I've received the odd email and comment or two about a news headline claiming that some parts of the Large Hadron Collider are now "colder than space". Surely, these emails ask, space has no temperature, so the headline must be wrong. Well, it's complicated. It depends on how you define temperature. It's really just an average measure of how much energy matter has. All the molecules in an object are wiggling and jiggling, and the amount of that motion is what we call temperature. The more energy they have, the more they move, and the higher the temperature. So, if empty space has no matter in it, it's a vacuum, and therefore has no temperature. Right? Well, it's complicated. Sure, if there's no there there, then there is no temperature. Of course, space isn't really empty, it's only mostly empty. Near the Earth, space actually has lots of subatomic particles per cubic centimeter. Even between galaxies, there are one or two particles per cc. But still, space is so empty that these hardly count when talking temperature. Right? Well, it's complicated. When the Universe formed, it was hot and dense. As it expanded, it cooled off. At first, your average photon -- a particle of light -- had lots of energy. But fighting the expansion of space itself takes its toll, and the photons lost energy. Flash forward 13.7 billion years, and today we see these photons have lost a lot of energy. What used to be a raging fireball from the moment the Universe formed is now a chilled brew, with the photons in the microwave part of the spectrum; very low energy indeed. You can convert that energy into a kind of temperature, and the number you get is about 2.7 Kelvin (-270 Celsius, or about -450 F).
That means that even if space is totally empty of matter, there are still photons at that energy flowing through it. If you took a mini marshmallow (and why not) and stuck it in deepest space, it would drop in temperature until it got to 2.7 Kelvin. It wouldn't get any colder than that, because those photons would warm it up to 2.7 K. So in a sense space does have a temperature. It may not satisfy purists, but realistically it's not too bad to think of it this way. So when parts of the LHC are cooled below 2.7 K -- and you can do this locally by pumping away any errant heat, but it takes a lot of energy to do -- it is in fact colder than space. So long story short, I don't have any real issues with the headline. And hey, it gave me a chance to explain some things, and to use the Mini Marshmallow of Science. So I'm glad it came up.