The Riddle of Black Holes
from Through the Wormhole; Take planet Earth and squeeze it down to the size of a marble. You'll create an object so dense that not even light, traveling at 186,000 miles per second, can escape its extraordinary gravitational pull. Its name, a black hole. Now, if it's more massive than a couple of times the mass of the sun, it will start to collapse even further. And there is no form of pressure that can resist this collapse. And it will continue to collapse down until it forms a black hole. Christian Ott and his theoretical-astrophysicist group at caltech are trying to discover whether exploding stars really do form black holes. There are two ways to find out whether black holes really form when stars blow up. A galactic supernova would provide us so much information, we wouldn't sleep for weeks. But after years of refining the physics and the math, he now thinks he may be the first to fully understand how a black hole is born. A black hole is the window into a world that we don't have the concept, we don't even have the mental architecture yet to be able to envision properly. You're in this strange world of strong gravity, where there are no straight lines anymore. You can't even see it. That is disturbing and exciting at the same time. The notion of a black hole is a natural extension of the laws of gravity. The closer you are to a massive object, the more the pull of its gravity slows down anything trying to escape from it. The surface of the Earth is 4,000 miles away from its center. So the force of gravity up here is not very strong. Christian Ott, an astrophysicist at the California institute of Technology, has been trying to understand how such strange entities as black holes might really form in the cosmos. It's not a bang but a whimper. Its name, not supernova, but unnova. It's basically just everything eventually sinks into a black hole, and the star slowly but surely just disappears. We have never seen an unnova. If Christian is right and black holes form silently, then these cosmic cannibals could be hidden in plain sight all around us, and we might never know it. Finding black holes is terribly difficult. Even if it wasn't black and would be radiating energy, it would still be only, let's say, 20 miles across. Astronomers began to wonder whether it might come from an object theorists had predicted but never detected, a black hole. The center of our galaxy is hidden from view by a thick veil of dust. A German astronomer, Reinhard Genzel, found a way to see through the fog. The problem is we are sitting in the Milky Way, and the galactic center is sort of just along the way through the entire plane of this big spiral galaxy we're sitting in. It's terrible at getting through the water vapor in Earth's atmosphere. Reinhard Genzel headed for the highest and driest place on Earth, the Atacama Desert of Chile. Beginning in 1992, he and his team at the Max Planck Institute began what would become an enduring campaign to find out exactly what was causing the strange noise at the center of the Milky Way. The black hole, you would think, is something, well, by definition, light can't escape from. Think of the solar system. There cannot really be any believable configuration which we know of other than the black hole. Reinhard Genzel had made the first definitive discovery of a black hole. But more than that, his team had found an object that must have swallowed millions of stars over its lifetime. Astronomers call it a supermassive black hole. The next was to figure out what goes on inside a black hole. What happens to stars, planets, even people if they get too close to this cosmic sinkhole? No telescope can ever see inside black holes. To understand how they twist reality, we have to stop looking and learn how to listen. Astronomers now believe almost every galaxy has a supermassive black hole at its core. Science fiction sees black holes as cosmic time machines or portals to a parallel universe. Astronomer Julie Comerford has been studying the centers of dozens of distant galaxies, trying to find signs of black holes, hoping to learn more about these mind-bending objects. It turns out that in all or nearly all galaxies, wherever we look, they have a central supermassive black hole at their heart. Supermassive ones are the ones that have masses of anywhere from a million to a billion times the mass of the sun. You can see a supermassive black hole when gas is falling onto it. And sort of right before the gas falls into it. When Julie investigates the glowing gas surrounding these giant black holes, she finds something totally unexpected. There's a cosmic dance going on on a scale that's almost unimaginable. You'd expect one from one black hole that's just sitting at rest in its galaxy, but we saw two peaks with different velocities. Julie began thinking about what would happen when two galaxies collide. And if both had black holes at their centers, what would happen to those massive objects? So, when two galaxies collide, the black holes at their center, instead of crashing in head-on, they begin this swirl, or dance. And the way that we can detect these waltzing black holes is by looking at the light that's emitted from them. So, for the black hole that's moving towards us, we detect light that is at smaller wavelengths, scrunched up together, so we see bluer light. And for the black hole that's moving away from us, we see stretched-out, longer-wavelength light that appears redder. So it's this redder and bluer light that is a telltale signature of a black-hole waltz. Every time we see it, we high-five in the observation room, and you just can't get over it. In galaxy after galaxy, black holes are paired up and dancing the cosmic night away. We identified 90 galaxies from when the universe was half its present age, and we found that fully 32 of them, or about a third, had black holes that exhibited this blue-and-red signature. So that was really surprising that such a high fraction of the black holes were not stationary at the center of the galaxy at all, that they were undergoing this waltz with another black hole. Scientists like Janna Levin believe the discovery of waltzing black holes opens up a whole new way to learn what's inside them, because their dance might not only be visible. The scientific visionary Albert Einstein saw space and time as a flexible material that could be distorted by gravity. A black hole is merely a very deep well in this material. When two black holes come close to one another, these two orbiting wells stir up space-time and send out ripples that can travel clear across the universe. And these waves will move out through the universe, traveling at the speed of light. So we can hope to not see black holes with light but maybe, in some sense, hear them if we can pick up the wobbling of the fabric of space-time itself. For the past several years, Janna and her colleagues have been trying to predict the sounds black holes make as they spin around one another. The calculations are not for the faint of heart. Modeling what happens when two giant objects create a storm in the sea of space-time takes some serious math and months of supercomputing. This is the orbit of a small black hole around a bigger black hole, and it's literally making a knocking sound on the drum, where the drum is space-time itself. Well, it really sounds like, sounds like a knocking. It starts to get a higher frequency and happen faster, until it falls into the big black hole and goes down the throat. And then the two will ring out together and form one black hole at the end of the day. Black holes stir up the space and time around them so much, the orbit of one black hole around another looks nothing like the orbit of Earth around the sun. An orbit can come in around a black hole and do an entire circle as it loops around before it moves out again. Black holes are like fundamental particles, and that's very surprising because they're huge, macroscopic objects. Right now, this idea is only a tantalizing hunch. But in just five years, super-sensitive detectors should be able to pick up the ripples in space created by two massive black holes spinning around one another. Stephen Hawking. The other began life as a plumber in the South Bronx and is now using black holes to develop the most revolutionary idea in physics since Albert Einstein, an idea that literally turns reality inside out. Black holes are the most massive objects in the universe. Around a black hole, there is an invisible line in space called the event horizon. Outside that line, the hole's gravity is just too weak to trap light. Inside it, nothing can escape its pull. If a pair of virtual particles fmed just outside the event horizon, then one of the pair might travel across that point of no return before being able to recombine, falling into the black hole and leaving its partner to escape as real radiation, Hawking radiation. If Hawking is right, black holes should not actually be black. They should shine ever so faintly. No one has ever detected Hawking radiation from the rim of a black hole. In fact, it's so faint, and black holes are so far away, that it will probably never be possible. Jeff Steinhauer thinks he's found a way to test Hawking's theory and send shock waves through the world of physics. He's the only person on the planet who has seen a black hole from up close. In fact, he's learned how to create one. My black hole is in no way dangerous. It's a sonic black hole that can only absorb sound waves. It's only made of 100,000 atoms, which is very little matter. And I'm sure that my neighbors would love that I would put a sonic black hole around my apartment, but it's not gonna happen. Leonard Susskind's fascination with black holes began 30 years ago when he listened to a talk by Stephen Hawking, a talk that triggered a violent reaction. I first heard Stephen Hawking give a lecture up in San Francisco, in which he made this extraordinary claim that black holes seem to violate the very fundamental principle of physics called conservation of information. Seven years after his groundbreaking work on black-hole radiation, Hawking had taken the idea to its logical conclusion. For every ounce of material a black hole absorbed into its core, it would radiate away an equivalent amount of energy from its event horizon. But since there is no physical link between the center of a black hole and its event horizon, the two processes could not share any information.
|
|
|
|
