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Stephen Hawking (1942–2018)

14 March 2018

Colleagues remember the leading cosmologist, whose influence expanded beyond the physics community.

Stephen Hawking
Credit: The Franklin Institute, courtesy AIP Emilio Segrè Visual Archives, Physics Today Collection

Editor’s note: This article was last updated on 22 March.

Famed theoretical cosmologist Stephen Hawking died on 14 March at age 76, a remarkable 55 years after he was diagnosed with amyotrophic lateral sclerosis.

Born on 8 January 1942 in Oxford, England, Hawking studied physics at Oxford University and astrophysics at Cambridge University. Much of Hawking’s research examined the interplay between general relativity and quantum mechanics. In 1974 he derived one of his most famous results: that quantum effects near a black hole’s event horizon will lead to the black hole’s emission of black body radiation. His 1988 book, A Brief History of Time, remains one of the most successful attempts to make modern cosmology accessible. James L. Anderson, in a review of the book for Physics Today, wrote that Hawking’s “exposition is usually so clear that one feels the missing equations [there is only one in the whole book] can be derived with just a bit of effort.”

Rather than have one person write an obituary, we asked many of Hawking’s colleagues to share remembrances. Their responses are below. If you would like to share a story about Hawking, please leave a comment or send us an email. We’d also love to hear from scientists whose early interest in STEM was sparked by Hawking’s popular writing.

  1. Don Page
  2. Marika Taylor
  3. John Preskill
  4. Thomas Hertog
  5. George Ellis
  6. Alan Guth
  7. William Unruh
  8. Andrew Strominger
  9. Martin Rocek
  10. Christophe Galfard
  11. Gordon Berry
  12. Bernard Carr
  13. James Hartle

Don Page
Distinguished University Professor, University of Alberta

Stephen Hawking was a brilliant scientist who proved that, under appropriate conditions that seem to be true for our universe, classical general relativity would imply that the universe had a singular beginning and that the area of the event horizon of a black hole would never shrink. Later Hawking showed that, going beyond classical physics, quantum theory implies that black holes would emit particles (now called Hawking radiation) and shrink, and that the universe might not in fact have a precise beginning. The latter idea, the Hartle–Hawking no-boundary wavefunction for the universe, though almost certainly not the last word on the subject, was an ingenious proposal for going beyond the dynamical laws of physics to a theory for the boundary conditions of the universe.

I was highly privileged to have had Hawking as my PhD co-supervisor (along with Kip Thorne, my main supervisor at Caltech) and as my postdoctoral supervisor at the University of Cambridge from 1976 to 1979. During that time I lived in his home and helped him get up, get dressed, eat breakfast, and go by wheelchair to the Department of Applied Mathematics and Theoretical Physics. He was a wonderful mentor and a close friend, as was his entire family. I shall miss him greatly.

Marika Taylor
Professor of Theoretical Physics and Head of Applied Mathematics, University of Southampton

Stephen lived with the debilitating disease of ALS for over 50 years and struggled with increasing disability and discomfort. Working with him, one could not fail to be influenced by his immense determination to carry on with his research and everyday life. And for him just carrying on was not enough: he wanted to enjoy all that life has to offer and to reach for the skies in everything he did.

As a student of his, many of my memories of him inevitably relate to physics discussions. He had an insatiable intellectual curiosity and a determination to solve longstanding physics puzzles such as information loss in black holes. Looking back, I appreciate how much he treated me as an intellectual equal even when I was a beginning graduate student. He did once joke that I wouldn’t get my PhD if I didn’t come around to his views on information loss, but he both allowed me and encouraged me to work on ideas in string theory that were quite far from his own interests. He and I would have long discussions about what I was doing in string theory—often while journalists and others waited outside, as Stephen liked to make the point that physics was his highest priority.

Nobody can talk about memories of Stephen without mentioning his impish sense of humor. He was the master of pithy one-liners and he had a wonderful smile (captured remarkably by Eddie Redmayne in The Theory of Everything). Stephen’s response when somebody called his office to say that God needed his help: “God has managed fine for over 13 billion years, so could God please wait until after lunch?”

John Preskill
Richard P. Feynman Professor of Theoretical Physics, Caltech

Stephen wrote one of his most famous papers (on the black hole information loss problem) while at Caltech during his sabbatical year in 1974–75, and began making regular visits here in 1991. What I’ll remember best about my time with Stephen is that we could make each other laugh. I sensed when we first met that he would enjoy being treated irreverently. So in the middle of a scientific discussion I could interject, “And what makes you so sure of that, Mr. Know-It-All?” knowing that Stephen would respond with his eyes twinkling: “Wanna bet?”

Our bets were facilitated by our friend Kip Thorne, and we were all quite surprised by how famous they became. Stephen conceded a bet about naked singularities in 1997 on a very public occasion, when I was giving a public lecture at Caltech. Stephen paid up by offering T-shirts to Kip and me, which carried a “suitable concessionary message.” I put on my shirt and wore it during my lecture, partially covered by my suit jacket. Finding it politically incorrect, I’ve been too embarrassed to wear it ever again; Stephen found my reluctance very, very funny.

Stephen conceded an even more famous bet (regarding whether black holes destroy information) in 2004, before an audience of 700 scientists in Dublin. He presented me with Total Baseball: The Ultimate Baseball Encyclopedia. You can’t buy one of those in Ireland, so Stephen’s assistant had arranged to have it shipped overnight just in time. Not knowing what else to do, I held the book over my head as though I had just won the Wimbledon final, while a million flashbulbs were popping (it seemed like a million anyway). One of those pictures wound up in Time magazine.

The bets were for fun, but the scientific issues in question are ones many physicists deeply care about, founded on some of Stephen’s most far-reaching contributions to physics. That combination of extraordinary depth of thought with an irrepressible sense of play, that’s what I’ll remember best about Stephen.

Thomas Hertog
Professor of Theoretical Physics, KU Leuven, Belgium

I first met Stephen in the late 1990s. I had come to Cambridge from Belgium to study theoretical cosmology. Stephen took me on as his PhD student to study what kind of universe comes out of his no-boundary model of the Big Bang. What followed was a very special collaboration, a dialogue as it were, about the origin of the universe, which continued for almost 20 years.

It turned out there are many universes coming out of his Big Bang model. We were led to the multiverse. But Stephen was not satisfied with this. “Let’s control the multiverse,” he said. So we set out to develop methods to transform the idea of a multiverse into a coherent testable scientific framework. This was Stephen: “to boldly go where Star Trek fears to tread,” as the screensaver on his old desktop used to read. Stephen was an adventurer, and science was his greatest adventure of all.

Over the years I came to know Stephen as a warm and generous friend. He had a dream. He wanted humankind to aim for the stars and for all of us to become cosmic citizens, agents of the universe. He paved the way for us through his brilliant science, his sparkling outreach, and above all his enormous courage.

George Ellis
Professor Emeritus, University of Cape Town

Stephen Hawking lived a life of extraordinary achievement, both scientifically and in terms of courage in facing a huge physical disability. His scientific success came from a combination of great technical ability and imagination, an inquiring mind always searching for answers to important issues, and extraordinary determination and focus. As to his disability, he was a huge inspiration to disabled people worldwide through his great achievements in the face of motor neuron disease.

His scientific work had three major epochs. I had the privilege of being one of his first collaborators, working together with him on the issue of whether there had been a start to the universe or not, which was a major issue at the time. We worked together on whether Bianchi (spatially homogenous) universe models were singular, and we were able to show there was indeed a start to the universe in those cases if suitable energy conditions held. The highlight of this part of Stephen’s career was his series of cosmological singularity theorems, developed from the highly innovative work of Roger Penrose on black-hole singularities, that transformed the field by using global methods and introducing the idea of closed trapped surfaces. Through these theorems, Stephen showed that when time-reversed closed trapped surfaces exist and suitable energy conditions hold, classical general relativity implies that there was indeed an initial singularity at the start of the universe, beyond which normal physics would not apply. He and I then showed that simply the existence of the cosmic microwave background radiation implies the existence of such closed trapped surfaces. The later discovery of the inflationary universe, however, showed that the required energy conditions will not in general be satisfied! These beautiful theorems in fact show that either a quantum-gravity era, or at least an era in which quantum fields dominated, occurred in the very early universe.

The technical tour de force of these singularity theorems was followed by Stephen’s developing a series of theorems about the behavior of black holes, one of the most unexpected and interesting outcomes of Albert Einstein’s theory of general relativity. Especially important was Stephen’s realization that the area of a black hole’s event horizon could never decrease, just like entropy in classical physics. The work was informed by ongoing discussions with Penrose and Bob Geroch in London, visitors John Wheeler and Charles Misner from the US, students and then colleagues Brandon Carter and me, and our mentor Dennis Sciama.

With this solid work, Stephen built his scientific reputation. He then worked on important theorems regarding black-hole geometry and uniqueness, and with Brandon Carter and James Bardeen established the four laws of black-hole thermodynamics. With Gary Gibbons he wrote one of the first papers on the analysis of gravitational-wave signals—Stephen’s famous area theorem had stemmed from his interest in gravitational waves that might be given off by colliding black holes. Stephen’s close friend Kip Thorne won the Nobel Prize in Physics last year for work in gravitational-wave astronomy.

The second epoch, from 1973 to 1979, saw Stephen’s adventurous and initially controversial—but later vindicated—work on quantum field theory in a curved spacetime. The core is his innovative paper integrating quantum field theory, general relativity, and thermodynamics to establish that black holes emit blackbody radiation—Hawking radiation—because of quantum effects. Although the paper developed from earlier work on black-hole thermodynamics, particularly by Jacob Bekenstein, the unexpected result is uniquely Stephen’s. It is his major achievement, one that has stood the test of time and served as the basis for a huge amount of further work. Initially rejected by major figures in the scientific community, Stephen’s seminal result has since become universally accepted, in particular because there are now many different ways to prove it. However, it raises major conundrums that physicists are still puzzling today: What happens when this radiation radiates away all the mass of a black hole? Does the black hole pop out of existence with a bang, or does it leave a remnant behind? Does information that falls in disappear, or does it somehow re-emerge? To fully answer those questions, we need a consistent theory of quantum gravity. We still do not have one.

Not as celebrated but just as important are Stephen’s contributions to studying the beginnings of the growth of structure during the inflationary expansion of the early universe. A key issue for cosmologists is to understand the origin of the primordial seeds that eventually developed into galaxies. Stephen and Viatcheslav Mukhanov independently proposed that the seeds formed via quantum fluctuations analogous to those involved in Hawking radiation from black holes. Stephen and colleagues in Cambridge hosted an important meeting in 1982 where such ideas were thoroughly discussed and a consensus view on the origin of structure emerged. That was a key development in modern cosmology.

The third epoch, from 1980 on, was more speculative and less rigorous. Stephen tested big ideas about the start of the universe in a creative way. With James Hartle he developed the no-boundary proposal, hypothesizing that the universe would start without a singularity in a timeless state: a domain in which only space existed. Stephen also introduced proposals for spacetime wormholes. Those ideas sparked a lot of interest and stimulated much activity, but they have not achieved the same level of acceptance in the scientific community as has Hawking radiation.

One of Stephen’s most important contributions was supervising and working with 40 research students who, while not enjoying the same eminence as he did, have become major figures in the field in their own right. Stephen also played a key role as a popularizer of science through the phenomenal success of A Brief History of Time, which has helped inspire young men and women to go into science.

Overall, Stephen had an extraordinary life that was full of extraordinary achievement. He lived as long as he did through sheer willpower, of which he had plenty. His impish humor will be much missed.

Alan Guth
Victor F. Weisskopf Professor of Physics, MIT

I first met Stephen Hawking in the summer of 1982, at the Nuffield Workshop on the Very Early Universe, which was organized by Stephen and Gary Gibbons. It was easily the most exciting conference that I have ever attended in my life.

At that time I was totally in awe of Stephen Hawking, and in some ways that has remained the case for my whole life. The workshop was a three-week program in Cambridge, UK, involving about 30 physicists. A key topic of discussion was the calculation of the density perturbations that would arise from a quantum mechanical treatment of cosmic inflation. Is it possible, we were all asking, that the vast tapestry of cosmic structures was actually the consequence of quantum fluctuations? Stephen was certainly one of the first people to take this idea seriously, and I first learned of the idea from hearing about a talk that he had given.

When Paul Steinhardt and I tried together to understand Stephen’s calculation, about a month before the workshop, we found that the description of the evolution of density perturbations was beyond us, but it nonetheless appeared that Stephen had made a trivial error at the end of the calculation. While Stephen found that the density perturbations would have just the desired amplitude, it looked to us that he was underestimating by a factor of about 104. We would have preferred Stephen’s answer, but could not see any way to justify it. At Nuffield we had a chance to discuss it briefly with Stephen, but he held rigidly to his calculation.

Stephen’s conference talk was in the middle of the second week. He impishly used an ambiguous title, “The End of Inflation,” which could maybe refer to how the period of inflation ended . . . or maybe to the death of the theory. Paul and I were anxiously awaiting the chance to raise our objections to Stephen’s calculations, but we were blindsided. When Stephen reached the part of the calculation that we disagreed with, he inconspicuously did an about-face, presenting the same result that we would have, without any indication that he had ever advocated anything different!

Paul and I had a hard time knowing what to make of this turn of events, but at least in hindsight, I think it shows two important aspects of Stephen’s personality. First, it showed his love of suspense, surprise, and showmanship. To Stephen, physics was always fun, and the fun was increased by building in elements of drama. Second, it showed his willingness to change his opinions when he saw convincing reasons to do so. (That may sound like something scientists should take for granted, but in my experience it seems rather special.)

All in all, Stephen Hawking’s contributions to physics are immense, including his impressive contributions to cosmology, and most importantly his discovery of black-hole radiation. Black-hole radiation continues to be a central theme in efforts to unify quantum theory and gravity, and the paradoxes it raises have caused us to view the nature of space and time as a wide open question. Stephen was also a monument to the strength of human persistence and determination. Despite his disabilities, Stephen managed to write as many papers, to make as many important contributions, and to travel the world and give as many talks as anyone I know. And he never lost his ability to smile. I was always amazed by Stephen, and I will miss him.

William Unruh
Professor of Physics, University of British Columbia

I first met Stephen Hawking in the early 1970s at a variety of UK conferences, including at a talk at the Rutherford lab where he first talked about his discovery that black holes have a temperature.

In 1984 my wife, Pat, and I were in Cambridge for a three-month sabbatical stay. That got extended to five months when our son, Daniel, was born by Caesarean. Jane Hawking advised us, based on their own travel experiences shortly after Robert was born, that the most important thing was for Pat to conserve her energy and not to go on to Paris so soon after the operation. I also remember Stephen playing football (soccer) with his 4-year-old son, Timmy, in their big back garden. Timmy would kick the ball to Stephen, and he would push it back with his wheelchair.

About a year later we were all in Cambridge again. Stephen had just come back from Switzerland, where he had had a tracheotomy, and he could no longer speak at all. He was still in the hospital, and Pat, Daniel, and I went to visit him in his hospital room. Stephen had a large Perspex sheet with letters and numbers on it for communication. When he saw Daniel, he smiled and immediately began to blow spit bubbles to him to amuse him—such was his delight to amuse a child, despite the collapse of his ability to communicate.

Stephen was a person who lived on will. Despite not having a working body, he was able to bend the world around him to his will and to live and see more than most. The biggest shame was that his inability for quick communication meant that he could not participate in those arguments, those disagreements, that we all need to refine and test our ideas. I recall in the late 1970s, just after he and Gary Gibbons had extended my work on accelerated detectors to de Sitter space, we were alone in his office arguing. Suddenly there was one word in the explanation he gave that I did not understand. He repeated the phrase at least five times, growing more and more frustrated, but my mind was simply unable to convert the sounds into words. To have accomplished as much as he did and more than almost all other physicists and despite such handicaps all around him was astonishing. It was amazing to have known him.

Andrew Strominger
Gwill E. York Professor of Physics, Harvard University

Hawking and Strominger in 1998. Credit: Strominger
Hawking and Strominger in 1998. Credit: Andrew Strominger

In 1974 Stephen Hawking derived an astonishingly simple formula for the number of gigabytes, or equivalently the maximal entropy, of a quantum black hole. The Hawking–Bekenstein area/information law reads

SBH = kBAreac34GNħ

Here Area is the surface area of the black hole, c is the speed of light, GN is Newton’s gravitational constant, ħ is Planck’s quantum constant, and kB is Boltzmann’s constant for statistical mechanics. If the information storage capacity on the right-hand side is measured in gigabytes, then kB = 10–9 / 3 log 2. Stephen wanted this equation inscribed on his gravestone.

The SBH equation is an enormous storage capacity. Black holes are probably the most space-efficient storage devices in the universe (of course once in, the information is hard to get out!). If we assume Moore’s law for the exponential growth in computer chip capacity, in three centuries all chips will be black holes. All the data now in the Google storage banks could be stored in a 10–25 cm black hole. Sagittarius A*, the black hole at the center of our galaxy, can store 1080 gigabytes!

The SBH equation stands alongside the Einstein equation and the Schrödinger equation among the most important equations of 20th-century physics. However, unlike the other equations, which describe distinct areas of physics, the area/information law unites disparate areas, as indicated by the striking appearance of nearly all the most basic constants of nature.

Also unlike the Einstein and Schrödinger equations, the area/information law has paradoxical features and is not well-understood. The basic paradox is the apparent absence of any quantum chips in which black holes store information. The Einsteinian description of a black hole as a curved spacetime provides no obvious place for the quantum chips to hide. It is striking that the storage capacity grows like the area, rather than the volume, of the black hole. Unravelling the mysteries springing from Stephen’s equation holds the promise of a revolution in our conception of spacetime and the universe as profound as those produced by relativity or quantum mechanics.

A giant of physics and humanity has left our world. The equation he gave us will remain forever.

Martin Rocek
Professor of Theoretical Physics, State University of New York at Stony Brook

I was Stephen Hawking’s postdoctoral fellow for two and half years, and so I am able to comment a bit both on his work and on other aspects of his life.

Stephen’s first big breakthrough was the realization that Roger Penrose’s theorems about the inevitability of singularities in black holes in Albert Einstein’s theory of gravitation could be applied in reverse to imply the inevitability of the Big Bang singularity and the beginning of time. His next, and most important, breakthrough was the realization that due to quantum effects, black holes are not black—they emit what is now called Hawking radiation. This shocking discovery implied that, despite the many orders of magnitude of scale that separated them, Einstein’s theory could not ignore the quantum world.

In 1979 Stephen hired me to teach him about supergravity, the remarkable extension of Einstein’s theory that Peter van Nieuwenhuizen, Daniel Freedman, and Sergio Ferrara had discovered two years earlier at Stony Brook—later I learned that Stephen hired me on Peter’s recommendation. Though I failed to teach Stephen supergravity, it was nevertheless a very productive time for Stephen. During this time, among many other projects, he explored the effects of gravitational instantons and performed calculations developing the consequences of his then recently proposed information paradox. Though his argument that Hawking radiation implied the breakdown of quantum mechanics is generally not accepted today (Stephen himself rejected it later in life), it stimulated a wealth of important research, some of which is described in Leonard Susskind’s entertaining book The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics.

My time with Stephen’s group let me see some things that are not as well known. Stephen always had a number of students and postdocs with him. He would work with them by asking them to write equations on the blackboard. At that time he was already confined to a motorized wheel chair and could not write himself, but he could still speak, albeit with such difficulty that only those who spent a lot of time with him could understand him. Nonetheless, he had a sharp sense of humor, and despite the effort it took, made jokes and displayed his knowledge in areas outside of physics. At a dinner where I was feeding him, he explained to me the proper way to fillet a braised trout; since a bone could have been quite dangerous to him, I had to be a quick learner. He tried to lead a normal life, with the necessary accommodation for his physical condition. Thus driving his wheelchair by himself was “walking,” etc. He spent time with his children, he went with the group for lunch at the “grad-pad,” went to the pub with us, and so on. All the students and postdocs did their part in helping to make this possible; Stephen would tell us what we should and shouldn’t do to help him.

Stephen would go on to propose that the universe began with a quantum fluctuation that replaced the singularity of the Big Bang, along with many other important and thought-provoking ideas. He was a great mentor—many of his students and postdocs went on to very successful academic careers. He was a role model for those overcoming physical adversity, and through his many books, a great popularizer of physics and science in general. He will be missed.

Christophe Galfard
Popular science writer

Stephen took me as his PhD student in 2000 to work on the so-called black hole information paradox. I had met him a couple of times during the preceding year, at Cambridge’s Department of Applied Mathematics and Theoretical Physics, and I still remember being struck by the depth of his gaze. His charisma was such that he could convince a student like me to spend the next five years of his life away from the world, buried inside black holes, looking for information. And to be happy about it.

The information paradox came about in the mid 1970s, when Stephen discovered that contrary to what everyone thought, black holes weren’t that black. Quantum effects, he showed, made black holes radiate their energy away and shine. This result, one of the most beautiful I’ve ever seen (if only because it mixed, for the first time ever, gravitational, quantum, and thermodynamics physics), was extraordinary. And puzzling. This radiation, today called the Hawking radiation, led to a rather deep problem: Stephen had shown that whatever came out of a black hole through his radiation was, at first sight, completely independent of what made the black hole in the first place.

This may not sound that bad, but as Stephen immediately realized, it was.

It meant that black holes bleached away some of our universe’s history. It meant that physics as it is known could not pretend accounting for the past: Anything that ever fell in a black hole (and there is plenty out there) was as good as erased from our universe’s memory. Forever. Obviously, if we could not know the past, it also meant that physics would never be able to forecast the future. This became known as the black hole information paradox. Do black holes really erase our universe’s past? And if they don’t, where does the information go?

Back then, the physicists who understood the problem obviously thought about the first possibility with great fear. One could joke that since the very power of physics was being questioned, their careers were at stake. But most did not pay much attention to such a heretic thought: Nothing could beat physics at understanding nature anyway, and Stephen’s calculation, after all, was only an approximation. It was widely believed that a more complete analysis of quantum effects around black holes should recover that lost information and physicists would then live happily ever after. But Stephen could always see farther, and he understood that to recover the lost information would have to mean giving up something equally dear to physicists such as, for instance, causality.

Stephen believed the problem to be of such a fundamental importance that he had to raise the community’s awareness of it. So he made a bet. In 1997, he and Kip Thorne bet John Preskill that information really was lost through the black hole evaporation process.

About a year later, theoretical physicist Juan Maldacena came up with a way to look at gravity as a quantum theory, and vice versa: the AdS/CFT correspondance.

In 2004 a huge conference was organized in Dublin for Stephen to announce his conclusion: Information could be retrieved from black holes after all. He conceded to Preskill. And funnily enough, even though Stephen knew I did not agree with his conclusion (nor did Thorne, who did not concede), he asked me to answer whatever questions the audience might have after his announcement. There were hundreds of people in the audience, most of them famed scientists. There was the BBC, CNN, and most television stations from around the world. I had never spoken in front of an audience before, let alone one of that caliber. I don’t think I have ever been so scared in my entire life. That’s, in essence, a good way to understand how Stephen was challenging his research students.

Subsequently, Stephen asked me to help him write some of his public talks. As I was finishing my PhD and toured the world with him, I discovered the impact he had on the general public. Even though his first passion was pure theoretical research, I understood why he found equally important to spread modern scientific knowledge to the rest of the world: it was for humanity’s sake.

Scientific knowledge belongs to the whole of humanity, and Stephen not only was one of the brightest theoretical physicists of the past half century, he also was a champion of the people. Millions throughout the world have dreamt about what is known and unknown thanks to him. He reached, and warned, and made us aware and alive. He showed us the best humanity can be. He gave hope. He gave me hope and the will to work and spread that knowledge. I am forever indebted to him for this, and for everything else he has done for humanity, as I believe many are.

Gordon Berry
Emeritus Professor of Physics, University of Notre Dame

I first met Stephen Hawking during the final experimental part of the entrance exam to University College, Oxford in March 1959. Only students who had done well on the written part took the experimental part. The experiment involved dropping ball bearings of different diameters down a long glass tube filled with oil and timing them as a function of the distance they fell. I suppose we then graphed the variables to see if the balls obeyed Stokes’s law. Lots of “professors” came around inundating two of us with questions: me and also a fellow at the next lab table. I later discovered that he was Stephen.

We met again in college in the autumn at the introductory beer bash for freshmen. There were only four of us entering University College as physics students. During the next three years, Steve and I shared many experiences. We became tutorial partners, the two of us meeting weekly with Robert Berman and later Patrick Sandars for physics, and with a Dr. G. in New College (his name I forget) for mathematics tutorials. Berman was a thermodynamics specialist, the first person to map the interface of diamond and carbon at very low temperatures, which led to the industrial production of synthetic diamonds. During our tutorials Steve and I had to cover every detail of Mark Zemansky’s book Heat and Thermodynamics. This knowledge certainly helped as Steve later developed his thermodynamic interpretation of black holes and discovered Hawking radiation. When we began work on general relativity, Steve and the tutor completely left me in the dust. He took to it like a fish (not just like a duck!) to water, and that topic became his life’s work.

Hawking's name misspelled
Credit: Gordon Berry

Outside the physics department, Stephen and I assembled together most evenings to play bridge or poker (pennies and shillings changed hands, and several bottles of port were consumed), or we just went down to the High Street Inn for darts and drinks (where the “prayer books” had handles on them, according to our college staff). We were both coxes on the river, almost every weekday afternoon, for the University College crews.

Our first visit to Cambridge was as members of the Oxford coxswains’ annual challenge with their Cambridge equivalents—unfortunately, Cambridge won. Stephen was not famous at that time, so a Cambridge paper spelled his name as “Hawkong.” Can you imagine an athletic Steve rowing in a race in an eight? He and I infrequently would row on the Isis in a “coxless pair”—we never wanted to be following orders from another cox! I have always claimed that the only thing I could do better than Steve was to cox the University College first eight; he coxed the second eight.

Steve and I both took the theory option for the Oxford examination finals in June 1962. Thus, we had just one year of laboratory physics to complete, which we took together as partners. We had to complete six separate experiments, essentially one per week. Since we were both coxing every afternoon on the river, we would complete each experiment in one three-hour morning and then write it up later. Most students would spend several days in the lab each week, so the graders—physics doctoral students—would be surprised when Stephen and I came in on Fridays to get our completed experimental reports approved. Admittedly, we did everything very rapidly, and the graders asked us lots of tricky questions, not quite believing that we had actually made the measurements. We passed! It is also true that neither of us went to many physics lectures during our three undergraduate years. The only set that we found valuable was “Quantum Mechanics” by a visiting American professor from Yale: Willis Lamb, of Lamb shift fame.

The story of Steve falling downstairs, hitting his head, and losing his memory has been described many times. There followed many hours of questioning with his friends, me among them, lasting until daylight the following morning. It was the beginning of the subsequent diagnosis of his debilitating illness. Successfully passing the Mensa test was an early verification that his brain was unharmed. The final diagnosis of ALS took place about 12 months later.

It is important to note that Steve has said he was fairly lonely and bored in his early undergraduate years. However, his 100 or so peers at University College recognized that he was the most intelligent person we had ever met; the rest of us were “just ordinary people.” Especially after the first year, he joined us in many College activities. He was after all two years younger than most of us, and we mostly did not have his strongly intellectual family background, which perhaps restrained him initially in all the non-physics adventuring of typical undergraduates.

Berry and Hawking
Credit: Gordon Berry

Steve went on to his first choice for graduate work, Cambridge, then the mecca for cosmology. The rest is well-known history. As physicists, we will be always grateful for his efforts in both developing detailed theories of our universe and driving our understanding of the cosmos forward. His explanations, which could be mostly understood by non-scientists, were able to excite the imaginations of the general population, making people value the sense that they could better understand the immensity of the universe and their place in it. Steve was the human “supernova of our time.”

We last met about four years ago in Cambridge in his office and home. It was delightful to find that Steve’s strong sense of humor was undiminished, which I think was a strong force in helping him overcome his health adversities. He immediately decided that the four 1959 University College physics undergraduates—Derek Powney, Richard Bryan, Steve and me—should get together again. We began the process, but tracking down Powney and Bryan took more time than we had available to us—the meeting never happened. I close with a photo of our meeting in his office, soon after the release of the movie Hawking (2013).

Bernard Carr
Professor of Mathematics and Astronomy, Queen Mary University of London

I was one of Stephen’s first PhD students, having worked with him from 1972 to 1976. He was not so famous in those days, but his brilliance was already clear to his peers. I found it rather daunting when, on becoming his research student, I was informed by one of my tutors that he was the brightest person in the department. The tutor was Jeffrey Goldstone, who might himself have been a contender for that position.

My relationship with Stephen was not the usual type of supervisor–student relationship. In those days, before he had his entourage of nurses and assistants, students would have to help him in various ways on account of his disability. That was not an arduous task, but it did mean that one’s relationship with him became quite intimate. I shared an office with him, lived with his family for a while, and accompanied him as he travelled the world giving talks and collecting medals. People sometimes ask me if working with Stephen was the high point of my career. I hope not—that would imply that it has been downhill since the start—but it was certainly a tremendous privilege.

I was fortunate to be working with Stephen when he discovered black hole radiation, surely one of the most important results in 20th-century physics. Even though it has not been confirmed experimentally—otherwise he surely would have added the Nobel Prize to his long list of awards—it is such a beautiful idea that most physicists agree that it must be true. John Wheeler once told me that just talking about it was like “rolling candy on the tongue.” I recall one tea-time conversation with Stephen when he mentioned that he was puzzled to be finding a large quantum flux from a black hole. I rather regret that I didn’t spot the implication and remark, “Obviously, Stephen, they’re radiating thermally.” Nevertheless, having a ringside seat during those developments was tremendously exciting and also enabled me to be one of the first people to study the cosmological consequences of the effect.

On matters of physics, I always regarded Stephen as an oracle, with just a few words from him yielding insights that would have taken weeks to work out on my own. However, Stephen was only human, and not all encounters led to illumination. Once I asked a question about something that was puzzling me. He thought about it silently for several minutes, and I was quite impressed with myself for asking something that Stephen couldn’t answer immediately. His eyes then closed, and I was even more impressed with myself because he was clearly having to think about it very deeply. Only after some time did it become clear that he had fallen asleep! Nowadays I also sometimes fall asleep while talking to students, so I recall this incident with amusement.

In 1975 Stephen visited Caltech for a year as a Fairchild scholar. I was in my final year as a PhD student and was invited to accompany him there. I also lived with his family for the year, which was an ideal arrangement because I paid no rent in exchange for helping Stephen at work and home. More importantly, the Hawkings welcomed me as a member of their family, and I’ve remained close to them ever since. After the rather cloistered life at Cambridge, I found Caltech exhilarating, and I know Stephen felt the same. In some ways the facilities were much better, since they provided him with a house and built ramps everywhere. It was quite a battle to get ramps in Cambridge.

One of the great excitements of visiting Caltech was meeting Richard Feynman, who was regarded almost like a god there. He used to visit our office quite often. Since Stephen’s voice was already quite weak, I would act as interpreter. On one occasion, I recall Stephen remarking that something was only a matter of a sign; Feynman retorted that he’d only became famous because he’d sorted out a sign! Stephen gave his first proper seminar about Hawking radiation at Caltech, and Feynman was in the audience. I was at the front helping Stephen show his slides, and afterward someone told me that Feynman had been drawing my portrait on the back of an envelope. So I contacted his secretary, only to discover that he’d thrown the envelope away. However, she kindly retrieved it for me from the garbage bin, and it now sits framed on my mantelpiece at home. My head is at the center, surrounded by Feynman’s notes on the seminar and some Feynman diagrams. In due course, Stephen became even more famous than Feynman, which made my envelope even more valuable.

Feynman envelope sketch
Credit: Bernard Carr

My PhD was about black holes that might form in the early universe. Stephen wrote a pioneering paper about such primordial black holes (PBHs) in 1971. But there was a problem because it was thought that any PBHs would grow enormously and by today would have attained a huge mass. However, the usual argument neglected the cosmological expansion. Stephen and I were able to show that such growth was impossible, thereby allowing the possibility that PBHs may have existed after all. We each made the discovery independently. I recall rushing excitedly to his office to give the good news and being rather dismayed to find that he had just come to the same conclusion by doing the calculation in his head!

Perhaps the most important aspect of the conclusion that PBHs might have formed was that it motivated Stephen to consider the quantum effects of small black holes—only primordial ones can be small enough for this to be important. After 40 years we still don’t know for sure whether PBHs formed, but this illustrates that it can be important to think about things in physics even if they may not exist. Of course, it will be even more interesting if PBHs do exist. There’s a lot of current interest in the possibility that the large (unevaporated) ones provide the dark matter.

The period in which I worked with Stephen was also interesting because it saw his transition from a brilliant physicist known only to professional colleagues to an international superstar. He clearly enjoyed his fame and valued the opportunity it brought him to popularize physics and highlight various sociopolitical issues. I recall one lunch at Caltech in 1975 when we were debating the nature of fame. He finally defined it as being a state in which one is known by more people than one knows. On the way back to the office, a passing stranger said hi. When I asked who that was, he answered, “That was fame”—a nice illustration of his quick wit. Ten years later the number of people who knew him was probably a million times the number he knew, so he probably modified his definition!

James Hartle
Research Professor and Professor of Physics Emeritus, University of California, Santa Barbara

With the death of Stephen Hawking, physicists have lost one of their greatest colleagues, and the world has witnessed the conclusion of an inspiring story of triumph over adversity. Personally I have lost a dear friend and matchless collaborator.

Stephen’s major contributions to science are well known, and there is no need for me to review them; I confine myself to a few personal remarks.

My association with Stephen began some 46 years ago during a many-month visit I made to Fred Hoyle’s Institute of Theoretical Astronomy (as it was known then). In residence were Brandon Carter, Martin Rees, Paul Davies, and Stephen—colleagues with whom I maintained lifelong personal and scientific contacts. In Cambridge I was warmly welcomed by Stephen and Jane. From that time on, I always felt that Stephen and I were on the same wavelength—not the same in ability or insight, of course—but rather similar in style and in views of what is important. Ten more joint papers were to follow that visit.

For me, the high point of our joint efforts is the paper on the no-boundary wavefunction of the universe. Stephen wanted to understand the universe in scientific terms. His deep interest in cosmology runs from his very first
papers around 1965 to his last paper with Thomas Hertog in 2017.

To understand the universe, it is necessary to understand how it began. The classical singularity theorems of Stephen and Roger Penrose showed that the universe could not begin with a classical Lorentzian geometry with one time and three spatial dimensions. An earlier joint paper with Stephen demonstrated the power of Euclidean geometry to help understand the Hawking radiation from black holes. If the universe couldn’t begin classically with a Lorentzian geometry, perhaps it could begin quantum mechanically with a Euclidean geometry. Perhaps it could start with four spatial dimensions and later make a quantum transition to a Lorentzian spacetime. The result was the no-boundary proposal for the quantum state of the universe.

I have often thought that the signature of a great problem in physics is that its solution generates more great problems. Certainly that is the case with the no-boundary wavefunction. The no-boundary wavefunction of the universe led me to numerous specific calculations, many with Hertog and Stephen, of what it predicts for our observations of the universe on the largest scales of space and time. It also motivated a new vision which I formulated with Murray Gell-Mann of how usual textbook quantum mechanics can be generalized to apply to cosmology. We called it decoherent histories quantum theory.

Working with Stephen was a wonderful experience. He had remarkably clear scientific insight. He always knew the right question to ask. He was able to cut through the clutter that characterizes theoretical physics and focus clearly on the essential points. Stephen also had the courage to discard cherished old ideas that are obstacles to progress, like the idea that black holes are black. Later, when looked at in the right way, these seem inevitable. But that was his genius.

How lucky then was my decision as a young assistant professor to take a long leave from Santa Barbara to work at the Institute of Theoretical Astronomy. As a consequence, many years later, I consider myself as most fortunate to have been able to count one of the great scientific figures of the age as a friend, and to have been able to work with him on something like an equal basis. I do not expect to meet his like again.

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