What temperature does a vacuum have? | Notes and Queries

What temperature does a vacuum have?

Torkel, Sydney, Australia

• What temperature does a question have? The question is a category error but it does raise an interesting point about heat transference. Heat is transferred by three means: conduction, convection and radiation. A coal fire convects most of its heat up the chimney (I think this may be an example of an 80/20 split), radiates almost all the rest into the room and if you put a poker into it, it will conduct heat into it. If you put a thermometer into the middle of a bottle containing no air, you would see a rise in temperature because the heat passes through the vacuum the same way light does. You would see virtually the same effect, however, if the vacuum were simply placed between the fire and the thermometer. And if the whole room were a vacuum and you turned on the telly you'd be warmed by the fire and see the pictures, but you wouldn't be able to hear the sound.

  Charlie Hartill, London

• The temperature of an object is usually defined as the average kinetic energy of its constituent particles, and since a vacuum has no constituents its temperature would be zero under this definition. However, microwave radiation left over from the big bang does have an associated energy, leading to the universe having a "background temperature" of about 3 degrees above absolute zero (ie. -271 C).

  Mark Lewney, Cardiff

• We have two answers suggesting that vacuum does not have temperature and is impervious to it and another two which say vacuum is immensely cold. If vacuum is cold how does a vacuum flask keep drinks hot? If it is hot how does it keep things cold? The evidence of our everyday experience seems to lie with those who say vacuum is impervious to temperature.

  Quentin Langley, Woking

• Iin my experience, if you run over a ball of wool, a vacuum can become hot enough to melt through its power cord and fuse the downstairs of a semi-detached house.

  Guy, Manchester, UK

• Quentin is no doubt confused by the idea that a vacuum could be cold or hot and also the idea that vacuum flasks would some how know!! As I see it the question is basically academic, since a vacuum is an empty space it cannot conduct or convect heat, radiated heat will pass through but won't affect it since there's nothing in the vacuum to take up the energy. Vacuum flasks work because they reduce the transfer of heat by conduction and convection. They're not perfect because a) You can't make a perfect vacuum b) Radiated heat passes through undisturbed. A vacuum basically has a temperature of 0Kelvin as stated earlier, a zero energy state. Stick a thermometer in and the thermometer will heat up according to the radiated heat passing through, this however doesnt measure the vacuum temp, as thermometers work on the basis of conduction of heat from the medium you want to measure. The NBMR that permeates the universe could be said to raise the temp above absolute zero other than that the perfect vacuum has no Kinetic energy and therefore zero temperature.

  Paul Norton, Worthing

• Further to the previous replies: As has previously been said, there is no such thing as a vacuum in practice. They simply cannot exist. This is not just because it is not possible to 'pump' all the matter out of a region of space, it is a consequenceof the fact that all the time, particle-antiparticle pairs are being created from nothing and annihilating each other. There are always particles in any given region of space.

  Perhaps the question was meant to be; what is the temperature of space? Then the answer is just about anything you like. It?s like asking, how long is a piece of string? Different parts of space are ?hotter? than other parts.

  David Pirie, Edinburgh, Scotland