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July 26, 2009

The quest for a 'Dumb Hole' — Will the first Hawking radiation be not seen but heard?


So it would appear.

Long story short: Scientists have created a sonic black hole and are now embarked on a quest for the Hawking phonon.

Here's a June 18, 2009 article from The Economist about the new new thing in cutting-edge desktop physics.


Dumb Insolence

Sound may offer a better way than light to test Stephen Hawking’s prediction that black holes emit radiation

When the Large Hadron Collider, a giant particle accelerator near Geneva, was switched on last September, the press was full of scare stories about the risk of it producing a tiny black hole that would, despite its minuscule size, quickly swallow the Earth. In fact, the first test runs could never have made such an object. And, just over a week later, the LHC broke and has not yet been repaired. But it is true that one of the things its operators would like to create, if and when they get it going again, are miniature versions of those fabled astronomical objects whose intense gravity means no light can escape them.

Among their reasons for this aspiration is a wish to examine Stephen Hawking’s famous but untested prediction that, despite their famous blackness, black holes do actually emit radiation, including light. But if they do not get the LHC running pronto, they may find themselves beaten to the prize of producing this so-called Hawking radiation by an experiment being carried out on a desktop in a laboratory in Haifa.

The desktop in question belongs to Jeff Steinhauer, a physicist at the Technion-Israel Institute of Technology. Dr Steinhauer has followed a suggestion made in 1981 by William Unruh of the University of British Columbia and created an analogue of a real black hole in the laboratory, using an extremely cold form of matter called a Bose-Einstein condensate. By studying this analogue, dubbed a “dumb hole” because it swallows sound rather than light, he and his colleagues hope to get a better understanding of the real thing—and, in particular, of Hawking radiation.

Real black holes’ light-swallowing abilities make them notoriously difficult to observe directly, though their gravitational effects on their surroundings can be seen. Some, such as those around which galaxies are believed to accrete, formed just after the beginning of the universe. A lot, though, are the result of huge stars collapsing in on themselves at the end of their lives. Whatever their origin, all black holes have an “event horizon”, within which gravity is so intense that nothing can escape.

In the 1970s, however, Dr Hawking, a physicist at Cambridge University, used quantum mechanics to predict that black holes are not, in fact, completely black. Heisenberg’s uncertainty principle, one of the central tenets of quantum mechanics, maintains that you cannot pin down all the physical properties of a particle at the same time. Among other curiosities, this leads to the startling idea (which has been proved experimentally) that a vacuum is not empty space. Instead, it is filled with pairs of “virtual” particles and their antiparticles, which pop into existence for a fraction of a second before recombining with one another and disappearing again.

But near a black hole’s event horizon things can go wrong. A virtual particle or antiparticle may be captured by the black hole’s gravity. Once within the event horizon, it cannot escape and its abandoned partner has no option but to become real. The energy needed for this “realisation” comes from the black hole itself, which thus shrinks and eventually evaporates. Meanwhile, the newly real particles decay, giving off (among other things) light, X-rays and gamma rays. Though this radiation is too weak to be detected by telescopes, it would be detectable if the LHC did make a tiny black hole. Indeed, any such hole would evaporate so fast that it would not have time to cause trouble.

A little while after Dr Hawking worked all this out, Dr Unruh pointed out that if you have a body of fluid which contains a region that is flowing supersonically, sound waves would be trapped inside that region. In other words, the boundary between the supersonic and subsonic regions would do for sound waves what the event horizon does for light.

What makes this interesting is that, just as light can be analysed as either waves or particles, the same is true of sound in some circumstances. And particles of sound, known as phonons, are just as susceptible to Heisenberg’s uncertainty principle as particles of light (photons)—or, indeed, any other fundamental particle. The upshot is that pairs of virtual phonons are appearing and disappearing all the time, and if one gets trapped inside a supersonic boundary, the other should be released into the wider world to make a noise.

Dr Steinhauer and his colleagues have not yet detected sonic Hawking radiation, but they have taken the first step: they have produced a dumb hole. Their tool, the Bose-Einstein condensate, is a superchilled soup of matter that can be created only when the temperature is near absolute zero. At this temperature, atoms almost stop moving. As a consequence, the wavelengths of the chilled atoms become very large (atoms, like everything else, are both particles and waves). That causes the waves to overlap and creates a kind of collective atom—as predicted by Satyendra Bose and Albert Einstein in 1924.

Since they were first made in 1995, Bose-Einstein condensates have become commonplace as experimental tools. (They are particularly valued for their ability to slow the speed of light all the way down to zero.) Dr Steinhauer and his colleagues created a condensate out of a gas of rubidium atoms held in a magnetic trap. They then compressed and decompressed the gas, which generated sound waves. They took photographs and used them to calculate both the speed of flow and the speed of sound within the condensate. Their data showed that the speed of part of the condensate exceeded the speed of sound, and they had therefore made an acoustic event horizon.

The next step is to look for Hawking phonons. That will mean making new apparatus, which could take a while. Nevertheless, with the LHC not scheduled to re-open for testing until the autumn—and, even after that, not guaranteed to produce any black holes—there is a good chance that the first Hawking radiation observed will be of a sort that even Dr Hawking never conceived of in his wildest dreams.


The Steinhauer group's paper appeared in the March 2009 issue of Nature Physics; the abstract follows.


Evidence for an oscillating soliton/vortex ring by density engineering of a Bose-Einstein condensate

When two Bose–Einstein condensates collide with high collisional energy, the celebrated matter-wave interference pattern appears. For lower collisional energies, the repulsive interaction energy becomes significant, and the interference pattern evolves into an array of grey solitons. But the lowest collisional energies, producing a single pair of solitons, have not been probed so far. Here, we report on experiments using density engineering on the healing length scale, to produce such a pair of solitons. We see evidence that the solitons evolve periodically between vortex rings and solitons. The stable, periodic evolution is in sharp contrast to the behaviour seen in previous experiments, in which the solitons decay irreversibly into vortex rings through the so-called snake instability. The evolution can be understood in terms of conservation of mass and energy in a narrow condensate.

July 26, 2009 at 04:01 PM | Permalink


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