by: Timothy Ferris
New York Times Magazine
September 29, 1996. Pages: 143-146
There Are Strange Subatomic Mysteries for a New Einstein to Solve. If the next Einstein were born today, what might he or she be doing in the year 2022, having reached the age, 26, at which Einstein formulated the theory of relativity?
My suggestion would be--solving the problem of guantum weirdness. The term is scientific slang. It stands for a conundrum, more properly known as the ”quantum observership” or ”quantum measurement” problem, that has defied some of our century’s strongest minds.
Quantum physics is a famously strange realm where matter, energy and knowledge are spooned out in indivisible units, the quanta--as if the world were a pub where you could guaff a pint of beer, or no beer, but never a half pint.
In quantum physics we have learned to accept such unlikelihoods as ”quantum leaps,” in which particles vanish from one place and reappear in another--instantly. (Raymond Chiao of the University of California at Berkeley has recently measured photons, the carriers of light, quantum-leaping at velocities that would amount to twice the speed of light if they crossed the intervening space, which they don’t.) But quantum physics itself is not the problem. It remains a bighly successful branch of science that promises to cruise with flying colors through the centennial, in 2000, of Max Planck’s discovery of the quantum principle. Weirdness arises when we try to reconcile some of the oddities of the quantum world with the dictates of common sense.
The history of that effort is littered with the bleached bones of mighty thinkers. Einstein pondered its paradoxes for decades and got next to nowhere. Niels Bobr didn’t get much further. David Bohm, John Stewart Bell labored mightily at it, and may someday be revered as pioneers, but all went to their graves with little more than obscurity to reward their efforts. Quantum weirdness is SO weird that just discussing it stands the normal rules of exposition on their heads: to understand it is to become not enlightened but confused. As the physicist John Archibald Wheeler says, ”The quantum is the greatest mystery we’ve got’.”
Some scientists dismiss quantum weirdness as a mere brain teaser. But it was a brain teaser that started Einstein on the road to relativity,’when at age 16 he wondered what he would see if he observed an electromagnetic field while traveling at the velocity of light.
With recent improvements in technology, physicists are actually conducting the thought experiments that Einstein and Bohr could only imagine, and their efforts have already led to intriguing technological breakthroughs--a new kind of laser in Germany, a demonstration of quantum cryptography in England and the promise of powerful microscopes, miniature particle colliders and computers fast enough to break any code. Where such diamonds are strewn on the surface, it’s reasonable to wonder what lies deeper down.
The essence of quantum weirdness can be summed up in the statement that quantum systems--typically, photons and electrons, things smaller than an atom--exhibit ”nonlocal” behavior. In all previous scientific investigations, nature acts locally. For a cause HERE to produce an effect over THERE, an intervening mechanism must link the two. Such a mechanism is ”local” in that you can identify it here and now; the waves that make a boat bob in its moorings can be traced to a passing ship. If no waves or other mechanisms could be found connecting the ship to the boat, we would have nonlocal behavior, which seems as inexplicable as if a car were to continue accelerating down the road after losing its drive shaft.
Newton worried that his theory of gravitation was nonlocal, since he never found a mechanism that could propagate gravitational force across space. Einstein cleared that up with the general theory of relativity, which revealed that gravitation results from the curvature of space. Changes in a gravitational field are conveyed, at the velocity of light, by ripples in space itself. As Einstein also found, neither this nor any other casual mechanism can work at faster than light speed. Yet quantum systems evidently behave in such a way that a cause here produces an effect over there instantaneously, with no discernible causal mechanism between the two points, and with insufficient time for such a mechanism, working at light speed, to have carried the news from one place to the other. Einstein regarded nonlocality as absurd. He called it ”spooky action at a distance.”
Quantum nonlocality first reared its weird head with Werner Heisenberg’s uncertainty principle. Heisenberg discovered that certain kinds of information about quantum systems can be obtained only at the cost of forsaking other kinds of information. You can ascertain exactly where a particle is (its position) or exactly where it is going (its momentum), but not both. The more precisely you pinpoint its position, the less you can know about its momentum, and vice versa. It is as if you were handed an autographed baseball in a lightless room and told you could take but a single flash photograph of it. The photo might show the signature on the ball, or it might show the manufacturer’s emblem embossed on the opposite side, or part of the signature and part of the emblem. But you could not learn everything about both the autograph and the emblem from a single photo. Bohr called these complementary aspects of the system. In real life, you could take the baseball out into the sunlight and examine it fully. But with quantum systems, what you see is all you get.
The central tenet of complementarity resides in the ”wave-particle duality.” Subatomic particles act like either waves or particles, depending on how they are examined. But common sense says they cannot be both at once, since waves and particles behave very differently. A wave is all over the place; a particle is in one place only.
The distinction is demonstrated by a simple test that the physicist:
Richard Feynman called ”the experiment with the two holes.” To run the experiment, punch two small holes in a sheet of steel, fire a stream of photons or other guanta at the sheet and record what comes through, using a detector of some sort on the far side of the steel sheet. (The detector can be something as simple as a sheet of photographic film.) When both holes are open, the detector records an interference pattern--the signature of interacting waves. Drop two stones in a pond and an interference pattern appears where the waves intersect. Wave peaks reinforce each other where they coincide, as do valleys, and where a wave peak intersects with a valley, the two cancel each other out. In the two-holes experiment, the interference pattern appears even if you send only a single photon through the apparatus: the photon finds its way through both holes and interferes with itself. Close one hole, however, and the photon’s wavelike behavior disappears. Now it acts like a bullet: either it emerges from the single open hole to register a point impact on the detector, or it misses the hole and hits the steel sheet, and nothing comes through.
The weird thing is that the photon does this--responds to whether one or both holes are open--instantly, even if you wait until the last moment, just before it reaches the steel sheet, before deciding to close one hole or leave both open. It is as if the particle (or wave, whichever you prefer) were everywhere at once, feeling out the entire setup and responding to it instantaneously, everywhere.
Bohr explained the wave-particle duality by declaring that subatomic systems don’t have either of their complementary states until they are observed. This view came to be known as the ”Copenhagen” interpretation of quantum mechanics, named for the city where Bohr and his colleagues set up shop. It might be summarized as .”Don’t ask, don’t tell.” Are photons particles or waves? Don’t ask! They are neither--or they are both. Their complementary states are only resolved, one way or the other, by their being observed.
In 1935 the Austrian physicist Erwin Schrodinger challenged Bohr’s view of the role of observation in quantum systems by proposing a famous thought experiment, known ever since as ”Schrodinger’s cat.” An unfortunate cat is placed in a sealed box with a guantum device that has a 50-50 chance of going to a particular state within, say, one hour. If the state is not achieved, nothing happens. If it is achieved, it explodes a cyanide capsule and kills the cat. At the end of the hour, but before we open the box, what has happened? If we accept the Copenhagen assertion that the system HAS no state until it is observed, we have to believe that the cat, until observed, remains in a ”superposed” state of both dead and alive. If you object that the cat itself made the observation, then let’s leave the cat out of it, run the experiment with an empty box, then clear the laboratory except for a graduate student wbo opens the box and will be killed if the cyanide has been released. As we stand outside the closed lab door, we reflect that according to Bohr the cyanide has been neither released nor unreleased until the student opens the box. The student’s own observation, and not the prior state of the cyanide canister, will decide whether he lives or dies. Does anybody believe that the world really works this way? Duncan note: This is only an analogy, quantum weirdness only seems to work on a sub-atomic scale.
To further highlight the weirdness of the Copenhagen view, Einstein and two of his Princeton colleagues constructed the E.P.R. (Einstein-Podolsky-Rosen) thought experiment. Let an atom spit out two particles, which then fly apart for an enormous distance. The physics equations tell us that the two particles must have opposite spin: if one is ”spin up,” the other must be ”spin down.” But according to the Copenhagenians, the particles have no spin state at all until observed. So now we observe one particle and find, say, that it is spin up. We have thus ”resolved” the spin state of a particle that allegedly had no such state until we made the observation. But that means that the other particle, millions of miles away, suddenly ”became” spin down. How did it know that its state was now supposed to be spin down?
Logic would seem to dictate that the particles actually had spin states all along. Einstein took that position, postulating that some underlying, causal agent must carry the spin. In other words, the spin state is not decided at the moment of observation after all, as Bohr claimed, but was in the particles to start with. But there is no evidence that any such mechanism exists. Theories like Einstein’s are therefore known as ”hidden variables” interpretations, since they propose the existence of something hidden in the quantum systems.
Since the Copenhagen view rejected hidden variables, the question remained at an impasse for decades. Which interpretation a given physicist preferred mainly depended on which seemed less repugnant, the hidden variables theory or spooky action at a distance.
Then the Irish physicist John Stewart Bell thought up an experiment that would decide whether hidden variables exist, as Einstein believed. The experiment, which involved obtaining the statistics of large numbers of photon interactions, was not technologically feasible when he proposed it in 1964. But in the 1970’s Bell’s experiment was performed, first by John Clauser and Stuart J. Freedman at Berkeley and later by Alain Aspect and his colleagues at the University of Paris. The verdict was clear: there are no hidden variables of the sort Einstein envisioned. Quantum physics really does exhibit nonlocality.
Some scientists reacted with a shrug of the shoulders. Quantum physics is simply like that, they asserted, and if it doesn’t fit our notions of common sense, too bad. But quantum weirdness remains disturbing insofar as we wish to reconcile quantum theory with the logic and language of the wider world--something that Bohr himself held to be a fundamental imperative of the scientific enterprise. And as Feynman noted, quantum weirdness ”is impossible, ABSOLUTELY impossible to explain in any classical way.”
Despite Bohr’s posthumous defeat of Einstein, the Copenhagen interpretation is beginning to fall out of favor. One reason is that theorists have begun to apply quantum physics to the study of the origin and early evolution of the universe it’s very difficult to imagine that observers existed in the fireball of the Big Bang, as the Copenhagen interpretation requires. A popular approach nowadays is to replace the concept of observation with that of ”decoherence,” an interference among particles that resolves quantum systems into one or the other of their complementary states. The jury is still out on decoherence, but neither its success nor its failure will do much to rescue the Copenhagen interpretation from its wider difficulties. To scientists dissatisfied with the Copenhagen interpretation, quantum nonlocality is not a bitter pill that must be swallowed but possibly the nimbus of a future physics.
The quantum handshake traces its roots to an older proposal made by Feynman one winter day in 1941, when he was a fresh-faced graduate student at Princeton giving his first scientific talk. Its subject was’an idea he’d worked up’ with the help: of his thesis adviser, John Wheeler. The Wheeler-Feynman theory,dealt with electrons. It depicted all the electrons in the universe as linked by spider webs of advanced plus retarded waves: jiggle one here and you produced an instant response far away.,’ Feynman already had a reputation as a bright young man, and his talk drew many luminaries, among them Wolfgang Pauli, a notoriously penetrating critic, and Einstein, the grand old man himself, who’d been briefed on what Feynman would have to say. Feynman recalled that when he had finished, Pauli spoke up first, dismissing the idea as a mathematical oddity. ”I do not think this theory can be right,” he declared. ”Don’t you agree, Professor Einstein?” But Einstein replied, Feynman wrote, ”’Nooooooooooooo,’ a nice, German-sounding ’No,’ very polite,” adding only that he could not see how to apply it to gravitation.
The Wheeler-Feynman approach stalled at that point, mainly because its creators could not tailor it to solve the technical problem they had hoped it would resolve in the first place, which was that some quantum equations yielded nonsensical, infinite energies. Feynman went on to win a Nobel Prize in 1965 for scotching the infinities by means of a mathematical procedure called renormalization. But he never fully approved of renormalization, and he always retained his fascination with the strange, if stillborn, theory of advanced and retarded waves. Accepting the Nobel Prize, Feynman sounded a note not of triumph but of ”regret” that the original idea hadn’t worked out. ”What happened to the old theory that I fell in love with as a youth?” he asked wistfully at the end of his address. ”Well, I would say it has become an old lady that has very little attractive left in her, and the young today will not have their hearts pound when they look at her anymore.”
Underlying handshake theories like those of Cramer and Chu is the curious fact that the laws of physics in general, and the equations central to quantum mechanics in particular, are time-symmetric: they run just as well backward as forward in time. Physicists customarily ignore the reverse-time capabilities of these eguations because the ”real world” is observed to proceed forward in time only. But what if that is only one side of nature? What if the equations are telling us that time runs backward on the other side of the looking glass? The existence of time-reversed waves could resolve the mysteries of quantum weirdness: part of the system over THERE responds instantly to changes HERE because the time-reversed waves shake hands with the famliar time-forward ones, producing instantaneous action. If so, clearing up quantum weirdness would be only one of the attainments of handshake theory. Such a theory might do much more, unveiling a startling new picture of nature, one as revolutionary as the visions of Copernicus, Galileo and Newton.
In a static, ageless universe it might seem inexplicable to speak of nature as in some way indifferent to space and time. But our universe is neither static nor ageless. It began in a singularity--the Big Bang, a state in which all times are the same time, and all places the same place. The Big Bang occurred not at some remote place but everywhere--right here, where I’m writing these words, and there, where you are reading them, and on Mars, and in the Silver Coin Galaxy. That is why astronomers studying the Big Bang find that the cosmic microwave background, which consists of photons released when the primordial material thinned out enough to become transparent, about a million years after Time Zero, comes not from one place but from all over the sky. Curiously, photons themselves pay no heed to time. Since, as Einstein’s relativity reveals, time comes to a halt at the velocity of light, from a photon’s point of view the moment that it departed and the moment, 15 billions years later, that it is observed on Earth are all the same. Conceivably photons bear a message from the other side of the looking glass.
The universe has been careening forward through time ever since it emerged from the original singularity, so we are accustomed to thinking of the cosmos as arrayed across vast starscapes of spacetime. But what if quantum weirdness proffers an enduring view of the way things were? Einstein liked to call God ”the Old One,” but we may come to apply the same term to the original universe, as it was in the beginning. The answer to the riddle of the quantum weirdness may be that the Old One is still around.
Timothy Ferris is professor emeritus of journalism at the University of California at Berkeley. His latest book is, ”The Whole Shebang: A State of the Universe(s) Report”.
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