What would someone who can alter gravitational (but not inertial) mass for a period of time be able to achieve?

A gravity mage is someone intrinsically able to alter the gravitational mass of objects, without affecting their inertial mass.

They are able to do so only to objects they touch, and when they change the G-mass of something, an equal and opposite change occurs to their own G-mass. This change is not the permanent - the G-mass 'wants' to return to its original object. So, the moment it is done, G-mass begins to move back to where it came from. However, depending on their skill and strength, the mages can stop this for a time, though even they can't stop a minuscule trickle back to the source, like small holes being made in a dam slowly, until a point comes where the dam breaks and the G-mass "floods" back to its original source. Anywhere between 5% - 30% of the mass is lost before the big return, depending on the skill of the mage. This big return generally happens within half an hour for an average mage, but skilled mages can make their alterations last for up-to 6 hours.

A mage cannot reduce their own G-mass to 0,or even that of another object, though stronger and more skilled mages can come close, but it is a tedious process that tires them greatly. They can, however, make themselves as heavy as they like. A gravity mage who does not express caution will soon find most of their bones broken in an accident.

However, they CAN get around these limitations: by altering another object. Even a moderately skilled mage can alter two objects simultaneously. This essentially allows them to transfer G-mass from one object to another. Less skilled mages can do this in stages, by altering one object first, then oppositely altering the other, repeatedly until the desired result is produced.

Altering too many objects can cause a problem. Different objects lose G-mass at different rates, and a mage may inadvertently give more attention to a certain object over another. This can cause too much change suddenly in their own body, which can be fatal. So only very experienced mages attempt something too complex.

What can be the applications of this power?

Already, I believe they will be used in tandem with siege weaponry - making very heavy rocks very light to throw them over a large distance, and then letting the mass return once it is over the enemy castle (though is the last part even necessary? As the inertial mass is unchanged, I believe it would apply the same force and thus cause the same destruction regardless. It WOULD fall faster though, so perhaps the increase in kinetic energy is enough of a danger). They will also be powerful assassins who can easily make structures collapse or drop heavy objects on people, and they will be useful when carrying heavy loads up mountains, for instance.

Some limitations:

1. They can only alter what they are touching. However, once they have altered it, they can let the G-mass return whenever they want, by not resisting it any longer. So while they can only cause a change in very close range, they can potentially reverse that change prematurely whenever they want.

2. They are limited by the magnitude of the changes made. The rate at which they tire, both physically and mentally, is directly related to how many kilograms of G-mass have they displaced, and how quick they have displaced it. A mage of the same power can either displace 50 Kg in a minute or two tons over multiple hours. However, in practice the latter is more tiring as the G-mass that they have already displaced (twice, as they had to put all the extra mass somewhere) would be pressing harder and harder to return to its original source.

3. They are not abundant. They are not present in large enough amounts so as to be readily available when something needs done, especially for civilians. This is in part due to the extermination of all mages other than a certain kind, that is being carried out by a large warmongering empire at the time. So most G-mages are either in hiding, or in an army fighting against the murderous empire.

With these limitations, what larger scale things would gravity mages be able to achieve?

(By larger scale, I mean something other than the immediate personal level or team level. For instance, if trained well, they would be amazing fighters, as they can not only alter their own mass but also that of their enemy, whenever they want. If you allow one to touch you, they can not only disorient you once, but tactically undo their changes a bit at time. However, I consider this small scale application. Examples of a larger scale application are what I gave a bit earlier)

• Can they drive this power into negative G? Like, they can take 2 10G objects and make them for a short time into 15+5G, or even 20+0G. But can they make them for any time at all, into 30+(-10)G? Commented Aug 22, 2021 at 10:50
• No. As I said, it is difficult for them to even approach 0 G, for any object. They can, at most, remove a vast majority of an object's mass and place it somewhere else (by increasing the mass of another object simultaneously) Commented Aug 22, 2021 at 10:58
• It seems this would make two types of mages, one that try and gain as much weight as possible to be able to add as much mass to objects as you can. And then another type of mage that is super strong to allow themselves to carry as much mass as possible. So sumo wrestler type and body builder type. Commented Aug 22, 2021 at 11:13
• Let's start with perpetual motion energy-producing machines, and go on from there. Commented Aug 22, 2021 at 16:09
• There is no difference between inertial and gravitational mass Commented Apr 21, 2022 at 9:56

So for example, you dont just take the mass away, throw a rock, then let the mass return. You transfer the mass to a rock, throw/drop, and slam all the mass back - along with all the mass in a nearby mountain, as well - in midair, just before impact.

But I think you have something the wrong way round.....

Its inertial mass that makes it hard to stop an object, or lets an object do damage, not gravitational mass. But it's also inertial mass that has to be overcome, to move it.

If you cant alter inertial mass, yes you can lift a big rock, but its still extremely slowww. Because F=ma - Newtons laws of motion. You can only exert so much force,so that huge rock will lift at 5mm per day, which you have to constantly sustain with no help from magic..... , and its velocity will be small unless you can accelerate it with much much more force, or much much more time. And, of course, your opponents mages have exactly the same ability to decelerate or oppose it.

Also, friction may depend on gravitational mass for large objects. But unless the object is perfectly flat and smooth, any irregularities in the surfaces of the object and whatever it's supported by, will still jam, because you have to overcome inertial mass to lift it above them.

Similarly, just because gravitational mass vanishes briefly a structure wont necessarily collapse. Gravitational mass reduction won't destabilise most structures. Greatly increasing it, might do so, by making the supports buckle, but all that means is that the rich and powerful dont go near structures that arent built to resist huge extra gravitational mass. Or their own mages oppose it.

Overall - pretty useless except in rare cases??

Infinite energy

1. Build a hydroelectric power plant
2. Enchant some mass of falling water to be heavier.
3. Pass that through the turbines
4. Disenchant that water
5. Use part of the energy to take that mass of water back up. You will still have a surplus of energy.
6. Rinse and repeat.
7. ????
8. Profit!!1!

On the one hand you will be lynched by the auditors of reality, but on the other hand you've solved the world's energy crisis.

• Good thinking, but it really doesn't work, for two reasons. The first is that they are not at the technological stage to consider hydroelectric power plants, and the second that there IS energy being used up in this transfer, it IS coming from somewhere (which I won't discuss because it's supposed to be a mystery for my story), but safe to say, while this WOULD solve the world's energy problems for a while, it would cause a lot more...existential problems for the people a decade or so down the line. Commented Aug 22, 2021 at 18:30
• @TheInfiniteOne neither of those points is clear in the question. Also water mills werea thing in ancient Babylon already. Commented Aug 22, 2021 at 20:34

I think the answer to this is "nothing". Gravity is a very weak force, so you need a lot of gravitational mass to do anything with it. The planet Earth has a mass of roughly 6 × 1024 kg, and that only produces a gravitational force of about 10 N / kg at the Earth's surface. The effect of changing some object's gravitational mass by 50 kg or 2 tonnes is completely negligible for all practical purposes. For intuition, next time you stand next to a truck, see if you can feel any gravitational pull on your body towards that truck. (You can't, of course.)

Your mages would have a hard time even proving to anyone else that they have such an ability, before the invention of extremely precise scales. As far as the rest of society is concerned, your mages are just as deluded as those fools who say they can turn invisible but only when nobody is looking.

If you want your mages to be able to telekenetically exert forces on objects to alter their movement, gravitational forces are not the way to do it. If your society's understanding of physics is primitive enough, then they can call it gravity; but if you want to do this in a science-based setting, you need it to be a different force, not gravity.

• I think the point is not to make the enchanted objects emit more or less gravity themselves, but rather that their response to Earth's gravity is changed.
– Qami
Commented Aug 22, 2021 at 17:23
• @Qami Well, that would violate conservation of momentum and conservation of energy. Since the question is tagged science-based, I think I have to interpret it in a way which at least doesn't clearly violate fundamental scientific laws. I guess I'll leave it to other answerers if they want to interpret the idea differently. Commented Aug 22, 2021 at 17:59

Consider a solid piece of wood laying on ground. There is no movement. Since there is no movement either no force act on it or if there are forces acting on it they must have balanced each other, if they havent then there would be movement but there is no movement so net force has to be zero.

Now remove one of the forces, gravity. The balance is now broken, net force is no longer zero.

If gravity is removed things explode.

If gravity is quickly restored the explosion is stopped quickly, parts dont get far from each other, they just fall where they reached. They dont combine together.

To combine them together you need a net force in opposite direction. You dont have that. So, things stay wherever they have reached in the explosion.

As long as you dont re-plug gravity, thing continue exploding, that is its parts continue moving away from each other, as long as they are not picked by other things. Remember you are in this world removing gravity of just one thing, not of all things. The gravity of non-affected things still work.

Gravity is like an endless pool of a resource. Put another moon in earth's orbit and earth's pull on moon dont reduce, it stay same. Therefore the gravity of other things can handle the increase in objects they have to pull, namely the moving-away-from-each-other parts.