The readership of science fiction gets smarter over time, so making your SF “smart” (even if it's not hard SF) is an ever-moving target. This earlier question makes me think about Conservation of Momentum.

Just as every modern reader understands that there is no air on the moon and we need that to survive, certain fundamental principles are becoming more widely known, even when specifics of physics are not. In particular, the Second Law of Thermodynamics is the subject of a famous quote on the reasonableness of any proposed idea, and there are the basic conservation laws.

Many SF stories feature some kind of “space drive” that is propulsion in some manner that goes beyond present day physics or makes extensions/changes to the physics in the story’s universe.

I submit that it would be “dumb” to handwave something that, without further clarification, violates the conservation of momentum. And, it would be “smart” to address this point in any description of the drive or the new physics, even if only to hang a bell on it.

Any new physics, as well as extensions to existing ideas such as gravity, will conserve momentum. It is quite fundamental: It is true because the laws of physics don’t vary with your position in space, and laws can be expressed via a principle of least action. For purposes of this discussion, consider the latter to be equivalent to there being laws of physics as we understand the notion.

So, if an author were making up some kind of “space drive”, how could it be made to conserve momentum? Related, what common ideas (like the gravity drive) have traps that need further work to produce a smarter idea in this respect?

I’m looking for various things that the author might use to handwave the problem in general or for certain classes of drive, as well as details for a sci-fi drive that is smarter and well thought out. Finally, details that lend themselves to being used as plot elements (rather than just paving over disbelief) are especially interesting.


4 Answers 4


To conserve momentum you need some kind of reaction mass. Something that will get momentum, too, for the net change of 0.

Modern rockets use exhaust for this, but we are using other mass much more commonly. It's planet Earth. Cars, trains, people - we "dump" momentum into our planet. Via friction, but if you can do it in contactless, long distance way, you're good to go. Pushing or pulling against the Sun or other stars might be neat, too.

Probably specific impulse of such drive, and its maximum "thrust" would depend on the distance to the body you want to use, and its mass. With gravity manipulation, you also could only push or pull, but nothing more. This would require three "anchors" for many maneuvers, making Sun—Earth—Moon nice set, but also making maneuvers trickier and time dependent in complicated way. "Can't go during full moon" things. Finally, if you know you need three anchors most of the time, you would want at least three engines running and one or two spare. In case of failure, maneuvers using only two would be extremely limited. But on the other hand, I can imagine sports in light one seat vessels, deliberately built with one or two push-pullers, and asteroids set up just for the game. Sports pilots would have unique place, wild and risky, but able to get the job done when others can't.


John Cramer's tachyon Drive

This is one of the best concepts for a space drive. Energy and momentum are conserved. The physics is more than a little speculative, but being speculative shouldn't be a problem for science-fiction.

Consider the central problem of rocketry: how can one burn fuel at a high enough exhaust velocity to provide reasonable thrust without an unreasonable expenditure of energy. This is the dilemma that plagues our space program, and the solutions we have developed are not very good.

So let's consider a device that makes great quantities of E=0 tachyons and uses them as the infinite velocity exhaust of a "rocket". Within the constraints of the conservation laws of physics, we can make all the tachyons we want for free, provided we make them in neutrino-antineutrino pairs to conserve spin and lepton number. Momentum conservation is not a problem because we want and need the momentum kick derived from emitting the neutrino-antineutrino pair. This leaves us to deal with energy conservation

The paradox here is that with a high-momentum exhaust of tachyons produced at no energy cost and beamed out the back of our space vehicle, the vehicle would seem to gain kinetic energy from nowhere, in violation of the law of conservation of energy. The solution to this paradox (as can be demonstrated by considering particle systems) is that the processes producing the tachyons must also consume enough internal energy to account for the kinetic energy gain of the system. Thus, a tachyon drive vehicle might be made to hover at no energy cost (antigravity!), but could only gain kinetic energy if a comparable amount of stored energy were supplied.

Cramer even provides a plausible mechanism for generating the tachyons to power this hypothetical drive.

How could we arrange for an engine to produce great floods of electron neutrino-antineutrino pairs beamed in a selected direction? All I can do here is to lay out the problems and speculate. Neutrinos are produced by the weak interaction, which has that name because is much many orders of magnitude weaker than electromagnetism. Neutrino production of any kind is improbable. On the other hand, in any quantum reaction process the energy cost squared appears in the denominator of the probability, and if that energy is zero, it should make for abig probability. The trick might be to arrange some reaction or process that is in principle strong but is inhibited by momentum conservation. Then the emission of a neutrino-antineutrino pair to supply the needed momentum with zero energy cost would make the process go. A string of similar atomic or nuclear systems prepared in this way might constitute an inverted population suitable for stimulated emission (like light, correlated neutrino-antineutrino pairs should be bosons), resulting in a beam from a "tachyon laser" that might amplify the process and produce the desired strong beam of tachyons.

There you are: the tachyon drive. A science-based hypothetical space drive running on speculative physics. Tachyons are out of fashion currently, but you never know when some old concept thought to be long discarded undergoes resurrection and is the latest hot, sexy science.

Source: Alternate View Column AV-61

  • $\begingroup$ The blockquote doesn’t really make sense on its own—what’s that about neutrinos? But the referenced essay is worth reading after this introduction. Thanks. $\endgroup$
    – JDługosz
    Commented Jan 4, 2017 at 23:39
  • $\begingroup$ The idea of tachyons for signaling is another thing I criticize, though it’s not in the common reader’s awareness (yet). But using them as reaction, not to communicate at any distance, could get around the issue. Very interesting. $\endgroup$
    – JDługosz
    Commented Jan 4, 2017 at 23:41
  • $\begingroup$ @JDługosz There's no suggestion a tachyon exhaust could be used for signalling. But as reaction mass, that's fine. I can see strong similarities between Cramer's tachyon drive and your dark matter rocket concept (which is a nifty idea). $\endgroup$
    – a4android
    Commented Jan 5, 2017 at 3:21

Invisible Reaction

There might be a fairly ordinary reaction as with a rocket, but it’s invisible and has no effect, so it can be disregarded. We know about dark matter, so why not a dark matter rocket?

However, the usual reason for having a “space drive” is to save on the energy needed to come up to speed, and allow accelerations that would crush the crew if done in a normal way. So we probably don’t want rockets at all.

If you have some novel way of getting up to speed etc. but handwave away the momentum issue by saying it’s carried away by dark matter emission, you still have the normal rocket equation applied to the energy needed to carry away the balancing mass.

Slower Than Light but still Warp Drive

Perhaps the ship doesn’t move through space at all, but a bubble of space containing the ship can move. Normally we want a ship that’s not in hyperspace or jumping or whatnot to be present in space so it can experience the universe around it (e.g. run into things). So having a warp/hyperspace/jump mechanism and then making it slower than light (due to causality rules?) is a bit odd.

Shared with Warp Technology

But we can get some mileage out of having a common mechanism between the Warp Drive (hyperspace, jumps, whatever) and the “sublight” engines. The ship moves through space in the normal way. But the engines use the FTL technology to transfer energy and momentum through wormholes, warp space, or whatever. This has the advantage of using common mechanisms, always good to do.

Think of an analogy of the Cable Cars, as San Francisco is famous for. It reaches below the street to grab a cable, getting its drive power that way from an engine located elsewhere in the city. The engine does not have to move itself or its fuel. The delivery of energy/momentum is invisible to normal cars on the road.

The engine on the space ship could reach into another dimension, through a wormhole, hyperspace, or whatever. The ship itself doesn’t travel through the subspace, but the energy and momentum does. The analogy provides what we specifically want for the space ship: it does not need to carry its own power source and fuel, freeing us from the tyranny of the rocket equation.

It can use a central power source, or it could use a system of energy and momentum buffers to save consumed power. Consider that a ship going one way and another ship going the opposite way will, as a pair, balance momentum. And ships do tend to go back and forth along the same route. The subspace cable system could connect to a buffer of some kind that allows momentum to be moved between opposite uses rather than simply lost. Likewise for energy: if you can recover the breaking energy, you will save a huge quantity of energy.

Imagine the subspace engines can connect to each other, dialed up at a desired address. Two ships can push against each other, through their engines via subspace. Or, a ship can get pushed by a huge reaction wheel, and likewise can break against the opposite side of the same wheel.

You can further note that using the hyperspace technology for this approach can always avoid causality violation rules that FTL travel would run into.

Another Universe

In general, if a conserved quantity appears to be violated, you just found another form of that quantity. Energy can be consumed in so many ways it is easy to lose track of. Angular momentum can go into magnetic fields (and invented new fields) as well as spinning objects. But linear momentum really has to be mass or massless particles moving, taking a certain amount of energy to do that. This brings us back to the invisible rocket.

So, lose the momentum in a different universe. But, as with the invisible rocket, you want to get energy and momentum, not spend it. So suppose this universe has momentum streams you can tap into, like rivers or the street cables. This is like the hyperspace tech sublight engines, but with a natural source. If you think of it like wind or water currents, you might “sail” them by probing to the desired depth, and this could give you limits as to what’s available: complete with storms and doldrums. That gives rich plot fodder right there.

Since the streams are everywhere, it is not some sneaky FTL, so you don’t have to worry about causality violation at all. And the same mechanism gives you unlimited energy more generally for your society.

Mach’s Principle

Maybe your drive “pushes” against all the mass in the universe. It appears to be a reactionless drive, but the balance is spread over everything. This has its own problems regarding FTL causality. It’s a quick handwave for whatever X design you already have and need to lampshade the conservation law.

In terms of using it as a principle to design a space drive, you still have the issue of bringing all the energy and generating a difference in momentum, so you might as well use a more ordinary invisible rocket.

  • $\begingroup$ The usual version of a Mach's principle drive has half the universe's mass pulling the vessel forward and other half repelling. Popular in 1930s & 1940s SF, but has fallen out fashion. Therefore, must be due for a comeback. $\endgroup$
    – a4android
    Commented Jan 4, 2017 at 12:03
  • $\begingroup$ Sublight warp drives make more sense than usually given credit. If spacetime can be curved by an appropriate piece of "magic" technology, then this will produce acceleration. There is no reason why it should break the lightspeed barrier. Physics isn't definitely against it. This could be close to an old-style field-drive where acceleration is no limit as every particle is accelerated in phase. EE 'Doc' Smith, art thou now forgotten? They knew how to do space drives back then. The science may been dodgy, but they had style and character. $\endgroup$
    – a4android
    Commented Jan 4, 2017 at 12:15
  • $\begingroup$ Doesn't anyone remember Arthur C Clarke's Stardrive, in Childhood's End, while the mechanism isn't explicated it appears Overlord starships pick up momentum and velocity from the star in the system it departs from and dumps it into its destination's star. By my estimate, it had a Lorentz gamma factor of 240. Now that's what I call time dilation. Implicitly, it could be a version of a Mach's principle drive. $\endgroup$
    – a4android
    Commented Jan 4, 2017 at 12:20
  • $\begingroup$ I remember that a “space drive” was a plot point, but not the detail about momentum. Good to see that Clarke considered this point, even back then. $\endgroup$
    – JDługosz
    Commented Jan 4, 2017 at 21:40
  • $\begingroup$ @a4android I’m under the impression that a couple sentences were introduced in The Skylark of Space after it was originally drafted, to hang a bell on the fact that it wildly contradicted special relativity. Readers were just becoming familiar with that, and the editor recognised the issue. $\endgroup$
    – JDługosz
    Commented Jan 4, 2017 at 23:37

At a handwavium level, a reactionless drive is simply explained by a unification theory. Just as mass and energy are not conserved separately (or nuclear reactions wouldn't work), so this advanced theory will explain that momentum and energy are not conserved separately, but can be converted from one to the other. At "normal" levels of experience, of course, the two are separate, just as mass and energy are separate and their conservation laws are considered to be independent.

The details are left as an exercise for the reader.

In the 50's and 60's, the conditions required for the unification to give practical results were usually described as interacting, opposed magnetic fields which "stress the fabric of space-time", but this has obviously fallen out of favor, and for very good reason - opposed magnetic fields may stress the generators, but the fields themselves really don't care.

High energy levels don't seem to do the trick, either, since momentum appears to be conserved in the LHC, and the local energy levels associated with its collisions are getting to within spitting distance of really fundamentally important levels.

Introducing such a theory is obviously a challenge for an author, and the best strategy seems to be simply to refer to it in passing. Details can be shrugged off as too abstruse for all but the greatest researchers, or perhaps dependent on such monumental computation levels as to require fabulously powerful computers (with similarly powerful software).

  • $\begingroup$ I want unified theory to combine momentum and entropy. Momentum is always increasing! Sounds like a fun universe. $\endgroup$
    – kingledion
    Commented Jan 4, 2017 at 15:39
  • $\begingroup$ Re your first paragraph: in my Q I explained why momentum is conserved and how more laws won’t get around that. Your premise, however it's acheived, would violate the conditions I detailed. You need to address that in order to have this make sense. $\endgroup$
    – JDługosz
    Commented Jan 4, 2017 at 23:53
  • $\begingroup$ So your answer doesn't answer the question at all but instead says “nevermind that.” $\endgroup$
    – JDługosz
    Commented Jan 4, 2017 at 23:57
  • 1
    $\begingroup$ @JDługosz - Guilty as charged. Since the alternative is to say, "No, you can't do that", I thought I'd try to be constructive toward your larger goal. $\endgroup$ Commented Jan 5, 2017 at 0:14

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