Black Hole Starships

posted by Jeff Lee on December 7, 2012

Black hole sun,

Won’t you come?

And wash away the rain,

Black hole sun,

Won’t you come?

Won’t you come?

(Black Hole Sun, by Sound Garden (from the CD Superunknown) )

 

When theoretical physicist John Wheeler coined the term “black hole” during his 1967 talk at the NASA Goddard Institute of Space Studies, he probably never expected it to become exceptionally pervasive in popular culture. He almost certainly didn’t expect that anyone would soon contemplate creating microscopic black holes, and using them as energy sources for interstellar starships. Today, black holes are familiar to many, but understood by few.

 

 
 Artist’s impression of a black hole in the Milky Way galaxy.

Artist’s impression of a black hole in the Milky Way galaxy.

 

Black Holes 101: All Good Things…

 

Pick a star, any star. Throughout its life, the outward push of its nuclear fusion staves off the relentless inward pull of its own gravity. However, once all of its hydrogen has burned into helium, death is close at hand. After all nuclear burning stops, its gravity now having no opposition, compresses the star in the “mother of all bear hugs”. All good things, including the life of a star, must eventually come to an end.

If a star starts with a mass less than about 1.4 solar masses (the so-called Chandrasekhar limit), its collapse under gravity would be stopped by the internal electron pressure of its atoms being squeezed together. The post-compression, feeble ember is a white dwarf. It’s a stellar corpse of electron-degenerate matter about the size of the Earth, but with a density of 1000 kilograms per cubic centimeter; put another way, one piece of white dwarf the size of a sugar cube has the mass of one car!

 

white dwarf Sirius B

The white dwarf Sirius B, seen as a small pinhole of light in the lower left of the photo, is almost entirely washed out by the light of its companion, Sirius A.

 

Consider a somewhat larger star, but one whose initial mass is less than about 3 solar masses (the Tolman–Oppenheimer–Volkoff limit). When this star collapses, its gravity is too strong to be stopped by electron pressure, and the electrons and protons merge to form neutrons. However, its gravitational tamping is then stopped by nuclear forces. What’s left is a neutron star, a core of hot neutrons, 30 kilometers across, with a surface gravity 200 billion times that of the Earth. Its density is 100 trillion times the density of water; one piece of neutron star the size of a sugar cube has the mass of a mountain range!

 

A neutron star

Artist’s impression of a black hole in the Milky Way galaxy.

 

For stars with initial masses beyond the Tolman–Oppenheimer–Volkoff limit, there is nothing that can hold the star up against its own unremitting gravity. The gravitational compression of the dying star continues past a neutron star, and cleaves the fabric of spacetime. Then, the star corpse, the singularity, drops into an abyss leading out of the visible universe. The resulting void of colossal gravity is a black hole.

 

Black Holes 201: Black Holes Do Not Keep to Themselves

 

The simplest model of black holes was discovered by Karl Schwarzschild in 1916, and bears the name Schwarzschild metric. He described the geometry of spacetime surrounding an electrically neutral, non-rotating, spherically symmetric body. Such a body would have an imaginary sphere surrounding it, the surface of which marks the location where the escape speed would equal the speed of light. This imaginary surface is the event horizon.

All matter and energy unfortunate enough to plummet past this point could not ever escape. If, for all eternity, it was possible to hover a road sign at the event horizon, perhaps the most apposite wording comes from Dante Alighieri’s early 14th century poem Divine Comedy – “Lasciate ogne speranza, voi ch’intrate” (“Abandon all hope, ye who enter here”).

However, the Schwarzschild metric almost certainly doesn’t accurately describe real black holes, just as the Bohr model of the atom doesn’t accurately depict real atoms. Real black holes are rotating, electrically charged bodies that are not necessarily spherically symmetric.

If a non-rotating black hole is electrically charged, then it would be described by the Reissner–Nordström metric. If an electrically neutral black hole is rotating, then the description is given by the Kerr metric. If a black hole is both rotating and electrically charged, the appropriate description is given by the Kerr-Newman metric. There are also other metrics to describe aspects of black holes not mentioned here.

A black hole does not keep to itself. Instead, this cosmic interloper “bullies” its surroundings and its neighbors. A rotating black hole causes the contiguous spacetime to rotate as well. An approximate analogy is a hand-holding group of people running in a circle in a swimming pool, and causing the surrounding water to rotate with them, making a whirlpool. If another bather enters the water and swims toward the running group, that person will be caught up in the whirlpool, and will travel in a circle at the same speed as the spinning water. Thus, the closer to a rotating black hole an observer ventures, the faster this observer (and spacetime) rotate, eventually reaching a location where the rotation speed is the speed of light. The region between this location and the event horizon is the ergosphere.

 

Black hole regions

 

Black Holes 301: Black Holes Are Not Really Black

 

Contrary to their name, black holes do actually radiate, and are therefore not wholly black. Some energy is “fortunate” enough to escape from the black hole, making its way out of the singularity’s gravity grip. This energy is Hawking Radiation.

However, this remarkable discovery by Stephen Hawking offers little solace to the passerby who is indomitably determined to personally prove this by falling into a black hole. Perhaps, after being fried by radiation, and torn apart by the immense tidal forces of a typical black hole (the term is spaghettification), some of the traveler’s remnants, in the form of Hawking Radiation, may be unshackled from gravity, and emerge from the hole. Nonetheless, I hardly think this experiment is worth performing.

When it comes to ingesting sustenance, black holes have the table manners of ravenous canines and the fastidiousness of cantankerous infants. Nor do they exhibit even the slightest tendency toward “culinary neophobia”; they will indiscriminately eat absolutely anything. They begin by gravitationally “chewing” on it. The byproduct of being unable to swallow everything at once is an orbiting disk of accreted matter, millions of miles across. When conditions, ostensibly within the disk, are suitable, these monsters “clear their throats” in the form of relativistic polar jets, thousands to hundreds of thousands of light years long. These streams of high energy particles, collimated probably by the twisting of magnetic fields, can reach speeds of 5 parts in 100,000 below the speed of light (0.99995c).

 

Black Hole Starships: Black Holes Have a Lot to Lose

 

Throughout human history, there has always been a list of what nature could accomplish, and mankind could not. However, as we have learned more about the world around us, we have been able to shorten that list, at least marginally. Nature routinely creates black holes; perhaps we’re approaching the point in our scientific evolution where we will be able to as well.

It’s easy, for nature at least – compress a large enough mass into a small enough volume, and voilà, a black hole materializes. The maximum radius to which an object would need to be compressed to form a black hole is the Schwarzschild Radius. To make a black hole from the Earth, merely compress it so that it becomes no larger than a marble. Fortunately, the technology to achieve this will probably continue to elude us for some time to come.

Along intersecting tracks, a recently suggested human approach to make a black hole would be to use a gamma ray laser. This currently theoretical device would focus sufficient energy onto a small enough region of spacetime so as to create a black hole with a Schwarzschild Radius equal to a thousandth the diameter of the nucleus of an atom (one attometer).

The Hawking Radiation of such a black hole would be in the terawatt range, and could theoretically be used to power a starship’s heat engines.

The particulars of this idea using the Schwarzschild metric have been discussed in the literature. My work involves extending this concept to black holes described by other metrics.

Furthermore, Kerr and Kerr-Newman black holes have rotational energy. Perhaps we could harvest some of this energy for ourselves. In principle, this could be done by surrounding the laser-created, microscopic black hole with a high intensity magnetic field, and then “offering” it the “culinary delicacy” of a relativistic plasma. The rotational energy would be extracted outward along the magnetic field lines by something called a torsional Alfvén wave. Due to the plasma’s resultant negative total energy, the rotational energy of the black hole will decrease when the plasma pierces the event horizon. This phenomenon has been successfully modeled for stellar mass black holes. I am examining the theoretical feasibility of “pilfering” some of the rotational energy of a minuscule, man-made Kerr or Kerr-Newman black hole, and using it as a power source for interstellar starships.

 

I thought I saw the light.

Was it real or somethin’ I’m imaginin’?

I thought I saw the light.

If it wasn’t, tell me did you see it too?

Did you see the light?

I guess that means I got a lot to lose.

(A Lot to Lose, by Tesla)

 

 


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24 Responses to Black Hole Starships

  1. lurscher says:

    Hi Jeff, i’ve been thinking about this subject recently too, so i find your article very timely.

    This is a list of Physics.SE questions i’ve been formulating in the past two weeks on this subject, not because i expect them to be answered quickly or simply, but rather because i had these same questions buzzing around my head, and i simply had to express them in a way to would invite others to think about them too

    regarding ABH creation:
    http://physics.stackexchange.com/q/45189/955

    Let’s assume an ABH of a mass range between 10^7 and 10^8 Kg is formed, it’s radius will be a few attometers but it’s temperature and power emitted will be more than enough to propel its own weight over 1g (assuming high reflectivity efficiency of the radiation with a sort of parabolic mirror ship)

    The daunting technical problem is mainly that, since the ABH will be too small to interact effectively with matter made of protons and electrons, it will be hard to push it back in order to accelerate it at the same rate that the parabolic mirror ship.

    The above subject is touched in this question:

    http://physics.stackexchange.com/q/45713/955

    The low cross section between this ABH and normal matter means also that the rate at which it can be feed will be astronomically lower than the rate it will burn its own weight. so, isolated Schwarzschild ABH big enough to be feedable (at the same rate that they emit) and manipulable (they can be pushed back with reasonable matter and energy configurations) will probably be big (over 10^15 Kg) and cold enough so that their power emission will be too low for quick interstellar propulsion. They will probably still make excellent engines for multigenerational ships, since that king of huge vessels will represent barely a small payload relative to the weight of the ABH.

    But for quick interstellar travels, we need to investigate ways to either enhance the Hawking radiation of heavier BHs,improve our ability to interact with the attoscale, or possibly switching on and off the Hawking radiation by switching the ABH between extremal and nonextremal states. These questions touch these subjects:

    http://physics.stackexchange.com/q/45448/955
    http://physics.stackexchange.com/q/45733/955
    http://physics.stackexchange.com/q/46154/955

    As you see, is not a easy subject, and we don’t know enough about quantum gravity at this moment to answer these in the definitive.

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  3. John Pattullo says:

    a somewhat simple question, if you created such a tiny black hole – its gravity field would be negligible – after having been artifically compressed to the point it achieves an event horizon – what stops it from expanding again and becoming whatever it was you compressed to begin with or more likely the energy it has been converted into?

    it would be far more comforting to think this than that someone creating a tiny black hole that proceeds to eat the earth

    • lurscher says:

      a black hole, regardless how tiny, has an event horizon, which is where it’s own gravity will be intense enough to avoid anything from getting out. gravity will not be negligible near this radius. Once it is compressed below that radius, nothing can reverse that compression.

      a black hole around 10^10 kilograms lying on the center of the earth would probably take over a million years to swallow a kilogram of matter. In fact, one of the problems i mentioned is not that the black hole eats anything dangerously, in fact, such black holes are actually bulimic; they won’t eat that much, and they will throw up faster than they’ll eat

      • John Pattullo says:

        so what your saying is that through hawking radiation the black hole would evaporate faster than it could consume and effectively not be dangerous? (defining dangerous as earth gobbling)

        next question if all that is true wouldn’t that make them more like tiny nuclear batteries more than anything else? since surely the compression of the original mass would likely take as much energy as will be released?

        • lurscher says:

          indeed, they will evaporate at increasingly higher rates until they explode (quantum gravity becomes important at this point, so there are unknowns at the very end of the evaporation process). In one way or the other, they will return all the energy that was used to create it (because energy is conserved).

          If you can find a sweet spot for the black hole to be kept stable by feeding at the same rate as it emits energy, it becomes the equivalent of Back To The Future “Mr. Fusion” reactor, where anything that you throw at it will be converted in usable energy (all mc^2 energy of it)

          • John Pattullo says:

            ok so lets say you do find this sweet spot or for the sake of safety slightly in favour of it evaporating

            the matter you feed into it by means of hawking radiation should evaporate out as matter and antimatter above the event horizon with half being sucked back in

            now when the antimatter is sucked in it should anhilate some of the matter in the black hole? or is antimatter made from the same subatomic particles just somehow with a different charge?

            anyway lets assume for my question that it does anhilate some of the matter, would that speed the rate of evaporation, if it did then as the black hole died surely it would release a titanic energy burst and the energy from half the mass of the black hole is released as pure radiation

            :) sorry i’m seeing doomsday weapons everywhere aren’t I!

          • lurscher says:

            @John: unfortunately, for the radiation rates and temperatures that would have the most interesting interstellar travel application, the black hole radiation is not only electromagnetic, but as you mentioned, is composed of electrons, positrons, and at even higher temperature, some mesons and hadrons.

            Just like in a standard antimatter rocket, your better chance to make use of those products is by channeling them in a matter-antimatter reaction chamber and push them through some magnetic noozle. The reason this is unfortunate is that massive products reduce the overall ISP of the rocket. Ideally, you would like all your radiation to be soft electromagnetic radiation that is easy to reflect. But physics realities do not allow things to be so easy.

  4. Petr Schumacher says:

    thats my teachers, great guy. Very interesting study.

  5. John Bensted says:

    Would a miniature black hole as described in this article produce more energy than was used to create it?

    • lurscher says:

      Never. But once it is formed, you can use it to convert mass to Hawking radiation, which is the most efficient energy conversion that is allowed by the laws of physics

      • John Bensted says:

        Thanks. So, it will have to be fed a steady stream of matter just as a fusion reactor. Would it be possible to have a terrestrial miniature black hole power plant?

        • lurscher says:

          It would be a lot safer to have on space. If it is big enough to feed at a reasonable rate, it will be heavy but small enough to exert incredibly big pressures over any material surface. It will probably fall through straight to the center of the earth

          • John Pattullo says:

            would there be any way to interact with it? or would it just sit in its orbit and spit out energy? basically could it be put in a spacecraft (not sure why you’d want to since it would be a considerble mass to try and move but might help with space warping both in energy and the act itself)

            and if you had it balanced on the point of evaporation/expansion shouldn’t it provide energy for as long as you feed it regardless of how much energy was used to create it?

          • lurscher says:

            John, thats the big question. We definitely want a way to interact with it: we want to use the radiation not only to accelerate a ship, but also to accelerate the black hole itself with the ship. We also want to keep feeding it so that the mass doesn’t decrease too much and the temperature becomes too hot or unstable. But at the scales such a black hole is energetically interesting, it is also extremely small and hard to interact with.

            If you make it big enough so you can safely interact with it and keep it feeding it, its temperature is low enough to be not practical for most purposes (specially not for interstellar propulsion)

          • reidh beallagh says:

            also don’t forget, you need a good security team to keep Gary Bussey’s brother from sabotaging it.

  6. Nelson Faulkenberry says:

    new space time crystal reminds me of the donut shape used in Dr. Harold ‘sonny’ White,s Warp Field paper.

  7. Joel Noir says:

    seems that there was talk about BH creation with the LHC. Couldn’t we first use a BH to create energy for power generation? A bit more dangerous than pure nukes but you could use trash for power.

  8. Treker says:

    Weren’t Romulan Ships on Star Trek Powered by black holes?

  9. reidh beallagh says:

    Before this exotic propulsion is used to make a space-like trip, why not try to use it to make a time-like trip?

  10. reidh beallagh says:

    So then why is space travel of greater interest than time travel? Surfing the black hole will get one somewhere across space and save time thereby, good and well, but so what? it doesn’t answer any major cosmological questions like what was the universe really like, and what will it be like some time in future. That is what I hear mostly about in astrophysics circles, not this exo-planet oooww what is living on it? Ohh damn another dry dead cold piece of shit orbiting a half dead star. That’s not very exotic. Star hopping from dead planet to dead planet.

  11. reidh beallagh says:

    say we succeed in producing a real black hole at a distance perpendicular to the elliptic plane of sufficient size equal to an asteroid the size of pluto. Then if you/we/they let a mass down into the black hole’s ergosphere on a sufficiently strong tether attached to our “ship” you could possibly get to circle the black hole at speeds approaching that of light by being tugged along behind that mass being accelerated to C by the ergosphere, and at just the right moment release the tether and go shooting off under centrifugal force and direction sufficient to get to say aldebaran in say 75 years intead of 68, only one way of course. Which would be sufficient for Dr. Sheryl Bishop, but too late to enjoy the arrival.

  12. Jeff Lee says:

    That’s fine…no problem.

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