I have seen some people talk about the idea of ships using their magical anti-gravity systems in a story in order to not only give ships a standard floor layout and inertial dampener systems, but to also propel the ship forward through space.

As much as this works for ideas like mass relays from mass effect as those give the ship something to push against, could it work as a stand alone engine system?
It won't have anything to push against in space, and without the gravity of a planet to hold it down making the ship lighter won't do anything either, so I can't tell if this would work or not.

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    $\begingroup$ There's always something in space or there's no point in going anywhere. However the details (for which no theory exists in reality !) depend on how the AG system works and what control it offers. $\endgroup$ Commented Dec 18, 2018 at 5:19
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    $\begingroup$ Asimov's Foundation series are a prime example of using gravitics to propel ships. Especially the novels Foundation's Edge and Foundation and Earth. $\endgroup$
    – JBH
    Commented Dec 18, 2018 at 5:30
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    $\begingroup$ I'm a bit confused as to what the question here is. Do you have some sort of engine design you want to ask about? or are these just musings you want to throw out into the open? $\endgroup$
    – Shadowzee
    Commented Dec 18, 2018 at 6:17
  • $\begingroup$ Gravity bends space-time - so if you have a "gravity generator", why won't it work for a ship in space? $\endgroup$
    – G0BLiN
    Commented Dec 18, 2018 at 7:45
  • $\begingroup$ @Shadowzee I don't have a specific engine idea as my only idea for ag that does not use a special element is two plates above and below the area where the gravity is being generated, but I don't think that would work for an engine. I'm basically asking if it can work without the engine being a fancy tow truck ala the pod racers from star wars $\endgroup$
    – Snydwell
    Commented Dec 18, 2018 at 8:46

5 Answers 5


As far as we know, any massless drive in space belongs to the realm of Troll Physics. Even NASA's EM drive. They all boil down to this:

Magic everywhere in this

However! Nature herself is the ultimate troll. Accept this as truth, and all boffinry becomes much clearer and easier to understand.

What you need in order to build a troll engine is not artificial gravity. Quite the opposite - literally! You need antigravity.

The Wikipedia entry on negative mass has this gem in it. The further you read, the more the troll nature of Nature becomes apparent:

Runaway motion

Although no particles are known to have negative mass, physicists (primarily Hermann Bondi in 1957, William B. Bonnor in 1989, then Robert L. Forward) have been able to describe some of the anticipated properties such particles may have. Assuming that all three concepts of mass are equivalent the gravitational interactions between masses of arbitrary sign can be explored, based on the Einstein field equations and the equivalence principle:

  • Positive mass attracts both other positive masses and negative masses.

  • Negative mass repels both other negative masses and positive masses.

For two positive masses, nothing changes and there is a gravitational pull on each other causing an attraction. Two negative masses would repel because of their negative inertial masses. For different signs however, there is a push that repels the positive mass from the negative mass, and a pull that attracts the negative mass towards the positive one at the same time.

Hence Bondi pointed out that two objects of equal and opposite mass would produce a constant acceleration of the system towards the positive-mass object, an effect called "runaway motion" by Bonnor who disregarded its physical existence, stating:

“I regard the runaway (or self-accelerating) motion […] so preposterous that I prefer to rule it out by supposing that inertial mass is all positive or all negative.”

— William B. Bonnor, in Negative mass in general relativity.

Such a couple of objects would accelerate without limit (except relativistic one); however, the total mass, momentum and energy of the system would remain 0.

This behavior is completely inconsistent with a common-sense approach and the expected behaviour of 'normal' matter; but is completely mathematically consistent and introduces no violation of conservation of momentum or energy. If the masses are equal in magnitude but opposite in sign, then the momentum of the system remains zero if they both travel together and accelerate together, no matter what their speed.


Forward used the properties of negative-mass matter to create the concept of diametric drive, a design for spacecraft propulsion using negative mass that requires no energy input and no reaction mass to achieve arbitrarily high acceleration.

So there you have it.

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    $\begingroup$ What, exactly, do the pictures add to this answer?... $\endgroup$
    – T.J.L.
    Commented Feb 1, 2019 at 13:45
  • $\begingroup$ @t.j.l. massless drives have always belonhed to the realm of troll physics. The first picture should therefore be self explanatory. The second one is a trollface, an internet meme you use as an interjection after someone has been trolled. In this post, I make a case that nature is trolling us, because if Einstein et. al are right then a massless drive (therefore a troll engine) is realistic and can be built. $\endgroup$ Commented Feb 1, 2019 at 14:36
  • $\begingroup$ I know what the images are, my question was how do they improve the post beyond what the text says? $\endgroup$
    – T.J.L.
    Commented Feb 1, 2019 at 14:44
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    $\begingroup$ the first image is barely useful. The trollface is totally pointless. $\endgroup$
    – L.Dutch
    Commented Feb 1, 2019 at 15:22
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    $\begingroup$ Images are helpful if they add something to the post. Moreover, not all users have the luxury of unlimited data connection. Adding pointless images to post is disrespectful toward them. $\endgroup$
    – L.Dutch
    Commented Feb 1, 2019 at 15:36

Theoretically, there is gravity in effect, no matter where you go. It may get to the point that our instruments can no longer detect it, but it is still there.

You could potentially create an A/B antigravity drive, where the "A" system "pushes" against the strong gravity of nearby objects, and the "B" system "pushes against" that or "pulls towards" the opposite of that. Or in other words, when the "A" system is powerful, the "B" system is weak, and vice versa.

  • $\begingroup$ Can you explain better your second period? It's totally obscure to me. $\endgroup$
    – L.Dutch
    Commented Dec 18, 2018 at 6:15
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    $\begingroup$ @L.Dutch - Either he means something about treating "weak" and "strong" gravity differently ("weak" as of being far off planets and stars, "strong" as being close to them), OR using a system which uses anti-gravity as a push effect or boosts gravity as a pull effect. $\endgroup$
    – Battle
    Commented Dec 18, 2018 at 8:05
  • $\begingroup$ So sort of like a space monorail with stations along the way to keep it moving? $\endgroup$
    – Snydwell
    Commented Dec 18, 2018 at 8:47
  • $\begingroup$ @L.Dutch I was envisioning a system where, in the presence of nearby mass (like a star or planet), the "A" system could use antigravity to "push" off the gravity created by the nearby mass. In deep space, the ship would turn off that drive, and turn on the "B" drive. This second drive would be essentially the polar opposite of the first: it could produce incredible thrust, but only when far away from significant mass. An alternate would be a drive that "surfs" the recently detected gravitational waves. But they are so faint that I cannot fathom how they could be utilized for propulsion. $\endgroup$ Commented Dec 18, 2018 at 15:45

The "Artificial gravity" will not work all by itself as you noted: You need something to pull yourself to. You will need Gravity assist.

The idea is that you use the planetary motion and gravity to transfer some of the momentum from the planet to the ship. This means, the planet looses kinetic energy, but is insignificant when a single space ship is used.

The "gravity engine" you describe may be used in the same way: Create gravity (there is no way to do this at the moment) to increase the attraction towards a planet. The ship should approach the planet at an angle, and not towards its center or it will crash on the planet. When the ship flies-by the planet, turn-off artificial gravity. The attraction will diminish and that way you will gain more speed than by using gravity assist alone.

Once in interstellar space, you will need a reaction drive (ion, rocket, anti-matter rocket, etc...) to accelerate further, to correct your trajectory and land on another planet eventually.


Gravity engines rely on how gravity works, gravity does not push or pull on things it is a distortion of of space itself, an object has gravity whether there is anything else in the universe or not. If you are having trouble visualizing how this works this is a great video simulation explaining it.

The "magic" part of the drives is creating gravity or negative gravity without have to have mass, or having it decoupled from mass. Which we haven't the slightest idea of how to do and it is highly unlikely to be possible.

It relies on the fact space has a sort of standing mass (zero point energy), and you can create small amounts of negative mass (casimir effect) by preventing virtual particles from appearing. Basically space is not starting at zero energy and you can move it closer to zero. But in this case even theoretically you need more zero point energy than the universe currently has to move anything. This is the classic warp drive.

but if it did it work it would be like a surfboard the magically creates its own wave to ride. it distorts space itself and rides the differential.

This video has a great discussion on the other problems with this, https://www.youtube.com/watch?v=tm1FvHRruUQ


Late response.

big edit.

So try to better explain this.


In general relativity, the dimensions space and time are connected, weaved together, forming a fabric, called space-time. anything in space time warps this fabric, this is what we perceive as gravity. And objects which warp space contract it, causing nearby objects to fall inward.

Negative mass, if it existed, is assumed to do the opposite-expand space, causing nearby objects to fall outward, or be repelled. Hence gravitational repulsion.

If you have two positive masses then the two contraction curvatures add up to one bigger contraction curvature, aka more positive gravity.

If you have two negative masses, then the two expansion curvatures add up to one bigger expansion curvature, aka more negative gravity.

*However, if you have both positive and negative mass near each other, the warping of space causes by both masses is equal(equal amounts of positive and negative mass), then the contraction and expansion of space will interact with one another, canceling each other out, creating a fabric with no space-time curvature, in other words, a region devoid of gravity. Thus negative mass would provide gravitational shielding as well.

How such masses operate and could be used for spacecraft propulsion are gone into more detail by propulsion physicist Marc Millis, in his book Frontiers of propulsion space flight.





More details

There are 3 types of mass (for this discussion anyway). 3 concepts: inertial, passive gravitational, and active gravitational. Inertial mass is the resistance of objects to changes in motion, passive gravitational mass is the extent to which an object responds to gravity, active is the field which produces gravity, the charge of gravity, analogous to electric charge. All these masses are equivalent and inseparable, known as the Equivalence principle. If negative mass exists, it is assumed to not violate such a principle and thus possess negative mass in all 3 aspects.

There are two types of negative mass formulated over the years. One type is always negative (it repels itself and all positive masses). This type repels positive mass while the positive mass attracts The negative mass. They move together in a line. The 2nd type has positive mass when interacting with other negative mass, (the two masses attract) but when interacting with positive mass is negative, aka the two masses repel each other.

Assuming you had some way (note: insert technobabble) of producing enough negative mass, you would have a form of matter that gravitationally repels positive matter. A conductor is a device that contains equal positive and negative electric charges that shield any flow of internal electric charge from any electric charge outside it.

Thus, analogous electric charges, positive mass is positive gravitational charge, negative mass is negative gravitational charge. In addition, gravity radiates outwards as a sphere, but the presence of other masses distorts this gravity region, creating more of a bubble. A ring of negative matter around a spaceship would generate negative gravity, aka antigravity, aka gravitational shielding. The bubble of negative gravity generated would shield anything inside it from any gravity outside it. – Shaquille Nelson 2 days ago
Inside the bubble is a spaceship, outside is the Earth, and the Earth's gravity has been negated. In addition, the only gravity affecting the spaceship is the bubble of negative gravity. Remember, regardless of what type of negative mass you are using it gravitationally repels matter. Thus, this negative mass, if moved, will also move the spaceship, producing thrust without rockets. How you move the negative mass can be done using gravity fields or gravitomagnestism(or some other unknown way). If you can create any amount/density of negative matter. Then you can create any level of speed.

Thus, such a propulsion method would both negate Earth's gravity, eliminating the need to attain orbital velocity, AND produce thrust without rockets.

  • $\begingroup$ Your explanation are pretty fuzzy. Can you provide more details and/or references? $\endgroup$
    – L.Dutch
    Commented Feb 1, 2019 at 11:32
  • $\begingroup$ I too would be interested in reading how a ring of negative matter would shield something. $\endgroup$
    – Willk
    Commented Feb 1, 2019 at 13:44
  • $\begingroup$ Here are some links.amazon.com/… $\endgroup$ Commented Feb 2, 2019 at 12:07
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    $\begingroup$ You need to edit all of that into your actual answer. Comments are transient and there's no guarantee that people will see them. $\endgroup$
    – F1Krazy
    Commented Feb 3, 2019 at 10:43
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    $\begingroup$ All right. I edited my answer. Is my explanation any better? $\endgroup$ Commented Feb 4, 2019 at 14:51

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