6
Before I’d started my visits to Cambridge, Stephen and I used the occasion of his annual visits to Caltech in 2005 and 2006 to create the plan for what our book would include. I wasn’t used to that level of planning. In my solo career, my editor at Pantheon, Edward Kastenmeier, had given me a lot of leeway. It might cause a problem if I’d said I was writing about quantum computing and then turned in a treatise on women’s soccer, but short of that, he was pretty flexible. I’d start with a rough outline and then figure things out as I went along. Stephen wrote his breakthrough book, A Brief History of Time, with even less of a plan. In contrast, we had plotted out Grand Design as if it were a physics research paper.
Stephen had apparently wanted to nail everything down before we started to write. But nothing was ever final. We’d make a decision on something one day and then revisit it a few days later—as we were still doing. I was beginning to think that we’d debate the contents of the book forever and never write anything. Then one day, when we were having a long, slow afternoon beating a few dead horses, out of the blue Stephen said, “It’s time to stop talking.” At first I wasn’t sure what he meant. Was it dinnertime? But he wasn’t talking about red wine and pot roast. What he’d meant was, it was time to start writing. There were still things to fill in and still things we were debating, but he’d apparently had enough. And so began my trips to Cambridge and our cycle of writing, exchanging drafts, and meeting to scrutinize every idea and every word.
In working with Stephen, you couldn’t be shy about defending your point of view. On the other hand, you could heatedly contest a point he was making and a half hour later be laughing with him in a pub. A few years before Stephen’s death, Neil Turok, a leading cosmologist and friend of Stephen’s, would write a series of papers criticizing some of the work Stephen was most proud of. It didn’t affect their friendship. That’s the culture of theoretical physics. When someone identifies a flaw in your reasoning, even if they say I believe you may have been laboring under a slight misconception that possibly impacts the validity of your otherwise brilliant argument, it can feel like they’ve said You’re an idiot. But deep down, you know they are doing you a favor—if an idea is not going to work, it’s better to see it before you and possibly others waste any more time marching down a dead end street. As a theorist you know that most of your ideas will be proved wrong—after all, if the majority of our ideas worked, all the open problems of physics would already have been solved. So there’s no beating around the bush about wrong ideas—and no hard feelings.
As we were writing, Stephen’s experience with Brief History was always in the background, sending us cautionary messages. With its discussion of concepts such as light cones and imaginary time, Brief History, despite all that editing—and despite its popularity—could be tough going. Once, when we were eating at Burger Continental, a cheap hamburger joint near Caltech, a student approached and told Stephen Brief History was one of his favorite books. Stephen replied, Thanks, but did you finish it? He believed that most people never got to the end, so he always made the same reply. It was one of those canned sentences he had stored on his computer because he had so much occasion to use it. He wanted Grand Design to be different. We knew there would be parts of the book describing advanced physics that most people would have trouble with, but we wanted people to finish it.
I was now in Cambridge on another critique-each-other trip. We’d just finished dinner at Stephen’s house. As usual, his wife, Elaine, wasn’t around and white-haired Joan had done the cooking. The look on Joan’s face made me think that the standing she had to do while cooking was causing some back pain, but she was always cheerful and never complained.
This time she’d made lamb stew, with lots of gravy. Gerald, who was Stephen’s carer that night, mixed in the liquid when he chopped up the meat for Stephen. The gravy made it easier for Stephen to swallow. As always, the serving plates were piled high. In addition to the lamb, there was mint jelly, greens, and mashed potatoes, one of his favorites. Afterward, berries with clotted cream. And wine, a bottle that I picked from his wine closet as usual, this time based on its pretty label. It wasn’t bad. Not that I’d know.
Stephen took his wine from a glass, but a spoonful at a time. It didn’t usually add up to much. But this night he went a bit overboard. Me too. Maybe it was the lamb. We ate, as always, at the table in the dining area, which opened on one side to a living room, with a patio just outside. On the opposite end was a galley kitchen, where Joan was starting on the dishes. Semiretired now, Joan still helped Stephen out in a variety of ways. She was devoted to him, and Stephen and his carers all loved her, too.
In some ways, it struck me, this was also Stephen’s family. He saw his daughter, Lucy, on Sundays, and they were close. He seemed to rarely see his younger son, Tim. His older son, Robert, lived in Seattle. At this point in their relationship, Elaine would come and go like a hummingbird just passing through. But Joan, Judith, and his carers saw Stephen and looked after him around the clock. They dined with him, put him to bed, took him to doctors, traveled with him around the world, and tended to his needs on all those rainy days.
When his carers had a family event, a birthday or anniversary, Stephen would often attend, and when they needed things, he’d sometimes give them the money to buy them. He lent one carer the money to buy a car. He promised the daughter of another that when she got married, he’d provide the fireworks. Back in his teens, Stephen and his friends used to make their own fireworks, and he still loved them. He often had them at his own big parties. Not amateur stuff like he used to make, but the kind they set off in stadiums. Sometimes someone would call the police, and they’d come and tell him to stop. Then the police would leave and Stephen would have the fireworks start up again.
Tonight’s dinner had been a feast, but I felt bad. Joan looked exhausted. After a while she left, and Gerald retired to the next room to read. Along the way Gerald turned on the television news. Stephen liked to watch the news, which I found odd because it often annoyed him. Right now he was making a face at a report about something or other that Parliament had voted on. I asked him if I should turn it off. He raised his brow, meaning yes, so I did.
We sat there looking at each other in silence for a minute. I suddenly understood why he liked my company in the evenings, how, after he was fed, his nights could quickly turn lonely. His eyelids drooped. I wondered if the wine was about to put him to sleep. And then he seemed to perk up, as if a thought had suddenly popped into his mind. He started to type.
“Are you healthy now?” he asked.
I nodded. After my last visit I’d undergone surgery for a small-bowel obstruction, and then one day, a while afterward, I’d fainted. At the hospital they said my blood pressure was 58/30. I’d had massive intestinal bleeding. They stuck an intravenous line in me and started infusing what would eventually total thirteen units of blood. That’s a complete oil change. But the bleeding came and went in spurts, and even after a dozen tests and procedures they couldn’t pinpoint the vessel that was causing it. One night while in intensive care I overheard my doctor telling a resident to keep an eye on me because I was in danger of “bleeding out” before morning. He’d apparently missed that medical school class on keeping your voice down. He also misread his crystal ball. After ten days my bleeding fits stopped as suddenly as they’d started.
Lying in that hospital bed, having heard the prediction that I might soon die, I’d thought about many things. About how I might have seen my family for the last time. How I’d never know whom my kids married or how their lives would turn out. How I’d no longer be there when they needed me. They were still so young—would they forget me? Had my life had any meaning?
My mind summoned random images, of ocean waves, a sunny beach, mountains covered with a blanket of snow. They were clichés but I didn’t want to let go of them. I looked out the window and thought about the great beauty of the blue California sky and the palm trees outside. Had I taken it all for granted? Should I have stopped more often to appreciate them? Was it now too late?
I wondered whether Stephen had such thoughts. In his many moments at the edge of death, did he think of the sky, or the stars he loved to gaze at? Did he want to hang on for the sake of his children? Did he have regrets? In time I realized that he’d had so many life-threatening incidents—a stoma breakdown, a lung infection, a sodium imbalance—that being at death’s door became routine. The plagues kept coming, but he’d made peace with his life and oft-expected death. He’d worked through all those thoughts that I, plunged into my first life-and-death crisis, suddenly had to confront.
When I was in that intensive care ward and feeling more vulnerable than I ever had, I thought of how vulnerable Stephen had seemed to me, and how wrong I’d been about that. It occurred to me that Stephen had proved himself to be an iron man in a fragile man’s façade. Science is about observation and evidence, and regardless of appearances or the pronouncements of doctors, the evidence suggested that nothing could fell Stephen. I, on the other hand, had proved to be fragile. Lying in my hospital bed, I’d thought about how ironic it would be if, of the two of us, I was the one who died before finishing our book. I had a dream. It was fuzzy, but in it Stephen and I were in a two-person race. I took off, happily sprinting, while he followed, pushed along slowly by his straining wheelchair motor. And then I toppled and lay still on the track as Stephen rolled past with his eyebrows raised and a smile on his face.
It seemed odd to me to have such thoughts and dreams on my potential deathbed, but I’d had them. I told Stephen that, and he appeared amused.
“You like to make bets,” I said. “We should bet on which of us will go first.”
He frowned.
“Why not?” I said.
He started typing.
“The loser won’t be around to pay,” he said.
He had a point. I sipped some wine and asked if he’d like me to wipe some drool that had collected on his chin. Another yes.
Over time the carers grew comfortable letting their guard down a bit when I was there, and I’d gotten used to doing minor carer-like tasks for him. I noticed that Stephen liked the attention. Sometimes, I thought, he asked for an adjustment to his sitting position or his glasses just to have human contact. He seemed to like to be touched, and I could understand his being hungry for it. He slept alone and couldn’t snuggle or trade caresses with his lover. He couldn’t even hug friends when greeting them or reach out to shake someone’s hand.
Stephen had gotten word of my bleeding incident and sent a kind get-well card signed by him, Judith, and several others. I thanked him for it. “Funny how being in the hospital made me think a lot about death,” I said. “Never thought much about it before.”
His look seemed to say welcome to the club.
“I know. You go through life-and-death scares all the time.”
He raised his brow. Yes. He started typing. “Then back to physics,” he said.
Physics. With Stephen no conversation was ever far from physics. “Doesn’t it frustrate you that you can’t write the equations?” I asked.
He grimaced. I wasn’t sure whether he meant no, it didn’t frustrate him, or that he didn’t like my prying. The issue was something I’d wondered about but hadn’t felt close enough to him to bring up. Now that I had, I hoped I hadn’t crossed a line.
He typed. “My disability was gradual,” he said. “I had time to adjust.”
“Imagine the physics you might have done if you hadn’t had your disability,” I said.
His frown said he disagreed. He started typing. It took a while, but I didn’t peek. “It helped. It helped me focus,” he finally said.
I marveled that he could see anything positive in his plight. And I marveled at his passion for physics. He’d met my son Nicolai on a couple of his trips to Pasadena. I told him that Nicolai, a basketball player who worked at it for hours each day, would always say, “Basketball is life.” I told Stephen it was amazing that, after so many decades of life and the ordeals he’d gone through, he still had that same passion. “For you, physics is life,” I said.
He wrinkled his nose. He disagreed again. He started to type.
“Love is life,” he said.
Stephen’s comment that love is life touched me, reminding me that though his disability was a barrier to developing both emotional and physical relationships, he thrived on human connection as much as anybody. Still, his comment surprised me, because in his choices he’d often chosen physics over human connection. Even before he became wheelchair-bound, he’d make himself essentially incommunicado for days on end while he puzzled out some important issue in his research. He hadn’t spent a great deal of time with his children when they were growing up. His wife, Jane, had felt neglected. Despite all that, I had no doubt that his family and relationships were central to his happiness. It struck me again that in his life, as in his physics, he embraced many contradictions.
To Stephen, contradiction in physics meant opportunity, that there was something that begged to be reworked, reconciled, or simply understood and accepted. As he moved on from his Ph.D. work, his capacity for physical adventure was fading but his adventures in physics were becoming ever braver and more audacious. In the years to come he would move on from his work on the origin of the universe and join that band of adventurers who were starting to explore the strange world of black holes.
Even among those black hole pioneers, Kip Thorne told me, “Stephen was an unusually bold thinker.” That was saying something. It was like being an unusually heavy sumo wrestler, or an unusually wet fish. Because in the early day of black hole research, when our knowledge of those strange and exotic objects was relatively new, no one in the field was timid. They couldn’t be. They were trailblazers in a world ruled by some of the strangest implications of relativity—that there is no universal “flow” of time, and no “present time” that is shared throughout the universe. We seem to experience that things “exist” in a common present and that events occur one after another, but, as Einstein said, “For us believing physicists the distinction between past, present, and future only has the meaning of an illusion, though a persistent one.”
The theory of black holes even allows for time travel. It predicts that if you fly to a black hole, hang out for a while, and return, when you get back you might find that you’ve jumped a few hundred or thousand years into your home base’s future. Do it repeatedly and you could watch civilizations rise and fall, as if you were watching the future of your planet on fast-forward. Today the science-fictionish world of black holes is more familiar, but back then, before every schoolkid knew that space could be curved, it was all new and mind-bending. And even among that band of daredevil thinkers, Stephen stood out.
Stephen’s first contribution to black hole physics concerned the black hole horizon, a key concept in the definition of those exotic objects. In colloquial terms, physicists have always thought of a black hole as a region in space that, due to its immense gravity, allows nothing to escape to the outside. One can think of that region as being defined by its horizon. In the words of Roger Penrose, the black hole horizon is “the outermost location where photons [light] trying to escape the hole get pulled inward by gravity.” The name comes from an analogy—just as we on our planet can’t see the sun after it passes our earthly horizon, an outside observer cannot see past the horizon of a black hole.
In his work on black holes, Roger Penrose turned his definition into a precise mathematical form. His formulation sounded reasonable and soon became the standard. But as Stephen studied the physics of black holes, he realized that the Penrose horizon was actually, as Kip put it, “an intellectual blind alley.”
Two issues tainted the Penrose approach. The first is an issue that touches the soul of relativity, the contradictions that arise among disparate observers. According to relativity, different observers—depending upon the strength of gravity in their environment and their movement in relation to each other—may not agree on the size and shape of regions of space and durations of time. This can make one’s analysis a mess. But there is a way around that: researchers can stick exclusively to concepts that are defined in a manner that is independent of the observer. To do so brings multiple blessings. First, it ensures that the laws and phenomena they discover apply to everyone. Also, it makes the mathematics simpler. Finally, and perhaps most important, it greatly enhances our ability to interpret the equations. According to Penrose’s definition, however, a black hole’s boundary would not be the same for all observers. Someone falling into the hole, for example, might see the Penrose horizon differently than would someone hovering outside it. So which is it? You’d have different horizons, depending upon who was looking.
The other problem in Penrose’s approach was that, if defined his way, the horizon can make discontinuous jumps. For example, when a glob of new matter falls in and the black hole becomes larger, the horizon will suddenly grow in size. In complex situations, such as when two black holes collide, those jumps can be bizarre and difficult to work with.
Penrose was aware of those drawbacks but stuck with his definition. Sometime in the early months of 1971, however, Stephen came to realize that a more productive way to think about the horizon of a black hole is to view it as a region in space-time, rather than as a region of space at a given time, as Penrose had done. So Stephen redefined the horizon as the boundary, in both space and time, beyond which signals such as light rays could not be sent out to the distant universe. He proved mathematically that this definition remedies both weaknesses of the Penrose approach—the black hole boundary would be the same for all observers, and it would always change smoothly and never jump around.
How can one understand the difference between the two definitions? Imagine a black hole with an immense mass of debris around it, outside it but imploding and soon to be swallowed up.*1 Imagine also a tiny rocket ship that is just outside the black hole, trying to escape its pull and leave the area. The rocket is firing its jets to propel itself away. Since it is outside the black hole, one might think it could be successful at that. However, the moment the immense mass has been swallowed up the black hole will grow larger, and if the mass is great enough, the black hole will then encompass the rocket, so that the rocketeer cannot escape.
In Penrose’s description of those events, the rocket is at first outside the black hole horizon. A moment later, as the immense mass falls in, the Penrose horizon jumps outward and the rocket is trapped inside. So though it was not possible for the rocket to escape the black hole, the Penrose horizon did not reflect that at first; it did so only after the immense mass had fallen in.
The way Stephen defines the horizon, if the rocket is fated to eventually be swallowed up by the enlarged black hole, this will come as no surprise, because the Hawking horizon will encompass the rocket from the start. The Hawking horizon, in other words, grows before the immense mass falls in. It depends not only on the present state of things, but on what will happen in the future. It thus violates the laws of cause and effect: in this case, the effect (the black hole’s horizon growing) precedes the cause (the mass falling in).
Stephen’s definition of the horizon requires that you know the full history of space-time, including its entire future, though in practical applications objects far away in space and time can be ignored. Physicists call a definition in which something is determined by future events teleological. They borrowed the term from philosophers, who use it to refer to the explanation of a phenomenon in terms of its eventual purpose rather than an immediate cause.
Philosophers have pondered teleological laws of nature at least as far back as Aristotle. Science teaches us that rain falls because the moisture in clouds condenses into droplets of water, which are heavier than air. But in Aristotle’s view there was another reason: it rains so that plants can grow, in order that people can eat them. The demands of the future, he believed, shape the present. We all make life decisions that way, to a greater or lesser extent, depending upon our personalities. For example, if you’re offered a slice of cheesecake at lunch, instead of reacting according to your current desires, you might take into account what you’re going to be eating for dinner. In physics, however, forces act and objects react according to present conditions, so although teleological concepts are part of our everyday lives, they are rarely used in physics. Stephen’s teleological definition of the horizon was a gem of creativity. It was his boldness that enabled him to embrace and explore the idea, unlike others, such as Penrose, who had quickly discarded it.
It may seem that the nuances of how theorists define the black hole horizon shouldn’t matter much because the definition of a term is a choice made by physicists, not a statement about nature. But the concepts we invent influence the ideas we generate and the conclusions we draw. In time, Stephen’s definition proved to be a powerful advance and was widely adopted by others. It guided their intuition and shaped their mental pictures of the processes black holes undergo. Stephen called his version of the horizon concept the absolute horizon and Penrose’s version the apparent horizon to distinguish between them. With that redefinition, Stephen had redefined not just the horizon but the way theorists think about black holes.
Armed with his new way of thinking about black holes, Stephen worked furiously to understand what general relativity tells us about the laws that govern them. He’d seal himself off for days at a time. Jane would approach to talk about some quotidian issue that had come up, but he’d stay in his physics world. She’d approach for reassurance that she was important to him, and he’d ignore her. He would blast Wagnerian opera for hours at a time on the record player as he worked, just as he’d done after receiving his diagnosis, just as his parents used to do when he was a child. Jane grew to hate Wagner. She thought of him as an “evil genius,” an alienating force in their marriage.
Whatever Wagner subtracted from their relationship, he apparently added to Stephen’s physics. After working for a year and a half and collaborating with two colleagues, he achieved a second important breakthrough, the laws of black hole mechanics. Formulated in August 1972—when Stephen was just thirty—this was a set of rules that govern how black holes grow when matter falls into them and what happens when black holes interact with other black holes.
Stephen’s laws were ahead of their time. The first experiments demonstrating indirectly but with high confidence that black holes even existed wouldn’t come until around 1990, based on observations of a heavenly body called Cygnus X-1. The first direct observations of the disturbances of space-time that are the signature of black hole collisions—a certain type of gravity wave—didn’t come until 2015, in the LIGO*2 experiment for which Kip Thorne shared the Nobel Prize. The first (almost) direct observation of a black hole didn’t come until 2019, the year after Stephen died.
Despite our inability at the time to see a black hole, Stephen believed that black holes could provide unique insight into the nature of gravity, space, and time, revealing secrets that don’t make themselves apparent in ordinary circumstances. His intuition would prove correct.
The discovery of the laws of black hole mechanics was an important step toward understanding those esoteric objects. But the laws had a strange peculiarity that would also turn out to be important: They looked a lot like the laws of another field, thermodynamics, the physics of heat. Each black hole law, in fact, was identical to a thermodynamics law if you replaced certain terms from black hole physics with a corresponding term in thermodynamics.
Consider one of the black hole laws in particular, the area increase theorem. It says that in any interaction black holes undergo—whether they are coalescing, swallowing matter, colliding with each other, and so on—the sum total of the areas of all the black hole horizons will always increase. That’s not true of ordinary objects. For example, suppose you take two identical balls of clay, merge them, and from that glob form a new ball. Simple high school math will then tell you that the surface area of the new ball will be about 20 percent less than the sum of the original surface areas. But due to the curvature of space, if you merge two black holes, the horizon—the analog of the surface area—will be greater than the sum of the originals.
Physicists immediately noticed that the area increase theorem is strikingly similar to what is known as the second law of thermodynamics. The area increase theorem says that in any black hole interactions, the sum of the black hole “horizon areas” always increases. The second law of thermodynamics says that, no matter what physical interaction takes place, the entropy (degree of randomness) of any closed system always increases. Just replace the term “horizon areas” with the word “entropy” and the black hole law becomes the thermodynamic law.
Virtually all physicists believed the correspondence of these laws was a weird but meaningless coincidence. But a graduate student at Princeton named Jacob Bekenstein didn’t agree. Bekenstein speculated that the connection between the laws should be taken literally—that the entropy of a black hole is proportional to the surface area of its horizon.
Entropy is a measure of disorder. In an ice cube, for example, the water molecules are arranged in orderly hexagonal rings, while in liquid water the molecules bounce around at random. An ice cube, therefore, has relatively low entropy, and its entropy increases when it melts. More generally, a low entropy system is an orderly system, or a simple system in which there aren’t many constituent parts that can become disordered. A typical high entropy system, on the other hand, is a complex system in disarray.
A black hole seemed to be too simple to ever be in a state of disorder. Once formed and in a settled state, a black hole in empty space was thought to be like a billiard ball. With no constituent parts, there was nothing that could ever be in a state of disorder. As a result, black holes were said to have no disorder at all—and hence an entropy of zero. Bekenstein’s ideas contradicted that picture and were met with widespread derision.
There was another reason people found Bekenstein’s theory unpalatable. According to the laws of thermodynamics, anything with an entropy greater than zero must have a temperature greater than zero—it cannot be completely cold. And anything with a temperature above absolute zero must emit radiation—it will glow.*3
That’s a problem, because when something glows, it releases—radiates—energy. That energy would have to come from the black hole’s mass. A glowing black hole, in other words, would be slowly converting its mass to electromagnetic energy (according to Einstein’s famous E = mc2) and radiating it away.*4 As a result, the black hole would gradually shrink and eventually disappear completely. It would, in a sense, “evaporate,” as everything inside eventually leaked out in the form of radiation.
Today we know of that radiation as Hawking radiation, but back then, ironically, Stephen didn’t believe in it, or in any of Bekenstein’s ideas. They contradicted the picture of black holes that Stephen and others had painstakingly derived from the equations of general relativity. Bekenstein recognized that his theory of black hole entropy seemed to demand that black holes radiate. Yet he also shared the opinion that black holes cannot radiate. He didn’t know how to resolve that issue, but he stuck with his idea that black holes have entropy.
The widespread attacks on Bekenstein illustrate why it takes courage to advocate for new ideas in physics. If you have convincing evidence, chances are you’ll win your battles. But though Bekenstein believed that black holes have entropy, he didn’t go far enough and accept the consequence of black hole radiation. Nor could he adequately defend his ideas. Virtually no one accepted his point of view. He got shot down, and Stephen was one of the lead attackers.
As it would turn out, when the old theory of black holes, based solely upon general relativity, was amended to incorporate the principles of quantum theory, the idea that black holes have entropy would be confirmed—and it would be Stephen himself who, admitting that he’d been wrong, would reluctantly flip sides in the argument and prove Bekenstein correct.
With all that wine and lamb and the talk about my near-death experience in the hospital, by the time I left Stephen’s home it was after ten. Still, I wasn’t ready to go to my room at ancient Caius college. Given Stephen’s work schedule, I tended to stay out late and return to the room only to collapse in bed.
This was winter, and my room, with its stone walls, small windows, and low ceiling, was tiny and dark. I suppose it had some charm, especially if you’re a bat. But I wasn’t in the mood to stare at the ceiling, so I walked the half hour or so from Stephen’s house to a pub I knew. The pubs in Cambridge were supposed to close at eleven, but “close” meant different things to different people. To the proprietor of this pub, a fortyish Chinese woman, it meant close your doors. That she did—she locked them tight promptly at eleven. What she and her British bartender husband didn’t do was ask their customers to leave. Instead, they kept serving until everyone had slowly filtered out. Sometimes that wasn’t until two, or even later. It was a business model of dubious legality, but it worked just fine.
Cambridge pubs were unlike any I’d encountered. The half-inebriated person guzzling pints next to you wasn’t just a drunk. He or she might also be an astrophysics graduate student or a renowned neuroscientist. One of my favorite evenings was spent listening to a beer-filled fellow talk about the agricultural economics of West Africa. Not my usual choice of topic, but it went surprisingly well with a few stouts and mixed nuts.
On this evening I made the mistake of letting the bartender steer the subject to my work with Stephen. Since I was a regular there, he and his wife knew I worked with him, but I usually managed to avoid the subject. Not this time. This time he poured me a free beer, then told me the price was that I’d have to tell him about black holes. I hated getting into such situations. I was there, after all, to forget about black holes. Now I’d been sucked back in.
Luckily, the bartender was one of those who’d rather talk than listen. He asked me a question, and just as I started to answer, he picked it up from there. And for the next twenty minutes he proceeded to tell me everything he knew about black holes. Most of it was correct, too.
We’d come far, I thought, from Stephen’s early days, when black holes were an exotic subject. Back then, few physicists cared to discuss them. Now you can get a lesson from your bartender. As he droned on and his wife occasionally rolled her eyes, my thoughts drifted to how Stephen was the person most responsible for that, how he’d had a huge effect, not just on the culture of physics but on the culture at large. He appreciated that, perhaps especially now, in his later years. For the questions he’d wanted to answer were questions not only for physicists, but for all of us. I realized then that if his discoveries in physics lent him a kind of immortality, so did the physics he shared with the public. That thought added to a feeling I’d had since my surgery that Stephen was indestructible.