My scifi world obeys the real world conservation laws. This presents a problem for spacecraft; due to the conservation of momentum, the spacecraft will need propellant, and a lot of it, for every single motion

However, I believe I have found a loophole in the form of negative mass. If I were to insert negative mass, and the ability to produce it, into my world then it seems like we could have a ship that can fly without energy: If equal and opposite masses are fired from opposite ends of the ship, then it seems like there would be an overall change in momentum (as momentum is directional) with no change in energy or mass (as they are scalars)

Could this work under the given rules? Or is this simply too good to be true?


4 Answers 4


With negative mass things get crazy

  • Mass is energy. If you can make negative mass, you're making negative energy. Since energy is conserved, that means you have positive energy "left over" that you can do whatever you want with
  • With positive mass, pushing on it causes it to accelerate in the same direction (F = ma). But if the mass is negative, pushing on it means it pushes back harder. If you can touch a negative mass, it pushes against your hand, and the harder it pushes the more force there is to accelerate it into your hand ... while this sounds like a bad thing, NASA says don't worry, it'll fall straight through your hand because the Pauli exclusion principle won't work between it and ordinary matter. But I don't like it - there's still ordinary electrical charge on those atoms. I'll let them try it.
  • The negative mass might neutralize the positive mass leaving nothing behind, which is one more reason not to handle it.
  • A negative mass is pushed away from a positive mass by gravity, so it falls toward it. The positive mass is pushed away from the negative mass by gravity, so it falls away from it. So you can build a ship with a negative mass chasing a positive mass through space, accelerating indefinitely with no fuel consumption. The kinetic energy of the positive mass increases without limit, and the negative kinetic energy of the negative mass always matches up with it.
  • Negative mass is also a sure-fire way to make stable wormholes, in case you want to get there faster. Most wormhole builders are looking for ways to not use negative mass.

In summary: yes, you can fly without energy - or make as much energy as you want - or get there without flying at all.

  • 2
    $\begingroup$ I image the hard part is NOT accelerating. $\endgroup$
    – John
    Jan 22, 2022 at 14:27
  • $\begingroup$ Actually we don't know. The kinematics of negative mass objects is unsolved, and the simple solutions proposed result explosions due to the normal force. $\endgroup$
    – Joshua
    Jan 23, 2022 at 4:46

Different question, same answer, so I'll just reuse what I had writte for Artificial gravity as an engine

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 an actual, scientifically accurate troll engine is 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.

  • 1
    $\begingroup$ I agree about the implausibility of the concept, but per my answer the physical coupling between the negative mass "magnet" and positive mass ship in your drawing has some issues of its own. (The troll doomsday bomb is more fun - produce a particle of negative matter moving at the speed of light, with infinite negative kinetic energy, and the corresponding positive mass particle should release a bang sufficient to annihilate the universe) $\endgroup$ Jan 22, 2022 at 16:23
  • 1
    $\begingroup$ The EM drive ended up being the funniest thing ever. It pushes on Earth's magnetic field. We could have tested it on the ISS and found it working and had quite the shock when it failed in deep space if we hadn't solved the why yet. $\endgroup$
    – Joshua
    Jan 23, 2022 at 4:49
  • $\begingroup$ @joshua the Sun's own magnetic field goes way beyond Pluto so we could at least colonize our neighbouring planets. $\endgroup$ Jan 23, 2022 at 13:31
  • 1
    $\begingroup$ The sun's magnetic field is more than a thousand times weaker so it would be a terrible drive, and without solar panels the EM drive would be useless. If you have stored fuel it's more efficient to use it directly. $\endgroup$
    – Joshua
    Jan 23, 2022 at 16:08

The idea of using negative mass to avoid physical laws is nothing new, but I assume you're not talking about FTL travel here? I think you'd first have to figure out where your unlimited source of negative mass was coming from, otherwise your ship is simply powered by unobtainium, and there are more sensible things to do with unobtainium. Rather than firing in two opposite directions, you might be better off creating a system which uses a negative mass drive to create localised regions of negative pressure which drive propulsion instead. Then your ship only needs half the amount of unobtainium...

  • $\begingroup$ Casimir plate engine. Just need unlimited electrical power which is what breeder reactors are for. $\endgroup$
    – Mazura
    Jan 22, 2022 at 21:57

I'm afraid this is a no-answer, I'd like to post both a practical and a theoretical objection..

Where to find material to build your ship, it will fly away from you..

Suppose for now, negative mass actually exists. A galaxy could consist entirely of negative mass.. you'd only notice the void around it, it will look the same. In that galaxy, the negative mass would attract other negative mass, nothing would really change in the observation.

To let your propulsion work, you'd need both types of mass, you'd only have a ship built of negative mass, the rest of the world would have to be positive mass. Suppose it works (hand-waive hand waive) with a negative ship mass, how would you build that ship, in the first place.. I wonder where the building material for the ship is harvested ? You can't have a negative mass planet in your positive mass solar system. This planet would not stay near the sun, it would be in acceleration, moving 1000s of km/second away from you. Same with comets and meteorites. They won't impact on your positive mass planet, when their mass is negative.

Double-sided space time ? a second antipode reality ?

According to Einstein, the slope of a vector in space time will yield the perceived gravity at any point. The attractor resides somewhere "down", everything falling in the direction, along the gradient. We visualize heavy things as a "steep hole" in a 3d plane, the bottom being a stable point.

Now suppose you would model negative mass as the opposite: a steep hill in space time. Everything that sits on top of that hill will be unstable, prefer to move away from it, in any downward direction ! The ship will not move away from the planet, it will move toward the planet with even larger velocity. Conclusion: a steep hill does not work, to model your ship.

Instead, your ship would be on the opposite side of space time. You need a mechanism to climb up hill in our space time.. That would only exist, if our space time has an opposite, mirrored reality, on the other side, where the same gradient is perceived as go down hill. How separate would these two realities be ? And suppose you reside inside the ship, are you still in your world, or did you enter the antipode side of space time..


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