In 1970, as we have 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 events 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 plughole of a bath, with gas piling up and getting hot as gravitational energy is converted into energy of motion. It would, 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 on to 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.

The 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 six 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 two decades has con-

vinced most astronomers, and the consensus today is that there is 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. 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 were 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 ago, 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 the 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 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 space-time 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, while a rotating hole bulges at the 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 twenty 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 space-time. 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.

This discovery may have made Hawking so excited that he could not sleep, and it may have impressed Roger Penrose when Hawking telephoned him the next day to discuss the idea, but initially it made very little impression on other astronomers and physicists, who regarded such notions as rather esoteric. After all, the X-ray observations that led to the identification of Cygnus X-1 with a visible star were made the next year, in 1971, and it was not until the end of 1972 that the consensus that the X-rays come from a black hole orbiting that star was reached. What really began to make other physicists sit up and take notice of Hawking’s ideas about the increasing area of a black hole was the seemingly outrageous suggestion that this might be connected with the branch of physics known as thermodynamics.

Thermodynamics is simply the study of heat and motion, as the name implies. It was developed as a branch of science during the nineteenth century and was of great immediate practical value in the age of steam engines. It rests upon some simple, basic rules, such as the fact that heat cannot flow from a cold object to a hot one (immortalized by the musical duo Flanders and Swann in the memorable couplet “Heat won’t flow from a colder to a hotter/You can try it if you like but you’d far better notter”). But thermodynamics goes far beyond the day-to-day practicalities of making steam engines work more effectively and leads on to fundamental truths about the nature of time and the fate of the Universe. One especially important concept, closely linked to the inability of heat to flow “from a colder to a hotter,” is known as entropy.

In everyday language, entropy is the law that tells us that things wear out. Hot things cool off as time passes, and heat flows out of them. Buildings fall down and crumble away; living things grow old and die. These changes are linked to the passage of time, marking a

distinction between the past and the future. They correspond to an increase in the amount of disorder in the Universe. This disorder is measured in terms of entropy. The flow of time from the past to the future means that the entropy of the Universe must always increase. The same applies to any closed system—the amount of entropy can only increase (or, at best, stay the same); it can never decrease. Now, obviously, the presence of living things on Earth runs counter to this rule. We create order out of disorder by building houses and so on. But the point is that the Earth is not a closed system. It “feeds” off the energy flowing from the Sun, dumping entropy as a result. If you take the whole Solar System and treat it as a closed system, the entropy does increase, just as the laws of thermodynamics require.

So Hawking’s dramatic realization, coming with such force that evening in November 1970, was to lead to the idea that the law which says that the area of a black hole can only stay the same or increase is equivalent to the law which says that the entropy of a closed system can only stay the same or increase. But even Hawking didn’t make that connection at first.

This is the kind of step that is quite often made in science by a junior researcher, not yet hidebound by tradition. The thought of trying to make a connection between the gravitational physics of black holes and the thermodynamic physics of Victorian steam engines would have daunted even the genius of a Hawking. But to a research student, just setting out on a scientific career and faced with two pieces of information that seem to say the same kind of thing in different ways, the similarity seemed worth remarking on.

Of course, research students very often remark on odd similarities and coincidences in science, and most of the time it turns out that there is nothing significant in the “discovery” at all. But when a student at Princeton University, Jacob Bekenstein, suggested that the size of the horizon around the singularity might literally be a measure of the entropy of a black hole, he started an avalanche of

investigation which led Hawking to the discovery that black holes are not necessarily black after all—they explode.

Just as research students are expected to come up with wild ideas (most of which prove fruitless), so it is a common theme in science that some of the most important developments are a result of somebody trying to prove that somebody else’s theory is wrong. This happened to good effect in the 1950s and early 1960s, when Fred Hoyle backed a rival model to the Big Bang, the steady state hypothesis, and became its most vocal proponent. Astronomers determined to prove Hoyle wrong worked much harder at establishing the accuracy of the Big Bang model than they might have done had there been no rival on the scene. But sometimes the effort can rebound.

Hawking was annoyed by Bekenstein’s suggestion. Even a research student ought to have realized that there is a direct connection between entropy and temperature, so that if the area of a black hole were indeed a measure of entropy it would also be a measure of temperature. And if a black hole had a temperature, then heat would flow out of it, into the cold (-270°C) of the Universe. It would radiate energy, contradicting the most basic fact known about black holes, that nothing at all—not even electromagnetic radiation—can escape from them. Together with Brandon Carter and Jim Bardeen, Hawking wrote a paper, published in Communications in Mathematical Physics, pointing out this seemingly fatal flaw in Bekenstein’s suggestion. It gave the formula for working out the temperature of a black hole according to this ridiculous notion and was published in 1973. But far from agreeing with Bekenstein, the team commented, “In fact the effective temperature of a black hole is absolute zero. . . . No radiation could be emitted from the hole.”¹

Within a year, however, Hawking had changed his mind. The reasons why he had second thoughts were related to another line of research on black holes he had been pursuing: the possibility, first

aired in 1971, that very small “miniholes,” smaller even than the nucleus of an atom, might have been produced in the Big Bang and could still be at large in the Universe today.

The critical mass needed to make a black hole simply by an object collapsing under its own weight is, as we have mentioned, about three times the mass of the Sun, and the Earth itself would become a black hole if it were squeezed down to about a centimeter. But absolutely anything will make a black hole if it is squeezed hard enough—a bag of sugar, a coin, the book you are reading, anything. The difficulty is that, the lighter the object you want to make into a black hole, the harder you would have to squeeze it.

Hawking reasoned that as we look back in time toward the beginning, we look back to higher and higher densities and pressures. So if we look back far enough, we come to a time when the pressure was great enough to squeeze any amount of matter you fancy, even a few grams, into a black hole.

The one snag with this argument is that, if the Universe were perfectly smooth and uniform back then, no miniholes could form; the only black hole would be the entire Universe itself. But provided there were some irregularities, some variations in density from place to place in the early Universe, then at the appropriate stage of the Big Bang a few grams of matter, any region that just happened to be a little denser than the average, could indeed get pinched off from the rest of space-time, forming tiny black holes that would last forever (or so Hawking thought in 1971) and still be around today.

We know that the Universe cannot have been perfectly smooth and uniform in the Big Bang because, if it had, there would be no way that irregularities such as galaxies could have formed as the Universe expanded. There must have been “seeds” in the form of tiny irregularities on which galaxies could grow by gravitational attraction. So Hawking’s notion of primordial black miniholes seemed plausible, even if there was no obvious way to test the idea.

In fact, although lightweight by the standards of conventional black holes, even a minihole may have rather a lot of mass by everyday standards. A black hole weighing about a billion tons, for example (the mass of a mountain here on Earth), would have a radius roughly the same as that of a proton. Less massive miniholes would be correspondingly smaller. And if you are dealing with objects as small as that, physicists knew, you have to use the quantum description of reality in order to understand what is going on.

Now the plot began to thicken. In 1969 Roger Penrose had shown that it is possible for a rotating black hole to lose energy and slow down as it does so. The way this happens is rather like the way in which space scientists sometimes use the gravitational pull of the planets to speed up spacecraft moving around the Solar System. For example, at the time of writing a probe named Galileo has just undergone a “slingshot” maneuver around the Earth and will eventually, if all goes well, end up in orbit around Jupiter. But in order to get there it will have followed a circuitous route.

After its launch, Galileo was sent not outward through the Solar System toward Jupiter but inward to fly by Venus. By diving around Venus on a carefully calculated orbit, the spacecraft gained energy and speed and was deflected toward the Earth. Venus lost a corresponding amount of energy but, being vastly more massive than the space probe, slowed down in its orbit by only a minuscule amount. At the end of 1990, the speeding Galileo carried out another slingshot maneuver, this time involving the Earth, and entered an orbit that will bring it back for a second slingshot past the Earth some two years later. Only then will it be moving fast enough to reach Jupiter in a reasonable time—and it is a sign of how much the probe’s speed will have been increased that it will reach Jupiter sooner, even after years of delicate maneuvering to take advantage of the three slingshots, than if it had gone straight out through the Solar System when it was launched.

Penrose showed that similar gravitational effects could boost the energy of electromagnetic radiation near a rotating black hole. The radiation gains energy; the rotation of the hole slows down. In 1973 two Soviet researchers, Yakov Zel’dovich and Alex Starobinsky, extended this idea to show that a rotating black hole should also throw off particles. Their argument had to do with the uncertainty principle of quantum physics, and we shall explain it shortly. They persuaded Hawking that the effect would be real, and he set about trying to find a precise mathematical treatment to describe the phenomenon. He was surprised, and at first annoyed, to discover that the equations said that the same process should be at work even for a nonrotating black hole.

“I was afraid,” Hawking wrote in A Brief History of Time, “that if Bekenstein found out about it, he would use it as a further argument to support his ideas about the entropy of black holes, which I still did not like.”² In 1977 he wrote in the January issue of Scientific American that he “put quite a lot of effort into trying to get rid of this embarrassing effect”³ but to no avail. In the end, Hawking had to accept the mathematical evidence rather than his prejudices. He had found that all black holes emit energetic particles and that therefore every black hole has a temperature. The temperature exactly matches the thermodynamic predictions related to the surface area of the black hole. We shall now describe how it works (leaving out the detailed mathematics).

Quantum uncertainty doesn’t just mean that human instruments are incapable of measuring any quantity precisely. It means that the Universe itself does not “know” any quantity with absolute precision. This applies to energy as much as to anything else. Although we are used to thinking of empty space as containing nothing at all and therefore having zero energy, the quantum rules say that there

is some uncertainty about this. Perhaps each tiny bit of the vacuum actually contains rather a lot of energy.

If the vacuum contained enough energy, it could convert this into particles, in line with E = mc². But things are not as simple as this. If the hypothetical energy of uncertainty in the vacuum were converted into particles and the particles became permanent features of the Universe, the rules of uncertainty would be violated—both human observers and the Universe would now be certain that there was something, in the form of a particle or two, where previously there had been nothing. Uncertainty works two ways: it is just as forbidden to be certain that the energy is nonzero, in these circumstances, as it is to be certain that the energy is zero.

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 that 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 USA. 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.