Open Quantum Dynamics of Mesoscopic Bose-Einstein ... - Physics

Chapter 1Condensation ‘without forces’The theory is pretty, but is there some truth in it?-A.Einstein 1 .1.1 An historical introductionWhat is now known as Bose-Einstein condensation was predicted by Albert Einstein in1924[139]. Einstein’s contribution to statistical mechanics was but a brief interlude inhis life as a physicist, but this alone, without relativity or his contributions to the earlyquantum theory would have guaranteed him fame, at least in scientific circles.Beforeany quantum theory had been fully developed, Einstein was investigating the quantumstatistical properties of massive particles.He generalised the work of Bose, who hadonly dealt with the statistical properties of photons, and in the process predicted a new,quantum state of matter.In deriving an equilibrium distribution of radiation, Bose had treated light as a quantumgas 2 of photons[14]. Einstein reasoned that if quanta of light could be treated in thisway, then so could particles of matter, and derived the properties of a quantum gas of‘molecules’. This was the beginning of quantum statistical mechanics.Among Einstein’s predictions was that below a certain temperature, part of the gasmust condense into the quantum state of lowest energy, thus creating a macroscopic occupationof a single quantum state[54]. For some years, the existence of this phenomenonwas treated with some skepticism, with Einstein himself wondering if the effect could be1 From a letter to Ehrenfest in December 1924, quoted on page 432 of [139].2 A quantum gas in the sense that its constituent particles are indistinguishable.14