The patient accretion of knowledge,
the focusing of all one’s energies on some problem in history or science,
the dogged pursuit of excellence of whatever kind
these are right and proper ideals for life.
~Michael Dirda

Nothing can escape from a black hole, not even light. This is why we call them “black.” One would imagine, then, that black holes are black invisible menaces, lurking out in the depths of space. Surprisingly, though, black holes glow. The cover image shows a radio photograph of the center of the Milky Way. The center glow, Sagittarius A, is partly due to a supermassive black hole, Sagittarius A*. (No, that doesn’t lead to a footnote…the name of the black hole actually is Sagittarius A*, pronounced “a star.”)

Black holes glow because they are very messy eaters. As a black hole sucks in surrounding matter, it pulls its food into a disk or a sphere around it, called an “accretion disk” or an “accretion shell,” as shown below. And it is partly this disk that generates the incredible glow. (There is another process, called a “jet,” which also produces a lot of light. I’ll briefly talk about it later.)

But why doesn’t stuff in the accretion disk just fall into the black hole? The answer, elegantly enough, is the same reason that the planets in our solar system don’t fall into the sun.

Centrifugal Force

Imagine that you tie a ball to a string and spin it over your head. The ball will fly out to stretch the string as much as possible and, if you let the string go, the ball will fly away from you in a direction tangential to the circle. This effect is so prominent that it can be used to make a weapon called a “bola.”

As Sir Isaac Newton predicted, objects like to travel in straight lines–you have to push or pull them to make them deviate. This resistance to change is called momentum. Thus, to make an object travel in a circle, you have to constantly pull it towards the center of the circle, forcing it to turn. The faster an object moves (or the more massive it is), the harder it is to turn, and the more force you have to use to pull it towards the center of the circle. Although the object’s tendency to fly out of the circle emerges purely from its momentum, for convenience, we often pretend it’s a separate “centrifugal force.”

Matter in accretion disks is often spinning too fast to fall into the black hole. The gravitational pull of the black hole isn’t strong enough to counteract the centrifugal force of the matter–partly because the black hole is spinning too and drags the matter with it, partly because the matter was spinning to begin with. (On the cosmic scale, most things in the universe are spinning.)

Over time, the black hole does win. The matter does lose outward momentum and fall into the black hole. (Like energy, momentum can’t be created or destroyed, but it can be transferred. Most of it is vented through the “jet” light-creating process that I’ll briefly explain later.) However, as stuff falls into the black hole, the gravitational pull of the black hole accelerates it up to incredible speeds, which in turn heats it up to incredible temperatures. And hot matter glows.

(Temperature actually contributes to the glow in another, less direct way. The in-falling matter is often so hot that it ionizes, its electrons separating from their nuclei. These charged particles follow the spin of the disk they’re in, which causes them to accelerate. Since accelerating charges emit light–which, incidentally, is how radios work–the disk glows even brighter.)

The glow has another surprising effect, though. We often imagine accretion disks to be very thin, flattened out by the spinning of the disk and the black hole, the same way that a pizza chef flattens out dough by spinning it. But they’re actually a bit thick. The secret is light.

A Quantum of “Push”

In the time of Sir Isaac Newton, there were two competing ways of understanding light. Newton believed that light was made out of tiny particles called “corpuscles” that carried kinetic energy and momentum and bounced off of things like any normal particle. In contrast, Christiaan Huygens believed that light was like sound: a wave that propagated through a clear medium, like air or glass.

Of course, we know now that Newton and Huygens were both right (to a degree). Quantum mechanics has shown us that light is both a particle and a wave. It bends and refracts like a wave, but it carries energy and momentum like a particle. This means it can bounce off of things and exert force. (Although light is a wave, it doesn’t need a medium like sound does. It can propagate in empty space.)

Imagine that a beam of light bounces off of a mirror, as shown below. One way to describe this is by using the equations of optics and electromagnetism. However, another way is to imagine a bunch of physical particles–which we now call photons–hitting the mirror and bouncing off of it. But Newton tells us that “for every action there is an equal and opposite reaction.” When the mirror pushes the photons, the photons must push back.

This effect is called radiation pressure. We don’t usually notice it because each individual photon doesn’t carry much energy compared to a human being. We need a lot of them to exert an appreciable force. However, we can harness radiation pressure to do some pretty cool things. The solar sail proposal for space travel is based on this idea.

(Experts know that the conception of light as a wave also predicts that it carries energy and momentum. However, we need to treat light as an electromagnetic wave, governed by Maxwell’s equations. Particle-wave duality lets me explain radiation pressure a lot more easily.)

Why Accretion Disks are Thick

So what does radiation pressure have to do accretion disks? As we now know, the matter in the accretion disk is producing quite a lot of light. When this light scatters, it exerts an outward force on the in-falling stuff, partly counteracting the pull of gravity and the flattening effect of the spin. If enough photons hit the in-falling gas, something amazing happens: the matter stops falling. The constant radiation pressure from within the disk completely counteracts the force of gravity.

The point when the glow of the accreting matter is bright enough to stop it from falling into the black hole is called the Eddington limit, after Sir Arthur Stanley Eddington. With rare exception, we never see accretion disks glowing brighter than this; if there’s enough glow to cause that, it means more matter is flying outwards than inwards, so the disk dissipates and the glow subsides. (The Eddington limit is usually lower than the brightness required to completely counteract gravity. The radiation pressure has some help from the centrifugal force, as discussed above.)

This is also why accretion disk are thick. The force of gravity and the incredible spin of the black hole should flatten the disk out like a pizza crust, and to a good extent, it does. However, the light from the glow of the disk pushes the matter outward and puffs it up a little bit, so that it looks more like a slightly squished donut. (Accretion disks seem to fall into several categories of shape–some thicker, some thinner. The factors involved are an ongoing area of research, but radiation pressure is often important.)

Jets

In the case of rotating black holes, there’s another source of light, the so-called “jets.” The plasma physics of the disk accelerates the in-falling matter to enormous velocities, ultimately launching it into space around the poles of the black hole and along the axis of rotation. These incredibly powerful jets of matter, which glow for the same basic reasons of centrifugal force as accretion disks, are another reason black holes are easy to spot. They also allow matter in the accretion disk to bleed off its outward momentum enough to fall into the black hole.

Further Reading

What I’ve given you is a very simplistic introduction to a very rich and difficult topic. Accretion physics is still an active area of research. To truly understand what’s going on, we need to simulate what happens to the stuff in the accretion disk, taking fluid dynamics, electromagnetism, and general relativity into account. I’ve tried to find some non-technical resources.

• Universe Today has a podcast on accretion disks.
• NASA has a nice introductory article.

Questions? Comments? Insults?

I am by no means an expert on accretion physics, so I could have gotten something wrong here. If I have, please bring it to my attention! And if you have any questions, please bring those to my attention, too–I’ll do my best to answer them!