One of the physics courses I took was an advanced laboratory. It contained two kinds of students: undergraduate whizzes in experimental physics, who were rendering helium superfluid and measuring sound waves, and those in the other half of the room, all of them standing agog. Required to be there in order to prove their bona fides as “real physicists,” this second half (excluding me, the historian) was composed of graduate students in theoretical physics who were only marginally more competent at manipulating voltmeters than I was.
My lab partner was one of these theorists, earning a joint PhD in physics and the history of science. Our lab reports contained the best “historical overviews” any of the instructors had ever seen, if not the best results. Perhaps the most comic of the three experiments we performed was an attempt to verify the irreducible weirdness of quantum mechanics, a quantity called “Bell’s inequality.” In brief, this result involves measuring two quantities that are related by a Heisenberg uncertainty relation in order to show that you can’t get around the quantum. After enduring ritual humiliation from the 18-year-olds levitating positrons (or something like that), we managed to get a serviceable result. I made sure this was the last physics course I ever took, while the theorist embarked on a stellar and perhaps unique career as both a practicing theoretical physicist and a historian of science. His name was David Kaiser.
Kaiser is today ensconced at the Massachusetts Institute of Technology, where he is the Germeshausen Professor of the History of Science in the Program in Science, Technology, and Society; professor of Physics in the Department of Physics; and associate dean of Social and Ethical Responsibilities of Computing. Alongside all that, which surely keeps him busy, he publishes books, including two award-winning ones — Drawing Theories Apart: The Dispersion of Feynman Diagrams in Postwar Physics (University of Chicago Press, 2005) and How the Hippies Saved Physics: Science, Counterculture, and the Quantum Revival (Norton, 2011). And, as if this weren’t more than enough, he also produces a stream of physics papers, mostly in inflationary cosmology and quantum theory, and a host of popular essays and mainstream book reviews that bring both science and its history to a wider audience.
Hence Quantum Legacies: Dispatches from an Uncertain World. The book consists of an introduction and 19 chapters, ranging in length from short to shorter, all of them (except the introduction and chapter eight, on quantum mechanics textbooks) adapted or merged from essays he has published in venues like The New Yorker, The London Review of Books, and academic journals. The essays flow together locally with their neighbors, but sometimes, over longer stretches, one is surprised to find oneself migrating from Schrödinger’s cat to President Eisenhower’s Science Advisory Committee. It is best read piecemeal over an extended period, rather than in three sittings as I did.
The result is a hybrid of genres. If forced at gunpoint to classify it, you could do worse than label it “popular science,” but the science comes along with carefully researched historical context — complete with scores of footnotes to unpublished archival documents — that proves indispensable to the narrative. For the same reason, the book isn’t “popular history of science” either. At several points, Quantum Legacies reads like a memoir: we witness Kaiser’s fascinating experiments in Vienna and the Canary Islands, see his personal reactions to the launch of the Large Hadron Collider at CERN or the death of Stephen Hawking, and learn a little bit about his twin children and (more briefly) his mother.
Nonetheless, the book has a coherent story to tell — or, rather, two coherent stories that interact with each other across the four sections of the book, somewhat-but-not-entirely-helpfully named Quanta, Calculating, Matter, and Cosmos. For the sake of convenience, I will call the first story the “quantum narrative,” which begins in terror and metamorphoses into sublimity, and the other the “scale narrative,” which starts out optimistically but ends in tears.
Beginning with a joke, the book turns dark fast. On the seventh line of the first page, we come across the familiar name of Albert Einstein, whose presence in books like this is something of a statutory requirement. He and his close friend, Leiden physicist Paul Ehrenfest, are passing snarky notes at a scientific conference in Brussels in 1927. Three pages and six years later, the friends are separated by an ocean, hurled apart by Hitler’s takeover of Germany in early 1933, and there is no room for laughter. In September, in a physician’s waiting room with his son Wassik (who had Down Syndrome), Ehrenfest took out a pistol, shot the boy, and then killed himself.
You might expect a rather somber book to unspool from here, but that is not the case (though a series of suicides does pepper the first 50 pages, disturbingly echoing Ehrenfest’s pistol). Instead, the Ehrenfest-Einstein peanut-gallery notes launch the beginning of the quantum narrative, starting with the full-fledged quantum mechanics of Werner Heisenberg and Erwin Schrödinger in 1925–1926, and landing with the “cosmic Bell” experiments that Kaiser ran with Austrian experimentalist Anton Zeilinger to generate some of the most precise and inventive tests ever conducted of that same Bell’s inequality.
At stake is the meaning of quantum mechanics. With very small scales of matter, strange things happen to the laws of nature that work in our everyday, “classical” world. Particles traverse space and time instantaneously, without seeming to travel the distance in between. We find ourselves unable to measure both the position and momentum of an electron to arbitrary accuracy. And, instead of the causal laws we know from the physics of Isaac Newton, the best we can do is to calculate probabilities: 30 percent chance the electron will be in SoHo, 70 percent it’s hanging out in Central Park — that kind of thing. (Kaiser is especially clear in outlining these issues, and I would recommend you turn to the book if what I just wrote has intrigued you.)
Einstein despised all of this. Even though he had been one of the earliest architects of quantum theory with his explanation for the photoelectric effect in 1905, he never accepted the probabilistic interpretation of nature as the final word. The upshot of the Kaiser-Zeilinger experiments, indeed of the entire quantum narrative, is that Einstein was wrong. The world is fuzzy when you look closely. Grappling with the scientific and philosophical implications of its fuzziness occupies roughly a third of Quantum Legacies, smeared like an electron’s orbital across the chapters.
A few themes in particular resonate through this tour of quantum mechanics. The first is that people matter. That might sound obvious — you can’t have any physics if you don’t have physicists — but the point is subtler. The specific personalities matter, and they matter to specific cohorts, making them different from all other cohorts. Even when Kaiser tours the most rarefied and abstract corridors of theoretical physics, he never finds a solitary theorist mooning over a blackboard. All physics happens in conversation, whether it is Einstein and Ehrenfest joking at a physics meeting, or Schrödinger working out his famous thought experiment about a cat — killed (or not) probabilistically with a dose of poison gas — in correspondence with Einstein and other quantum dissidents. As Kaiser points out, it is no accident that, as Europe descended into fascist chaos and warmongering, “Schrödinger’s thoughts turned to poison, death, and destruction.” Even the main character of the first chapter, Paul Dirac, notorious as the weirdest weirdo to ever do theoretical physics, is presented as enmeshed in the communities that found him so inscrutable. Kaiser, in his memoir moments, always credits the graduate students and collaborators who helped make the science possible. The story of quantum entanglement is about scientists being entangled too.
A second more geopolitical theme emerges as well. The book begins in Europe in the 1920s, and it ends there as well: with the discovery of the Higgs boson at CERN in 2012 and the cosmic Bell experiment in Vienna in 2016. The intervening century mostly takes place in the United States, and it is a somewhat dispiriting story. The wonderful philosophical quandaries of quantum mechanics are scrapped for a “shut up and calculate” pragmatism, and it takes a catastrophic crash of the physics job market (and a few hippies) to bring those questions back. In the book, Cold War America is not the place for pure inquiry. Enter the “scale narrative.”
Historians of science use a standard expression to characterize postwar physics, especially in the United States: “big science.” That sounds like a tragic failure of poetic imagination, but it is simply plain speaking. Science was big. It was big because it was centered on experimental physics: table-top experiments were mutating into gigantic bubble chambers, beakers were being replaced by nuclear reactors, the intimate laboratory was being transformed into a factory floor where assembly lines of postdocs extracted data about the very minute. Kaiser guides the reader through the main stops of this familiar story. His more central point, however, is to stress what this change of scale does to American theoretical physics.
The meat of this narrative occupies the second part, “Calculating,” which begins with an illuminating chapter entitled “From Blackboards to Bombs.” Kaiser’s quarry here is the oft-repeated chestnut that World War II was “the physicist’s war,” in contrast to the canonical “chemist’s war,” World War I, forever stained by gas warfare. With the advent of radar and the nuclear destruction of two Japanese cities at the second war’s end, the appellation obviously fits. The puzzle is that the term “physicist’s war” was tacked onto the conflict by Harvard President (and veteran of chemical weapons research in the Great War) James Bryant Conant before the Americans even commenced hostilities in December 1941, and long before either radar or the atomic bomb (classified topics, not suited for nicknames) were realities. What were these people referring to?
They meant classrooms and classical physics. In an age of submarine warfare, aerial bombing, and mortar fire, officers needed to know how to calculate trajectories and repair electronics. They needed the kind of physics that is now the staple of high school education, but it was a scarce resource in those days. Physics instructors were spared the draft so they could drill recruits in Newton’s laws, not so they could build nukes. (That came later.) The term’s currency spiked in 1943, before Hiroshima, and declined precipitously after 1945. It sowed the seeds for what followed.
Decision makers in the United States were convinced that they needed lots of physicists, oodles more than before. Some of the reasoning for this made sense: electronics and nuclear weapons meant this esoteric profession was in greater demand. Some of the reasoning, however, was fabricated, based on a spurious reading of scientific “manpower” in the Soviet Union. (Kaiser’s deconstruction of these statistics is engrossing.) American policy became to overproduce physicists. You never know when you are going to need an egghead to build a gizmo, so make lots of ’em (both eggheads and gizmos). The ensuing physics “bubble” — Kaiser consciously develops the analogy from economic bubbles such as tulips and subprime mortgages — burst in the early 1970s, producing a recession in the physics job market. All academic fields suffered, but physics, which had risen fastest, crashed hardest.
The central theme of this section is understanding the making of scientists as an act of training. We train schoolchildren to spell correctly; we also train vines to grow the way we want them to, and lop them off if they go awry. We train physicists in both senses. Training a physicist drawn from the pool of a dozen or so smart men (women barely figure in Kaiser’s text) discussing philosophical conundrums about causality over cigarettes and coffee into the wee hours is something quite different from UC Berkeley commandeering the largest lecture halls to deal with the postwar onrush of aspiring scientists. Somewhere in the middle, in Kaiser’s telling, American physics instruction transitioned from the schoolchildren to the vines.
The key to these central chapters is a graph of the annual production of American physics PhDs from 1900 to 2005 (a segment of it is reproduced a few pages away, in case you missed it). It’s a striking curve: a modest but slight rise into the low hundreds until World War II, then a dip as physicists are drafted into the conflict, and then a quadrupling by the late 1950s, and more than a doubling of that by 1970. “In fact, more physicists were trained during the quarter century after the Second World War than had ever been trained, cumulatively, in human history,” writes Kaiser. To train them, the science itself changed. Quantum mechanics shed the philosophical puzzles so entrancing to Einstein, Zeilinger, and Kaiser and became a matter of manipulating formulas to get results. The system eventually collapsed under the strain of its scale.
The bursting of the physics bubble proved no less transformative than the boom had been. Quantum philosophy came back, midwifed by the groovy stylings of Fritjof Capra and his surprise best seller The Tao of Physics, a fusion of Copenhagen and Lhasa that was at first mocked by the mainstream and then embraced when it lured flower children into physics courses. The bust also changed the ancillary fields of science. No longer able to find a job at particle accelerators, theorists indulged previously marginal interests like cosmology. The synergies that happened in this rapprochement of subdisciplines still animate today’s physics world.
Kaiser is a product of this post-bubble era, trained during the collapse of the reprise Reagan bubble. Unusual for a popular science book, we are not introduced to just one area of contemporary physics — say, the irreducible quantum entanglement revealed by Bell’s inequality — but also to neutrino physics, high-energy theory, cosmology, the Higgs boson, gravitational waves, and more. Kaiser has worked in all of these areas, which is possible because he is a theorist at a particular nexus of the post-bubble landscape. The scale narrative ends with ruined careers, but a new physics rose from the ashes.
To the extent there is a hero to Quantum Legacies, it is not Kaiser, nor Einstein, nor the Italian-physicist-turned-Soviet-spy Bruno Pontecorvo, nor the curve of the rise and fall of physics PhDs. It is … the textbook. That maligned genre of students and teachers everywhere repeatedly takes its star turn not only as a source of historical and scientific information but as an actor in its own right. The foreword by Kaiser’s MIT colleague Alan Lightman foreshadows this stardom in what at first seems a strange aside: “My college textbook on heat, titled Thermal Physics, is full of equations describing the modern understanding of heat as the random motion of atoms and molecules.” (Oddly, given that we are of very different generations, I used the same textbook in another dive into the salt mines of graduate physics education encouraged by the well-meaning advisor.)
It is not the last textbook we will encounter. There are several chapters devoted to them: to Capra’s Tao of Physics, to Richard Feynman’s and others' on quantum mechanics, to Charles Misner, Kip Thorne and John Wheeler’s triumph Gravitation, and even to creationist textbooks that attempt to argue away the time-scales needed for Big Bang cosmology. Kaiser tells us up front that he is
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.
By the end of the book, we see the point. Textbooks build communities within a generation of students who study from them, but they are also generated by the community of teachers from whom their authors are drawn. Those chapters that explore the relationships between teachers and students — including the sections about Kaiser’s relationships with his own teachers and students — leave us with a richer picture of physics as a lived activity than either the number-crunching about citations and papers or the nonmathematical explanations of astonishing physical phenomena. Kaiser was well trained.
Michael D. Gordin is a professor in Princeton’s department of history. His latest book, Einstein in Bohemia, is just out.