Is artificial gravity impossible?

[Dark Flame]

Well, this is my first post, so in addition to asking a question I will introduce myself. I am 17 years old, in case anyone is curious. I have been reading these forums (mostly PSW) for the past several months, until I realized that I have a paid-for email account. I am a SW fan, but I've never really liked any other form of science fiction.

Anyways, I was thinking the other day, and I realized that artificial gravity (as seen in SW) is impossible.

For example, say there is a square room in space, and it is under the influence of artificial gravity. Assume that there is a box sitting on the floor, where it has no potential or kinetic energy. Now, all of a sudden there is a shift in the artificial gravity, making the floor into the ceiling. Now the box sitting on the floor is now on the ceiling, and it has potential energy, which turns into kinetic energy. This energy doesn't come from anywhere, and so I believe that it violates the Law of Conservation of Energy.

The only solution I thought of was that the power output of the artificial gravity might be the energy that is transferred to the box.

Am I totally off-base, or am I right? I'm a high school student so my understanding of physics may not be totally correct.

[Ghost Rider]

Off to SLAM as it has more to do with a particular subject matter, rather then something SW specfic.

[Admiral Valdemar]

As with most soft SF, artificial gravity, FTL and telekinesis are all pretty much impossible. To have a ship that can artificially create a gravity well like that of Earth in such a small space defies all known physics today without said ship using degenerate matter, somehow.

[Teleros]

Well if the gravity generator suddenly flipped to anti-gravity then the energy to move the box would come from the generator. If the gravity generator was turned off and the one on the ceiling turned on then the energy to move the box would come from the second generator. Problem solved .

The real problem with artificial gravity is that we don't have a clue how to turn say electricity into gravity. We can generate gravity artificially today, but only because objects have mass - and we can simulate gravity (rotating hull segments, ala Babylon 5 EarthForce ships) - but we've a long way to go before someone can flip a switch and turn gravity on / off.

[Darth Raptor]

If gravitons are real, and you know how to make them, there's no reason why you can't turn energy into gravitons. The problem with this is that it would take just as many gravitons as it would normal mass to achieve the same effects. Like Valdemar said, it would probably be easier and less energy intensive to use a chunk of neutron star or something. The problem with THAT is that your ship is permanently as massive as an actual planet (this creates its own host of problems). The only possible advantage to generating gravity with nothing but energy is that you could switch it off. But it makes way more sense to just spin a portion of your ship or medically adapt your crew to constant freefall. Barring some kind of magic, it just isn't feasible.

[Kuroneko]

I hope you've had high-school physics. Forgive me for making this a bit complicated, but frankly capacitors are a lot easier to understand than gravitons.

For example, say there is a square room in space, and it is under the influence of artificial gravity. Assume that there is a box sitting on the floor, where it has no potential or kinetic energy. Now, all of a sudden there is a shift in the artificial gravity, making the floor into the ceiling. Now the box sitting on the floor is now on the ceiling, and it has potential energy, which turns into kinetic energy. This energy doesn't come from anywhere, and so I believe that it violates the Law of Conservation of Energy.

If the artificial gravity is limited to the volume of the room, then in classical terms, what you've described is a standard capacitor, except with a gravitational field instead of an electric field. Of course, this would imply large opposing aggregates of positive and negative gravitational charge (mass), but we can deal with it as a thought experiment. The system will actually have energy stored in the field itself, just as in the case of the capacitor. Gravitational charges behave a bit differently from electric charges in regards to sign, but still sufficiently close for intuition regarding capacitors to be useful.

The only solution I thought of was that the power output of the artificial gravity might be the energy that is transferred to the box.

Changing the field inside a capacitor involves work on the charges, or more plainly, charging a capacitor requires energy. "Shifting" the artificial gravity in this manner would be analogous to changing the field in a capacitor and therefore moving the charges in it (those are equivalent, although with some caveats that aren't relevant here). One shouldn't expect any charged test particle inside the capacitor to be unaffected; it too will respond to the changes in the field and have work done on it. In your case, the box is like a test particle with gravitational charge.

[Kuroneko]

A note on the gravitational capacitor: having virtually no net mass, there is no problem moving it around. Rather, it has the opposite problem of not being able to stop without thrust--it is essentially a self-accelerating reactionless drive with a runaway hyperbolic motion. It is obviously unphysical (although interesting to think about); energy is conserved because the negative-mass portion of the setup will have negative kinetic energy.

Note that having a constant gravitational field limited to a certain volume, like those frequently seen in sci-fi, requires the field to be analogous to that of a capacitor, which in turn requires negative charge (mass) density (this is fairly intuitive when one considers how to have zero field outside of the given volume, but can be made precise via Poisson's equation). Of course, that's without hypothesizing any entirely new physics, which, like all handwaving, is somewhat legitimate in science fiction.

[RedImperator]

So just to make sure I understand the implications of this properly: if you have a sci-fi artificial gravity system, you also have a sci-fi reactionless drive because negative mass will accelerate unless acted upon by an outside force?

[Dark Flame]

Thanks for all the help everyone. I think I understood most of it, but I've never heard of negative mass and negative kinetic energy. Could you explain those a bit, Kuroneko?

[Kuroneko]

So just to make sure I understand the implications of this properly: if you have a sci-fi artificial gravity system, you also have a sci-fi reactionless drive because negative mass will accelerate unless acted upon by an outside force?

As long as the sci-fi artificial gravity system works by pseudomagically modifying the gravitational field, rather than using some other means to mimic gravity, yes. Since most sci-fi artificial gravity devices do not use centrifugal force, it's either this or some entirely new fictional mechanism. There are actually quite a few parallels to the Alcubierre drive, almost like a Newtonian version of it (but never superluminal if under special relativity).

I think I understood most of it, but I've never heard of negative mass and negative kinetic energy.

That's not a surprise; negative mass and negative kinetic energy aren't known to exist. In Newtonian physics, there is no theoretical reason for mass to be always non-negative, so one could carry out the same with negative mass as with positive mass. A negative-mass particle would accelerate opposite to the applied force (as per F = ma, but with m<0), and therefore if Newton's law of gravitation holds, gravitationally repel all other particles.

[Surlethe]

As long as the sci-fi artificial gravity system works by pseudomagically modifying the gravitational field, rather than using some other means to mimic gravity, yes. Since most sci-fi artificial gravity devices do not use centrifugal force, it's either this or some entirely new fictional mechanism. There are actually quite a few parallels to the Alcubierre drive, almost like a Newtonian version of it (but never superluminal if under special relativity).

I'm not sure I understand this. Why would a gravitational capacitor accelerate itself if an electric one doesn't? Is it because of the differences between gravitational and electric fields?

[Feil]

Wasn't there some valid research a year or two ago that indicated that extrapolated from Heim-Droscher theory to conclude that rotating electromagnets in the ridudiculous numbers of teslas might produce a gravitational force? Some variety of unification of the electroweak with the gravitational?

I really lack the science / math background (so far I'm working on it) to investigate the theories myself. Anybody able to comment?

[Spoonist]

In the Star Wars universe it would be "relatively" simple to create artificial gravity through Shield technology.
Shields generate a physical resistance, if that would be apllied as a resistance from the ceiling, then you would have a slight push towards the floor. Then you could adjust the gravity shield depending on your species preference.

This would also explain why you would have gravity in larger ships and not in fighters and such.

[Then as a sidenote I would have to admit that shields are not that 'realistic' either.]

[Kuroneko]

I'm not sure I understand this. Why would a gravitational capacitor accelerate itself if an electric one doesn't?

Because the electric one doesn't have any negative mass. If you look at Newton's law of gravitation F = -GMm/r², you'll find that the positive mass M>0 attracts the negative mass m<0 (outward force but inward acceleration as per a = F/m), while the negative mass repels the positive mass. Hmm... a possible loophole is that this assumes that gravitational and inertial mass are identical (which general relativity affirms, but in the end is an empircal matter), which might not be the case, particularly in a sci-fi universe. If it isn't, the device might not self-accelerate after all, at the cost of GTR being false.

As for Heim-Droescher, I don't know. Electromagnetism gravitates even in GTR, but presumably in H-D it does so differently.

[Son of the Suns]

Another interesting thing about sci fi artificial gravity is that it seems to only affect things in a certain direction. I'm assuming that if it was possible to increase the amount of gravity something has without increasing it's mass, that gravity would affect everything in all directions, not just everything in a plane above a plate for example like in most sci fi.

[Teleros]

Well if they can replicate gravity it's possible they can neutralise or contain it (and yes, I know generator gravity =/= neutralising or containing it, before you say anything), in which case the ceilings, walls etc could have such technologies built into them.

[Surlethe]

I'm not sure I understand this. Why would a gravitational capacitor accelerate itself if an electric one doesn't?

Because the electric one doesn't have any negative mass. If you look at Newton's law of gravitation F = -GMm/r², you'll find that the positive mass M>0 attracts the negative mass m<0 (outward force but inward acceleration as per a = F/m), while the negative mass repels the positive mass. Hmm... a possible loophole is that this assumes that gravitational and inertial mass are identical (which general relativity affirms, but in the end is an empircal matter), which might not be the case, particularly in a sci-fi universe. If it isn't, the device might not self-accelerate after all, at the cost of GTR being false.

Self-acceleration would seem to violate conservation of momentum through Newton's third law, though, wouldn't it? So if you were a sci-fi author and wanted consistency, you'd have to go the route of saying inertial mass is not equivalent to gravitating mass -- perhaps say that inertial mass m_i = |m_g|, where m_g is gravitating mass. This seems similar to the treatment of areas, where an area can be oriented positively or negatively, but when you just want to compare areas, you take the modulus of the area vector.

[Kuroneko]

Self-acceleration would seem to violate conservation of momentum through Newton's third law, though, wouldn't it?

Not quite; it's at least mathematically consistent with all of the laws of mechanics, since for a negative mass, the momentum vector will be opposite the velocity vector. The construction will self-accelerate, but the total momentum will still be zero.

So if you were a sci-fi author and wanted consistency, you'd have to go the route of saying inertial mass is not equivalent to gravitating mass -- perhaps say that inertial mass m_i = |m_g|, where m_g is gravitating mass.

Yes, there would be no self-acceleration in this case. However, it would be inconsistent with the equivalence principle (which may be an acceptable loss), as the behavior of a negative mass could locally distinguish acceleration from gravitation simply by comparing the behavior of free positive and negative masses (identical for acceleration, opposite for gravitation).

[metavac]

As with most soft SF, artificial gravity, FTL and telekinesis are all pretty much impossible. To have a ship that can artificially create a gravity well like that of Earth in such a small space defies all known physics today without said ship using degenerate matter, somehow.

Sort of. Artificial gravity does violate energy-momentum conservation if the gravitational field exists without the requisite mass. A crude example uses wormholes with weird, flat mouths approachable from only one side, one underneath the deck and another in orbit around a body with the desired surface gravity. In that case, it's a question of whether or not it's cheaper and more practical just to bunk the crew in a centrifuge. Besides, your deckhands--constantly moving heavy equipment and cargo--might appreciate working in freefall more often than not.

[Teleros]

Artificial gravity does violate energy-momentum conservation if the gravitational field exists without the requisite mass.

Surely not if you can convert say electricity into a gravitational field? If it's just a magical anti-gravity material then you're right, but if it requires a power supply...

[metavac]

Surely not if you can convert say electricity into a gravitational field? If it's just a magical anti-gravity material then you're right, but if it requires a power supply...

Well, run an electric current through a material and measure its mass as it heats. If kept a constant volume, it gains mass (E = mc^2) which in turn increases the surface gravity. There's your conversion, and it does not avail you of accounting for the masses and distances necessary to generate a desired surface gravity and its gradient.

[Napoleon the Clown]

Surely not if you can convert say electricity into a gravitational field? If it's just a magical anti-gravity material then you're right, but if it requires a power supply...

Well, run an electric current through a material and measure its mass as it heats. If kept a constant volume, it gains mass (E = mc^2) which in turn increases the surface gravity. There's your conversion, and it does not avail you of accounting for the masses and distances necessary to generate a desired surface gravity and its gradient.

Uh, no. It will not gain mass. If anything, it'll lose mass as it bleeds off into the eviroment, and even that's going to be undetectable.

Does a hot light bulb way more than a cool one? No. Mass doesn't come out of nowhere. Heating will make it's volume and surface area increase. That'll decrease surface gravity. Mass can't be "poofed" into existence using only an electric current. If it could we'd be using that to increase the amount of gold in gold nuggets. We'd use it to minimize the weight of ammo on navel ships. Matter can neither be created nor destroyed. The extra energy you added by running an electric current? That's where the heat comes from.

[Lord Zentei]

Surely not if you can convert say electricity into a gravitational field? If it's just a magical anti-gravity material then you're right, but if it requires a power supply...

Well, run an electric current through a material and measure its mass as it heats. If kept a constant volume, it gains mass (E = mc^2) which in turn increases the surface gravity. There's your conversion, and it does not avail you of accounting for the masses and distances necessary to generate a desired surface gravity and its gradient.

Uh, no. It will not gain mass. If anything, it'll lose mass as it bleeds off into the eviroment, and even that's going to be undetectable.

Energy has a mass equivalent, which is what metavac meant, methinks. A heated filament does indeed weigh more than a cold one, though the difference is neglible, and is not useful in this context.

[Napoleon the Clown]

Uh, no. It will not gain mass. If anything, it'll lose mass as it bleeds off into the eviroment, and even that's going to be undetectable.

Energy has a mass equivalent, which is what metavac meant, methinks. A heated filament does indeed weigh more than a cold one, though the difference is neglible, and is not useful in this context.

I see.

[metavac]

Uh, no. It will not gain mass. If anything, it'll lose mass as it bleeds off into the eviroment, and even that's going to be undetectable.

Hence, I noted this works only if the object is kept at a constant volume.

Does a hot light bulb way more than a cool one?

Because a hot bulb radiates into its environment. It's losing mass.

No. Mass doesn't come out of nowhere.

No, it doesn't. It's conserved; the additional mass is do to the heat into the system.

Heating will make it's volume and surface area increase.

Hence, I said "constant volume."