With all the ruckus going on about Black Holes at the LHC, and my attempts to explain it all to my friends, I’ve realized that many people probably have little idea of what a black hole is… or worse, have misconceptions about them. I’d like to take a shot at explaining a bit about black holes.
You already know what happens when you clump bits of matter together. The combined gravitational pull of the matter will hold it together. The more matter you put on this thing, the bigger the gravitational force will be, and the more tightly it will hold itself together. You might have read my post about how space and time are curved. If you factor in this idea of matter curving spacetime then eventually you get a limit for the amount of mass you can pack into a certain volume. This volume is defined as a sphere of a certain radius called the Schwarzschild Radius. The size of this radius depends on the amount of mass in question.
So what is this so called Schwarzschild limit? Well, if you cram all of the mass involved inside a sphere with a radius equal to the Schwarzschild radius, then all this stuff you have crammed in must travel towards the center. I feel like I have to stress the word must here, because I don’t mean it in the same way that things must fall towards the center of the earth. Sure, things must fall towards the center of the earth, but eventually the thing will hit the ground, and the ground will stop it from moving any further. On the other hand, if something is falling inside this Schwarzschild radius, the object must move towards the center, and must keep moving! It can’t stop moving towards the center of the “sphere” in the same way that everything must travel slower than the speed of light. This means that if something is inside the Schwarzschild radius, regardless of what the thing is, regardless of how fast it’s going, regardless of what acceleration it has, it will always move and keep moving towards the center. In other words, all the stuff your cramming together is doomed to end up at a single point: the center of the sphere. So if you get a large amount of stuff, like a giant star, and this star begins to stop producing enough energy to counteract the pull of its own gravity, then the stuff might compress too much (within the Schwarzschild radius) and will be forced to compress to a single point. This is a Black Hole.
The term Black Hole has two different meanings. It can mean the single point where all of the mass has been compressed. This is also called the singularity (it’s a mathematical term). Black Hole can also mean “the sphere who’s radius is the Schwarzschild radius”. This is what people are talking about when they say things like “size of the Black Hole”. The size here means the Schwarzschild radius. It’s not the same idea as the “size” of the earth. When it comes to measuring the earth, we measure things like the radius that contains the earth’s matter. In a Black Hole, all of its mass is at a single point. So, to follow that same meaning of size we would have to say that the size of a Black Hole is zero. In other words, a Black Hole has infinite density.
So what would it be like to get up close to a Black Hole? Firstly, I should say that it is not the scary, cosmic vacuum cleaner as it is portrayed in some sci-fi. Planets, stars, and all things could happily orbit a Black Hole much like they orbit anything else. For example, if our sun suddenly became a Black Hole (which it can’t, it’s not massive enough), all of the planets, including the earth, would keep their same orbital trajectories, like nothing happened. Secondly, anything that falls into the Schwarzschild radius of a Black Hole (including light!!!) is subject to the same fate as the stuff it is made of, namely, it would be forced to fall towards the center, with no hope of escape (unless you consider Hawking Radiation an escape…). Lastly, theories that Black Holes are gateways to other universes can safely be ignored as speculation.
How would we observe a Black Hole if even light is doomed to fall into it? Well, we can infer the existence of a Black Hole (as we do with many things in astrophysics) by looking at effects of its gravitational influence. We can look for orbital trajectories of stars orbiting Supermassive Black Holes at the center of galaxies and see if they look like orbital trajectories things around a Black Hole might have. Recently there has been an effort to measure the mass of a certain Black Hole by looking at variations in temperature around galactic centers. There are tons of neat facts about Black Holes, so for more, visit your local Wikipedia page.
