When hit by sun rays, the moon creates excess energy, it increases the energy & reflects it back to earth as mana rays to make magic possible (the exact process is not important). Most of it quickly vanishes into the atmosphere if not used or stored. I imagine mana to work similar to light, but capable of being temporarily "absorbed" by matter like heat to make it linger on earth for practical reasons.

Magic users(about 5% of the population, but dependent on the nation), gods(not literally, but practically just immortal wizards in this context) & some animals & plants can store it or use it for spells, ranging from changing the environment to healing, creating magical artifacts, etc. Unless magic users receive extreme amount of schooling, they are only really capable of very limited feats like creating sparks or dubiously helpful lucky charms. It takes more than a lifetime of commitment to use more than the residual mana left inside ones body.

The (potential) problem

In the process of creating mana, the moon breaks the first law of thermodynamics. Gods (and very talented humans) can also imitate this process, although the amount is limited by the mana they can utilize(equivalent of 1 TWH for gods, 1MWH for humans), the meager results(10-20% increase) and the effort it takes(a day for maximum amount). The reverse is also possible under similar rules, but doesn't see much use. I also assume creating/destroying matter with energy is practically impossible to avoid the world filling up over time without adding new rules.

What possible problems could arise from such a system for breaking the laws of thermodynamics?

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    $\begingroup$ That system seems like an unnecessary complication. Why not just have the moon radiate magic energy by itself? You yourself said the sun doesn't radiate enough magic energy to make magic possible, so instead of having the moon reflect and somehow multiply magic energy, just have the moon radiate it directly. Alternatively, you could say the moon has a certain material that, when exposed to magic rays (from the sun) decays into further magic, turning mass energy into magic energy. No more breaking of thermodynamics. $\endgroup$
    – M S
    Commented Aug 10, 2023 at 11:07
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    $\begingroup$ It seems like you first invented an arbitrary rule about magic, and then are concerned about it not following a physical law. This seems like something you can easily fix: either get rid of that first rule, or let it operate independent of physics. $\endgroup$
    – Joachim
    Commented Aug 10, 2023 at 11:34
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    $\begingroup$ (1) "Most of it quickly vanishes into the atmosphere" and thus the atmosphere breaks the First Law too, not only the moon. (2) But the good thing is that the First Law does not say anything about magical energy, whatever that may be. The First Law is only about mugglish physical energy, which is only a quantity expressing the capacity of a physical system to do mechanical work and has nothing to do with changing the environment, healing, and creating magical artifacts. $\endgroup$
    – AlexP
    Commented Aug 10, 2023 at 11:45
  • $\begingroup$ @M S I clarified my first sentences to hopefully be clearer. The sun doesn't create magic, only the moon. I set it up that way to make magic dependent on the lunar cycles instead of sun exposure, leading to very different mana amounts being around depending on the time of month. Your second idea seems like a good solution, but I still prefer my original one for a few reasons unless someone points out big problems with it. $\endgroup$ Commented Aug 10, 2023 at 11:47
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    $\begingroup$ Title: yes. Body: all of them. This is a dupe of, I want to explain magic; bad idea? "the exact process is not important" - then neither is the question. $\endgroup$
    – Mazura
    Commented Aug 11, 2023 at 1:11

7 Answers 7


There are zero problems with your idea.

Here's a simple solution that doesn't violate the laws of thermodynamics: All energy originates in the sun, as it does in real life. However, if that is the case, how could the moonlight in your universe be a special mana-containing light when sunlight isn't?

Answer: Some of the energy from the sun is invisible. For real-life examples, consider UV and infrared light, radio waves, etc. The sun is emitting all of these all the time.

What happens when you shine UV light at something fluorescent? It glows (appearing to create energy out of nowhere). The energy WAS there already in the form of invisible UV radiation, but you can see it now.

So, the moon's rocks are special. When sunlight hits them, a handwavium process equivalent to fluorescence occurs. This converts invisible energy from the sun into mana-containing moonlight, which then shines onto the Earth.

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    $\begingroup$ Great analogy with fluorescence. Other luminosity related phenomenon like phosphorescence ("visible-mana" keeps shining after "invisible-mana" stops hitting it) or bioluminescence (organisms that naturally produce magic) could be interesting to replicate as well. $\endgroup$ Commented Aug 11, 2023 at 12:00
  • $\begingroup$ Nitpicking: in my life not all energy originates in the sun, there are other sources that did not originate on the sun. You may be living in a different universe than me, though. $\endgroup$
    – Ángel
    Commented Aug 11, 2023 at 23:27
  • $\begingroup$ Well ok, there's hydrothermal vents, star light, gravitational waves, the cosmic microwave background, radioactive isotopes, fusion, and so on... List not exhaustive, point taken. Maybe OP is even inventing a world which has both mana and nuclear reactions - then it's not just a nit-pick, it's actually really important. $\endgroup$
    – matt_rule
    Commented Aug 12, 2023 at 0:00
  • $\begingroup$ +1 because I really like the idea that the moon is doing nothing more than converting sunlight into another form of energy - not unlike a solar panel converting sunlight into electricity. That doesn't play with the math at all and completely rationalizes the presence of "mana" in a way that reflects Real Life, which makes it more relatable to the proverbial reader. $\endgroup$
    – JBH
    Commented Aug 12, 2023 at 5:09

A candle in infinite darkness

Breaking the laws of thermodynamics isn't necessarily an end of the world scenario. I would compare it to having an identical universe as we live in, with one single candle on Earth that burns on nothing and outputs extra energy. It breaks the laws of thermodynamics, but if all it can do is add this pitiful amount of energy, it doesn't change anything in the grand scale of the universe. At least not to the lives on Earth.

Adding magic that from your story sounds limited in nature doesn't need to change anything. Sure they add some energy here and there, but even the Earth is massive in comparison. It'll radiate the resulting energy sooner or later, having an energy equilibrium of intake and output insignificantly higher than before.

Make a good story

Let's say the above isn't true. Let's say the magic absolutely destroys the laws of thermodynamics and would destroy the world in a month. Why would you write about that? It's a story about magic. There's already some suspension of disbelief. It'll stretch a bit further. Implement it in a way that looks scientifically plausible, or do not mention it at all.

A quick example are the books if "A song of ice and fire", better known from the first book title "A game of thrones". Most of it is rooted in a world from late medieval periods we all understand. Yet it has seasons that can last years. How would most creatures survive a 3 year winter without stockpiling food as the humans need to do? Not to mention dragons and other magical creatures. The point is not to talk about how it's possible, but that a good story will not have you question it.

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    $\begingroup$ Great answer! The most unbelievable thing in ASOIAF for me is the fact that the world has been locked into late-medieval tech for several millenia. But, like you said, it doesn't matter to the story being told. $\endgroup$ Commented Aug 11, 2023 at 13:10

Frame Challenge: It doesn't need to be in defiance of thermodynamics.

The Courtship of the Moon and the Land

When the Sun's rays strike the Moon, they bounce off towards the Earth to create Moonlight. The Moon sends her love along with those reflected beams in the form of mana, which is absorbed greedily by the Earth, who cares as little for the Moon's love as it does for the scraps mages and gods use up for themselves.

But the Earth is sloppy in her greediness, and so when the Sun's rays strike the Earth and bounce towards the Moon as "Earthlight", the scraps and the refuse of the Moon's gifts are carried back to her as "hollowed mana" that has little use to mortals since there are no mortals on the moon.

This dance has carried on for thousands of years, the Moon pouring her love into mana, the earth sending it back wasted, and mortals benefitting from the excess, and it shall carry on for thousands more, but the gods fear the day that the Moon's love runs out, or that she turns her eye towards another, for how then will they maintain their power?

Or, in scientific terms:

The mana system is a closed loop between the Earth and the Moon similar to the water cycle, where mana bounces back and forth without ever changing in amount, carried along by reflected light and restored to full power by the reaction between the moon's surface and the sun's light (because, uh, magic moon rocks contain handwavium). When a mage uses a spell, after the effects are finished, the mana used in its casting is hollowed out and eventually makes its way to the moon to be refilled.

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    $\begingroup$ I want my system to be more based on logical principles & energy accumulation is actually something I care about, so I would prefer it if I can make it work. A version of this story will definitely end up somewhere in my folklore though, thanks! $\endgroup$ Commented Aug 10, 2023 at 12:27
  • $\begingroup$ Sure, feel free, all I ask is that you upvote the answer if you found it helpful. Happy worldbuilding! $\endgroup$ Commented Aug 10, 2023 at 12:30

As others mentioned, you don't have to worry about this, regardless of the mechanics of your mana.

Consider, for instance, sugar. Sugar is stored sunlight. If it becomes adequately ubiquitous in an environment, then creatures will start using it as a primary energy source, taking it from creatures that make it. Those creatures turn the sugar into heat. Every time a lightning bug puts 50k volts through its prey, the heat of electrical release will go into the environment.

If the environment receives a surplus of heat, it radiates into space long before it can build up. Burning our coal reserves is an excellent example of this. Coal is sunlight that was captured hundreds of millions of years ago. The heat released from the coal isn't meaningful in the planetary heat equation, even if the CO2 blanket is.

Mana from the Moon would be no different. If it builds up, it's because nothing has evolved to use it. The more it builds up, the more likely that something will evolve to extract it from the environment and use it metabolically or effectually.

Thus, the only reason you'd have thermodynamics issues is if you purposely write them into your world.


The moon doesn't need to reflect the sunlight

And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years: And let them be for lights in the firmament of the heaven to give light upon the earth: and it was so. And God made two great lights; the greater light to rule the day, and the lesser light to rule the night: he made the stars also. And God set them in the firmament of the heaven to give light upon the earth, And to rule over the day and over the night, and to divide the light from the darkness: and God saw that it was good.

(Gen 1:14-18, KJV version)

The sun and the moon are simply described as lights in the firmament of the heaven to divide the day from the night, to give light [and magic] upon the earth.

In your world, the moon may be a source of light on its own. The moon doesn't need to reflect the light of the sun. It could be a floodlight set up by martians in the moon. Or a spaceship. The sun and the moon could be giant lanterns that get switched on and off every day.


The sun just moves the mana from the moon

Lore says that in the old days, a number of powerful wizards/gods created the Moon to move around the Earth. So, the moon is part of the Earth that was magically put into the space in an old age.

(They could have done that asked by a king, for their own glory, to remember a glorious event, to jail an enemy/devil there, because they wanted to see in the night...)

Also, we don't know the origin of their magic (was it floating on the Earth? it exhaled from a volcano? it could be extracted from some <materials> that are no longer available now? we just don't know).

The Moon was moved into orbit by magic, and magic is -according to some people- moving it through the space (a potential contradiction to Newton's First Law, although we don't know if the moon of your world varies its speed, or corrects its route like satellites do), others think the magic around it is a field intended to protect the moon (or its inhabitant?) from meteorites/visits/escaping, and finally others think it's just residual magic from the orbiting act (on the Earth it would quickly disappear, but there is no atmosphere in the Moon, so it may stay there, trapped by the Moon's gravity field).

This large amount of magic/mana in the Moon is slowly being eroded by the sunlight. And when the sun rays reflect there and travel to the Earth, they carry a bit of that in mana form.


Short idea: the moon reflects sunlight (so you're not violating any laws of thermodynamics), but there's some special magical part of the moon that laces it with mana.


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