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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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  • $\begingroup$ The temperature of the vacuum is usually defined as the temperature of the thermal radiation in it. If the spectrum doesn't follow the Planck curve reasonably well, then it's meaningless to speak of temperature of the vacuum. A monochromatic light source, for instance, doesn't have a temperature. $\endgroup$
    – CuriousOne
    Jan 1 '16 at 15:29
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    $\begingroup$ Possible duplicate of Temperature in space $\endgroup$ Jan 1 '16 at 15:33
  • $\begingroup$ Probably related as well: physics.stackexchange.com/q/2059 $\endgroup$
    – Kyle Kanos
    Jan 1 '16 at 21:02
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If you stick a thermometer in a vacuum it will eventually read the same temperature as the container of said vacuum. If it is in deep space it will eventually read as the background temperature of the universe which is about 2.7K.

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  • $\begingroup$ Isn't the first sentence the same as what OP is asking? $\endgroup$
    – Kyle Kanos
    Jan 1 '16 at 19:24
  • $\begingroup$ No. The measured temperature is always that of the container. The only exception would be an accelerating reference frame seeing Unruh radiation en.wikipedia.org/wiki/Unruh_effect $\endgroup$
    – user56903
    Jan 1 '16 at 20:58

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