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Physicists in Israel designed an analogue of the black hole – only, rather than being a gravitational fortex that does not allow light to escape, this particular object is a sound black hole that does not allow sound waves to escape. escape. In the process, the researchers measured the long-theorized Hawking radiation, with important implications for physics.
A black hole is a region of space-time where gravity pulls so hard that even light cannot exit once it passes through a point of no return, known as the event horizon – but you probably already knew that. What may be new to you is that almost 50 years ago Stephen Hawking proposed a more nuanced view in which black holes can also generate light.
According to Hawking, black holes can spontaneously emit photons on the event horizon through transient quantum fluctuations called virtual particles. Despite their name, virtual particles are indeed real particles – it’s just that they appear and disappear for a fleeting time.
Virtual particles appear in pairs and in the vast majority of cases they annihilate almost immediately. However, if they appear near a black hole, Hawking has suggested that it is possible that one particle from one pair is absorbed by the black hole, while the other escapes into space.
This flow of particles is known as stationary Hawking radiation, but because this phenomenon is so subtle, it is virtually impossible for our instruments to detect it. But, by thinking outside the box, it is possible to better understand this elusive cosmic phenomenon.
In order to study Hawking’s radiation, scientists at the Technion-Israel Institute of Technology designed a scaled-down version of a black hole, or the like, in the lab. An example of a black hole analogue can be found in your own home: a tub whirlpool. The water swirling down the drain can be compared to the accretion of matter by a black hole – but that’s not what physicists in Israel used.
Instead, the team cooled 8,000 rubidium atoms to near absolute zero and trapped them in place with a laser beam. This almost static gas was in an exotic state of matter known as Bose-Einstein condensate (BEC), in which atoms become so densely packed that they behave like a superatom, acting in unison.
A second laser beam created a flow of potential energy that caused the BEC gas to flow like water tumbling down a waterfall. The boundary between the region where half of the gas flowed faster than the speed of sound while the other half flowed slowly was the event horizon of the sonic black hole.
Rather than spontaneously forming pairs of photons in gas, the researchers looked for pairs of phonons – particles of quantum sound waves. Phonons in the faster half of the gas flow, beyond the event horizon, are trapped by the speed of the circulating gas. Just like in a black hole that has trapped light particles crossing the event horizon, the phonon cannot return to the other side of the sound black hole’s event horizon.
“Essentially, the event horizon is the outer sphere of a black hole, and inside there is a small sphere called the inner horizon,” Professor Jeff Steinhauer of the Technion’s Department of Physics said in a statement. . “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 would be in a more “normal” environment, because the pull of gravity would be weaker, so you wouldn’t feel it anymore.
It took 97,000 iterations of this experiment over 124 consecutive days for physicists led by Steinhauer to confirm Hawking’s radiation. Fortunately for them, their patience has paid off.
“Professor Steinhauer’s experimental results are of great importance and interest,” Professor Amos Ori, an expert in general relativity and black holes in the physics department at the Technion, said in a statement.
“Jeff measures the stationary Hawking radiation emitted by a sound black hole, in agreement with Hawking’s theoretical prediction. This gives very important experimental support to Hawking’s analysis, which obtains experimental approval for the first time in Jeff’s experiments.
The experiments also revealed new information that had not been predicted during Hawking’s lifetime. After a while, the radiation emitted by the system began to intensify. This is likely due to the development of stimulated radiation following the formation of the inner horizon, the physicists explained in the journal. Physics of nature.
“Our new long-term goal,” concludes Steinhauer, “is to see what happens when we go beyond the approximations used by Hawking, in which Hawking’s radiation is quantum, but space-time is classical. In other words, we would take into account that the analog black hole is made up of point atoms. “
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