I'm developing a world where certain people have affinity to modify/control certain aspects of the material plane. In the early stages I'd included the good old "element control" premise, but as I developed it further I started exploring ways to involve physics in it. I looked into this but couldn't find anything too reliable for the time being - would someone able to shift molecules and affect their density in space be able to change certain materials' physical state? (for instance, solidify/vaporize water)

Many thanks~

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    $\begingroup$ What exactly do you mean by that? You might benefit from researching what vaporizing and so on means, intermolecular forces, what density is and so on. Here is a well-meaning tip for you: Don't build your worlds on things you do not understand. If you do not have a solid background in physics or chemistry, messing with molecular physics is a disaster waiting to happen. It is ok to handwave certain things because nobody can be a master of everything $\endgroup$ – Raditz_35 Aug 10 '18 at 9:05
  • $\begingroup$ True, I guess part of the question was actually how farfetched something like this might sound, even if it somehow physically made sense. Many thanks for replying :) $\endgroup$ – Vmf Aug 10 '18 at 9:24
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    $\begingroup$ I highly recommend viewing this from the perspective of Sanderson's First Law of Magic: "An author’s ability to solve conflict with magic is DIRECTLY PROPORTIONAL to how well the reader understands said magic." Above all else, that is the defining rule to let you know whether you should let your magic do something or not. $\endgroup$ – Cort Ammon Aug 10 '18 at 21:49

Different molecules stick together into a solid because there is a certain amount of attractive force between them. To break them apart, you must exert enough energy to overcome this force. As a metaphor, imagine pulling on a rubber band. If you only pull a little, the rubber band snaps back, but if you pull hard enough it will break.

You mentioned vaporizing water. The kind of energy we put into water to vaporize it (a.k.a. boil it) is heat energy. What we call "heat" is a shorthand for how tiny particles vibrate or move around and bounce off each other. If you put a cold object next to a hot object, the molecules from the hot object bump into the molecules of the cold object, transferring some of their energy and making the cold object's molecules vibrate more. Seeing this, we say that the heat is flowing from the hot object into the cold object. We can also transfer heat energy without physical contact: a hot object shoots out some of its energy as electromagnetic radiation (visible light, infrared, etc.), and a cold object absorbs it.

The point is that once a molecule is vibrating fast enough, the intermolecular forces can't hold it any longer—the rubber band snaps, so to speak—and it flies off. The water boils.

What does this mean for your theoretical "molecule-shifter"? Well, you could say that their real power is the power to control heat. Maybe they have to obey most of the laws of physics, and so they have to "spend" a certain amount of their own personal energy to make an object hotter, or suck heat out of one object before they can pour it into another. Or maybe they can tap into some other source of energy like <insert hand-wave here> that allows them to do as much heating as they want. It's up to you.

(P.S. I've been talking mostly about heating or vaporizing objects. Cooling or solidifying them is just the same process in reverse: a particle loses energy and moves slower until it is slow enough to get trapped by those intermolecular forces. Also I didn't talk much about the solid-to-liquid transition: a liquid is a state where the particles have enough energy to leave their fixed position and start sliding around past all their neighbors, but not enough energy to fly off somewhere.)

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  • $\begingroup$ Welcome to Worldbuilding! I think this is a great answer! Just to clarify for anyone reading who isn't aware of this part of thermodynamics - changing the temperature of a system can indeed be used change the density of the system. Feel free to join us in Worldbuilding Chat and have a look around Worldbuilding Meta $\endgroup$ – Mithrandir24601 Aug 10 '18 at 21:19

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