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If we consider a vacuum stripped from all particles except the virtual particle-antiparticle pairs, the vacuum has temperature zero, assuming that this vacuum energy is zero (despite the fact that no particles are present to support the temperature the vacuum still has a temperature; a black body placed in it will lose energy to the vacuum, which means the vacuum has a temperature).

Now if we look (for example) between the two plates involved in the Casimir effect, there are less virtual particle-antiparticle pairs, will the temperature still be zero or less than zero? In other words, will a black body give off its heat (by means of radiation) at the same rate to a region with negative energy or at the same rate? Let's assume the black body isn't so hot that it emits high energy photons which can create real particles out of the sea of particle-antiparticle pairs.

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    $\begingroup$ Do you think that an ideal gas in contact with a casimir vacuum thermal reservoir could reach a negative temperature? $\endgroup$ – user126422 Jun 15 '17 at 1:32
  • $\begingroup$ @WillyBillyWilliams (you should become a singer!)- Got your point!! (i.e. NO) Though, doesn't this only means that the ideal gas can't reach a negative temperature (while the vacuum has)? $\endgroup$ – descheleschilder Jun 15 '17 at 6:47
  • $\begingroup$ Actually, my question is not useful. I just remembered that is systems with "negative" temperature heat flows from the negative to the positive system. So, I have no idea! $\endgroup$ – user126422 Jun 15 '17 at 12:34
  • $\begingroup$ This reminds me of the following: There are 5 people on the bus. If 7 people leave the bus, how many have to enter for the bus to be empty? $\endgroup$ – Aziraphale Jun 16 '17 at 7:10

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