Quantum Philosophy

© 1992 SCIENTIFIC AMERICAN, INC © 1992 SCIENTIFIC AMERICAN, INC with these issues. Albert Einstein, who in 1905 showed how Planck's electro magnetic quanta, now called photons, could explain the photoelectric effect (in which light striking metal induces an electric current), insisted later that a more detailed, wholly deterministic the ory must underlie the vagaries of quan tummechanics. Arguing that "God does not play dice," he designed imaginary "thought" experiments to demonstrate the theory's "unreasonableness." De fenders of the theory such as Niels Bohr, armed with thought experiments of their own, asserted that Einstein's objections reflected an obsolete view of reality. It is not the job of scientists, Bohr chided his friend, "to prescribe to God how He should runthe world." Until recently, the prevailing attitude of most physicists has been utilitarian: if the theory can foretell the performance of a doped gallium arsenide semicon ductor, why worry about its epistemo Revealing the Split Personality of Light logical implications? In the past decade or so, however, a growing cadre of re searchers has been probing the ghostly Two-slit experiments reveal that photons, the quantum entities giving rise to underpinnings of their craft. New tech light and other forms of electromagnetic radiation, act both like particles and nologies, some based on the very quan like waves. A single photon will strike the screen in a particular place, like a par- tum phenomena that they test, have enabled investigators to carry out ex periments Einstein and Bohr could only imagine. These achievements, in turn, of interest in "interpretations" of quan was formed by the overlapping, or in have inspired theorists to dream up tum mechanics, which attempt to place terference, of the wavelet pairs. The even more challenging-and sometimes it in a sensible framework. But the cur screen was bright where crests coincid bizarre-tests. rent interpretations seem anything but ed and dark where crests met troughs, The goal of the quantum truth-seek sensible. Some conjure up multitudes canceling each other out. ers is not to build faster computers or of universes. Others require belief in a But more recent two-slit experiments communications devices-although that logic that allows two contradictory suggest that Newton was also right. could be an outcome of the research. statements to be true. "Einstein said Modern photodetectors (which exploit And few expect to "disprove" a theory that if quantum mechanics is right, the photoelectric effect explained by that has been confirmed in countless then the world is crazy," says Daniel Einstein) can show individual photons experiments. Instead their goal is to lay Greenberger, a theorist at the City Col plinking against the screen behind the bare the curious reality of the quantum lege of New York. "Well, Einstein was slits in a particular spot at a particular realm. "For me, the main purpose of right. The world is crazy." time-just like particles. But as the pho doing experiments is to show people The root cause of this pathology is tons continue striking the screen, the how strange quantum physics is," says the schizophrenic personality of quan interference pattern gradually emerges, Anton Zeilinger of the University of tum phenomena, which act like waves a sure sign that each individual photon Innsbruck,who is both a theorist and one moment and particles the next. went through both slits, like a wave. experimentalist. "Most physicists are The mystery of wave-particle duality is Moreover, if the researcher either very naive; most still believe in real an old one, at least in the case of light. leaves just one slit at a time open or waves or particles." No less an authority than Newton pro moves the detectors close enough to So far the experiments are con posed that light consisted of "corpus the two slits to determine which path firming Einstein's worst fears. Photons, cles," but a classic experiment by Thom a photon took, the photons go through neutrons and even whole atoms act as Young in the early 1800s convinced one slit or the other, and the inter sometimes like waves, sometimes like most scientists that light was essential ference pattern disappears. Photons, it particles, but they actually have no def ly wavelike. seems, act like waves as long as they inite form until they are measured. Young aimed a beam of light through are permitted to act like waves, spread Measurements, once made, can also be a plate containing two narrow slits, il out through space with no definite po erased, altering the outcome of an ex luminating a screen on the other side. sition. But the moment someone asks periment that has already occurred. A If the light consisted of particles, just where the photons are-by determin measurement of one quantum entity two bright lines should have appeared ing which slit they went through or can instantaneously influence another on the screen. Instead a series of lines making them hit a screen-they abrupt far away. This odd behavior can occur formed. The lines could be explained ly become particles. not only in the microscopic realm but only by assuming that the light was Actually, wave-particle duality is even even in objects large enough to be seen propagating as waves, which were split more baffling than this explanation sug with the naked eye. into pairs of wavelets by the two-slit gests, as John A. Wheeler of Princeton These findings have spurred a revival apparatus. The pattern on the screen University demonstrated with a thought 96 SCIENTIFIC AMERICANJuly 1992 © 1992 SCIENTIFIC AMERICAN, INC terfering with each other at the detector. that the theory made as much sense as Then the workers installed a cus Lewis Carroll's poem "Jabberwocky," in tomized crystal called a Pockels cell in which "slithy toves did gyre and gimble the middle of one route. When an elec in the wabe." Unfortunately, the jargon tric current was applied to the Pockels of quantum mechanics is rather less cell, it diffracted photons to an auxilia lively. Anunobserved quantum entity ry detector. Otherwise, photons passed is said to exist in a "coherent superpo through the cell unhindered. A random sition" of all the possible "states" per signal generator made it possible to mitted by its "wave function." But as turn the cell on or off after the photon soon as an observer makes a measure had already passed the beam splitter ment capable of distinguishing between but before it reached the detector, as these states, the wave function "col Wheeler had specified. lapses," and the entity is forced into a When the Pockels-cell detector was single state. switched on, the photon would behave Yet even this deliberately abstract like a particle and travel one route or language contains some misleading im the other, triggering either the auxiliary plications. One is that measurement detector or the primary detector, but requires direct physical intervention. not both at once. If the Pockels-cell de Physicists often explain the uncertainty tector was off, an interference pattern prinCiple in this way: in measuring the would appear in the detector at the end position of a quantum entity, one in of both paths, indicating that the pho evitably knocks it off its course, losing ton had traveled both routes. information about its direction and ticle (left). But as more photons strike To underscore the weirdness of this about its phase, the relative position of the screen, they begin to create an effect, Wheeler points out that astron its crests and troughs. interference pattern (center). Such a omers could perform a delayed-choice Most experiments do in fact involve pattern could occur only if each pho experiment on light from quasars, ex intrusive measurements. For example, ton had actually gone through both tremely bright, mysterious objects found blocking one path or the other or mov slits, like a wave (right). near the edges of the universe. In place ing detectors close to the slits obvious of a beam splitter and mirrors, the ex ly disturbs the photons' passage in the periment requires a gravitational lens, two-slit experiments, as does placing a a galaxy or other massive object that detector along one route of the de experiment he devised in 1980. "Bohr splits the light from a quasar and refo layed-choice experiment. But an experi used to say that if you aren't confused cuses it in the direction of a distant ob ment done last year by Mandel's team by quantum phYSiCS, then you haven't server, creating two or more images of at the University of Rochester shows really understood it," remarks Wheeler, the quasar. that a photon can be forced to switch who studied under Bohr in the 1930s from wavelike to particlelike behavior and went on to become one of the most Psychic Photons by something much more subtle than adventurous explorers of the quantum direct intervention. world. The astronomer's choice of how to The experiment relies on a paramet Inthe two-slit experiments, the physi observe photons from the quasar here ric down-converter, an unusual lens that cist's choice of apparatus forces the pho in the present apparently determines splits a photon of a given energy into ton to choose between going through whether each photon took both paths two photons whose energy is half as both slits, like a wave, or just one slit, or just one path around the gravitation great. Although the device was devel like a particle. But what would happen, al lens-billions of years ago. As they oped in the 1960s, the Rochester group Wheeler asked, if the researcher could approached the galactic beam splitter, pioneered its use in tests of quantum somehow wait until after the light had the photons must have had something mechanics.

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