Is there any generally useful magic that can be done if all it could do was violate the Third Law of Thermodynamics?

The Third Law of Thermodynamics just seems to relate to absolute zero? And it says that:

It is impossible for any process, no matter how idealized, to reduce the entropy of a system to its absolute-zero value in a finite number of operations.

Physically, the Nernst–Simon statement implies that it is impossible for any procedure to bring a system to the absolute zero of temperature in a finite number of steps.[3]

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    $\begingroup$ What do you consider to be "generally useful"? $\endgroup$ – Frostfyre Nov 26 '15 at 3:06
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    $\begingroup$ Interestingly enough, a lot of sci-fi technology (or "magic" with some pseudo-scientific basis) is based around violating (at least) this law of thermodynamics, or (and this is not the same thing) assuming they can somehow violate the third law would provide a decent explanation for how they can do what they do. But the law itself isn't mentioned in a lot of fiction outside of very "hard" sci-fi, because people balk at these tersely stated scientific statements. $\endgroup$ – allquixotic Nov 26 '15 at 7:35
  • $\begingroup$ @Frostfyre - I guess some power that most people would want, if they had that magic, but also not something that seems non-useful on first glance, like the power to change the color of any object to green. $\endgroup$ – Malady Nov 26 '15 at 13:23
  • $\begingroup$ If all it could do was violate the third law? Instant super-conductor spell maybe? $\endgroup$ – Kaithar Nov 26 '15 at 21:24
  • $\begingroup$ Greg Bear's novella Heads posits that at T=0, quantum field distributions go flat, space-time undergoes a phase change not seen since the Big Bang, information flows in funny ways, and things get hideously weird. $\endgroup$ – Beta Nov 26 '15 at 22:48

Violating any of the laws of thermodynamics would be very useful.

If you can violate the third law then you can get a substance to reach (or since it's magic, go below) absolute zero.

The laws are occasionally humorously defined like this:

enter image description here

The efficiency of a heat engine follows this formula from Carnot:

$$\eta = 1 - {{T_C}\over{T_H}} $$

If you can get $T_C$ to zero, the efficiency is unity. If you can get it lower than zero, efficiency is greater than unity. That means if you can actually reach zero, you can get 100% efficiency. If you can magic something to be below absolute zero, you can get more than 100% efficiency. This is known as free energy.

In general, free energy is considered useful.

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    $\begingroup$ "In general, free energy is considered useful." $\endgroup$ – iAdjunct Nov 26 '15 at 3:55
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    $\begingroup$ +1 just for the "you can't win, you can only draw" reference, I always loved that. I know we're talking "magic" so hand waving is more or less a given, but even so it seems like T < 0 should have no meaning, since atoms would have to be moving "less" than not at all. Also, to get there you have to break the laws of Quantum Mechanics as well as thermodynamics, otherwise you're not allowed to know where your extremely cold thing is, which could get messy. $\endgroup$ – Joseph Rogers Nov 26 '15 at 9:33
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    $\begingroup$ If we can live in a world where negative interest rates are a thing, I can kind of get my head around negative motion, in a hand-wavy, magic-y way... $\endgroup$ – nwhaught Nov 26 '15 at 13:29
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    $\begingroup$ @JosephRogers Actually, supposedly negative absolute temperature is at least a theoretical possibility. Of course, one can still argue about what this really means... $\endgroup$ – user Nov 26 '15 at 17:31
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    $\begingroup$ @MichaelKjörling not just a theoretical possibility - it's demonstrated in every laser, for example. But the third law doesn't apply in that case, since the third law is only for certain types of matter. (Either perfect crystals or solids and liquids, depending on how its stated.) The degrees of freedom that have negative temperatures in a laser are neither of those things, so it's OK. But it does suggest that the third law is nowhere near as fundamental as the other two. $\endgroup$ – Nathaniel Nov 27 '15 at 6:52

As far as I understand, you'll also get rid of the quantum uncertainty: since a particles do not move, you can know both their momentum (zero) and their position. There already are some workarounds that reduce the uncertainty, and they are linked to the possibility of the quantum computing - so, I guess your magic could be used in quantum computer.

By the way, getting the system lower than zero does not require magic:

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  • $\begingroup$ If you had the rule of uncertainty broken, you could teleport, create matter(/energy) from nothing and turn matter(/energy) to nothing. $\endgroup$ – mg30rg Nov 27 '15 at 15:37

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