Stephen Hawking, His Life and Work

by Kitty Ferguson

  Another radio astronomer, Bernard Burke, heard about Penzias’s and Wilson’s puzzle with the antenna, and he knew, as they did not, of current work by Robert Dicke at Princeton. Dicke, following up on a proposal made in the 1940s by the Russian-born George Gamow and Americans Ralph Alpher and Robert Herman, was building an antenna to search for radiation surviving from not long after the origin of the universe, from an era when, if the Big Bang theory was correct, the universe would still have been very hot. Gamow, Alpher and Herman had theorized that such radiation should exist, and that by our own era its temperature should have cooled to about five degrees above absolute zero. Burke brought Penzias, Wilson and Dicke together, and they concluded that Penzias and Wilson had discovered by accident the radiation that Dicke had been hoping to find.

  The discovery of what soon became known as the ‘cosmic microwave background radiation’, or CMBR, was dramatic support for the Big Bang theory, for clearly the universe had once been very much hotter and denser than it is now. Hawking and his friend George Ellis wrote a paper in 1968 emphasizing how strongly this evidence supported the Big Bang.6 However, it also posed a problem for that theory. In repeated measurements, taken out as far as possible in every direction, researchers found that the temperature of the radiation was the same and failed to find small variations in the CMBR that could have resulted in the structure we see today.

  Those problems aside, support for the Big Bang kept coming. There was a discovery that quasars, which theorists were realizing might be an early stage of galaxy formation, exist only at enormous distances from Earth. If the Steady State theory is correct, with galaxies continually spreading further and further apart and the emptiness between being filled by the formation of new galaxies, and if quasars are part of the process of galaxy formation, we ought to find quasars fairly evenly distributed near and far throughout the universe. They are not. Quasars’ vast distance from Earth in space (and, by virtue of that fact, in time) means they must have existed only when the universe was much younger than it is now. This particular stage of galaxy formation must have occurred only in the distant past, has not occurred again in later epochs of the universe’s history, and is not going on today.

  Yet another nail in the Steady State theory’s coffin was hammered into place in 1973 when balloon experiments at Berkeley found that the spectrum of the cosmic microwave background radiation was the spectrum that Big Bang theory predicted. And studies of the abundances of various elements in the Milky Way and other galaxies showed that Big Bang predictions of these abundances were on target.

  Nevertheless, in the 1970s the Big Bang theory still had stumbling blocks to overcome. While Hawking had diverted his attention to black holes, the question of how the universe began had also never been far from his mind; and how to solve the lingering problems of the Big Bang theory had remained high on the agenda for many of his colleagues around the world. These problems became known as the ‘horizon problem’, the ‘flatness problem’ and the ‘smoothness problem’.

  The horizon problem had to do with the observation that the cosmic microwave background radiation is the same in all directions in areas of the universe too far apart for radiation ever to have passed from one to the other, even in the earliest split seconds after the Big Bang. The intensity of radiation is so remarkably close to identical in those remote areas that it seems they must somehow have exchanged energy and come to equilibrium. How?

  The flatness problem had to do with the question of why the universe has not either long ago collapsed again to a Big Crunch or else experienced such runaway expansion that gravity wouldn’t have been able to pull any matter together to form stars. A universe somehow poised, as ours seems to be, between those possibilities is so unlikely as to boggle the imagination. The expansive energy (resulting from the Big Bang) and the force of gravity would have had to be so close to equal that they differed from equality by no more than 1 in 1060(1 followed by sixty zeros) at a time less than 10−43 seconds after the Big Bang (a fraction with 1 as the numerator and 1 with 43 zeros as the denominator).

  The smoothness problem was that, judging by the CMBR, the early universe must have been smooth, without any lumps, clumps, ridges or other irregularities. The question became one of the foremost challenges of astrophysics – a ‘missing link’ in Big Bang thinking: how did a universe that looked so uniform in the era from which the CMBR comes to us, when the universe was 300,000 years old, become so diverse and clumpy all these years later – with stars, galaxies, galaxy clusters, planets – even such small clumps of matter as you and I? Why was it not possible to see even faint beginnings of that differentiation in the cosmic microwave background radiation?

  If that last seems an unlikely problem, think about Wheeler’s democracy: the closer to one another the particles are, the stronger they feel each other’s gravitational pull. If all particles of matter in the universe are equidistant and there are no areas in which a few particles have drawn together even slightly more densely, then every particle will feel equal pull from every direction and none will budge to move closer to any other particle. It was this sort of gridlock researchers seemed to have discovered in the early universe where matter appeared to have been distributed so evenly that it could never yield and form the structure evident in the universe today.

  In the mid-1970s, when Hawking first visited Caltech, no theorist had been successful in overcoming any of these stumbling blocks.

  Inflation to the Rescue!

  In the late 1970s a young particle physicist at the Stanford Linear Accelerator in California, Alan Guth, came up with a significant revision to the history of the universe as it had been written by cosmologists up to that time. Recognizing right away that he had hit on a good thing, Guth wrote ‘SPECTACULAR REALIZATION’ in his notebook and drew two concentric boxes around the words. His realization offered a brilliant solution to still stubborn problems in Big Bang theory, and also suggested a way in which the universe could have got to be like the present universe without requiring that its initial state be chosen with such exquisitely precise care.

  Guth proposed that the universe might early on have undergone a brief interval of stupendously rapid growth, before settling down to continue expanding at the rate we find in our own era. It was the ‘settling down’ that particularly set his idea apart. Others had found solutions to Einstein’s equations that produced a universe in which the expansion would accelerate during the whole of its existence, or where the expansion began by decelerating and then started to accelerate and never stopped. Guth’s universe had only a short surge of accelerated expansion in its extreme infancy.

  Guth worked out a process in which the universe, at a time less than 10−30 seconds after the Big Bang (a fraction with 1 as the numerator and 1 followed by thirty zeros as the denominator), was subject to a huge repulsive force that for only a short interval behaved like the cosmological constant that Einstein rejected. During a period that lasted only an unimaginably small fraction of a second, this force would have accelerated the expansion, causing violent runaway inflation in the dimensions of the universe from a size smaller than a proton in the nucleus of an atom to about the size of a golf-ball.

  In the more than thirty years since Guth first suggested it, physicists have been embroidering on his idea, coming up with new versions, and trying to work out why and how this process might have happened. In order to understand this work, you must get your head around some terminology.

  Begin with symmetry-breaking: a simple example is a rod set on end. It could fall in any direction. Gravity, which makes it fall, is ‘symmetrical’; it has no preference which direction the rod falls. All directions are equally likely. But when the rod falls it will fall one way or another, not every way at once. When the rod falls, the symmetry is broken. Hawking used another example in A Brief History of Time: picture a roulette wheel. The croupier spins the wheel and the ball rolls round and round. The situation is ‘symmetrical’. Although you, who have placed a bet,
may have a preferred outcome for this spin, the physics of the situation has no preferred outcome. The wheel slows and the speed (the high energy) of the ball decreases. Finally it falls into one of the slots in the wheel. The symmetry is broken.7

  To demonstrate the meaning of ‘false vacuum’ and ‘true vacuum’, both of which are relevant in understanding inflation, physicists often like to picture a man’s hat, the kind with a brim around the edge and a depression in the crown. Put a marble into the depression in the crown of the hat. It will settle down at the lowest spot available. This is not the lowest it could get on the hat, but it’s the lowest on the crown of the hat. Likewise, elementary particles can ‘land’ in a number of temporary energy levels. These resting places are ‘false vacuums’. Jostle the hat or let the marbles bump around against one another and the marbles may roll from the crown of the hat to the brim. In this analogy the brim represents the ‘true vacuum’. We can recognize it as the lowest possible energy level in this system.

  Something like that could have happened as the universe cooled down. Some of the matter started moving towards a new state with lower energy, in the process liberating the gravitationally repulsive stress that caused the rapid acceleration that was inflation. But, to return to the analogy, the marbles probably didn’t all roll down at once. How they rolled, how fast they rolled, how they made it over the edge of the crown, or through the edge of the crown – or whether inflation happened in an entirely different way – these questions have been occupying cosmologists for decades. Nearly all, however, agree that inflation occurred. Inflation has become part of the ‘standard model’.

  Another useful bit of vocabulary is ‘phase transition’. An everyday example of a ‘phase transition’ is the freezing of water (which also involves a breaking of symmetry). Water as a liquid is symmetrical, the same everywhere and in every direction. Lower the temperature sufficiently and ice crystals form. No longer is everything the same everywhere. The crystals have certain positions and not others, and they line up in some direction and not another. The symmetry is broken. However, if you reduce the temperature of water very carefully, it can go below freezing without ice forming, without the symmetry being broken. The name for this is ‘supercooling’. It happens in nature when liquid drops of rain fall during a winter storm and remain liquid even though the air temperature is below freezing, until they encounter something – a tree, a pavement – and immediately freeze.

  Alan Guth’s suggestion rested on the idea that the universe immediately after the Big Bang would have been extremely hot, with all the particles moving around very rapidly, at high energies. The four forces of nature discussed in Chapter 2 – gravity, electromagnetism, the strong nuclear force and the weak nuclear force – were still united at that time, undifferentiated, as one superforce. The universe expanded slightly, cooled slightly. The particle energies became slightly less, and, as the universe cooled, the forces became separate and distinct from one another. Their initial symmetry was broken. One by one they ‘froze out’. It didn’t all happen at once, however, because supercooling occurred. There was a phase transition, with bits of the universe going through this transition separately in the form of bubbles where the temperature dropped below some value (analogous to water going below the freezing point) without the symmetry between the forces being broken. The result was that the universe entered an unstable, supercooled state, with more energy than it would have had if the symmetry among the forces had been broken.

  Bubbles, marbles, hats, frozen water … the bottom line of early inflation theory was, Hawking explains, that during this interval all regions of the universe – including those with more particles than average, and those with less – expanded at an enormous rate, faster than the speed of light. Even where there were more matter particles than average and you might expect gravity to be drawing them together, that could not have happened. As matter particles got farther apart, the result would have been a universe, still expanding, with all its particles few and far between. The expansion would have smoothed out irregularities, meaning that the universe’s modern-day smooth and uniform state could have evolved from a great variety of different initial states. The rate of expansion would automatically have become close to the critical rate, solving the ‘flatness problem’ without the initial rate having been so carefully chosen.8

  How, then, did the expansion slow down again? Like the rain drops that resist turning to ice but must eventually freeze, the universe had to complete its temporarily arrested phase transition. Guth’s original proposal had it that when supercooling occurred, in this overall milieu of unbroken symmetry, bubbles of broken symmetry formed, and these bubbles expanded and joined with one another until everything everywhere was in a new phase of broken symmetry, and the universe was expanding more or less as we discover it doing today.

  Initially, there was a problem with this idea. The bubbles would have expanded so quickly that they would have collided with one another, resulting in many irregularities and enormous variations in density and in the expansion rate from one part of the universe to another. This situation could never have developed into our universe.

  Nevertheless, Guth went ahead and announced his theory. It promised too much to be allowed to flounder on a glitch he was certain he or others would be able to solve later. It offered solutions to the remaining problems in Big Bang theory: our visible universe could have emerged from a region originally so tiny that it had the opportunity to reach equilibrium before it inflated. The period of runaway inflation could have wiped out the imbalance between the expansive energy and the contracting force of gravity. The prediction that inflation would generate areas of slightly higher and slightly lower density – the seeds of future galaxies, supergalaxies and all the other structure that has evolved in the universe – was particularly promising. Technology for observing the cosmic microwave background radiation had not yet been able to reveal those ‘density perturbations’, but Stephen Hawking and others had been thinking about them and the smoothness problem since the mid-sixties when Wilson and Penzias had discovered the CMBR. Could inflation provide the answer cosmologists had been looking for?

  Hawking, like Guth himself, was not satisfied. Hawking’s objection to inflation theory was not that the bubbles would collide and cause havoc instead of a smooth universe, but that in the inflationary phase the universe would have been expanding too rapidly for bubbles of broken symmetry ever to join with one another. He thought they would have scattered apart too quickly, even if they were growing at the speed of light. The result would have been a universe where the symmetry among the four forces was broken in some areas and not in others – again, definitely not our universe. With that in mind in October 1981, Hawking set off for a conference in Moscow.

  A Debate in Moscow

  The Russian physicist Andrei Linde, aged thirty-three, a graduate of Moscow University and the P. N. Lebedev Physics Institute in Moscow, was about to encounter Stephen Hawking for the first time under rather teeth-grinding circumstances at that Moscow conference.

  A few years before Alan Guth developed and published his model of inflation, Linde himself had been thinking along those same lines but recognized that this type of theory had problems. Not so reticent as Linde, Guth of course had recognized the same problems but daringly and, as it turned out, wisely gone ahead and published his paper anyway, beating Linde to the draw. That setback notwithstanding, it would not take long for Linde to catch up and move to the head of the roster of cosmologists working in the field of inflation theory. After 1990 he would also be famous among colleagues at Stanford for his sleight-of-hand magic, acrobatics and hypnosis, but in 1981, when he met Hawking at the conference in Moscow, he was still relatively inexperienced, barely known in the West, and had never been to America or Europe. Stephen Hawking was a distinguished and honoured attendee.

  Linde and Hawking both presented papers. In his presentation Hawking spoke of his recent findings that inflation would have generated densi
ty perturbations that were too large to result in the universe as we find it today. Linde, in his presentation, explained a way he had worked out the previous summer to solve the problems in his and Guth’s original inflation models. Because of the Soviet Union’s long delays due to censorship before a paper could be released, Linde’s ‘new inflation’ paper would not be published until early in 1982. Linde had no opportunity at the conference to discuss his ideas with Hawking, but after it ended, circumstances brought them together. At the celebration of Hawking’s sixtieth birthday in 2002, Linde vividly described both the trauma and the eventual success of that first meeting.9

  The Sternberg Astronomy Institute in Moscow had invited Hawking to give a lecture the day after the conference ended, and he had chosen as his subject problems in Alan Guth’s inflation theory. At the last minute, Linde, who was fluent in English and Russian, was asked to translate. This was during the time when it was customary for Hawking to have one of his students deliver his talks while he listened and occasionally intervened to make a comment or correction. For some reason this talk had not been prepared in that way. Linde recalled the tedious two-stage translation process. Hawking said something in his garbled voice; Hawking’s student struggled to understand and then repeated it clearly in English; Linde translated it into Russian. It all moved at a glacial pace. However, Linde knew the subject well and began adding explanations in Russian. Hawking would make a statement; the student would repeat it; Linde would expound, saving Hawking the trouble of explaining what he had said. Hawking seemed not to object and all went much more smoothly and rapidly, as long as they were talking about the old inflation theory.

  There came a moment, however, when Linde was startled to hear the student say, for Hawking, that Andrei Linde had recently ‘suggested an interesting way to solve the problems of inflationary theory’.10 Linde was delighted to translate that announcement into Russian. Top Russian physicists were about to hear Stephen Hawking explain his (Linde’s) theory! His future in theoretical physics seemed exceedingly bright, but only for a few seconds. Hawking proceeded to tear apart Linde’s new inflation scenario. For a painful and embarrassing half-hour Linde ‘was translating for Stephen and explaining to everyone the problems with my scenario and why it did not work’.11 At the end of the talk, Linde summoned extraordinary courage and told the audience that he had translated but did not agree with Hawking, and he explained why. Then he suggested that he and Hawking continue the discussion in private. Hawking might have interpreted that to mean ‘Step outside and say that!’ But no, they found an empty office, and while the Institute officials searched in a panic for the ‘famous British scientist who had miraculously disappeared’,12 Hawking and Linde talked for two hours and then moved to Hawking’s hotel to continue their debate. By then things were looking up for Linde. ‘He started showing me photos of his family and invited me to Cambridge. This was the beginning of a beautiful friendship.’13