Fluid art by Vera de Gernier. |

Hawking notably was first to derive that black holes are not entirely black, but must emit what is now called “Hawking radiation”. The temperature of this radiation is inversely proportional to the mass of the black hole, a relation that has not been experimentally confirmed, so far.

Since the known black holes out there in the universe are very massive, their temperature is too small to be measurable. For this reason, physicists have begun to test Hawking’s predictions by simulating black holes in the laboratory using superfluids, that are fluids at a few degrees above absolute zero which have almost no viscosity. If a superfluid has regions where it flows faster than the speed of sound in the fluid, then sound waves cannot escape the fast-flowing part of the fluid. This is similar to how light cannot escape from a black hole.

The resemblance between the two cases more than just a verbal analogy, as was shown first by Bill Unruh in the 1980s: The mathematics of the two situations is identical. Therefore, physicists should be able to use the superfluid to measure the properties of the radiation predicted by Hawking because his calculation applies for these fluids too.

Checking Hawking’s predictions is what Jeff Steinhauer and his group at Technion in Israel are doing. They use a cloud of about 8000 Rubidium atoms at a temperature so low that the atoms form a Bose-Einstein Condensate and become superfluid. They then use lasers to confine the cloud and to change the number density in some part of it. Changing the number density will also change the speed of sound, and hence create a “sonic horizon”.

Number density (top) and velocity (bottom) of the superfluid. The drop in the middle simulates the sonic horizon. Figure 2 from arXiv:1809.00913 |

Using this method, Steinhauer’s group already showed some years ago that, yes, the fluid black hole emits radiation and this radiation is entangled across the horizon, as Hawking predicted. They measured this by recording density fluctuations in the cloud and then demonstrated that these fluctuations on opposite sides of the horizon are correlated.

Three weeks ago, Steinhauer’s group reported results from a new experiment in which they have now measured the temperature of the fluid black hole:

**Observation of thermal Hawking radiation at the Hawking temperature in an analogue black hole**

Juan Ramón Muñoz de Nova, Katrine Golubkov, Victor I. Kolobov, Jeff Steinhauer

arXiv:1809.00913 [gr-qc]

The authors also point out in the paper that they see no evidence of a black hole firewall. A black hole firewall would have been conflict with Hawking’s prediction according to which radiation from the black hole does not carry information.

In 2012, a group of researchers from UCSB argued that preserving information would necessitate a barrier of highly energetic particles – the “firewall” – at the black hole horizon. Their argument is wrong: I demonstrated that it is very well possible to preserve information without creating a firewall. The original proof contains a mistake. Nevertheless, the firewall issue has arguably attracted a lot of attention. The new experiment shows that the fluid black holes obey Hawking’s prediction, and no firewall appears.

Of course the fluid black hole does not reproduce the mathematics of real black hole entirely. Most importantly, the emission of radiation does not reduce the mass of the black hole, as it should if the radiation would carry away energy. This is the lack of “backreaction” (which this blog is named after). Note, however, that Hawking’s calculation also neglects backreaction. So for what the premises of Hawking’s calculation are concerned, fluid analogies should work fine.

The fluid analogies for black holes also differ from real black holes also because they have a different symmetry (it’s a linear system, a line basically, rather than a sphere) and they have a finite size. You may complain that’s a rather unrealistic case, and I would agree. But I think that makes them more, not less, interesting. That’s because these fluids really simulate lower-dimensional black holes in a box. And this is exactly the case for which string theorists claim they can calculate what happens using what’s known as the AdS/CFT correspondence.

Now, if the string theory calculations were correct then the information should leak out of the black hole. If you want to avoid a black hole firewall – because that hasn’t been observed – you need to break the entanglement across the horizon. But this isn’t compatible with the earlier results of Steinhauer’s group.

So, this result documents that black holes in a box do not behave like string theorists think they should. Of course the current measurement results have large uncertainties and will have to be independently reproduced before the case can be considered settled. But I have little doubt the results of the Steinhauer group will hold up. And I’ll be curious to hear what string theorists say about this.