Reprinted from Physics World David Kaiser, “Turning physicists into quantum mechanics,” Physics World (May 2007): 28-33 Feature: Teaching quantum mechanics physicsweb.org Turning physicists into Einstein, Bohr and the other pioneers of quantum mechanics loved to debate its strange philosophical implications with their students. David Kaiser thinks he knows why this approach to teaching the subject largely vanished from university curricula after the Second World War Quantum computing, quantum encryption and quan- tum teleportation: today’s physics journals sound more like Star Trek than ever. Based on the weird and coun- terintuitive features of quantum mechanics, these con- cepts have been brought to fruition in recent years by theorists and experimentalists alike. Indeed, these ideas are now leaving physics laboratories and being used by industry to make real products. Who knows what revolutions await in computation and communications from these budding breakthroughs? But as they move from theory to application, these developments have rekindled for many physicists an interest in some long-standing questions about quan- tum mechanics – interpretive, philosophical questions that, on the face of it, seem light-years away from the grubby worlds of engineering and manufacturing. Can something be in two places at once? Can an object in- fluence another more quickly than the time it takes light to travel between the two? Is it really impossible – as the uncertainty principle suggests – to fix the prop- erties of a quantum-mechanical object such as its posi- tion and momentum at the same time? These philosophical issues are not new. Perhaps the most famous physicist to agonize over the implications of quantum mechanics was Albert Einstein, who did not like the inherent randomness of the theory and dubbed the seeming ability of one particle to instantaneously influence another as “spooky action at a distance”. But Niels Bohr also worried about such questions; so too did Werner Heisenberg and Erwin Schrödinger. In fact, most of the architects of quantum theory, toiling away during the 1910s and 1920s, thought that quantum me- secure the peace. The hardening of the Cold War a few chanics – our description of matter and forces at the years later added new urgency. The massive training atomic scale – demanded new ways of thinking. mission that ensued – bolstered in the US by tens of Yet these complex puzzles largely disappeared from thousands of new federal fellowships in physics and David Kaiser view during the mid-20th century, despite Einstein’s allied fields – radically changed how physics was taught is a historian and hope that some means might be found to regain a de- both in American universities and elsewhere. physicist at the terministic description of nature. They had not been Even though many new physics professors were hired, Massachusetts solved – as lively discussions in recent books like Seth the ratio of students to staff ballooned in the US, with Institute of Lloyd’s Programming the Universe make plain – but the pre-war ratio trebling by the mid-1950s and then Technology, US, e-mail dikaiser@ were simply sidestepped. Why was this? widening further after that. During the boom years, the mit.edu. He is A large part of the answer to that question, unex- number of students studying physics rose faster than any currently writing pected though it may be, is the huge impact on physics other field in the US, peaking at about 15000 graduate- a book entitled of the Second World War. The success of military pro- level students in 1969. As class sizes grew, however, the American Physics jects, such as the development of the atomic bomb and philosophical aspects of quantum mechanics got and the Cold War radar, convinced influential policymakers that what squeezed out of the lecture hall. The goal of physics Bubble they needed after the war was many more physicists to became to train “quantum mechanics”: students were 28 Physics World May 2007 physicsweb.org Feature: Teaching quantum mechanics quantum mechanics Calculating minds Richard Feynman, shown here lecturing at the California Institute of Technology in 1963, was one of many physicists to sidestep the philosophical aspects of quantum mechanics when teaching the subject after the Second World War. C L L , y c n e g A n o s k c a J e i n a l e M / y g o l o n h c e T f o e t u t i t s n I a i n r o f i l a C , s e v i h c r A to be less like otherworldly philosophers and more like from its post-war peak. As class sizes shrank, funda- engineers or mechanics of the atomic domain. mental questions of quantum theory returned to the The focus on learning to calculate, unburdened by lecture room and textbooks, as my historical research speculative philosophical concerns, certainly brought has revealed. In surprising and often subtle ways, these some good pay-offs. Students from this era successfully changes in the student ranks shaped how US physicists applied quantum theory to tackle nuclear forces, super- grappled with quantum mechanics, with similar trends conductivity and more. Indeed, the huge success of the apparent in other countries too (see box on page 33). Standard Model of particle physics is in large part based on our fundamental knowledge of the rules of Focusing on philosophy quantum mechanics. Even more than relativity – with its talk of shrinking Yet by the early 1970s, the effects of economic recess- metre sticks, slowing clocks and twins who age at differ- ion, détente between the West and the Soviet Union, ent rates – quantum mechanics is a science of the bizarre. and immense cuts in defence and education spending Particles tunnel through walls. Cats become trapped, led to a decline in student numbers, with physics plum- half dead and half alive. Objects light-years apart retain meting faster than any other field. By the end of that telepathic links with one another. The seeming solidity decade, the number of physics students had halved of the world evaporates into a play of likelihoods. Physics World May 2007 29 Feature: Teaching quantum mechanics physicsweb.org s e During the 1920s and 1930s the architects of quan- r u t c i tum mechanics – most of whom were European – P e f i tackled the deep philosophical implications of the sub- L e m ject in their classrooms and textbooks head on. No clean i line separated calculation from interpretation. The- T orists like Bohr, Heisenberg, Hermann Weyl, Max Born and Arnold Sommerfeld each paused within their text- books to relate the latest discoveries in atomic physics to long-standing trends in philosophical inquiry, such as the extent to which we can ever gain trustworthy knowledge about the physical world (let alone unob- servable entities), the role of language in shaping our concepts, or the active filtering of seemingly direct ob- servations by our prior concepts such as space and time. Some invoked the philosophers Immanuel Kant or Ernst Mach; others even turned to Eastern mysticism and Jungian psychoanalysis to help interpret the latest physics research. But all agreed that the new physics demanded serious philosophical scrutiny. Even in the US, where physicists tended to be more practically minded than those in Europe, it was gener- Strange thoughts Before the Second World War, physicists did not shy ally felt that the philosophical implications of quantum from the philosophical implications of quantum mechanics. mechanics needed to be examined. During the late J Robert Oppenheimer’s famous course at the University of California at 1920s and through the 1930s, young American physi- Berkeley emphasized hard questions of philosophy and interpretation cists such as Edwin Kemble, Arthur Ruark and Henry alongside more practical calculations with the new formalism. Margenau paraded their philosophical convictions in the Physical Review and in their textbooks. Indeed, it pretations. Long before going through the first practical became common for reviewers to compare and contrast calculation with the new equations, he even indulged in the latest US quantum-mechanics textbooks based on Einstein-styled thought experiments to circumvent the their philosophical approaches. uncertainty principle, each of which, Oppenheimer Many of this pre-war generation’s most influential showed, was destined to fail. teachers likewise focused on philosophical material in Oppenheimer was not alone. Lecture notes from their classrooms. Emblematic was Robert Oppen- other graduate-level courses on quantum mechanics heimer’s popular course at the University of California in the 1930s reveal similar attention to matters of inter- at Berkeley. Graduate students routinely sat through pretation. Copies of “general” exams from the same his quantum-mechanics course more than once; one period, which students had to pass if they wanted to do desperate student even went on a hunger strike until a PhD, survive from a dozen top-ranked physics de- Oppenheimer relented and let her attend the class for partments across the US. Common to each of them are a fourth time. Well into the late 1930s, Oppenheimer essay questions probing such thorny issues as how a still introduced quantum mechanics as a “radical solu- wavepacket gets reduced from a superposition of pos- tion” to pressing philosophical issues that have their sibilities to a single measured result. Others ask how roots in physics. the role of the observer differs between classical and On page after page of his lecture notes, Oppenheimer quantum mechanics or press students to explain how focused not only on the new formalism of quantum the uncertainty principle challenges the nature of mechanics – centred on Schrödinger’s wavefunction – physical explanation. but also on its physical interpretation, lavishing atten- tion on the origins and meaning of probabilistic inter- Enrolments and interpretation after the war After the Second World War the unmistakable import- At a Glance: Teaching quantum mechanics ance of nuclear and solid-state physics to the success of military projects like the atomic bomb pushed quantum G Quantum mechanics is a theory with many philosophical ramifications that mechanics to the forefront of US physics curricula.