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If my Hot Pizza is in deep space and there is no gas molecules around it, how would it dissipate it's energy? There are neither conduction nor conviction, and since the Pizza is solid it can't radiate its heat will it stay hot?

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It will still radiate heat in the infrared part of spectrum. It doesn't matter whether it is solid or not. See Black-body radiation for details.

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Heat may be considered to be movement of particles. The particles of your hot pizza oscillate. An accelerating electric charge creates an electromagnetic field. The oscillating particles of the pizza emit photons in the wavelength of the field the particles create, generally infrared radiation. As the particles on the surface of the pizza radiate these photons, they become less energetic. For a while, conduction from hot particles within the pizza keep the surface particles oscillating. But eventually, all the paricles of the pizza become less energetic, their heat vibration slows, and the pizza cools.

See the first answer to this similar question: https://space.stackexchange.com/questions/7830/how-is-heat-in-space-dissipated.

Were heat never radiated away from the pizza as photons of electromagnetic radiation, the pizza would remain hot forever. Such a state would contravene the increasing entropy of our universe.

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  • $\begingroup$ @AccidentalFourierTransform : I edited the answer to make clear that there is radiation, so the heat energy of the pizza is accounted for as vibration slows. $\endgroup$
    – Ernie
    Sep 24 '17 at 18:38
  • $\begingroup$ "The particles of your pizza will gradually slow their heat vibration as friction against each other attenuates their movement." This doesn't make sense, as it implies that internal friction causes cooling. Also, an object that simply remains hot as a result of poor heat transfer does not constitute a perpetual motion machine. I think these statements could be reworded. $\endgroup$ Sep 26 '17 at 0:21
  • $\begingroup$ @Chemomechanics : I reworded the answer and hopefully eliminated the misleading statements. $\endgroup$
    – Ernie
    Sep 26 '17 at 2:00

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