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I want to use a handwavium gravity field generator on my spaceships, but this has several problems.

Generating gravitational fields of peculiar shapes is not a difficult thing to do in this setting. We have graviton generators and space-time sheet modulators to create gravity fields of all shapes and sizes. So that's not the problem.

I also don't want to use "Spin-Gravity" or artificial gravity from thrust for reasons I won't go into here.

But, by generating a field of gravity, even if it only affects everything within the ship, all that mass being accelerated downwards will cause the ship to be pushed down, taking them off course.

If I wanted to counteract this, what would be the best way to do it?

Notes:

  • Superintelligent AI is not available.
  • This needs to be as believable as possible within the established tech.
  • Established tech includes exotic matter, anti-matter, plenty of power generation, (hydrogen, helium, deuterium and other forms of) fusion, superconductors, advanced carbon allotropes, high content-batteries, and kugelblitzes.
  • Resources are not a problem, but the cheaper the better.
  • Low gravity is fine, as long as the ship doesn't go off course and the characters and their stuff will "feel" gravity.
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    $\begingroup$ Could you use a different force to simulate gravity? Make everyone wear ferrous clothing perhaps? $\endgroup$
    – Matt Bowyer
    Commented Aug 18, 2017 at 14:28
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    $\begingroup$ Magboots are a thing but they don't allow for pets, personal items, weaponry... etc $\endgroup$
    – Disgusting
    Commented Aug 18, 2017 at 15:15
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    $\begingroup$ If you're willing to handwave artificial gravity, you can also handwave the effect you mentioned. Because just as you just realized, artificial gravity means free thrust for the spaceship which uses it. Actually you can incorporate that into your world, having a spaceship which moves it's gravity generator around instead of using low power thrusters for fine navigation. $\endgroup$
    – Tomáš Zato
    Commented Aug 19, 2017 at 0:13
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    $\begingroup$ That's known as "a reactionless drive" AKA total annihilation of the conservation of momentum. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Aug 19, 2017 at 19:40
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    $\begingroup$ Alternatively, any force generated by the gravity re-appears as an equal and opposite force on the gravity generator. $\endgroup$
    – SomeoneSomewhereSupportsMonica
    Commented Aug 20, 2017 at 3:26

9 Answers 9

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Have two gravity generators, running along the middle of the ship. From someone outside the ship, looking in, it would appear that that there is Artificial Gravity pulling items from the bottom of the ship to the middle, and from the top of the ship to the middle.

From the point-of-view of people inside the ship, it won't really matter which half of the ship they are in, except that it makes defining the 'port' and 'starboard' of the ship more interesting.

To cross between the two halves, there could be a a tube with a gravity-free zone in the middle. Persons crossing would take a lift across, at the half way point they would be free-floating. The lift would stop here, the people would turn head-about-feet, and then the lift would resume. When they enter the Artificial Gravity zone on the other side, they would again feel gravity pulling them feet-wards.

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  • $\begingroup$ This is the exact sort of thing I was looking for. Thank you very much. Don't know why this didn't occur to me, seems so simple now. The best ideas always are I suppose. $\endgroup$
    – Disgusting
    Commented Aug 20, 2017 at 22:33
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Unless you plan to suspend Newton's Third Law (with an enormous dose of wandwavium) or you will have the Gravity Generator pulled in the opposite direction of ship internals, with exactly the same force. Net effect should be null (otherwise you could use this effect to accelerate the ship without reaction mass).

If you have the G.Generator solidly fixed to spaceship You won't have any course deflection as the two pulls will even out.

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  • $\begingroup$ This is what I thought he meant until he said "all that mass being accelerated downwards will cause the ship to be pushed down". $\endgroup$
    – Aric
    Commented Aug 18, 2017 at 14:16
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    $\begingroup$ @AricFowler: ... and all that Generator being accelerated upward will cause the ship to be pushed up (if connected to it)... unless you kill Newton who worked remarkably well to date ;) $\endgroup$
    – ZioByte
    Commented Aug 18, 2017 at 14:25
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    $\begingroup$ Screw Newton. My setting has wizards. $\endgroup$
    – Disgusting
    Commented Aug 18, 2017 at 15:18
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    $\begingroup$ I instantly regretted saying that. $\endgroup$
    – Disgusting
    Commented Aug 18, 2017 at 15:18
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    $\begingroup$ This answer is quite right. Unless your goal is to have this 'balancing', there's no need for it. $\endgroup$
    – Pyrotrain
    Commented Aug 18, 2017 at 15:26
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Change the meaning of "down".

Artificial gravity allows you to arbitrarily define where down lies. What seems to be in your head is the Star TrekWars ship definition of down. There's no reason down shouldn't be towards the bow, stern or even some point or plane in the middle decks. The orientation of your decks is also free to change as you see fit relative to your current convenience.

Building a small spherical ship with down in the centre, or even on the outside, will mess with your head (and with the engineers) but it's as valid as any other setup under artificial gravity.

What this means in practice is that for a long distance bulk carrier down should be forward. For a combat vessel, down should only be used to counter acceleration effects from maneuvering with a possible low basic setting to stop things floating away.

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  • $\begingroup$ You could have "down" as the direction you are moving in, so the ship is powered by the weight of its inhabitants. Of course, you'd need a way to reverse this to slow down again. $\endgroup$
    – Aric
    Commented Aug 18, 2017 at 14:14
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    $\begingroup$ @AricFowler, if it does actually affect your course then direction of travel is in practice the only logical option $\endgroup$
    – Separatrix
    Commented Aug 18, 2017 at 14:17
  • $\begingroup$ This is something I had considered, but my players designed their ship with no thought to science and I'd rather not change it too much. Also, I liked the idea of the gravity generator failing during a mission, but this may be the only sensible option. $\endgroup$
    – Disgusting
    Commented Aug 18, 2017 at 15:17
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    $\begingroup$ The enemy's gate is down $\endgroup$
    – SethWhite
    Commented Aug 18, 2017 at 16:47
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    $\begingroup$ Doesn't have to be spherical - if the ship has multiple parallel decks, have "down" be a plane cutting through the middle. There's just a bit of awkwardness when you have to move from one half to the other. $\endgroup$
    – Gregor Thomas
    Commented Aug 18, 2017 at 19:10
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Put your mystical gravitation machine (you said wizards, right?) in the center of the ship. It pushes things away, rather than pulling them toward itself. Thereby creating a gravity-esque force on the "underside" of the floors and the inside of the hull. It would balance out as the force is equal in all directions.

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  • $\begingroup$ Or have it pull towards the generator in the centre of mass of the ship, no net resultant gravitational forces on the external inertial frame. $\endgroup$
    – KalleMP
    Commented Aug 19, 2017 at 20:17
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    $\begingroup$ Sure. I mean, that's pretty much how we interact with natural gravity. I proposed reversing it because I think it has some fun implications for an adventure narrative. For instance, a hull breach could cause people to "fall" out of the ship. The repulsive force could also serve as a defense against minor collisions and projectiles. Also, its kind of weird, which I tend to gravitate toward....haha.....hah. At least I amuse myself. $\endgroup$
    – Neocognitron
    Commented Aug 20, 2017 at 21:15
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When you put gravity on a room, it's possible to calculate the thrust generated according to the room mass.

So, since you can shape the gravity field as you like, it's possible to equilibrate the thrust alternating up and down gravity on different room.

Thereby, one part of the ship would have a 'normal' gravity and the other part would have a 'upside down' gravity.

This solution would require only a calibration of graviton generator in a zero-G environment.

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It's not a problem because forces, even handwavium forces don't act that way. The reason is in the artifical gravity field, the bodies aren't actually accelerating

A person is in a room with artifical gravity. He is pulled towards the "floor" His weight pushes down on the floor, and (by Newton's third law) the floor pushes back on him. The two forces are exactly balanced.

If you could get your ship to accelerate by turning on the anti-gravity field, well done! You have just invented a reaction free propulsion device. It can push a ship without a rocket shooting stuff the opposite direction. Reaction free propulsion breaks the law of conservation of momentum so it doens't exist.

So an antigravity device won't be able to cause the ship to accelerate in any direction, because it isn't actually accerating anything.

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  • $\begingroup$ Either the gravity field affects the floor as well (as natural gravity does), and the forces aren't balanced; or the artificial gravity selectively doesn't affect the floor, or whatever the floor rests on, and there isn't even a balancing aspect. I think your conclusion is backwards: it's artificial gravity that is impossible precisely because it must imply reactionless acceleration. $\endgroup$
    – Alex Celeste
    Commented Aug 19, 2017 at 12:48
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The feeling of gravity doesn't come from being pulled down, it comes from the floor pushing you back up.

Whether the gravity generator is moving the ship or not, it well feel like Earth gravity as long as the amount it accelerates the ship's chassis is different to the amount it accelerates the crew's bodies by 9.8m/s/s.

If the gravity generator is cheap enough to leave on for comfort when you're not moving, people would use it as a drive too, unless your universe's physics has a conservation law that says the generator has to push something up with the exact same force that it pushes stuff down. Which would be a very plausible law to have. Even with that law, people might want to investigate whether it's economical to use a gravity generator to drop propellant out of the ship instead of using a thermal rocket engine or an ion drive.

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Gravitoelectromagnetism might be a interesting avenue for consideration. Some high order effects of this phenomenon allow you to theoretically generate gravity fields or equivalent effects. For example two wheels connected to the same axis will experience stronger gravitational attraction to each other if those wheels are spun in opposite directions. This is an expression of gravitoelectromagnetic attraction, if the wheels are spun in the same direction the opposite is true and a repulsive gravitoelectromagnetic effect is generated.

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Once you have artificial gravity you sort of throw out the laws of physics... that being said let's see what you think about this:

There's the crew, and there's the floor of the ship. Newton's third law says that any force has an equal and opposite reaction force. So let's imagine a gravity generator in the floor that pulls 'down' on a 200 pound crewman.

We want the crewman to feel a 200 lb force down, towards the floor. Then we have:

if (crewmanForce = 200 lb): then: reactionForce = 200 lb

and

if (crewmanForceDirection = down): then: reactionForceDirection = up

That's the logic of Newton's third law. What's the reaction force? Well the floor will feel a force up, towards the crewman. That's the reaction force here. Since these two forces cancel out, there is no external force that can pull your ship off-course.

This can be extended to the entire crew, and the entire ship. Newton's third law saves your navigation issue automatically.

That being said, think about how much of a physics lesson your readers are actually looking for. It might be better to keep that stuff in the background, depending on the audience.

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