Jakub Scholtz of Durham University and James Unwin of the University of Illinois recently published a paper with a twist, a twist given away by the title: What if Planet 9 is a Primordial Black Hole? The authors propose that the hypothetical Planet 9, whose existence would explain some anomalous wobbles in the orbits of trans-Neptunian Objects, as well as some lensing events, might be…well, you probably guessed from the title.
Finding a five-Earth-mass, ten-centimetre-diameter, 0.004 Kelvin object somewhere in the outer boroughs of the Solar System should be easy—I’m sure that some grad student or professor angling for tenure is hard at work right now! But what would be the use to the rest of us of a five-Earth-mass, ten-centimetre-diameter, 0.004 Kelvin primordial black hole (PBH) orbiting somewhere in the outer boroughs of the Solar System?
OK, sure: if it’s there, it offers us the chance to do some wonderful science; we’d be able to run experiments in regions of intense gravity. But people in general don’t seem to care all that much about pure science. So, what applied applications are there?
(A) Old-time SF (published back when open-chested polyester shirts were sexy) knew one cool thing to do with black holes: use them as a shortcut across space-time. We learned how to use shortcuts in space-time in A Wrinkle in Time, and Starman Jones, for example. Both Joe Haldeman’s The Forever War and Joan D. Vinge’s The Snow Queen used black holes as the McGuffin to provide such shortcuts. Please feel free to name other books/examples in comments.
There are only two minor issues preventing us from using Scholtz and Unwin’s hypothetical PBH as a space-time teleport booth. The first is its size: at ten centimetres wide, you won’t be cramming a full-size spacecraft down its gullet. Now you might point out the Space Eater solution (which if you don’t know what that is, you’re probably better off) or you could suggest we use it, the way A. A. Jackson IV and Howard Waldrop did in “Sun Up,” for communications purposes. It’s too bad that black holes don’t work the way that SF authors have blithely hypothesized. Probes (and ships) and laser beams wouldn’t just pop into a hole here and out of another hole there; tidal forces would spaghettify our poor probe before it got to the event horizon. The pastafied remnants would then be plasmafied in the PBH’s accretion disc. If anything made it past the event horizon (which, thanks to relativity, outside observers would never see), it would emerge as Hawking radiation. Such a transformation would probably invalidate your health insurance.
(B) Another use for black holes, one that popped up in Disco Era stories penned by authors from Sheffield to Somtow to Varley, is to use the object as a combination oubliette/power station. Material dropped towards the PBH would be shredded and heated into a plasma, which could then be used to generate power (by the same methods as proposed for fusion reactors). There may be other ways to extract energy from a black hole, which may or may not work in the case of this particular PBH. But…even the less ambitious schemes would produce a surprising amount of power.
This use of PBHs seems doable if we assume some future super-science. That given, the main issue would be that since a five-Earth-mass black hole isn’t all that maneuverable, the power will be generated between 300 to 1000 AU from Earth. Getting it from there to here would seem dicey. Still, maybe some scaled-down version of the Nicoll- Dyson Laser could be used by Primordial Black Hole Power and Ravening Death Ray, LLC. to deliver power across the gulfs of space. Or maybe there’s a use for unthinkable terawatts of energy 300 to 1000 AU from the Sun. We could make anti-matter, useful in both very high-density batteries and pocket WMDs!
(C) The hypothetical PBH would have high mass and insanely high density. That means that it should be even more useful for flyby maneuvers than is Jupiter. It would have less mass, but it would be lot closer to being a point source. A starship zooming past the black hole at a safe distance (whatever that is) would end up with an escape velocity that might approach 20,000 km/s. A sufficiently robust probe with a sufficiently high thrust, high exhaust velocity propulsion system could use a PBH flyby to achieve otherwise unattainable velocity. Why, we could travel from the Solar System to the nearest star systems in mere… centuries!
Flybys are an important strategy for real-world space exploration (see list). I know that they’re occasionally featured in SF. I can’t think of any works featuring flybys around a black hole (although Niven used a neutron star to similar effect in a couple stories) but…I could be missing something, so feel free to point out my oversights in comments.
Figuring out just how close one could get to the PBH would be interesting. A centimetre-wide object passing within a kilometer of the PBH would be stressed: the bit of the object closest to the PBH would want to move about 10 km/s faster than the bit farthest from the PBH. The object would have to be damn robust. Also the sufficiently high thrust, high exhaust velocity propulsion system mentioned above can be imagined but not yet built.
(D) As demonstrated in 1919 by Eddington and company, mass can deflect light. The PBH could be used as a gravitational lens, to better gather information on distant objects. Thanks to the combination of mass and high density this has to be easier than using the Sun’s mass for the same purpose—or it would be if the PBH wasn’t at about the same distance from the Sun as the minimum distance one would have to dispatch a probe to exploit the Sun’s mass in this way.
(E) Finally, if there’s one thing we know about human nature, it’s that people will spend surprising amounts of money to make other people miserable. This is particularly true when it comes to the death penalty, which (depending on jurisdiction) can cost several times more than lifetime incarceration. Sending someone 300–1000 AU to be crushed into oblivion by an object roughly the size of a golf ball would be a spectacularly improvident use of resources in the service of spite. It follows therefore that this might turn out to be the most common use.
In the words of Wikipedia editor TexasAndroid, prolific book reviewer and perennial Darwin Award nominee James Davis Nicoll is of “questionable notability.” His work has appeared in Publishers Weekly and Romantic Times as well as on his own websites, James Nicoll Reviews and Young People Read Old SFF (where he is assisted by editor Karen Lofstrom and web person Adrienne L. Travis). He was a finalist for the 2019 Best Fan Writer Hugo Award, and is surprisingly flammable.