WASHINGTON, D.C.--Scientists have triggered a miniature explosion that may resemble a tiny supernova by enticing a newly created form of matter, called Bose-Einstein condensate, to collapse in the lab. The phenomenon, described here yesterday at a meeting of the American Physical Society, could be a boon to physicists probing mysterious interactions of atoms when they are suddenly crammed together.
Bose-Einstein condensates are dense collections of certain atoms cooled to a fraction of a degree above absolute zero, where their identities merge and they act as a single particle. Members of a team led by Randall Hulet of Rice University found that they were unable to load more than 1400 atoms into their condensate, created from chilled lithium trapped in a magnetic field. The researchers believe that when too many atoms are in the trap, all the lithium atoms rush together and somehow spin off from the magnetic trap. "It's like the lithium is digging a deeper and deeper hole," says Robert Dodd, a physicist at the University of Maryland. "When the hole is deep enough, they all rush in."
The lithium condensate behaves this way because lithium atoms have a peculiar property: They attract each other when close together, instead of repelling each other, as most other elements do. "They want to suck together rather than push each other apart," says team member Charles Sackett, a physicist at Rice University. Nobody is sure why lithium acts this way, says Eise Tiesinga, a physicist at the National Institute of Standards and Technology. "There's no 'why'--it just is," he says.
Until the number of atoms in the trap reaches a threshold, however, the zero-point energy--quantum-mechanical fluctuations of energy that are a consequence of the Heisenberg uncertainty principle--helps prop up the condensate. But when there are too many atoms in the trap, their attraction overwhelms the zero-point energy and the condensate collapses.
The heat generated by the collapse warms the atoms enough to muck up the condensate, says Sackett. That process of collapse and expansion resembles what happens when the core of a giant star collapses into a black hole or neutron star, triggering a supernova. Stars collapse when gravity overcomes the pressure exerted by electrons obeying the Pauli exclusion principle, a law of quantum mechanics that says that certain particles cannot be in the same place at the same time. The lithium condensate collapse "is the exact same thing," says Dodd. "You're just overcoming Heisenberg instead of Pauli."