what is the temperature of Vacuum since temperature of a system is related to the average of the molecular kinetic energy and there is no molecule in a vacuum? i know there could be radiations but i want to know how much the contribution is?
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1There are no perfect vacuums, either man-made or in nature. So, there are particles (and as pointed out below, photons) in the volume. – Jon Custer Dec 31 '15 at 16:20
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This is not about chemistry... – Mithoron Dec 31 '15 at 21:13
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4I'm voting to close this question as off-topic because it's about physics, not chemistry. – bon Dec 31 '15 at 22:01
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It really depends on how the vacuum is defined. If you, for instance, pump all the air out of a steel container, you will have a vacuum in there. Yet, there will be photons constantly radiated off of the walls. This interplay of photons will be in thermal equilibrium with the walls and therefore will have a contribution to the temperature. For instance, if you would place a thermometer inside this container, the material of the thermometer would be hit by the photons of the container wall and therefore the thermometer would give a temperature equal to the temperature of the wall. As a matter of fact, even in the deepest of deep space, there is some radiation to be found which gives a temperature of about 3K. (This radiation is left over from the Big Bang)
Nevertheless, if you make a true vacuum inside some container of which the walls are at 0 K that cannot be penetrated by any kind of radiation or matter, then the temperature therein is 0 K.
Ivo Filot
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Well... yes and no. Indeed, the Casimir effect would prevent the creation of a true vacuum, yet see this Science article for an example of a sub-zero Kelvin temperature. :-) – Ivo Filot Sep 18 '16 at 19:07
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