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One of the main characters of my book has gravity magic, and I'm struggling to find out how it would work. From what I've read, gravity is the result of immense mass bending space-time, so I have no idea how to explain that. I know it's magic, but I want magic in my book to be limited by the laws of the universe. Does anyone know how to explain gravity powers as they're normally depicted in fiction (levitating people, creating anti-gravity areas)?

Edit: To further explain things, is there a way to have gravity powers without the density of a planet?

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    $\begingroup$ If your magic is limited by the laws of the universe, it's physics, not magic. $\endgroup$ – Nuclear Wang Nov 18 '19 at 21:08
  • $\begingroup$ @NuclearWang Clearly the physics system of the universe supports something we'd consider magic. Similar to how chemistry and alchemy were, at one point, hard to distinguish from one-another -- it might become physics at some point but for now it is magic. $\endgroup$ – Zwuwdz Nov 18 '19 at 21:11
  • $\begingroup$ Isn't magic, by definition, something that is outside the laws of physics? Are you asking how levitation & antigrav can be consistent with the current understanding of physics / general relativity? $\endgroup$ – Richter65 Nov 18 '19 at 21:14
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    $\begingroup$ @Richter65 isn't it just defined as "sufficiently advanced technology"? $\endgroup$ – Starfish Prime Nov 18 '19 at 21:14
  • $\begingroup$ @starfish: ah, yes, the old Arthur C Clarke definition. The problem with that is that while you can have advanced technology doing stuff that looks like magic, it's much harder to go the other way around. $\endgroup$ – Richter65 Nov 18 '19 at 21:16
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Does anyone know how to explain gravity powers as they're normally depicted in fiction (levitating people, creating anti-gravity areas)?

In all of these works of fiction, does anyone ever bother to distinguish between "gravity" and "telekinesis", beyond "well, its gravity innit" and "its mind powers, innit"? I cannot think of any that do. Unless you can think of a way that makes gravity powers somehow clearly distinct from telekinesis, there's no reason to call it gravity or to invoke gravitation at all other than the fact that "gravity" just sounds cooler.

Anyway, you seem to have dug yourself into that hole already, so I'll suggest one possibility for you.

gravity is the result of immense mass bending space-time

is only kinda correct. Gravity, of the sort that's strong enough to be interesting is the result of lots of mass in a small space bending space-time. A cloud of gas the mass of the sun worth of matter spread out over a lightyear isn't really very interesting. Compress it down into a space 3km across and suddently it becomes very interesting indeed.

The force of gravity is defined as $$F = {GM_1M_2 \over r^2}$$

Lets imagine a chunk of rock with the same density of Phobos, that weighs a mighty 300 million tonnes. Its surface gravity will be a miniscule fraction of Earth's... about 1mm/s2. Compact all that down inside a handy warp field such that you can fit all the mass within a 1m radius area and suddenly its surface gravity is twice that of earth. Inside the warpfield the rock still has its original density and mass, but from the outside it appears to be exceedingly dense.

By conjuring and controlling mere mountain-masses of material stored in miniscule warp fields, very strong gravitational fields can be developed, but unlike planet-sized gravitational fields their strength will drop off very rapidly so you won't end up wrecking the entire planet you're standing on. The number of these warps that you can conjure, the strength of the gravitational fields that they generate and the finesse with which a cloud of warps may be manipulated would of course depend on the skill of the magician, but all of their powers would derive from this one building block. Levitation, for example, involves forming and positioning enough warps above a target to provide an upward force that exceeds the pull of regular gravity downwards.

The Orion's Arm universe has Halo Drives, a reactionless motor inside a warpfield that tows around a ship by gravitational and gravitomagnetic effects, because the mass of the drive in such a tiny volume generates a very strong gravitational gradient. They talk more about the nature of the space-time metrics (the geometry of the warp fields) involved here, with some academic references at the bottom, if you were interested in a non-magical explanation. In that case you wouldn't be able to conjure up a warp at will, but assemble it with great difficulty ahead of time, and then when you weren't using it you'd have to park it up in orbit so it didn't tear up everything around you. Sometimes there's something to be said for magical handwavium for keeping things neat...

You may also be able to do the same trick by handwaving the warps as wormholes instead, but really it just boils down to the technobabble you prefer. And maybe wormhole metrics can't survive close proximity to intense gravitational fields. You decide.

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The ability to adjust the gravitational constant

As pointed out by Starfish Prime, your assessment of gravity is partially correct. Gravity is the force that governs the attraction of two objects of mass in proportion to the distance between the objects. Or just F = G*M1*M2 / d^2 - the force is the first mass multiplied by the second mass multiplied by the gravitational constant, and then all that divided by the squared distance between the mass. The gravitational constant is pretty small - G = 6.67 * 10^-11, or 0.0000000000667. This is good, as only objects with very large mass can affect gravity.

But let's say that for two given objects, say a rock and your target, you can increase that constant by a factor of 1 trillion, or even more. (Not the object's gravity to itself, because then it might implode, just to the target mass.) Then the rock (and the head) would accelerate to each other at equal speeds, and very large speeds at that. There's no need to adjust density to increase mass when you can just affect the G component of the formula instead.

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