Black holes for beginners part I
What are they?
I'll start with what they aren't.
What aren't they?
- Cosmic vacuum cleaners – black holes do not suck material up. Sucking implies a difference in pressure. There is no air in space.
- Tunnels to other universes – black holes lead no-where. What happens when you fall into a black hole will be discussed later.
- Giant swirling purple things – not swirling in the usual sense, and definitely not purple. They are black, hence the name.
An introduction to gravity.
What goes up must come down, right? Wrong. There is something called the 'escape velocity'. What this is is that if I stood on Earth's surface and threw a ball up, it will never come down provided I throw it faster than Earth's escape velocity (about 11.2 kilometres per second, or 25 000 miles per hour). If I constructed some giant cannon to fire a ball faster than this upwards, it would carry on forever.
What does escape velocity depend on? It depends on the mass of the Earth. The bigger the Earth, the faster you have to go to escape the pull of gravity (and never come down). It also depends on the size of the Earth, though.
The smaller the Earth is, the closer you are to most of the stuff the Earth is made of. If you halve the size of the Earth, you're twice as close to the opposite side when you stand on it, right? You're also twice as close to the centre, where most of the matter that makes up the Earth is.
This means gravity acts more strongly on you (the closer you are to something, the stronger the pull of gravity). A smaller Earth (but one weighs the same i.e. has the same mass) would have a stronger pull of gravity and so a bigger escape velocity.
Where does a black hole fit into this?
Suppose I were to take the Earth and crush it so it becomes smaller and smaller (but still weighs the same). It would be very difficult, that's for sure. For a long time, not a lot would happen. Eventually, I would run into some more serious difficulty. All the atoms in the Earth would be essentially touching each other. I would have to force the electrons closer to the centre of the atom. Eventually, the electrons would be forced into the centre of the atom where they would combine with protons to form neutrons. You would end up with a mass of neutrons, so dense that a lump the size of a sugar cube would weigh 2 billion tons! The Earth would have become a neutron star.
Nothing in the universe can travel faster than light. Einstein showed us this. So how small would the Earth have to be for the escape velocity to be faster than light?