Historical research guide · 0742 · 2019

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

In one minute

Muñoz de Nova, Golubkov, Kolobov and Steinhauer made a sonic horizon in a flowing condensate of about 8,000 rubidium atoms. Across 7,400 runs they measured how density fluctuations on opposite sides of the horizon varied together. Those correlations identify outward-moving Hawking phonons and their inward-moving partner modes. From the measured flow and sound-speed gradients, the team calculated a Hawking temperature of 0.35 nanokelvin. They then extracted a correlation spectrum and found it followed the thermal curve at that temperature without fitting an extra parameter. The matched spectrum tests a quantitative prediction for waves in this analogue system. It does not measure a gravitational black hole’s temperature; the condensate supplies an experimentally accessible horizon for sound.

Connection

The group separately determined the condensate’s flow and sound-speed profile and measured phonon correlations across its sonic horizon. The first gave a predicted 0.35-nanokelvin Hawking temperature; the second yielded a spectrum that followed that thermal curve without an extra fit parameter. This tests the temperature relation in an analogue fluid while leaving gravitational black holes as a separate physical case.

From the source

The 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
The 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
The Hawking temperature predicted from the measured density and sound-speed profiles, averaged over successive five-run groups, 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
The 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
The measured correlations between Hawking phonons and their partners have the predicted magnitude. The authors infer from this that there is apparently no analogue firewall at this sonic horizon that reduces those correlations; they relate the thermal spectrum to the information paradox for gravitational black holes.Abstract, final sentence; concluding paragraph, citing reference 44
The 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

The way in

https://doi.org/10.1038/s41586-019-1241-0

Historical source-access record. This guide links to the original; it does not reproduce the paper text.

LICENCE. 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 fluid