A guide to living at a black hole

By Paul M. Sutter

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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Hanging out just above the event horizon, however, doesn’t allow a stable orbit. There are benefits and drawbacks to this approach. You might learn solutions to some of the most mystifying aspects of modern physics. You’ll probably also die.

The problem is, as usual, gravity. The gravitational environment around a black hole isn’t all that different from the gravitational environment around a star or a planet. Mass is mass and gravity is gravity. What makes black holes so weird is how that mass is distributed. Or rather, isn’t. According to Einstein’s general theory of relativity, which is how we determined black holes must exist in the first place, all the matter that originally formed the black hole and all the material that fell in since its birth is concentrated into an infinitesimally tiny region at the center, a literal zero-dimensional geometric point, called the singularity.

According to general relativity, that singularity has infinite density. This is obviously wrong, but for now we’ll leave it as it is.

With that extreme concentration of mass, normal everyday gravitational effects get cranked up to 11. Take, for instance, tidal forces. The gravity of the Moon exerts a tidal force on the Earth: the ocean nearest to the Moon gets an extra tug, pulling it upwards, and the ocean farthest from the Moon gets less tugging than average, and so tends to float away. The end result is a bulge in the Earth’s oceans that follows the Moon.

Black holes can exert tidal forces too—this is how they are capable of tearing apart stars that wander too closely.

If you fall into a stellar-mass black hole, the tidal forces will rip you to shreds before you even reach the event horizon. For the great supermassive black holes, however, you can actually approach the event horizon without even noticing: the infinitely-dense singularity is sufficiently far away from the horizon that its tidal forces are negligible.

According to general relativity, there’s absolutely nothing special about the event horizon. There’s no wall, no sign, no flashing lights. Food tastes the same. You only ever realize you’ve fallen into a black hole when you try to turn around and leave…and discover that you can’t.

But there’s more to say about event horizons than just what we learn from general relativity. When we examine the microscopic nature of the horizon with the tools of quantum mechanics, all hell breaks loose.

Stephen Hawking discovered in the 1970’s that event horizons can separate pairs of particles that are spontaneously generated in the quantum foam of the Universe. This causes black holes to be not entirely 100 percent black—they emit radiation slowly (less than one photon per year), and eventually evaporate entirely.

This realization led to the birth of a paradox. In Hawking’s original formulation, the radiation emitted by black holes is thermal; it’s just random noise. But we also know from quantum mechanics that information cannot be copied or destroyed—we can always reconstruct initial states from final states, and vice-versa. So if you toss a bunch of information into a black hole, and the radiation it emits is completely information-free, and then the black hole disappears…what happens to the information?

One solution to this black hole information paradox is called the firewall. In this highly speculative proposal, the event horizon is far from another boring patch of space. Instead, it’s a wall of searing quantum energies. The energies work to rip apart anything falling into the black hole, burning it to a crisp while keeping its information content on the horizon. The Hawking radiation proceeds, but in a modified form that slowly carries away all that information.

We don’t know if the firewall exists. If it did exist, it would sit right at the event horizon, making it unobservable to the outside Universe. The only way to observe it would be to take the plunge and see if you make it through to the other side.

Stephen Hawking in Princeton, NJ, October 1979.
Enlarge / Stephen Hawking in Princeton, NJ, October 1979.

Beyond the horizon

The event horizon of a black hole represents a concrete place in the Universe where known physics breaks down—we simply don’t have the sophistication to understand what really happens at the horizon. But assuming you make it through with nothing more than a queasy stomach, more adventures, and more mysteries, await you inside.

The space inside a black hole isn’t any different than the space outside. If you were freely falling before, you’re still freely falling now. If you were taking a bite of your sandwich before, you’re still chewing on it now. But due to the extreme gravity, your future becomes much more restricted upon entering the black hole.

In short: all roads lead to the singularity. The singularity of the black hole now lies in all your possible futures. No matter how you move or turn, the singularity will always appear in front of you, and it will always grow larger.

The singularity itself is still an infinitely tiny point, but due to the extreme gravitational tidal forces, your perception of the world around is strongly distorted. Light falling in behind you gets compressed into a thin belt at your waist. Ahead of you, the singularity stretches to become a black world, and eventually a featureless black plain.

Before you touch the singularity, though, those same tidal forces will destroy you. And they will do so in finite time. You can indeed survive inside a giant black hole, but only for a while. Your time to reach the singularity depends on the mass of the black hole. For stellar-mass black holes, you have only a few microseconds; for the supermassive ones, you have a few handfuls of seconds to divine the deepest secrets of the Universe (and contemplate your fate).

You really do not have a choice. According to the watch on your wrist, you will reach the singularity, no matter what you do. Outside a black hole, you have complete freedom to explore any direction in space you please, but you cannot avoid moving forward into your own future. Inside a black hole, every single possible movement leads you towards the singularity. And since you can’t help but move inside the event horizon, the singularity is literally unavoidable.

No matter the size of the black hole, approximately 1/10th of a second before reaching the singularity, the tidal forces will be strong enough to overwhelm any other known force. You will be destroyed—atomized—before reaching the infinitely tiny point.

As to the singularity itself, it too is a region where known physics breaks down. We know that the infinity that appears in the equations of general relativity is a signal that we need a full quantum theory of gravity to properly describe what happens at the center of a black hole. But we currently lack such a theory, and so we can’t say for certain what happens.

Speculation abounds, here in the deepest void of the black hole. If you do find yourself inside one, you’ll come face-to-face with the most inscrutable physics known to humanity. It’s a shame you won’t be able to tell anyone about it.

Paul M. Sutter is an astrophysicist at Stony Brook University and the Flatiron institute in New York City. He is the host of the "Ask a Spaceman" podcast and the author of "Your Place in the Universe" and "How to Die in Space".