In a Bose–Einstein condensate no one can hear you scream!Posted: June 18, 2009
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ok perhaps not as catchy as “in space no one can hear you scream!” but its a pretty apt description of the work of a number of condensed matter physisits who have recent created the worlds first acoustic black hole. Now I know what you are thinking … it was the same thought I had as soon as I heard about this work : acoustic black hole + coldplay = a better world for us all. In my head its like those flying flat prison things from superman … but we let out the brutal superhuman dictator and replace him with the far greater evil of coldplay
Seriously though this is some really cool stuff. As we all know in our current theory of gravity, matter and energy bend space and time around it and in turn light and matter will move along the contours of this curvature. In a black hole the density of the matter/energy doing the warping of space time is so great that light (and everything else) is unable to escape from that region. Now what you might not have known (I certainly didn’t!) is that it turns out that in a certain state of matter, known as a Bose-Einstein condensate, the equations which govern the paths of sound through the matter are the same as those which govern how light travels along the curvature of space. This throws up an pretty intriguing idea: could we create within a Bose-Einstein condensate a region from which no sound can escape? A form of coldplay prison acoustic black hole?
Well it turns out that not only is the answer a resounding “hell yeah!” but some scientists form the Israel Institute of Technology, in Haifa have done just that. They created a region within the Bose-Einstein condensate which had a supersonic flow, that is the material which makes up the condensate is moving faster than sound. Within this region little packets of sound called phonons (an analogous version of photons but for sound instead of light) can never escape this region. Its sort of like running on one of those flat escalators that you get at airports, if its moving faster than you can run you can never get off it. The border between the supersonic region and the rest of the condensate then acts like an event horizon in regular black holes.
So why, apart from the fact that it is just unbelievable that we can do this kind of thing in the first place, is this interesting? Well there is a property of black holes which has been long theorised but never observed. It turns out that with a wild and quite frankly rude disregard for their names, black holes are not entirely black. That is they do emit some light, infact they probably glow faintly ( not in anyway you could see with your eyes but more like the way that you are currently glowing with infrared radiation). So how can this be? How can a region of space from which nothing can escape actually give out light?
Well to understand it we turn from the science of the very large to the very the science of the very small: quantum mechanics. It turns out that empty space probably isnt quite as empty as we would like to think. On incredibly short time scales from the nothingness two particles can be created: a particle and its gottie wearing anti-particle twin. Normally these two particles come in to creation and then annihilate each other poping back out of existence so fast that it is imposable to ever detect them directly. Near the event horizon of a black hole however one of these particles may in its brief life venture beyond the event horizon, doomed never to return. The other particle now left alone in the cold dark universe has nothing to annihilate with and so goes about its merry way. A constant stream of these particles are thought to emanate from a black hole as whats known as Hawking radiation and so black hole appears to glow just like a solid object with a temperature which depends on the black hole mass. As there is no such thing as a free lunch though we need to conserve energy and as a result the black hole loses some mass in the proses. The emission of Hawking radiation from a black hole means that black holes should slowly evaporate away. Measuring the precises nature of the radiation that a black hole emits could give us all kinds of new exciting new insights in to how physics works at a fundamental level but unfortunately the Hawing radiation is emited at such a low level that noone has ever observed it.
The interesting thing about the acoustic black hole is that it should have a similar effect happening at its event horizon and this effect should be measurable. Instead of particles being emitted, small packets of sound should be and while this wont tell us very much about the fundamental properties of the Universe it will be the first time in any situation that this specific mechanism has been observed.