system
Given the following post, respond as if you're commenting on Reddit:
human
I know of absolute zero at -273.15°C, but is there an absolute hot?
gpt
We don't know if there's a maximum temperature.

In certain models -- string theory is one of them -- there is a maximum temperature called the *Hagedorn temperature*.  This arises because the number of possible high energy states increases sufficiently fast that as you put more energy into the system, it gets spread out over more and more states in such a way that the temperature decreases less and less for a given amount of heat put in.  Net result is that there's a temperature that is the upper limit of the temperatures that can be reached.

I'll add that even in string theory, some people think the Hagedorn temperature might not be an actual limit, but more an indication that there is a phase transition (like when a liquid turns to gas), but that's even more speculative.