Gothos: Jillian's Guide to Black Holes

Jillian's Guide to Black Holes: Forming - Types - Outside - Inside - Finding - References - Websites

Accretion Disks

Artist's rendition of an accretion disk around a black hole.

The funny thing about black holes is that they're very rarely seen at all. There's usually all this stuff surrounding 'em. That makes sense, when you think about it. Black holes form either from stars or from a lot of matter being in one place. Old, large, black-hole-candidate-sized stars usually have a lot of their outer atmosphere floating around them when they collapse; and just because a black hole formed from some matter doesn't mean it used up all the matter in its formation. Besides, the universe is a very dirty place---all the stars you see and all of those you don't were formed by dust and gas. So. There is dust and gas around the black holes. I can imagine astronomers hold grudges against the dust in our own galaxy that prevents us from seeing the core of the milky way galaxy---so, don't underestimate the power of dust to conceal something interesting!

What, am I getting side-tracked? What does this dust have to do with anything, I can see you want to ask. This dust that happens to be around a black hole will eventually be disturbed by the black hole. I won't say that it will all fall in, for black holes aren't terribly efficient for "cleaning" up the universe. Still, there's all this dust revolving around the black hole. Not only is it revolving, but it is doing so at tremendous speeds! Why should it go so fast, and why shouldn't it just tumble in instead of forming yon great mucking disk? There's this thing in physics called gravitational energy. The higher up in a gravitational field, the more gravitational energy something has.

Take a bit of dust way above a black hole. It's got a lot of gravitational energy. Say that the piece of dust is starting to fall towards the black hole. It is now lower in a gravitational field, so it must have less gravitational energy. Where did that energy go? Well, some of the energy changed into kinetic energy, and the bit of dust sped up, which means it follows some kind of shrinking, erratic (precessing, I believe it's called), elliptical orbit. Some of the energy also went into heat. Hot objects radiate light, from a hot stove emitting infrared to a hot poker emitting visible light. The friction between the bit of dust and the other bits of dust around the black hole translates into more heat and more light being emitted. After a while the bit of dust starts to visibly glow. It falls some more, and soon it gets so hot that it emits x-rays. Astronomers have telescopes that scan the sky for x-ray sources and have found no few black holes that way.

So, that's why it goes so fast. But, what about the disk shape? Wouldn't it make sense for stuff to form a sphere around the black hole? Well, consider this: most black holes rotate just because it's very difficult to get things in this universe to stop moving. That and it's quite difficult to tell whether something is completely sessile or whether it's just stopped moving relative to you. It also has to do with the structure of rotating black holes. There's this ellipsoid (three dimensional oval) around the black hole called the ergosphere (the dark purple area in the picture to the right), and it connects with the black hole's outer event horizon at the poles of its axis of rotation. Got that image in your head? It's a lot like a squished orange on a stick: the peel billows out and connects with the pulp where the stick punctured the orange. Something like that, you see.

ANYway, in the area where the accretion disk forms is the ergosphere at its fullest. At the two poles of the black hole there is no ergosphere (that or it's too tiny to matter) and there is just the outer event horizon. Stuff in the ergosphere starts moving with the rotation of the black hole (it can't not move, that's the power of the ergosphere). So, the dust in the ergosphere moves faster than the stuff at the poles, which means it takes longer for it to cross the event horizon. Stuff at the poles crosses the event horizon sooner than it would have had it been at the equator. Naturally, a disk forms. Ta-dah!

What about those two jets?

With all the dust rushing around the black hole it's not surprising that a little static electricity might build up around the black hole, too. When an electric field moves, it generates a magnetic field. It's just one of those physics things. A magnetic field, so what? Black holes are tremendously powerful, and the field it generates is also powerful. Electrons that were going to fall into the black hole get caught up in this field. Another physics thing is that, whenever an electric field generates a magnetic field, the force of the magnetic field goes in directions perpendicular to the electric field. To use a popular teaching device from my circuits class, the Right-Hand Rule: curve the fingers of your right hand in the direction of the electric field, and the thumb of your right hand points in the direction of the magnetic field.

Those electrons that move in the magnetic field, they streak out along the axis of rotation, propelled to incredible speeds and energy by the magnetic field. They pick up so much energy that they become x-rays and sometimes even gamma rays! Gamma rays are the highest-power form light can take. The twin jets shooting out from the accretion disk are like insanely huge fountains of light whipped around and away from the black hole. Astronomers can look to those jets to lead 'em back to the black hole.

The energy from the accretion disk and the twin energy jets aren't the only thing a black hole radiates. There's also...

Hawking radiation

You can make a black hole out of anything, really, it would just take tremendous pressure. That's why the sun could never become one---it couldn't compress past the pressure of the electrons not wanting to be close to one another and then past the pressure of the neutrons not wanting to decompose. However, if you could compress the sun that much (IF!), it would shrink down to its Schwarzschild radius and become a small black hole. Small black holes such as this theoretical one have very strong tidal forces, recall. Famed astrophysicist Stephen Hawking knew these facts and more. He knew that space wasn't really empty: it was full of virtual particle and antiparticle pairs. He realized that the tidal forces of small black holes were strong enough to tear apart the pair, producing enough energy to make the virtual particle materialize in real space. According to this theory, black holes give off Hawking radiation of particles and antiparticles. What happens when the particles "form" inside the event horizon, aren't they trapped there? Well, not really. Due to quantum mechanics there is a certain probability that the particles can tunnel through to the outside universe.

Since black holes radiate particles, physicists can give 'em a temperature (albeit a very small one). This doesn't really matter for the large black holes (those with a mass of the earth²), for they do not radiate many particles and therefore have a temperature that is so frustratingly close to absolute zero that it's inconsequential³. Small black holes radiate more particles by comparison. Should mercury (the planet, not the element and not Sailor Mercury) *pwoof* into a black hole, it would have a temperature of about 10° Kelvin. Many people find this quite odd that a black hole, something thought of as only absorbing matter and never letting anything go, should shine particles like a star. I still think it's weird.

Black holes are forever The mortal black hole!

This radiation is the key to the black hole's mortality. As a black hole radiates particles and antiparticles, it loses mass. Eh? Well, the energy it took to materialize the virtual particle come from somewhere. The smaller the black hole gets, the more it radiates. The more it radiates, the smaller it gets. People like to call this process evaporation. Eventually, it will get so small that it releases massive amounts of energy and loses mass at an incredible rate. In the last second of the black hole's existence, it releases the same energy as billion one megaton hydrogen bombs would². After this I've been told that either the black hole ceases to exist or it leaves a particle with a mass of Planck mass (Planck's constant being 6.63x10^-34 and perhaps the unit of measure is grams); except, I forget where I read that bit of information, so it could be chimerical speculation (a fun phrase to say).

Black hole energy-generators: the wave of the future!

Don't laugh, it works much better than anything we've got now! Okay, there are two ways to generate energy with a black hole: dumping stuff into them and throwing stuff near them. Actually, you can only do the second with a rotating black hole; the first is applicable to any type of black hole. I consider dumping stuff into a black hole dangerous, for it only makes the black hole larger. Eventually, you would have to move your dumping station, or it would be swallowed by the black hole.

I should explain how the system works before I criticize it, I imagine. I won't go into painful detail about the workings of relativity, so you're going to have to accept my word on this (or go read up on the topic yourself to see if I'm right!). The formula for the energy released by an object that falls into a black hole is as follows:
E_released = (mass)(speed of light)²[1 - (1 - R_s/radius)^1/2]
It looks complicated enough, but it's kinda simple. I'm sure you've heard Einstein's famous equation E = mc². It just means that the energy of anything is just the mass times the square of the speed of light. The speed of light is a rather large number, 3 x 10⁸ to be precise. That means there's a lot of energy locked up in matter. Anyway, the energy unlocked by that matter as it falls in the black hole is mc² times a certain factor describing how close to the black hole the matter is.

It has to do with gravitational energy, which I explained when I was describing an accretion disk. So, the closer an object gets to a black hole, the more energy it releases. How much? Well, it can't release it all. How much energy it releases depends on the R_s/r factor. When the object is far away from the black hole, that factor is less than one. That means the energy released is only a certain percentage of mc². The closer an object gets to the black hole, the closer it gets to 100%.

Very nice, very mathematical, but how does that generate energy?? Imagine a really big conveyor belt attached at one end to a generator station (a station with big ol' rockets to keep it from moving closer to the black hole). At the other end the conveyor belt goes real close to the black hole (how close depends on how much energy you want to generate). Attached to this belt is an electric motor. What will we be dumping into the black hole? Trash. Nuclear waste. Those little tabs from aluminum soda cans. It doesn't matter, we just dump what we want to get rid of. Each time some trash falls into the black hole, the conveyor belt gives a mighty jerk that turns the generator and generates a staggering amount of energy. This process is much more efficient than using nuclear fission or even fusion and utterly dwarfs the energy generated by using fossil fuels. Very clean, very safe energy.

What if we don't want to make a black hole bigger by throwing stuff in it? Easy enough, that's the second way to get energy from a black hole. Imagine a rotating black hole. As I explained up above, there's this area outside of the event horizon called the ergosphere. Stuff in the ergosphere has to move with the rotation of the black hole. If you put a delicate glass statue in the ergosphere, it would start moving. Perchance, since it is glass after all, the statue breaks in half. Half falls closer towards the black hole and half flies out of the ergosphere with a more energy than when it came in. Where did the extra energy come from? What really happened was that the part that flew out stole rotational energy, kinda like kinetic energy but a little more complicated. That made the black hole spin a little slower, though. At least the event horizon didn't get any bigger.

Superradiance

You don't have to do this process with matter; it also works for light. Throw a light ray in the ergosphere with a certain energy (say, a visible light ray), and it comes out of the ergosphere with more energy (say, an x-ray ray). The extra energy came from the black hole---it rotated a little slower after this energy-theft, but it didn't slow down as much as it would have with the glass statue. At the price of a little rotational energy, you can amplify light rays. This process is called superradiance.

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