Black holes for beginners part II
So what is a black hole?
A black hole is an object whose escape velocity is faster than light. If Earth were crushed into a ball only 1.8 cm across, it's escape velocity would be the speed of light, about 300 000 kilometres per second, or 67 million miles per hour. Because nothing can travel faster than light, nothing would be able to escape. The Earth would have become a black hole.
It's difficult to explain why here, but the Earth would also collapse into a point with no dimensions. That is, its diameter is zero. It would be smaller than an atom, or a proton, or an electron, or anything. It would have no size and infinite density. This is a black hole.
So what is a black hole like?
If the sun suddenly became half the size it was, what would happen to Earth's orbit? Nothing, because half the sun would be getting closer to us and half would be getting further away, so we would still feel the same overall attraction due to gravity.
Suppose you were standing on the surface of the Earth when it has a diameter of 1.8 cm. This means you are 9 mm from the centre of the Earth. Now, suppose the Earth collapse like I have already said, to a single point. what happens to the amount of gravitational attraction you feel from the Earth? It doesn't change. Escape velocity from this point is still the speed of light.
This means that there is a region of space around the black hole which was Earth that nothing can escape from, not even light. This is called the 'event horizon'. If you fall inside the event horizon, you have no hope of getting out. You have fallen into the black hole.
Can the sun collapse into a black hole?
No. When we were crushing the Earth, it took a large force to crush it. The stages the Earth went through to become a black hole were similar to the stages a star must go through. The force is the star's own gravity. Remember the stage where the electrons are crushed into the nucleus to form neutrons and it ends up with a density of 2 billion tons per square centimeter? That is called a neutron star.
Our sun will eventually end up as a neutron star. It takes a lot of force to crush a neutron star into a black hole, and the gravity of our sun is simply not strong enough to do that. To collapse into a black hole, a star must have a mass that is at least 1.44 times the mass of the sun (known as the Chandrasekhar limit).
If we did somehow manage to collapse the sun into a black hole, Earth's orbit would be unaffected. There would be the same amount of mass (nothing has been lost or gained) and it would the same average distance from the Earth, so the Earth would feel exactly the same gravitational pull, and so would orbit in exactly the same way (it is gravity that holds Earth in its orbit), albeit it would get very cold and baked in X-rays.