If temperature is 'The average kinetic energy of
particles' (i.e. if you measure the temperature of a cup of water it is the average of all the
water molecules in the cup), then how does one determine the temperature of a vacuum? (Division by
Asked by: Jeff Retty
One doesn't determine the temperature of a vacuum. Just
as 'nothingness' has no color, taste, smell, etc. it also
has no temperature. That is because, as you point out in
your question, there are no particles whose kinetic
energy can be measured or averaged.
Only objects within a vacuum can have a temperature, and
that temperature will depend on the balance of incoming
and outgoing radiation. Electromagnetic radiation can
travel through a vacuum, so objects in space of any
temperature above the near absolute zero (0 Kelvin = about -273
deg C.) temperature of cosmic background radiation (which is about 3 Kelvin)
will radiate energy into space. Without another source of
energy replacing that loss (a nearby Sun, for example)
the object's temperature will decrease. That is why you
read about 'the coldness of outer space'.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor
'He who finds a thought that lets us even a little deeper into the eternal mystery of nature has been granted great grace.'