Coldest Man-Made Material Could Do for Atoms What Lasers Did for Light
Dec. 08, 1996
For a physicist, a good day is creating stuff that's never existed before in the history of the universe.
Eric Cornell had one of those days in June 1995, when a clever trick he devised produced the Bose-Einstein condensate, the coldest material ever made by humans. The material is expected to help physicists manipulate an ultrasmall world they've had little control over in the past.
But before it does, Cornell and his colleagues have to figure out exactly what they've got.
Experiments over the last 18 months have shown that what was a very difficult substance to generate turns out to be remarkably easy to work with _ even though it exists at temperatures within a millionth of a degree of absolute zero and lasts only 20 to 30 seconds.
``We are now able to do all sorts of experiments,'' said Wolfgang Ketterle, a physics professor at the Massachusetts Institute of Technology in Cambridge, Mass. ``We can shake it, we can cut it, we can drop it.''
The experiments are showing that some of the hopes physicists hung on the Bose-Einstein condensate when they discovered it last year may stick.
Most exciting is the idea that the condensate could do for atoms what lasers have done for light.
``It won't cure cancer, but it can be useful in the way that a laser is useful,'' said Cornell, a physics professor at the University of Colorado in Boulder.
He doesn't mean Bose-Einstein condensates will bring better compact disc players and supermarket checkout scanners. But they could improve atomic clocks and measuring instruments that work in incredibly tiny realms. And physicists have a feeling they could do much more.
``We know where to steer the boat, but the final destination is by no means clear,'' Ketterle said.
Named for the physicists Satyendra Bose and Albert Einstein, who predicted the unusual material might be possible, a Bose-Einstein condensate is essentially a group of atoms that act as a single ``superatom.''
Just about any atom with the right properties can be prodded into the Bose-Einstein state but, in practice, physicists experiment with just a handful of elements, including sodium, rubidium and chromium.
The first experiments to create the substance couldn't take pictures of it directly. But now Ketterle has found a way to shine a laser through a Bose-Einstein condensate without disturbing it. The condensate bends light a tiny bit, the same way a lens would and, from measuring that redirection, Ketterle can construct an image of what the material looks like.
Using that method, Ketterle has made stop-action photographs of Bose-Einstein condensates forming at less than a millionth of a degree above absolute zero, the point at which all atomic motions stop.
``Tremendous things have gone on in a year,'' Ketterle said.
Many of the experiments have supported the predictions made when the condensate first was created. For example, Ketterle has done an experiment where he essentially drops a chunk of Bose-Einstein condensate to produce something like an extremely short-lived atom laser.
``We try to stay low-key about it and say this is a step toward an atom laser, but some people regard it as an atom laser,'' Ketterle said.
Cornell said his early experiments with the material, which basically have been to ``thump it and look at it wiggle,'' suggest it may help physicists explain a bizarre phenomenon known as superfluidity.
Suppose, Cornell said, you were sitting in a nice cold bath of liquid helium and you pulled out the plug. The helium would start to swirl down the drain.
Now suppose you put the plug back in. The helium would keep swirling indefinitely.
``You would call that really weird,'' Cornell said.
But that's what happens to helium at extremely low temperatures, because the flowing motions in liquid helium don't gradually dissipate the way they do in water and other liquids. They just keep going like the stupid rabbit in those battery commercials.
Physicists have known since the 1930s that liquid helium can flow forever without losing heat to internal friction. But they haven't been able to understand why, partly because liquids are complex and difficult to describe with mathematical equations.
Because they're gasses, Bose-Einstein condensates are much more amenable to mathematical description. And Cornell's experiments suggest that if physicists can make a Bose-Einstein condensate just a little bit colder than the ones now possible, the substance will become superfluid.
That means physicists soon could have a much simpler venue in which to study superfluidity, Cornell said.
But some of his colleagues don't expect that venue to be a very interesting place.
The primary virtue of a Bose-Einstein condensate _ that it's a gas, and thus much more simple to study than a liquid or solid _ makes it less likely to offer insights into the deeply complex problem of superfluidity, said John Doyle of Harvard University in Cambridge, Mass.
Doyle is more interested in experiments that plumb the bizarre material's properties. In one of those, Ketterle generates two superatoms of Bose-Einstein condensate and then brings them together to see if they'll mix together or interact in some way. By and large, they don't.
``They find each other repulsive,'' Cornell said.
Physicists can't explain exactly why the two clumps react to each other the way they do, because nobody's ever seen the subatomic rules that Bose-Einstein condensates play by applied to whole substances.
``It's a fascinating experiment,'' Doyle said. ``I don't know what's going on. That's what's fascinating.''