Stephen Hawking (22 page)

In fact, the precise version of the uncertainty rule says that energy can only be “borrowed” from the vacuum for a very short time, a time determined by Planck's constant. This is related to the uncertainty inherent in the measurement of time itself. The only way in which this energy can then be converted into particles is if particles are always created in pairs, which then interact with one another and annihilate themselves before the Universe has time to “notice” that the
energy has been borrowed. This means that the particles created out of the vacuum are matched in a special way.

Every variety of particle, such as an electron, has a counterpart known as an antiparticle (in the electron's case, a positron). Antiparticles have been manufactured in experiments using particle accelerators, and they are also found in cosmic rays (energetic particles reaching the Earth from space), as well as being predicted by quantum theory, so there is no doubt that they exist. In many ways, an antiparticle is a mirror image of its particle equivalent: the positron, for example, carries positive charge, whereas the electron carries negative charge. And whenever a particle meets its antiparticle counterpart, the two annihilate each other.

So according to quantum theory, the vacuum is a seething sea of “virtual” particles. Pairs such as electron-positron are constantly being created, interacting with one another, and disappearing in accordance with the quantum rules. Overall, no energy is released, but virtual pairs flicker in and out of existence all the time, below the threshold of reality.

What Hawking showed was that, even for a nonrotating black hole, this process can drain off energy from a hole and release it into the Universe at large. What happens is that a pair of virtual particles is created just outside the horizon of the hole. In the tiny fraction of a second allowed by quantum uncertainty, one of the particles is captured by the hole. So the other particle has nothing to annihilate with, and escapes, carrying energy with it.

Where has the energy come from? In effect, it is the gravitational energy of the hole. The energy of the hole creates two particles, but it captures only one of them, so only half the
energy debt is repaid and the net effect is that the hole loses mass. Other things being equal—if the hole does not gain mass from somewhere else—it will steadily shrink away as a result, evaporating like a puddle in the sunshine. This process is slow but sure, taking billions of years to shrivel even a proton-sized minihole to the point where it explodes. Hawking had contradicted his own earlier conclusion that the surface area of a black hole cannot decrease. Having established a link between black holes and thermodynamics by showing that—according to general relativity alone—black holes cannot shrink, he had now found that if you add quantum theory to the brew, the link with thermodynamics is strengthened, but now black holes
must
shrink.

For ordinary black holes, made out of dead stars, this effect would be of no real importance. A black hole with three or four times the mass of our Sun and a horizon roughly as big as the surface of a neutron star will be constantly swallowing traces of gas and dust from its surroundings, even in the depths of space, and it is simple to show that the mass lost by Hawking Radiation is much less than the mass gained by this accretion. If nobody had thought of the notion of miniholes, nobody would have been very interested in Hawking Radiation. But since Hawking had already come up with the notion of miniholes, the idea of quantum evaporation of black holes made an immediate impact.

A hole smaller than a proton will not eat up much material from its surroundings, even if it happens to be inside a planet. To a hole that small, even solid matter is mostly empty space! So the Hawking Radiation from the surface of a minihole will actually dominate its behavior. Hawking showed that the
radiation produced in this way gives the hole a temperature, exactly the temperature suggested by the work of Bekenstein. For a black hole with the mass of our Sun, this temperature is about one ten-millionth of a degree K (with the resulting ultra-feeble Hawking Radiation easily overwhelmed by infalling matter); but for a minihole with a mass of a billion tons and the size of a proton, the temperature is about 120 billion K. As these examples indicate, the temperature depends on one over the mass of the hole, so as it loses mass and gets smaller, such a hole gets hotter and radiates energy faster, until it finally explodes in a burst of X-rays and gamma rays.

Science fiction fans may be intrigued to know that if we could find a proton-sized minihole today, it would be a more than useful energy source. The output from such a hole would be about 6,000 megawatts and could make a substantial contribution to the energy requirements of even a large country. Unfortunately, though, holding on to such a hole if you found it would be tricky—remember that it would weigh a billion tons and gravity would tend to pull it down toward the center of the Earth.

The lifetime of such a minihole depends on the exact mass it starts out with, but roughly proton-sized black holes born in the Big Bang should be exploding here and there in the Universe today.

Intriguingly, detectors flown on satellites have reported occasional bursts of gamma radiation coming from the depths of space, and there is no universally accepted explanation for this phenomenon. It is just possible that the Hawking Radiation from exploding black holes has actually been discovered, although it will be almost impossible ever to prove this.

Hawking had achieved something that even he had thought to be almost impossible, using a combination of general relativity and quantum physics (plus a smattering of thermodynamics) in one package to describe a physical phenomenon. It was this work that made his name outside the specialist circles of mathematicians and astronomers, and any physicist today can tell you what Hawking Radiation is and why it is important. But in a quirky gesture which is in some ways typical of Hawking's attitude toward established conventions, the astonishing discovery that “black holes are not black” was announced first not in the pages of a scientific journal such as
Nature
but in an essay that Hawking entered for a somewhat obscure competition organized by the Gravity Research Foundation in America.

The Gravity Research Foundation runs an annual competition for articles describing new research into the nature of gravity. Until the 1970s, it had been almost exclusively a domestic U.S. competition, with very few entries from abroad, although it had once been won by an expatriate Briton living in the United States. Then, with his last contribution to academia, one of us (J.G.) won the prize in 1970. So when Stephen Hawking won the same prize a year or two later for an essay describing black holes, J.G. quickly sent him a congratulatory note. It was nice, the note said, to see Hawking's name on the list of prizewinners because this added to the prestige of the award and gave previous winners a chance to bask in the reflected glory. “I don't know about the prestige,” Hawking wrote in reply, “but the money's very welcome.”

The “official” version of the exploding-black-hole story appeared first in
Nature
on March 1, 1974.
⁴
While the Gravity
Research Foundation essay carried the dogmatic title “Black Holes Aren't Black,” the
Nature
paper, uncharacteristically for Hawking, was equivocally headed “Black Hole Explosions?” It sparked a furious debate, as we saw in
Chapter 8
, with some opponents of the idea suggesting that this time Hawking really was talking rubbish. John Taylor and Paul Davies, of King's College in London, combined to produce a retort in the issue of
Nature
dated July 5, 1974,
⁵
headed “Do Black Holes Really Explode?” and answered their own question with an unequivocal “No.” Even Taylor and Davies, though, were soon persuaded that they were wrong and Hawking was right.

More important even than the specific idea that black holes explode was the underlying basis for this discovery—that quantum physics and relativity could be fruitfully combined to give us new insights into the workings of the Universe. Soon Hawking would be using that insight to focus, once more, on the puzzle of the singularity at the beginning of time. But it seems, with hindsight, singularly appropriate that his election as a fellow of the Royal Society, Britain's highest academic honor, should have come in the spring of 1974, within a few weeks of the publication of the
Nature
version of the exploding-black-hole paper. Ten years after being given just two years to live, however (and scarcely five years after the deterioration that had seemed likely to cut short his promising career), Hawking's research was really getting into its stride. In the second half of the 1970s, he moved on to investigate the origin of the Universe itself, going back to the beginning of time.

10

THE FOOTHILLS OF FAME

R
eflecting on his achievements during the first thirty-two years of his life, Stephen Hawking must have felt a deep sense of pride in what he had accomplished. The 1970s were the years when he established himself as a world-class physicist, and they marked the beginning of five decades of startling success in the disparate worlds of arcane research and popular writing.

Soon after becoming a fellow of the Royal Society, Hawking was invited to spend a year away from Cambridge at Caltech, in Pasadena. The research year, funded by a Sherman Fairchild
Distinguished Scholarship, was to study cosmology with the eminent American theoretician Kip Thorne.

Pasadena is a leafy suburb of Los Angeles, nestling up against the San Gabriel Mountains to the northeast of Hollywood. The wide boulevards intersecting the district are lined with grand old houses, and in the heyday of Hollywood it was a favorite haunt of film stars. The main street, Colorado Boulevard, was immortalized in the Jan and Dean song “Little Old Lady from Pasadena,” and there has been no shortage of celebrity names who have taken up residence there over the decades. However, in the summer Pasadena is one of the smoggiest areas of Los Angeles because the mountains inhibit the escape of ozone. If a Stage 2 Smog Alert is sounded, citizens are advised to stay indoors unless on essential business, and the authorities have the power to make industry and commerce temporarily shut down. Smog-alert warnings are broadcast on the radio, and illuminated signs are switched on over freeways. Perhaps the American Indians displayed great powers of premonition when, long before white men arrived, they named the region “Valley of the Smokes.”

Caltech itself is unique in that, for such a prestigious institution, it is tiny. In the mid-seventies it was home to no more than fifteen hundred students and was a tenth the size of colleges with comparable reputations such as Harvard or Yale. But despite its small size, Caltech is the West Coast's mecca for science and technology. Throughout its history, it has attracted the leading people in their fields from all over the world. Nobel Prize–winning physicist Robert Millikan arrived there in the twenties and was frequently visited by Albert Einstein. Money simply pours into the place from
benefactors ranging from private individuals fascinated with scientific research to multinationals such as IBM and Wang. With some of the best telescopes in the world a matter of miles away on Mount Wilson and the massive Jet Propulsion Laboratory as a gargantuan “annex” dwarfing the mother campus, it has everything a scientist could wish for.

Some of the world's best physicists were based at Caltech in the seventies. Kip Thorne headed the relativity group there, and the charismatic Nobel laureate Richard Feynman still taught there and played bongos in college bands during the evenings. Academic quality aside, the contrast between Caltech and Caius could not have been starker. The buildings making up the campus, although tastefully designed and constructed in sand-colored stone, are all Spanish-style, light and airy, with the nine-story Millikan Library block rising at the center. Those admitted to Caltech are among the very best students in the country, and they are driven hard. There is very little social life on campus, and the suicide rate among students ranks almost as high as its academic reputation. Having said that, there was no shortage of colorful characters around the place at the time of Hawking's sabbatical.

Feynman, a physics professor, had already acquired a formidable reputation as an amiable eccentric and once took on the local authorities who were trying to close down a topless bar in Pasadena. In court he claimed that he frequently used the place to work on his physics. Feynman and Hawking shared an offbeat sense of humor, and although their work rarely overlapped, they had a lot of time for each other. Both men have achieved international fame as scientists and live-wire characters, and each has acquired cult status in the wider
world outside his own discipleship of graduate students and fascinated laypeople. When Feynman died of cancer in 1988, the whole of Caltech mourned and the global village of science felt the loss.

Kip Thorne, now viewed as the West Coast's relativity guru, favors floral shirts, beads, and shoulder-length gray hair. He introduced Hawking to another physicist who was to play a significant role in collaborations and become one of Hawking's lifelong friends—Don Page. Page, who was born in Alaska and graduated from a small college in Missouri, was working on his Ph.D. at the time of Hawking's visit. The two of them immediately hit it off, and before Hawking's year at Caltech was over they had written a black-hole paper together.

The family was excited by the move. Jane organized all the details, booking airline tickets, packing, and arranging schedules, as well as managing to transport a severely disabled husband and two young children to the other side of the world almost single-handedly. At Caltech, Hawking was treated with the respect he should have received at his own college in Cambridge. Wooden ramps were fitted against the curbs in the vicinity of his office so that he could get around easily in his wheelchair, and he was provided with an office and every aid and resource he would need to help him with his research. The work was satisfying, and he found collaboration with Thorne's team both stimulating and scientifically rewarding. Jane and the children enjoyed the Southern California climate. Despite the air pollution, noise, and traffic congestion of Los Angeles, the beaches and the blue Pacific made a welcome change from the often monotonous lifestyle and erratic weather of Cambridgeshire.