Liquid Bose-Einstein condensate found
(This story appeared on page 95 of the
Feb. 7, 1998, Science News.)
By Sid Perkins
Science News
A new analysis of data obtained 8 years ago confirms a decades-old suspicion that a measurable fraction of the atoms in liquid helium fall into the peculiar quantum state known as a Bose-Einstein condensate.
Such a condensate forms when elementary particles and atoms occupy the same quantum state. Several teams of researchers have shown recently that low-density gases can form Bose-Einstein condensates (SN: 5/25/96, p. 327; SN: 7/15/95, p. 36).
Scientists discovered in 1938 that liquid helium-4, which has two protons and two neutrons, becomes a superfluid when cooled sufficiently - flowing without any resistance or viscosity. They immediately suspected that some proportion of the atoms might form a Bose-Einstein condensate, but direct evidence has been hard to find.
In a 1990 experiment, Adrian F.G. Wyatt of the University of Exeter in England and his colleagues sent phonons - packets of vibrational energy analogous to photons of light - in specific directions through a pool of liquid helium-4.
The new analysis of that study shows that all of the helium atoms knocked from the liquid by the phonons came from the same quantum state. Wyatt reports his results in the Jan. 1 NATURE.
"It's clear and unambiguous that there's a [Bose-Einstein] condensate there," says physicist Allan Griffin of the University of Toronto, "but you can't tell from the data the actual fraction of helium atoms that are in that state." Upcoming experiments by Wyatt may answer that question, Griffin adds.
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Copyright 1998 by Science Service.
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