The Perfect Theory (21 page)

In the late 1940s a Russian physicist working in the United States, George Gamow, predicted the existence of a very cold bath of light permeating the universe. He started from the Abbé Lemaître's idea that the universe started in a hot, dense soup from which all the elements eventually emerged. The argument goes as follows: Imagine a universe in its simplest state, just full of hydrogen atoms. The hydrogen atom is the elementary building block of chemistry, a proton and an electron held together by electromagnetic force. If you bombard a hydrogen atom with enough energy, you can rip the electron away from its nucleus, leaving a lone proton floating in space.

Now imagine a gas of hydrogen atoms pushed together in a hot bath. They will collide, move around, and be bombarded by energetic photons, beams of light whizzing around. And the hotter they are, the more likely it is that the electrons will rip away from the protons. If the environment is very hot, very few hydrogen atoms will remain intact. Instead of a gas of hydrogen, the universe will be full of free protons and electrons. Early in the life of the universe, when the universe's temperature was greater than a few thousand degrees, you would find very few atoms and mostly free protons and electrons. As time passes and the universe cools, electrons stick to nuclei, leaving mostly hydrogen and helium atoms, an almost insignificant smattering of heavier elements, and a faint, almost invisible background of light. This is what Arno Penzias and Robert Wilson saw—clear evidence for a hot, dense state at early times. It was as close as one could get to proving the existence of a Big Bang, as Hoyle had disparagingly called it, and it would be another of Dennis Sciama's students, Stephen Hawking, who would take that final step.

There was something of Einstein in the young Hawking, and indeed his childhood friends would often call him that. He hadn't shone at school, and if anything he had been relaxed, playful, and naughty, a slight, untidy boy who delighted in entertaining his colleagues. Hawking had become increasingly interested in science and, on applying to Oxford, had aced the entrance exam and interview. He found Oxford ridiculously easy and had done well enough to impress his tutors and lecturers. It was at Cambridge as a PhD student, under Sciama's tutelage, that Hawking would be steered toward the cosmos and, finding his scientific voice, would spell out one significant consequence of Penzias and Wilson's discovery.

Stephen Hawking was a year older than Martin Rees and became fascinated by the mathematics of general relativity. Early on during his PhD studies, he had been diagnosed with Lou Gehrig's disease and given just a couple of years to live. The initial news had been profoundly demoralizing, yet two years into his PhD he was still alive and well. His continued health galvanized him to focus on his work and try to understand what actually happened at the beginning of the universe's expansion—at the Big Bang itself. Could it be that the singularities were inevitable at the beginning of time as well as in Wheeler's final state?

As he raced against the possible onset of his illness, Hawking was able to show that, indeed, an expanding universe under normal conditions should have inevitably started off with a singularity. Over the years, he proved, with a South African physicist and fellow talented Sciama student named George Ellis, that a universe with relic radiation like that found by Penzias and Wilson must have started in a singular state. Finally, with Roger Penrose, he constructed a complete set of theorems that covered almost any possible model of an expanding universe that could be cooked up at the time. Singularities were inevitable, or so Penrose and Hawking's math seemed to say, both in the future as well as in the past.

 

At the first Texas Symposium, there had been speculation that the distant, copious sources of radio waves in Ryle's catalogue might somehow be related to the general relativistic collapse of supermassive stars. Chandra had once pointed out that superheavy white dwarfs would be unstable and might implode, and Oppenheimer and Snyder had shown that if stars were even heavier, the next stage in the inexorable collapse would be via neutron stars. While there was pretty convincing evidence for white dwarfs, there was no sign of neutron stars. That changed in 1965, when Jocelyn Bell arrived in Cambridge to start her PhD in Martin Ryle's group.

Bell didn't work with Ryle himself but with one of his more junior colleagues, Antony Hewish. Hewish had her build a radio telescope out of an assortment of wooden posts and chicken wire that she could use to pinpoint and study the position of quasars at 81.5 megahertz. As she puts it, her
“first couple of years involved a lot of very heavy work in the field, or in a very cold shed.” But the job had its perks: “When I left I could swing a sledge hammer.” By 1967, Bell was taking data on a chart recorder, analyzing over 30 meters of chart paper a day, looking for the telltale signals of quasars. About 120 meters of paper would cover the whole sky.

There was something odd in the recording she was making. For each 120 meters of paper, there was a quarter-inch spike of data that Bell couldn't understand. She couldn't figure out what the signal was or where it was coming from. It was undoubtedly there, a set of chirps in a very specific direction of the sky. “We had begun nicknaming it ‘little green men,'” Bell recalls. “I went home feeling very fed up.” The team decided to go ahead and publish their mysterious finding.

In February 1968, a paper appeared in
Nature
titled “Observation of a Rapidly Pulsating Radio Source.” In it, Bell, Hewish, and their coauthors announced their discovery, declaring, “Unusual signals from pulsating radio sources have been recorded at the Mullard Radio Astronomy Observatory,” and then went on to make a bold claim: “The radiation seems to come from local objects within the galaxy, and may be associated with oscillations of white dwarf or neutron stars.” They speculated that the spikes in the chart paper were the oscillations or pulsations in these dense, compact radio sources.

The press took to the discovery, interviewing Hewish about its importance. But, as Bell recalls, “journalists were asking relevant questions like was I taller than or not quite as tall as Princess Margaret.” She says,
“They'd turn to me and ask me what my vital statistics were or about how many boyfriends I had . . . that was all women were for.” The
Sun
headlined the news piece with “The Girl Who Spotted the Little Green Men.” It was the
Daily Telegraph
that came up with a name for the outlandish objects; a journalist suggested calling the objects “pulsars,” short for “pulsating radio stars.”

Yet again, radio astronomy had delivered in spades, and yet again, it was by chance. The discovery was momentous, and in 1974 the Nobel Prize was awarded to Bell's supervisors, Tony Hewish and Martin Ryle. Bell was left out entirely, in what is seen by many as one of the greatest injustices in the history of the prize. Almost twenty years later, Bell attended the prize ceremony as the guest of another astronomer, Joseph Taylor Jr., when he won the Nobel Prize in 1993. “I did get to go in the end,” she recalls without any bitterness.

Pulsars were the first tangible evidence for neutron stars. They don't actually pulsate—they rotate, which causes them to emit a periodic signal. But they were the fabled missing link in gravitational collapse, posited by Landau, studied by Oppenheimer, and explored in meticulous detail by Wheeler and his disciples. They were the final step before the formation of Penrose's inevitable singularities.

 

When Yakov Zel'dovich switched fields, he did so fearlessly. One of his students recalls Zel'dovich's advice: “It is difficult, but interesting to master ten percent of . . . any field. . . . The path from ten to ninety percent is pure pleasure and genuine creativity. . . . To go through the next nine percent is infinitely difficult, and far from everyone's ability. . . . The last percent is hopeless,” from which Zel'dovich concluded, “It is more reasonable to switch to a new problem before it is too late.”

Like Wheeler, Zel'dovich turned from nuclear research to relativity in his forties, and he went on to set up one of the most focused research groups in the world. The papers that Zel'dovich wrote with his students were almost impressionistic, often with quirky openings such as “The Godfather of psychoanalysis Professor Sigmund Freud taught us that the behavior of adults depends on their early childhood experiences. In the same spirit, the problem is to derive the . . . present . . . structure of the universe . . . from . . . its early behavior.” They read like condensed essays, with a smattering of equations, just enough to flesh out his point. When translated into English, they could be difficult to decipher. But over time they were appreciated for what they were: veritable gems of relativistic astrophysics.

When Zel'dovich switched fields, he went looking for frozen stars, as Schwarzschild and Kerr's collapsing stars were called in the East. These frozen stars are invisible, emit no light, and have no surface that can reflect or shine. Yet Zel'dovich couldn't accept that these strange objects would be hidden from view, for they were dramatic, distorting space and time about them. In fact, as he began to discuss with his students, they should exert an inexorable pull on anything that gets near them. And so, he surmised, by looking at the effect of the frozen stars on other things, it just might be possible to see them, not directly but indirectly. For example, if the sun got too close to a frozen star, it would be forced to orbit around it, much like the moon around the Earth. The frozen star would be invisible, so the sun would look as if it were dancing around on its own, wobbling about in a strange orbit with no center. Look for wobbling stars, Zel'dovich and his team proposed: stars that appear to be on their own but behave like half of a binary system.

But, Zel'dovich conjectured, frozen stars shouldn't just nudge their partners around; they should positively rip them apart. He made a very simple assumption: as stuff falls into the gravitational field of a frozen star, it should approach the speed of light, condensing and heating up in the process. As the material mixes and collides, heating up as it falls onto the frozen star in a process that has been dubbed accretion, it radiates energy. The accretion near the Schwarzschild horizon is so efficient it can emit up to 10 percent of its rest mass energy, an astounding amount of energy that makes it the most efficient energy-generating process in the universe. And so, in a short paper published in
Doklady Akademii Nauk
in 1964, Zel'dovich went on to speculate that the production of energy around a frozen star would be overwhelming, enough to explain the intensely bright quasars that were being found by radio astronomers. At exactly the same time, an American astronomer at Cornell University, Edwin Salpeter, was coming to the same conclusion, that copious radio emissions could come from a massive object that weighed more than a million solar masses or, as he put it,
“extremely massive objects of relatively small size.”

Zel'dovich didn't stop there. With his young colleague Igor Novikov he applied his argument to binary systems such as a normal star circling a frozen star. They speculated that the immense gravitational pull of the frozen star would strip the outer layers of the normal star of all its gas and fuel. It is like, as Roger Penrose once put it, “having to drain . . . a bath the size of Loch Lomond through a normal size plughole.” The forces that the gas would experience would be so tremendous that copious amounts of light at very high energy, known as x-rays, would be emitted. Look for the x-rays, Zel'dovich and his pupils told the world.

The name Schwarzschild was constantly popping up in scientific articles by astronomers and astrophysicists as the link between collapsed or frozen stars and quasars became more and more compelling. But, as Wheeler recalled years later, the name that he and his colleagues in the United States were using—“completely collapsed gravitational object”—was cumbersome, and “after you get around to saying that about ten times, you look desperately for something better.” At a conference in Baltimore, in 1967, a member of the audience helped him out and proposed the term
black hole.
Wheeler adopted it, and the name stuck.

In 1969 one of Dennis Sciama's colleagues at Cambridge, Donald Lynden-Bell, stated in the introduction to one of his papers, “We would be wrong to conclude that such massive objects in spacetime should be unobservable, however. It is my thesis that we have been observing them indirectly for many years.” He argued that massive black holes at the center of galaxies would suck in the surrounding material just as Penrose had described it, like water falling down a drain, gurgling around and around. The rotating gas around the hole would form a flat disk, just like Saturn's rings, and the whole system would be locked spinning on its axis. The nuclei of galaxies, fueled by these accretion disks, would then be veritable beacons of light, and Lynden-Bell could show how the energy was created and emitted. Martin Rees had also, with Dennis Sciama, set to work trying to build detailed models of quasars that could explain all the different strange properties—their size, their distances, how quickly they would flicker and pulsate, and what ranges of energy would be pumped out. Over the next few years, Rees, with Lynden-Bell and their students and postdocs at Cambridge, were able to come up with a beautiful, meticulous model of the fireworks surrounding quasars and radio sources. All the pieces were falling into place.

And then, finally, Zel'dovich and Novikov's x-rays started trickling in. Starting in the 1960s, a team led by the Italian physicist Riccardo Giaccone sent rockets up out of Earth's atmosphere where, for a few minutes, they would look for x-rays. They found them, bright spots of x-rays spread across the sky that outshone the planets in the solar system. In the early 1970s the
Uhuru
satellite was launched from a platform near Mombasa in Kenya with the sole goal of mapping out the x-ray sky. It was a resounding success, making exquisite measurements of over three hundred x-ray objects.