Concepts and Misconceptions In Physics - Can We Define The Temperature Of Vacuum?
A perfect vacuum contains neither matter nor radiation. Even if we consider that there are some particles at negligible pressure then these don’t perform any activities. As temperature is the measure of average translational kinetic energy due to random motion of the particles,therefore according to this definition, the temperature of the vacuum must be zero kelvin. But according to the third law of thermodynamics,Zero kelvin temperature is not possible. So finally we conclude that either there is no perfect vacuum or if it is then its temperature will be zero.