Quantum collapse of an atomic cluster.
In a supernova, immense gravitational forces drive a star's collapse, which in turn triggers the explosive disintegration of the star. Recent theoretical work suggests that a sufficiently large number of lithium atoms congregated in a state of matter known as a Bose-Einstein condensate can likewise collapse into a denser state, then explode.
Randall G. Hulet and his coworkers at Rice University in Houston described indirect evidence of this atomic phenomenon this week at an American Physical Society meeting in Washington, D.C.
At room temperature, atoms of a gas move about independently, bouncing around and traveling in random directions. At extremely low temperatures, however, atoms of such elements as lithium, sodium, and rubidium collectively enter the same quantum state and act as a single, coherent unit, creating a Bose-Einstein condensate (SN: 2/8/97, p. 87).
In such a state, rubidium and sodium atoms weakly repel each other, whereas lithium atoms attract each other (SN: 9/9/95, p. 164). "Lithium atoms tend to clump together," Hulet says. "They want to collapse into a denser state."
Nonetheless, a cloud of lithium atoms confined in a magnetic trap can remain stable because of a quantum effect that generates enough internal pressure to stave off collapse. Quantum theory suggests that a trapped clump can contain a maximum of about 1,400 atoms.
Recent experiments by the Rice group demonstrate the existence of such a limit. "We never see more than about 1,000 atoms in the condensate," Hulet says.
"If you put in more atoms than that, the condensate is unstable," he notes. In this case, the extra atoms could simply leak away, leaving the clump to return to its stable state. Alternatively, the additional atoms could trigger the rapid collapse of the entire condensate.
"Here, we're talking about the possibility of observing all the atoms in this coherent entity collapsing together at once," Hulet says. "This is similar to what happens in a supernova."
Theoreticians predict that such an outcome is possible because of a process called macroscopic quantum tunneling, which makes possible a wholesale transformation of the condensate from a low-density gas to a higher-density state.
During the collapse, the atoms would collide and stick together as molecules. Molecule formation releases energy, and the collapsing condensate would quickly heat up and blow apart, shooting atoms out of the magnetic trap.
The observation that lithium condensates never contain more than about 1,000 atoms indicates that such a phenomenon may be occurring, Hulet says. The Rice team is now trying to observe directly the collapse of a lithium condensate and its aftermath.
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|Title Annotation:||chilled lithium atoms form Bose-Einstein condensate|
|Article Type:||Brief Article|
|Date:||Apr 26, 1997|
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