Sonic analogue of black holes in the lab

      Black holes cannot be produced in the lab, but they can be mimicked and their properties studied, as demonstrated by Jeff Steinhauer of Technion-Israel Institute of Technology. In a paper published recently in Nature Physics, the author describes observing the equivalent of Hawking radiation in a “sonic black hole” created in his lab.
Black holes consist of extremely dense matter giving rise to such a strong gravitational attraction that nothing, not even light, can escape from its edge — called the event horizon However, in 1974, Stephen Hawking proposed that there was a way in which the black hole could emit a faint stream of particles from its edge. This phenomenon is now known as Hawking radiation. This happens in the following way.

In quantum field theory, a vacuum is not completely empty but contains fluctuations — particle-antiparticle pairs form and annihilate each other in a twinkling, and this happens all the time. In the context of a black hole, if such a particle-antiparticle pair formed at the event horizon, it could happen that one member of the pair escapes the edge and the other is swallowed up, thereby leaving a weak flux of particles. This would give rise to Hawking radiation.

This effect is difficult to measure experimentally. So, while there is a theoretical prediction that it would take place, it has not been observed directly. Now, Dr. Steinhauer has created and observed a sonic version of this in the lab using what is called a Bose-Einstein condensate (BEC). The BEC is formed when a gas of bosons is cooled to very low temperatures, close to absolute zero. The gas condenses into a quantum liquid, namely, a liquid in which the quantum effects which normally take place at a microscopic level are observable macroscopically. In other words, the entire fluid behaves like a single entity.

He confined the BEC to a narrow region using a laser. Using another laser, he created a step like “waterfall” potential which accelerated a part of the fluid to supersonic speed while the rest of the fluid remained moving at subsonic speed. This marked off two horizons. The boundary of the supersonic part was like the event horizon because a quantum of sound moving in the BEC cannot escape the boundary of the fluid moving at a supersonic speed.

Llike in the case of a black hole, when pairs of phonons are created near the horizon, there is a chance that one escapes and the other gets trapped within the condensate. This is what happened. Further the trapped phonons kept bouncing between the two horizons and getting amplified in the process, much as a laser does. This phenomenon is called a black hole laser, and the observations strengthen the belief that real black holes would emit Hawking radiation.

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