Quantum Legacies: Dispatches From an Uncertain World Read online

  The particularities of time and place can shape scientific research in many ways, across many scales. At the individual level, hiccups of individual biography can produce lonely thinkers like Paul Dirac or the heroic perseverance of Stephen Hawking. Forces acting across larger scales, from changes in specific institutions to epochal geopolitical rifts, shape scientific research as well. During the years since Ehrenfest and Einstein exchanged their playful notes in Brussels not quite a century ago, the story of physics and physicists has been dramatically reshaped by these kinds of forces several times over: by the rise of Nazism and cataclysmic world war, by the relentless calculation of nuclear brinksmanship during the Cold War, and by the whiplash suddenness with which the Cold War sputtered to an end. With hindsight, we can trace how such turning points helped to propel unexpected insights about the natural world, just as often as they limited individuals’ horizons.

  Captivated by the ways in which the world of ideas remains tethered to more earthly concerns, I pursued graduate study in theoretical physics and in the history of science. After that, I had the great fortune to join the faculty at the Massachusetts Institute of Technology (MIT), where I have taught both subjects for twenty years. Drawing on this pair of perspectives, I have aimed in much of my writing to reconstruct the never-straight path of physics research over the past century by focusing on scientists’ training. How have particular approaches come to seem natural for researchers and their students in a given time and place? How have generations of young physicists learned to broach questions and evaluate results, and how have those methods shifted—sometimes subtly, sometimes by great leaps—between one setting and another? Teaching, training, undertaking the hard work of trying to forge some shared understanding between individuals and across generations—these are the “quantum legacies” to which I refer in the book’s title. Some of these legacies are captured well by focusing on the efforts of individuals and their exchanges with small circles of colleagues; others are clarified by scrutinizing new machines, like the first electronic, programmable computers or hulking particle accelerators like the Large Hadron Collider. I am particularly fascinated by textbooks as legacy-making engines: objects crafted expressly to try to smuggle forward, into the future, bundles of hard-won skills and insights. Chasing down these legacies has offered me an opportunity to reflect on my own training, as I wonder about what sorts of legacies my colleagues and I might pass along to our students.

  The essays in this volume explore episodes in which physicists have grappled with subtle uncertainties of the natural world, even as they have labored within the inevitable uncertainties of the social and political worlds. Though the settings of the essays range across space and time, many center on developments within the United States during the Cold War years. Physicists’ fortunes during that era lurched between periods of plenty and want. The boundless optimism coming out of the war years was countered, for some, by McCarthyist red-scare anxieties. Enormous new tools helped to reshape physicists’ intellectual landscape—some of them legacy equipment from the massive wartime projects, others underwritten by a newly generous federal government, especially its military divisions. A flood of eager new students rushed into the field, making physics the fastest-growing academic specialty in American higher education. But then came crippling reversals. The first occurred in the early 1970s, years into the slog of the Vietnam War, combined with détente, “stagflation,” and substantial cuts in defense and education spending. Another came in the early 1990s, after a second wave of defense-related spending, which had accelerated under the Reagan administration, vanished quickly, and unexpectedly, after the Soviet Union dissolved.

  These turning points, driven by the unsteady push-pull of world events, changed the texture of everyday life for members of what had been, not long before, a rather sleepy academic field. Einstein and Ehrenfest had traded notes across the conference table at the 1927 meeting in Brussels. After the war, physicists had to adapt to rather different communication habits. That American workhorse of a journal the Physical Review swelled from three thousand pages published in 1951 to more than thirty thousand pages published in 1974. One of my favorite photographs shows particle physicist Val Fitch, who received the Nobel Prize in 1980, in danger of being crushed by the sheer mass of the journal, as the stacks of each year’s volumes climbed higher and higher. The journal’s longtime editor Samuel Goudsmit explained to a colleague in the mid-1960s how he and his editorial team attempted to manage the changes. The journal, he wrote, “is no longer similar to the neighborhood grocery story where old customers get personal attention.” Rather, it had become “more like a supermarket where the manager is hidden in an office on the top floor. As a result, lots of things are just done by routine rather than by human judgment.” He meant it literally: by that time, the editorial office was experimenting with a new punch-card computer system to mechanize tasks like matching referees with submissions, tracking the progress of referee reports received, and recording responses sent to authors.6

  Figure 0.3. Particle physicist Val Fitch poses with stacks of the Physical Review, arranged by decade, in the late 1970s. (Source: Photograph by Robert P. Matthews, courtesy of AIP Emilio Segrè Visual Archives, Physics Today Collection.)

  The effects were palpable. Goudsmit himself noted in the mid-1950s that each issue of the journal had become “almost too bulky to carry.” A few years later, he observed that “we have long ago passed the psychological limit above which the subscriber is overwhelmed by the bulk and looks only at the few articles in his own narrow field.” At a time when most professional physicists in the United States still purchased their own subscriptions to the journal, several wrote to Goudsmit to complain about the runaway volume: back issues of the journal filled their office shelves and threatened to overrun closet space at home. Goudsmit advised his colleagues to stop being “overly sentimental” and simply rip out those articles they wanted from each issue, tossing the rest. “There is really little reason to keep more than about ‘six feet’ of The Physical Review at home,” he concluded. Though some felt “revolted” by “such destruction of the printed word,” others took up the editor’s suggestion. One Caltech physicist reported with pride that he had reduced two feet of shelf space, which had been taken up by his copies of the 1963 Physical Review, to a few inches, though he wondered whether the journal might switch to a different glue for its binding, to make it easier to tear out the desired articles.7

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  As I have glued the essays for this volume together, my thoughts have often returned to Goudsmit’s advice. Various essays will no doubt resonate more strongly for some readers than others. In collecting them for this volume, I have updated most and merged others together. I have also tried to put the essays more directly into dialogue with each other by grouping them into four sections. My hope is that each essay can be enjoyed on its own, while together they may yield insights, partial and patchwork-like, into broader transitions within physicists’ continuing quest to understand space, time, and matter. Like an old issue of the Physical Review, the picture they present is more kaleidoscopic than a formal portrait.

  The essays in “Quanta” address discrete moments in the transformation of physicists’ understanding of quantum theory, from the heady days of the 1920s, through the dark times of the 1930s, to some bizarre twists early in the nuclear age. The section culminates with my own group’s efforts—first in Vienna and most recently at the observatory on La Palma—to build upon this century-long legacy by testing quantum entanglement as thoroughly as our imaginations and toolkits would allow.

  The second section, “Calculating,” focuses on some of the changes in how—and why—new generations became physicists within the United States, during and after the dramatic disruptions of the Second World War. The early years of the Cold War fostered a new type of calculation among many defense analysts and policymakers in the United States, as they scrutinized the unsteady standoff with the Soviet Union. To prepar
e for the nation’s defense—in case the Cold War ever tipped into outright warfare between the superpowers—these analysts and policymakers concluded that the United States needed many more physicists, trained and at the ready, who would be available to staff massive projects like a next-generation Manhattan Project. The defense intellectuals’ calculations, and their relentless calls for “scientific manpower,” drove enormous shifts in enrollment patterns and the rhythms by which new generations entered higher education. These institutional changes, in turn, reshaped how young physicists learned to calculate and how they grappled with quantum theory.

  In “Matter,” I turn to physicists’ more recent efforts to understand the world of electrons, quarks, and more fleeting constituents of the subatomic realm, such as the long-elusive Higgs boson. Over the past half century, physicists around the world have cobbled together a remarkably successful account of subatomic particles and the forces between them. The so-called “Standard Model” is built within the framework of quantum theory but harbors conceptual surprises that neither Einstein nor Heisenberg could have foreseen. Meanwhile, the specialty of high-energy physics inherited its own particular legacy from the Cold War: within the United States, political priorities and unprecedented federal investment fostered an era of gigantism, as physicists built larger and larger machines to probe matter at smaller and smaller scales. That investment—and the political arguments that had sustained it—collapsed soon after the Soviet Union fell apart, in the early 1990s. I was an undergraduate at the time. An internship at the Lawrence Berkeley National Laboratory provided me with a crash course in particle physics as well as in the changing political realities of “big science.”

  The final section, “Cosmos,” explores moments in physicists’ changing conceptions of space and time on the largest scales—phenomena described by that other great pillar of modern physics, relativity. Physicists’ efforts to understand Einstein’s relativity, and to use it to model the evolution of our universe as a whole, have paralleled their concerted efforts on quantum theory over the past century. These efforts have yielded some stunning insights into the universe and our place within it, even as these insights have resisted physicists’ every attempt to combine relativity and quantum theory into a conceptually consistent whole. Were Einstein alive today, he might be excused for passing new notes to a friend, still laughing at our naiveté.

  QUANTA

  1

  All Quantum, No Solace

  Physics became “modern” at breakneck speed. Only twenty years separated Albert Einstein’s formulation of special relativity, in 1905, and the development of quantum mechanics in 1925–26. The two events have attracted rather different kinds of stories. Einstein’s achievement is typically portrayed as an epic tale of one man’s obsession. The creation of quantum mechanics, on the other hand, required an ensemble cast, more Heinrich Böll’s Group Portrait with Lady (with a nod to Marie Curie) than Melville’s Moby Dick.

  And quite a cast it was. The fatherly Niels Bohr dressed like a banker and mumbled like an oracle. Werner Heisenberg, a gregarious Bavarian, thrived as the life of any party, banging out Beethoven piano sonatas into the wee hours and traipsing up mountain paths in lederhosen. Louis de Broglie, a young French aristocrat, flitted from studies of literature and history before brazenly introducing the notion that solid matter might consist of waves. Erwin Schrödinger, the dapper Austrian, led a surprisingly bohemian lifestyle. Openly promiscuous, he sustained a string of affairs with much younger women—his biographer felt compelled to list “Lolita complex” in the index—and raised, with his wife, a child he fathered with the wife of one of his assistants. Then there were the children who never grew up: practical jokesters like the brilliant Russian physicist Lev Landau and the acid-tongued Wolfgang Pauli.1

  The creators of quantum mechanics formed a tight-knit community. When not visiting together at Bohr’s Institute for Theoretical Physics in Copenhagen or at one of the informal conferences sponsored by the industrialist-turned-philanthropist Ernest Solvay, they kept up their conversations by letter. Tens of thousands of their letters have survived. Over the years, scholars have dutifully inventoried, archived, microfilmed, and translated these letters, subjecting them to the kind of line-by-line scrutiny once reserved for scripture.2 And yet for all the attention lavished on the founding generation of quantum physicists—several biographies each of Bohr and Heisenberg, lengthy treatments of Schrödinger, Max Born, and others—few have paid much attention to the brilliant British physicist Paul Dirac. Of all the strange characters who paraded through Bohr’s Copenhagen institute, Bohr called the waifish and withdrawn Dirac “the strangest man.”3

  As a young man, Dirac had dreamed of studying relativity. By the time he arrived at Cambridge in 1923 to study for a doctorate, however, the local expert on the subject had stopped taking students. Dirac was assigned to Ralph Fowler instead, at the time Britain’s foremost expert on the strange physics of atoms. Quantum theory was still an ungainly patchwork of models and heuristics, and physicists across Europe had been struggling for decades to make sense of matter at the smallest scales. The tendency was to begin with the familiar laws that govern everyday objects—the motion of planets in the solar system or the interaction of electric charges and radiation—and then append this or that ad hoc rule to cover instances when the usual equations broke down.

  Dirac’s first glimpse of a new approach arrived by mail in September 1925. Werner Heisenberg had sent page proofs of a new article to Dirac’s adviser Fowler, who in turn sent them to Dirac, still at his family home in Bristol for the summer vacation. Fowler appended the casual note, “What do you think of this? I should be glad to hear.”4 In the short article, Heisenberg aimed to establish a new quantum mechanics, a first-principles treatment of matter and radiation rather than the tattered, hand-me-down quilt of his teachers’ generation. Heisenberg was convinced that it was a mistake to rely on intuitions or models taken from ordinary physics. Electrons orbiting a nucleus in an atom were not just like planets orbiting the Sun, he declared: the electrons’ paths could not be observed, even in principle. The best way forward, he announced in the opening paragraphs of his brief article, written when he was just twenty-three years old, was to construct a new theory “in which only relations between observable quantities occur.” In Heisenberg’s new formulation, arrays of discrete numbers replaced the smoothly varying quantities usually found in physicists’ equations, and he filled his arrays with observable quantities, such as the color and brightness of light emitted by atoms that had been excited by some outside source of energy.5

  Making his way through Heisenberg’s page proofs, Dirac brushed aside the opening philosophical challenge about sticking only with observable quantities. Instead, Dirac focused on something interesting later in the article. Some of the arrays of numbers in Heisenberg’s new scheme behaved in a curious way: their product depended on the order in which they were multiplied. A times B did not equal B times A. Unlike Heisenberg, Dirac had a degree in pure mathematics; such unconventional rules of multiplication reminded him of similar relationships that crop up in ordinary mechanics—the physics of tops, balls, and orbiting planets—when written in a particularly advanced, mathematically elegant way. This buried mathematical analogy, rather than Heisenberg’s opening salvo about unobservable quantities, prodded Dirac forward. Nine months later he completed his dissertation, using the mathematical analogy to clarify and generalize Heisenberg’s work.

  By then, Heisenberg’s approach to quantum mechanics was no longer the only game in town. During the winter of 1926, Erwin Schrödinger—ten years older than Heisenberg and Dirac, and far more conservative in his approach to physics, if not in his personal life—had produced an independent formulation. Instead of Heisenberg’s discrete arrays of numbers, so jarringly unfamiliar to most physicists, Schrödinger borrowed the familiar mathematics of waves, usually enlisted to describe such phenomena as ripples spreading on the surface of a pond or the wailing scre
ech of a passing siren. The contrast between Heisenberg’s and Schrödinger’s rival approaches stirred strong emotions. In one of his early articles on wave mechanics, Schrödinger wrote that he “felt discouraged, not to say repelled,” by Heisenberg’s methods. Heisenberg shot back, in a letter to a friend, that the more he thought about Schrödinger’s work, “the more disgusting I find it.”6

  Dirac’s dissertation had earned him a scholarship to spend the 1926–27 academic year on the Continent. His first stop was Bohr’s institute in Copenhagen. There he ignored most of the jabs and jokes of his colleagues and sequestered himself in the library, where he set about demonstrating that Heisenberg’s and Schrödinger’s approaches were mathematically equivalent, the name-calling notwithstanding. Other people produced independent proofs of the equivalence, but most physicists were quick to admire Dirac’s approach as the most powerful and elegant.

  Figure 1.1. Paul Dirac (left) with Werner Heisenberg, early 1930s. (Source: AIP Emilio Segrè Visual Archives.)

  Dirac now produced a steady stream of breathtaking results. Just before leaving Bohr’s institute in January 1927, he extended the quantum formalism beyond atoms to the treatment of light, including the interaction of charged particles with radiation, thus creating a whole new physical theory. He dubbed it “quantum electrodynamics,” or QED. Next he set about rectifying Heisenberg’s and Schrödinger’s equations with Einstein’s special relativity, seeking to create a quantum mechanics that would hold together even as the objects under study moved at speeds closer and closer to the speed of light. Back in Cambridge in autumn 1927 (having been elected a fellow of St. John’s College), he derived his relativistic equation for the electron, clarifying, along the way, the notion of quantum “spin” that had puzzled colleagues for years.