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Investigating the ‘Halo Drive’

One of the interesting things about gravitational assists is their ability to accelerate massive objects up to high speeds, provided of course that the astrophysical object being used for the assist is moving at high speeds itself. Freeman Dyson realized, as we saw yesterday, that a pair of tightly rotating white dwarfs could offer such an opportunity, while a binary neutron star carried even more clout. When Dyson was writing his “Gravitational Machines” paper, neutron stars were still a theoretical concept, so he primarily focused the paper on white dwarfs.

Get two neutron stars in a tight enough orbit and the speeds they achieve would make it possible to accelerate a spacecraft making a gravity assist up to a substantial percentage of lightspeed. But what an adventure that close pass would be — the tidal forces would be extreme. I don’t recall seeing a neutron star propulsive flyby portrayed in science fiction (help me out here), though Gregory Benford offers a variant on the white dwarf idea in his early work Deeper than the Darkness (1970, later re-done as The Stars in Shroud, which ran in Galaxy in 1978), where a neutron star and an F-class star comprise a system he calls a ‘Flinger,’ with potential uses both for acceleration and deceleration.

David Kipping’s thoughts on extending Dyson to black holes were partially triggered by a fascinating paper from William Stuckey in 1993 entitled “The Schwarzschild Black Hole as a Gravitational Mirror” (citation below). The gravitational mirror in the title is what happens when photons skim near the event horizon and return to the source, a photon ‘boomerang’ that gains propulsive impact because the returning light rays receive a blueshift thanks to the black hole’s relative motion.

Thus the Kipping concept: Use laser beaming technologies to beam toward a precisely targeted member of a black hole binary. The light is returned in blue-shifted form to the spacecraft, the extra energy being used to push the craft. What’s especially intriguing here is that in this scenario, it is not the spacecraft that is making the messy plunge between two rapidly moving, tightly orbiting black holes, but rather the photon beam that is used for propulsion.

It is the light beam that gets the gravitational assist as photons are re-emitted and re-absorbed, for as the paper puts it, “…the halo drive transfers kinetic energy from the moving black hole to the spacecraft by way of a gravitational assist.” A less harrowing experience for any crew, to be sure, and one in which extreme time dilation can be avoided, not to mention the dangers of tidal disruption and radiation referred to above.

Image: David Kipping (Columbia University), creator of the ‘halo drive’ concept.

Kipping described the idea to me in an email recently:

The idea is to essentially to perform a Dyson slingshot remotely, by firing a collimated particle/energy beam just to the side of the event horizon of a Schwarzchild black hole. If you choose the angle carefully, the beam loops around (like a halo) and comes back to you. If the black hole is moving towards you (I envisaged a compact binary like Dyson), then the beam returns blue shifted. When you initially fire the beam, your ship receives a small momentum kick and when the beam returns and strikes your ship you get another. This is how the ship is propelled, much like a light sail. But the beam actually returns with more energy than it departed, since it siphoned some of the kinetic energy from the black hole. So not only did you accelerate, but your ship actually gained stored energy.

Do this right and the speed of your spacecraft eventually matches that of the black hole, but the cumulative blue shifts allow the craft to continue firing the laser past that point. No new ‘free’ energy is gained but energy from the stored cells aboard the craft can keep the acceleration going up to, the author calculates, 4/3 the speed of the black hole. It’s interesting to note that we are not limited to small masses in such a calculation. Unlike the Breakthrough Starshot energy issue we discussed yesterday, we can drive arbitrarily large masses up to potentially relativistic speeds. All of this by exploiting the fantastic energies available through astrophysical objects.

Image: This is Figure 1 from the paper. Caption: Figure 1. Outline of the halo drive. A spaceship traveling at a velocity βi emits a photon of frequency νi at a specific angle δ such that the photon completes a halo around the black hole, returning shifted to νf due to the forward motion of the black hole, βBH. Credit: David Kipping.

As you would imagine, for the gravitational mirror to function in this way, the beam must be precisely oriented. From the paper:

In order for the deflection to be strong enough to constitute a boomerang, this requires the light’s closest approach to the black hole to be within a couple of Schwarzschild radii, RS ≡ 2GM/c2. Light which makes a closest approach smaller than 3GM/c2 becomes trapped in orbit, known as the photon sphere, and thus typical boomerang geodesics skim just above this critical distance [italics mine].

Kipping calls this a ‘halo drive’ because the photons returning to the craft appear as a halo around the black hole. Single black holes as well as binaries can be used (and evidently Kerr black holes as well as Schwarschild black holes, though the author plans future work on this), but the paper notes that the potential for tight configurations at relativistic speeds makes binaries preferable. 10 million binary black holes are thought to exist in the galaxy [I’ve seen this figure reduced to 1 million recently — clearly, the issue is still open], raising the possibility of a network of starship acceleration points or, for that matter, deceleration stations.

A range of possible uses for binary black holes emerges, as the paper notes:

Although not the focus of this work, it is worth highlighting that halo drives could have other purposes besides just accelerating spacecraft. For example, the back reaction on the black hole taps energy from it, essentially mining the gravitational binding energy of the binary. Similarly, forward reactions could be used to not only decelerate incoming spacecraft but effectively store energy in the binary like a fly-wheel, turning the binary into a cosmic battery.

Another possibility is that the halos could be used to deliberately manipulate black holes into specific configurations, analogous to optical tweezers. This could be particularly effective if halo bridges are established between nearby pairs of binaries, causing one binary to excite the other. Such cases could lead to rapid transformation of binary orbits, including the deliberate liberation of a binary.

A natural question is why an interstellar civilization, one already capable of reaching a black hole binary and manipulating it in this manner would need to establish a galactic network of transit points. A possible answer is that the amount of energy liberated from such a black hole binary is, as with other kinds of gravitational assist, arriving essentially ‘free’ at the spacecraft. Thus a transportation network on the cheap could be established between specific locations, a mechanism for cost-savings that may well be too efficient to ignore.

Freeman Dyson’s interest in using astrophysical objects for propulsion included, of course, his abiding fascination with the technosignatures of advanced civilizations. Would a galactic infrastructure at work in the galaxy exhibit evidence of its use to distant astronomers? In my next post, I’ll look into the possibility.

The Dyson paper is “Gravitational Machines,” in A.G.W. Cameron, ed., Interstellar Communication, New York: Benjamin Press, 1963, Chapter 12. The Kipping paper is “The Halo Drive: Fuel-free Relativistic Propulsion of Large Masses via Recycled Boomerang Photons,” accepted at the Journal of the British Interplanetary Society (preprint). The Stuckey paper is “The Schwarzschild Black Hole as a Gravitational Mirror,” American Journal of Physics Vol. 61, Issue 5 (1993), pp. 448-456.


Comments on this entry are closed.

  • Gary Wilson March 5, 2019, 14:43

    Just for curiosity’s sake where are the nearest known black hole and neutron star binaries?

    • Jens March 6, 2019, 14:29

      Strictly speaking; I don’t get much sense out of this list: https://en.wikipedia.org/wiki/List_of_nearest_black_holes It says the problem is to detect black holes, so we usually find binaries. Understandable. Still, 3000 light years to the nearest known black hole, and it is not a binary, is not inviting. Can’t we use some usual stars instead? They might even be found in systems that might be somewhat hospitable to us :)

      • Steve Bowers March 6, 2019, 14:52

        V616 Mons is a binary, Jens.

        • Jens March 7, 2019, 12:33

          Yes, I see it. It has a star companion. Maybe it would be useful anyway. The only challenge left is to get there then :)

          • J. Jason Wentworth March 8, 2019, 0:51

            From an engineering perspective, reaching the stars is quite easy, and it has been possible since almost the dawn of the Space Age. (Amazingly, it was actually *physically* possible, although with an inert payload, as early as December 1946 [see: http://www.drewexmachina.com/2017/10/16/fritz-zwickys-solar-orbiting-pellets/ ]. Professor Fritz Zwicky’s sounding rocket-lofted, gun-launched artificial meteor experiments, whose second try succeeded in October 1957, just days after Sputnik’s launch, shot at least one to 33,000 mph *upward*, far in excess of the Earth’s escape velocity and 1,000 mph faster than needed to reach Jupiter. If that artificial meteor–or another, deliberately-aimed-and-timed one–passed close enough [a few Jupiter radii are ample for the gravity assist boost] above Jupiter’s receding–in its orbit–hemisphere, it could have reached another star.) Using the Zwicky method, and with Jupiter’s help, we could send artifacts–and even tiny, resin-“potted” acceleration-hardened interplanetary/planetary/interstellar probes (akin to Gerald Bull’s gun-launched Martlet suborbital space probes: https://en.wikipedia.org/wiki/Project_HARP ) to the stars. Also:

            It’s almost twice as hard to reach our own Sun than it is to reach the next nearest star (66,000 mph vs. 37,000 mph to reach Proxima Centauri). Even getting to Mercury requires more energy than escaping from the Solar System. As well:

            Escaping from the Solar System (which Jupiter is very helpful, but not essential, for doing) isn’t the problem. The problem is purely biological; crossing the interstellar abyss–with automated probes or crewed ships–in periods of time that are acceptable to all-too-mortal, short-lived human beings. If there are immortal or very long-lived intelligent extraterrestrials, to them the stars are–psychologically and socially–not far away at all, and velocities that we would consider far too slow for such voyages (one percent or even a tenth of one percent of the speed of light) would be “economy cruising speed” to them. Plus:

            If there are macroscopic, substellar-size black holes (perhaps “sublimed down” from ‘smaller-end,’ stellar-sized ones formed long ago; substellar ones evaporate via Hawking radiation emission fairly rapidly [in astronomical time scales]) scattered fairly commonly through the galaxy, we might reach them sooner and make some use of them eventually. But it’s probably just as well if they aren’t around, because of the death and destruction that even such smaller black holes would cause if our Solar System crossed paths with one. A collision with our world would be devastating, at least locally, and such an object–even if it didn’t collide with a planet or moon–might disrupt asteroid and comet orbits, setting up another episode of planetary bombardment.

            • Charley March 8, 2019, 15:12

              @JJ – you went over an awful lot of topics in your above paragraph. I simply wanted to add something on the black holes; it was brought up other places that artificial black holes the size of asteroids would be non-usable due to tidal forces.
              In the absence of calculations which I’m not willing to do principally because I’m kind of lazy, instead, I’d be willing to venture a guess that title forces might not be a dealbreaker.
              Whether or not a situation is a dealbreaker with regards to tidal forces depends upon what you’re trying to accelerate and how tough you can make a structure (as well as many, many other considerations). I can’t help but wonder whether or not that once we had started obtaining some sort of mastery over nature, we should not create ‘artificial black holes’ in see if we can exploit them to our desires. What do you think?

              • Charley March 8, 2019, 15:15

                Just had an additional thought right now; what if we used “the halo effect” in conjunction with artificial black holes that we make and control? Then we get the best of all worlds with out tidal force considerations. Around and around and around the light beam goes continuously pumping energy into a space probe.

    • Steve Bowers March 6, 2019, 14:51

      The closest black hole binary is V616 Mon, about 3000 ly away. The closest neutron star (RX J1856.5-3754) is not a binary, but is approx 400 ly away.

      There are probably solitary black holes at similar distances, but they are tricky to find, because they are black. This propulsion technique seems only likely to be useful long after we’ve figured out other ways of getting to the stars.

      • Theo April 17, 2019, 13:51

        ..is that a ‘Red Dwarf’ reference ;-)

  • Michael March 5, 2019, 15:09

    Would be better to do this with pellets I.e. mass, much more energy in a volume.

    • Alex Tolley March 5, 2019, 15:39

      I considered this too. I think the value is that “smart pellets” could also guide themselves back to the star ship, thus reducing the requirement for extremely accurate aiming. The pellets can also travel more slowly, thus avoiding the extreme gravity gradients when skimming just above the Schwarzchild radius which would tear them apart. The pellets would thus provide a constantly recycling propellant load, eliminating the limitations of the rocket equation.

      A further advantage is that the lower speed of the pellets might allow any body to be used as a gravitational boomerang and for a lot longer than a conventional gravitational assist.

      • J. Jason Wentworth March 6, 2019, 13:07

        “Souped-up light”–what an intriguing (and free [except for the initial laser beam], as in regular solar sailing) idea! If a light-emitting star, perhaps even a red dwarf, was in the vicinity (not necessarily very close by), a parabolic mirror in orbit around it might provide a reasonably tight beam of light–without a laser–that the black hole could bend and blue-shift to provide high-acceleration propulsion to a lightsail. Or, via various energy conversion methods, it could provide high-intensity light to a “solar thermal rocket” or to a photovoltaic cells/electrical propulsion spacecraft (these latter two types might be preferable for fast “local” travel, to more like interplanetary than interstellar distances). Also:

        The late Jordin Kare’s pellet propulsion could be greatly enhanced by such black hole assistance, too. Due to the tremendous velocity with which the pellets would reach the ship (they needn’t necessarily be fired from the ship, but could come from a pellet launcher located elsewhere in the vicinity), they could be much smaller and lighter, as their velocity would easily make up for their lower mass. Also, being much smaller (more compact) and lighter, they would be less affected by tidal effects, making it possible–within reason, of course–for them to pass closer to the black hole’s event horizon, which would enable even higher pellet velocities (and thus more energy to be transferred to the ship, via impact [including impact nuclear fusion] and/or interaction with a powerful magnetic or electric field on the ship).

      • J. Jason Wentworth March 8, 2019, 1:08

        Yes. The ship itself could utilize a black hole flyby gravity assist to greatly increase–or decrease, for arrival–its velocity (especially if the black hole was part of a binary star–or even binary black hole–system). Even a solitary black hole would be useful for large delta-v departure (or arrival) if the ship conducted an Oberth effect prograde (or retrograde) burn at periastron (while taking care, of course, to avoid approaching to within the “spaghettification radius” :-) of the black hole(s) [and having good radiation shielding, especially for crewed vessels, would probably also be advisable]).

    • Harry R Ray March 5, 2019, 16:15

      Correct me if I am wrong, but I think that the pellets would be spigettified due to tidal stress.

      • Alex Tolley March 5, 2019, 17:23

        If the pellet velocity was c and they took the same trajectory as the light beam, yes they would. But the pellet velocity is arbitrarily below c and can take a trajectory well beyond the event horizon.

        Kipping notes that a light beam has a finite width, so that beam divergence also occurs due to the gravity gradient.

      • Michael March 6, 2019, 14:42

        It depends on the size of BH, large massed ones have quite a gentle gradient preventing spaghettification.

      • J. Jason Wentworth March 7, 2019, 23:39

        That’s why I included the qualifier “within reason, of course.” At some distance, tidal forces would stretch out *any* material object, no matter how dense and hard it was. But the more dimensionally compact (strong; a larger object is easier to pull apart via tension than a smaller object made of the same material) and dense a Kareian propulsion pellet was, the closer it could pass to a black hole without becoming like Arthur C. Clarke’s “star-mangled spanner.”

    • Michael Fidler March 5, 2019, 16:47

      No, particle beams would work much better or some of the weapons used on the Enterprise, like solitons.

  • Gareth Wilson March 5, 2019, 15:13

    The closest fictional example I can think of is Niven’s Protector, where an ramscoop starship uses a neutron star to make a right-angle turn.

  • George King March 5, 2019, 16:05

    Would gravitational waves from a binary black hole have any practical impact on Kipping’s YT-1300F Corellian light freighter at the working distance in his diagram?

    • George King March 5, 2019, 23:06

      The question arises for me because Wikipedia suggests that: (1) “[a]s a gravitational wave passes an observer, that observer will find spacetime distorted by the effects of strain,” (2) “[d]istances between objects increase and decrease rhythmically as the wave passes, at a frequency equal to that of the wave;” (3) “[t]he magnitude of this effect decreases in proportion to the inverse distance from the source;” (4) “[i]nspiraling binary neutron stars [and I’m sure also binary black holes] are predicted to be a powerful source of gravitational waves as they coalesce, due to the very large acceleration of their masses as they orbit close to one another;” but (5) “due to the astronomical distances to these sources, the effects when measured on Earth are predicted to be very small.”


      In contrast, Kipping’s craft (which looks suspiciously like the Millennium Falcon) no doubt will have to get much closer than astronomical distances for the halo drive to work, potentially exposing the craft to larger gravitational wave effects than we measure here on Earth from far across the galaxy or universe. So I wonder whether gravitational waves could have a practical impact on the craft, including on all the equipment used to precisely send – and then receive – a laser beam shot ‘round a black hole. Distances between objects – on and in the craft – increasing and decreasing rhythmically as the wavelike distortion in spacetime passes through the ship doesn’t sound necessarily conducive to accurate pointing and receiving, at least in theory.

      If I understand correctly, any such potential effect from gravitational waves would be a consideration separate and apart from the direct gravitational impact of tidal forces closer in.

      • Alex Tolley March 6, 2019, 11:39

        Very good point. I don’t think that is addressed in the paper at all. At a minimum, it adds an extra complication to the aiming system. It might even disrupt the symmetry of the aiming, ending the “halo” effect.

  • Harry R Ray March 5, 2019, 16:35

    “The idea is to esentialy to perform a Dyson slingshot remotely by firing a collimated particle/energy beam…”. Wouldn’t any massive particles get caught up in the black hole’s magnetic field and transported to the poles to be ejected quasar-like there? Unless you want to travel in that SPECIFIC DIRECTION, I think ONLY massless photons could do the trick.

    • Ron S. March 5, 2019, 18:16

      BH have no magnetic field.

      • Bruce D. Mayfield March 6, 2019, 11:09

        But they can have charge. Something that has an electric charge would also have a magnetic field, would it not?

        • Ron S. March 6, 2019, 12:34

          No. However it is possible to experience magnetism depending on how ones moves through a static electric field, such as one that can (theoretically) be possessed by a BH. You need a gradient to induce a magnetic field. That said, a BH formed in the usual way is unlikely to have anything but a negligible electric field since it is formed from a net electrically neutral mass, and subsequent mass inflow.

          • Michael March 6, 2019, 14:17

            The BH can only have an electric field internal to the BH not outside.

            • Ron S. March 6, 2019, 15:16

              Sorry, no. Regardless, we’re only talking about outside the horizon since the inside has no external influence.

              • Bruce D. Mayfield March 6, 2019, 23:39

                I see your points Ron and agree. Thanks for explaining.

    • Alex Tolley March 5, 2019, 18:28

      Why would non-magnetic or uncharged particles be affected by any local magnetic field?

      • Brett Bellmore March 6, 2019, 13:41

        Well, neutrons, for instance, have dipole moments, and so are effected by the derivative of the local magnetic field. Not much of an issue in weak or slowly varying fields, though. But it could be significant near a magstar.

        Atoms have much larger dipole moments, and can even be torn apart by the induced voltage from moving through a strong magnetic field.

        • J. Jason Wentworth March 8, 2019, 1:12

          Even light (polarized light) is affected, to some extent, by a magnetic field.

          • Michael March 8, 2019, 16:47

            It should not be, unless the frequency of an alternating magnetic field is similar to the light wave.

      • Michael March 6, 2019, 14:31

        It depends on the velocity of the magnetic field, energy can be given by induction to ionise atoms.

    • Jens March 5, 2019, 18:52

      Use a neutral beam?

    • Adam March 6, 2019, 6:58

      The case that Kipping considers is a Schwarzschild black hole space-time geometry – that is, it’s not rotating. Thus there’s no spinning space-time to support a magnetic field close to the black hole. Any orbiting matter has to be a considerable distance from the black hole, as the last stable circular orbit is at 3 Schwarzschild radii.

      If the black hole is rotating, then magnetic fields might be closer to the event horizon and they might deflect charged particles. Neutral particles and photons, of course, will experience no deflection.

      • Ron S. March 6, 2019, 10:33

        BH have no magnetic field.

        • Bruce D. Mayfield March 6, 2019, 12:52

          “The no-hair conjecture postulates that, once it achieves a stable condition after formation, a black hole has only three independent physical properties: mass, charge, and angular momentum; the black hole is otherwise featureless. If the conjecture is true, any two black holes that share the same values for these properties, or parameters, are indistinguishable from one another. The degree to which the conjecture is true for real black holes under the laws of modern physics, is currently an unsolved problem.[43]

          These properties are special because they are visible from outside a black hole. For example, a charged black hole repels other like charges just like any other charged object.”

          So if the charge of a BH is non zero it will have a magnetic field.

          • Michael March 6, 2019, 14:18

            The charge is internal to the BH, it can’t reach out.

            • Adam March 6, 2019, 15:56

              It’s like black hole gravity. A ‘memory’ of the gravity and charge of the objects that slip beyond the event horizon.

              • Michael March 7, 2019, 13:15

                This creates a problem, if the black hole exerts an electric field and it is accelerated it will create a magnetic field. It would be interesting if we could observe this happening. The gravity of a black hole is the distortion of ST the concept of mass breaks down as it goes over the EH, we just don’t know what it is anymore.

  • Ashley Baldwin March 5, 2019, 19:05

    Thanks Paul. Great post . In answer to your question concerning a neutron star propulsive assist there is an example to be found in Sci- Fi genre. Cooper uses one at the end of the movie “Interstellar” . This to help “Endurance” escape the supermassive black hole Gargantua, before slinging it and the lovely Dr Amelia Brand on to the bucolic safety of Edmunds’ planet.

  • Gary Wilson March 5, 2019, 20:35

    I suppose the Halo drive could fall into the category of really, really advanced propulsion systems but other than an interesting thought exercise ala Einstein I don’t think it helps us move forward in any realistic way. Am I being too pragmatic here? What is the actual purpose of proposing a strategy that would require some incredibly powerful drive system to get us to a binary black hole or pulsar system or incredibly powerful tools to create miniature black holes which we can then manipulate very precisely? Maybe we should focus on getting somewhere in the real universe in a realistic amount of time. Even the analogies to moving mass on Earth which gradually grew more powerful and efficient followed a logical pathway. I see no pathway that leads to a functioning Halo drive to gad around the galaxy.

    • J. Jason Wentworth March 8, 2019, 1:56

      There is talk–and some theoretical work–directed toward one day developing a Schwarzschild Kugelblitz Drive (described here, among the Top 5 realistic stardrives: http://www.youtube.com/watch?v=EzZGPCyrpSU ; kugelblitz–German for ball lightning–being the name given to the artificial black holes [not astronomical, matter ones, but ones made in the laboratory using manipulations of light]). Its performance is the best yet determined for a realistically build-able stardrive, but it’s also considered a distant technology, perhaps not achievable in this century. BUT:

      Since what human beings consider an acceptable interstellar transit time (even for unmanned starprobes) is based entirely on the typical human lifespan, that figure will rise, at least somewhat, as the typical lifespan increases. While I don’t think lifespans of 1,000, or even 150, years will occur–at least not for the foreseeable future–even a modest incremental improvement could make a huge difference. For example:

      The backers of the Breakthrough Starshot project are very antsy to have the probes reach the Alpha Centauri system a little over 20 years after launch (moving at 20% of c), so that they could live to see returned pictures and data from the probes. But that transit time requirement is pushing the state of the art to–and well beyond–the bleeding edge of even foreseeable technologies. If people lived, on average, just 5 – 10 years longer than they do today, and had good functional levels of health to the age of 90 or so, lowering the requirements (a probe velocity of 10% of c, for arrival 43 years after launch [those are the notional figures for NASA’s 2069-launched, Sun-diver *solar* sail starprobe, incidentally]) would still enable those who sent the probe to be on hand to see its results, while considerably lowering the complexity and cost of the venture.

  • Josh E March 5, 2019, 21:53

    While this is an interesting propulsive mechanism, what interests me more is the power generation potential.

    If a civilization were to set up a long-term infrastructure for shooting lasers/pellets/whatever at a BH and get that energy back with profits, the potential range of this application might stretch out to the parsec scale, meaning no need to figure out how to get close to the BH – not to mention survive anywhere near the harsh environment of a BBH.

    Even though a matter/energy stream might take years or decades to complete a full boomerang, it’ll be easier and cheaper to cast, catch, and collect that energy with infrastructure not housed in a vehicle optimized for moving places, fast.

    • Harry R Ray March 6, 2019, 10:26

      In that case. Breakthrough Initiatives should IMMEDIATELY start to look for technosignatures around black holes!

      • Harry R Ray March 6, 2019, 12:30

        I have a great idea for a science fiction novel. Assume that some time in the distant future, astronomers on Earth detect a black hole that is headed directly at us. Earth has one hundred thousand years to build a Shkadov Thruster to maneuver our solar system far enough away to be protected from the black hole’s gravity and radiation. This maneuver proves to be successful, but then, in a flash of brilliance, someone realizes that if sunlight can somehow now be beamed to the black hole in a way that a mega halo drive would be created, beaming photons of MUCH HIGHER ENERGY than the sun produces BACK to the Shkdov Thruster which is now PEPOSITIONED to focus the high energy photons on to the Sun, causing an acceleration that allows our solar system to EXACTLY MATCH the velocity of the black hole, allowing us to tap the black hole’s halo energy indefinitely. If there is, somewhere in the GAIA database, a star that is doing EXACTLY THAT with a black hole, such a star should be INTENSELY MONITORED for any Shkadov Thruster transits!

    • Bruce D. Mayfield March 6, 2019, 13:08

      > While this is an interesting propulsive mechanism, what interests me more is the power generation potential.

      That was my thought too Josh. The notion of being able to extract energy from Black Holes is awe inspiring.

      • Josh E March 6, 2019, 20:23

        I might have felt a few spinal chills when I thought of what would happen if a hypothetical conductive particle or pellet stream arriving back at its starting spot were to complete an electrical circuit, allowing a current to flow through it. I’m not sure there’s enough numbers in my calculator to figure out the voltage that might flow, albeit in what would probably be in an incredibly destructive fashion splashed out across countless AU.

        Either examples of this constitute a sufficiently advanced technology so as to appear to be a natural event, or it highly unlikely to be viable – given the sheer magnitude of energies involved, it’s also pretty likely that the failure modes for this type of system would have some… spectacular consequences

        • Harry R Ray March 8, 2019, 10:25

          Even MORE spectacular would be DELIBERATE WEAPONIZATION! Hypothetical case: The Death Star approaches a black hole binary at nearly the speed of light, fires its planet destroying ray in a way so that it would boomerang back, immediately veers away so that the ENHANCED beam misses it and speeds on to a target planet. What is the MAXIMUM distance a Death Star can be from a planet when it fires its death ray and wind up destroying the planet using this mechanism?

  • Adam Crowl March 6, 2019, 2:04

    Hi Paul
    One advantage of black holes, especially the binary variety, is they can be very big. Neutron Stars have a more or less fixed size (else they’re black holes) and thus they inspiral due to gravitational wave energy loss very, very quickly when their speeds approach relativistic speeds. Black Holes can get really fast and really big, thus the lifetime isn’t quite so fixed.

    Dyson was well aware of that fact as his original preprint notes close neutron Stars last but seconds.


    • Paul Gilster March 6, 2019, 10:01

      Yes, and we’ll be talking about neutron star mergers in the next post; Dyson was keenly interested in how they might be detected.

    • Josh E March 6, 2019, 19:55

      Your comment and the paper’s almost off-hand mention of binary orbital manipulation made me think of the neutron star computational engine described by Alastair Reynolds in his brilliant novel Revelation Space, wherein a neutron star is maneuvered into a state of indeterminacy between collapsing into a black hole or remaining a neutron star by sending streams of matter on interesting trajectories around the collapsar.

      Power and travel potential aside, one could really plumb the depths of quantum mechanics, relativity, etc. from just observing what would be the one of the biggest particle accelerators in the universe.

  • Harold Shaw March 6, 2019, 13:20

    Would binary black holes support debris disks? If so, then a civilization with access to self-replicating manufacturing technology could turn them into shipyards. For instance, a civilization in a more conventional star system could could could get a small seed probe to the BHH, construct telescopes that observe at the BHH focal point and send probes to interesting targets. Self-replicating/assembling technology could also allow a civilization to send a much smaller craft to the BHH that would increase its size and complexity at the BHH before using the BHH to propel itself elsewhere.

  • Gary Wilson March 6, 2019, 14:43

    I think for the foreseeable future we already have an enormous power source available that we haven’t used properly or efficiently yet, our own Sun. Instead we have burned enormous amounts of coal, oil, and natural gas, pushing our climate out of its moderate and safe range and forcing us to now make drastic changes or face the consequences. Forget about black holes as energy sources for now and focus your obviously considerable talents to helping us convert our energy systems before we enter a climate regime we cannot survive. Read some science fiction that deals with these problems. Try The Water Knife by Paolo Bacigalupi or New York 2140 by Kim Stanley Robinson if you want to read about our near term future. Check out the temperatures being reached all around the world in summer. Over 50 C in India and many other places now. This may seem a little contrite but we need a massive change and it doesn’t include black holes as power sources. As a family we are converting to solar power for our home and electric cars and transit and walking for our transportation. Have fun with black holes as long as you are also committed to making the real changes necessary for our children’s and grandchildren’s survival.

    • Jens March 7, 2019, 12:36

      Yes :)

    • J. Jason Wentworth March 8, 2019, 11:55

      Being a survivor of the Ice Age of the 1970s (and other assorted predicted ends of the world as we know it), I am profoundly skeptical of today’s new-fangled predictions of doom, BUT:

      I agree that we should pursue and implement–to an extent and at a pace that make economic sense–OTH (Other-Than-Hydrocarbon) energy sources, but for three very different reasons:

      [1] It’s a shame that we burn the stuff, when it is so useful as a chemical feedstock for countless agricultural, industrial, consumer, and medical products. Solid, liquid, and (many sources of) gaseous fossil fuels (coal, petroleum, and natural gas) *are* renewable, but only slowly, over geological time scales (we extract them at much higher rates than they are produced). Methane can also be produced in renewable ways (from compost, etc.), but not in such large quantities at a time.

      [2] Getting the energy we need from OTH sources would also obviate the need to be deferential to the largely-odious nations that happen to be sitting atop large hydrocarbon reserves (ditto for rare earth elements, which our technological civilization requires, but I’m getting ahead of myself here–but not below).

      [3] Bringing Gerard K. O’Neill’s vision to fruition (either with or without the space colonies, in the beginning; the SPSs–Solar Power Satellites–are the important part), which can be done in smaller steps, so as not to break banks and/or overwhelm the logistical managers, would bring us both abundant energy from the Sun and abundant–although rare on Earth in many cases–strategic industrial metals (such as rare earth elements), “ordinary” industrial metals, minerals, and volatiles from Near-Earth and main belt asteroids, for use on the Earth and in space. Doing this would not only improve the standard of living for potentially everyone in the world, but it would also make Solar System exploration and colonization–and interstellar probe and starship ventures–financially, technologically, and logistically achievable.

  • Adam March 6, 2019, 16:32

    Hi Paul
    The speed of the orbiting black holes is clearly of relevance in this discussion. Which does significantly constrain the required size of the black holes in the binary. In the case of an equal mass binary, the speed depends on the distance between the black holes in proportion to their Schwarzschild radii – when the distance is at 3 times the Schwarzschild radius, their speed is about 0.4 c around their common centre of mass. Though when they’re that close General Relativity makes my semi-Newtonian calculation an approximation.
    At that distance the binary doesn’t have very long to live, thanks to loss of orbital energy to gravitational waves. The total time left (also approximate, but less when full GR is used) is 30 milliseconds for two 30 solar mass black holes. Increasing the mass means greater separation and longer lifespans. Two 300 solar mass black holes last 300 milliseconds. Beef it up to 3 billion solar masses each and the time remaining is 3 million seconds.
    That’s not a very useful lifespan for a starship accelerator. If we increase the distance to 30 times the Schwarzschild radius (Rs), then the lifespan increases to 30 billion seconds, but the orbital speed drops to 0.129 c. Increase the distance to 300 Rs and the speed is 0.04 c, while the useful lifespan is 10 million years.

  • Charley March 6, 2019, 19:17

    I went ahead and looked through a good portion of the authors paper, and was able to get up through page 6. Then I proceeded to skim ahead to the last page of 14; up to page 6, he was fairly engrossed in momentum conservation calculations and how it related to blue shifting a light ray. The only thought I had concerning this was the potential problem associated with aiming the beam from the ship around the black hole and back to the ship.

    He indicates that the only refined answers can be found by numerical computation, not surprisingly, and I’m assuming from the maintenance of momentum balance around the black hole that the step-by-step integration relies heavily on knowing the mass of said black hole. So the question arises how much inaccuracy can you have within the mass of the black hole, such as to allow accurate return of the accelerated beam? The mass of a black hole would undoubtedly be an enormous and there would have to be a considerable mass uncertainty which might greatly affect beam convergence back on the ship. I didn’t find anything in the paper that seem to address that issue.

    • Ron S. March 6, 2019, 21:20

      Calculation won’t cut it for pretty well the reasons you describe. Kipping talks about a probe laser to fine tune and find the “sweet spot” which is going to be a very, very small target. The practicality is suspect, especially since in a relativistic fast rotating binary the sweet spot is going to moving fast over a difficult to track path. Each BH can only be used over perhaps no more than 25% of its orbit for acceleration when the spacecraft is in the plane of the orbit. A higher percentage would require rapid switching between each BH. Calling this challenging is overly generous.

  • Michael March 7, 2019, 13:21

    Just thinking we could combine the laser propulsion with a kick like matter by recycling the photons by reflective back around and so on.

  • Ron S. March 7, 2019, 13:59

    When I found a few minutes my curiosity led me to produce a spreadsheet to calculate the attributes of these binary systems. It’s easy to do, if you are careful with units, etc. For example, I kept messing up the conversions between meters and kilometers while using MKS units.

    Here are a few numbers that I found interesting. In all cases we are measuring in the orbital plane, which is best for propulsion.

    For neutron stars they are always almost of equal 1.4 Msun mass. For an orbital velocity of 1% c:
    -Orbital radius: 5200 km
    -Orbital period: 5.5 seconds

    10% c:
    Orbital radius: 52 km
    Orbital period: 5.5 ms

    There is of course no event horizon for neutron stars and therefore no “boomerang” photon orbits. But it’s interesting nonetheless. For a pair of orbiting BH of equal 10 Msun the result is more interesting. First for an orbital velocity of 1% c:
    -Orbital radius: 37,000 km
    -Orbital period: 38.8 seconds

    10% c:
    -Orbital radius: 370 km
    -Orbital period: 3.9 ms

    As Adam pointed out elsewhere these higher velocities occur in rapid in-spiralling binaries that won’t last very long. So you would be limited to binaries with orbital velocities of less than 3% (approximately), which isn’t much use.

    The Schwarzchild radius of a 10 Msun BH is 88.5 km, and I did not calculate the ergosphere radius for a realistic BH. At least this seems to indicate that the horizon radius of the front BH doesn’t “shadow” the one you want to use for propulsion. I had thought that might be a problem.

    • Alex Tolley March 7, 2019, 16:01

      @Ron. Good calculations. As you point out, to get the high fractional c velocities you want, the BH binaries are orbiting at very fast orbital periods – milliseconds!) and the system will not last long anyway, so there cannot be a galactic “highway” without constant maintenance. (The neutron stars could still be used for conventional gravity assists, but even then the timing has to be exquisitely precise and accurate.)

      There certainly wouldn’t be some existing system like a “Stargate” abandoned by a long dead civilization that we could find and use.

      A highway based on coherent em radiation driven sails powered by the local star seems far more achievable and flexible. Branch roads to minor civilization stars can be easily set up. [ As we could build a system at Sol, maybe signaling to ETI that we would like a decelerating beam set up at their end would be possible. Even if it proves unlikely that we would even send a crewed ship to the stars, such light sails could deliver small payloads for scientific research, or even information and AIs to communicate directly with ET civs.]

      I would be interested in Dr. Kipping’s response to Ron’s calculations, as it seems to pretty much break the usefulness of the concept of the halo drive as described.

      • Ron S. March 7, 2019, 17:32

        Just in case it isn’t obvious from the numeric pattern (hat tip to Kepler), I used a purely Newtonian model. A post-Newtonian approximation of GR is necessary for accuracy at the smaller radii shown, but I was just aiming for a rough estimate to see what we’re dealing with.

        That millisecond orbit for the BH binary is indeed in the process of (to use LIGO lingo) ringing down. You really don’t want to be nearby when that happens. ;-)

      • Paul Gilster March 8, 2019, 9:34

        Dr. Kipping passed along this response, Alex:


        “It’s certainly true that relativistic binaries are short lived but it’s not a requirement that a hypothetical network is static. There will always be other binaries which inspiral over time into a compact state, thus ensuring that is a population of compact binaries at any given time. The size of that population is something we are starting to get clues at from LIGO and should be measured quite well in the near future.

        “The other factor to keep in mind is that the this system should be applicable to isolated spinning black holes too as I discuss in the paper, by exploiting the frame dragging around the black hole. I focused on the binary case mostly for mathematical convenience, but there is growing evidence now of relativistic spinning super massive black holes (https://www.nature.com/news/spin-rate-of-black-holes-pinned-down-1.13512) – again LIGO should have something to say about smaller black holes in the future.”

        • Alex Tolley March 8, 2019, 13:13

          Using Al Jackson’s referenced value for binary BH density of 1 million in 8 trillion cu ly, that is about 30 ly between each BH binary. But if the useful binaries are rarer, this distance must increase, therefore requiring even further travel to reach each way point.

          However, the situation is worse than just distance. To use any pair of binaries they must last for at least as long as 2x their separation distance in time for acceleration and deceleration. (detection + travel time) For 30 ly, that is at least 2×10^9 s. That is assuming travel starts at an existing binary, rather than searching for it. To plan longer distance travel the binaries must last much longer. IMO, this makes any sort of “highway” so transient that it meaningless. Even sailing ships could rely on trade winds and ocean currents. Even knowing the location of a longer lived BH binary, the orbital decay must be predicted so that on arrival, the orbit is close enough and fast enough for the slingshot to be useful for either acceleration or deceleration. This might require being ably to manipulate the orbital distances of known BH binaries to ensure there slingshot suitability, but this is rather like building a disposable highway. This seems like the technology of KII (at least) civilizations.

          [I have to think that miniaturization of agents and vehicles is the more likely path to interstellar voyages, rather than Star Wars sized ships carrying biological multi-cellular, human-sized persons. We are already heading in that direction and it seems to me far more likely we will have the necessary technologies in place to do that well within a millennium.]

          Rotating BHs might well be a better option, especially if they are more numerous and less difficult to manage. I look forward to reading a paper on this approach.

  • Alex Tolley March 8, 2019, 13:16

    OT question. If dark matter affects gravity, does that mean that it is also influenced by gravity and therefore could collapse into black holes, or does it remain as a low density medium of some sort?

    • Ron S. March 8, 2019, 15:19

      DM gravitates just like baryons. Since it formed alongside baryons they gravitate into galaxies. But that’s where their histories diverge.

      The difference between the posited non-baryonic DM and baryons is that of interaction. Baryons forms nebulae then stars and so on due to EM driven interactions. That is, they rub against each other, exchanging momentum, and exchange photons (heat, etc.).

      DM particles appear to have no or very small interactions with both baryons and themselves. They don’t rub against each other or exchange photons (no EM interaction). So they can never coalesce into nebulae and stars. There can also be no DM chemistry. Particles sail past each other the way neutrinos stream through your body without either noticing. They are bound to galaxies but mostly remain diffuse.

      DM can fall into a BH, adding mass to it, but only if a particle path intersects the event horizon. They cannot form or participate in an accretion disk. You could take a small BH and (if you have several eons to spare) go around collecting DM particles to, eventually, form a BH that is mostly DM. You’d only be able to tell you’re making progress by the BH gaining mass.

      For this reason it is difficult to detect DM particles to discover what they are. At least with the low interaction cross section of neutrinos a reliable detector can be built since we know what they are. Not so with DM particles. You make a guess, hopefully one that is theoretically plausible, build an experiment and cross your fingers.

    • Michael March 8, 2019, 15:44

      Dark matter feels the force of gravity but it appears not to like getting together to make objects. It’s quite rarefied but will be taken up by BH’s but to a lesser extent than ordinary matter because of the rarefied nature. My feelings is that dark matter/energy may not exist and it could be due to excess of electric charge, black holes may have swallowed some of it.

  • George King March 8, 2019, 19:19

    I imagine that — as is the case here — early on in the moon shot effort there were substantial scientific and engineering issues that remained to be worked out — some of which at the time potentially might even have looked like possible deal-breakers based on our nascent level of knowledge then.

    I do hope and trust that we always encourage creative scientific thought and expression like this, even if such thought doesn’t directly pertain to what very well may be pressing, even highly pressing, issues of the day closer to home.

    We are a species that has a drive not merely to survive but also to explore, including exploration at the constantly expanding bounds of human thought.

    I trust that we will never seek to quell our drive to explore even as we seek to secure our survival. Over and above being inherent in our very nature, the former may well be the path to the latter one day.

  • Dmitry Novoseltsev March 11, 2019, 4:02

    This thesis is not quite clear.
    “But the beam actually returns with more energy than it departed, since it siphoned some of the kinetic energy from the black hole. So not only did you accelerate, but your ship actually gained stored energy.”
    If the first part of the path (from the source to the black hole) the beam falls into the gravitational well and acquires additional energy, then on the next, ascending part of the trajectory it rises along the same gravitational well and loses the energy accumulated earlier. Moreover, if the environment around a black hole is not empty and is filled, for example, drawn to her by the gas (or, in the limiting case, with a cloud of virtual particles at the event horizon), the further beam loses energy in case of partial dispersion in the environment and returns to the source is less powerful and more “red”.
    It is not clear what is the advantage of such a technical solution over the actual reactive photon thrust from the emitted laser beam.

    • Charley March 11, 2019, 16:31

      The simple answer to your dilemma is the fact that this is a ‘gravity assist’ which uses a beam of light to perform momentum transfer. The fact that the light beam and the black hole are in relative motion with respect to each other permits energy and momentum transfer.

  • Alex T. March 12, 2019, 3:23

    Wandering , are astronomers ever detected any “blue” shifted light source (stars) images, that can be explained by effects of this “halo drive”?
    Same shift should be seen for many objects that are passing by in proximity of prognozed BH … If number from article are correct there shoulb be multiple observation of this effect (photons that are stealing energy from BH).
    I am not sure that such effect can be proved by astronomers.

    • Arda Karaduman April 19, 2019, 9:16

      Since this is a directed beam, I don’t think it is possible to observe this in action from anywhere except the point of origin (ie the ship). Unless the extraterrestrials miss their aim :)

  • Alex T. March 12, 2019, 3:49

    Wandering again, Does autor takes in account the “gravitational redshift” – that is a relativistic effect observed in electromagnetic radiation moving out of gravitational fields.?

  • John Hunter April 8, 2019, 14:50

    IMHO, comments regarding Halo technology for global power extraction are spot-on and could possibly yield useful results for our planet sooner than fusion. Search for the closest BH should be elevated to top priority ASAP.

  • Arda Karaduman April 19, 2019, 9:14

    One thing about this though, if you fire your beam to bend around the black hole, wont it take like million years to come back because of time dilation ? I’m not sure how close it should be fired to the event horizon, but the closer it needs to get, the longer time it will be required.

  • Arda Karaduman April 20, 2019, 4:42

    The more I think about this, the more improbable it seems. I think there is a maximum bending angle you can get from around black holes, which should be 90-180 degrees (depending on black hole size maybe, and the original shooting angle). If light is bent more than 180 degrees, it will forever be trapped around the event horizon and form a photon sphere. I don’t see how it could leave the orbit and come back to source of origin.

    This assuming a single black hole, for binaries, it could be pulled back by the gravity of the pairing black hole. But it would be a really complex operation.