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The cryogenic magic I am considering functions like this:

  • When the magic is applied to an object (a stone, some water or air), it forces the molecules, atoms or other subatomic particles of the object to release its thermal energy as photons, thus causing the object to glow and cool.
  • In reality, all objects with non-zero degrees kelvin in terms of temperature do this, but an object only cools down if it receives less incoming radiant energy than it radiates out. The magic somehow changes the point of equilibrium so that the object is effectively cooler than its environment.

You may assume that some mysterious storage (like an m.p. bar) of energy is required for this magic. If this becomes important or it must function in a certain way to preserve the laws of physics, be sure to specify in what way.

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I think there are two aspects to your question.

The first law of thermodynamics states that energy cannot be created or destroyed, but it can change form and it can move from one place to another. Another way to state this is that within any given closed system, the total energy is constant, known as the law of conservation of energy.

What you describe sounds like it could possibly work within those constraints. If you consider the entire universe to be a closed system, then the object affected by the magic is releasing thermal energy (in the form of photons) to its surroundings, causing its own temperature to drop. The total energy of the system is maintained, so there is no violation of the first law of thermodynamics in this process.

However, there is also another matter. As phrased by Wikipedia,

Heating is a natural process of moving energy to or from a system other than by work or the transfer of matter. Direct passage of heat is only from a hotter to a colder system.

This becomes a problem because in this case, we can no longer consider the entire universe to be a single closed system. Each component that is involved in the exchange is its own system, illustrated by the fact that each component has its own temperature, and heat flows from "hotter" to "colder", not the other way around. This is why getting anything to a temperature below that of the cosmic background radiation, which is about 3 Kelvin -- not 0 K, as you imply -- requires a heat pump to continually remove the energy being transferred from the cosmic background radiation to someplace other than the supercooled object.

Basically, by the laws of thermodynamics, in order to cool an object below the ambient temperature, you have to supply energy, and you need to somehow use that energy for the work of transferring energy from the object you want to cool to somewhere else. (That's a heat pump.) If it wasn't for this, someone would likely have made a killer refrigerator or freezer that doesn't draw any power except for the light when you open the door, and would have made a fortune selling them.

Absent any process either adding or removing thermal energy, by the laws of thermodynamics, any object will eventually stabilize at the temperature of its surroundings; not higher and not lower. This will happen by thermal energy flowing from a point of higher temperature to one of lower temperature; you can accelerate the process, but not cause it to continue once the two sides are in thermal equilibrium. Your cryogenic magic violates this by causing the thermal transfer to continue after the two sides reach equilibrium, so requires some form of heat pump, even if magical, to work.

Realistically speaking, the user of your magic should need to supply energy to maintain the lower temperature. If you are willing to go with that (and it could make for an interesting plot device), I can see no violation of the laws of thermodynamics. In fact, as Jim pointed out in a comment, we are doing something very similar with technology and call it laser cooling.

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    $\begingroup$ Nicely done Michael. Readers might want to look into laser cooling as an example of "cryogenic magic" and a momentum / heat pump, albeit on a much more modest scale than is proposed here. $\endgroup$ – Jim Aug 13 '16 at 3:43
  • $\begingroup$ @Jim Good point. I added a link to the Wikipedia article on laser cooling as well; it seems to fit quite well with what the OP wants, with perhaps the exception that energy must be continually supplied to maintain the lower temperature. $\endgroup$ – a CVn Aug 13 '16 at 12:48
  • $\begingroup$ You also might want to add explicit references to the second law of thermodynamics (as you did implicitly with the quote) and the fact that this magic would decrease entropy (impossibe by the third law of thermodynamics), which means that, to remain consistent with all laws of thermodynamics, the magic has to consume more energy (possibly in the form of mana) than is transferred from the colder object to the hotter one. $\endgroup$ – Annonymus Aug 13 '16 at 13:35
  • $\begingroup$ Maybe magic is based on entropy. $\endgroup$ – JDługosz Aug 24 '16 at 9:37

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