Observation of thermal Hawking radiation and its temperature in an analogue black hole
Juan Ramón Muñoz de Nova · Katrine Golubkov · Victor I. Kolobov · Jeff Steinhauer
Abstract and summary · read the original at the source
In one page
Jeff Steinhauer’s group at the Technion built a black hole out of flowing atoms and then took its temperature. About 8,000 rubidium atoms are chilled into a Bose-Einstein condensate — a cloud that behaves as one quantum wave — and driven over a sharp step of laser light, so that downstream the atoms flow faster than their own speed of sound. A sound wave downstream can never climb back out, which is exactly what a horizon is. Hawking predicted that such a horizon glows, and that the glow is thermal at a temperature fixed by the surface gravity. Muñoz de Nova, Golubkov, Kolobov and Steinhauer measured the correlations between each escaping wave and the partner left behind inside, over 7,400 repetitions of the experiment, and the spectrum they recovered matches the thermal prediction at 0.35 nanokelvin with no free parameters. The partner inside carries negative energy exactly as the theory says, and the correlations are strong enough that this horizon has no analogue firewall.
Why it matters hereChapter 5 reads the vacuum as a quantum fluid whose flow carries a metric, and this is the measurement that puts a number on that reading: the temperature Hawking derived for a spacetime horizon, recovered from a fluid horizon, with no parameter left free to tune. Chapter 2 gains a second demonstration that vacuum fluctuations become real particles when the medium is driven, and chapter 4 gains the cleanest laboratory statement that the metric felt by waves is a property of the medium carrying them. The group’s 2016 paper, which first caught the radiation and showed the pairs entangled, is on this site at /library/stm-a3449e5f7e.
What it claims
01The correlation spectrum of the Hawking radiation emitted by the analogue black hole is thermal at the Hawking temperature implied by the analogue surface gravity, and the measured spectrum agrees with the predicted thermal curve with no free parameters.Abstract; Figure 4b, measured curve against the dashed thermal prediction
Published and peer-reviewed02The horizon is a waterfall potential swept through a Bose-Einstein condensate of 8,000 rubidium-87 atoms: the oscillating-horizon measurement of the dispersion relations gives a sound speed of 0.52 mm per second against a flow of 0.23 mm per second outside the horizon, and a sound speed of 0.31 mm per second against a flow of 0.90 mm per second inside it — subsonic outside, supersonic inside.Section describing the condensate and the step potential; Bogoliubov fits to Figures 1b and 1c
Published and peer-reviewed03The Hawking temperature predicted from the measured density and sound-speed profiles, averaged along the horizon, is 0.35 nanokelvin, equal to 0.12 times m c-out squared in units of Boltzmann’s constant — low enough that the radiation sits in the linear part of the dispersion relation, where the analogue of Hawking’s derivation applies.Equation 1 and the estimate that it holds for k-B T-H not more than about 0.14 m c-out squared; Figure 2c
Published and peer-reviewed04The radiation inside the black hole is composed of negative-energy partner modes only: the measured correlation pattern lies along the Hawking-and-partner pairs identified by the driven oscillation experiment, and no correlations appear along the Hawking-and-copropagating pairs.Abstract; Figure 4a, green circles carrying the correlations and red circles carrying none
Published and peer-reviewed05The correlations between the Hawking particles and their partners are of the predicted magnitude, so there is no analogue firewall at this horizon that would serve to reduce them — the thermality of the spectrum being the root of the information paradox for real black holes.Abstract, final sentence; concluding paragraph, citing reference 44
What to watch06The extraction of the Hawking-and-partner correlations from the density-density correlation function rests on one stated assumption, that modes of different frequencies are uncorrelated; and the two highest points of the negative-energy partner branch depart from the Bogoliubov form, which the authors attribute to off-resonant stimulation of excitations near the ultraviolet cutoff.Paragraph following Equation 2; discussion of Figure 1c and 1d
What to watch
Read it · abstract
Abstract
We measure the correlation spectrum of the Hawking radiation emitted by an analogue black hole and find it to be thermal at the Hawking temperature implied by the analogue surface gravity. The Hawking radiation is in the regime of linear dispersion, in analogy with a real black hole. Furthermore, the radiation inside of the black hole is seen to be composed of negative-energy partners only. This work confirms the prediction of Hawking’s theory regarding the value of the Hawking temperature, as well as the thermality of the spectrum. The thermality of Hawking radiation is the root of the information paradox. The correlations between the Hawking and partner particles imply that the analogue black hole has no analogue firewall.
Juan Ramón Muñoz de Nova, Katrine Golubkov, Victor I. Kolobov and Jeff Steinhauer, Department of Physics, Technion — Israel Institute of Technology, Haifa. Published as Nature 569, 688–691 (2019); preprint arXiv:1809.00913.
(Abstract only — see the rights note above for why the full text is not reproduced here. The complete paper, with the four figures carrying the dispersion relations, the density and sound-speed profiles, the correlation function and the Hawking spectrum, is at the source. The group’s 2016 observation of the radiation and its entanglement is on this site at /library/stm-a3449e5f7e.)
The way in
https://doi.org/10.1038/s41586-019-1241-0LICENCE. Published as Nature 569, 688–691 (2019); the Crossref record carries only Springer Nature’s text-and-data-mining terms, and the preprint, arXiv:1809.00913 submitted 4 September 2018 under the title ’Observation of thermal Hawking radiation at the Hawking temperature in an analogue black hole’, carries the arXiv.org perpetual non-exclusive distribution licence rather than a Creative Commons licence. No Creative Commons statement appears in the text or on the arXiv record, so this page holds the summary, the claims and the authors’ own abstract and sends the reader to the source. The claims below are read from the preprint text. The group’s 2016 paper, the first observation of the radiation and its entanglement, has its own sheet at /library/stm-a3449e5f7e.
How to cite it
Juan Ramón Muñoz de Nova, Katrine Golubkov, Victor I. Kolobov, Jeff Steinhauer (2019) Observation of thermal Hawking radiation and its temperature in an analogue black hole. doi:10.1038/s41586-019-1241-0
Where it sits in the curriculum
The vacuum as a quantum fluidWhat the vacuum isThe metric, warp drives and wormholes