One team used one laser to confine the BEC to a narrow tube, and another to accelerate some of it faster than the speed of sound. This fast flow created two horizons: an “outer” one at the point where the flow became supersonic, and an “inner” one further on where the flow slowed down again.
The Hawking effect comes from quantum noise at the horizon, says William Unruh at the University of British Columbia in Canada, one of the first to propose fluid-based black hole analogues. The horizons create pairs of particles of sound, or phonons. One phonon escapes the horizon, and the other is trapped inside it.
A single phonon is too weak to observe, but the phonons inside the black hole bounce back and forth between the inner and outer horizons, triggering the creation of more Hawking phonons each time, much like a laser amplifies light. Physicists call this effect a black hole laser.
“The Hawking radiation exponentially grows, it self-amplifies,” Steinhauer says. “That allows me to observe it, because the amplitude has grown.” In the future he hopes to improve his detectors to sense radiation from a single horizon, which could help determine whether the pairs of phonons are entangled – another predicted feature of real black holes that may have fiery consequences.
Self-amplifying Hawking radiation. The density-density correlation pattern is shown.
In conclusion, we have observed self-amplifying Hawking radiation. The emitted Hawking radiation is clearly visible in the density-density correlation function. The lasing mode between the horizons is clear in both the average image as well as the correlation function. The negative energy particles are seen to lase with exponentially increasing amplitude. The frequency of the lasing mode is found to be about 0.3 of the Hawking temperature. This is reasonable in the sense that the horizon should only mix modes with frequency less than the Hawking temperature. This work suggests a method for probing the inside of a black hole. Specifically, if strong laser-like Hawking radiation emanating from an astrophysical black hole were observed, it would implythe existence of an inner horizon. Even more interestingly, it would imply a superluminal dispersion relation.The analog black hole laser is achieved in a weakly trapped, low density, elongated Bose-Einstein condensate with very low temperature, which is floating on a magnetic field gradient. The sharp black hole horizon is created by high-resolution optics. This configuration could be used for additional analog gravity experiments, such as simulating the expansion of the early universe.
The black hole lasing phenomenon and the experimental technique. The black hole and inner horizons are indicated by BH and IH, respectively. The Hawking radiation is indicated by HR and the negative energy partners of the Hawking radiation are indicated by P.
Abstract – Observation of self-amplifying Hawking radiation in an analog black hole laser
By a combination of quantum field theory and general relativity, black holes have been predicted to emit Hawking radiation. Observation from an actual black hole is, however, probably extremely difficult, so attention has turned to analogue systems in the search for such radiation. Here, we create a narrow, low density, very low temperature atomic Bose–Einstein condensate, containing an analogue black-hole horizon and an inner horizon, as in a charged black hole. We report the observation of Hawking radiation emitted by this black-hole analogue, which is the output of the black-hole laser formed between the horizons. We also observe the exponential growth of a standing wave between the horizons, which results from interference between the negative-energy partners of the Hawking radiation and the negative-energy particles reflected from the inner horizon. We thus observe self-amplifying Hawking radiation.
Abstract – Ultracold atoms: A black-hole laser
Astrophysical observations of Hawking radiation may be out of reach, but evidence for the self-amplification of Hawking radiation has now been observed in a sonic analogue of a black hole.
SOURCES – Arxiv, Nature Physics, New Scientist
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