by John Gribbin
The two men talked for longer than any of the other guests. Finally the Pope stood up, dusted down his cassock, and gave Hawking a parting smile, and the wheelchair whirred off to the far side of the stage. There were a number of offended Catholics in the hall that afternoon, misinterpreting the Pope’s gesture as undue respect. Many of the non-scientists present were unfamiliar with Hawking’s latest proposals, but his reputation as a scientist with irreligious views was well known. They simply could not understand why the Pope should kneel before him; to them Hawking’s opinions were at the opposite end of the spectrum from orthodox Catholic doctrine. Why had John Paul not taken more interest in them, the faithful?
Back at the DAMTP, work continued as usual. Hawking’s third book for Cambridge University Press was published soon after his return. However, this time things did not run so smoothly, and there was a whole series of arguments between Hawking and Simon Mitton before the book saw the light of day. It was to be called Superspace and Supergravity, aimed at about the same level as The Large Scale Structure of Spacetime, and was expected to sell in similar numbers to its predecessor—between five thousand and ten thousand copies over a period of years. The source of the dispute between Hawking and the publishers was the choice of cover for the book.
Hawking wanted a drawing from the blackboard in his office to be photographed and used on the dust jacket of the hardback edition, as well as on the cover when the book was issued in paperback. The trouble began when Simon Mitton realized that the picture, a bizarre cartoon covered with in-jokes and witticisms done by a group of colleagues after a recent conference at the DAMTP, had been drawn in color and required full-color printing. Hawking would not consider a black-and-white photograph of the illustration and was absolutely adamant about using a full-color representation.
Cambridge University Press insisted that they had never done a four-color cover for a book such as Hawking’s, which, even accepting his international fame as a scientist, would not sell enough copies to warrant the expense. The cover, they stated, would make absolutely no difference to the number of copies the book sold. At this point Hawking saw red and declared that unless they agreed to use his cover he would withdraw the book completely. After a hastily convened editorial meeting, Mitton capitulated, but he’d been right—Superspace and Supergravity sold marginally less than The Large Scale Structure of Spacetime.
While the dispute with Cambridge University Press was in full flow and Hawking miraculously found time to work, travel, see his family, and engage in bureaucratic wrangles with the city authorities and university, the world at large was going through its usual turmoil. Riots hit British cities; there was intensified fighting in Beirut; and President Anwar Sadat of Egypt was brutally assassinated on October 6 during a military parade in Cairo. In December, doctors in the United States were alerted to a deadly new illness that appeared to attack the body’s immune system. But the news in 1981 was not all bad. In July an estimated 700 million TV viewers tuned in to see Prince Charles marry Lady Diana Spencer in St. Paul’s Cathedral; England claimed a remarkable cricketing victory against Australia; and the New Year Honors List announced at the end of December included a wheelchair-bound Cambridge physicist who had pioneered important work on black holes—Stephen Hawking was made a commander of the British Empire by Queen Elizabeth II.
As the 1980s progressed, awards and honors continued to be bestowed on Hawking. In 1982 alone he was made honorary doctor of science by no fewer than four universities: the University of Leicester in Britain, and New York, Princeton, and Notre Dame universities in the United States.
The interest of the media intensified as Hawking’s recognition grew. In 1983, a BBC Horizon program profiled him at work at the DAMTP. For the first time, the British public was given a chance to see Professor Hawking whirring around Cambridge in this wheelchair, talking in his strangely contorted way with his students and coworkers, at home on West Road with Jane and the children, and attending official functions. The public was captivated. One magazine article after another appeared in rapid succession. The London Times and Telegraph newspapers ran pieces about him, and in-depth interviews turned up in the New York Times, Newsweek, and Vanity Fair. A few short years into the decade, and “black hole” and “Stephen Hawking” had become synonymous in the eyes of the media and the general public.
Hawking has never been a man to shy away from publicity, and he thoroughly enjoyed his growing fame. However, fame alone does not pay the bills, and in the early eighties there were intensifying financial pressures on the Hawking household. A professor’s salary is not large compared with equivalent positions in industry or commerce, and occasional monies from prizes and awards were erratic and usually too small to make any real difference. With the strain of running a home and maintaining her own career, Jane was finding that the little nursing help they could afford was growing increasingly inadequate. She desperately needed more private nursing assistance, and that would be expensive.
That was not all. They had managed to finance their eldest son Robert’s education at the fee-paying Perse School in Cambridge since the age of seven. He had been highly successful academically and was scheduled in a few short years to go to university. Grants were available, but they would not cover all the expenses of a three-year degree course. Coinciding with these problems was the fact that, in 1982, Lucy was in her final year at a junior state school, Newnham Croft. Stephen and Jane both wanted her to attend the Perse School as her brother had done. With Timothy growing and everyday family expenditures increasing, there seemed to be no way for them to afford school fees for two children.
And what of the future? Stephen’s illness had been stable for a number of years, but things could begin to slide again at any time—that was the nature of the disease. If he could no longer work, the prizes would soon dry up and his pension from the university could not sustain them comfortably. There was another great fear: if Jane could no longer look after Stephen and earn a salary, what would become of him? They did not like to discuss the awful possibilities, but they were there and had to be faced. They needed money, quickly. The last thing any of them wanted was for Stephen to end up in a nursing home, if his condition should degenerate further, simply because they could not afford to look after him at home.
Something had to be done, and fast. Hawking had the germ of an idea in the back of his mind. He had mentioned it to no one but had allowed it to grow and develop. Now, he realized, he would have to put his idea into action. It would be a number of years before Hawking’s secret plan would come to fruition and, with one stroke, solve the family’s financial problems. When it did, it was to change everything. But first there were intriguing developments to follow up in the field of inflationary cosmology.
13
WHEN THE UNIVERSE HAS BABIES
Even though Hawking has offered us an image of a self-contained Universe, with no boundaries and no edges, either in space or time, many people still wonder what might lie “outside” such a Universe. The analogy between the closed surface of the Universe and the closed surface of the Earth does, after all, encourage us to speculate that there might be other universes, just as there are other planets.
Within the framework of Hawking’s no-boundary Universe, any such other worlds would have to be embedded in some strange form of space which has more than the three dimensions we are used to: the surface of a sphere, after all, is actually a two-dimensional surface wrapped around in the third dimension, but spacetime is four-dimensional; you always need at least one extra dimension to wrap up anything into a closed surface. But there is another model—or rather series of models—developed from the inflationary scenario which offers us another way to imagine many worlds coexisting, without having to try to wrap our brains around the higher geometries of five or more dimensions (four of space plus one of time). Although Hawking himself has expressed reservations about the idea, which goes by the name of continual inflation, it is in fact based on his dramatic breakthrough
discovery from 1974: that black holes explode.
Just after the Planck time, according to the inflationary scenario, the vacuum itself was in a “false” state, excited and full of energy, like supercooled water. When the false vacuum underwent a transition into its stable, lower-energy state, this energy went into the phenomenal burst of expansion that is known as inflation, creating the smooth Big Bang out of which the Universe as we know it has evolved. But suppose this transition did not happen everywhere at the same time.
Almost as soon as Alan Guth came up with the idea of inflation, researchers such as Alex Starobinsky and Andrei Linde realized that different regions of the primordial false vacuum might have made the transition into the low-energy state independently. The effect would be rather like unscrewing the cap of a bottle of fizzy drink—a myriad of bubbles would appear throughout the fluid, each corresponding to a stable vacuum expanding in its own way. Unlike the bubbles in your fizzy drink, though, each of these bubbles would carry on expanding, until all the fluid had gone and only bubbles remained.
This possibility raised serious technical problems for early versions of the inflationary scenario because if two or more expanding bubbles were to merge, they would create disturbances that would spread right through both bubbles. If we lived in a universe that had formed in this way, it would not be perfectly uniform, because these disturbances would leave their mark—for example, on the microwave background radiation.
There are ways around this problem. The notion that Hawking himself favors is that of “chaotic inflation,” in which the world beyond our Universe (the infinite “meta-universe,” now usually called the Multiverse) is in a messy state, with some regions expanding, some contracting, some hot and some cold. In such a chaotic meta-universe, there must inevitably be some regions just right for inflation to take place. We just happen, in this picture, to be in a universe produced by a random fluctuation within the chaos.
But you don’t have to invoke chaos to explain our existence. Maybe we just happen to live in a bubble that hasn’t (yet!) merged with any of its neighbors (if this sounds like an extraordinary coincidence, it may not be, as we shall see later in this chapter). Or perhaps some law of physics prevents bubbles from forming very close together in the “fluid” of the false vacuum. This is where the proposal that Hawking Radiation might be involved comes in.
Hawking Radiation, as we saw in Chapter 9, is produced by the interplay of quantum effects and gravity at the horizon surrounding a black hole. But Hawking and his colleague Gary Gibbons, who shared an office with him in Cambridge in the late 1970s, realized that this kind of radiation must be produced wherever there is a horizon of this kind, and that such horizons do not always surround black holes.
Because of the way the Universe expands, the more widely separated two regions are, the faster they recede from each other. So regions of space that are far enough apart can never “communicate” using light beams (or, indeed, anything else) because the space between them expands faster than light can travel. If light cannot travel from one region to another, then in effect there is a horizon which light cannot cross, separating the two regions of space as effectively as the horizon surrounding a black hole separates the inside from the outside.
Hawking and Gibbons showed that this kind of horizon will also produce radiation, just like the radiation at the horizon around a black hole, spreading out from the horizon into both regions of space. In the Universe as it is today, spread thin by expansion, the effect of this radiation is tiny, but it could have played a much bigger role in the early stages of the expanding Universe. The expansion of the Universe is steadily slowing down, as the gravity of all the matter in the Universe tries to pull everything back together in a Big Crunch. So the expansion rate was much faster, and the effect of Hawking Radiation from horizons therefore more pronounced, when the Universe was younger. Long ago, even rapidly separating regions had not had time to move far and were much closer together.
The notion that radiation produced by horizons might affect the expansion of the Universe has been enthusiastically taken up and combined with the idea of inflation by Richard Gott of Princeton University. Andrei Linde has also investigated it, but he has made less noise about the idea than the ebullient Gott.
It turns out that under the right conditions, the Hawking Radiation produced in a volume of space filled with horizons of this kind can provide the energy that drives inflation and makes the Universe (or rather the Multiverse) expand super-fast. The super-fast expansion then creates more horizons, which in turn produce more radiation, driving the super-fast expansion in a self-sustaining continuing process of inflation. The bubbles of ordinary low-energy stable vacuum that form within this infinite sea of inflationary expansion grow at a slower rate; and so even if two bubbles form next to each other, they will be kept apart by the rapid growth of the false vacuum of the Multiverse between them.
The “right” conditions for this process to work are mind-boggling. The temperature of the Hawking Radiation has to be about 1031 K, and the density of mass-energy in the false vacuum has to be an even more staggering 1093 grams per cubic centimeter. And everywhere throughout this extraordinary, rapidly expanding false vacuum, bubbles of stable vacuum are forming and becoming universes in their own right.
In this scenario, there is not just one Universe but an infinity of universes, forever separated from one another by the impenetrable walls of the super-dense false vacuum. In a sense, such a concept is meaningless. The existence of other universes which we can never observe, and which can never have any interaction with our Universe, is a matter more suitable for discussion among philosophers than astrophysicists. But it turns out that there are more ways than one to make a universe and that in some scenarios universes can interact with one another, producing consequences of interest to everybody, not just to astrophysicists and philosophers.
With all this talk of superdensity and superenergy, and numbers like 1093 grams per cubic centimeter being bandied about, it is natural to wonder how much mass-energy our entire bubble Universe contains (assuming, that is, that any of these scenarios have a grain of truth in them). The answer is perhaps even more startling—none at all! Let us leave the discussion of continual inflation to the philosophers and look again at Hawking’s no-boundary model of the Universe to see how this can possibly be true.
We are used to thinking of mass-energy chiefly in terms of lumps of matter: stars, planets, and so on. Each of them contributes its own amount of mc2 to the total mass-energy of the Universe. But there is another, equally important contribution (exactly equally important, if Hawking’s ideas are correct): it comes from gravity. And there is a strange thing about gravitational energy—it is negative.
To understand what this means, physicists talk in terms of the gravitational energy of a hypothetical collection of particles. This is zero if the particles are dispersed to infinity, spread apart from one another as far as possible. But if the collection of particles falls together under the influence of gravity, perhaps eventually to make a star, it loses gravitational energy. Since the particles start with zero energy, this means that by the time they have collected together to form a star or a planet they have negative energy. And if all the matter in the entire Universe could be collected together at a single point, its negative gravitational energy (–mc2) would exactly cancel out all the positive mass-energy (+mc2) of all the matter.
But that is exactly how we think the Universe did start out: with all its mass-energy concentrated in a point. The closed Universe scenarios actually describe a situation in which a point of zero energy becomes separated into matter (with positive energy) and (gravity with negative energy), expands out to a certain size, and then collapses back into a point of zero energy again. At first, the idea seems ridiculous. However, this is not some crackpot, lunatic-fringe theory, but a respectable cosmological idea, backed up by the equations of relativity.
The Universe, it seems, is the ultimate free lunch. And if
the Universe contains zero energy, how much energy does it take to make a universe? Not a lot—certainly not very much compared with the amount of mc2 contained in your body or the pages of this book. For according to Alan Guth and his colleague Edward Fahri, all you need is enough energy to squeeze some matter into forming a black hole. Then the new universe comes free—one universe free with every black hole. In a tour de force to rank with the great conjuring tricks, Guth and Fahri have shown that the two great threads of Hawking’s life’s work are really one and the same: black holes are big bangs.
In principle, the seeds of entire universes could be produced out of nothing at all, in a manner reminiscent of the way pairs of virtual particles can be produced out of nothing at all by quantum uncertainty (as we saw in Chapter 9). Such a baby universe would be in the form of a super-dense concentration of mass, smaller than a proton but containing no energy because the mass is balanced by negative gravitational energy. Of course, according to the ideas of the 1970s and before, such tiny super-dense seeds would immediately collapse back into nothing under their own weight. But inflation provides a way to blast out such a seed to form an expanding universe before gravity can make it collapse. It would then take many billions of years for gravity first to halt the expansion and finally to make the universe disappear into a Big Crunch.
