Historical research guide · 0551 · 2008
Numerical observation of Hawking radiation from acoustic black holes in atomic Bose–Einstein condensates
Iacopo Carusotto · Serena Fagnocchi · Alessio Recati · Roberto Balbinot · Alessandro Fabbri
In one minute
Hawking showed in 1974 that a black hole should glow faintly, and the glow is far too weak for any telescope. Unruh pointed out in 1981 that any horizon does the same thing to any kind of wave, which opened the door to building one in a laboratory. Carusotto, Fagnocchi, Recati, Balbinot and Fabbri simulate that experiment in an ultracold atomic gas — a Bose–Einstein condensate flowing along a waveguide with a step in its interaction strength, so the flow goes from slower than the speed of sound to faster than it. Past that point sound cannot swim back upstream, which is exactly what a horizon does to light. Instead of counting emitted sound quanta, they compute how the gas density fluctuates at two separate places at once. Two symmetric tongues of negative correlation appear, linking points on opposite sides of the horizon. They are steady in space and time, their strength matches the black-hole prediction, and they survive a starting temperature twice the Hawking temperature.
Connection
This is chapter 5’s central move made concrete: treat the vacuum as a fluid, and a horizon becomes something you can build on a bench rather than only observe in the sky. It is also the paper that showed how to see the effect — in the correlations between two points, not in the raw emission — which is the technique later analogue-horizon experiments went on to use.
From the source
Hawking emission from a sonic horizon shows up as long-range correlations in the density of the gas: the two phonons of each pair travel in opposite directions from the horizon, so their quantum correlation appears as a signal linking distant points on opposite sides of it.Sect. 1, closing paragraphs; Sect. 4, features (iii) and (iv)
In the hydrodynamic limit the computed peak of the correlation signal matches the analytic prediction of the gravitational analogy, and the inverse width of the correlation tongue tracks the surface gravity of the horizon, which is what fixes the Hawking temperature as h-bar times kappa divided by two pi k-B.Sect. 5, Eqs. (7) and (8); Fig. 5
The simulation never invokes the gravitational analogy: it is a microscopic many-body Wigner calculation of a realistic experimental system, so the emission it finds is independent evidence for the Hawking effect and answers the standing concern that the result depends on unknown short-wavelength, trans-Planckian physics.Sect. 1; Sect. 3; Sect. 8, second paragraph
The signal is robust to heat: starting from a temperature twice the expected Hawking temperature, the Hawking tongues stay clearly visible and are slightly strengthened by stimulation, while the separate feature caused by thermal phonons reflecting off the horizon has a different slope and can be told apart from them.Sect. 6, Fig. 6(a)
As a control, a flow kept everywhere below the speed of sound produces no Hawking tongues, while the transient dynamical Casimir fringes and the thermal feature persist unchanged — the correlation signature tracks the presence of the horizon and nothing else.Sect. 6, Fig. 6(b)
The measurement is small — the rescaled correlation peak is about 5 parts in a thousand — so the named route to seeing it is large statistics from a continuously reloaded atom-laser beam plus a non-destructive single-atom detector such as a high-finesse optical cavity.Sect. 7, Discussion of some experimental issues
The way in
https://doi.org/10.1088/1367-2630/10/10/103001How to cite it
Iacopo Carusotto, Serena Fagnocchi, Alessio Recati, Roberto Balbinot, Alessandro Fabbri (2008) Numerical observation of Hawking radiation from acoustic black holes in atomic Bose–Einstein condensates doi:10.1088/1367-2630/10/10/103001