This week's headlines about the Large Hadron Collider operating at a temperature colder than that of deep space (see previous blog posts here and here) have inspired some to ask: If deep space is empty, with no atoms in it to jiggle around and produce what we define as heat, how can it be said to have any temperature at all?
Bad Astronomy tackles that question in a most interesting way involving emptiness, photons and the Mini Marshmallow of Science:
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.
Be sure to check out the comments, many of them deliciously marshmallow-related.