finding aliens by the black holes they make?
As the world keeps moving forward, our energy requirements are constantly increasing. Our cities and towns are consuming terawatt after terawatt, and as new technology comes online and old technology improves, the rate of consumption only grows. Surely, an incredibly advanced alien civilization that had a fairly sophisticated infrastructure for the last few million years would have energy needs far, far exceeding ours, measuring their demands in yottawatts and beyond, right? This seems to be the argument in a paper which suggest that one more SETI strategy we should consider is to look for black holes smaller than just a few solar masses since they could well be artificial, created by an extraterrestrial species which needs a constant stream of energy to run its exceptionally advanced society as per Kardashev’s predictions. After all, feeding black holes blast out torrents of extremely energetic beams that could be harnessed by an advanced enough species and all that energy could well exceed the output of an entire star, often concentrated in primarily two beams around what we could call their magnetic poles. So yes, why not look for alien life around oddly lightweight black holes?
Well, the problem is that we don’t really know the lower limits of a natural black hole and the smaller they are, the harder they are to detect. How can you prove that a black hole is artificially made? Drawing a simple line between 3.5 solar masses and below, then saying that a gravitational well as dense as a black hole but any lighter than that is suspect raises the spectrum of primordial black holes, gravitational collapses of matter as small as a marble but with the mass of our planet, thought to have formed just after the Big Bang, first. An old alien civilization that can simply build a gravitational singularity is going to require a lot more proof than a very rudimentary weigh-in of a quirky astronomical object. Condensing the mass of three suns into a sphere just shy of 17 kilometers across requires an awful lot of energy, roughly just an order of magnitude less than your typical hypernova, so the creation of an artificial black hole on an astronomically relevant scale requires some very, very mighty aliens who can summon the energy of entire solar systems to do their bidding. But of course the paper’s author, Clement Vidal, doesn’t tell us to look for the biggest possible artificial black holes but that there may be something intrinsically interesting about a solar system in which a small black hole is accreting matter from an active star because that’s a very good way to get a torrent of raw energy for a long time.
However, his rather vague mechanism for manipulating matter into collapsing to a black hole state seems a little lacking since it equates our current ability to make nanomaterials by manipulating how chemical bonds come together on a molecular level, with the ability to compress atoms out of material existence, arguing that we can think of the latter as an extension of the former. In reality, making a string of carbon nanotubes and a tiny black hole are completely different capabilities. That said, I could see why a hypothetical alien species is going to be interested by manipulating microscopic black holes. As we discussed once before, an atom-sized gravitational singularity would be a great engine for a relativistic spacecraft, emitting enough energy to just balance on the edge of stability, evaporating quickly enough to do very little damage if things go wrong while it emits plenty of energy from the accretion of raw fuel keeping it alive. And the great thing about black holes in cores of reactors is that you can feed them with pretty much anything gaseous. Want to get rid of some highly noxious fumes you’ve trapped in a factory? Feed it to a tiny black hole. It will turn it into a shower of radiation to be turned into electricity that will power the closest city. However, triggering that initial collapse that will give a tiny black hole a fleeting chance at life is very, very difficult. The smaller the black hole, the smaller the energy input required, and the harder they’d be to detect here on Earth over all the noise made by natural objects.
And that brings us to a very important point that often goes unmentioned about the Kardashev scale. It’s not a set of good predictions, it’s a set of extrapolations which assume that an alien species jumps several orders of magnitude in its energy requirements at each level, sort of like a character in a video game ranking up one more notch. What if truly advanced alien species just don’t exist in very large numbers and don’t have such an insatiable thirst for energy that they need to build a black hole to siphon it form stars? What if they’re fine with just a tiny cluster of black hole reactors supplying all their needs? The very event of creating a black hole on a scale we could detect would be immediately flagged by astronomers as an immensely powerful and puzzling gamma ray burst (GRB) comparable to a Type Ia supernova in its might so if anything, that should be what we should consider suspect rather than a black hole on the lowest ends of scale appearing near alien suns. But then again, we’re back to assuming than an alien species would actually want to, or need to do something on such a drastic, astronomical scale to keep its civilization’s immense demands for energy sated…
Clement Vidal (2011). Black Holes: Attractors for Intelligence? arXiv: 1104.4362v1