# Q & A: hottest temperature?

Q:
I know there is an absolute zero where all motion stops, but is it possible to reach an "absolute hot"? In other words, is there a limit to how fast particles can move?
- Grace (age 15)
Fishers, IN, USA
A:
That's a really great question.

Yes, there is a limit (the speed of light) to how fast particles can move but that doesn't help much with your main question. The reason is that when particles start going fast you need a different formula to describe their energy than the one you use for slow motions. The energy can just get bigger and bigger without limit as the speed gets closer to the speed of light.

In some sense we can define a maximum temperature, what you call "absolute hot", but it would take an infinite amount of energy for any physical system to reach it. One way to think of temperature is that it tells us something about how likely you are to find a particle in a state with low energy vs. a state with high energy. For low temperature, the low energy state is much more likely. For high temperature, the low energy state is only a little more likely.  The limit where each state is equally likely would be what you call "absolute hot". However, since there are always some things around with more and more states at higher and higher energy, it would take an infinite energy to get to that "absolute hot".

Mike W.

(published on 02/28/2011)

## Follow-Up #1: Planck temperature

Q:
For this question, there is also another explanationAs temperature of a body increases, the wavelength of the light that it emits decreases. But wavelength cannot go beyond planks length(shortest possible distance). The temperature at which a body will emit light whose wavelength length is just about to go smaller than planks length is the maximum temperature.I don't know whether both answers are related. Just a add on.
- Deepak (age 23)
603202
A:

Yes, as temperature gets large it reaches a point where thermal fluctuations in the spacetime itself (gravity) become important. That's the Planck temperature. A description of that would require a quantum theory of gravity, but we don't yet have a good one. So it may be that the whole picture breaks down at that temperature, about 1032 K, so that we can't talk about higher T.

Mike W.

(published on 08/08/2016)