First post here, but longtime lurker :3

My question is this: if you have a civilization that is using black holes as a battery/pseudo power source, and they want to weaponize it -without- shooting the thing at a target (due to ethics of this particular civilization, they don't want to screw up spacetime by blowing the things up near inhabited systems), can they use the penrose process to fire a laser/particle beam at the black hole, using the black holes gravity to accelerate the kinetic energy of the photons, and then fire that beam at an enemy ship?

In my mind I'm picturing a pencil beam ( seen here http://www.projectrho.com/public_html/rocket/spacegunexotic.php )where the particle weapon is being guided by the laser, with the two being bent and accelerated by the black hole. I don't know how realistic this is though, and thus wondered if perhaps using a matter beam via accretion disk is the better choice?

Which is the more viable method for an advanced civilization? Pencil beam(laser+particle beam) acceleration around a tiny black hole, or using the accretion disk of said black hole to create a matter beam?


  • $\begingroup$ Welcome to the site, Veronw. Please note that we strongly encourage users to wait at least 24 hours before accepting an answer. The site has users all over the world and accepting early may deter other, better answers from appearing. $\endgroup$ – Frostfyre Apr 5 at 20:02
  • $\begingroup$ Oh! Alright, I'll go ahead and wait on that then! Thanks for letting me know ^_^ $\endgroup$ – Veronw Apr 6 at 0:12

Yes, with the "Black Hole Bomb", AKA Black Hole Death Laser

Far more effective than only superradiant scattering once!

The Penrose Process can be used amplify electromagnetic waves using a spinning black hole through superradiant scattering. However, by reflecting waves that have been already amplified back at the black hole again, a runaway energy growth can be initiated, with said energy being supplied by the rotational energy of the black hole.

Simply surround the black hole with a spherical mirror. Supply some electromagnetic radiation of your choosing (say, gamma rays) through a small opening in this mirror, and have another opening that focuses the amplified radiation towards a target. While a significant portion will fall beyond the event horizon, the rest will be amplified many times more before it arrives at either opening, with an extraordinary amount of energy. You can use this radiation to essentially evaporate anything that looks at your black hole funny.

By making the openings smaller, you can make the radiation scatter more before being extracted for theoretically unlimited output, though this is dependent on the strength of your mirror.

See: Black Hole Bomb on Wikipedia

  • $\begingroup$ Oooooh, now that's an interesting thought! So it would be a radiation weapon, not as much a laser or a particle beam? You'd basically fire a gamma ray at the black hole, let it circle a bunch of times, then channel it at some unfortunate target? Hm, any idea on the actual #'s for the power of something like that? Would it make the laser more powerful by one order of magnitude? 10? Sorry for the long delay, worlds been busy xD $\endgroup$ – Veronw Apr 22 at 16:30
  • $\begingroup$ The maximum strength of your beam is entirely dependent on the strength of the mirror, where the maximum is the amount of radiation pressure the surface can withstand before being shattered / torn apart. With a mirror that is strong enough, you could theoretically evaporate planets. Note that since the Penrose Process converts energy from the black hole's spin, it isn't unlimited and will eventually run out depending on which black hole you use (albeit over an extremely long period of time). $\endgroup$ – kouwei32 Apr 22 at 22:29
  • $\begingroup$ Good to know! Thanks for that! I believe this answers the question for me ^_^ $\endgroup$ – Veronw Apr 23 at 1:45

can they use the penrose process to fire a laser/particle beam at the black hole, using the black holes gravity to accelerate the kinetic energy of the photons, and then fire that beam at an enemy ship?

First of all, photons would always move at c, because of the physics we know today. We can't accelerate nor decelerate them. At most they would blue shift while approaching the black hole and red shift when moving away from it.

But even if c wasn't a hard limit, or if you are using particles and not photons, the black hole would at most deviate them, because if it could accelerate them while they approach, it would similarly decelerate them when they move away, in the same way a body in an elliptical orbit move faster when it approaches the central attractor than when it departs from it. Unless the target is closer to the black hole than the emitter, in which case it might work.

  • 1
    $\begingroup$ Ahh, I see, okay then it sounds like maybe the matter weapon variant is the better choice. I had assumed that because of the way the black holes worked that the photons of the laser would pick up additional kinetic energy, but my knowledge of such things is very poor. Would the particle beam gain effective traits from it or do you think it would still be essentially non-viable? $\endgroup$ – Veronw Apr 5 at 16:40
  • $\begingroup$ True, the black hole cannot cause a net increase in the speed of the particle beam. False, the black hole can amplify the particle beam by way of the mentioned penrose effect. Same speed, greater mass exiting than entered. (only by about 20-29% though, depending on black hole rotation and charge.. but it is a net gain in energy) $\endgroup$ – PcMan Apr 5 at 16:45
  • $\begingroup$ You can accelerate light if it strikes a light sail - it reflects and transmits F = ma to the sail, where m is the relativistic mass. More to the point, the "Penrose process" is decelerating the rotation of the black hole, so I certainly hope it's putting force on the light, i.e. accelerating it. The light can't go faster but it can blue shift. I'll admit it's an abuse of language to say that you can accelerate without going any faster, but you almost do that whenever you accelerate a relativistic particle.in an accelerator. $\endgroup$ – Mike Serfas Apr 5 at 16:47
  • $\begingroup$ Hmmm, okay so it -can- work, sort of kind of XD When describing it I'm guessing I should avoid the use of 'acceleration' and instead use something like, amplification/blue shifting of the laser, altering the mass of the particle beam? $\endgroup$ – Veronw Apr 5 at 16:52
  • $\begingroup$ Superradiant Scattering actually allows the Penrose Process to amplify Bosonic fields, which includes electromagnetic radiation, meaning that using the Penrose Process to make death lasers is fully viable. I wrote about this a bit in my answer to this question. $\endgroup$ – kouwei32 Apr 16 at 7:42

That's not really what Penrose process is for.

The "Penrose process" has the rather limited effect of extracting some rotational energy that surrounds a spinning black hole. It isn't a tremendous amount of energy extracted, and I think you have other ideas how to get energy from small black holes. Once you have the energy in fungible form, use it to run any model of Big Honkin' Space Gun you like. (The black holes might be handy as micro magnet-free particle accelerators. I suspect though that someone with an unlicensed particle accelerator on his back would rather it use magnets.)


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