Q1: Length contraction | Q2: Time dilation | Q3:Gravitational T.D. | Q4: Spaghettification | Q5: black hole ends |

Black Holes Before we talk about black holes, we need to make a digression into relativity. This is what Einstein is most famous for... There are two theories of relativity: Special Relativity and General Relativity. These two theories work together, and revolutionized the way that we view the world, almost as much as removing the Earth from the center of the solar system in the 1600s... Special Relativity: Special Relativity has two postulates: (a postulate is something that you assume is true, to see how things turn out. You know if the postulate is false because it leads to a contradiction later on. But you never know for SURE if your postulates are correct. Usually we invent postulates because we have an intuition for how things should turn out, or because they just make sense.)

- No reference frame is special. This means that the laws of physics should hold everywhere in the Universe, no matter what speed you are travelling. So, for example, gravity acts on you whether or not you are in your car (thankfully!), and while the value of gravity may be different in different places, such as on the moon, it still works in the same manner.
- The speed of light,
*c*, is a constant in all reference frames. This means that if you are travelling in your car at close to the speed of light, and you turn on the headlights, you will measure the light travelling at*c*. So will somebody watching you drive by! This is completely counter-intuitive. You expect that when you are travelling at some speed,*v*, and you throw something at a speed*t*, it will move away at*v*+*t*. But light doesn't work that way. Instead, it appears bluer when emitted in the direction you are travelling, and redder when it's emitted in the opposite direction.

Well, aside from the tidal forces, which would stretch you out into a long thin line and tear you apart and then twist you up into a big knot (this process is known as spaghettification!)---aside from that, as you fell into the black hole, not much would happen to you. You would be travelling, and you'd cross the event horizon, and you'd be travelling, and everything would be fine. But, because of gravitational time dilation, someone on the outside would see you slowing down. They observe your seconds getting longer... You approach the event horizon, approach the event horizon, keep approaching the event horizon, more and more slowly all the time, and never quite cross over. Crazy.

Now, there are a couple of ways to think about this. The first is simply observationally. Imagine that you are just outside of the region where the pair creation happens, so that you view the whole operation (inside the yellow line) as a single system:

As far as the observer is concerned, light (energy) is coming out of the black hole. In order for this to balance out, energy must be removed from inside the yellow line. If all of this happens at the event horizon, then energy is being removed from the black hole.

But that sounds fishy, right, so let's look at it from another point of view. Pair creation outside of the event horizon will leave an 'energy hole'. If the particles annihilate immediately, the hole is filled. But if one zips away, and the other zips into the black hole, then energy must come from somewhere to fill the hole. One way to think about it is that the energy is stolen from the gravitational field of the black hole, making the hole shallower. Thus the black hole effectively loses mass.

But again, this sounds like it's not quite right. But to get at what's really happening, we need some quantum mechanics. Usually, we are used to thinking about particles as having a discrete location. For example, an electron is AT point x on a line:

But in quantum mechanics, this is not the right way to think about it. Instead, we need to think about particles as having a

The particle is

So what does this have to do with our problem? Well, the energy hole that is left by the pair production actually gets filled by photons that 'tunnel' out of the black hole. This is the actual

Because the situation is not really symmetric. The pair creation always happens

We don't know! Our laws of physics break down completely! We don't even have any math for this part of the Universe. Nothing applies here. Cool, huh? Lots of people are working on this. It's all tied up with sub-fields of physics and astrophysics like quantum gravity, quantum field theory, etc., etc.

Also, we observe the centers of galaxies as huge sources of X-rays. We can look and see how fast stuff is rotating near the center, and that helps us to know the density in the very center, and it's too high to be anything but a black hole (that we know of). So we're pretty sure that the centers of a lot of galaxies, including our own, are large black holes.