Scientific Method / Science & Exploration

Exploring black holes

The sonic analogue of a black hole maybe realizable in the lab.

One of the problems with astrophysics is that much of our information relies on indirect observations combined with modeling based on basic theory.  Between the simplifications that make modeling possible and the inferential nature of the observations, cosmology can be a rather frustrating field.  This becomes even more true when the observations relate to black holes, where our knowledge is constrained by the event horizon.  Black holes only emit Hawking radiation, and this radiation is thermal in nature, conveying no information about the interior.  This is paradoxical since nature on the microscopic scale (as black holes often are) is quantum in nature and quantum mechanics preserves information (even though measurement may cause some information to become unknowable).  Thus we have an object in nature which, from the scale, should be quantum in nature, however it appears to violate our basic understanding of quantum mechanics.  So what should physicists do when they can't poke a black hole to see what it does?

Well, a recent suggestion has been to investigate analogues in the lab.  Initial investigations showed that the analogues themselves were experimentally inaccessible and interest in the topic died away.  However, with the realization of Bose Einstein condensates, Fermi degenerate gases, and the absurd level of control exhibited over their properties, these experiments are no longer the stuff of science fiction.  The basic experiment for Fermions would consist of cooling the atoms until they form a very cold gas.  This gas is then accelerated to a speed in excess of the speed of sound within the gas, which is quite slow since the gas is dilute.  The gas is then illuminated by a laser from the side (like photographing a bullet in flight) which creates a barrier through which the atoms can tunnel.  The atoms that successfully tunnel will set up sound vibrations that have the same spectral properties as Hawking radiation from black holes.  The difference being with a black hole all we can observe is the Hawking radiation, but in the lab we have exact control over the internal properties of the black hole and can tune these parameters to observe the effect on the sonic Hawking radiation.  From the analogue we may then be able to infer something about the internal properties of real black holes.

The paper from which I obtained this information is not that interesting for the general reader.  However, what is very cool is the unique approach taken to understanding some of the most inaccessible physical systems known to man.  I hope that even if the analogy is shown to be faulty that we learn a great deal about quantum physics in doing these experiments.

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