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I am thinking about what would happen in a very-very distant future when most of the stars have already died out into black holes or other catastrophic events, leading to severe lack of light in the sky of remaining star systems with life. And I'm thinking about recent discoveries of gravitational waves made by combining black holes that release energy at the time of combination in range of several percent of combined mass. That energy has to eventually hit something. I wonder if there's a plausible mechanism of focusing a wave emitted from a detected distant black hole pair's combination that could provide enough intensivity in the focal point to make the potential energy create at least electron-positron pairs? At best it would be proton-antiproton pairs, meaning if that's somehow going to happen, there would be a way to make more simple matter for fusion.

A civilization in this question is expected to be a K3 one, operating star-sized power with relative ease, maybe at prolonged intervals, after all real world is VERY slow to accept global changes. So in theory they can move stars or help them move so that the relative positioning required for focusing a gravitational wave is achieved with rough precision, but still produces some output if theoretically possible.

Related: Possible methods to convert gravitational waves into storable energy? yet that question is focused on harvesting energy without any kind of focusing.

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Focusing low density G-waves ?

Related, not the same..

Could gravitational wave interference patterns be used for long range sensors detecting advanced spaceships?

GW energy proportional to distance, not distance squared

There could be interference patterns, maybe detectable.. but I wonder if your focusing plan would work. Gravitational waves have the peculiarity their energy decreases proportional to distance, not the square of distance. As a result, the energy of the GW is spread over a much larger distance from the event that causes it. We can witness events hundreds of millions of light years away.

Size issues with your focusing mirror

If you want to focus the energy,suppose some mirror can be made, or field generated for that, you would need the square of the mirror size i.r.t. electromagnetic radiation, such as light. Suppose you'd make a light mirror of 100km, the same energy yield would result from a GM mirror 10.000km wide.

Gravitational wave lenses don't focus anywhere near

The other way to focus is place a gravitational lens somewhere.. Unless your K3 civilization can play billiards with black holes, they won't be able to concentrate enough mass to do that. And the lens will actually focus on a place on the other side.. millions of light years away from you.

I'm not a physicist.. maybe merging of positrons and electrons occurs as a result of the gravitational distortion and this distortion (making matter) does not require that much energy ? K3 can invent ways to handle gravitational waves we can't think of.. but I'm afraid there is no "science based" method to engineer the focussing.

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  • $\begingroup$ Actually I thought about whether a gravitational lens in a certain place close enough to black holes that are about to combine could focus spacetime distortions to a point behind it if looking from the source, and there the civiliztion would try to employ something that would allow energy transformation from those waves into spacetime in form of matter. I wonder if that lens would have to be a bigger BH... $\endgroup$
    – Vesper
    Jan 18 at 14:23
  • $\begingroup$ Well @Vesper suppose K3 might engineer (or set up) black holes.. And if not.. such attractors and their resulting focus points could exist in this universe, for light and for GW.. but keep in mind the energy you can harvest from GW's in such a place will be limited. It could be more effective to put some solar panels up and harvest the energy from the light arriving at the focus point ! It will probably arrive at (about) the same spot but I'm really not sure. It's a challenge to find them and.. these focus points move ! things are always in orbit, not stationary. $\endgroup$
    – Goodies
    Jan 18 at 15:01
  • $\begingroup$ I rather mean that a K3 civilization can detect black holes better than us, and can also probably travel to the place where the focus of GWs over a known gravitational lens would be. It might also be possible that there exists a ready-made pair of BHs and a lens BH so that their focal point if unmoved is within reach of the civ. Can that be harnessed, at least in theory? Based on others' answers I say a likely no, yet this would still be a valid answer. $\endgroup$
    – Vesper
    Jan 19 at 6:12
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    $\begingroup$ How can GW "energy" (intensity, presumably) decrease proportionally to distance? doesn't that mean the total power increases over distance? Free energy from nothing? $\endgroup$
    – user253751
    Jan 19 at 12:11
  • $\begingroup$ @user253751 the energy will spread over an enormous range. The event that has caused the GW has a certain energy. That is a LOT of energy.. and it is also the energy contained in the field. When the energy propagates linearly, the range of the field will be huge, spreading the energy over hundreds of light years. Because the range is huge, the GW manifests as a tiny energy reaching the observer. It is all a matter of range. You have to be far away to survive the event... $\endgroup$
    – Goodies
    Jan 21 at 9:39
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Have you ever read the laymen explanation of how can black holes produce Hawking radiation?

To make it short, in the vacuum there is a constant production and annihilation of virtual matter/antimatter pairs. Normally this production goes unnoticed, however when the pair happens to be produced on the event horizon of a black hole, an element of the pair will end up trapped in the black hole, the other instead will be free to leave. As a result, the virtual particle has now become real.

Something similar might happen with gravitational waves. Gravitational waves "stretch" the space-time into which they propagate; if the concentrated wave happen to pass in a volume of space when a virtual pair is produced, the pair can be stretched far apart enough to not annihilate any more. As a result, matter would be produced.

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  • $\begingroup$ Yes I've heard about Hawking radiation, however it's highly improbable for a "normal" black hole of mass around 1 mass of sun to produce a pair of particles this way, so if a gravitational wave would pass through a black hole's edge, they would make a highly improbable event slightly less improbable. Maybe a source of matter though, as the black hole's edge is decently big. Thinking... $\endgroup$
    – Vesper
    Jan 18 at 14:15
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    $\begingroup$ Physicist here. This mechanism sounds plausible at a words level, but you have to be very careful about reasoning from these heuristic explanations of Hawking radiation, which can be very misleading. The key thing that makes Hawking radiation work is a horizon (region of space from which light cannot escape). Gravitational waves are not associated with a horizon, and so do not produce Hawking radiation. $\endgroup$
    – Andrew
    Jan 18 at 18:32
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    $\begingroup$ To add to Andrew's concerns, gravitational waves interact with matter extremely weakly. Even if you were in the same star system as a binary BH merger, the GWs would pass harmlessly through you. To overpower the electromagnetic force and separate two particle pairs would need GWs probably not seen in our universe. $\endgroup$
    – BMF
    Jan 18 at 18:43
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    $\begingroup$ The generation of Hawking radiation requires an event horizon. More precisely a horizon is required for the radiation to propagate to infinity. Gravitational waves cannot produce Hawking radiation. $\endgroup$
    – John Rennie
    Jan 19 at 6:03
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    $\begingroup$ The question is tagged science-based, not hard-science, so it doesn't have to rigorously follow the mathematics of quantum physics. $\endgroup$
    – user253751
    Jan 19 at 12:11
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Yes and No.

A Gravitational Wave does not interact with matter the same way say a light wave does. Gravitational Waves are changes in the geometry of Spacetime. As such, you can build Mechanisms which exploit the fact Gravitational waves change the geometry of spacetime.

However, this is only energy extraction. You cant focus / Mirror them. They are not light waves and as a matter of fact really not "waves" either. Its a bit more complicated. But the takeaway is that there is no known way to reflect Gravitational Waves.

Besides, there are infinitly better ways to extract energy out of a black hole.

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If you can get gravitational waves to constructively interfere to create a region of space-time with a great enough energy packed into a small enough radius, the Schwarzschild radius, then you'd get a black hole. Black holes will release black body radiation—photons, probably in the gamma range—which may collide to form electron-positron pairs via the Breit-Wheeler process.

The next step involves somehow separating the charged particles.

I don't believe this is possible in nature. BH merger GW energy is severely diluted over extragalactic distances. There just isn't much to work with by the time it reaches Sol (let alone the "materials" used to reflect/focus GWs.) Even if you could produce a BH from GWs, only a tiny, tiny, tiny fraction of the energy would end up as electrons/positrons (and some subset of those would annihilate, too).

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    $\begingroup$ I really doubt that a gravitational wave could be focused into a Planck radius, as its source is FRIKKING WIDE, namely several light-hours to several light-years, depending on SMBHs' masses. And that "conservation of etendue" principle seems to apply to these waves as well. $\endgroup$
    – Vesper
    Jan 18 at 9:23
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    $\begingroup$ @Vesper I really doubt it too. I was just answering the question in the title, but now I see there's an additional question about whether natural GW sources can be focused to produce the effect. I think that's highly unlikely, too, and for the same reasons. The GW energy is severely diluted over extragalactic distances. There just isn't much to work with (let alone the "materials" used to reflect GWs.) Even if you could produce a BH from GWs, I think only a tiny, tiny, tiny fraction of the energy would end up as electrons/positrons (and some subset of those would annihilate, too). $\endgroup$
    – BMF
    Jan 18 at 9:28
  • $\begingroup$ As a sidenote, I have heard of some exotic metamaterials that might be able to generate high-frequency gravitational waves in a lab (citation needed). They might also be used to detect them, too. Problem is that most natural GW sources are extremely low frequency, which is why we need big detectors like LIGO. $\endgroup$
    – BMF
    Jan 18 at 9:40
  • $\begingroup$ And as a side-sidenote (super citation needed), it's possible that high frequency gravitational waves, when passing through a very strong magnetic field, may produce very weak electromagnetic signals as a result (the kind you need cryogenic detectors to see...). Another possible avenue for electron-positron pair production. $\endgroup$
    – BMF
    Jan 18 at 9:46
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I'm going to take a step back and ask, "is it possible to focus gravity waves." Our science currently tells us that you can't make gravity do anything it isn't already doing. That's why anti-gravity, or grav plating on star ships, or gravity based putt-putt golf is on the "big lie" list of science fiction.

If you could control/create/nullify gravity, then you would have a level of control over the sub-atomic froth that would probably have resulted in matter creation much earlier. Matter is basically self-perpetuating vortexes in the froth. I could see using gravity to induce those vortexes, creating matter/anti-matter pairs, then separating them with magnetic fields.

This, however, is well into the "sufficiently advanced technology" zone that, no matter what you make up, it's just technobabble.

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