Einstein’s frustrations reached the boiling point in the summer of 1935. He exchanged a series of letters that summer with Erwin Schrödinger, each egging the other on with his discontent over the direction quantum physics had taken. Building on suggestions from Einstein, Schrödinger crystallized their position with a thought experiment that came to be known as “Schrödinger’s cat.” In what he called a “ludicrous example,” Schrödinger pushed the problem of only being able to calculate probabilities to the extreme. Imagine a cat, Schrödinger instructed readers of his resulting article, “enclosed in a steel chamber, together with the following infernal machine”: a small source of radioactive material next to a Geiger counter, which would be able to detect any radioactive decays. Rigged up to the Geiger counter would be a hammer. Should the Geiger counter detect even a single radioactive decay, it would release the hammer, which would strike a bottle of poison, killing the cat. Suppose, Schrödinger continued, that the radioactive material had a probability of one-half to decay within an hour. The best that quantum mechanics could say was that after one hour had elapsed, the cat locked inside the box would be in the strangest of conditions: “in equal measure, the living and the dead cat are (sit venia verbo [pardon the expression]) blended or smeared out.” Neither dead nor alive, the cat would be in some weird quantum mixture of half-dead-and-half-alive, a condition with no analogue in ordinary experience. But, Schrödinger and Einstein emphasized, no one had ever seen a cat in such a horrid state. Surely, they were convinced, there must be more to physics than mere probabilities.22
Bohr, in contrast, delighted in the new probabilistic framework, reaching back to his undergraduate studies of Kant and Kierkegaard to craft a new quantum worldview. Heisenberg, too, found ample fodder for philosophizing in the turn to probabilities. The son of a classicist, Heisenberg enlisted ancient concepts of being and becoming, or “potentia,” from the likes of Plato and Aristotle. Puzzling through the uncertainty principle, he liked to recall later in life, had sent him scrambling for his copy of Plato’s Timaeus. (To Heisenberg’s close friend and collaborator Wolfgang Pauli, such claims smacked of mere posturing. Pauli declared in a letter to Bohr that Heisenberg was in fact “very unphilosophical.”)23 Indeed, Bohr, Heisenberg, Pauli, and their colleagues like Max Born became convinced that their new quantum theory ushered in an entirely new philosophical age. Bohr announced at every opportunity that his “either-or” interpretation of the quantum realm, complementarity, was a “general epistemological lesson,” to be applied liberally across the entire gamut of human learning, from biology and psychology to anthropology. Typical example: according to Bohr, we can either experience the free flow of our own thoughts, or observe ourselves in the process of thinking, but not both at the same time. Soon after the onset of the Cold War, Max Born was moved to liken capitalism and communism to particle and wave, destined for a quantumlike complementarity.24
Einstein would have none of it. “This epistemology-soaked orgy ought to come to an end,” he wrote to a colleague at one point. Setting aside the wider speculations in which the quantum theorists indulged so freely—traipsing from natural sciences to social sciences, religion, politics, and beyond—Einstein still harbored deep reservations about their interpretation of the physics. Their embrace of probabilities was especially troubling. Such a probabilistic description might well be useful, Einstein granted, but it was hardly fundamental. “My own opinion,” he confided to a correspondent late in 1939—nearly fifteen years after the breakthroughs by Heisenberg, Schrödinger, Bohr, and Born—was that “we will return to the task to describe real phänomena in space and time (not only probabilities for possible experiment).” By that time, most of the younger generation had stopped worrying about Einstein’s quibbles. Yet others, closer in age to Einstein (such as Schrödinger), came to share Einstein’s dissatisfaction with quantum mechanics. All agreed that mysteries like the double-slit experiment demanded serious philosophical attention. The fate of physics depended on it.25
The creators of quantum mechanics formed a tight-knit community. At its center, roughly a dozen physicists occupied what sociologists would call a “core set.” Surrounding the core, only a few dozen more published on the topic anywhere in the world during the critical period of the mid-1920s. The main players knew each other well. They continually crossed paths at Bohr’s institute in Copenhagen, Born’s center in Göttingen, or the informal conferences sponsored by the industrialist-turned-philanthropist Ernest Solvay. Quantum physicists criss-crossed Europe by rail, dropping by for visits that lasted days, weeks, or months. “Kramers was here for eight days,” Born wrote to Einstein in typical fashion in July 1925, “and Ehrenfest…. Last week Kaptiza from Cambridge was here, and Joffé from Leningrad.” “If it is agreeable to you,” Schrödinger wrote to Einstein a few years later, “I would be glad to come over sometime to talk” more in person about Bohr’s latest ideas.26 When not in the same town, they kept up their conversations by letter, tens of thousands of which 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. The letters reveal just how earnestly the early quantum physicists worked to interpret their new formalism, day in and day out. Clustered in small, informal groups, they struggled to put flesh on the new equations, to wrap their heads around how the world could possibly work that way.27 (Fig. 1.4.)
The same philosophical impulse shaped their earliest pedagogical writings. Some textbooks included entire chapters with titles like “Quantum mechanics and philosophy.” Other textbook authors paused within their expositions to pronounce the death of the Kantian “thing-in-itself,” or to weigh the consequences of Heisenberg’s uncertainty principle for scientists’ age-old quest for objectivity.28 The young American physicists who learned quantum mechanics at the feet of the European masters likewise agreed that the material demanded philosophical attention. They often broke with their teachers’ preferred philosophies—American instructors turned most often to the homegrown philosophy of Harvard physicist Percy Bridgman, rather than the rarefied heights of Plato, Kant, or Kierkegaard. But they, too, demanded that their students sit with the quantum weirdness during the 1920s and 1930s and hone their own philosophical response. General examinations from across the country, required for graduate students to advance to candidacy for a PhD, routinely pressed students to compose essays about wave-particle duality, the double-slit experiment, and related matters. Throughout the 1930s, reviewers held the latest American textbooks on quantum mechanics accountable for their philosophical orientation and exposition.29
FIGURE 1.4. Niels Bohr and Albert Einstein deep in conversation about the mysteries of quantum mechanics while visiting the house of a mutual friend in 1930. (Photographs by Paul Ehrenfest, courtesy Emilio Segrè Visual Archives, American Institute of Physics.)
The landscape changed sharply after World War II. In the early 1950s, Einstein—having moved to the United States twenty years earlier, fleeing fascism in Europe—surveyed the scene with despair. The problem was no longer his colleagues’ “tranquilizing philosophy”; it was their ardent lack of interest in philosophy altogether.30 Graduate students at Caltech were caught equally off guard. Having dutifully pored over reports from their predecessors about what to expect on the general examination, the new generation felt cheated. One complained that all the effort he had “invested in analysis of paradoxes and queer logical points was of no use in the exam.” Others recorded how their questions had avoided matters of interpretation altogether, focusing instead upon a narrow set of stock problems. (Forget about philosophy and just give the “usual spiel,” came one student’s advice to those who would take the examination after him.) Essay questions disappeared from graduate students’ written exams across the country, replaced by a coterie of standard problems to calculate. Textbook reviewers in the United States began to praise books on quantum mechanics that “avoided philosophical d
iscussion” or omitted “philosophically tainted questions.” Enough with the “musty atavistic to-do about position and momentum,” stormed MIT’s Herman Feshbach in 1962.31
Much had changed. The hateful policies of Mussolini and Hitler had chased scores of intellectuals out of Europe. Nearly a hundred physicists and mathematicians followed Einstein’s lead and resettled in the United States during the 1930s. Born and Schrödinger rode out the war in Edinburgh and Dublin, respectively, while a few—including, most famously, Heisenberg—remained behind in the Nazi Reich. By the close of the 1930s, quantum physicists had been scattered across the globe, their days of riding the rails in pursuit of further banter gone forever.32
The new world that these émigrés found, meanwhile, was changing fast under their feet. With memories of fascism still fresh, dozens of them joined the Allied war effort, alongside their new American and British colleagues. During the war, physicists all over the world—but especially in the United States—received a crash course in “gadgetry,” their new shorthand for the special flavor of research and development conducted side by side with engineers and military planners. Radar, the proximity fuse, solid-fuel rockets, and especially the atomic bomb project ripped academic physicists from their ivory towers and thrust them into a grubby world of grease and pumps, gauges and lathes. The round-the-clock pressure to produce working gadgets in time to impact the course of the war left little leisure for philosophizing. Physicists learned to put their heads down, ignore philosophical tangents, and wring numbers from their equations as quickly as possible. When Edward Teller lectured on quantum mechanics at Los Alamos—the central scientific laboratory of the atomic bomb project—for the gaggle of students and lab hands whose education had been interrupted by the war, he raced through the interpretive material so quickly that he replaced the fabled double slit with a single slit on the blackboard, from which the crucial interference pattern would never arise! Here, in stark relief, was the new face of war-forged pragmatism.33
The wartime relationships continued unabated after the war, especially as the Cold War with the Soviet Union hardened into a fact of life in the late 1940s. Defense agencies swamped the previous sources of funding for physics, keeping physicists’ attention tethered close to the demands of national security. Only a small minority spent the bulk of their time working on weapons after the war. Yet across the United States, from bustling research universities to tiny liberal-arts colleges, nearly all academic physicists became enrolled in a massive Cold War project: to produce more physicists, at an ever-increasing rate, to ensure that the nation’s supply of technical workers was trained and ready should the Cold War ever turn hot. Leading policymakers freely equated the country’s population of physicists with a “standing army.” In the course of a single speech in 1951, for example, a top member of the Atomic Energy Commission managed to describe physicists as a “war commodity,” a “tool of war,” and a “major war asset,” to be “stockpiled” and “rationed.” Analysts at the Bureau of Labor Statistics agreed. “If the research in physics which is vital to the nation’s survival is to continue and grow,” they asserted in a 1952 report, “national policy must be concerned not only with keeping the young men already in the field at work but also with insuring a continuing supply of new graduates.”34 Adding fuel to the fire, a series of reports published in the mid-1950s, which had been bank-rolled secretly by the Central Intelligence Agency, seemed to suggest that the Soviet Union was training new scientists and engineers even more quickly than the United States. Coming at a propitious moment politically—one was published just two weeks after the Soviets’ surprise launch of the first Sputnik satellite, in October 1957—these reports helped shake loose another billion dollars from Congress (more than $7 billion in 2010 dollars) to support graduate training in “defense” fields like science and engineering.35
The Cold War imperative for scientific “manpower” had immediate effects on enrollments. Backed by expansive fellowship programs and special draft deferments, classrooms in American physics departments bulged faster than any other field. Nearly all fields were growing exponentially after World War II, thanks to a backlog of veterans returning to the nation’s campuses, supported by programs like the GI Bill. Yet physics outpaced them all, its graduate-level enrollments doubling nearly twice as quickly as all other fields combined. By the outbreak of fighting in the Korean War, American physics departments were producing three times as many PhDs per year as the prewar highs—a number that would only climb higher, by another factor of three, after Sputnik.36
The astronomical growth had an immediate effect on teaching. Enrollments in stock courses for graduate students, such as introductory quantum mechanics, swelled to more than 100 students in physics departments from MIT to Berkeley. Such classroom numbers, Berkeley’s department chair exclaimed to his dean, were “a disgrace and should not be tolerated at any respectable university.”37 Despite a frenzy of faculty hiring, student-to-faculty ratios ballooned in physics departments across the country. Professors routinely complained that the bloated enrollments trampled out any sense of the prewar “intimacy” between faculty and students. Students agreed. “The classes are so large that there is little or no individual contact between student and teacher,” complained one graduate student in Harvard’s department after the war.38
Faced with such runaway growth, physics professors across the country revamped their teaching style. They began to accentuate those elements that could lend themselves to high-throughput pedagogy, pumping record numbers of students through their courses. First to go was the discussion-based, qualitative, philosophical inquiry into what quantum mechanics meant. Staring out at the sea of faces in their stadium-seating classrooms, many instructors felt they had little choice. (Fig. 1.5.) “With these subjects,” explained one frustrated professor in 1956, “lecturing is of little avail.” He had in mind once-central topics like the meaning of the uncertainty principle, Bohr’s complementarity, and the consequences for causality of the probabilistic turn. “The baffled student hardly knows what to write down, and what notes he does take are almost certain to horrify the instructor, who perspicaciously usually resolutely refuses to question his students on these topics.” And so, this commentator concluded with regret, when it came to “the philosophical issues raised by quantum mechanics…the student never has a chance to gauge their depth.” A few years later, another critic weighed in. A lion of the interwar era who had emigrated from Europe to the United States, he accused his American colleagues of confusing what was “easy to teach”—the “technical mathematical aspects” of quantum mechanics, which could be chopped up and parceled out on problem sets and exams—with the conceptual, interpretive material that students needed most.39
FIGURE 1.5. Enrico Fermi lecturing to physics graduate students in the early 1950s. (Photograph by Samuel Goudsmit, courtesy Emilio Segrè Visual Archives, Goudsmit Collection, American Institute of Physics.)
The few traces that remain from the nation’s physics classrooms bear these observations out. Comparing lecture notes from graduate-level courses on quantum mechanics from across the country, each dating from the 1950s, reveals a stark pattern. An increase by a factor of three in enrollments correlated with a decrease by a factor of five in the proportion of time spent on interpretive or philosophical material. In short, the larger the class, the less time spent talking through the big issues at the heart of quantum mechanics. Textbooks followed a similar trend. As physics enrollments continued to climb well into the 1960s, the proportion of essay questions plummeted to around 10 percent of all problems embedded in new textbooks. Faced with skyrocketing enrollments, no one had time to grade such verbiage. What students and faculty needed, opined a Berkeley physics professor in 1965, were more textbooks like Leonard Schiff’s successful Quantum Mechanics. The Berkeley physicist had used the first edition, from 1949, as a student, and he looked back on it fondly. “The book kept me sufficiently busy to prevent pseudo-philosophical speculations abo
ut the True Meaning of quantum mechanics”—just the ticket for the new classroom realities. He urged the publisher to bring out a new edition of Schiff’s book. By trimming what had already been paltry discussion of interpretive matters, the new edition could be larded even more fully with tough calculations, to keep the new generation busy. (The publisher brought out the new edition in 1968 to widespread acclaim from reviewers; it sold well.)40 Countries that had similar physics enrollment patterns—major Cold War players like the United Kingdom and the Soviet Union—produced remarkably similar textbooks. Other European countries, like France, West Germany, and Austria, spent much more time rebuilding after the war and did not experience the same bulge in physics classrooms. Physicists in those countries continued to write textbooks in the prewar fashion, featuring long excursions into philosophy and stuffed with juicy essay questions.41
How the Hippies Saved Physics: Science, Counterculture, and the Quantum Revival Page 4