled 8,000 rubidium atoms to almost absolute zero and trapped them with a laser beam. This almost static gas was in an exotic state of the material known as Bose-Einstein condensate (BEC), in which atoms become so densely packed that they act like one superatom and act in harmony.
A second laser beam created a stream of potential energy that caused the BEC gas to flow like water flowing down a waterfall. The boundary between the region where one half of the gas flows faster than the speed of sound, while the other half flows slowly, was the event horizon of the sonic black hole.
Instead of pairs of photons spontaneously forming in the gas, the researchers were looking for pairs of phonons – quantum sound wave particles. Phones in the faster half of the gas flow, outside the event horizon, are caught by the speed of the flowing gas. Just like in a black hole trapped by light particles that cross the event horizon, the phonon cannot return to the other side of the sonic black hole’s event horizon.
“In essence, the event horizon is the outer sphere of a black hole, and within it there is a small sphere called the inner horizon,” said prof. Jeff Steinhauer of the Department of Physics of Technion said in a statement. ‘When you fall through the inner horizon, you’re still sitting in the black hole, but at least you do not feel the strange physics of being in a black hole. You would be in a ‘normal’ environment, as gravity would be lower, so you would not feel it anymore. ”
It took 97,000 repetitions of this experiment for 124 consecutive days before physicists led by Steinhauer confirmed Hawking radiation. Fortunately for them, their patience paid off.
“The experimental results of Prof. Steinhauer are of great importance and interest,” said Prof. Amos Ori, a general expert on relativity and black holes at the Department of Physics at Technion, said in a statement.
“Jeff measures stationary Hawking radiation emitted from a sonic black hole, in accordance with Hawking’s theoretical prediction. This gives very important experimental support to Hawking’s analysis, which gets experimental approval for Jeff for the first time. ”
The experiments also revealed new insights that were not predicted during Hawking’s lifetime. After a certain time, the radiation emitted by the system began to increase. This is probably the result of the development of stimulated radiation after the formation of the inner horizon, the physicists explained in the journal Natural Physics.
“Our new long-term goal,” Steinhauer concluded, “is to see what happens if one goes beyond the approaches used by Hawking, in which Hawking radiation is quantum but space-time is classic. In other words, we will be in consider that the analog black hole consists of point-like atoms. ”