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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, 2018 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, 2018 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, 2018 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, 2018 at 2:25

2 Answers 2

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D&D is not designed as a completely simulationist game

The first thing to note is that D&D is not a designed to be a strictly simulationist game. It contains many rules issues that break down if you try to apply them in a strictly 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\$ 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, 2018 at 1:28
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    \$\begingroup\$ @ValleyLad Who says matter in D&D is composed of atoms? D&D explicitly operates on the four-elements theory of matter and metaphysics, not the table-of-elements one. Gravity and time in D&D also don’t obey the our-world theories of relativity. D&D physics and metaphysics are not our world’s, and conservation of mass is a law that interdepends on our world’s other physical laws. \$\endgroup\$
    – SevenSidedDie
    Sep 4, 2019 at 19:41
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    \$\begingroup\$ @ValleyLad It remains that any objection founded on “but X violates the laws of physics” regarding how D&D magic and worlds work is the bigger non sequitur. “Creation ex nihilo” of matter and energy is done by many spells—grease, web, light, fly, goodberry, feast, to name a few. Enlarge isn’t exceptional in that regard. \$\endgroup\$
    – SevenSidedDie
    Sep 5, 2019 at 6:33
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    \$\begingroup\$ D&D usually says “nothing” about where the matter and energy come from, and “ex nihilo” means “from nothing”… I don’t know what you want from the game then. It just doesn’t care about this detail. Perhaps you want to call it “creation from apathy” if the evident “from nothing” is unacceptable unless it’s literally printed so. But “from apathy”… that’s getting meta-textual, not metaphysical. \$\endgroup\$
    – SevenSidedDie
    Sep 5, 2019 at 14:11
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    \$\begingroup\$ Saying nothing about where it comes from is not saying that it comes from nothing. I am merely trying to confirm if the rules are silent about this. There is an ambivalence in some of the comments: after saying the rules say nothing, then there's a jump to make that mean that matter must come from nowhere. But the rules don't say that either, that I've seen. I was asking if I'd missed a RAW statement like "your Patron creates new matter in the shape you desire". We should probably move this to chat. \$\endgroup\$
    – Valley Lad
    Sep 5, 2019 at 17:33
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RAW is silent on (even indifferent to) conservation of mass

Spells do what they say they do, and they do it by magic.

  1. The scientific explanation of magic is "         ".
  2. 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, 2018 at 19:52
  • \$\begingroup\$ Matched the title to the content and the question, and accounted for the rewording of the question by ValleyLad \$\endgroup\$
    – KorvinStarmast
    Sep 4, 2019 at 17:03

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