Such possibilities provoke consternation in some quarters. Edwin T. Jaynes of Washington University, a prominent theorist whose work helped to inspire Scully to conceive the quantum eraser, has nonetheless dubbed it "medieval necromancy." Scully was so pleased by Jaynes's remark that he included it in a recent article on the quantum eraser. Necromancy cannot hold a candle to nonlocality. Einstein,Boris Podolsky and Nathan Rosen first drew attention to this bizarre quantum property (which is now often called the EPR effect in their honour) in 1935 with a thought experiment designed to prove that quantum mechanics was hopelessly flawed. What would happen, Einstein and his colleagues asked, if a particle consisting of two protons decayed, sending the protons in opposite directions? According to quantum mechanics, as long as both protons remained unobserved their properties remain indefinite, in a superposition of all possible states;that means each one travels in all possible directions.
But because of their common origin, the properties of the protons are tightly correlated, or "entangled." For example, through simple conservation of momentum, one knows that if one proton heads north, the other must have headed south. Consequently, measuring the momentum of one proton instantaneously determines the momentum of the other proton- even if it has travelled to the opposite end of the universe. Einstein said that this "spooky action at a distance" was incompatible with any "realistic" model of reality; all the properties of each proton must be fixed from the moment they first fly apart.
Until the early 1960s, most physicists considered the issue entirely academic, since no one could imagine how to resolve it experimentally. Then, in 1964, John S. Bell of CERN, the European laboratory for particle physics, showed that quantum mechanics predicted stronger statistical correlations between entangled particles than the so-called local realistic theory that Einstein preferred. Bell's papers triggered a flurry of laboratory work culminating in a classic (but not classical) experiment performed a decade ago by Alain Aspect of the University of Paris.
Instead of the momentum of protons, Aspect analysed the polarization of pairs of photons emitted by a single source toward separate detectors. Measured independently, the polarization of each set of photons fluctuated in a seemingly random way. But when the two sets of measurements were compared, they displayed an agreement stronger than could be accounted for by any local realistic theory-just as Bell had predicted. Einstein's spooky action at a distance was real. Until recently, no experiment had successfully shown that the EPR effect held true for momentum, as Einstein, Podolsky and Rosen had originally proposed. Two years ago John G. Rarity and Paul R. Tapster of the Royal Signals and Radar Establishment in England finally achieved that feat.
The experiment began with a laser firing into a down-converter which produced pairs of correlated photons. Each of these photons then passed through a separate two-slit apparatus and thence to a photon detector.Through conservation of momentum, one could determine the route of each photon if one knew the route of its partner. But the arrangement of mirrors and bean splitters made it impossible to determine the actual route of either photon.
Next, the workers slightly lengthened one of the four routes, as Chiao did in his quantum eraser experiment. Although the rate at which photons struck each detector did not change, the rate of simultaneous firings recorded by a coincidence counter oscillated, forming a telltale interference pattern like the one observed by Chiao. Such a pattern could occur only if each photon, the one on the left and the one on the right, was passing through both slits to its respective detector, its momentum fundamentally undefined and yet still correlated with the momentum of its distant partner.
Still more ambitious EPR experiments have been proposed but not yet carried out. Greenberger, Zeilinger and Michael Home of Stonehill College have shown that three or more particles sprung from a single source will exhibit much stronger nonlocal correlations than those between just two particles. Bernard Yurke and David Stoler of AT& T Bell Laboratories have even suggested a way in which three particles emitted from separate locations can exhibit the EPR effect. Unfortunately, the EPR effect does not provide a loophole in the theory of relativity, which prohibits communications faster than light, since each isolated observer of a correlated particle sees only an apparently random fluctuation of properties. But the effect does allow one safely to transmit a random number that can then serve as the numerical "key" for an encryption system .In fact such a device has been built by Charles H. Bennett of the IBM Thomas J. Watson Research Center.
A die-hard realist might dismiss the experiments described above, since they all involve that quintessence of ineffability, light But electrons, neutrons, protons and even whole atoms-the stuff our own bodies are made of-also display pathological behaviour. Researchers observed wavelike behaviour in electrons through indirect means as early as the 1920s, and they began carrying out two- slit experiments with electrons several decades ago.
Superposed Philosophers A new round of electron experiments may be carried out soon if Yakir Aharonov of Tel-Aviv University has his way. Noting that superposition is generally inferred from observations of large numbers of particles, Aharonov contends that a single electron bound to a hydrogen atom could be detected smeared out in a relatively large cavity- say, 10 centimetres across- by very delicately scattering photons off it. Aharonov has not yet published his idea- " I am a very fast thinker but a very slow writer," he says-and some physicists he has discussed it with are sceptical. On the other hand, many were sceptical in 1958, when Aharonov and David Bohm of the University of London suggested a way in which a magnetic field could influence an electron that, strictly speaking, lay completely beyond the field's range. The so-called Aharonov - Bohm effect has now been confirmed in laboratories.
Since the mid-l970s various workers have done interference experiments with neutrons, which are almost 2,000 times heavier than electrons. Some 15 years ago, for example, Samuel A. Werner of the University of Missouri at Columbia and others found that the interference pattern formed by neutrons diffracted along two paths by a sculpted silicon crystal could be altered simply by changing the interferometer' s orientation relative to the earth's gravitational field. It was the first demonstration that the Schrödinger equation holds true under the sway of gravity. Investigators have begun doing interferometry with whole atoms only in the past few years. Such experiments are extraordinarily difficult. Atoms cannot pass through lenses or crystals, as photons, electrons and even neutrons can. Moreover, since the wavelength of an object is inversely proportional to its mass and velocity, the particle must move slowly for its wavelength to be detectable. Yet workers such as David E. Pritchard of the Massachusetts Institute of Technology have created the equivalent of beam splitters, mirrors and lenses for atoms out of metal plates with precisely machined grooves and even standing waves of light, formed when a wave of light reflects back on itself in such a way that its crests and troughs match precisely.
Pritchard says physicists may one day be able to pass biologically significant molecules such as proteins or nucleic acids through an interferometer.In principle, one could even observe wavelike behaviour in a whole organism, such as an amoeba. There are some obstacles,though: the amoeba would have to travel very slowly, so slowly, in fact, that it would take some three years to get through the interferometer, according to Pritchard. The experiment would also have to be conducted in an environment completely free of gravitational or other influences-that is, in outer space. Getting a slightly larger and more intelligent organism, for instance, a philosopher, to take two paths through a two-slit apparatus would be even trickier." It would take longer than the age of the universe," Pritchard says.
While physicists may never nudge a philosopher into a superposition of states, they are hard at work trying to induce wavelike behaviour in objects literally large enough to see. The research has rekindled interest in a famous thought experiment posed by Schrödinger in 1935. In a version altered by John Bell, the EPR theorist, to be more palatable to animal lovers, a cat is placed in a box containing a lump of radioactive matter, which has a 50 per - cent chance of emitting a particle in a one-hour period. When the particle decays, it triggers a Geiger counter, which in turn causes a flask of milk to pour into a bowl, feeding the cat. (In Schrödinger 's version, a hammer smashes a flask of poison gas, killing the cat.)
Common sense dictates that a cat cannot have a stomach both empty and full. But quantum mechanics dictates that after one hour, if no one has looked in the box, the radioactive lump and so the cat exist in a superposition of indistinguishable states; the former is both decayed and undecayed, and the latter is both hungry and full. Various resolutions to the paradox have been suggested. Wojciech H. Zurek, a theorist at Los Alamos National Laboratory, contends that as a quantum phenomenon propagates, its interaction with the environment inevitably causes its superposed states to become distinguishable and thus to collapse into a single state. Mandel of the University of Rochester thinks this view is supported by his experiment, in which the mere potential for knowledge of a photon's path destroyed its interference pattern. After all, one can easily learn whether the cat has been fed-say, by making the box transparent-without actually disturbing it.
But since the early 1980s Anthony J.Leggett, a theorist at the University of Illinois, has argued that one should be able to observe a superconducting quantum interference device, more commonly called a SQUID, in a superposition of states. A SQUID, which is typically the size of a pinhead and therefore huge in comparison with atoms or other quantum objects, consists of a loop of superconducting material, through which electrons flow without resistance, broken by a thin slice of insulating material called a Josephson junction. In a classical world the electrons would be completely blocked by the insulator, but the quantum indefiniteness of the electrons' positions allows hordes of them to "tunnel" blithely through the gap.
Inspired by Leggett's calculations, Claudia D. Tesche of the IBM Watson center proposed an experiment that could show the superposition quite directly. Given certain conditions, Tesche notes, the current in a SQUID has an equal chance of flowing in either direction. According to quantum mechanics, then, it should flow both ways, creating an interference pattern analogous to the one formed in a two-slit experiment. Tesche's design calls for placing two extremely sensitive switches around the SQUID, each of which is tripped when the current is going in a different direction. Of course, once a switch is tripped, the wave function collapses, and the interference pattern is destroyed. Tesche hopes to infer the pattern from those microseconds during which the switches are not activated- making measurements, in effect, by not making them.
Orthodoxy under Attack Other theorists note that Tesche's experiment is extremely difficult, since even minute disturbances from the environment can cause the SQUID's wave function to collapse. In fact, Tesche recently turned to other, more conventional pursuits, at least temporarily setting aside the experiment. " It wasn't working very well," she concedes. Yet less ambitious experiments by John Clarke of the University of California at Berkeley, Richard A. Webb of IBM and others have produced strong circumstantial evidence that a SQUID can in fact exist in a superposition of two states. The experiments involve a property known as flux, which is the area of the superconducting ring multiplied by the strength of the magnetic field perpendicular to the ring. In an ordinary superconducting ring the flux would be constant, but measurements with magnetometers show the flux of the SQUID spontaneously jumping from one value to another. Such jumps can occur only if the flux is in a superposition of states-hungry and full at the same time, as it were.
All the recent experiments, completed and proposed, have hardly led to a consensus on what exactly quantum mechanics means. If only by default, the "orthodox" view of quantum mechanics is still the one set forth in the 1920s by Bohr. Called the Copenhagen interpretation, its basic assertion is that what we observe is all we can know; any speculation about what a photon, an atom or even a SQUID "really is" or what it is doing when we're not looking is just that-speculation. To be sure, the Copenhagen interpretation has come under attack from theorists in recent years, most notably from John Bell, author of the brilliant proof of the divergence between "realistic" and quantum predictions for EPR experiments. In a television interview just before his sudden death from a stroke two years ago, the Irish physicist expressed his dissatisfaction with the Copenhagen interpretation, noting that it "says we must accept meaninglessness." Does that make you afraid? the interviewer asked." No, just disgusted," Bell replied, smiling.
Bell's exhortations helped to revive interest in a realistic theory originally proposed in the 1950s by Bohm.In Bohm's view, a quantum entity such as an electron does in fact exist in a particular place at a particular time, but its behaviour is governed by an unusual field, or pilot wave, whose properties are defined by the Schrödinger wave function.The hypothesis does allow one quantum quirk, nonlocality, but it eliminates another, the indefiniteness of position of a particle. Its predictions are identical to those of standard quantum mechanics. Bell also boosted the standing of a theory developed six years ago by Gian Carlo Ghirardi and Tullio Weber of the University of Trieste and Alberto Rimini of the University of Pavia and refined more recently by Philip Pearle of Hamilton College. By adding a nonlinear term to the Schrödinger equation, the theory causes superposed states of a system to converge into a single state as the system approaches macroscopic dimensions, thereby eliminating the Schrödinger 's cat paradox, among other embarrassments.
Unlike Bohm's pilot-wave concept, the theory of Ghirardi's group offers predictions that diverge from those of orthodox quantum physics, albeit subtly. " If you shine a neutron through two slits, you get an interference pattern," Pearle says. " But if our theory is correct, the interference should disappear if you make the measurement far enough away." The theory also requires slight violations of the law of conservation of energy. Zeilinger of the University of Innsbruck was sufficiently interested in the theory to test the neutron prediction, which was not borne out. "This approach is one of those dead end roads that has to be walked by someone," he sighs.
Yet another view currently enjoying some attention, although not as a result of Bell's efforts, is the many-worlds interpretation, which was invented in the 1950s by Hugh Everett III of Princeton.The theory sought to answer the question of why, when we observe a quantum phenomenon, we see only one outcome of the many allowed by its wave function. Everett proposed that whenever a measurement forces a particle to make a choice, for instance, between going left or right in a two-slit apparatus, the entire universe splits into two separate universes; the particle goes left in one universe and right in the other.
Although the theory was long dismissed as more science fiction than science, it has been revived in a modified form by Murray Gell-Mann of the California Institute of Technology and James B. Hartle of the University of California at Santa Barbara.They call their version the many-histories interpretation and emphasize that the histories are "potentialities" rather than physical actualities. Gell-Mann has reportedly predicted that this view will dominate the field by the end of the century. An intriguing alternative, called the many-minds view, has been advanced by David Z. Albert, a physicist-turned- philosopher at Columbia University, and Barry Loewer, a philosopher from Rutgers University. Each observer, they explain, or " sentient physical system," is associated with an infinite set of minds, which experience different possible outcomes of any quantum measurement. The array of choices embedded in the Schrödinger equation corresponds to the myriad experiences undergone by these minds rather than to an infinitude of universes. The concept may sound far-fetched, but it is no more radical, Albert argues, than the many histories theory or even the Copenhagen interpretation itself.
The It from Bit Other philosophers call for a sea change in our very modes of thought.After Einstein introduced his theory of relativity, notes Jeffrey Bub, a philosopher at the University of Maryland, "we threw out the old Euclidean notion of space and time, and now we have a more generalised notion." Quantum theory may demand a similar revamping of our concepts of rationality and logic, Bub says. Boolean logic, which is based on either- or propositions, suffices for a world in which an atom goes either through one slit or the other, but not both slits. "Quantum mechanical logic is non-Boolean," he comments. " Once you understand that, it may make sense." Bub concedes, however, that none of the so-called quantum logic systems devised so far has proved very convincing.
A different kind of paradigm shift is envisioned by Wheeler.The most profound lesson of quantum mechanics, he remarks, is that physical phenomena are somehow defined by the questions we ask of them. " This is in some sense a participatory universe," he says. The basis of reality may not be the quantum, which despite its elusiveness is still a physical phenomenon, but the bit, the answer to a yes-or-no question,which is the fundamental currency of computing and communications. Wheeler calls his idea "the it from bit." Following Wheeler's lead, various theorists are trying to recast quantum physics in terms of information theory,which was developed 44 years ago to maximise the amount of information transmitted over communications channels. Already these investigators have found that Heisenberg's uncertainty principle, wave-particle duality and nonlocality can be formulated more powerfully in the context of information theory, according to William K. Wootters of Williams College, a former Wheeler student who is pursuing the it-from-bit concept.
Meanwhile theorists at the surreal frontier of quantum theory are conjuring up thought experiments that could unveil the riddle in the enigma once and for all. David Deutsch of the University of Oxford thinks it should be possible, at least in principle, to build a "quantum computer," one that achieves superposition of states. Deutsch has shown that if different superposed states of the computer can work on separate parts of a problem at the same time, the computer may achieve a kind of quantum parallelism, solving certain problems more quickly than classical computers.
Taking this idea further, Albert - with just one of his minds - has conceived of a quantum computer capable of making certain measurements of itself and its environment. Such a "quantum automaton" would be capable of knowing more about itself than any outside observer could ever know-and even more than is ordinarily permitted by the uncertainty principle.The automaton could also serve as a kind of eyewitness of the quantum world, resolving questions about whether wave functions truly collapse, for example. Albert says he has no idea how actually to engineer such a machine, but his calculations show the Schrödinger equation allows such a possibility.
If that doesn't work, there is always Aharonov's time machine.The machine, which is based not only on quantum theory but also on general relativity, is a massive sphere that can rapidly expand or contract Einstein's theory predicts that time will speed up for an occupant of the sphere as it expands and gravity becomes proportionately weaker, and time will slow down as the sphere contracts. If the machine and its occupant can be induced into a superposition of states corresponding to different sizes and so different rates of time, Aharonov says, they may "tunnel" into the future.The occupant can then disembark, ask physicists of the future to explain the mysteries of quantum mechanics and then bring the answers-assuming there are any-back to the present. Until then, like Plato's benighted cave dwellers, we can only stare at the shadows of quanta flickering on the walls of our cave and wonder what they mean.