Statistical-physics textbooks typically present Bose–Einstein condensation in a way that’s similar to Albert Einstein’s original description. In a gas of identical bosons, they say, the statistical weighting of each state is such that the total occupation of the excited states is capped at a critical value Nc, which depends on temperature. If the temperature is decreased, or the particle number is increased, so that the number of identical particles exceeds Nc, the thermally excited portion of the system becomes saturated, and all additional particles must occupy the ground state.

The textbook picture doesn’t include interparticle interactions, but it does assume thermal equilibrium, which cannot exist without interactions of some form. It’s no surprise, therefore, that the picture doesn’t describe real systems exactly. But how far wrong is it? Physics is full of simplified descriptions that neglect some of the complexities of real systems but are still...

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