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I could see this question being asked on Physics SE as well, since it concerns the law of conservation of energy at its core.

Wizards in the world of Harry Potter seem capable of almost effortlessly performing feats of destruction. Concentration may be required, but they don't put hardly any physical work into their actions. Dark wizards destroy buildings, explode stuff, and fly around at whim. While magic in Harry Potter seems to usually create entropy, it may also reverse entropy. Slughorn used magic to clean his room on Dumbledore's visit without any physical work put in. Obviously they're doing work which requires energy, but where does this energy come from?

Even if JK Rowling has some sort of extra-dimensional pool of magic energy the wizards pull from, other universes don't do this.

  • Pokémon may cast Blizzard and Ice Beam as regularly as Fire Blast, Hyper Beam, and Flamethrower. Ice techniques in Pokémon obviously remove significantly more energy than the caster put into the attack. I think Mewtwo also created his island fortress almost at whim in the first Pokémon movie.
  • Almost any Square Enix game (except FF7. They literally had a pool of magic energy).
  • X-Men telepaths can effortlessly assemble mansions and destroy continents. Iceman can also create endothermic reactions on a large scale just by thinking about it.
  • Star Wars with force users
  • Insert your favorite universe here

It follows that if we were capable of creating energy from nothing, then we could also effortlessly undo entropy and restore order. Say we had a group of magic users capable of violating the laws regarding conservation of energy at whim in our universe. By extending their hands they can freeze stuff, blow stuff up, and assemble buildings. Could these people set out to stop heat death of the universe?

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    $\begingroup$ Just pointing out that the issues with the energy source for Pokemon moves has already been discussed on the site. It's not as implausible as you might think. $\endgroup$ – Frostfyre Aug 3 '16 at 14:07
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    $\begingroup$ One major correction: Entropy is not the same as disorder. Also, regarding magic and thermodynamics, see this question. $\endgroup$ – HDE 226868 Aug 3 '16 at 14:08
  • $\begingroup$ I suppose you're right. I was using the concepts almost interchangeably. $\endgroup$ – DeepDeadpool Aug 3 '16 at 14:30
  • $\begingroup$ This question doesn't really make sense. By definition, magic is something that can't be explained within the laws of physics. In other words, once you invoke magic, you can do whatever you like. There is no right or wrong answer. $\endgroup$ – JBentley Aug 3 '16 at 19:57
  • $\begingroup$ In the fanfiction "Harry Potter and the methods of rationality" (hpmor.com) the issue of magic breaking physic's law of energy conservation is broadly discussed with an unexpected scientific skepticism. Spoiler on conclusion: basically "I have no idea why it works so weird, but I'm going to find out! I'm a scientist after all!" $\endgroup$ – SK19 Mar 16 '18 at 10:20
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Let's state the first law of thermodynamics: $$\mathrm{d}U=\delta Q-\delta W\tag{1}$$ where $U$ is internal energy, $Q$ is heat, and $W$ is work done. This is another way of stating the law of conservation of energy. Now, according to the second law of thermodynamics, $$\mathrm{d}S=\frac{\delta Q}{T}\tag{2}$$ for an idealized reversible process, where $S$ is entropy and $T$ is temperature. Normally, this would be expressed with $>$, instead of $=$, but those are for non-ideal irreversible processes - which obviously do not encompass magic.

However, your magic requires that there is energy added to the system, so $(1)$ should be $$\mathrm{d}U=\delta Q-\delta W+\text{added magic energy}\tag{3}$$ and so $$\mathrm{d}S=\frac{\mathrm{d}U+\delta W-\text{added magic energy}}{T}\tag{4}$$ Therefore, assuming that the process is reversible (as I think we can assume the magic is), $\mathrm{d}S$ may in fact be negative. I think. This could then avoid the heat death of the universe.

Oh, but if you violate conservation of energy, you violate Noether's theorem, and so, long story short, you've broken classical mechanics in the immediate region, as well as quite a lot of other physics. I recommend avoiding this.

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    $\begingroup$ I'm not sure that this answer is that clear. You state that adding magic energy could offset entropy, but not by how much or how feasible it would be. And your last paragraph recommends against breaking classical mechanics, but isn't that the whole point of magic? $\endgroup$ – D.Spetz Aug 3 '16 at 18:00
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    $\begingroup$ @D.Spetz I can't calculate anything because the question doesn't state anything about how powerful the magic is; therefore, I can't calculate the numerator of $(4)$. Also, yes, magic wants to break laws of physics, but selectively, i.e. only for certain objects at certain times. $\endgroup$ – HDE 226868 Aug 3 '16 at 20:53
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    $\begingroup$ I second the recommendation on avoiding breaking the laws of physics. Please don't break reality while I'm using it: that'd just be rude. $\endgroup$ – Ghotir Aug 3 '16 at 22:21
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    $\begingroup$ Breaking energy conservation somewhere does not require breaking it everywhere. Even if classical mechanics isn't perfect (which it isn't anyway), it can still be good enough on appropriate scales, including local conservation, in a universe without global energy conservation. Yes, allowing energy non-conservation means you lack exact time-translation symmetry, but you can still have an approximate symmetry which is exact in most cases on appropriate scales. Our own universe is a good example of this, as energy is in fact not globally conserved in GR, as cosmologists are well aware. $\endgroup$ – Logan R. Kearsley Aug 3 '16 at 23:02
  • $\begingroup$ @LoganR.Kearsley That's a good point, and one I should have considered. $\endgroup$ – HDE 226868 Aug 4 '16 at 2:05
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Well, since the physics has been covered, let's cover your assumption and rationalize magic, shall we.

First, let's start by thinking of alchemy, essentially slightly more magical chemistry. If we look at a show like Full Metal Alchemist, the energy used for alchemy is (in part), taken from a parallel universe (at least in the original show, I haven't finished Brotherhood). If use this logic, then assume that the universe is this combined group of parallel universes, then no laws of conservation of energy are being broken, the energy is merely coming from an alternate and unexplained source, but in this case the wizards/alchemists are actually accelerating heat death, because they are using up valuable energy.

Next, let's look at the basic assumption being made, all this energy from nowhere! Let's start with pokemon and realize that the energy does not come from nowhere. Moves have a limited PP in the games, and in the show pokemon do grow tired from continued fighting. This would mean that just like humans, pokemon eat and produce energy, but they are much more efficient and can use the energy to use blizzard. Also, note that pokemon are level based, only very high level pokemon can use such moves. Level corresponding to innate ability to convert energy i.e. increased metabolism from training, something that DOES happen to humans, although not to quite as impressive degree. Again, no rules are broken.

Finally, Harry Potter specifically. I think you are forgetting that most Wizards need a wand to do magic. They, at least, need wands to blow up bridges etc. Wands, as explained in the series are made with a powerful core of dragon heartstring/ unicorn hair/ phoenix tail feather. It is these items that provide the power for such powerful magic. Therefore, wizards again are just using pre-existing power for their magic.

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  • $\begingroup$ In HP world, there are examples of many types of magic which does not require use of wand. $\endgroup$ – Sp0T Aug 4 '16 at 5:00
  • $\begingroup$ Correct, but this is all simple magic, more complex magic requires a wand. Worth noting that the North American schools teach only wand less magic, but it's much more like tricks rather than bridge exploding. This simple magic could just use in ate magic, since it's never really described, it could leave a wizard hungry or tired, because it requires inate power. Remember that the most common case of wand less magic is children who have the most overflow of energy, (see any child running around like a maniac) $\endgroup$ – EvSunWoodard Aug 4 '16 at 11:10
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Let's look at the basic laws of Thermodynamics:

  • In the absence of magic, yada, yada, yada

Basically, you can never make a coherent answer about physics in the presence of not-physics. I can always postulate the existence of spells like "Accelerate Heat-death" or "Retard heat-death."

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Anything that ignores the laws of thermodynamics for its own advantage could easily slow the entropy (and eventually heat death of the universe). In fact whenever such an entity leaves matter/energy at a lower state of entropy at a net-sum gain just slowed the heat death of the universe.

Not by much, since the universe is a big place, but they did help.

The degree or size of the spell/ability/superpower and how often it was used would affect the outcome. I imagine there's multiple Marvel and DC heroes to that end that could help us out.

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Magic negates heat death. In a universe where magic works, the laws of thermodynamics don't exist. Without the laws of thermodynamics, there's no reason to think that heat death is the ultimate fate of the universe.

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  • $\begingroup$ The laws of thermodynamics can still exist, they just might need to be adjusted. We don't throw the baby of Einstein's General Theory of Relativity out with the bathwater when we find out pieces of it don't mesh with quantum mechanics. Magic, if it exists, can be studied by science, it just might have separate laws describing its behavior. $\endgroup$ – Ranger Aug 3 '16 at 13:59
  • $\begingroup$ @NexTerren But adjusted laws of thermodynamics won't have the same consequences as the current laws. And you're wrong about general relativity; its incompatibility with quantum mechanics means that at least one of them is wrong, and is just a good approximation that works in some circumstances. And so any conclusions based on pushing either of them to their ultimate limits are suspect. $\endgroup$ – Mike Scott Aug 3 '16 at 14:37
  • $\begingroup$ Sure one of them are wrong, but we still actively use both of them. It'd be silly to dismiss the laws of thermodynamics that still seem to describe everything in the universe but magic. The laws need worded in the nature exception once we pin it down (and how magic roughly works), but that doesn't mean that the descriptors (the laws) no longer describe. $\endgroup$ – Ranger Aug 3 '16 at 14:45
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    $\begingroup$ @MikeScott You could argue that "Relativity" proved most of Newton's laws are wrong, but we still use them a lot, because they tend to be close enough as long as you're dealing with slower things. In this imaginary world, I could see they adjusting the usual physics assumptions to: "Assuming the object is a rigid body, friction is negated, and the object is in a null-magic field." $\endgroup$ – Patrick M Aug 3 '16 at 15:37
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    $\begingroup$ @PatrickM We use Newton's laws as a decent approximation, but with the understanding that they are wrong and therefore must be used only in appropriate contexts. If you want a universe with magic where the laws of thermodynamics are a good approximation, then that too must come with the understanding that they are wrong (and therefore heat death is not a foregone conclusion, as per this answer). $\endgroup$ – JBentley Aug 3 '16 at 20:01
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neither. The heat death of the universe is a conceptual misundertanding about how the laws of thermodynamics apply to the universe as a whole. If you think the universe is a closed system, then heat death seems inevitable. However, there's absolutely no reason to assume the universe is a closed system.

Certainly the observable universe has a boundary. This is the event horizon beyound the rest of the cosmos is unobservable. There is nothing to prevent matter and energy crossing the event horizon, so this doesn't constitute a closed system.

Therefore, magic will neither accelerate or slow down the heat death of the universe, because there is no heat death of the universe.

Unless, of course, there is a magic spell to make the heat death of the universe actually happen, then all bets are off. (That would require an enormously powerful magic spell, beyond anything any single magic caster or whole legions of them could possibly cast or even spell.) This does make sense, so forget it. Also, it turn the fantasy into some kind of science fiction, therefore, it's totally impossible!

Anyone experiencing sleepless nights worrying about the heat death of the universe you can rest easy tonight.

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