I'm looking into magic systems for my fantasy world, and among other things, I want to make sure that I have the effects right.

One such magic system allows the user to create a small 'bubble' of space nearby. All objects within this bubble (including the ground and air) have their mass reduced or increased. This works on an atomic level. The magic isn't adding new atoms or removing them, just increasing/descreasing their mass, and therefore increasing/decreasing their gravitational pull.

To the best of my knowledge, the mass of an atom is governed by how many neutrons it has, and changing that number would actually change what the atom is. Disregard that for this question. All atoms somehow stay the same with a new mass. Note that this effect only lasts as long as the objects remain within the bubble of magic. So if the mage stops the effect or the material leaves the bubble, mass returns to normal.

Question: I wanted to be sure I have the effects of this magic right, as in what would happen when the mass of objects was increased/decreased. I've listed my theories below, and would love to hear if they are right, or if they are wrong, why (and obviously what the right effect is). And of course, if there's an effect you know of which I haven't considered, I would like to know that as well. For the theories below, assume there's no limit on how far you can increase/decrease the mass of an object. I'm just trying to get a feel for what would happen; limitations come later.


  • Increasing the mass of objects would make them get heavier. Due to the increased gravity of the atoms, I would imagine they would also get a bit smaller if the magic was powerful enough.
  • Decreasing the mass of objects would make them lighter. I'm not sure about increasing size.
  • Extreme: You could potentially (assuming you had enough magic) increase the mass of an object (and thus shrink it down) to generate heat due to pressure. Thus you could possibly heat things up, turn them into dim lights, or if you go far enough, cause them to explode. Would I be correct in this?
  • Extreme: If you decrease the mass of an object far enough, the gravitational bonds of the molecules will be unable to hold onto each other, and the object will simply float apart as particles. I know atomic bonds are very strong, so I'm assuming the molecules would stay intact (and thus the material would remain the same), meaning something like a brick would just float away as a fine dust. Am I correct in this?
  • Assuming the previous is true, what happens when a low-mass dust leaves the bubble, and mass returns to normal? It won't reassemble into a brick, but would it reassemble at all? Would a rock form in mid air and fall to the ground? A bunch of pebbles? Nothing?

Note: This isn't a comprehensive 'would this work' question for this magic system. I just want to be sure I have the effects of it correct. How it works and related questions are separate and may be asked later.

  • $\begingroup$ I am not an expert but, are you looking to change the mass or weight of the objects? w = m*g $\endgroup$
    – V.Aggarwal
    Commented Jul 6, 2018 at 15:46
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    $\begingroup$ You didn't mark your question as "science-based", but if you start messing with the mass of electron, all material properties will be thrown out of whack. Imho, extensive handwaving is needed here. $\endgroup$
    – Alexander
    Commented Jul 6, 2018 at 17:24

3 Answers 3


There's a bit of a disconnect between atoms and objects, so here's some fundamentals in Nuclear Chemistry:

  1. Each Atom is comprised of Protons, Neutrons, and Electrons.
  2. The Nucleus of an Atom contains Protons and Neutrons
  3. The number of Protons define what element the atom is.
  4. The number of Neutrons define what isotope of the element the atom is.
  5. Both Protons and Neutrons give a substantial mass to the Nucleus
  6. Electrons orbit the Nucleus in crazy patterns since they move so fast.
  7. Protons attract electrons and vice versa.

Unfortunately, the "Neutron = Mass" approach is more of a problem than it seems. While Protons and Neutrons do dictate the atomic mass, they also define what kind of isotope and element the atom is. A lot of isotopes are not stable, and many can even be dangerous. Nuclear Decay could then easily occur, which would render the mass change void.

Furthermore, atomic mass doesn't necessarily translate to a mass of an object. That is governed by how tightly the atoms hold each other together. These are known as intermolecular forces. The tighter they are held together, the more of them in a specific space, meaning the mass of the object is higher.

For instance, what would you consider heavier: Iron or Plastic?

A layman would consider Iron obviously heavier. Even though you can reasonably have a kilogram of each without turning any heads, the layman is considering the density each object.

Depending on the plastic, they're held together by Van der Waal Dispersion Forces while Iron is held together with Metallic bonds. There are some nuances here, but Metallic bonds tend to be pretty strong, and thus more atoms in a smaller space result in a denser object.

So you may want to consider a density and space approach. All objects are contained within a boundary of space. Thus, if objects aren't changing size, you could manipulate mass by magically withdrawing atoms and exerting some sort of magical energy to have it retain the same volume.

(Fun fact: High Density Polyethylene, a plastic, is used as body armor!)

A way in which your Neutron = Mass approach could maybe work is if you were able to retain the same Intermolecular Forces in the object, add a Neutron to each atom, and prevent Nuclear Decay. Which, it's magic, so within the realm of possibility.

  • $\begingroup$ Thanks for taking the time to break this down in layman's terms. It looks like I need to take a different approach to this magic. $\endgroup$ Commented Jul 6, 2018 at 17:44
  • $\begingroup$ No problem! Glad to help! $\endgroup$
    – jedmeyer
    Commented Jul 6, 2018 at 19:07

Actually, your first two effects are wrong. More massive atoms are much larger than small atoms--to the point that small atoms (notably Hydrogen) can simply float through the gaps in more massive atoms. Larger atoms mean that as you add mass to objects by adding mass to their atoms, items would expand. Gravity only comes into play once the objects get very massive (on the scale of stars massive). Likewise, reducing mass will mean the objects shrink.

Another way to think of it is if you have a 1lb block of wood and a 2lb block of wood. The 2lb block has approximately twice the mass (depending on the accuracy of measurements), and is larger, even though the extra mass means (very, very slightly) more gravity within the elements.

  • $\begingroup$ Welcome to Worldbuilding, Hosch250! (Fancy meeting you here!) If you have a moment please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox (both of which require 5 rep to post on) useful. Have fun! $\endgroup$ Commented Jul 6, 2018 at 15:27
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    $\begingroup$ Massive atoms are more sizeable because of their expansive "electron clouds". Mass of atomic nuclei (with constant change) has virtually no effect on atom's size. $\endgroup$
    – Alexander
    Commented Jul 6, 2018 at 17:21
  • $\begingroup$ Hmm. In that case, the first two points may be right. $\endgroup$
    – user39548
    Commented Jul 6, 2018 at 17:31

When I originally read the question, I understood it differently than the first two answers. One answer seems to assume that the atomic particles (protons, neutrons, and electrons) stay the same size and mass, but how many of them are in each atom is what changes. The other assumes that the number of them in each atom stays the same, but their size changes.

I understood the question to mean that each particle stays the same size, and the number of particles in each atom stays the same, but the mass (and therefore the density also) of each particle changes. For example, a single proton in a bubble that increases mass, would be the same size as a single proton outside the bubble, but it's mass would increase and therefore the effect of gravity on it would also increase. (this is actually similar to what the selected answer suggests that you consider). I'm working with little more than layman's level knowledge of sub-atomic particle physics here but, as I understand it the effects of mass at that level are negligible unless the change is EXTREMELY severe (the only thing I know of that is so extreme is when a star's mass becomes so great that its gravity becomes so strong that it crushes Electrons down to the point that they come in to contact with Protons, instead of orbiting them, and become Neutrons, creating a neutron star). If this is the case, then until you get to that extreme level of change in mass, nothing much noticeable is going to happen to size of objects and distances between atoms and molecules within the bubble.

HOWEVER, since you've indicated that individual particles and objects can freely pass into and out of the "bubbles", even (relatively) small changes in mass would create some interesting effects on the overall objects themselves. For example, assuming the bubble was a sphere, and mass were reduced, the air inside the sphere would be much lighter (due to reduced pull of gravity on its reduced mass) than the air outside of it, so it would rise up out of the top of the bubble. This would leave space in the bubble for more air, which would be pulled up from any of the underside of the bubble that is not in contact with the ground, creating an updraft for as long as the magic remained active. A person inside the same bubble would also be lighter, and could jump higher (until they begin to exit the top of the bubble and start to regained their normal weight). Thrown objects would be similarly affected. For a bubble that increased mass, you would get a downdraft of air, a person would feel heavier, anything they are wearing would feel heavier. Thrown objects would have a steeper downward arc until they exit the bubble, etc. Extreme mass increase could cause someone to collapse under their own weight, in a fantasy setting, something like maille armor could actually crush the wearer, branches of trees could be ripped off under their own weight, bridges collapsed, etc.

For this type of magic system, the shape, size, specific positioning of the "bubble", as well as how fast they can be created, and how fast their effects are felt, then become interesting factors to consider. Can you create one that appears and takes immediate effect, and appears in such a way that only half of a person is affected at the time of creation? That would cause serious balance issues, or force a tool or weapon to be yanked from the hand, even if the intensity of the change was very low. Or a long, wide, low, flat shaped one could make a marching army feel like it was walking through thick mud even on a grassy field or paved road.

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    $\begingroup$ I'd give +1 for the final example of the army alone: pure genius. $\endgroup$ Commented Sep 20, 2018 at 23:02
  • $\begingroup$ Thanks. I just threw in a few random examples that came to mind in the moment. But what has always been the most fun, to me, about magic systems is not so much what the magic does, but what the creativity of the magic user can use it for. $\endgroup$
    – Harthag
    Commented Sep 21, 2018 at 13:20

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