Originally Posted by

**exchemist**
Originally Posted by

**billvon**
Originally Posted by

**mastermind2007**
What temperature does vacuum have?

None. However you can measure the temperature of an object IN that vacuum. Such a measurement would depend on energy in and out of the object (via EM radiation, gravitational tidal forces etc)

While I agree that, by the kinetic theory definition of temperature a vacuum has no temperature, due to having no matter in it with a Boltzmann energy distribution, I would have thought that by the broader thermodynamic definition, it

** does** have a temperature of absolute zero.

If you were to introduce a body with a temperature >0K into this vacuum, it will radiate heat and its temperature will drop asymptotically towards absolute zero, will it not? So it seems to me that inasmuch as a temperature gradient is that which determines the direction of the spontaneous flow of heat, the vacuum does indeed behave as if it is at absolute zero.

What do you think?

This is, indeed, an interesting point. However, I think Gere is right with his statement about degrees of freedom. There is also temperature which is "in the spins" of particles. I might be wrong, but I think, due to the fact that, in absence of any magnetic field, the chance of spin-1/2-particles to be spin-up or spin-down is 50%, leads to a statistical distribution (also with magnetic field, but then it's different). The mean value should be zero, but it's not that the whole time 50% is spin-up and 50% spin-down. There are fluctuations which lead to values different than zero and a standard deviation. Now beta (= 1/kT) is related to these spin distributions and thus the temperature.

This means that a body consisting of fermions cannot reach absolute zero due to statistical fluctuations (if I am right; maybe I'm wrong). If, of course, temperature is defined as the mean value or by means of the partition function, then it might be that one can reach absolute zero. Ah...I forgot the exact definitions!

However, we were talking about vacuum. If you go by degrees of freedom, then you don't need an outside reference. If you don't have any degrees of freedom any more, then you probably can't define a temperature, as was said beforehand. Then there would remain the question if spacetime itself has any degrees of freedom, or not. If one could quantize spacetime and then apply some notions of quantum statistics, wouldn't that be possible?