This question already has an answer here:
Of course I know what black-body radiation is, like everyone else who has taken a thermal or statistical physics course. But it was recently pointed out to me that one thing that is rarely taught (including to me) is what the mechanism for the radiation is.
At the particle level, how does black-body radiation arise (which radiative process is this)? In other words, where are the photons coming from?
To perhaps clarify a bit further, borrowing a comment from Kevin Driscoll on an answer:
The point of the question is that there are materials whose emission spectra are quite well described by a black-body spectrum. And yet we know that at the atomic level photon emission are caused by some quantum mechanical transitions. So, how is it that such an underlying quantum mechanical description gives rise to a black-body spectrum at everyday temperatures? What is the mechanics that causes light to be absorbed/emitted at all frequencies rather than at the discrete set we might expect from the electronic transitions in isolated atoms?
With a caveat from me:
That's a good re-statement, though I'd caution that the description need not a priori be quantum mechanical in nature - there is radiation in classical electrodynamics.