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Comment by AdamN | Hacker News Reader

I'm kind of confused. For example, if you have a glass sphere and then you take all of the atoms out of the sphere so that it's a complete vacuum, isn't that some sort of absolute zero? Or does that not count since you need at least one particle to 'have' the temperature (i.e. a measurement of thermal energy)?

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A perfect vacuum still displays activity in the form of "vacuum energy" - a counterintuitive property of the universe where particles apparently blink into existence for a short duration in otherwise empty space.

https://en.wikipedia.org/wiki/Vacuum_energy

Right -- it's probably more accurate to say that a perfect vacuum has no temperature than to say it's at absolute zero.

Wow!

In technical terms temperature is conventionally understood to relate to the average kinetic energy of particles in a region.

If a region contains no particles it would be more accurate to say that it doesn't have a temperature than to say that its temperature is zero.

There would still be the Higgs field permeating the universe and any radiation emitted nearby permeating that transferring energy to your sphere's surface, and normalizing the temperature inside.

In its most general sense temperature describes how the entropy of a system increases/decrease as a reaction to temperature, if there is only 1 possible state then entropy is 0 and can neither increase or decrease so the notion of temperature doesn't really make sense.

Some of your replies are pointing out that empty space actually still has some states, but even ignoring that, it's just not really sensible to talk about the temperature of nothing.

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