The thing about black holes

A visualisation of a simulated black hole and its accretion disc. The distortion in the disc is caused by the black hole’s immense gravity. Image: NASA’s Goddard Space Flight Center/Jeremy Schnittman.

The news that astronomers have discovered the closest known black hole to Earth, just a thousand light years away, sent bells of recognition ringing in my memory of a prediction made by a theory. Ironically, I can’t remember exactly what it was – perhaps readers can help – but I have a suspicion that it may have been related to theoretical physicist Lee Smolin’s ‘Fecund Universe’ idea.

It’s kind of a natural selection for universes on a cosmological scale. Smolin’s concept is that every time a black hole forms, it spawns a brand new universe that inherits most of its parent universe’s properties, so that universes that are able to produce lots of black holes are naturally selected for, whereas those that produce few or none do not replicate and become evolutionary dead ends, cosmologically speaking. Naturally, if the model is correct and our Universe is proficient at producing black holes, then Smolin’s model predicts that the population density of black holes across our Universe would be great enough that there should be black holes relatively close to our Solar System.

Anyway, I digress; the Fecund Universe isn’t actually what I want to talk about (although if anyone could confirm whether that specific prediction is related to Smolin’s theory or not, I’d be grateful!). The discovery of this new closest black hole is interesting for all kinds of other reasons beyond cosmological speculation. It is a stellar-mass black hole, meaning that it has a mass equivalent to that of a star, as opposed to supermassive black holes with millions, or even billions, of times the Sun’s mass (2 x 10^30 kilograms), which lurk at the centres of galaxies. A stellar-mass black hole forms when one of the most massive stars, with at least 40 times the mass of the Sun, explodes as a supernova. As nuclear reactions within the doomed star cease, its core collapses and compacts to a point of infinite density – a black hole. The remainder of the star is blasted apart in the supernova explosion.

The newly-discovered black hole exists as a member of a triple system named HR 6819, alongside two ordinary stars. Although we cannot see the black hole directly, astronomers are able to infer that it’s there by its gravitational pull on the other two stars. Based on the strength of its gravity, it has a mass four times greater than our Sun.

A dark black hole

Before the discovery of the black hole in the HR 6819 system, the closest known black hole was found in the V616 Monocerotis system, which is about 3,000 light years distant. 

Most of the stellar mass black holes currently known were revealed by their interactions with companion stars. The black holes and their ferocious gravitational pull are able to rip matter from their neighbouring stars. This matter flows towards the black hole, forming a disc of gas around it as the black hole feeds on the matter. The disc grows so hot that it emits X-rays, and variations in the amount of matter flowing onto the disc can cause brightness fluctuations that astronomers can detect. We call such a system an ‘X-ray binary’.  

An artist’s impression of an X-ray binary system, wherein a black hole (left) is at the centre of an accretion disc of gas that it is pulling off a companion star (right). Image: NASA/CXC/M. Weiss.

The black hole in the HR 6819 system is, however, different. From what we can tell, it isn’t interacting with its stellar companions, and therefore doesn’t have an accretion disc of gas, and hence isn’t emitting any X-rays – it is completely dark.

Which is interesting. We only detected the black hole in HR 6819 via its gravitational pull on its companions, but how many other black holes are out there, lonely without companions, remaining dark and hidden? The team that discovered the HR 6819 black hole, led by Thomas Rivinius of the European Southern Observatory, think that HR 6819 is just the tip of the iceberg. “There must be hundreds of millions of black holes out there,” says Rivinius. Maybe, some are even closer to Earth.

Astronomically, it’s fascinating. A census of stellar-mass black holes in the Milky Way Galaxy would tell us about previous populations of massive stars that created them. Having black holes on our doorstep could make it easier for us to study them, so long as we can see or detect them somehow. One way of spotting these lonesome black holes would be to look for random gravitational microlensing events, which would occur when one of these black holes moves in front of a background star, as seen from our point of view, and the gravity of the black hole briefly bends and magnifies the light of that background star, betraying the black hole’s presence.

“There must be hundreds of millions of black holes out there”

Thomas Rivinius, European Southern Observatory astronomer

Why SETI should target black holes

The discovery of a very large population of black holes is an intriguing proposition for SETI too. Modern radio SETI doesn’t ask anything too ambitious of aliens – they just need to be sat on their planet, like us, beaming signals into space. However, extraterrestrial societies with perhaps millions or even billions of years of technological advances over us might have grander aims, and black holes might be their targets.

For one, black holes are laden with energy that is just waiting to be tapped. First, there’s the easiest of the black hole’s energy to gather – radiation emitted by hot gas that is spiralling into the black hole’s maw. If the black hole is in the middle of diet, a sufficiently technologically-developed society might want to chuck material at it – asteroids, planets, whole stars – to spur on this process. Remember that Albert Einstein taught us that mass and energy are equivalent, and the mass to radiative-energy conversion rate for a black hole is suggested to be about 10 per cent, which is considered to be quite a high value as these things go.

Alternatively, the gravitational energy of the black hole itself could be mined. Swoop down close to a black hole and tip some material into it – maybe an asteroid, mounds of toxic waste, rubbish from the tip, whatever. The action of dumping the material will produce a recoil, pushing your spaceship away from the black hole and in the process (because of the conservation of angular momentum) your spaceship will acquire some of the black hole’s rotational energy, which is determined in part by the black hole’s mass, and because you’ve just added some mass to the black hole, the rotational energy increases, so that your spacecraft departs with more energy than it had when approaching the black hole. (See Kip Thorne, Charles Misner and John Wheeler’s 1973 textbook, Gravitation, for more.)

There’s enough energy in the most massive black holes to power a technologically-advanced extraterrestrial civilisation for trillions of years. We’re talking about a long-term energy source to drive a civilisation towards the deep future. Furthermore, there are speculative suggestions that the Hawking radiation leaking from a black hole could be manipulated to turn that black hole into a giant computer capable of handling all the computations that an information-hungry society would ever need. Who knows, if by some chance Lee Smolin does turn out to be right after all (and that is a very big ‘if’ at this stage) in his supposition that black holes can give birth to new universes, then perhaps a highly-technological-advanced society could manipulate that process to create a universe to their own liking.

So, a long-lived society might head to its nearest black hole and set up base camp around it, and once there they could potentially have all the energy and data processing capabilities that they would ever need. This postulation, of course, depends on several assumptions, such as that it is indeed possible to venture close enough to a black hole and extract energy and turn it into a giant computer without being ripped apart by the immense gravitational tides. It also assumes that a technological society can achieve interstellar spaceflight, a capability that seems feasible, although only at significant sub-light speeds. This means that they will need a black hole reasonably close to them in order to be able to get to it, so the more black holes that exist, the greater the likelihood that they will be in reach of one. 

Can a sufficiently advanced technology be indistinguishable from nature?

The discovery of the black hole in HR 6819 and the likelihood that there are hundreds of millions of black holes wandering in the dark between the stars suddenly makes this assumption seem a little more plausible. 

However, we’ve gotta be honest – this discussion is at present little more than high-concept speculation. Nevertheless, as well as continuing to listen or watch for signals from planets orbiting stars, SETI could also turn its attention to known nearby black holes, just in case. Clément Vidal, of the Free University of Brussels, has had some ideas in this regard. He has suggested that it could be difficult to distinguish a natural X-ray binary from an artificial one created by technologically-advanced extraterrestrials as a means of harnessing its energy. Indeed, suggests Vidal, we may have already observed extraterrestrial activity, in known X-ray binary systems such as Cygnus X-1, and not realised it.

X-ray binaries (circled)in the galactic centre, imaged by NASA’s Chandra X-ray Observatory. Image: NASA/CXC/UCLA/M.Muno et al.

Vidal has also pointed out that there is a known over-density of X-ray binaries clustered in the galactic centre, surrounding the supermassive black hole called Sagittarius A*, which sits at the hub of the Milky Way, 26,000 light years away. Some of these X-ray binaries have been discovered by astronomers using NASA’s Chandra X-ray Observatory, and it is estimated that there could be as many as 20,000 stellar-mass black holes within a few light years of Sagittarius A*. If a technologically advanced society could reach the galactic centre, and somehow survive the extreme radiation environment there, then they would have access to a tremendous amount of energy, all confined to a relatively small volume.

Although these ideas are speculative, it wouldn’t do any harm for SETI to look at black holes more closely. If they really are attractors of intelligence, then perhaps technological life migrates to them once they achieve the ability to do so – it would explain why we’re not detecting signals from planets orbiting stars. On the other hand, if the black hole–intelligence hypothesis turns out to be a load of hooey, our study of black holes wouldn’t go to waste – astrophysicists could still learn a great deal about the nature of black holes in the process. So either way, any discoveries, whether it be aliens, the birth of new universes, or a greater understanding of how black holes operate, could be profound.

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