Black Hole Starships

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.

 

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)