Wednesday, 29 June 2011

Bose-Einstein Condensates

Named for the theorists Satyendra Nath Bose and Albert Einstein who predicted its existence, a Bose-Einstein condensate is an unusual state of matter that arises because of quantum mechanical effects on a collection of entities called bosons.
     Everything is either a boson or a fermion. The spin of an object determines whether it is a boson or a fermion.
     The reason why is important to differentiate between bosons and fermions is that they have vastly different quantum mechanical behavior. Identical fermions cannot occupy the same place. This is called the Pauli exclusion principle. For example, you cannot put two electrons spinning in the same direction on top of one other. It is forbidden and never happens in nature. Bosons behave in almost the opposite way. They like to overlap.
     In quantum mechanics, the position of an object is uncertain. An object has a definite probability of being at any given point in space. This probability is encoded in what-is-called a wave function. It is like a "cloud" that tells you the probability that an object has a certain location. The object is more likely to be found in denser parts of the "cloud" and is less likely to be found in the less dense parts. If a region of space has no "cloud," then there is zero chance that the object is there.
     If one concentrates a large number of identical bosons in a small region, then it is possible for their wave functions to overlap so much that the bosons loose their identity. If a dozen clouds are well separated in the sky, then it is easy to determine where each one is. But if you look up and the 12 clouds have already joined to form one large cloud, it is no longer possible to tell which part comes from the original 12 clouds. A collection of bosons can do the same thing. When this happens, a Bose-Einstein condensate forms. This exotic state of matter is only possible at low temperatures. At high temperatures, the individual bosons not only have small wave functions but they move rapidly and fly apart. In summary, in a Bose-Einstein condensate, the individual bosons become indistinguishable.
     Two examples of materials containing Bose-Einstein condensates are superconductors and superfluids. Superconductors conduct electricity with virtually zero electrical resistance: Once a current is started, it flows indefinitely. The liquid in a superfluid also flows forever. In effect, there is no friction. The nucleons in a neutron star are believed to form a superfluid. Scientists have been able to make superfluids in the laboratory by cooling materials to very low temperatures. Helium-4 below a temperature of 2.17 Kelvins is an example.



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