Stephen Hawking (31 page)

The helpful image is the old familiar one of the Universe represented by the skin of an expanding balloon. What we have to imagine now is that a tiny piece of that skin is pinched off, forming a little blister connected to the Universe by a narrow throat—the black hole. That little blister can now, in turn, expand to enormous size, while all that any resident of the parent universe sees is the tiny black-hole throat in the fabric of spacetime. And the whole process can repeat indefinitely, producing an infinite foam of bubbles, each one a universe in its own right. Quantum cosmology actually allows the possibility of creating not just one Universe but an infinite number of universes, out of nothing at all.

And this raises another question. At one level, physics operates by finding out the rules according to which the Universe operates and using them to make predictions about how systems will interact. We find, for example, that the speed of light has a certain value, and that this is the ultimate speed limit. That enables us (or at least it enabled Einstein) to work out how our view of the world changes when we move at high speed. But at another level, some physicists puzzle over why the rules should have the precise form that we find.

Why, for example, is the speed of light 300,000 kilometers a second, rather than, say, 250,000 kilometers a second? Why does Planck's constant have the precise value it has, and not one a little bigger or a little smaller? What would happen if gravity were weaker (or stronger)? And so on. We live in a world that seems to be just right for life-forms like us—which is in a way tautological, since obviously if the world were very different we would not be here to wonder about these things. But as far as anyone is yet able to tell, the rules of physics that came out of the era of inflation could have been different from the rules we know, either subtly different or dramatically different. Is it, then, just a coincidence that these rules have produced a Universe suitable for people like us to live in? The idea of an infinity of bubble universes, either formed out of an eternally expanding false vacuum or pinched off from one another by the baby process, says that it is not—and explains other cosmic coincidences as well.

The idea of trying to understand the nature of the Universe in terms of the relationship between the laws of physics and ourselves is known as “anthropic cosmology.” It has a long history, but in its modern version it stems mainly from a revival
of interest triggered by Martin Rees, of the University of Cambridge, in the 1970s and continuing to the present day.

Rees is an exact contemporary of Hawking. He was born on June 23, 1942, when Hawking was six months old. They were working for their Ph.D.s in Cambridge at the same time, and Rees became Plumian Professor of Astronomy and Experimental Philosophy in 1973, at the remarkably early age of thirty-one, just six years before Hawking became Lucasian Professor. He was elected a fellow of the Royal Society in 1979, five years after Hawking. But where Hawking has made his reputation by investigating in great detail one particular set of problems—the singularities and horizons around black holes and at the beginning of time—Rees is known and respected for the breadth of his work, ranging from quasars and pulsars to the influence of black holes on their surroundings, cosmology, and the nature of the dark matter that holds the Universe closed. When Rees turned his attention to anthropic cosmology and stirred the revival of serious interest in the subject by scientists in the 1970s and 1980s, for once Hawking was prepared to follow somebody else's lead.

Rees has developed a particularly nice example of the nature of anthropic reasoning in cosmology. He has worked out in detail the evolution of a universe in which gravity is stronger than in our Universe, but every other rule of physics is the same. Galaxies, stars, and planets can all exist in this model universe, but they are all very different from their counterparts in our Universe. In particular, everything is speeded up to such an extent that it is doubtful whether intelligence (which has taken more than four billion years to emerge on Earth) could ever evolve.

For the particular value of the strength of gravity chosen by Rees, each star has a mass about the same as that of an asteroid in our Solar System (much less than the mass of the Moon) and a diameter of about two kilometers. The typical lifetime of such a star is just one of our years, and it burns with a brightness a hundred-thousandth that of our Sun. The Earth has an average surface temperature of about 15°C, and a planet in this other universe, orbiting around its parent star at a distance roughly twice as great as the distance from the Earth to the Moon, would have a similar surface temperature. It would take about twenty of our days for the planet to orbit the star. So with the star itself having such a short lifetime, it would be burned out in just about fifteen of the planet's “years,” whereas the lifetime of our Sun is likely to be at least ten billion of our years.

Life on the surface of such a planet would be short, in more ways than one. The biggest mountains on the tiny planet could be no more than 30 centimeters high, while the maximum mass of any creatures roaming its surface would be just one-thousandth of a gram—any bigger than this and their bodies would break if they fell over in the strong gravity of that world.

And all of these dramatic changes stem, remember, from making a change in just
one
of the constants of physics, the strength of gravity! It is possible to imagine very many changes that would ensure that the universe that emerged from the inflationary phase would be quite inhospitable for life-forms like us.

If ours is the only possible Universe, then the existence of the cosmic coincidences that permit our existence is a real
puzzle. But if there are many possible universes, then there is a straightforward explanation. Every different bubble universe may have its own laws of physics. In some cases, that will mean that the bubbles are held together very tightly by gravity and re-collapse before life can evolve. In others, gravity may be so weak that material is never pulled together to form stars and planets at all. But there will be a range of possibilities—a range of universes—where stars, planets, and intelligences can evolve. The same argument applies to each and every one of the exact values of the laws and constants of physics.

If this picture is correct, it means that there may be an infinite number of universes in the Multiverse, and out of that infinite number life-forms like us will exist only in universes where the laws of physics are just right. The fact that we exist preselects, to some degree, the exact rules of physics that we will discover the Universe operates on. This idea is known, rather grandly, as the “anthropic principle,” a term coined by Bernard Carr, who worked with Rees on a seminal paper on the topic.

Of course, because the different universes can never communicate with one another, this is still largely a matter for the philosophers to debate. Except for one thing. Remember that the crucial ingredient of Hawking's no-boundary model is the sum-over-histories quantum approach. When we mentioned this earlier, we rather glossed over the explanation of what, exactly, the different histories that were being “summed” were. Now we can set the record straight.

Instead of regarding all the different possible universes that could have emerged out of inflation, each with its own set of
physical laws, as “real,” we can regard them as mathematical possibilities, like the many different paths that an electron can take from A to B. And using the sum-over-histories approach, Hawking shows not only that our Universe is one of the
possible
histories, but also that it is one of the most
probable
ones:

. . . [I]f all the histories are possible, then so long as we exist in one of the histories, we may use the anthropic principle to explain why the universe is found to be the way it is. Exactly what meaning can be attached to the other histories, in which we do not exist, is not clear.
²

Nevertheless, using the “no-boundary” condition, Hawking and his colleagues have found that the Universe must start out with the maximum amount of irregularity allowed by quantum uncertainty, and that inflation and the subsequent more leisurely expansion of the Universe then make these irregularities grow to become the clouds of gas that then contract to become galaxies of stars within the expanding Universe.

All of this is very much research at the cutting edge of science today. The choice of different variations on the theme—bubbles in a continually inflating false vacuum, baby universes, a choice of quantum histories—reflects not an inability of physicists to make up their minds, but an attempt to push ahead on many different fronts, not yet knowing which (if any) will turn out to hold the most promise in the long term. But it is already clear that in the 1990s the basic
premises underlying cosmological thinking changed dramatically from those of what we might call the “pre-Hawking” era. Fifty years ago, it was generally accepted that our Universe was unique. Today, it seems to be generally accepted that, one way or another, it is just one among many. Is it any wonder that, when Hawking presented these ideas in a book in 1988, the book took the world by storm?

14

A BRIEF HISTORY OF TIME

T
he dying notes of Tears for Fears' “Mad World” lead into Radio One's 12:30
P.M.
news as Simon Mitton walks into the DAMTP and a car with its window down and radio turned up parks in front of the building on the other side of the cobbled courtyard. The news report is full of peace protesters at Greenham Common, British troops in the troubled city of Beirut, and the biggest Christmas film ever,
E.T.
, but Mitton has other astronomical thoughts on his mind. He is visiting Stephen Hawking to discuss the imminent publication of the professor's new book for Cambridge University Press,
The Very Early Universe
. Unexpectedly, however, after talking through the latest details of the book over tea and biscuits, the two of them fall into a discussion about something altogether different—a popular cosmology book, which Stephen has been mulling over for some time.

For almost as long as he had known him, Mitton had been intimating to Hawking that he should attempt a cosmology book aimed at the popular market. Hawking had displayed little interest in the idea, but by late 1982 he had come to recognize that such a project might provide the answer to his looming financial difficulties, and he decided to revive the idea. The two of them had enjoyed a fruitful publishing relationship for many years, and, despite the problems over
Superspace and Supergravity
, Hawking's first thought was to approach Cambridge University Press with the proposal. Mitton's original intention was for Hawking to attempt a book on the origin and evolution of the Universe. Cambridge University Press had enjoyed a long tradition of publishing popular science books written by eminent scientists, such as Arthur Eddington and Fred Hoyle, whose titles had sold well. A popular book by Stephen Hawking would, he believed, neatly follow on from these.

According to Mitton, Hawking laid things on the line immediately. He wanted a lot of money for this book. Mitton had always known him to be a tough negotiator; that was clear from the fracas over the cover for
Superspace and Supergravity
. When it came to financial matters he was prepared for some intransigence, but in the event even Mitton was surprised by Hawking's suggestions. At their first organized meeting to discuss the book, Hawking opened the conversation by
explaining his financial situation, making it clear that he wanted to earn enough money to continue financing Lucy's education and to offset the costs of nursing. He was obviously unable to provide any form of life insurance to protect the family in the event of his death or complete incapacitation, so if he was going to spend a considerable amount of his valuable time away from research writing a popular book, he expected an appropriate reward.

Mitton is philosophical about the whole matter, pointing out that Hawking was showing remarkable loyalty toward Cambridge University by staying there. There is absolutely no doubt that he could have commanded a huge salary from any university in the world. A number of colleges in the United States would have offered him six-figure sums simply for the prestige accompanying his international fame, not to mention the enormous kudos of cashing in on the important breakthroughs he would almost certainly make in the near future. The fact that he remained in Cambridge for a fraction of the salary he could command elsewhere is, Mitton believes, a great credit to him. The simple fact is that the Hawkings loved Cambridge. They had lived there for nearly two decades, and Stephen had spent practically all of his academic life at the university. The DAMTP is, without doubt, one of the best theoretical physics departments in the world, and he would have left it only as a last resort.

In the early eighties, Simon Mitton's office was based in the same courtyard as the DAMTP on Silver Street, so the two of them had plenty of opportunity to talk about the proposed project. One afternoon, Hawking went to see him with the rough draft of a section of the proposed book. Mitton knew
the commercial market as well as any publisher. In fact, he was by that time the author of several successful popular science books himself. He had a very clear idea of the type of book the general public would want and which would earn Hawking the sort of money he was after. After looking through the section Hawking had shown him, he came to the conclusion that it was far too technical and highbrow for the general reader. “It's like baked beans,” he told Hawking. “The blander the flavor, the broader the market. There simply isn't a commercial niche for specialist books like this, Stephen.”

Hawking went away and thought about Mitton's comments; Mitton went to the Cambridge University Press Syndicate to see what they thought of the idea. The two men met up again shortly afterward. Mitton had the encouraging news that the Syndicate had accepted the idea of the book with glee and had handed over to him all negotiations. Hawking, for his part, had done a little editing of the section he had written earlier. Mitton sat back and flicked through the manuscript as Hawking remained motionless in his wheelchair on the other side of the room, patiently awaiting his opinion. Finally, Mitton put the typescript down on his desk and looked across at him.