On a really hot day somebody is bound to say “Well, it can’t get much hotter than this!” That could be true as far as the weather is concerned, but it raises the interesting question “How hot can it get?”

Adding heat makes atoms and molecules move faster. So the question of how hot something can get, is really asking whether there’s a limit to how fast its components can move as we heat them up.

When something gets hotter, it eventually turns into a gas. The hotter the gas becomes, the faster the atoms and molecules zip around. Eventually even electrons break loose from atoms, and we get something called a plasma. That’s what’s inside stars. Star plasma has a temperature in the tens of millions of degrees.

But is there anything to stop a plasma from getting hotter and hotter, making its fragments move faster and faster? Yes! It’s that universal speed limit – the speed of light that sets the upper limit.

Even electrons in a plasma follow the rule that Einstein told us about – that close to the speed of light, particles get heavier! If they could reach the speed of light they would become infinitely heavy. So that’s what sets the ultimate theoretical temperature limit. It would correspond to a temperature of around one hundred and forty, with thirty zeros after it!

So next time someone says “It can’t get much hotter than this” tell them how hot it really can get, at least in principle!