When I think about the structure of an atom and its tightly bound subatomic particles, it is a different regime than the 'world of molecules' and their motion. The early theory of light emission (Bohr, etc) described the emission of light as a phenomenon associated with the transition of an electron from a high energy state to a lower energy state. And the frequency of such light is dependent on the specific energy of the transition. Now when I think of heat, I imagine it to be a jiggling of atoms and collisions of atoms. Typically this is something I associate with a solid although certain liquids and gases can be made to glow but usually not with heat.Rather electricity.So how is it than simply jiggling atoms can send their electrons into high energy states? Certainly it's a question of degree (pun?) Certainly, Enough collision energy can get nuclei to misbehave(fission, fusion). But then I think of the quantized behavior of the inverse process- - how much light is needed to eject an electron (photoelectric effect? The ultraviolet catastrophe paradox was resolved with the assertion that no matter how much light you shined on an object, you could not get electrons to leave those atoms unless the light had a high enough frequency (energy per photon). But I guess I'm asking why light emission caused by heat is not quantized in the same way as electron emission caused by light? Is not the same quantum rule in play? You might say 'how do you quantize heat' in the same way as light' - I do not have the facility with quantum mechanics to construct a reasonable model.