In 1924 Satyendra Nath Bose from Dacca University, in what was then India, wrote to Einstein asking for his help in getting a paper published. Bose had already sent it to the Philosophical Magazine, where it had been turned down. The paper showed how Planck's distribution law for photons could be derived from first principles. Duly impressed, Einstein translated it into German, and the paper was published in 1924 in Zeitschrift für Physik.
As a result, Einstein temporarily turned away from his dogged but unsuccessful search for a unified theory of gravitation and electromagnetism and started work on the quantum theory of radiation. Thus was born the concept of "Bose-Einstein" statistics for quanta ("bosons") carrying an integer value of intrinsic angular momentum (spin). There is no limit to the number of bosons that can simultaneously occupy any one quantum state.
Einstein noted that if the number of such particles is conserved, even totally non-interacting particles should undergo a change of behaviour at low enough temperatures - Bose-Einstein condensation. Bose had not predicted this because he was looking at photons, which can simply disappear when the energy of the system is decreased.
The condensation that Einstein predicted derives from the fact that the number of states available at very low energy becomes exceedingly small. With less and less room for all of the particles when the temperature is decreased, they accumulate (condense) in the lowest possible (ground) energy state.
From CERN COURIER
Dec 4, 2001
Bose-Einstein condensation revisited
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