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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?

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    $\begingroup$ 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. $\endgroup$ – SJuan76 May 29 '16 at 23:06
  • $\begingroup$ 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? $\endgroup$ – Justin Thyme Dec 16 '17 at 4:19
  • $\begingroup$ 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. $\endgroup$ – 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.

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