Also see "Being Stephen Hawking," the profile of Stephen Hawking by former

Nature editor John Maddox.

Two decades after rocketing to scientific stardom with his book A Brief History of Time, Stephen Hawking still knows how to make an entrance. On a mild March evening in Pasadena, California, 4,500 people fill the convention center to hear him give a talk called “Why We Should Go Into Space.” Shortly after 8 p.m. the lights dim, a few thousand conversations stop, and the soaring trumpet fanfare from Richard Strauss’s Also Sprach Zarathustra (better known as the theme from 2001: A Space Odyssey) fills the room. Hawking is in the house. The crowd turns to watch the frail physicist being wheeled at a good clip down the center aisle. He is wearing a charcoal gray suit and an open-neck white shirt; his head slumps toward his right shoulder; his hands are folded neatly in his lap. The music segues to The Blue Danube Waltz as he rolls up a ramp to the stage.

Hawking sits silently for a few moments, alone at center stage, before a member of his Cambridge, England, posse appears. Sam Blackburn, a graduate student who manages the beeping, bulky communications complex that is Hawking’s wheelchair, runs over and makes a few adjustments to his boss’s Lenovo ThinkPad X61 laptop. The iconic synthesized voice kicks in. “Can you hear me?” Hawking asks. The crowd cheers.

That cheer follows Hawking wherever he goes, in part because he shouldn’t even be here. In 1963, while a graduate student at the University of Cambridge, he was told he had amyotrophic lateral sclerosis, or Lou Gehrig’s disease, a diagnosis that usually proves fatal within five years. Yet he pressed on, becoming one of the world’s leading physicists and a best-selling author. He has plumbed the depths of black holes and sought to explain the beginning of time. He has been a guest on Star Trek: The Next Generation and has met the Dallas Cowboys cheerleaders. He has been married twice, has three children (“The disease only affects voluntary muscle,” he reportedly once commented), and is now a grandfather. Judith Croasdell, Hawking’s ruthlessly devoted personal assistant, says his schedule is fully booked through 2012. Oh, yes—and he’s also working on a new theory about the origin of the universe.

Hawking’s latest work has received mixed reviews. Some of his colleagues compare it unfavorably with the achievements that made his reputation. At the beginning of his career, in the late 1960s, Hawking proved that the universe must have started from a singularity, a point of infinite density. A few years later he found that black holes gradually shrink, fade, and die—a paradigm-shifting insight with thorny theoretical consequences that physicists are still pondering. His subsequent ideas have stirred controversy and skepticism, however, and Hawking’s influence on cosmology and physics is clearly not what it once was. In the popular realm, too, his star has dimmed. As the Pasadena event testifies, Hawking can still pack a room, but he has lost much of his iconic status. None of his books since Brief History have come close to its runaway success. The master of black holes is himself becoming steadily less visible.

Late last year reports circulated that Hawking would be retiring from Cambridge in 2009 and that he might even leave England to join the Perimeter Institute, an innovative research center just outside Toronto. Hawking, Croasdell assured me, will neither be retiring nor abandoning Cambridge, but this year will bring a significant transition. On September 30 he will relinquish his prestigious post as the Lucasian Professor of Mathematics at Cambridge, a position once occupied by Isaac Newton, which Hawking has held since 1979. He turned 67 on January 8, the maximum age for holding the Lucasian chair, and so will continue at the university with a new title, Emeritus Lucasian Professor.

No one should have believed the rumors. Retiring is no more of an option for Hawking than ceasing to think would be. What he is reaching for now is closer to the opposite: a defense and extension of the kind of sweeping cosmological ideas that first shot him to fame. In other words, a capstone to his career—or, to be blunt, a comeback.

It is challenging for any academic in his sixties to reclaim the aura of youthful genius, and especially so for a man whom the popular media routinely likened to Albert Einstein. And then there is Hawking’s most formidable antagonist: his own withered body. “I imagine it has become very difficult for him to work, and that’s been the major cause of his being out of the game, so to speak,” says Leonard Susskind, a theoretical physicist at Stanford University. “In the last number of years, he has been so incapacitated that it has been very difficult for him to keep up with what is happening in the field.”

Nevertheless, Hawking continues his almost ludicrously grand program. “My goal is simple,” he famously explained. “It is a complete understanding of the universe, why it is as it is, and why it exists at all.”

Into the Abyss As an undergraduate at Oxford University, Stephen William Hawking was a wise guy, a provocateur. He was popular, a lively coxswain for the crew team. Physics came easy. He slept through lectures, seldom studied, and criticized his professors. That all changed when he started graduate school at Cambridge in 1962 and subsequently learned that he had only a few years to live.

The symptoms first appeared while Hawking was still at Oxford. He could not row a scull as easily as he once had; he took a few bad, clumsy falls. A college doctor told him not to drink so much beer. By 1963 his condition had gotten bad enough that his mother brought him to a hospital in London, where he received the devastating diagnosis: motor neuron disease, as ALS is called in the United Kingdom. The prognosis was grim and final: rapid wasting of nerves and muscles, near-total paralysis, and death from respiratory failure in three to five years.

Not surprisingly, Hawking grew depressed, seeking solace in the music of Wagner (contrary to some media reports, however, he says he did not go on a drinking binge). And yet he did not disengage from life. Later in 1963 he met Jane Wilde, a student of medieval poetry at the University of London. They fell in love and resolved to make the most of what they both assumed would be a tragically short relationship. In 1965 they married, and Hawking returned to physics with newfound energy.

Also that year, Hawking had an encounter that led to his first major contribution to his field. The occasion was a talk at Kings College in London given by Roger Penrose, an eminent mathematician then at Birkbeck College. Penrose had just proved something remarkable and, for physicists, disturbing: Black holes, the light-trapping chasms in space-time that form in the aftermath of the collapse of massive stars, must all contain singularities—points where space, time, and the very laws of physics fall apart.

Before Penrose’s work, many physicists had regarded singularities as mere curiosities, permitted by Einstein’s theory of general relativity but unlikely to exist. The standard assumption was that a singularity could form only if a perfectly spherical star collapsed with perfect symmetry, the kind of ideal conditions that never occur in the real world. Penrose proved otherwise. He found that any star massive enough to form a black hole upon its death must create a singularity. This realization meant that the laws of physics could not be used to describe everything in the universe; the singularity was a cosmic abyss.

At a subsequent lecture, Hawking grilled Penrose on his ideas. “He asked some awkward questions,” Penrose says. “He was very much on the ball. I had probably been a bit vague in one of my statements, and he was sharpening it up a bit. I was a little alarmed that he noticed something that I had glossed over, and that he was able to spot it so quickly.”

Hawking had just renewed his search for a subject for his Ph.D. thesis, a project he had abandoned after receiving the ALS diagnosis. His condition had stabilized somewhat, and his future no longer looked completely bleak. Now he had his subject: He wanted to apply Penrose’s approach to the cosmos at large.

Physicists have known since 1929 that the universe is expanding. Hawking reasoned that if the history of the universe could be run backward, so that the universe was shrinking instead of expanding, it would behave (mathematically at least) like a collapsing star, the same sort of phenomenon Penrose had analyzed. Hawking’s work was timely. In 1965, physicists working at Bell Labs in New Jersey discovered the cosmic microwave background radiation, the first direct evidence that the universe began with the Big Bang. But was the Big Bang a singularity, or was it a concentrated, hot ball of energy—awesome and mind-bending, but still describable by the laws of physics?

Hawking found the answer. If Einstein’s laws of general relativity were applied to our expanding universe, then at some time in the past all matter and energy must have been concentrated at a point of infinite density. The entire cosmos had once been a singularity, beyond physical explanation.

“After Roger Penrose proved that there are singularities inside black holes, Stephen applied the same idea to the universe,” says Don Page, a University of Alberta physicist who was one of Hawking’s postdocs and has remained a lifelong friend. “He showed that there is an edge to space-time, that you can’t extrapolate back indefinitely.”

A singularity at the cosmic zero hour was bad news for physicists. It meant their theories could not explain how the universe began. It left too much mystery, resisted rational explanation. And that rankled Hawking.

Vanishing Act By the early 1970s Hawking’s speech had become unintelligible to all but his closest friends. He could no longer walk, even with a cane. Yet no one who knew him dared call him weak. “He is one of the most determined people I’ve ever met,” Penrose says. “I remember staying with Stephen and his then wife, Jane. They had this house on Little St. Mary’s Lane,” a quiet lane in Cambridge directly across from a medieval church. “I was spending the night there. My room was on the top floor, and Stephen’s room was up one floor from the street. When it came time for him to go to bed, he absolutely insisted on doing everything himself. He crawled up the stairs, which took him about a quarter of an hour, and put himself to bed. He just refused to let anyone help him in any way. I think this absolute determination to keep his muscles working might well have stopped them from atrophying to the degree they might otherwise have done.”

Hawking turned the same formidable will to his work. Since he could no longer use the simple, essential tools of his trade—slide rules, pencils, chalk—he adapted.

“He learned to think in ways that other people don’t,” says Kip Thorne, a theoretical physicist at Caltech who has known Hawking for 44 years. “Because he couldn’t write equations and stare at them and do long calculations by hand, he developed a geometrical approach to thinking about things where he could manipulate shapes in his head in order to get insights that other people couldn’t get. I think it is very likely true that he has done greater work as a result of this handicap than he would have done other­wise, and I think he believes that as well.”

Since it began to take long minutes to utter a sentence, Hawking became a master of concision. “He had to make his statements as terse as possible,” says Bernard Carr, a cosmologist at Queen Mary, University of London, who in 1974 became Hawking’s first live-in grad student. “A 15-minute conversation with Stephen was like speaking with someone else for several hours.”

Carr’s years as a grad student coincided with Hawking’s greatest work. Hawking was developing a strange new theory about black holes, a theory that would foment arguments among theoretical physicists that have yet to be resolved. Black holes are usually described using Einstein’s equations of general relativity, which dictate how gravity molds the shape of space-time. Hawking wanted to see whether quantum mechanics, which governs the behavior of atoms and fundamental particles, could provide any insight about the nature of black holes. “Stephen’s work was tremendously important,” Carr says. “It combined relativity, thermodynamics, and quantum mechanics, and that had never been done before.”

The results of his efforts were completely unexpected, even to Hawking, and the work is now generally acknowledged as his greatest achievement. In 1974 Hawking published an essay with a koan of a title: “Black Holes Aren’t Black.” He argued that physicists had been wrong about one of their central assumptions about black holes: namely, that nothing can escape their grasp. He proved that black holes actually emit a stream of what is now called Hawking radiation.

According to Hawking, this radiation does not originate inside the black hole; it comes from outside, just beyond the region where escape would be impossible. The radiation consists of particles that spontaneously materialize in empty space, a quantum mechanical phenomenon that occurs everywhere, all the time. These “virtual particles” arise in pairs that normally cancel each other out almost immediately, releasing their energy back into the vacuum that spawned them. Near a black hole, though, the pairs can get split up. One particle can fall into the black hole while the other feeds on the gravitational energy of the hole and flies away to safety. The transformation of gravitational energy into particles (think

E = mc^2

) gradually causes the black hole to shrink. Eventually, at an inconceivably distant time in the future, the black hole will vanish entirely. What is left behind is a problem that physicists still have not fully solved.

Cosmic Wager Even as Hawking continued to cope with the possibility that he himself would soon fade to black, he realized that black hole evaporation posed a crisis for science. It seemed to violate basic notions about the nature of time and of cause and effect. Physicists, like the rest of us, assume that the present evolves from the past, the future from the present. Moreover, the laws of physics hold that it should always be possible to follow processes backward in time—as physicists do, for example, when they reconstruct particle collisions by studying the debris created in accelerators. In principle, physicists should be able to do the same thing with a shrinking black hole: piece together its past by looking at the Hawking radiation. But Hawking claimed that was not possible.

Unlike the subatomic crack-ups in particle accelerators, where the colliding particles fragment directly into their components, nothing that falls into a black hole—gas, stars, people—has a direct connection to the Hawking radiation it emits in the present. Whatever falls into a black hole stays there; the Hawking radiation dribbles away from outside the hole and contains no hint of what is inside. The entire history of the black hole is forever hidden from the rest of the universe. Physicists call this the information loss problem.

“God not only plays dice,” Hawking wrote, paraphrasing one of Einstein’s objections to quantum mechanics, “but also sometimes throws them where they cannot be seen.” There seemed to be only two possible explanations for the mystery. Either Hawking was wrong about black holes’ destroying all traces of their past, or something was wrong with quantum mechanics, whose equations require that information never be lost. For years Hawking thought that quantum mechanics—the foundation of modern physics—would have to change.

“It seems to me that the indication that the information is lost is very powerful,” Penrose says. “And that is what Stephen originally thought. But more recently he changed his mind, at a meeting in Dublin where he publicly retracted and agreed to lose a bet about this. In my view he was completely wrong to retract. He should have stuck to his guns.”

Penrose is referring to a wager that Hawking and Thorne made in 1997 with John Preskill, a theoretical physicist at Caltech. Hawking and Thorne bet that black holes truly did destroy information, severing the links between past and present. Quantum mechanics, therefore, was probably wrong at some level. Preskill took the opposite view, betting that quantum theory would someday be vindicated. They put the bet in writing, specifying that the winner would receive “an encyclopedia of the winner’s choice, from which information can be recovered at will.”

In July 2004 at a physics meeting in Dublin, Hawking conceded the bet and gave Preskill an encyclopedia of baseball statistics. Hawking had made new calculations showing that black hole radiation could contain subtly encoded information about the past. His new result made use of a pillar of quantum theory called the uncertainty principle, according to which the detailed properties of an object—its position or velocity, for example—can never be completely pinned down. Hawking argued that when quantum uncertainty is taken into account, the dreaded edge of a black hole, called the event horizon, becomes fuzzy. Consequently, there is no sharply defined, inescapable boundary surrounding black holes. “A true event horizon never forms,” Hawking said at the Dublin meeting. The wiggle room provided by the uncertainty principle is just large enough to allow information to escape in black hole radiation. Or so he claimed.

Many physicists remain unconvinced that Hawking has solved the problem (and Thorne, unlike Hawking, refuses to concede the bet). Even now, the status of the information loss problem and the ultimate validity of quantum theory remain...uncertain.

In fact, uncertainty looms over Hawking’s entire legacy. Unlike Einstein’s theories, which have been confirmed many times by experiment, Hawking’s ideas about singularities and black hole evaporation will probably never be observed. There is a small chance—Hawking himself puts the probability at less than 1 percent—that the Large Hadron Collider, the enormous new particle accelerator near Geneva, might detect miniature black holes. If Hawking is right (and for the sake of those who fear the LHC might spawn a planet-devouring mini black hole, he’d better be), those black holes would evaporate almost as soon as they appeared. Such a discovery would validate one of Hawking’s signature insights and could easily provide the tangible evidence needed to snag a Nobel Prize.

Prometheus Unbound Refuting his own previous accomplishments—or attempting to, anyway—has become something of a habit with Hawking. His latest work circles back to his earliest. It is, in truth, more of a theory of a theory, a mathematically rigorous guess about what a complete theory of the universe might look like. One thing that final theory absolutely cannot allow is a singularity at the Big Bang. To achieve his goal of understanding why the universe is as it is, the singularity—whose existence at the beginning of time Hawking had once proposed—would have to go. Like all his work, this most recent effort is bold and imaginative. But also like nearly all his work, it falls on the very edge of testability. It seems unlikely to be the magic intellectual bullet that will allow him to regain the Einsteinian stature that he once held.

Hawking’s latest quasi-theory has had a long genesis and is in fact a direct attempt at resolving the old conundrum of that Big Bang singularity. The singularity was a creature of general relativity: Squeeze enough matter and energy into one spot and Einstein’s equations of general relativity predict that the density will become infinite. With general relativity an initial singularity was inevitable.

For years Hawking has argued that a better theory—one that physicists have been seeking ever since Einstein’s heyday—would one day inevitably combine quantum mechanics with general relativity. If that synthesis is accomplished, it is likely that the uncertainty principle would make it impossible for singularities to form. A singularity, by definition, is an infinitesimally small point. But quantum theory forbids such exactitude. Nothing can be located at a precise point in space or time, preventing the formation of a blip of infinite energy and density. Quantum uncertainty would thus blur the singularity into something tamable; it offers the possibility that the beginning of the universe could still fall within the domain of physics and rationality. In that case, there would be no singularity, no place or time where the laws of physics did not hold.

Hawking’s original theory of cosmic origin, which he called the “no-boundary proposal,” clarified how to dispense with that initial singularity. More shocking, it posited a universe that would explain its own existence. In the early 1980s he began to flesh out this idea. In collaboration with James Hartle, a theoretical physicist at the University of California at Santa Barbara, he developed a quantum mechanical framework for the entire universe. One of the uncanny principles of quan­tum theory is that the constituents of physical reality—protons, electrons, and all other fundamental particles—are not solid, pointlike objects. They behave instead like both waves and particles. In the quantum realm there are no fixed positions; atoms and other particles are blurry, shimmying things, each capable of being in many places at any given time. The creators of quantum mechanics developed a powerful mathematical tool—the wave function—to predict how a fluctuating particle/wave moves through space and time. Hawking and Hartle borrowed the basic math of quantum theory and sprinted with it.

The Hartle-Hawking wave function, as their creation is called, describes how the universe evolves—or, more accurately, all of the possible ways it could evolve. Just as the quantum wave function for a single particle gives every possible path the particle could follow between two points, the Hartle-Hawking function represents all the physically possible histories our universe might have. The universe we see is just one possible outcome among many. It was an intriguing idea, but for years it remained just that.

Hawking’s most recent work explores the implications of the notion that the universe is a giant quantum phenomenon. The problem with conventional attempts to understand the cosmos, he now believes, is that researchers have failed to appreciate the full, bizarre implications of quantum physics. These efforts to create a unique theory that would explain all the properties of the universe are therefore doomed to fail. Hawking refers to such attempts as “bottom-up” theories because they assume the universe had a unique beginning and that its subsequent history was the only possible one.

Hawking is now pushing a different strat­egy, which he calls top-down cosmology. It is not the case, he says, that the past uniquely determines the present. Because the universe has many possible histories and just as many possible beginnings, the present state of the universe selects the past. “This means that the histories of the Universe depend on what is being measured,” Hawking wrote in a recent paper, “contrary to the usual idea that the Universe has an objective, observer-independent history.”

This idea could cut through some long-standing scientific mysteries. One debate now roiling the physics community concerns string theory, currently the leading candidate for a so-called theory of everything. String theory holds that all the particles and forces in the universe can be explained as arising from the vibrations of vanishingly small strands of energy. But it has one huge problem: Its fundamental equations have a near-infinite number of solutions, each corresponding to a unique universe. Hawking’s idea provides a natural context for string theory. All those universes might simply represent different possible histories of our universe. This notion is as daring and exotic as anything Hawking has ever proposed. Even better, it just might be testable.

If Hawking is right, the alternative quantum histories of the universe (the ones we have not observed) may have left a subtle imprint on the cosmic microwave background, the faint radiation left from the hot glow of the Big Bang. Physicists believe that the slight temperature variations in the microwave background were caused by quantum fluctuations in the early universe. Hawking suspects that if other quantum histories really do exist, they may have made their own measurable contribution to the background radiation. Over the next few years, the European Space Agency’s new Planck spacecraft may be able to detect the sort of microwave patterns that Hawking is predicting.

Slow Fade While Hawking’s colleagues universally acknowledge the significance of his work on black holes and his early work on singularities, his more recent work has not had the same impact.

“Without any question, Stephen’s work on black hole evaporation was hugely important, because it linked relativity theory, quantum theory, and thermodynamics,” says physicist George Ellis of the University of Cape Town, in South Africa, who worked with Hawking at Cambridge when both were starting their careers. “His first period”—Hawking’s proof of a singularity at the Big Bang—“was very solid, classical relativity. His second-period stuff on black hole radiation seemed very speculative at first and was disbelieved for quite a while, but then so many other people proved it by different methods that we all agree now that it is correct. That was really a unique breakthrough. The third period—his work on the wave function of the universe—is much more speculative. It is much less solidly grounded in experiment and much less agreed on.”

Leonard Susskind says Hawking—like all fundamental theorists today—is struggling with impossibly difficult questions. “It’s the main muddle of physics and cosmology: How is quantum mechanics to be correctly used to study the universe as a whole?” Susskind says. “That, I think, is one of the biggest, most profound, most conceptually confusing questions that we face. Am I interested in Stephen’s ideas? Of course. Do I think they’re the solution? No. Do I think they’re part of the solution? Perhaps. I think Stephen would say the same thing, frankly.”

The media often portray Hawking as the Einstein of our time (in his Star Trek appearance, Hawking was matched up with Einstein—and with Isaac Newton, too, for good measure). Hawking himself dismisses such comparisons. His accomplishments have not been as wide-ranging, and his most important work may never be confirmed by experiment. So where does he rank in the pantheon? Judged only by his contributions to physics, he cannot match the giants of the last century: not just Einstein but also Bohr, Heisenberg, and Feynman.

I ask Stanford cosmologist Andrei Linde what the state of physics would be like today without Hawking’s contributions. “That’s a tough one,” he replies. “Nature abhors empty spaces. Stephen made big jumps to new theories. Maybe somebody else would have come and done something comparable. It probably would not have happened for quite a while—for how long, I don’t know. But this combination of enormous creativity and honesty and fighting with external circumstances, this is something that doesn’t happen often, and it influences all of us. You start thinking, measuring yourself with people like that; it creates an atmosphere of high science. So while the discoveries may have happened, his combination of qualities is something unique.”

Hawking’s flurry of public appearances and his renewed attack on the fundamentals of cosmology suggest that he is not terribly interested in being remembered just for his inspirational biography. His paradox, then, is that much of the public’s fascination with his science seems to rest squarely on the nonscientific aspects of who he is. Were it not for the tragedy and drama of his life, would so many of us have become interested in black holes, the beginning of the universe, the nature of time?

In his latest incarnation, Hawking is hoping both to rekindle that fame and to transcend it. He clearly relishes encouraging public understanding of science, but simply rewriting the text and ideas of A Brief History of Time (as he has been doing for much of the past 20 years) is not enough. He wants more big jumps, more deep insights; he wants to make serious headway in deciphering the secret code of physics. These are perhaps the greatest demands that a man could make of himself. Such achievements do not come quickly or easily, and time—brief enough for any of us—is something that Hawking does not have a lot of.

But right now, right here in the Pasadena Convention Center, Hawking is exactly where he likes to be: onstage, the still center of attention, about to explain our place in the universe.