Zero - Charles Seife [69]
As pressure in the star increases, the exclusion principle states that electrons inside must move faster and faster to avoid one another. But there’s a speed limit: electrons cannot move faster than the speed of light, so if you put enough pressure on a lump of matter, electrons cannot move fast enough to stop the matter from collapsing. Chandrasekhar showed that a collapsing star that has about 1.4 times the mass of our sun will have enough gravity to overwhelm the Pauli exclusion principle. Above this Chandrasekhar limit a star’s gravity will pull on itself so strongly that electrons can’t stop its collapse. The force of gravity is so great that the star’s electrons give up their struggle once and for all; the electrons smash into the star’s protons, creating neutrons. The massive star winds up being a gigantic ball of neutrons: a neutron star.
Further calculations showed that when collapsing stars are a little more massive than the Chandrasekhar limit, the pressure of the resulting neutrons—similar to the pressure of electrons—can stave off collapse for a little while; this is what happens in a neutron star. At this point, the star is so dense that every teaspoon weighs hundreds of millions of tons. There is a limit, though, to even the pressure that neutrons can bear. Some astrophysicists believe that a little more squeezing makes the neutrons break down into their component quarks, creating a quark star. But that is the last stronghold. After that, all hell breaks loose.
When an extremely massive star collapses, it disappears. The gravitational attraction is so great that physicists know of no force in the universe that can stop its collapse—not the repulsion of its electrons, not the pressure of neutron against neutron or quark against quark—nothing. The dying star gets smaller and smaller and smaller. Then…zero. The star crams itself into zero space. This is a black hole, an object so paradoxical that some scientists believe that black holes can be used to travel faster than light—and backward in time.
The key to a black hole’s strange properties is the way it curves space-time. A black hole takes up no space at all, but it still has mass. Since the black hole has mass, it causes space-time to curve. Normally, this would not cause a problem. As you approach a heavy star, the curvature gets greater and greater, but once you have passed the outer edge of the star itself, the curvature decreases again, bottoming out at the center of the star. In contrast, a black hole is a point. It takes up zero space, so there is no outer edge, no place where space begins to flatten out again. The curvature of space gets greater and greater as you approach a black hole, and it never bottoms out. The curvature goes off to infinity because the black hole takes up zero space; the star has torn a hole in space-time (Figure 52). The zero of a black hole is a singularity, an open wound in the fabric of the universe.
This is a very troublesome concept. The smooth, continuous fabric of space-time might have tears in it, and nobody knows quite what happens in the region of those tears. Einstein was so disturbed by the idea of singularities that he denied the existence of black holes. He was wrong; black holes do exist. However, the singularity of a black hole is so ugly, so dangerous, that nature tries to shield it, preventing anyone from seeing the zero at the center of a black hole and returning to tell the tale. Nature has a “cosmic censor.”
Figure 52: Unlike other stars, a black hole tears a hole in space-time.
The censor is gravity itself. If you toss