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If we want to be realistic, we imagine ships pushing stuff out the back and relying on the conservation of momentum to move forward.

Gravitational waves carry momentum. Are they viable as a kind of "reaction mass"? How might a propulsion system emit them? How efficient can they be?

MackTuesday
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    If we want to be realistic, we need to study A LOT (and that's not enough emphasis) of physics to understand what gravitational waves are/can be useful for... My advice, do not get too carried away in the details of the physics part unless there is a reason for it in the narrative, whatever you do it will be unrealistic and may be boring for the reader; otherwise a little of handwavium may be very useful. – SJuan76 May 29 '16 at 23:06
  • At first, I thought you were asking about surfing the gravitational waves produced by other celestial bodies, like pulsars. On re-reading it, I realize you are postulating that the spaceship itself produces the gravitational waves. Quite a different situation. On the other hand, you did not mention that you wanted to use them in a space ship. Perhaps if you were talking about moving planets, stars, or even entire solar systems? – Justin Thyme Dec 16 '17 at 04:19
  • If your setting succeeded in creating a grand unified theory than its possible they've figured out a way to create gravity using electromagnetism or something like that. Though how it would work is beyond me, as we've still yet to have a unified theory. – rclev Dec 16 '17 at 15:25

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Not at all. Very violent events cause a very tiny amount※ of energy to be converted to gravitational radiation. If you are manipulating that much energy, you can use it in other ways like pair production or lasers.

And, in order to generate gravitational waves, you need to move large masses. Which means you already have some way to move very massive objects with great force, and using the indirect result of that for propulsion is pointless.

(See also this Answer)

※ E.g. the Earth moving around the sun radiates about 200 watts (joules/second), which is minuscule compared to the energy of the system: 1.14 ×1036 joules.

Merging black holes, unlike simply moving mass around, will generate substantial gravitational radiation. E.g. GW150914 had a peak emission of 3.6 × 1049 watts, which is more power then the light from all the stars in the observable universe.

JDługosz
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Gravitational waves are massless and move at the speed of light, so they carry exactly the same amount of momentum per unit energy as light. In other words, a rocket producing gravitational waves would essentially the same performance characteristics as a photon rocket.

If you can convert mass entirely into kinetic energy, that is literally the best bang-for-your-buck that you can get. But if you can't--if you have a choice between using pure luxon propulsion with some useless exhaust mass, or using the energy you get from your fuel to accelerate the exhaust mass (like a regular rocket does), then accelerating massive exhaust is always the better option.

So, let's assume that have a perfect mass-to-energy converter, so expelling material exhaust is not your best option. Do you build a photon rocket, or a gravitational wave rocket? Well, that'll depend on how efficiently you can convert the available energy into either photons or gravitational waves. If you wave a magic wand and declare that you have a device that efficiently generates directed gravitational waves, there are some definite advantages to that; gravitational waves interact weakly with matter, which means they won't cause massive devastation if you point the terawatt watt beam of your exhaust at the ground for takeoff! But realistically, just building a photon rocket would be a lot easier and just as effective.

Logan R. Kearsley
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    "that'll depend on how efficiently you can convert the available energy into either photons or gravitational waves": and since all you need is for the photons to be going in more or less the same direction, you don't need anything fancy like a laser. Realistically, any gravity beam device you build is likely to make a lot of waste heat, and to be a lot heavier than what's essentially an incandescent lamp heated directly by your power source and some mirrors/fresnel lenses. – Christopher James Huff Oct 23 '22 at 22:37
  • @ChristopherJamesHuff If you want a decent thrust-to-weight ratio, though, you will need something fancier than just an incandescent bulb, if you want to keep the mirrors/lenses from melting. Something that could dump the equivalent of multiple kg/s into a beam of vaguely-collimated light without turning the rest of the ship into a cloud of plasma is fancy enough that I could suspend disbelief on a graviton emitter being not-that-much-more-complicated. – Logan R. Kearsley Oct 24 '22 at 14:47
  • I highly doubt you can get higher T/W than an incandescent system. A simple silver-surfaced mirror will reflect all but 1-2% of the light across the visible range and well into the infrared range. Any other light source will dump an order of magnitude more waste heat into the ship than the mirrors. And then there is the mass of the power conversion system you're using to feed whatever alternative you're looking at. If you're going to produce electrical power to feed a laser, that's going to take mass and produce waste heat. An incandescent system wouldn't need any conversion. – Christopher James Huff Oct 24 '22 at 17:24
  • @ChristopherJamesHuff Probably not, but if you want a thrust-to-weight ratio comparable to traditional mass-expelling rockets, and not turn your ship into plasma, you need to do better than incandescence; 1% of even a few terawatts will still destroy your mirror! Hence, if you posit a device that can do that, that's already enough handwaving that I'd be almost-equally willing to accept that you've got a magic graviton transducer instead. – Logan R. Kearsley Oct 24 '22 at 17:59
  • "if you want a thrust-to-weight ratio comparable to traditional mass-expelling rockets": I don't need such a thing, any more than I need a propeller-driven biplane that can fly into orbit. It's not a capability I expect photon rockets to have, and I would find it extremely unrealistic and unbelievable. The power source alone would be enough to vaporize the spacecraft many times over, even if it had a magically perfect-efficiency photon drive. And the question has no such requirement of the gravitational wave system it proposes. – Christopher James Huff Oct 24 '22 at 18:15
  • @ChristopherJamesHuff So in other words... you agree with me. A high-thrust photon rocket already requires suspension of disbelief. What exactly are we arguing about? You may not need a high-thrust photon rocket for your own purposes, but published science fiction exists that posits such things, so it's worth addressing them. – Logan R. Kearsley Oct 24 '22 at 18:26