In a more detailed analysis, a proposal to create an artificial black hole and using a parabolic reflector to reflect its Hawking radiation was discussed in 2009 by Louis Crane and Shawn Westmoreland.[2] Their conclusion was that it was on the edge of possibility, but that quantum gravity effects that are presently unknown will either make it easier, or make it impossible.[3] Similar concepts were also sketched out by Alexander Bolonkin.[4]
Advantages
Although beyond current technological capabilities, a black hole starship offers some advantages compared to other possible methods. For example, in nuclear fusion or fission, only a small proportion of the mass is converted into energy, so enormous quantities of material would be needed. Thus, a nuclear starship would greatly deplete Earth of fissile and fusile material. One possibility is antimatter, but the manufacturing of antimatter is hugely energy-inefficient, and antimatter is difficult to contain. The Crane and Westmoreland paper states:
On the other hand, the process of generating a BH from collapse is naturally efficient, so it would require millions of times less energy than a comparable amount of antimatter or at least tens of thousands of times given some optimistic future antimatter generator. As to confinement, a BH confines itself. We would need to avoid colliding with it or losing it, but it won't explode. Matter striking a BH would fall into it and add to its mass. So making a BH is extremely difficult, but it would not be as dangerous or hard to handle as a massive quantity of antimatter. Although the process of generating a BH is extremely massive, it does not require any new Physics. Also, if a BH, once created, absorbs new matter, it will radiate it, thus acting as a new energy source; while antimatter can only act as a storage mechanism for energy which has been collected elsewhere and converted at extremely low efficiency. (None of the other ideas suggested for interstellar flight seems viable either. The proposal for an interstellar ramjet turns out to produce more drag than thrust, while the idea of propelling a ship with a laser beam runs into the problem that the beam spreads too fast.)
Criteria
According to the authors, a black hole to be used in space travel needs to meet five criteria:[5]
has a long enough lifespan to be useful,
is powerful enough to accelerate itself up to a reasonable fraction of the speed of light in a reasonable amount of time,
is small enough that we can access the energy to make it,
is large enough that we can focus the energy to make it,
has mass comparable to a starship.
Black holes seem to have a sweet spot in terms of size, power and lifespan which is almost ideal. A black hole weighing 606,000 metric tons (6.06 × 108 kg) would have a Schwarzschild radius of 0.9 attometers (0.9 × 10–18 m, or 9 × 10–19 m), a power output of 160 petawatts (160 × 1015 W, or 1.6 × 1017 W), and a 3.5-year lifespan. With such a power output, the black hole could accelerate to 10% the speed of light in 20 days, assuming 100% conversion of energy into kinetic energy. Assuming only 10% conversion into kinetic energy, it would take 10 times more.[2]
Getting the black hole to act as a power source and engine also requires a way to convert the Hawking radiation into energy and thrust. One potential method involves placing the hole at the focal point of a parabolic reflector attached to the ship, creating forward thrust, if such a reflector can be built. A slightly easier, but less efficient method would involve simply absorbing all the gamma radiation heading towards the fore of the ship to push it onwards, and let the rest shoot out the back.[5][6] This would, however, generate an enormous amount of heat as radiation is absorbed by the dish.
Criticism
It is not clear that a starship powered by Hawking radiation can be made feasible within the laws of known physics. In the standard black hole thermodynamic model, the average energy of emitted quanta increases as size decreases, and extremely small black holes emit the majority of their energy in particles other than photons.[7][8] In the Journal of the British Interplanetary Society, Jeffrey S. Lee of Icarus Interstellar states a typical quantum of radiation from a one-attometer black hole would be too energetic to be reflected. Lee further argues absorption (for example, by pair production from emitted gamma rays) may also be infeasible: A titanium "Dyson cap", optimized at 1 cm thickness and a radius around 33 km (to avoid melting), would absorb almost half the incident energy, but the maximum spaceship velocity over the black hole lifetime would be less than 0.0001c (about 30 km/s), according to Lee's calculations.[8]
Govind Menon of Troy University suggests exploring the use of a rotating (Kerr–Newmann) black hole instead: "With non-rotating black holes, this is a very difficult thing...we typically look for energy almost exclusively from rotating black holes. Schwarzschild black holes do not radiate in an astrophysical, gamma ray burst point of view. It is not clear if Hawking radiation alone can power starships."[5]
In the MMOEve Online, starships designed by the Triglavian faction utilize naked singularities contained on the external hull as their vessel's primary power source.
In Foundation (TV series), jump ships appear to use black holes to power their jumpdrives, enabling faster-than-light (FTL) travel over interstellar distances. This differs from the Foundation series on which the TV series is based, where FTL travel is facilitated through hyperspace travel.
In the TV series, Doctor Who, the TARDIS is powered by a black hole and that is not just how it is capable of being bigger on the inside but also how it travels through time.
In the book, How High We Go In The Dark, by Sequoia Nagamatsu an interstellar starship is used to take passengers in cryo-sleep 582 light years from Earth at 10% the speed of light. The starship uses an engine powered by Hawking radiation.
