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Black Hole Propulsion as Technosignature

When he was considering white dwarfs and neutron stars in the context of what he called ‘gravitational machines,’ Freeman Dyson became intrigued by the fate of a neutron star binary. He calculated in his paper of the same name (citation below) that gradual loss of energy through gravitational radiation would bring the two neutron stars together, creating a gravitational wave event of the sort that has since been observed. Long before LIGO, Dyson was talking about gravitational wave detection instruments that could track the ‘gravitational flash.’

Image: Artist conception of the moment two neutron stars collide. Credit: LIGO / Caltech / MIT.

Observables of this kind, if we could figure out how to do it (and we subsequently have) fascinated Dyson, who was in this era (early 1960s) working out his ideas on Dyson spheres and the capabilities of advanced civilizations. As to the problematic merger of neutron stars in a ‘machine,’ he naturally wondered whether astrophysical evidence of manipulations of these would flag the presence of such cultures, noting that “…it would be surprising if a technologically advanced species could not find a way to design a nonradiating gravitational machine, and so to exploit the much higher velocities which neutron stars in principle make possible.”

He goes on in the conclusion to the “Gravitational Engines” paper to say this: “In any search for evidences of technologically advanced societies in the universe, an investigation of anomalously intense sources of gravitational radiation ought to be included.”

Searching for unusual astrophysical activity is part of what would emerge as ‘Dysonian SETI,’ or in our current parlance, the search for ‘technosignatures.’ It’s no surprise that since he discusses using binary black holes as the venue for his laser-based gravity assist, David Kipping should also be thinking along these lines. If the number of black holes in the galaxy were large enough to support a network of transportation hubs using binary black holes, what would be the telltale sign of its presence? Or would it be observable in the first place?

Remember the methodology: A spacecraft emits a beam of energy at a black hole that is moving towards it, choosing the angles so that the beam returns to the spacecraft (along the so-called ‘boomerang geodesic’). With the beam making the gravitational flyby rather than the spacecraft, the vehicle can nonetheless exploit the kinetic energy of the black hole for acceleration. Huge objects up to planetary size could be accelerated in such a way, assuming their mass is far smaller than the mass of the black hole. No fuel is spent aboard the spacecraft which, using stored energy from the beam, continues to accelerate up to terminal velocity.

Image: Simulated image of the two merging black holes detected by LIGO, viewed face-on. LIGO’s gravitational-wave detection is the first direct observation of such a merger. Credit: LIGO / AAS Nova.

Kipping likes to talk about the process in terms of a mirror. Because light loops around the approaching black hole and returns to the spacecraft, the black hole exhibits mirror-like behavior. Thus on Earth, if we bounce a ping-pong ball off a mirror, the ball returns to us. But if the mirror is moving towards us quickly, the ball returns faster because it has picked up momentum from the mirror. Light acts the same way, but light cannot return faster than the speed of light. Instead, in gaining momentum from the black hole, the light blueshifts.

We exploit the gain in energy, and we can envision a sufficiently advanced civilization doing the same. If it can reach a black hole binary, it has gained an essentially free source of energy for continued operations in moving objects to relativistic speeds. Operations like these at a black hole binary result in certain effects, so there is a whisper of an observable technosignature.

I discussed the question with Kipping in a recent email exchange. One problem emerges at the outset, for as he writes: “The halo drive is a very efficient system by design and that’s bad news for technosignatures: there’s zero leakage with an idealized system.” But he goes on:

The major effect I considered in the paper is the impact on the black hole binary itself. During departure, one is stealing energy from the black binary, which causes the separation between the two dead stars to shrink slightly via the loss of gravitational potential energy. However, an arriving ship would cancel out this effect by depositing approximately the same energy back into the system during a deceleration maneuver. Despite this averaging effect, there is presumably some time delay between departures and arrivals, and during this interval the black hole binary is forced into a temporarily contracted state. Since the rate of binary merger via gravitational radiation is very sensitive to the binary separation, these short intervals will experience enhanced infall rates. And thus, overall, the binary will merge faster than one should expect naturally. It may be possible to thus search for elevated merger rates than that expected to occur naturally. In addition, if the highway system is not isotropic, certain directions are preferred over others, then the binary will be forced into an eccentric orbit which may also lead to an observational signature.

Tricky business, this, for a black hole binary in this formulation can be used not only for acceleration but deceleration. The latter potentially undoes the distortions caused by the former, though Kipping believes elevated merger rates between the binary pair will persist. Our technosignature, then, could be an elevated binary merger rate and excess binary eccentricity.

I was also interested in directionality — was the spacecraft limited in where it could go? I learned that the halo drive would be most effective when moving in a direction that lies along the plane of the binary orbit. Traveling out of this plane is possible, though it would involve using onboard propellant to attain the correct trajectory. The potential of reaching the speed of the black hole itself remains, but excess stored energy would then need to be applied to an onboard thruster to make the course adjustment. Kipping says he has not run the numbers on this yet, but from the work so far be believes that a spacecraft could work with angles as high as 20 degrees out of the plane of the binary orbit and still reach an acceleration equal to that of the black hole.

For more on the halo drive, remember that Kipping has made available a video that you can access here. The other citations are Kipping, “The Halo Drive: Fuel-free Relativistic Propulsion of Large Masses via Recycled Boomerang Photons,” accepted at the Journal of the British Interplanetary Society (preprint); and Dyson, “Gravitational Machines,” in A.G.W. Cameron, ed., Interstellar Communication, New York: Benjamin Press, 1963, Chapter 12.


Comments on this entry are closed.

  • Michael March 6, 2019, 16:25

    It’s not just binary BH’s that can do this, a single fast rotating BH that is tumbling could do it as well.

    • Andrei March 8, 2019, 8:20

      Then the question is if one such could be tumbling, and what process that caused it.
      A few Millisecond pulsars got quakes in the crust that make the changes in the pulse, but that’s not exactly tumbling – only the crust of heavy elements that settle and adjust.
      Can any of you here write the paper: ‘Do Neutron stars have continental drift?’ I could not find any! ;)

      • Michael March 8, 2019, 10:35

        Most likely with interactions with other BH’s, perhaps at the centre of the galaxy. As for neutron stars the crust is solid on top of a superfluid, they do under go quakes which can be very violent.

        • Andrei March 8, 2019, 11:46

          I think you are right there, black hole can’t tumble on their own.
          But multiple system or the galactic center with chaotic orbits with multiple sources of gravity – it’s possible.

          Yes it’s the superfluid that is frictionless that had me think the parts of the crust could move around easily – and that’s my joke it might it’s similar to continental drift on Earth. (First joke I made here I think.)
          I found a paper on the real thing, only 12 reads?

  • Michael March 6, 2019, 16:28

    These BH can also be used as very powerful weapons that are able to throw large amounts of energy and matter about.

    • Alex T. March 9, 2019, 3:13

      BH do that every moment, so what?

      • Michael March 9, 2019, 11:58

        Not really, they are quite and unassuming until they’re ticked off…much like me.

        • Alex T. March 12, 2019, 3:35

          BH supposed to be not quiet at all , and supposed to radiate every moment huge amount of killing energy, for example famous “jets” comming from galaxy centers – that are detected by astromonets.

          • Michael March 13, 2019, 3:36

            There is predicted to be over 100 million BH’s in our galaxy and we are only seeing a few active ones, so i would say they are very quiet.

  • Adam March 6, 2019, 16:45

    Hi Paul
    On Dyson’s original paper, the Gravitational Research Foundation has a copy online from when Dyson submitted it for their essay competition. However it isn’t a very clear reproduction, since the ink has clearly faded from the old typescript. Mallove & Matloff essentially reproduce it in their “The Starflight Handbook” if the readers can’t access a copy of Cameron’s anthology.

  • Geoffrey Hillend March 6, 2019, 16:57

    A civilization has to solve the interstellar travel problem and develop fast spacecraft to get to the black hole. Consequently, a gravitational assist from a black hole would be unnecessary for civilizations which already have that capability.

    • Zachary March 7, 2019, 0:24

      A civilization has to solve the intercontinental travel problem and develop fast boats to get to the next continent. Consequently, an aerial assist from an airplane would be unnecessary for civilizations which already have that capability.

      I think you can see the obvious fallacy there. As long as a means of transportation offers an advantage, it will be used. I don’t think we can rule this idea out.

      • Brett Bellmore March 9, 2019, 8:43

        Yeah, we see the obvious fallacy, and it’s yours. Getting to the next continent didn’t enable the production of airplanes.

        The basic problem here is that just getting to the black hole pair is so difficult that it’s probably always going to be better to just apply that effort to making your ships faster on their own, unless you just happen to have such a pair next door.

        It’s like flying to the top of the mountain to get a boost from the winds up there. Unless it’s already on your path, it’s a waste of time.

  • James M Essig March 6, 2019, 19:39

    Black holes are an amazing potential source of kinetic energy. An ultra-massive black-hole spinning at a better part of light-speed could come in handy for cosmic travelers.

  • Ron S. March 6, 2019, 19:41

    “The latter potentially undoes the distortions caused by the former, though Kipping believes elevated merger rates between the binary pair will persist.”

    Even if there are equal and perfectly efficient accelerations and decelerations the binary’s merger is hastened. That’s because these gravitational events are not quite reciprocal due to the non-linearity of gravitation under general relativity. The effect grows as the gravitational gradient is increased to extreme values. The same non-linearity is how event horizons are formed.

    In any case I find it difficult to imagine using binaries for spacecraft propulsion in this way can have any measurable effect on a system containing tens of solar masses.

  • Alex Tolley March 6, 2019, 19:45

    I would think looking for the reverse of infallible (our falling?) would be the signature that would be indicative of technology. While I’m falling happens naturally, any increase to accelerate a starship would not be definitive. The reverse is not natural and would therefore be indicative.

    The instruments would have to be insanely sensitive to detect this, even if planets were being decelerated.

    The detection of leaked laser light from the halo might be easier to detect, especially if instabilities meant some loss of light capture by the ship. It would require full sky surveys to detect transient signals from such technology, even if the star lanes were as busy as terrestrial shipping lanes.

  • Andrew Palfreyman March 6, 2019, 19:56

    The expiration of a kugelblitz *might* be viewed as a technosignature, but more convincingly so, were there to be multiple observations. I’m not aware of a single one being reported thus far.

  • Adam March 7, 2019, 7:04

    Hi Paul
    Of course, Googling can turn up fresh surprises…
    Gravitational Machines

    Seems to be part of an Astronomy Course’s references. Essentially the same text as the GRF essay.


    • Paul Gilster March 7, 2019, 9:29

      Good catch!

  • Al Jackson March 7, 2019, 9:13

    The problem here is that one wants to gain the highest flyby boost as possible . Interstellar flight needs boosts up to near the speed of light. However in neutron star and black hole binaries this only occurs when the pair are close together. The disadvantage of that is gravitational radiation will collapse close neutron star and black binaries very fast. The window of opportunity for using such a binary is quite short with respect to interstellar travel times, this is a hard constraint of neutron star or black holes as flyby amplifiers. Also there are only so many neutron star and black hole binaries int the Galaxy, approximately 1 million black hole binaries* in a volume of about 8 trillion cubic light years!

    Counting black holes: The cosmic stellar remnant population and implications for LIGO, Monthly Notices of the Royal Astronomical Society, Volume 473, Issue 1, p.1186-1194, 2018.

  • Harry R Ray March 7, 2019, 11:32

    Paul Gilster: This may be a bit OT, but I hope not(more on this later). arxiv.org/abs/1902.09985 “Searching for Extraterrestrial Civilizations Using gamma Ray Telescopes”. by Louis Crane. I tried to read the entire PDF, but it is way over my head. Please take a stab at it, and alert Dr Kipping to its existance. He could actually wind up being a referree on this because of HIS paper. The ONE THING I did get out of this which MAY make this OT, is the following: “To produce an artificial black hole, we would need to focus a billion ton gamma ray laser to nuclear dimensions”. The question I want you to ask Dr Kipping, is; could this be done with halo drive technology instead? ALSO: for all of you Centauri Dreams readers, an excellent synopsis of this is presently up on https://www.phys.org.

  • Robert March 7, 2019, 14:53

    It’s not clear from the paper and video whether or not Kipping considers in addition to the returning light being blue shifted, if the ship can mirror the same light back to the black hole to reuse that same beam a number of times thus magnifying the momentum transfer. This is what Young Bae’s Photonic Laser Thruster does on a somewhat shorter scale…

    • Alex Tolley March 7, 2019, 16:23

      By recovering the energy of the returning beam and re-emitting it, this is reusing the beam. This is what Kipping is suggesting.

      One cannot just reflect the beam as it is diverging. It would also need re-aiming. Re-emission would be fine if the conversion efficiency was close to 100%, but as I noted on a previous post, we could not get anywhere close to the needed efficiencies with our technologies.

      [While a photonic drive can work in the lab with short distances and locked mirrors, this is going to be very much more difficult in space with light mirrors/sails. Breakthrough Starshot is going the route of very high energies and accelerations, and just keeping the sail stable in the beam is a work in progress. The lower the acceleration, (e.g. 1g for crewed ships) requires the beam to run for very much longer. Beam divergence and reflection accuracy become extremely hard over the huge distances involved. We are pretty much forced to use basic beaming technology rather than exotic beam reuse. The difficulties around a rapidly orbiting BH magnifies the already “challenging” aiming and collection problem.]

      • Robert March 8, 2019, 14:02

        Thanks. Yes, it’s all depressing. I fear by mid-century we’ll have quite a catalogue of fantastic starships we can’t build.

  • T. Pliska March 7, 2019, 17:20

    Any thoughts about what kind of detectable signal the beam might produce? I would think there would have to be some scatter unless the surface of the BH was perfectly predictable. Maybe some odd flashes of coherent light?

  • Michael Fidler March 7, 2019, 23:40

    Louis Crane
    February 27, 2019
    ABSTRACT We investigate the possibility of searching for black hole star-ships using very high energy gamma ray telescopes. We find that they would be near the lower threshhold for observability, although there are many unknown factors.


    Gamma Ray Telescopes could Detect Starships Powered by Black Hole.

    In the course of looking for possible signs of Extra-Terrestrial Intelligence (ETI), scientists have had to do some really outside-of-the-box thinking. Since it is a foregone conclusion that many ETIs would be older and more technologically advanced than humanity, those engaged in the Search for Extra-Terrestrial Intelligence (SETI) have to consider what a more advanced species would be doing.

    A particularly radical idea that has been suggested is that spacefaring civilizations could harness radiation emitted from black holes (Hawking radiation) to generate power. Building on this, Louis Crane – a mathematician from Kansas State University (KSU) – recently authored a study that suggests how surveys using gamma telescopes could find evidence of spacecraft powered by tiny artificial black holes.

    The study, “Searching for Extraterrestrial Civilizations Using gamma Ray Telescopes“.


    • ljk March 8, 2019, 10:16

      When Cocconi and Morrison were first considering how ETI might communicate between star systems in 1959, they first looked at gamma rays as a method before settling on radio waves.

      In this new era of the alien “technosignature”, perhaps they should have gone with their first instinct.

      Their landmark 1959 Nature paper that started modern SETI:


      • Michael Fidler March 8, 2019, 13:22

        They had evidence of gamma rays emissions as far back as 1949 in relation to technosignatures.

    • Adam March 10, 2019, 4:17

      Hi Mike
      I discussed the Black Hole Starship flight performance with Louis Crane & his student of the time, Shawn Westmoreland, in this preprint…
      Are Black Hole Starships Possible
      If the Black Hole is acting as a Photon Rocket, then it will mass about a million tons to get reasonable acceleration. If the Hawking Radiation is being thermalised by adding matter to the stream, then a broader range of black hole starship masses become possible. Eventually the black hole needs to be ejected to a safe distance so it can detonate in its final decay stages.
      An older discussion can be found in Robert Freitas’ “Xenology” here:
      17.3.5 Total Conversion Drives

      • Alex Tolley March 10, 2019, 13:36

        The “Are BH Starships Possible” is a very accessible paper. The calculations are simple, well explored, and the results put into a good context for starship propulsion. If these subatomic sized BHs can be made as advertised, then they seem far more tractable than anti-matter rockets. It struck me that if feeding them at the rate needed to maintain them for the journey was not possible, they would still make excellent power sources for deep space ships in our system, similar to the one Clarke proposed for the Sirius in “Imperial Earth” (except that Clarke did not envisage the emission of Hawking radiation, so that the BH could be seen as a object through a microscope, and its mass was so small that it would have almost instantly evaporated despite the fuel feed).

      • Charley March 10, 2019, 16:34

        “Are Black Hole Starships Possible”is a fascinating paper, and one of the best I’ve read up to this point. I disagree that the black hole needs to be ejected to an extremely great distance, rather it should be utilized in its final detonation as a energy source on a pusher plate to extract the last bit of energy as it goes off.
        Can anyone explain to me how the black hole is utilized in its normal radiation emission state as a source of momentum? To convey momentum being that you must have some kind of physical tie to your payload, I would think, how would you have a payload attached to the black hole, so as to permit momentum extraction? Anybody out there who can explain that to me?

        • Michael C. Fidler March 11, 2019, 11:30

          Possibility of hypothetical stable micro black hole production at future 100 TeV collider.

          Anton V. Sokolov, Maxim S. Pshirkov
          (Submitted on 15 Nov 2016 (v1), last revised 10 Dec 2017 (this version, v2))
          “We study the phenomenology of TeV-scale black holes predicted in theories with large extra dimensions, under the further assumption that they are absolutely stable. Our goal is to present an exhaustive analysis of safety of the proposed 100 TeV collider, as it was done in the case of the LHC. We consider the theories with different number of extra dimensions and identify those for which a possible accretion to macroscopic size would have timescales shorter than the lifetime of the Solar system. We calculate the cross sections of the black hole production at the proposed 100 TeV collider, the fraction of the black holes trapped inside the Earth and the resulting rate of capture inside the Earth via an improved method. We study the astrophysical consequences of stable micro black holes existence, in particular its influence on the stability of white dwarfs and neutron stars. We obtain constraints for the previously unexplored range of higher-dimensional Planck mass values. Several astrophysical scenarios of the micro black hole production, which were not considered before, are taken into account. Finally, using the astrophysical constraints we consider the implications for future 100 TeV terrestrial experiments. We exclude the possibility of the charged stable micro black holes production.”


          If micro black holes can be created this easily then Active Galactic Nuclei (AGN) Jets must be producing everything from micro up to supermassive BH’s. The amount of energy in these jets can have direct interaction with spacetime and could be creating new matter and new TIME in the universe. Mini black holes could be alot more common then general theories predict since AGN’s are spewing them out in massive numbers. Our home the Milky Way has been an AGN in the past and small black holes may still be orbiting around it. Reading about the interaction they would have with matter indicates that some may even be in the cores of jumbled asteroids such as Bennu and Ryugu.

  • LJC March 8, 2019, 1:34

    There’s also the possibility that some systems are used as redirects and acceleration points between two other systems, meaning energy is leached in both directions. Nothing would be added, only removed. The counterpoint is that a civilization this advanced could throw large but useless objects into the intermediate system to add energy back.

  • Michael March 8, 2019, 15:17

    Just thinking slightly off topic we could move BH’s about into orbits or prevent them from colliding. If we sent a package one way around the object and another around the other way we could get them to collide at the back at highest peri velocity to produce an almighty explosion. If the collision is controlled a jet would go into the BH and the other away from it and so move it, the other jet could be used to move the other BH away as well. Just thinking about electric charges and BH’s, they may be able to be opened if we charge them up. We could do this with two orbiting black holes by feeding electrons into one and the protons into the other, the electric force is extremely powerful.

  • Brett Bellmore March 9, 2019, 8:47

    So, if you’re close enough to a tight black hole pair to feasibly use it for a boost, what are the consequences for your home system if it merges?

    Is it possible that the best use of such a pair under those circumstances is to aid in the evacuation effort?

  • Bruce D. Mayfield March 9, 2019, 19:05

    The merger of BH’s vibrates space just before it happens and then it settles down extremely rapidly. What would that do to nearby objects?
    That’s a good question, but the answer could be not much.

    The merger of two Neutron Stars otoh, in addition to ringing space’s bell will produce a kilonova. Yeah, they’re a thousand times less powerful than a full supernova, but still they spray local interstellar space with radiation and highly radioactive debris. Evacuation of nearby systems might be called for.

    • Michael Fidler March 9, 2019, 21:49

      I have a question for you on the spacetime and the superfluid vacuum. The merger of BH’s have a profound effect on spacetime so could can they tell from the mergers effects if the superfluid vacuum exist or what exactly is spacetime made up of?

      • Michael Fidler March 9, 2019, 21:52

        My smartphone keeps trying to correct but it’s pretty dumb. ;-}

    • Michael March 10, 2019, 6:02

      It should produce a gravitational potential over the planet dependent on its wavelength which would compress it and then expanding it. The wave of energy which can be huge for large massed BH mergers would destroy near by objects.

    • Brett Bellmore March 10, 2019, 7:24

      True, I suppose in a system with two black holes, and nothing else, their merger would, by definition, emit nothing but gravitational radiation. Which doesn’t interact very much with matter.

      And if there’s an accretion disk around one or both, well, there goes your chance to do a gravitational maneuver around them. No way your hypothetical ship is going to survive crashing through one of those at a healthy fraction of light speed.

  • Michael March 10, 2019, 6:06

    It is interesting that when a BH collapses it creates a GW going outwards, it will also likely create one going inwards. Perhaps this inward GW is what prevents a singularity from forming, the GW is bouncing back and forth supporting the collapse to infinity.

    • Michael March 10, 2019, 16:37

      Just a thought experiment here but say a neutron star starts to collapse into a BH the GW should start at the center and move outwards rapidly and get bigger as more material falls in. Now as the density in the doomed star increases even the GW will struggle to get out, think of light going though a high refractive material. There will be a point where the GW could stall the inward falling material and to the outside observer freeze, supporting the total collapse. Inside though things go on at a pace but the waves can’t get out anymore so just bounce around holding it up.

  • Jim Baerg March 11, 2019, 16:12

    See the interstellar transportation system in ‘Rails Across the Galaxy’
    IRC it has laser beam ‘rails between pairs of black holes separated by many light years, no binary black holes involved.

  • Harry R Ray March 15, 2019, 10:41

    Speaking of technosignatures: “A Shiny New Method for SETI: Specular Reflections from Interplanetary Artifacts.” by Brian C. Lacki. To quote from the abstract: The resulting glints can be very bright, but they will be seen only if the mirror happens to reflect sunlight to Earth.” I STRONGLY believe this is what happened with A10bMLz. The FIRST observation showed the object to be MUCH BRIGHTER than all SUBSEQUENT observations, leading astronomers to believe that perigee was only 600 kilometers instead of the later confirmed perigee of 334,000 miles. I wonder if Dr. Lacki would be willing to do a guest post on his method here on this website sometime in the near future?

  • Harry R Ray April 9, 2019, 10:02

    Looking forward to black hole propulsion IN ACTION tomorrow as images of Dr Kipping’s Halo Drive Megastructures near the event horizon are finally made public. lol.