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Since some spells (such as enlarge/reduce) seem to reduce or increase the amount of mass that is present in game-play with no explanation as to how this happens, some interpret that to mean that conservation of mass is "thrown out the window" in 5e.

However, another view is that the RAW is silent on how those spells work, which leaves it up to the DM to either enforce or discard conservation of mass. The detailed mechanics of enlarge, for example, might be that it draws molecular matter from deep within the earth and/or from the environment or atmosphere, or even from another plane, such that conservation of mass holds true. Or then again, it might not. The spell description doesn't say.

In a couple of answers about 3.5e (such as this one and this one), it has been softly asserted that conservation of mass is not really respected in D&D, and the topic was touched upon in comments to an answer to a 5e question, but the issue has not been definitively put to rest, that I can see.

Is this indeed ambiguous according to RAW, perhaps intentionally (perhaps the rules authors are avoiding over-managing the campaign settings of various tables)? To put it differently: Is the RAW silent on conservation of mass? Or is there a definitive RAW answer to this somewhere that I have not come across?

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    \$\begingroup\$ I look forward to these answers, though I think it is pretty much hopeless. D&D and physics/biology/chemistry don't mix. How do elves and humans interbreed? Where does the energy of a fireball come from? How does telepathy work? What is "magical force"? --A wizard did it. :) \$\endgroup\$ – keithcurtis Nov 22 '18 at 1:20
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    \$\begingroup\$ What do you mean by "RAW agnostic"? If you're simply asking whether there are rules for conservation of mass then this definitely needs to be reworded to make that clearer. \$\endgroup\$ – Purple Monkey Nov 22 '18 at 1:24
  • \$\begingroup\$ I think that the OP meant "Are the 5e rules as written agnostic about the conservation of mass." What precisely that means I am not sure. \$\endgroup\$ – Gandalfmeansme Nov 22 '18 at 2:21
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    \$\begingroup\$ @Gandalfmeansme that's right, and agnostic means that one neither confirms nor denies something. So if the RAW neither confirms nor denies conservation of mass, then it's agnostic about conservation of mass. But no need to get hung up on that word, and I've re-worded the main question without using it. \$\endgroup\$ – Valley Lad Nov 22 '18 at 2:25
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D&D is not a simulationist game

The first thing to note is that D&D is not a simulationist game. It contains many rules issues that break down if you try to apply them in a simulationist way.

The books I have reviewed do not discuss that matter, but the safest assumption is that conservation of mass, and most of the rest of physics, is effectively ignored, at least once magic becomes involved. As an example, spells like enlarge most definitely ignore the full implications of the square-cube law.

The RAW answer is that the spell does exactly what it says and nothing more.

Spells do exactly what they say they do, no more, no less. Enlarge/reduce, the many other shape-changing effects, and indeed every spell I have reviewed do not say they draw mass from elsewhere - so they do not. The spell effect simply happens by magic.

Unless some book I don't have contradicts me, I feel safe in saying that the definitive rules as written answer is that the entirety of the explanation is "magic" - nothing more or less.


Disclaimer: my access to 5e books is somewhat limited and in particular I have not read the 5e Dungeon Master's Guide. That said, based on both the 5e books I have read and the earlier editions from which I have reviewed a great many of the books, I think it is safe to say D&D does not care about physics, especially when magic is concerned.

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    \$\begingroup\$ @TimothAWiseman +1 for the valuable answer, but I think saying "DnD is not a simulationist game (period)" is too sweeping a statement, in that it expresses just one among various possible play styles. The DMG on p. 34 under "Play Style" tells the DM to ask him/herself several questions to decide play style, the very first question of which is: "Are you a fan of realism.... or are you more focused on making the game seem like an action movie?" Either response to this question is acceptable according to RAW. Just something to consider. \$\endgroup\$ – Valley Lad Nov 23 '18 at 23:11
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    \$\begingroup\$ It might be more accurate to say "D&D is not a perfectly simulationist game". \$\endgroup\$ – V2Blast Nov 23 '18 at 23:26
  • \$\begingroup\$ Re: "every spell I have reviewed do not say they draw mass from elsewhere - so they do not." Those spells also don't say they grant the god-like power of "creation ex nihilo", creating matter out of nothing. They don't say the spell is eliminating matter from the entire multiverse, down to the atomic level with no remainder. \$\endgroup\$ – Valley Lad Nov 25 '18 at 1:28
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Agnostic is not quite the right word. Atheistic — no a-mass or a-physics would be better.

Spells do what they say they do and the do it by magic. The scientific explanation of magic is "         ". Similarly, the magical explanation of mass is "         ".

In terms of magic the expression "conservation of mass" is equivalent to "purple monkey dishwasher" and makes just as much sense.

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    \$\begingroup\$ +1 and I've re-worded the question so that it does not use "agnostic" but, by the way, that word does not necessarily relate to atheism (that's just the most popular application of it). Examples: New York Times Magazine Feb 3 '91 "Some greenhouse agnostics have used the report's conclusions to..." or J.E. Mercer's 1921 book *Alchemy" p. 61 "As regards alchemy, he was an agnostic." \$\endgroup\$ – Valley Lad Nov 22 '18 at 19:52

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