Black holes are one of those objects that every member of the public seems to be interested in, and rightly so! Lots of weird things happen around black holes, and they are so hard to study that there are lots of theories as to what happens in and around black holes.

Several days ago I got an email question from Mike in Maryland
about black holes. I'll paraphrase the question:

Different astronomy books, both at the general public level and the college textbook level, describe black holes differently. Some say it has a certain radius, and some say a black hole is a singularity (an infinitely dense, infinitely small point). Which is it?

This is a very good question, and the answer is both simple yet very subtle ("subtle" is astronomer speak for "complex and something we don't really understand, so we sweep the details under the rug"). The simple part of the answer is that the two descriptions of black holes are talking about two different parts of the black hole: the event horizon and the material in the black hole itself.

The *event horizon* is the smallest part of the black hole we
could ever hope to see. At the event horizon, the "escape velocity,"
or the speed at which something needs to move to escape the black
hole, is the speed of light. Since nothing that we know of can go
faster than the speed of light, anything that gets inside the event
horizon stays there. Another way of thinking about it is that
anything that happens inside the event horizon is invisible to the
rest of the Universe.

The size of the event horizon depends on whether the black hole is spinning (most probably are), but is around 6 km (about 3.5 miles) across for every solar mass of material in the black hole. In other words, if we wanted to turn the sun into a black hole, we'd have to squeeze all of its matter into a ball less than 6 km across. Black holes that form from the explosions of big stars are between about 3 and 100 times the mass of the sun, and so their event horizons are 18 to 600 km (about 10 to 375 miles) in diameter. The black hole at the center of the Milky Way galaxy is thought to be about 6 million times the mass of the sun, so its event horizon is about 36 million kilometers, or 22 million miles, in diameter. If we were to put this black hole in the center of our solar system, it would easily fit inside the orbit of Mercury!

Since we can never see what happens inside of a black hole's event horizon, most astronomers are happy to say a black hole is 6km is diameter per solar mass. But this doesn't tell the full story. What happens to matter that falls inside the event horizon?

At present, we don't know of any physical force that can stop the
collapse of material inside of a black hole. So, it is theorized that
material is squeezed into an infinitely dense, infinitely small point
at the center of the black hole. This is called a
*singularity*. But singularities are nasty things.
Mathematically they are okay, but in the real world, we never see
things go to infinity. Some process always tends to show up that
keeps things from going to infinity. In the case of black holes, that
process may be quantum mechanics.

Quantum mechanics seems to be the process that rules things at atomic sizes. In quantum mechanics, the smallest size that you can talk about is called the "Plank length." The plank length is extremely small -- If a yardstick (meterstick) were the size of the Universe, then the Plank length would be many times smaller than a single atom. Despite being so incredibly tiny, the Plank length is still a far cry from being "infinitely small." So, many physicists suspect that a true singularity could never be smaller than the Plank length.

General Relativity, Einstein's theory that seems to describe gravity in the Universe, and the theory that astronomers use to describe black holes, does not include quantum mechanics. So, many physicists think that Einstein's theory cannot hold once you get down to the sizes of atoms. Many physicists are working on a quantum theory of gravity, but this is proving extremely difficult to work out. Given all of this, I think most astronomers would admit that we don't know what happens inside of a black hole's event horizon. Maybe there is a singularity down in there. Maybe there is a new form of matter that we don't know about, and stuff that falls into the black hole is transformed into this new matter. Or maybe space and time itself rip, and the material that falls in a black hole is transported somewhere else in our Universe or into another Universe. Or maybe the giant robot Maximillian is collecting matter to try and take over the Universe. We just don't know.

To summarize all of this, the answer to the above question is that
a black hole's *event horizon* has a known size, about six
kilometers in diameter for every solar mass of material in the black
hole. But the size and makeup of the lump of matter inside a black hole is
completely unknown at this point.

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