If a medieval-level society with access to things like magic (which itself follows the laws of thermodynamics) were to have some understanding of the laws of thermodynamics, how would the 2n law be stated?

I ask this because, in a medieval world the physical sciences are not advanced enough to have mathematically precise definitions of these laws.

The first law could be stated as: "Nothing can be created or destroyed, only change form." You get a vague idea about how this works. You need not define it using 'dU = dQ - W'.

How would the 2nd law be stated in a similar way?

I was thinking something like:

"The natural evolution of all things is towards decay and this process is unstoppable".

The thing is, this doesn't seem really like a re-statement of the 2nd law as much as it is a consequence of the 2nd law. Obviously the word 'decay' doesn't quite capture a full understanding of entropy but we're not going to be making use of natural logarithms in this kind of fantasy/magic using society.

Thanks in advance for any ideas!

  • $\begingroup$ I have some questions for you: a) What is the intention behind whoever formulates them? Those "laws" are constantly expressed differently depending on the context. b) In what way does the magic/fantasy change the nature of the laws? c) The laws are completely arbitrary. Why do you need 2 laws instead of perhaps one idea or concept that includes both of those "laws"? Also I wouldn't agree with your formulation of the 1st law, please explain it a bit more, it might have to do with the magic stuff which you have not established $\endgroup$
    – Raditz_35
    Aug 10, 2018 at 13:23
  • $\begingroup$ @Raditz_35 The two laws are just the 1st and 2nd law of thermodynamics. But yeah, they would likely be discovered in the context of magic. My statement of the 1st law is essentially correct. And, in the context of magic, would relate to how a mage would have to source energy equal in magnitude to the energy 'in the spell'. For example, the thermal energy in casting a simply kind of fireball spell. The 2nd law is relevant because no energy conversion (not even magic, in my world) is 100% efficient. Therefore entropy increases. $\endgroup$ Aug 10, 2018 at 13:47
  • $\begingroup$ (2/2) What I kind of want is a way to express this 2nd law without directly referring to 'entropy' or 'efficiency' since these concepts most likely wouldn't be well defined to the society. $\endgroup$ Aug 10, 2018 at 13:48
  • $\begingroup$ If you have the 2nd law, you have a concept of entropy, there is no other way. So if basically you are just looking for formulations that don't include certain words, there are many formulations that can be found on wikipedia for example. Perhaps make it perfectly clear what we can't work with to formulate them for you. Btw, I wanted to ask earlier but thought it was too much, but now you started theorizing about it: It would really depend on how they were discovered. A description of what rule they've discovered = the law $\endgroup$
    – Raditz_35
    Aug 10, 2018 at 13:50
  • $\begingroup$ Yeah, this is really a linguistic problem more than a physical one. Entropy obviously exists in the world. I wager that building one without entropy would be impossible. $\endgroup$ Aug 10, 2018 at 13:59

5 Answers 5


The answer will depend on what they do have. You'll have to decide what sorts of models they use to describe their world. Your definition will also need to relate to what conclusions you want to draw. The more informal the conclusions, the more informal the definition can be.

Personally, I like the prior art of Aristotle's approach to motion. Obviously this predates the mathematics for modern thermodynamics. What I find fascinating about this is that he divides energy into two classes. There's energia, which is the energy to "do stuff" which is immediately visible. Then there's entelechia, which is the energy required to stay the same. The famous example provided in that article is that a fish must expend energy to remain a fish. If it fails to spend that energy, it becomes meat.

It is an approach which fits quite well with the human experience, but is effective at showing many of the laws of thermodynamics in some manner.


In the real world, the amazingly photogenic Rudolf Clausius presented an early version of the second law of thermodynamics in the 1850's:

Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time.

In other words, in order for energy in a dispersed, high entropy state to transfer to a more concentrated, lower entropy state, some outside force must be applied.

For your magical world, this can be reworded to:

Energy can never pass from a chaotic to an ordered body without some other change, connected therewith, occurring at the same time.

In other words, in order for the ambient magical energy of the world to transfer into an ordered, tangible state such as a spell, some outside force must be applied by a spellcaster.

  • 1
    $\begingroup$ Yeah, I think this is something like what I'll go with. Referring to 'chaos' and 'order' as kind of universal forces fits in fairly well with the fantasy genre. Cheers. $\endgroup$ Aug 10, 2018 at 20:53

According to Physics for Dummies,

Essentially entropy is the measure of disorder and randomness in a system.

2nd Law = “The entropy of the universe tends to a maximum.”

According to historical myths/legends and religious creation myths

In the beginning there was Chaos/disorder...and it wants to come back.

So if your 1st Law of Thermodynamics is

Nothing can be created or destroyed, only change form.

Then your 2nd Law of Thermodynamics would be a collection of related concepts

Without a guiding force, Order always falls into Chaos.


Chaos seeks out more Chaos.


Order cannot come from Chaos, unless firmly directed.

It gives a lovely justification for your ruling class to lord it over the uncivilized masses. (please note the heavy sarcasm)

  • 2
    $\begingroup$ As I said to someone above, this is the kind of thing I think I'm going to go with. More vague discussions of chaos/order than precise statements about physical quantities. $\endgroup$ Aug 10, 2018 at 20:54

If you're talking about the magic with aether of your last question I think it would be something like God or whatever in charge of magic and energy is egalitarian and want the same amount of those everywhere so if there is some energy or aether to snatch from the spell you casted, he will do it. Your godlike entity must not be to powerful tho otherwise it will just equalize everything once and for all.

  • 2
    $\begingroup$ You mean the 2nd law then states that there is a tax to be paid for every spell you cast? $\endgroup$
    – Raditz_35
    Aug 10, 2018 at 14:53
  • $\begingroup$ Something like that, a god want to redistribute the aether to everywhere there is less than the mean $\endgroup$
    – Jean-Abdel
    Aug 11, 2018 at 11:32

The Second Law of Thermodynamics (in our real, non-magical universe) can be well stated without recourse to Mathematics, as follows:

"Heat cannot, of itself, pass from one body to a hotter body."

I think this may address how to express the 2nd Law in terms that non-scientists can understand. But I can't take credit for this statement of the 2nd Law; it (and a concise, understandable statement of the 1st Law) are from Flanders & Swann - 'First And Second Law' available online as of this posting at: https://www.youtube.com/watch?v=VnbiVw_1FNs

Enjoy! (And be edified ;-)


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