A guide to living at a black hole


Even with today's real estate boom, a supermassive black hole in the neighborhood has to drive the asking price down a bit, right?
Enlarge / Even with today’s real estate boom, a supermassive black hole in the neighborhood has to drive the asking price down a bit, right?

Black holes flood the Universe. The nearest one is a mere 1,500 lightyears away. A giant one, Sagittarius A*, sits in the center of the Milky Way about 25,000 lightyears away. While your typical space traveler might look for a home around a calm G-type star, some celestial citizens are brave enough to take up refuge around one of these monsters. It’s not an easy life, that’s for sure, but being neighbors with a black hole does mean you’ll almost certainly learn more about the fundamental nature of reality than anybody else.

Interested? What follows is a guide of what to expect should you make your home around a black hole. Good luck.

Black hole basics

Upon first arriving at a black hole, you will most likely be struck by how utterly, completely…boring it is. The black hole itself is simply a fathomless black orb hanging out somewhere in the distance. Black holes don’t really do anything except sit there and gravitate. In fact, they’re famously easy to miss: Unless they’re actively feeding on material or coincidentally bending/blocking the view to a star in the background, you simply can’t see them. Once you know one is there, though, you can start to have some fun.

The size of the orb is determined by the black hole’s mass in a famous equation first derived by German astronomer Karl Schwarzschild, and the radius of that orb is named after him (the Schwarzschild radius). The smallest black holes have Schwarzschild radii no bigger than Manhattan; the largest ones could encompass our entire Solar System.

The orb itself represents the event horizon of the black hole. This is the region where the inward pull of gravity becomes so strong that nothing, not even light, can escape. While gravitating objects are constantly pulling spacetime towards them, black holes pull so intensely that, at the event horizon, spacetime itself rushes in faster than the speed of light. If you want to escape, you have to fight against that extreme current of spacetime. Since you can’t, you’re trapped.

Beyond the weirdness of the event horizon, however, there’s nothing strange about orbiting a black hole.

That’s because gravity is just gravity. Your gravitational attraction to the Sun, for example, depends entirely on the mass of the Sun. Same for a black hole. You could replace our Sun with a one-solar-mass black hole and the orbits of the planets would be completely unperturbed (sure, all the plants would die and everything would freeze from the lack of light, but that’s a different problem).

As long as you are far enough away from the black hole itself, nothing seems out of the ordinary. You can maintain a stable orbit around a black hole for eternity if you wanted to. And thankfully for anyone wanting to take up residence there, we can calculate what “far enough away” really is. It’s called the innermost stable circular orbit (ISCO), which is pretty much exactly what the name implies. For a simple, non-rotating black hole, it’s three times the Schwarzschild radius. Within that distance, stable circular orbits are impossible, and you either have to eject yourself to the freedom of empty space or allow yourself to plummet below the event horizon.

For a more realistic situation where the black hole is rotating, the ISCO is much harder to calculate, and depends on how quickly the black hole is rotating and whether your orbit is going with the spin of the black hole (prograde) or against it (retrograde). In general, though, as long as you’re more than 10 times the Schwarzschild radius away from the black hole, you’re good.

Artist's impression of a star being tidally disrupted by the powerful gravity of a supermassive black hole.
Enlarge / Artist’s impression of a star being tidally disrupted by the powerful gravity of a supermassive black hole.

Gravity in all its glory

While black holes themselves may seem boring, life around them is anything but. And that’s because black holes do one thing and do it well—pull.

No matter the size of the black hole, they tend to collect accretion disks—something they share with pretty much any massive, compact body, like neutron stars. When gas and dust finds its way into the vicinity of a black hole, conservation of angular momentum squashes that material into the form of a thin, flat disk. This material can come from anywhere: random interstellar gas clouds, the atmosphere of a nearby body, or even torn-apart remnants of other stars. Whatever the origins, the material gets shredded to pieces, and those pieces follow winding paths, known as tendex lines, towards the open maw of the event horizon.

The ferocity of their surrounding environments depends on the mass of the black hole. By far the most common kind of black hole is relatively small; only a few times more massive than the Sun. If a black hole of this mass happens to orbit a companion star, and that star wanders too close, the black hole can siphon off the star’s atmosphere. As the gas approaches the black hole, it must compress to make it to the relatively small black hole, like too many people crowding into an open elevator. When gas compresses, it heats up, and that hot gas glows in X-rays. It’s through this copious X-ray emission that astronomers discovered our very first black hole, known as Cygnus X-1.

The largest black holes, known as supermassive black holes, are truly gargantuan, easily topping hundreds of millions of even billions of solar masses. The physics of accretion work around these monsters too, appropriately scaled up. The accretion disks around supermassive black holes can reach a million Kelvin. At those temperatures, they emit so much radiation that they can outshine millions of galaxies combined.

Those accretion disks are a curse and a blessing for any potential visitors. You’re going to need that energy if you want to set up shop around a black hole, as the black hole itself won’t be providing any kind of light for you if not for the disk. But the gravitational forces around black holes are strong enough to literally tear apart stars, and the electric and magnetic fields within accretion disks are some of the strongest in the entire Universe. If you’re up for the challenge of surviving in this kind of hellish environment, you’ll find more than enough energy to spare for generations.

However, even naked black holes can give you a power source. This process is known as the Penrose mechanism in honor of its discoverer, Nobel prize-winning physicist Roger Penrose. While it only works on rotating black holes, this isn’t much of a problem. Black holes are formed when massive stars die, and stars are always rotating, and that momentum gets transferred to the black hole. So the Universe is not lacking in rotating black holes.

The Penrose mechanism takes advantage of a peculiar facet of rotating black holes: the ergosphere. Rotating objects drag on the spacetime around them, like trying to turn a heavy coffee table on top of a rug. All objects do this, as it’s a normal part of gravity. But like everything else black holes do it in excess. Surrounding the event horizon proper is a region of constantly moving spacetime, dragged into rotation by the black hole itself.

Penrose discovered that if you drop an object into the ergosphere and then let it split apart, you can extract energy. You let one of the pieces fall into the event horizon, never to be seen again. You then allow the other piece to escape the ergosphere. The piece that escapes gets a boost from the rotating spacetime, and you get a net gain in energy from the maneuver. The Penrose mechanism pulls energy from a rotating black hole, slowing it down in the process. You can’t do this forever, of course (eventually the black hole stops spinning), but seeing as how the Penrose mechanism is capable of launching material up to tens of thousands of lightyears away from giant black holes, I wouldn’t worry.

If the energy from the accretion disk or the Penrose mechanism aren’t enough, you can take advantage of one other feature of black holes: their extreme gravity. When light falls into a black hole, it ramps up in energy as it nears the event horizon, just like a ball begins to speed as it rolls down a hill. If you manage to hang out just above the event horizon, you’ll be bathed in a swarm of high-energy radiation.



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