# Why does earth cool? [duplicate]

This question already has an answer here:

This question is boggling me for some time. We know that heat can be transferred from matter to matter and heat is nothing more than tiny atoms vibration intensity (correct me if I'm wrong). But space is a vacuum, and so arises the question: Can you heat vacuum? :)) How can it be that earth loses it's heat to space itself?

Thanks

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## marked as duplicate by Kyle Kanos, Ali, Chris White, Brandon Enright, John RennieAug 1 '14 at 7:01

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## 1 Answer

Earth can lose heat to space through radiation.

The earth behaves roughly as a blackbody and so radiates electromagnetic radiation into space at a rate of roughly 120 PW.

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I suspected that. But that's another solution that's boggling me :D I'll ask it here if you don't mind. As far as I know radiation has no mass, and according to Einstein's famous equation, mass=energy. So how come that if radiation has no mass, it is carrying energy (heat)? Thanks again :) – Davita Aug 1 '14 at 0:01
@Davita See this question – alemi Aug 1 '14 at 0:03
@Davita: $h$ = Planck's constant. – Kyle Kanos Aug 1 '14 at 0:14
@Davita PetaWatt, or $10^{15}$ W – alemi Aug 1 '14 at 0:28
@Davita A photon has 0 rest mass ($m=0$), but $E=mc^2$ and $E=pc$ are both simplifications; the latter applies when $m=0$. The full formula is $E = \sqrt{m^2c^4 + p^2c^2}$. When we say a photon is a "massless" particle, we mean that it has no rest mass, not that it has no mass-energy at all. – Tim S. Aug 1 '14 at 2:31