I ask this question because, I have seen many places where they say the average temperature of the universe is some 2 degrees K and this somehow relates to mass present within a given volume of space.
So if there is a relation between temperature and the mass present inside a volume, i want to know that if a cubic meter of space is said to be at absolute zero(no approximations) then can it be said that there is no mass bearing object inside this volume?
Also conversely, if in this cubic volume at absolute zero, if one were to introduce say an electron or any other mass carrying particle; could it be said that the system is no longer at absolute zero?
If so can we conclude that for any system with temperature above zero kelvin, there has to be something with mass inside the system?
Answer
A cubic metre void of anything cannot be described with a temperature. Spacetime itself does not have the property of temperature, so it would be incorrect to say such a void is at absolute zero.
However, it is not necessary that any volume not at absolute zero has mass. The property of temperature could be held by photons or other massless particles. For example, the photons from the Cosmic Microwave Background (CMB) - the 2.7K background you referred to - are described as having a temperature. These photons, when they are the only inhabitants of a given volume, would make the mean temperature of that volume the mean of their temperatures.
We can say, though, that any volume containing thermal particles (I mean particles describable as having a temperature and not specifically phonons or photons) is not at absolute zero. Thus sayeth the almighty Third Law of Thermodynamics.
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