In 1974 Stephen Hawking showed that general relativity and quantum field theory together suggested that photon pairs should be created at the event horizon of a black hole. One photon, carrying "negative energy", would fall into the black hole; the other, carrying positive energy, would be emitted as radiation, giving the black hole a well-defined temperature. The theoretical implications of this are revolutionary, as it is unclear how the statistical thermodynamic definitions of temperature and entropy could apply to a black hole. Physicists have not been able to study Hawking radiation because the temperatures of all known black holes are lower than that of the cosmic microwave background radiation. This makes their Hawking radiation effectively undetectable. However, in 1981 William Unruh of the University of British Columbia pointed out that the production of quantized sound waves – or phonons – in a Bose–Einstein condensate (BEC) could be made mathematically equivalent to the predicted photon production at the event horizon of a black hole. Since then, various researchers have used this to build analogue black holes in the laboratory. In 2014 atomic physicist Jeff Steinhauer of the Israel Institute of Technology (Technion) in Haifa replicated a particular type of hypothetical black hole with two horizons in a BEC. He showed that, if radiation was excited, it would bounce between the two horizons, amplifying itself continuously to produce a type of laser. It was by no means certain that such a phenomenon could occur in a real black hole, however, as this would require radiation to travel faster than the speed of light between the horizons. In the new research, Steinhauer has extended his model to cover the general case of a black hole. He swept a potential-energy step along a flowing BEC of rubidium-87 atoms. On one side, the flow was slower than the speed of sound in the condensate – allowing phonons to flow against the condensate. As the condensate travelled over the potential step, however, its speed became supersonic thus preventing phonons from travelling against the flow. Steinhauer measured the spectrum of phonons in the condensate that are created by quantum density fluctuations at near-zero temperature. These are analogous to the photons created by fluctuations in the quantum vacuum (i.e. Hawking radiation) in a real black hole. The spectrum matched Hawking's prediction. As Steinhauer explains, the measurement reported here verifies Hawking’s calculation, which is viewed as a milestone in the quest for quantum gravity. http://physicsworld.com/cws/article...king-radiation-spotted-in-analogue-black-hole
Great stuff Plasma and again an example of science, particularly cosmology, going from strength to strength. In the couple of Hawking Radiation threads we have had on this forum, I see it as a reasonably logical scenario and am happy that now that the whole concept appears gaining in credibility and certainty.
http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3863.html Observation of quantum Hawking radiation and its entanglement in an analogue black hole Jeff Steinhauer Abstract We observe spontaneous Hawking radiation, stimulated by quantum vacuum fluctuations, emanating from an analogue black hole in an atomic Bose–Einstein condensate. Correlations are observed between the Hawking particles outside the black hole and the partner particles inside. These correlations indicate an approximately thermal distribution of Hawking radiation. We find that the high-energy pairs are entangled, while the low-energy pairs are not, within the reasonable assumption that excitations with different frequencies are not correlated. The entanglement verifies the quantum nature of the Hawking radiation. The results are consistent with a driven oscillation experiment and a numerical simulation.
