My magic system is based on science. “Nothing can be done with magic that a scientist couldn’t eventually accomplish. Casting a spell is just using the Will to manipulate atoms and energies within the rules of science to create some effect.” As such conservation of energy is in play and so creating something from nothing is impossible without converting energy into mass or reverse.

So I have two questions on how to make spells:

  1. In this magic system, how can a spell caster create a shield to stop kinetic energy (specifically bullets but hopefully whatever process stops bullets could work for slower/heavier impacts).

    • The handwavium answer I have for this to simply steal the kinetic energy of the object since my magic system allows for the direct manipulation of any form of energy.
    • That has problems though. Firstly, the caster now has all this kinetic energy, something has to be done with it. Secondly, I don’t understand kinetic energy well enough to figure out how that complicates the story. (If someone could give me some insight as to how stealing kinetic energy might complicate the story rather than offering an alternative answer, I'll take that gladly).
  2. With this system and the rule listed below, how can a caster unleash something equivalent to a telekinetic attack?

    • Since magnetism is easy enough to generate and then amplify, that is a possibility but I would like something that doesn’t care if the object is magnetic and that can affect humans even if only indirectly.

Some more quick rules about my magic system because they may be important to answers.

  • It’s impossible to manipulate atoms/energies within another person’s aura (3-5 inches from their skin).
  • It’s easier to amplify energies than suppress them.
    • to increase a magnet’s power, the energy comes from the caster and to comply with that ‘for every reaction and equal and opposite reaction’ law, the reaction is in the cost of the spell on the caster.
    • To suppress an energy still takes something from the caster but there is the addition that, they have to do something with that energy being suppressed – it cannot go back into the caster for later use.
  • Atoms can be moved around at relatively little cost but to cause any sort of chemical reaction requires the same energy that would be otherwise required to catalyze it.
  • Energies like electricity are easier/cheaper to manipulate than more fundamental forces (gravity, the strong force, etc).
  • If there are any other rules that needs to be known ask, I have lots.

EDIT; the aura is an extension of the human will (unconscious and out of control) and as such all the atoms and energies in that field are always being touched by a person's will and can't be overridden by another person's. It's not a function of any other rule (like suppressing energy being harder than amplifying it).

  • 4
    $\begingroup$ Energy is easy, because only total energy is conserved, and energy can be converted from one form to another. Momentum on the other hand is very hard; both linear and angular momentum are conserved quatities, and you cannot convert it into anything else. For example, a bullet has both a certain amount of kinetic energy and a certain momentum; you must do something with both those quantities. $\endgroup$
    – AlexP
    May 5, 2018 at 18:32
  • $\begingroup$ Hi. I edited your question to try make it easier to read and to make it easier to refer back to when looking for a particular line. feel free to rollback (undo) if you don't like the changes. I didn't change any of the meaning just made some bullet lists to separate out questions from the examples and rules :) $\endgroup$ May 5, 2018 at 21:00
  • $\begingroup$ the science is probably more than unfounded, in the marvel movie the new suit made for Black Panther by his inventor-genius sister did something like that. storing kinetic energy for later :) $\endgroup$ May 5, 2018 at 21:19
  • $\begingroup$ Suggesting a useful benchmark for this question - taking the AK-47 as an arbitrary reference for bullet's speed, mass and energy, you get the following values (in SI units): muzzle speed: 718m/s, bullet weight: 0.0079kg, muzzle energy: 2,036j. wikipedia source. Also - remember that kinetic energy is 1/2mv^2, where m is the mass and v is the velocity. $\endgroup$
    – G0BLiN
    May 6, 2018 at 10:39

7 Answers 7


I'll try for the simplest answer.

Freeze the air around you. This way you have a proper shield and you don't have to focus on bullets, whether they are just one or many.

Like sound and light, bullets also experience refraction when they go from one medium to another. By having a layer of frozen air around you, besides having to go through much more compact air, a bullet could also be refracted away twice.

All the energy taken away from air molecules to freeze them could go into a mass of air anywhere else.

As for the retaliation attack, just throw the frozen air at whomever shot at you.

  • $\begingroup$ I considered that, but my understanding is that air at STP isn't a whole lot of atoms per volume and you would need something rather solid to stop a bullet meaning it would take a lot of air to condense to even a modest sized shield. Or am I wrong in this thinking? $\endgroup$
    – Seserous
    May 6, 2018 at 14:38
  • 2
    $\begingroup$ @seserous liquefy air and it gets a thousand times denser. Solid, it gets even denser. A couple cubic meters of air would make for a very thin shield. But that shield would be be hard to penetrate (it would be like piercing the sonic boom barrier). Might not stop a sniper rifle bullet at point blank but should do for pistols and revolvers, for example. $\endgroup$ May 6, 2018 at 15:20
  • $\begingroup$ In a science based way, please explain how a person stays alive inside of a lake of liquid air? Alternately, explain how frozen or liquefied air avoids falling into a lake around you? $\endgroup$
    – kingledion
    May 6, 2018 at 18:09
  • $\begingroup$ @kingledion if you are telekinectic, you can hold it around you, no? $\endgroup$ May 6, 2018 at 18:21

Momentum is the real problem…

AlexP's comment hit it right on the nose. If all Noether's conservations still apply, then energy, linear momentum, and angular momentum all have to be conserved.

Kinetic energy is the first one you have to deal with. Fortunately, it's also the easiest. Only the total amount of energy is conserved, and it can be freely changed from one form to another (with a bit of loss to entropy). So you could deflect it in a slightly different direction without trouble.

Linear momentum, though, is harder. You can't freely change its direction or its form; it's always momentum, and nothing can change that. Fortunately, though, momentum is proportional to $v$ rather than $v^2$, so the momentum of a bullet is much less dangerous than its kinetic energy.

Angular momentum is the last concern: if you want to deflect a bullet, you need to change its direction slightly, which requires balancing out that angular momentum somewhere else.

…but spreading it out helps a lot.

Let's say a bullet is flying toward you. You need to deal with the kinetic energy, the linear momentum, and the angular momentum. The kinetic energy can be dealt with by changing its direction slightly, so let's not worry about that.

But now the difference between its initial velocity and its new velocity, including direction, causes a change in momentum. This has to be balanced out by an equal and opposite change in momentum somewhere else. (A change in momentum, notably, is Newton's definition of a force.) So perhaps our mage applies this force evenly across their entire body. Even if they wanted to stop the bullet dead in the air instead of kicking it slightly to one side, they'd only be hit by exactly the same force that the would-be assassin felt when they fired: the recoil of the gun (minus some energy lost to heat and air resistance). If they sent it straight back where it came from, they'd feel a force equal to twice the recoil.

Finally, the angular momentum: it's much harder to change something's direction when it's spinning. That's why gyroscopes stay up. A rifle will give the bullet a significant spin (angular velocity), which will make it harder to redirect. But fortunately, a bullet is small, and thus has a very small moment of inertia compared to a person. The mage could take the bullet's angular momentum into themself and feel only a very slight kick compared to the recoil of the deflection.

  • $\begingroup$ Ooh, very insightful here! $\endgroup$
    – Alendyias
    Jan 12, 2021 at 14:13

If you allow this magic system to move kinetic energy around even for shields and to stop bullets, conservation of momentum goes out the window, or is made much more complicated (momentum is conserved along the psi dimension where magic moves it to, or some other handwavium).

I'm not saying that this is wrong for your purposes, I'm just saying that there are many conserved qualities in our world and that changing even one of them changes a whole lot of things.

If this is for a book, well, if the writing is good enough and you don't invoke science, then it could work. I've had problems with magic in the urban fantasies I've read where the author has tried to tie magic too closely to science as we know it. The better authors just say something like "magic belongs to a superset of physics as we know it" or don't even bother to explain it.

It’s easier to amplify energies than suppress them

Then why is it impossible to manipulate things within a few inches of a person? This involves some sort of suppression, which is more difficult than amplification.

That's my opinion. I think you have an interesting question.


The answer applies to both your questions: the spell caster can transfer kinetic energy from a body A to a body B.

If he has to dissipate the kinetic energy of a bullet, he will transfer it via the spell to a close by body (i.e. the water of a lake or the sand of a pit). He will basically turn motion into heat, which is also what the bullet would have ended up doing in the target's body.

On the other hand, to create a kinetic attack, he can harvest kinetic energy from a body (again, it can be a lake) and convey it to a bullet. Kinetic energy can be harvested at the expenses of the molecular motion, thus the supplying body will be somewhat colder.

  • $\begingroup$ I have the image of fantastimorphic glass creations being created if you are directing heat into sand :) curiosity.com/topics/… $\endgroup$ May 5, 2018 at 21:07
  • $\begingroup$ Remember that harvesting energy from heat only works if the source is above "room temperature". Entropy is a bitch. $\endgroup$
    – Draconis
    May 5, 2018 at 23:53
  • $\begingroup$ You're still missing the science-based way to transmit KE from a lake to a bullet. Unless you are dropping thermodynamics from your list of 'things that are science.' $\endgroup$
    – kingledion
    May 6, 2018 at 1:01
  • $\begingroup$ @kingledion kinetic and heat energy aren't interchangeable then, with kinetic being able to transfer in the same way as heat? (I know heat is movement on the atomic level, kinetic movement on a larger scale) $\endgroup$
    – Seserous
    May 6, 2018 at 16:22
  • $\begingroup$ @Seserous Not exactly. Energy conversion has leftovers, ∆S. aka entropy. Every time your spell caster does this trick a little heat is left over, and this heat can't be reused, so a side effect of his magic is the flattening of energy gradients in his location. Fortunately, he has the Earth to draw from. Heat engines work better the hotter they are. Do this same bullet trick with say, chunks of earth crust falling into the mantle during subduction, and he'll have plenty of energy to work with. Move to Hawaii, or Icleand perhaps $\endgroup$
    – chiggsy
    May 22 at 15:56

It's all about Vectors

Say a bullet is coming straight to your mage, simply stopping the bullet could be too much work, so instead simply change the force vecto and suddenly the bullet is going straight up.

Of course, more absent-minded mages could have some problem if they forget where their allies are.

Living Revolver

Here how a mage attacks, he quickly and efficiently converts chemical energy into kinetic energy, flinging small darts of steel towards the enemy.

The mages are known for carrying around small packs of alcohol, powder, or other high energy density material to use when they need.

  • $\begingroup$ Conservation of energy doesn't care if you change the direction, but conservation of momentum certainly does. $\endgroup$
    – Draconis
    May 5, 2018 at 23:51
  • $\begingroup$ @Draconis The mages described probably could solve this. $\endgroup$
    – Sasha
    May 6, 2018 at 0:52

Your telekinetics draw off the kinetic energy into their own bodies. Consider: when I fire a pistol the equal and opposite reaction of the bullet firing is transmitted through the pistol to my hands.

Kinetic energy is $1/2mv^2$ where $m$ is mass and $v$ is velocity.

You could get away with taking the kinetic energy of the bullet into your body because it is not very massive. If I telekinetically stop a bullet I can brace myself and transmit the energy to the ground, just like if I catch a baseball. I might be slid backwards by something with a lot of kinetic energy. Stopping a moving car would send my body flying - I am less massive than the car and so on absorbing its energy I would move even faster than it was moving. But a team of us together could stop the car.

Likewise if I telekinetically move a ball, I confer that kinetic energy - maybe by swinging my arm as though I threw it. Maybe I run full speed and when I transfer my energy to the ball I stop and it goes.

This opens the possibility of a trick for stopping the car. I absorb its energy and go flying. Then I confer my kinetic energy back to the car (or something else). My body stops moving and the car goes off in a different direction, with a substantial fraction of its original speed (kinetic energy).

  • $\begingroup$ A good idea, but you're conflating energy and momentum: the equal and opposite reaction is a force aka change in momentum, whereas the energy of the bullet comes from the chemical potential of the gunpowder. Sending a car in a new direction might not change its kinetic energy, but all that momentum still has to go somewhere. $\endgroup$
    – Draconis
    May 5, 2018 at 23:54

Its all about thermodynamics

Everyone wants to talk about physics, because you learn that stuff in, like, 8th grade, and the equations are pretty simple. Momemtum and energy are conserved, etc.

But the real problem is with thermodynamics. No energy transfer is going to be without loss. Consider the equation for the efficiency of a Carnot engine: $$\eta_{max} = 1-\frac{T_c\Delta S}{-T_h\Delta S}.$$

If you are a human operating at 37 C; and you happen to be in an enviornment where nothing else is above 37 C (a pretty common occurance if you are outside in the winter, for example), how are you expected to draw energy from your environment? If you can't draw energy, how are you going to expend that energy using telekineses or a kinetic shield? Likewise, if the bullet is over 37 C (which is probably is after coming out of a gun), how can you transfer energy to it?

Even if you can find some slightly warmer objects to absorb energy from, and a cold bullet to deflect (maybe you are in a wizard duel with Elsa?), the rate of transfer of such energy will be very slow, and since entropy loss will be non-negative (and in real practice, non-zero), you are going to lose some of your energy transfer to the enviornment anyways as 'friction' losses.

Lord Kelvin denies your 'science-based' magic

Forget physics and momentum and all that. The second law of thermodynamics and time's arrow mean that you can't just move energy around all willy nilly.

  • $\begingroup$ Can you be more specific about "can't just move energy around all willy nilly" because in normal every day life, that's what happens all the time. Pick up a rock, put it on a hill, watch it roll down. That's moving energy around isn't it? Chemical in my body becomes potential becomes kinetic, right? $\endgroup$
    – Seserous
    May 6, 2018 at 14:40
  • 1
    $\begingroup$ @Seserous Energy operates on a gradient. For heat transfer, there is a temperature gradient, where energy naturally flows from high temp to low. Gravity is a gradient field (the gradient being acceleration), where things move from high potential energy to lower potential energy. Electric energy operates on a gradient (which is electric potential or voltage) where charge flows from high potential to low. There is no gradient where the bullet is at a lower potential than the person trying to deflect it, so energy doesn't go that way. $\endgroup$
    – kingledion
    May 6, 2018 at 18:07

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