Question: Is there a temperature in a vacuum

Steve Marsden answered on 14 Mar 2016:

That’s a really challenging question. It feels like it should just have a yes or no answer, but it’s far more subtle than that. It really depends on what you mean by “vacuum”.

If you have a system containing lots of atoms, the temperature is a measure of how much they are bouncing off of each other. If you give the atoms more energy, they move quicker and bounce off of each other more. We refer to this as a higher temperature.

So, if we don’t have any atoms, there’s no temperature right?… Well not quite. Because the particles in the system don’t have to be atoms. In space there are a large number of photons (particles of light). Many of the photons travelling through the universe were created soon after the big bang (actually about 400,000 years after it, but this is pretty soon compared to the age of the universe). The temperature of these photons is about 3 K (about -270°C). So even in the vacuum of space, there’s a temperature.

What if you block out the photons though. Let’s make a box in which there’s no atoms, and no particles of any kind… The problem is that subatomic particles are constantly popping in and out of existence for brief moments in time. These particles have some energy associated with them, so have a temperature… even though the box is as empty as possible.

So the vacuum of space has a temperature associated to it. If you were to make a complete vacuum, then there would still be particles bouncing round, giving the region a temperature. If you could create a complete vacuum, and somehow stop particles popping in and out of existence, then no, it would not have a temperature. But as far as we know, this type of vacuum is not possible.