The coldest temperature possible is −273·15°C, otherwise known as absolute zero: but is there a corresponding maximum possible temperature?
Seems like an innocent enough question, right? Absolute zero is 0 on the Kelvin scale, or about minus 460 F. You can’t get colder than that; it would be like trying to go south from the South Pole. Is there a corresponding maximum possible temperature?
Well, the answer, depending on which theoretical physicist you ask, is yes, no, or maybe. Huh? you ask. Yeah, that’s how I felt. And the question doesn’t just mess with the minds of physics dummies like me. Several physicists begged off of trying to answer it, referring me to colleagues. Even ones who did talk about it said things like "It’s a little bit out of my comfort zone" and "I think I’d like to ruminate over it." After I posed it to one cosmologist, there was dead silence on the other end of the line for long enough that I wondered if we had a dropped call.
Candidate temperatures range from 1017 K to 1032 K, the latter being one quadrillion times hotter than the former (although both are really quite warm).
I once asked if there was such a thing as a maximum temperature when learning about absolute zero in my A-level chemistry class; happily, it seems that it’s not as stupid a question as I assumed at the time.