I had an idea for my story for an advanced civilization to capitalize on possibly the most energetic events in their universe which is the merger of supermassive blackholes.

From current observations of mergers it seems that ten percent of the mass of the smaller blackhole is turned into gravitational wave energy so for SMBHs (supermassive blackholes) this will be a tremendous amount of energy. It is predicted that, unlike the currently observed mergers of a few ten solar mass blackholes which produce its energy in less than a second but SMBHs will give off the large amounts of energy over a week or longer depending on their size.

My idea is similar to a Dyson swarm but a very large swarm to cover the orbital distance of the blackholes as they reach the merger and the swarm must be large to have enough ships to convert what will be a small amount of the total energy but combined in their trillions over they can capture a high amount of the gravitational waves, if that is even possible?

What method could be used to covert the waves into usable/storable energy using current or near future methods?

Bounty notes:

I had thought of @Goodies idea of creating a devise to turn the motion of parts into electrical energy as this method is similar to how they currently detect waves from the position of protons but I did wonder if there were better methods or what a large high tech version of that method would be like?

I am looking for hard science answers but will also accept detailed theoretical ideas such as @Goodies suggestion of a more direct transfer of gravity to energy. I had thought that the concentration of space during the wave could increase virtual particle density which could be extracted with a sort of Casimir plate device for increased zero point energy. I would rather Hard-science answers but theoretical answers are welcome.

On another note for bounty answers, can a gravitational wave dissipate in the same way that water waves can be dissipated or would it still pass through most matter even if some energy is extracted? As if it does dissipate by the swarm this could be used as defence method for civilizations in the merged galaxy.

  • $\begingroup$ Related: worldbuilding.stackexchange.com/questions/148476/… $\endgroup$
    – Goodies
    Dec 31, 2021 at 22:21
  • 1
    $\begingroup$ Alan Davies you were very good on QI last week! $\endgroup$
    – Daron
    Jan 7, 2022 at 20:16
  • $\begingroup$ What do you understand a "gravitational wave" to mean? There are several theoretical ways to harvest energy from a black hole. But what do you mean by harvesting the gravitational waves? $\endgroup$
    – Daron
    Jan 7, 2022 at 20:18
  • $\begingroup$ The Penrose Process is one example of how to harvest the rotational energy of a rotating black hole. $\endgroup$
    – Daron
    Jan 7, 2022 at 20:21
  • $\begingroup$ @Daron True and I like that method in other situations but for a supermassive blackhole merger I cant see how the process can be used. $\endgroup$ Jan 7, 2022 at 22:54

3 Answers 3


Ok gravitational wave season, say it persists for 2 weeks, energy can be harvested for 2 weeks..

How much you'd get

Depend how far away you are, of course.. these waves have huge energies, but after 1 billion light years space time distance.. there wasn't much left for little Earth. Forbes calculated it,

"From a billion light-years away, two black holes of 36 and 29 solar masses merged, converting about three Suns worth of mass into pure energy. By the time those waves arrived at Earth, they had spread out so only 36 million J of energy impacted the entire planet: about as much energy as Manhattan receives from 0.7 seconds worth of sunshine. "

From https://www.forbes.com/sites/startswithabang/2020/05/08/ask-ethan-can-we-extract-energy-from-gravitational-waves/?sh=3559d37f1211

Now.. suppose such a gravitational wave persists for two weeks (you propose that, I'll handwaive), and the Forbes numbers are accurate, the wave that occurrered came from 1 billion light years away and it provided Manhattan (59.2 square km2 ) with 100% extra sunshine, for 0.7 seconds.

(disclaimer: below calculations are not accurate, more like "in the order of..")

What would be needed in terms of distance, to harvest that energy ? how near should your planet be, to get 100% extra sunshine, during two weeks, everywhere ? That would be 510,100,000 km² Earth's surface, not 59.. we'll need a factor 10 million times more energy to warm up the whole Earth. Also, you'll need to multiply the 0.7 seconds to get 2 weeks continuous energy so together, you're about 0.7/(24 * 3600 * 14)=5.78e-7, multiplied say you would need about 10e14 times more energy from the gravitational wave..

From an earlier topic I learned energy of gravitational waves is proportional to square distance so you need to take the square root of 10e14, that would be about 10 million. You should be 10 million times nearer.. the extra sunshine would happen if the two black holes would collapse at a distance of ca 100 light years, that is one billion light years (the original distance) divided by 10 million.

Concluding: at 100 light years distance, you can harvest 2x sunlight for 2 weeks everywhere

But again, your assumption about the two weeks spread MUST work.. if you don't have that spread, you'd be completely fried chicken when distance is only 100 light years and you'll get all energy at once, you'd have 10 million suns in the sky for 0.7 seconds.

To harvest it..

Night side of the planet would receive the extra energy too, it would be available 24/7

The Newton answer would be: you'd have to transfer very small mechanical displacements to larger ones. Tiny, but moves that allow huge, near-infinite reaction force. A very strong force. You could try to devise a gear system, to transfer that force to a rotator (generator) in some way.

The Einstein answer I don't know. It may be possible to harvest energy directly, from the distortion of space time itself, gravitational deviations.. Say you drop a large weight, will it bounce back up again, and how to derive energy from that.. and how to prevent your workers from bouncing up and down too?

Creating perpetual motion (newtonian) using the gravitational wave

With gravitational waves, spacetime will show a standing wave in a certain direction, like you have at the sea shore, blue is background, red is the wave added by the collision,

enter image description here

Gravitation will vary, in a certain pace. How to harvest this ? Let's say the wave will be in a certain direction, you'll have a band over your planet where the variation is perpendicular to the planet surface. Space time theory sais, you'll have a gravitational force proportional to the gradient of the spacetime surface. An observer will notice a variable gravitational force when spacetime has a hight gradient (goes steep).

Now suppose a large weight will be pushed upward, during a phase of small gravitation.. it would cost little energy, when the gravitational wave is in the downgoing position..

enter image description here

When this thing would fall back with enhanced gravity, it would bounce back with more force, getting up higher than before. If you repeat it a few times, in the same rhythm, your heavy object oscillates ever higher up. Perpetual motion. You could harvest the energy when it is touching the ground and bouncing back, or in the end, collecting all energy gained in the process.

enter image description here

The mechanism could also be used to launch objects into space.

  • $\begingroup$ Goodies, I have opened a bounty for this question. If you could expand on your ideas your answer could be accepted. $\endgroup$ Jan 6, 2022 at 13:44
  • $\begingroup$ I've added an option to harvest the energy in the Newtonian way. Of course, it's highley speculative whether this is will work. It is based on assuming you can harvest kinetic energy from a weight, that is it will enable a kind of "perpetual motion". However, this will only be temporarily. It will work while the gravitational wave is octive, with a certain frequency. So there are lots of assumptions.. $\endgroup$
    – Goodies
    Jan 7, 2022 at 4:17
  • $\begingroup$ Thanks for the update, you mentioned a planet a few times but it is a swarm around the two merging blackholes and you say"sunshine" which confused me but I glossed over that as the concept is still the same. $\endgroup$ Jan 7, 2022 at 13:25
  • $\begingroup$ There's no real context in my answer yet.. and I haven't found a proper engineering solution for harvesting this energy.. Maybe I'll do a third edit later when I think of something. $\endgroup$
    – Goodies
    Jan 7, 2022 at 15:37

This is a good question, and a difficult task.

Part of the problem is that most objects in the universe are effectively transparent to gravitational waves - they don't really absorb or scatter them, so energy transfer is quite difficult. In 1969, Freeman Dyson calculated that Earth should absorb a fraction $\varepsilon\approx10^{-21}$ of the energy flux of a 1 Hz gravitational wave passing through (although there is a dependence on the frequency $f$ of the wave, scaling as $\varepsilon\propto f^{-2}$).

That said, under the right conditions, gravitational waves can dissipate some energy. There was some extremely speculative work about a decade ago (Li et al. 2012) arguing that gravitational waves emitted by a supermassive black hole binary (SMBHB) could deposit energy in an accretion disk and in nearby stars. A merging SMBHB of mass $\sim10^9M_{\odot}$ could induce a luminosity increase of about 1% of the Sun's luminosity in a nearby star about 10% the mass of the Sun, lasting for about a week. This might seem like a pittance - and it arguably is - but it would still be significantly larger than the normal luminosity of that star by an order of magnitude or so.

Therefore, one possible way to convert the energy of gravitational waves to something useful would be to indeed build something like a Dyson swarm, but have it surround nearby stars and take advantage of the luminosity increases in the aftermath of the merger (as well as the stars' typical output).

This is arguably fairly inefficient and is extremely speculative - and if you're building a Dyson swarm, you might as well surround a much more luminous star than a red dwarf - but it would indeed convert the energy of gravitational waves into electricity, or some other type of energy of your choosing.


Guiding the collapse of a binary Black hole:

I will open with the fact that I am not a physicist and am shamelessly raiding Wikipedia for material. Since this is somewhat uninformed, I post it here mostly to see what ideas it provokes. PLEASE feel free to leave constructive comments.

The final Parsec: Collision of two such black holes is an unlikely event. Instead, under ideal conditions (which are likely to require the intervention of a high-level civilization) You would have two supermassive black holes using the ejection of matter to cushion the stabilization of an orbit. More matter would be needed to guide the two stars closer and closer to each other (inspiral), and they would be likely to eject all available matter before the effective end of the universe as we know it.

An advanced interstellar civilization could guide this process by manipulating stars as breaking and mass for this process. I have little doubt that the energy of this mass being ejected from the system of the black holes would be both vast and occur over an extremely long time. Harvesting energy from moving matter is a well-established process doubtlessly better done by a high-end civilization than anything we could manage.

The continuing narrowing would involve the constant feeding of matter into the system and the release of energy. So we're milking the event from start to finish. The final cataclysmic event is supposed to release insane amounts of energy, and continuing gravity waves as the shapes of the black holes stabilize. The challenge is to figure out a useful way to store even a fraction of this power for a meaningful amount of time.

I'm spit-balling a bit here, but can this energy be used to stretch a sort of cosmic string? It could be as "simple" as your Dyson swarm containing connected pieces with "elastic" chords that are then stretched, but I doubt an advanced civilization would depend on something so crude. SO what if you maneuver a ring black hole around your collapsing SMBH collision at the moment of maximum output? I don't know how large you could inflate one without blowing it apart, or if it would expand and immediately contract, but if you could make a super-wide, super-fast spinning black hole around your new SMBH singularity, and milk the energy of it as it shrunk, the potential energy stored in the system would be insane.

  • $\begingroup$ Interesting ideas, food for thought. Cosmic string tension could be an issue with the first idea. A ring black hole around the merger sounds tricky but cool also, can a ring singularity be so wide? How do the gravitational waves transfer to the ring, does it add to its rotational speed so then Penrose process can be used milk it? $\endgroup$ Jan 10, 2022 at 12:27
  • $\begingroup$ @AlanDavies That was my question - could you inflate the ring singularity like a giant rubber band using the force of the gravitational shockwaves, or would explode/collapse? Logic say it would spin faster, but not sure how the mechanics would work. But such a massive event seems to scream for a completely insane-scaled energy storage device. Who knows? Maybe it would let you make a ring black hole large enough to use as a wormhole or something fun that shouldn't be able to exist otherwise. Just trying to think outside the box. Fun idea. $\endgroup$
    – DWKraus
    Jan 10, 2022 at 16:08

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