by John Gribbin
Sciama dashed off to see Hawking. By the end of the conversation, he too was convinced and persuaded his former student to announce his results at the conference he was organizing in February at the Rutherford-Appleton Laboratory outside Oxford.
Hawking was driven to the laboratory through the icy chill of midwinter Oxfordshire and assisted into the building by one of his research students. Sitting patiently to the side of the main group, he listened to the other speakers announcing their latest news. As usual, he asked his customary penetrating questions, trying hard to control his great feeling of excitement. He had a hunch, now supported by a number of his respected colleagues and peers, that he was on to something very big. At last he was wheeled to the front of the lecture theater, and his illustrations were projected onto the back wall while he delivered his talk in the almost unintelligible tones to which his colleagues had become accustomed. His final line was delivered. A stunned hush fell over the entire room. You could have heard a pin drop as the audience of scientists tried to absorb the astonishing news. Then the backlash began.
The moderator of the meeting, the English theorist John G. Taylor, jumped up from his seat and proclaimed that what Hawking had said was complete nonsense. Pausing only to drag one of his colleagues from the seat beside him, Taylor stormed from the room and immediately started writing a paper denouncing Hawking’s claim. Hawking had expected a reaction, but nothing like this. He simply sat at the podium in shocked silence.
John Taylor’s paper was dashed off and sent to the scientific journal Nature for publication. The editor of Nature sent the draft manuscript to Hawking for his comments before making the decision to publish it. Hawking wrote back to recommend publication. He would not want to stand in the way of anyone rash enough to disclaim his work without having investigated the matter thoroughly.
A month after the meeting outside Oxford, Hawking published in Nature his own paper describing the newly discovered phenomena. Within weeks, physicists all over the world were discussing his work, and it became the hot topic of conversation in every physics laboratory from Sydney to South Carolina. Some physicists went so far as to say that the new findings constituted the most significant development in theoretical physics for years. Dennis Sciama described Hawking’s paper as “one of the most beautiful in the history of physics.” This radiation, which he had discovered could be emitted by certain black holes, was from then on known as Hawking Radiation.
However, not everyone was convinced, and it was quite a while before many groups working around the world came to terms with this revolution in black-hole physics. It took until 1976 for Zel’dovich’s team in Moscow to accept the new ideas. Zel’dovich ran his institute in an extremely dictatorial manner. What he said went. When he finally gave his endorsement to the theory, his team was compelled to go along with it, just as they had followed him when he had disagreed with it.
At the time of Zel’dovich’s change of heart, Roger Penrose was invited to Moscow to give a talk that Zel’dovich, as Penrose’s colleague and head of the institute, would be attending. In his lecture notes, Penrose had assumed the validity of Hawking’s deductions and had built his talk around them. When he arrived, a day before the lecture, he was told bluntly that Zel’dovich did not agree with Hawking, nor did any of his students. Not only that, but he would prefer it if Penrose did not mention Hawking’s findings. Penrose was completely thrown. It meant, quite simply, that he had to rewrite his lecture; he set to work, laboring into the small hours. Then, a few hours before he was due on the podium, an assistant turned up at his hotel to inform him that Zel’dovich had changed his mind about Hawking—and so had all his students.
Another story relates how the American physicist Kip Thorne was in Zel’dovich’s apartment when the transformation in his thinking actually occurred. Zel’dovich was pacing the room when Thorne arrived, and in a theatrical display of resignation the Russian physicist threw up his arms in despair and said, “I give up, I give up. I didn’t believe it, but now I do.”8
The mid-seventies saw the beginnings of a renaissance in the public awareness of science, and the idea of such exotic objects as black holes that could eat whole solar systems for breakfast caught the public imagination. It was at about this time that the name of Stephen Hawking first impinged on the popular awareness. It was also the time when a great deal of hot air was circulated around the serious theories by writers overpopularizing the ideas the physicists were propounding.
Hawking himself began to pass as a metaphor for his own work. He was becoming the black-hole cosmonaut trapped in a crippled body, piercing the mysteries of the Universe with the mind of a latter-day Einstein, going where even angels feared to tread. With the arrival of black holes in the public consciousness, the mystique that had begun to gather around him in Cambridge at the end of the sixties started to extend beyond the cloistered limits of the physics community. Newspaper articles and TV documentaries about black holes started to appear, and Stephen Hawking began to be seen as the man to talk to.
It was not only the media that were beginning to register what was going on; Hawking’s achievements had also been noticed by the scientific establishment. In March 1974, within weeks of the announcement of Hawking Radiation, he received one of the greatest honors in any scientist’s career. At the tender age of thirty-two, he was invited to become a fellow of the Royal Society, one of the youngest scientists in the society’s long history to be given such an honor.
The investiture took place at the headquarters of the Royal Society, at 6 Carlton House Terrace, a white-colonnaded mansion overlooking St. James’s Park in the West End of London. It is traditional for new fellows of the society to walk to the podium in the large meeting room that dominates the building in order to sign the roll of honor and shake the president’s hand. However, in this case, the president at the time, Nobel Prize-winning biophysicist Sir Alan Hodgkin, brought the membership book down to the front row for Hawking to sign. It took an age for Hawking to sign his own name. The letters were slowly and agonizingly formed on the page alongside the others invested at the same ceremony. As he wrote, the room was completely silent. Then, as he finished the last letter and Hodgkin lifted the book from his lap, the gathered scientists burst into thunderous applause.
The local newspaper, the Cambridge Evening News, reported the great event on the day of Hawking’s investiture, and a party was thrown at the DAMTP after the ceremony in London. Friends, family, and colleagues in the department were all invited to celebrate his achievement. As one of the senior members of the gathering and Hawking’s old supervisor, Dennis Sciama was invited to give an impromptu toast to his most successful student, in which he paid tribute to Hawking’s achievements and raised his glass to future successes.
As his friends and family joined Sciama in the toast, Hawking surveyed the room. He had come a long way, he knew that, but this was just the beginning. Although he would always believe his investiture into the Royal Society to be the proudest moment of his career, there were plenty more rungs to climb on the career ladder. And, despite the adversities—or perhaps, as some have suggested, because of them—he would continue to climb. Where his feet could not go, his mind would soar.
9
WHEN BLACK HOLES EXPLODE
In 1970, as we mentioned in Chapter 7, Hawking had shifted the focus of his scientific attention from what goes on at the heart of a black hole, at the singularity, to events that occur on the horizon surrounding the black hole, the nearest thing it has to a “surface.” A key difference between these studies and the investigation of singularities is that, whatever your theory predicts about things going on at a singularity, you can never test the theory by looking at a singularity, because they are all hidden inside black holes (except, of course, the Big Bang singularity at the beginning of time, which Hawking was to investigate more fully later in his career). But when you apply your theory to predict what goes on at the surface of a black hole, at the horizon, then whatever strange eve
nts it describes ought to make their mark on the outside Universe and might even produce effects that could be detected by instruments here on Earth or on satellites in orbit around the Earth.
It was, in fact, satellite-borne instruments that identified, at about this time, the first really plausible black-hole candidate in our Milky Way Galaxy. Just as great new discoveries in astronomy had come about in the 1960s through the investigation of the radio part of the spectrum, at wavelengths longer than those of light, so great new advances came in the 1970s through the investigation of the X-ray part of the spectrum, at wavelengths much shorter than those of light. Unlike radio waves, however, X-rays from space are blocked by the Earth’s atmosphere and do not reach the ground (which is just as well, or we would all be fried). So X-ray astronomy came of age as a branch of science only when suitable detectors were placed in orbit around the Earth. These unmanned satellites transformed astronomers’ view of the Universe, showing it to be a much more violent and energetic place than they had thought. And at least some of that violence, they are now convinced, is associated with black holes.
It happens like this. An isolated black hole is, of course, undetectable, except by its gravitational pull—the way it distorts space in its vicinity. It is, after all, black. But a black hole in a binary system, orbiting around a more ordinary star, could make its presence highly visible. Matter torn off the companion star by the gravitational influence of the black hole would funnel down into the hole and be swallowed up. On the way in, it would form a swirling accretion disc, like water going down the drain of a bath, with gas piling up and getting hot as gravitational energy is converted into energy of motion. It would, in fact, calculations showed, get hot enough to emit X-rays.
But how likely is it that a black hole will just happen to be orbiting a companion star? In fact, binary-star systems are very common—most stars probably have at least one close stellar companion, and in this our Sun is an exception to the rule. Binaries are also easy to identify because the tug of the two stars on each other makes them wiggle about, producing regular changes that can be observed using telescopes on Earth. The orbital variations also give astronomers a clue to the masses of the two stars, and that turned out to be crucial in identifying black-hole candidates.
The snag, for seekers of black holes, is that it is not enough just to identify an X-ray source in a binary system. Both white dwarfs and neutron stars are also compact enough, with a strong enough gravitational pull, to strip matter from a companion and pull it onto themselves, creating hot spots that radiate at X-ray wavelengths.
Several of the first binary X-ray sources found could indeed be identified as white dwarfs because the orbital variations showed that their masses must be comfortably less than 1.5 solar masses. But four reasonable black-hole prospects did emerge from the first X-ray surveys of the sky, carried out in the early 1970s. A first examination showed that all were X-ray sources in binary systems—small, energetic, compact objects orbiting normal stars. More detailed investigations gradually eliminated three of the candidates. One had a mass 2.5 times that of the Sun and might very well be a neutron star. Another had a mass three times that of the Sun, which seemed a little high for a neutron star but left room for doubt about its black-hole status. The third had a mass only twice that of the Sun. But the fourth had a mass estimated at between 8 and 10 solar masses.
That source is called Cygnus X-1. Only the most tortuous explanations could be invoked to avoid the inference that it harbored a black hole. For example, some astronomers suggested that the unseen companion in the binary system might actually consist of two stars—a faint, unseen, ordinary star (too dim to be visible) with a mass 6 times that of the Sun, itself orbited by a neutron star of 2 solar masses. But the contrived explanations flew in the face of the attractive argument that the simplest explanation was probably the best. Ultimate proof that Cygnus X-1 harbors a black hole would only come if we were able to go and look at it close up; but the weight of evidence that has accumulated over the decades has convinced most astronomers, and the consensus today is that there is at least a 95 percent chance that Cygnus X-1 is the first black hole to be identified. Several other promising candidates are also now known, which strengthens the case—we would hardly expect there to be just one detectable black hole in our Galaxy.
The identification of Cygnus X-1 itself as a black-hole candidate was the occasion of a famous bet, which sheds intriguing light on Hawking’s character. Hawking, whose career has been founded on the study of black holes, made a bet with Kip Thorne of Caltech: that Cygnus X-1 does not contain a black hole. The form of the bet was that, if it were ever proved that the source is a black hole, Hawking would give Thorne a year’s subscription to Penthouse; but if it were ever proved that Cygnus X-1 is not a black hole, Thorne would give Hawking a four-year subscription to the satirical magazine Private Eye. In June 1990, Hawking decided that the evidence was now overwhelming, and paid up—although, being Hawking, he did so in a typically mischievous fashion, enlisting the aid of a colleague to break into Thorne’s office at Caltech. They extracted the document recording the bet and officially “signed” his admission of defeat with a thumbprint before returning the paper to the files for Thorne to discover later. Over the following months, Thorne duly received the promised issues of Penthouse.
The disparity between the subscriptions wagered simply reflected the different cover prices of the two magazines. But why did Hawking bet against black holes? He called it an insurance policy, hedging his bet. If black holes didn’t exist, he had been wasting his time for most of his career, but at least he would have had the consolation of winning the bet. On the other hand, the only way he could have lost the bet would be if he turned out to be right about black holes, so he was happy to offer Thorne some consolation.
In the eyes of most astronomers, Hawking erred on the side of extreme caution in waiting so long to pay up; he had lost his bet, they reckoned, several years before, for there is no reasonable doubt that Cygnus X-1 is indeed a black hole. And since black holes do exist, that makes Hawking’s investigation of their properties during the early 1970s one of the most important pieces of scientific research ever carried out. This work succeeded not only in partially uniting the general theory of relativity and quantum theory, but also in bringing into the fold the great development of nineteenth-century science, thermodynamics.
Just as Hawking and Penrose had shown that the physics of the Big Bang actually gets simpler, not harder, the closer you delve back toward the beginning, so in the late 1960s other research had shown that collapsing black holes are much simpler than the objects that collapse to form them. You could, in principle, make a black hole out of anything: by squeezing the Earth to the size of a pea; or by adding scrap iron to a heap until you had enough for gravity to take over; or by watching a star much heavier than our Sun run through its life cycle, explode, and die. But however you make a black hole, what you end up with is a singularity surrounded by a perfectly spherical horizon, with a size (surface area) that depends only on the mass of the hole, not on what it is made of.
This basic truth about black holes was established in 1967, by the Canadian-born researcher Werner Israel. When he first developed the equations, Israel himself thought that because black holes had to be spherical, what the equations were telling him was that only a perfectly spherical object could collapse to form a black hole. But Roger Penrose and John Wheeler found that an object collapsing to form a black hole would radiate away energy in the form of gravitational waves—ripples in the fabric of spacetime itself. The more irregular the shape of the object, the more rapidly it would radiate energy, and the effect of this radiation would be to smooth out the irregularities. Penrose and Wheeler showed that any collapsing object would end up perfectly spherical by the time it formed a black hole. The only thing that could affect the appearance of the horizon surrounding the hole, apart from the amount of matter inside it, is rotation. A nonrotating hole is perfectly spherical, whi
le a rotating hole bulges at its equator.
So it was established by the early 1970s that a black hole could rotate, but it could not pulsate (Hawking played a small part in this work, too). The size and shape of a black hole depend only on its mass and the speed at which it rotates; the horizon, all that we can see from the outside Universe, carries no identifying features that can tell us what the hole was made of. Physicists call this lack of identifying features the “no hair” theorem. A black hole has no “hair” in the sense that it has no identifying features, and because all we can ever know about it is its mass and its rate of rotation, this makes the mathematical study of black holes much simpler than scientists had feared it would be.
As nothing can get out of a black hole, its mass can never decrease. So the discovery that the surface area of the horizon can never decrease may not seem that dramatic to ordinary mortals. But Stephen Hawking tells how the moment this hit him was so dramatic that it has stuck in his memory for more than forty years. It happened, as we mentioned in the last chapter, one evening in November 1970, not long after the birth of his daughter, Lucy, as he was getting ready for bed. The idea was so exciting that he spent most of the night thinking about the implications.
He was so excited largely because he and Penrose had only just, at that time, come up with a practical mathematical definition of a black-hole horizon in terms of the tracks of light rays through spacetime. With this definition, he realized, the surface area of the black hole would always increase if matter or radiation fell into the hole, and even if two black holes collided with one another and merged, the area of the new black hole would always be greater than (or, just possibly, the same as) the areas of the two original black holes put together.