Bose-Einstein condensate is a
state of matter, in the sense that
solid,
liquid,
gas and
plasma are states of
matter. Bose-Einstein condensates form from matter that has been cooled to near
absolute zero. Predicted in the
1920s by
Satyendra Nath Bose and
Albert Einstein based on Bose's work on rules for deciding when two
photons should be counted up as either identical or different. Einstein formalized and generalized these ideas and the result of theirs efforts is the so called
Bose Einstein statistics. This is the description of the statistics of identical particles
that don't mind sharing an
quantum energy level with each other (as opposed
to
Fermi Dirac statistics which describes identical particles of which you can only put one in each energy level). One of the results that one can derive from this statistics is the existense of
stimulated emission of photons, which is the effect that is used in creating
lasers.
Einstein also applied the statistics to
atoms instead of photons, and discovered that at a certain very low temperature, all of the atoms tend to drop into the lowest accessible energy level.
Velocity-distribution data that confirm the discovery of a new state of matter, the Bose-Einstein condensate. The two right-most images, corresponding to lower temperatures, show multiple atoms coalescing into a single macroscopic quantum state.(Larger Version)
The effect can be understood in broad outline by considering the Heisenberg Uncertainty Principle which states, roughly, that it is impossible to know both a particle's velocity and a particle's position simultaneously with certainty. When a group of atoms is cooled to a low enough temperature, however, their velocities become very certain; they must be moving very slowly, or stated more technically they must have low quantum energy levels. This causes their positions to "smear out," effectively causing the individual atoms to overlap each other. In a Bose-Einstein condensate, the many overlapping atoms can be considered to be a single super-atom, with all of its constituent atoms sharing a single quantum state.
A Bose-Einstein condensate was not actually created in a lab until June 5, 1995, when Eric Cornell[?] and Carl Wieman used a combination of laser cooling (a technique the invention of which won Steven Chu[?], Claude Cohen-Tannoudji[?], and William D. Phillips[?] the 1997 Nobel Prize for Physics) and magnetic evaporative cooling[?] to cool a cloud of approximately 2000 rubidium atoms to 20 billionths of a degree above absolute zero, the lowest temperature ever achieved at that time. This was cold enough to form a Bose-Einstein condensate. (Cornell, Wieman and Wolfgang Ketterle[?] won the 2001 Nobel Prize in Physics for this achievement.)
Bose-Einstein condensates are extremely fragile. The slightest interaction with the outside world can be enough to warm them past the condensation threshold, causing them to break back down into individual atoms again; it will likely be some time before any practical applications are developed for them. However, several interesting properties have already been observed in experiments. Bose-Einstein condensates have extremely high optical densities, resulting in extremely low measured speed of light with in them; some condensates have slowed beams of light down to mere meters per second, slower than a human can move on a bicycle. A rotating Bose-Einstein condensate could be used as a model black hole, allowing light to enter but not to escape. Condensates could also be used to "freeze" pulses of light, to be released again when the condensate breaks down. Research in this field is still young and ongoing.
See also
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