Why does the vacuum of space have a temperature at all? What is it that has the temperature?
In: Physics
Preface: I’m not a physicist. I just watch lectures and read books on this stuff to help me sleep. I’m pretty sure that what I’m saying is accurate, though the actual application of this stuff is way over my head. Take what I say with a grain of salt. Also, this will be a bit more complex than an ELI5, but it’s the best I can do.
At least one comment here (I haven’t read them all, so there might be more) referenced the Cosmic Microwave Background (CMB) as the source of the temperature of space. This is the best answer for what we usually measure as the background temperature of space, but if you were to perfectly shield a portion of space from the CMB and create a “perfect” vacuum inside, you’d still have a very small amount of energy no matter what you do. That energy isn’t useful, though. Work can only be done when there’s an energy differential, and all of space is filled with this tiny energy density.
Quantum Field Theory (QFT) can be summarized by saying that space is permeated by “layers” of energy fields with discrete components. Computer monitors are a great analogy. What looks like fluid movement across the screen when viewed from afar is actually the energization and de-energization of individual pixels. In QFT, a particle is nothing more than an excitation of a particular quantum field, and the percieved motion of the particle is just the transfer of that energy to a different, indivisible location in the field.
Now, this is where things get really weird. According to the Heisenberg Uncertainty Principle, there is a fundamental limit to the precision with which you can measure complimentary variables. In this case, we’ll be dealing with position and momentum. The more precisely you measure a particle’s position, the more uncertain its momentum becomes (and vise versa). This isn’t a limit imposed by our technological ability to measure, but instead by the nature of reality itself.
So, how does the background temperature of space emerge from these two principles? Well, temperature is a measurement of particle vibration. But particles themselves are nothing more than vibrations in the quantum fields. So, to reach absolute zero temperature, you would have to eliminate all vibration in the quantum fields. But if you do this, you have perfectly defined their momentum and position (the latter being due to the fields themselves being made of discrete components), which the Heisenberg Uncertainty Principle does not permit. So, as you approach a perfectly defined measurement of position, the momentum would reach infinity. This gives the quantum fields a non-zero minimum temperature.
Edit: forgot to specify that absolute zero means a perfectly defined position *and* momentum
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