How the Hippies Saved Physics: Science, Counterculture, and the Quantum Revival Page 3
The meeting did not get off to an auspicious start. Sarfatti felt restless, uninterested in the meeting; he had never heard of Erhard. Erhard’s gaudy outfit, accessorized by a beautiful female admirer hanging on his sleeve, put Sarfatti off even more. Sarfatti asked what Erhard did. Erhard grinned and replied, “I make people happy.” It was more than Sarfatti could take. Itching to leave, he said in a strong Brooklyn accent, “I think you’re an asshole.” As Sarfatti remembers it, Erhard rose from his chair—smile stretching from ear to ear—embraced Sarfatti right there in the hotel lobby, and said, “I am going to give you money.” Without knowing it, Sarfatti had used one of the catchphrases associated with Erhard’s sprawling self-help venture. Soon the money began to flow: thousands of dollars, all from this most eager new patron of quantum physics.3
Erhard was not the first to seek enlightenment from the strange subject of quantum theory. Even more than relativity—with its talk of shrinking meter sticks, slowing clocks, and twins who age at different rates—quantum mechanics is a science of the bizarre. Particles tunnel through walls. Cats become trapped, half dead and half alive. Objects separated light-years apart retain telepathic links with one another. The seeming solidity of the world evaporates into a cloud of likelihoods. Long before Erhard, Wolf, or Sarfatti had arrived on the scene, the world’s leading physicists had struggled to come to grips with quantum theory, to tease out just what it might mean. Many of their ideas sounded no less peculiar than the half-formed inklings that inspired Erhard on that fateful spring day.4
Quantum mechanics emerged over the first quarter of the twentieth century, honed primarily by Europeans working in the leading centers of theoretical physics: Göttingen, Munich, Copenhagen, Cambridge. Most of its creators—towering figures like Niels Bohr, Werner Heisenberg, and Erwin Schrödinger—famously argued that quantum mechanics was first and foremost a new way of thinking. Ideas that had guided scientists for centuries were to be cast aside. Bohr constantly spoke of the “general epistemological lesson” of the new quantum era. The disjuncture of cause from effect, Heisenberg’s uncertainty principle, wave-particle duality—all required explicit, extended philosophical engagement, so these leaders proclaimed. They differed, often passionately, over which philosophical schools of thought might best clarify the new material. Some invoked the writings of eighteenth-century scholar Immanuel Kant; others quoted aphorisms from Hindu holy scriptures, or “Upanishads”; some even dabbled in Jungian depth-psychology. The subject’s leading detractors, such as Albert Einstein, likewise agreed that quantum mechanics had to meet stringent philosophical tests. Mathematical self-consistency and agreement with experiments were important, but hardly sufficient.5
During this heady period, grown men argued into the night, trying to make sense of a series of puzzles and paradoxes. Names were called; tears were shed. At one point, an ailing Schrödinger sought refuge in bed while visiting Bohr’s Institute for Theoretical Physics in Copenhagen. Unable to let a disputed matter of interpretation rest, Bohr hounded the poor Austrian at his bedside, repeating, “But surely Schrödinger, you must see…”6
That style of working on quantum mechanics faded fast after World War II. Especially in the United States, the war and its aftermath shaped how generations of new physicists were trained. Ultimately, the war changed what it meant to be a physicist. The Cold War completed the transformation, winnowing the range of acceptable topics and admissible approaches. Very quickly, philosophical inquiry or open-ended speculation of the kind that Bohr, Einstein, Heisenberg, and Schrödinger had considered a prerequisite for serious work on quantum theory got shunted aside. “Shut up and calculate” became the new rallying cry.7
Yet the Cold War consensus proved to be no more eternal than the prewar style had been. As the fortunes of physics plummeted in the late 1960s and early 1970s, sending academic physics departments into a tailspin, new intellectual possibilities opened up. Buoyed by cash from new patrons like Erhard, small clusters of physicists, including Wolf, Sarfatti, and their colleagues in the Fundamental Fysiks Group, labored to carve out a new identity for themselves and for the science they loved so much.
Back in the 1920s, sticking points seemed to abound in the new quantum theory. Every time physicists tried to make sense of their hard-won equations, new and bizarre challenges tumbled forth. One experiment captured the lion’s share of peculiarities. It came to be known as the “double-slit experiment.” Champions of quantum mechanics trotted it out time and again to sharpen their understanding of the issues involved. Bohr and Heisenberg, for example, featured it in some of their earliest expositions of quantum mechanics.8 Critics likewise saw much of value in the experiment, goading their colleagues to admit how preposterous their explanations sounded. Schrödinger—caught between the warring camps, with his own uneasy relationship to the equations he had produced—recognized the pedagogical value of the double-slit experiment for clarifying many of the core mysteries of quantum mechanics, and featured it prominently in lectures during the 1930s.9 Since that time, generations of physicists have followed Schrödinger’s lead. In fact, readers of the trade magazine Physics World recently voted the double-slit experiment the single most beautiful experiment of all time. In their view, it edged out heavyweight contenders from Galileo to Newton, and even a classic dating from ancient Alexandria, all of which also made the top ten.10
In an essay for Einstein’s seventieth birthday, published in the late 1940s, Bohr used the double-slit experiment as the leitmotiv of his decades-long debate with Einstein.11 Years earlier, Einstein had helped to launch the quantum revolution, introducing several crucial concepts. In fact, the Nobel Prize committee cited only his contributions to quantum theory when granting his award in 1921, remaining mum on relativity. Then, in one of the delicious ironies of the history of science, Einstein reversed course and turned his back on his own creation. (The irony was not lost on Einstein. “After all,” he wrote to Schrödinger, “many a young whore turns into an old praying sister, and many a young revolutionary becomes an old reactionary.”) He brandished the double-slit experiment in private correspondence to drive home his criticisms as early as April 1926, and in more public settings the following year.12
Fearing that their friendly squabbles over quantum theory had become too ethereal or detached from the real world over the years, Bohr worked with an artist to make his position more concrete when preparing his essay for Einstein’s birthday. The resulting images had the look and feel of engineering diagrams, all bulky bolts and heavy planks. In Bohr’s reconstruction, the double-slit experiment centered around an apparatus like the one in Figure 1.1, a thick wall with two slits hollowed out. A sliding latch was installed in front of one of the slits, so that physicists could choose whether to leave that slit open or shut. Behind the wall stood a recording screen—it could be photographic film or some other means of detection—bolted securely in place.
FIGURE 1.1. Niels Bohr’s depiction of the double-slit apparatus. (Cropped from Bohr [1949], 219. Reproduced with permission of Open Court Publishing Company, a division of Carus Publishing Company.)
Einstein and Bohr each knew well what would happen if they shined a light on the wall when both slits were open. Bohr included a picture in his birthday essay. (Fig. 1.2.) If the light source were far enough away, the light waves would approach the wall-with-slits in a simple configuration that physicists call a “plane wave,” with all the crests and troughs lined up neatly in rows. Most of the light from the source would be blocked by the wall. The light that passed through the narrow slits would fan out in a new pattern, arcing in semicircular waves toward the recording screen. The crests and troughs of the two curving light waves, emanating from the open slits, would no longer be lined up with each other. In some locations along the recording screen, the crest from one wave would arrive in step with the crest from the other, adding up to make a bright spot on the photographic film. In other locations, however, the crest from one wave would arrive with the trough of th
e other. At those spots, the light waves from each slit would cancel each other out, leaving no mark on the film. And so it would go as one moved down the recording screen: alternating light and dark bands known as an “interference pattern.”
FIGURE 1.2. The double-slit apparatus and interference pattern. (Cropped from Bohr [1949], 216. Reproduced with permission of Open Court Publishing Company, a division of Carus Publishing Company.)
Bohr pressed on. One of the biggest surprises in quantum physics was that the same quintessential interference pattern arose when one fired tiny particles, such as electrons, at a wall with two slits. Each particle seemed to behave like a tiny billiard ball when released from the source on one side of the room and detected at the screen on the other side. Yet upon shooting tens, hundreds, or thousands of electrons at the twice-slitted wall, the locations at which each tiny electron was detected matched the wavelike interference pattern. That would never happen with ordinary billiard balls. When thrown at a wall with two slits, the balls would cluster in two clumps, one behind each of the open slits. The billiard balls would never arrange themselves in the alternating interference pattern. Even more strange, physicists could choose to shoot a thousand electrons at the wall one at a time, an hour apart. After all the electrons had made their way through the apparatus, the pattern of light and dark patches on the recording screen—marking where each individual electron had arrived, one at a time—would appear just as if physicists had sent light waves to interfere. (Fig. 1.3.)
Physicists had managed to conduct laboratory demonstrations of the effect as early as 1927.13 Einstein pressed his colleagues at an informal conference that year to explain: what did the waving? Certainly not the electrons themselves, at least not without straining credulity. Each had been fired one at a time, so no two electrons could have interacted with each other (say, by repelling each other with their electric charge). Each had been detected as a tiny particle; none showed up at the recording screen as a washed-out wave. The distance between the slits was much larger than the electrons themselves, so it hardly made sense to think that an electron passed through both slits at the same time and interfered with itself on the other side. Einstein clearly enjoyed watching his colleagues squirm. Like two giddy schoolboys, Einstein and a close friend passed notes back and forth while one defender of quantum theory after another tried to fend off Einstein’s challenges. “Don’t laugh!” his friend scribbled. Einstein’s prescient reply: “I laugh only at the naiveté [of the proponents of quantum theory]. Who knows who will be laughing in the coming years.”14
FIGURE 1.3. Three snapshots of the detection of individual photons after they have passed through a barrier with slits. The photographs show results after 1/30 of a second (left), 1 second (middle), and 100 seconds (right). Each photon, or quantum of light, gets detected as an individual particle, and yet the pattern that builds up over time reveals wavelike interference. (Courtesy Robert Austin and Lyman Page, Princeton University.)
Einstein’s sparring partners were laughing soon enough. Bohr, Heisenberg, and their colleagues cobbled together an interpretation of what was happening in the double-slit experiment. Every quantum system, they reasoned, had an associated “wavefunction,” which they labeled with the Greek letter, (pronounced “psi”). The values that the wavefunction assumed in different locations, and the way those values changed over time, were governed by a new equation first introduced by Schrödinger in 1926. Schrödinger’s equation was similar in mathematical form to well-known equations that described wave behavior, such as water waves on the ocean. Max Born—Einstein’s friend and Heisenberg’s mentor—advanced an interpretation that same year that was related to probability. In particular, the probability for detecting a quantum object at a particular time and place was given, in Born’s account, by the absolute square of the associated wavefunction: Probability = ||2. In the double-slit experiment, according to this interpretation, the electron’s wavefunction spread out like a wave and went through both slits, leading to the characteristic interference pattern.15
So were the electrons behaving like particles or waves? The answer—which brought a smile to Niels Bohr’s face every time he walked a new audience through the experiment—was “all of the above.” Einstein was less amused. “The Heisenberg-Bohr tranquilizing philosophy—or religion?—is so delicately contrived,” he complained in a letter to Schrödinger in May 1928, that “for the time being, it provides a gentle pillow for the true believer from which he cannot very easily be aroused. So let him lie there. But”—he left no doubt—“this religion has so damned little effect on me.”16
Heisenberg and Bohr had more tricks up their sleeves; they weren’t finished with the double slit yet. They considered modifying the apparatus, to be able to measure through which slit an individual electron passed. Despite all the talk of wavefunctions, after all, each electron was emitted and detected like a tiny particle; surely each electron must have passed through one slit or the other, just like ordinary billiard balls would do. That notion could be tested, they explained, by placing some other tiny particles behind one of the slits. If an electron passed through that slit en route to the recording screen, then some of the test particles would get scattered, like pins tossed about by a bowling ball, signaling the electron’s passage through the slit. If, on the other hand, none of the test particles were scattered, then the electron must have passed through the other slit.
It sounded simple enough. And it would have worked, too, but for one catch, known as Heisenberg’s uncertainty principle. Soon after Schrödinger and Born worked out the basic rules for manipulating , Heisenberg demonstrated that the new equations behaved in some unexpected ways, totally unlike the usual physics of particles or waves. Certain pairs of quantities, such as position and momentum or energy and time, could never be specified with unlimited precision at a single instant. The more precisely a quantum object’s position was specified, the less precisely its momentum could be, and vice versa. According to Heisenberg, in other words, we can never know exactly where an object is and where it is going at the same time.17
During lectures at the University of Chicago in 1929, in one of his earliest deployments of the uncertainty principle, Heisenberg demonstrated why the slit detector could not work as advertised. To yield a reliable measurement of whether an electron passed through a particular slit, the test particles would have to be clumped tightly behind that slit. The uncertainty in their position, in other words, would have to be much smaller than the distance between the two slits. That small uncertainty in position, in turn, would correspond to a large uncertainty in their momentum. The incoming electron thus would careen into a collection of test particles that already had some large uncertainty in their momentum; this would translate into a correspondingly large uncertainty in the electron’s momentum following the collision. Heisenberg needed just a few lines of algebra to show that the collision would jostle the electron’s path just enough to smear out the sharp peaks and valleys of the interference pattern. In fact, if every electron could be measured to pass through one slit or the other, the resulting detection pattern would revert to two broad peaks, one behind each slit; all wavelike interference would vanish. On the other hand, reducing the uncertainty in the electron’s momentum after scattering, to retain the interference pattern, could only be done by increasing the uncertainty of the test particles’ position—by such an amount that no one would know whether they had been clumped behind one slit, the other, or both.18
To Bohr, the paradox of the slit detector exemplified a more general feature of quantum mechanics. Ask a “particle-like” question—“through which slit did the particle pass?”—and you will always receive a particle-like answer (“slit A” or “slit B”). Ask a “wavelike” question—“how does behave in the region between the slits and the detectors?”—and you will always receive a wavelike response (“in a state interference, crests canceling troughs in some places and amplifying crests in others”). Bohr coined the term “complem
entarity” for his emerging philosophy. Explanation in the quantum realm, he maintained, required the constant juxtaposition of statements that were themselves mutually exclusive, the particle “yin” always paired with the wavelike “yang.” (In 1947, when the king of Denmark anointed Bohr with the prestigious Order of the Elephant, Bohr needed to produce a family coat of arms for display in the Frederiksborg Castle near Copenhagen. He placed the classical Chinese yin-yang symbol at its center.) Einstein had little patience for this kind of talk. The goal of physics, he maintained his entire life, was to determine how the world works on its own, independent of the questions we happen to ask of it. Writing to Schrödinger, Einstein mocked Bohr’s increasingly oracular outbursts as those of a “a ridiculous little Talmudic philosopher.”19
Einstein had other bones to pick. Max Born had suggested—and nearly all quantum physicists came to agree—that the square of the wavefunction yielded a probability. But neither Born nor anyone else had succeeded in pressing beyond mere probabilities. For Einstein, this seemed an intolerable shortcoming. He made a few false starts of his own, at one point jotting a rushed note to Born to announce that he had found an interpretation of that did not resort to probabilities; but each of these efforts fell short of the mark. In the meantime, Einstein only accorded quantum mechanics what he called “transitory significance,” despite his many contributions to the subject. “I still believe in the possibility of giving a model of reality,” he explained in a lecture at Oxford in 1933, “a theory, that is to say, which shall represent events themselves and not merely the probability of their occurrence.”20 Writing to Born, he was even more direct. “Quantum mechanics is certainly imposing,” he began. “But an inner voice tells me that it is not yet the real thing. The theory says a lot, but does not really bring us any closer to the secret of the ‘old one.’ I, at any rate, am convinced that He is not playing at dice.” Einstein had no beef with the logical self-consistency or the empirical successes of quantum mechanics. In the right hands, he acknowledged, Schrödinger’s equation and Born’s interpretation of could produce stunningly accurate descriptions of the overall outcomes of large collections of events, such as where, on average, thousands of electrons that had been fired at a barrier would be detected. But the quantum formalism could never reconstruct those aggregate results on a case-by-case basis; it could never explain why the electron in experimental run 867 happened to pass through one slit rather than the other and wind up at a particular location.21