Bose-Einstein condensation

In 1924 Bose introduced a counting rule for the states of a gas of photons which explained Planck’s law for thermal radiation at one stroke. Einstein not only recognised the importance of this idea but immediately applied it to a more conventional gas like helium. In this case, unlike that of radiation, the number of particles is held fixed. He derived the stunning conclusion that a finite fraction of the particles could settle in the lowest energy state even above absolute zero. These ideas had to wait fourteen years for their first application. Seventy years later they are being used in atomic physics laboratories all over the world.