Zero - Charles Seife [70]
Even though a black hole is a star, none of the light it shines ever escapes past the event horizon; that’s why it’s black. The only way to view a black hole’s singularity is to go beyond the event horizon and see for yourself. However, even if you had an impossibly strong spacesuit that kept you from being stretched into a piece of astronaut spaghetti, you could never tell anybody about what you saw. Once you pass the event horizon, signals you broadcast can’t escape the black hole’s pull—neither can you. Traveling beyond the edge of the event horizon is like stepping off the edge of the universe. You will never return. This is the power of the cosmic censor.
Even though nature tries to shield the singularities of black holes, scientists know that black holes exist. In the direction of the constellation Sagittarius, at the very center of our galaxy, sits a supermassive black hole that weighs as much as two-and-a-half million suns. Astronomers have watched stars dance around an invisible partner; the stars’ motions reveal the presence of the black hole even if the black hole is not visible. However, though scientists can detect black holes, they still haven’t spotted the zeros at their centers, since the ugly singularities are shielded by the event horizon.
This is a good thing. If there were no event horizon, no cosmic censor that shields the singularity from the rest of the universe, very strange things might happen. In theory, a naked singularity with no event horizon might allow you to travel faster than light or backward in time. This could be done with a structure known as a wormhole.
Back in the rubber-sheet analogy, a singularity is a point of infinite curvature; it is a hole in the fabric of space and time. Under certain circumstances that hole can be stretched out. For instance, if a black hole is spinning or has an electric charge, mathematicians have calculated that the singularity is not a point—a pinpoint hole in space-time—but a ring. Physicists have speculated that two of these stretched-out singularities might be linked with a tunnel: a wormhole (Figure 53). A person who travels through a wormhole will emerge at another point in space—and perhaps in time. Just as worm-holes can, in theory, send you halfway across the universe in the blink of an eye, they can send you backward and forward in time (see appendix E). You might even be able to track down your mother and kill her before she meets your father, preventing you from being born and causing a terrible paradox.
A wormhole is a paradox caused by a zero in the equations of general relativity. Nobody truly knows whether or not wormholes exist—but NASA is hoping that they do.
Figure 53: A wormhole
Something for Nothing?
There’s no such thing as a free lunch.
—“THE SECOND LAW OF THERMODYNAMICS”
NASA hopes that zero might hold the secret to traveling to distant stars. In 1998, NASA held a symposium entitled Physics for the Third Millennium, where scientists debated the merits of wormholes, warp drives, vacuum-energy engines, and other far-out ideas.
The problem with space travel is that there is nothing to push against. When you swim through a pool, you push against the water, forcing it backward and pushing you forward. When you walk on the ground, your feet are pushing against the floor, providing the force to drive you forward. In space, there is nothing