FALLING IN

Muse, May/Jun 2004 by Folger, Tim

Andrew Hamilton probably knows more than anyone about what it would be like to fall into a black hole. Not that he has ever been inside one himself; the nearest black hole is about 15 light-years from Earth. And even in the distant future it's unlikely that explorers will ever dare enter one. As Hamilton will quickly tell you, a voyage into a black hole would be a one-way trip. Whatever falls in-even light-never comes out. "Once you're inside, that's it," he says. "You're doomed."

Hamilton is an astrophysicist, and black holes are his specialty. From the safety of his office at the University of Colorado he can describe in detail what no astronaut would ever live to tell about: a close encounter with a black hole, the strangest object in the universe. So hang on for a guided tour into a realm where gravity is so strong it warps not only space but time as well.

First, says Hamilton, you would have to be very careful to pick the proper sort of black hole to explore. Black holes come in many different sizes. Most are only a few miles wide. Some, like the enormous black hole that lies at the center or our galaxy, measure millions of miles across, and contain more mass than 3 million suns.

Scientists aren't sure how the giant black holes form, but the smaller ones are born when huge stars burn out and die. Most stars, including our sun, aren't big enough to turn into black holes at the end of their lives. A star must be at least 20 or 30 times larger than the sun to form a black hole. Once such a giant star M has burned all its fuel it caves in on itself, leaving behind a black hole with a gravitational pull so strong that nothing can escape its grasp.

The most dangerous black holes to visit-those that kill you before you even get inside them-are the small ones. If you tried to approach a small black hole, say one about two miles wide, you would be stretched like a rubber band and then ripped apart while still outside it. What would kill you is not the strength of the black hole's gravity, but the difference in the gravity's pull on your head and feet. Near a small black hole, that difference would be enormous, strong enough to stretch and break steel-or bones. But near a large black hole, one that's thousands of miles in diameter, the pull of gravity doesn't change much over the length of something as small as a human body.

So you could remain in one piece, for a little while anyway, if you happened upon a black hole of the right size.

A good choice would be the monster that astronomers have found at the heart of our galaxy, a gash in space 10 times as wide as the sun that has probably swallowed thousands of stars. Here's what would occur if you somehow accidentally became trapped by that black hole's gravity.

Odd things would start to happen even before you reached the edge of the black hole. If the crew of a nearby spaceship happened to be witnessing the terrible accident, they'd see you change color and become redder and dimmer, says Hamilton.

You'd become redder because the black hole's immense gravity actually stretches the light waves that reflect off your body, and longer waves are redder. If the light waves were stretched enough, no one would be able to see them any longer. Even before you reached the black hole, you would fade away, becoming invisible. So the crew on the spaceship would never actually see you enter the black hole.

Meanwhile you would be seeing some pretty strange things, too. Before falling into the black hole you would pass a ring of light trapped in an orbit around the hole. Spacetime is so warped here that light travels in a circle instead of the usual straight line. What difference would that make? While crossing the ring you would be able to see the back of your own head!

If you could somehow remain suspended above the black hole-if you had a powerful jet pack with an inexhaustible supply of fuel-the entire future of the universe would unfold before your eyes. "You'd have to be firing your rockets like crazy just to stay put," says Hamilton. But the black hole's gravity would distort spacetime in such a way that "the universe would then appear speeded up and concentrated in a tiny piece of the sky just above you."

Unfortunately, no fuel supply is inexhaustible, so you would continue to plummet downward. Strangely enough, you wouldn't notice anything when you actually entered the hole. Its outermost edge-which physicists call the event horizon-isn't a solid surface. It is just the boundary of no return. The black hole and even the stars behind it would still seem to be in front of you after you fell in. The only clue you would have that you had crossed a fatal boundary would be radio silence: your transmissions, trapped with you, would no longer be getting through to the crew aboard the spaceship.

Once across the event horizon, you'd have no more than an hour to live. Eventually you'd reach the very center of the black hole, the singularity, a point of infinite density where all known laws of physics break down. Some physicists think the throat of a black hole may open out into a mirror-image throat connected to another universe. No one knows for sure. But even if a new world lies at the bottom of a black hole you would probably never see it. No matter how big the black hole, you would be stretched and shredded.