Black holes are definitely some of the most mysterious things in the universe. A hole that looks the same from all sides is quite a puzzle!
If I were to go up into the attic and saw a circular hole around me, I would fall down into the living room. Likewise, if there were a hole in my living room floor, I might keep falling down into the basement. This is easy to imagine. It is not too much harder to picture gravity reversing, so that I would fall up from the basement through the hole in the living room floor, ending up back on the couch where I started. In each of the three places I had been – the living room, the attic, and the basement – I could look through (or fall through) a hole to another space.
Before I start talking about black holes, I should say that everything we know about them is based on math. Until 1971, there was no observed evidence that they even existed. Today, there are many known black holes, but we will never see one of them directly, so all our knowledge of black holes is based on measurements of things happening in space that cannot be explained without the math model of a black hole. Most of my readers are not math professors, so you may be wondering what I mean by “math model”. If I tell you that I am thinking of an object that is 5 cm long, 5 cm wide, and 5 cm tall, you could guess that it is a cube. But what if it is a sphere? All you really know is how big it is. Now, if I tell you that the object has six square sides, you know it is a cube. You can picture it in your mind. There is no real cube, but your image of it is based on numbers I gave you. The cube in your mind is a math model.
Remember the holes in my ceiling and floor? Now imagine a hole in the middle of empty space. It is not a hole in a wall or anything else; it is a hole in space. You might be able to see the hole if you got close enough, but you would not be able to see through it. If you moved in a big circle around the hole, it would look the same from any angle! If you were to throw an object – like a marshmallow – through a hole in a wall, you could look over the wall (or through the hole) and see the marshmallow on the other side. But if you threw a marshmallow into a black hole, there is no other side. It is gone forever!
How can this be? About 250 years ago, a scientist named John Michell imagined a thing nobody had thought of before.
(Scientists of his time already knew that objects had to reach a certain speed to escape the gravity of any planet or star; this speed is called the “escape velocity”. The more massive a planet or star is, the faster an object has to move to escape out into space. This is why a rocket can get to the moon, but a bullet from a gun cannot. It is not fast enough. A rocket fast enough to escape the Earth’s gravity would still not be able to escape the Sun, because the Sun is so much more massive than the Earth. You would need a much faster rocket. On the other hand, if you have seen pictures of the Apollo missions to the Moon, the rocket they used to get off the Moon was not very fast at all. It didn’t have to be; the Moon is much less massive than the Earth, so its gravity is much weaker.)
John Michell imagined a star so massive that even light would not have enough speed to escape its gravity. If the light from the star could not escape out into space, then nobody would be able to see it! John Michell called his imaginary star a “dark star”. Later scientists, including Albert Einstein and Karl Schwarzschild, made math models of dark stars to describe the behavior of light and of objects that got close to them. It was a scary but fascinating idea! In 1964, a journalist named Ann Ewing wrote a report about these math models. The report was called “Black Holes In Space”. Since then, people have been calling them “black holes”.
Even though many scientists made lots of math models of black holes, nobody had ever seen one. They are, after all, invisible! Between 1971 and 1973, a team of astronomers watched a giant star far out in space. It behaved unlike any other star. By 1973, they knew from their measurements that the star had a black hole next to it, just like you knew (after getting enough information) that the object I was describing was a cube. The star system fit the math model: it could only be a black hole!
That first observed black hole is called Cygnus X-1. Since then, we have observed many other black holes. Although they are invisible, we know they are black holes from observing what happens around them. If it fits the math model, it must be a black hole.
So what is the answer to the question we started with? If you fell into a black hole, where would you go? By now, you know that black holes are not really holes at all. They are objects with such strong gravity that nothing, not even light, can escape them, which is why they are perfectly dark. Because its gravity is so strong, anything that falls into a black hole is crushed into zero volume. Since one of the properties of matter is that it takes up space, objects falling into a black hole would really not even be objects anymore. The center of a black hole is called a “singularity”, and the math model for a singularity seems to break the rules for what we know about the universe.
Black holes may always be one of the universe’s mysteries!