system
Given the following post, respond as if you're commenting on Reddit:
human
Is There a Limit To How Hot Something Can Get?

I know that there is and absolute zero, but is there an absolute hot? A point where something is so hot that it can't get any hotter? Also what is the hottest thing in the universe?
gpt
It depends on whether you are speaking of a small finite system or not. For a small finite system (like a handful of atoms), you can end up with negative temperatures which are hotter than any positive temperature, and so there is an 'absolute hot' which happens to be 0 (or arbitrarily close to it at least, just like absolute zero). This is because temperature is defined as the inverse ratio of the change in entropy of a system over the change in energy. If you end up in a situation where adding energy lowers entropy, as can happen in finite systems, then the system will have negative temperature and then be hotter than any positive temperature, in the sense that heat will naturally flow from the system to any other system with positive temperature.

Outside of 'small' finite systems, the question is a little more difficult to answer. There is a very high (positive) temperature called the Planck temperature, after which energies are so high that we need a theory of quantum gravity just to say what is happening. This isn't an 'absolute hot' though, it is just saying that we don't know what happens when you raise the temperature beyond it. It could be possible that you can still achieve negative temperatures in a large finite system, but we don't really know what that would mean anymore.