December 12, 2000 

Quantum Theory Tugged, and All of Physics Unraveled


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Quantum theory ignited a scientific revolution 100 years ago, giving rise to paradoxical notions of lumpy light, wavelike particles and the disconnection of cause and effect. Leading physicists were among those who debated the theory at a 1927 congress in Brussels. The discussion continues in Berlin this week.

1900 Max Planck proposes that atoms emit energy in discrete amounts, called quanta, rather than in continuous waves.

1905 Albert Einstein explains the photoelectric effect (light strikes an atom and dislodges an electron) by suggesting that light is made of little energy bundles, which are later called photons.

Compact disc players work when the light (photons) from a laser strikes a sensor (photodiode) to generate electrical current (electron release).

1913 Niels Bohr proposes a planetary model of the atom in which electrons orbit the nucleus and jump between orbits as the atom absorbs or emits energy.

1924 Louis de Broglie develops the idea that matter, like light, can behave as waves. According to de Broglie's formula, the wavelength of an electron is only about one-10,000th the wavelength of a photon of light.

In electron microscopes, beams of matter, electron beams, explore spaces far smaller than those accessible to light. At right, the mouth of the common housefly.

1924 Einstein and Satyendra Nath Bose develop a set of statistics recognizing a class of particles, called bosons, which can collectively exist in the same state of energy.

Photons are bosons, so they can collectively occupy a single state, allowing them to coalesce as an intense laser beam.

1925 Wolfgang Pauli develops the exclusion principle, stating that no two electrons in an atom can occupy the same state of energy simultaneously. This explains the movement of electrons into successive orbits around the nucleus of an atom.

1926 Erwin Schrödinger proposes that an electron is best described by the mathematical function of all its possible energy states, a wave. Max Born later proposes that this wave is not the particle itself, but the probability of finding the particle in a particular place.

1926 Enrico Fermi and Paul Dirac describe the statistical properties of particles that obey the Pauli exclusion principle. Fermions, as they are known, include protons, neutrons and electrons and are distinct from particles that obey Bose-Einstein statistics.

Atoms in certain solids, semiconductors, will collectively fill out their energy orbits with electrons. When excited from a burst of energy, these electrons can move about. Semiconductors are at the heart of the circuits in microprocessors.

1927 Werner Heisenberg arrives at his uncertainty principle, theorizing that it is impossible to measure both the position and momentum of a particle at the same time.

1928 Paul Dirac predicts antimatter by proposing that fermions, including electrons, can inhabit states of negative energy. When matter and antimatter collide, they annihilate each other and release energy.

Positron emission tomography relies on collisions between electrons and their anti-particles, positrons, that occur in chemical compounds injected into the body.

Sources: Science, Dorling Kindersley

They tried to talk Max Planck out of becoming a physicist, on the grounds that here was nothing left to discover. The young Planck didn't mind. A conservative youth from the south of Germany, a descendant of church rectors and professors, he was happy to add to the perfection of what was already known. 

Instead, he destroyed it, by discovering what was in effect a loose thread that when tugged would eventually unravel the entire fabric of what had passed for reality.

As a new professor at the University of Berlin, Planck embarked in the fall of 1900 on a mundane sounding calculation of the spectral characteristics of the glow from a heated object. Physicists had good reason to think the answer would elucidate the relationship between light and matter as well as give German industry a leg up in the electric light business. But the calculation had been plagued with difficulties.

Planck succeeded in finding the right formula, but at a cost, as he reported to the German Physical Society on Dec. 14. In what he called "an act of desperation," he had to assume that atoms could only emit energy in discrete amounts that he later called quanta (from the Latin quantus for "how much" ) rather than in the continuous waves prescribed by electromagnetic theory. Nature seemed to be acting like a fussy bank teller who would not make change, and would not accept it either. 

That was the first shot in a revolution. Within a quarter of a century, the common sense laws of science had been overthrown. In their place was a bizarre set of rules known as quantum mechanics, in which causes were not guaranteed to be linked to effects; a subatomic particle like an electron could be in two places at once, everywhere or nowhere until someone measured it; and light could be a wave or a particle.

Niels Bohr, a Danish physicist and leader of this revolution, once said that a person who was not shocked by quantum theory did not understand it.

This week, some 700 physicists and historians are gathering in Berlin, where Planck started it all 100 years ago, to celebrate a theory whose meaning they still do not understand but that is the foundation of modern science. Quantum effects are now invoked to explain everything from the periodic table of the elements to the existence of the universe itself. 

Fortunes have been made on quantum weirdness, as it is sometimes called. Transistors and computer chips and lasers run on it. So do CAT scans and PET scans and M.R.I. machines. Some computer scientists call it the future of computing, while some physicists say that computing is the future of quantum theory.

"If everything we understand about the atom stopped working," said Leon Lederman, former director of the Fermi National Accelerator Laboratory, "the G.N.P. would go to zero."

The revolution had an inauspicious start. Planck first regarded the quantum as a bookkeeping device with no physical meaning. In 1905, Albert Einstein, then a patent clerk in Switzerland, took it more seriously. He pointed out that light itself behaved in some respects as if it were composed of little energy bundles he called lichtquanten. (A few months later Einstein invented relativity.)

He spent the next decade wondering how to reconcile these quanta with the traditional electromagnetic wave theory of light. "On quantum theory I use up more brain grease than on relativity," he told a friend.

The next great quantum step was taken by Bohr. In 1913, he set forth a model of the atom as a miniature solar system in which the electrons were limited to specific orbits around the nucleus. The model explained why atoms did not just collapse — the lowest orbit was still some slight distance from the nucleus. It also explained why different elements emitted light at characteristic wavelengths — the orbits were like rungs on a ladder and those wavelengths corresponded to the energy released or absorbed by an electron when it jumped between rungs.

But it did not explain why only some orbits were permitted, or where the electron was when it jumped between orbits. Einstein praised Bohr's theory as "musicality in the sphere of thought," but told him later, "If all this is true, then it means the end of physics."

While Bohr's theory worked for hydrogen, the simplest atom, it bogged down when theorists tried to calculate the spectrum of bigger atoms. "The whole system of concepts of physics must be reconstructed from the ground up," Max Born, a physicist at Göttingen University, wrote in 1923. He termed the as-yet- unborn new physics "quantum mechanics."

Boy's Mechanics

The new physics was born in a paroxysm of debate and discovery from 1925 to 1928 that has been called the second scientific revolution. Wolfgang Pauli, one of its ringleaders, called it "boy's mechanics," because many of the physicists, including himself, then 25, Werner Heisenberg, 24, Paul Dirac, 23, Enrico Fermi, 23, and Pascual Jordan, 23, were so young when it began.

Bohr, who turned 40 in 1925, was their father-confessor and philosopher king. His new institute for theoretical physics in Copenhagen became the center of European science.

The decisive moment came in the fall of 1925 when Heisenberg, who had just returned to Göttingen University after a year in Copenhagen, suggested that physicists stop trying to visualize the inside of the atom and instead base physics exclusively on what can be seen and measured. In his "matrix mechanics," various properties of subatomic particles could be computed — but, disturbingly, the answers depended on the order of the calculations.

In fact, according to the uncertainty principle, which Heisenberg enunciated two years later, it was impossible to know both the position and velocity of a particle at once. The act of measuring one necessarily disturbed the other.

Physicists uncomfortable with Heisenberg's abstract mathematics took up with a friendlier version of quantum mechanics based on the familiar mathematics of waves. In 1923, the Frenchman Louis de Broglie had asked in his doctoral thesis, if light could be a particle, then why couldn't particles be waves? 

Inspired by de Broglie's ideas, the Austrian Erwin Schrödinger, then at the University of Zurich and, at 38, himself older than the wunderkind, sequestered himself in the Swiss resort of Arosa over the 1925 Christmas holidays with a mysterious woman friend and came back with an equation that would become the yin to Heisenberg's yang.

In Schrödinger's equation, the electron was not a point or a table, but a mathematical entity called a wave function, which extended throughout space. According to Born, this wave represented the probability of finding the electron at some particular place. When it was measured, the particle was usually in the most likely place, but not guaranteed to be, even though the wave function itself could be calculated exactly. 

Born's interpretation was rapidly adopted by the quantum gang. It was a pivotal moment because it enshrined chance as an integral part of physics and of nature.

"The motion of particles follows probability laws, but the probability itself propagates according to the law of causality," he explained.

That was not good enough for Einstein. "The theory produces a good deal but hardly brings us closer to the secret of the Old One," Einstein wrote in late 1926. "I am at all events convinced that he does not play dice." 

Heisenberg called Schrödinger's theory "disgusting" — but both versions of quantum mechanics were soon found to be mathematically equivalent.

Uncertainty, which added to the metaphysical unease surrounding quantum physics, was followed in turn in 1927 by Bohr's complementarity principle. Ask not whether light was a particle or a wave, said Bohr, asserting that both concepts were necessary to describe nature, but that since they were contradictory, an experimenter could choose to measure one aspect or the other but not both. This was not a paradox, he maintained, because physics was not about things but about the results of experiments.

Complementarity became the cornerstone of the Copenhagen interpretation of quantum mechanics — or as Einstein called it, "the Heisenberg- Bohr tranquilizing philosophy."

A year later, Dirac married quantum mechanics to Einstein's special relativity, in the process predicting the existence of antimatter. (The positron, the antiparticle to the electron, was discovered four years later by Carl Anderson.)

Dirac's version, known as quantum field theory, has been the basis of particle physics ever since, and signifies, in physics histories, the end of the quantum revolution. But the fight over the meaning of the revolution had just barely begun, and it has continued to this day.

Quantum Wars

The first and greatest counterrevolutionary was Einstein, who hoped some deeper theory would rescue God from playing dice. In the fall of 1927 at a meeting in Brussels, Einstein challenged Bohr with a series of gedanken, or thought experiments, designed to show that quantum mechanics was inconsistent. Bohr, stumped in the morning, always had an answer by dinner.

Einstein never gave up. A 1935 paper written with Boris Podolsky and Nathan Rosen described the ultimate quantum gedanken, in which measuring a particle in one place could instantly affect measurements of the other particle, even if it was millions of miles away. Was this any way to run a universe? 

Einstein called it "spooky action at a distance."

Modern physicists who have managed to create this strange situation in the laboratory call it "entanglement."

Einstein's defection from the quantum revolution was a blow to his more conservative colleagues, but he was not alone. Planck also found himself at odds with the direction of the revolution and Schrödinger, another of "the conservative old gentlemen," as Pauli once described them, advanced his cat gedanken experiment to illustrate how silly physics had become.

According to the Copenhagen view, it was the act of observation that "collapsed" the wave function of some particle, freezing it into one particular state, a location or velocity. Until then, all the possible states of the particle coexisted, like overlapping waves, in a condition known as quantum superposition.

Schrödinger imagined a cat in a sealed container in which the radioactive decay of an atom would trigger the release of cyanide, killing the cat. By the rules of quantum mechanics the atom was both decayed and not decayed until somebody looked inside, which meant that Schrödinger's poor cat was both alive and dead. 

This seemed to be giving an awful lot of power to the "observer." It was definitely no way to run a universe.

Over the years physicists have proposed alternatives to the Copenhagen view. 

Starting in 1952, when he was at Princeton, the physicist David Bohm, who died in 1992, argued for a version of quantum mechanics in which there was a deeper level, a so-called quantum potential or "implicate order," guiding the apparent unruliness of quantum events. 

Another variant is the many- worlds hypothesis developed by Hugh Everett III and John Wheeler, at Princeton in 1957. In this version the wave function does not collapse when a physicist observes an electron or a cat; instead it splits into parallel universes, one for every possible outcome of an experiment or a measurement. 

Shut Up and Compute

Most physicists simply ignored the debate about the meaning of quantum theory in favor of using it to probe the world, an attitude known as "shut up and compute."

Pauli's discovery that no two electrons could share the same orbit in an atom led to a new understanding of atoms, the elements and modern chemistry.

Quantum mechanics split the atom and placed humanity on the verge of plausible catastrophe. Engineers learned how to "pump" electrons into the upper energy rungs in large numbers of atoms and then make them all dump their energy all at once, giving rise to the laser. And as Dr. Lederman said in an interview, "The history of transistors is the history of solving Schrödinger's equation in various materials."

Quantum effects were not confined to the small. The uncertainty principle dictates that the energy in a field or in empty space is not constant, but can fluctuate more and more wildly the smaller the period of time that one looks at it. Such quantum fluctuations during the big bang are now thought to be the origin of galaxies.

In some theories, the universe itself is a quantum effect, the result of a fluctuation in some sort of preuniversal nothingness. "So we take a quantum leap from eternity into time," as the Harvard physicist Sidney Coleman once put it.

Where the Weirdness Goes

Bohr ignored Schrödinger's cat, on the basis that a cat was too big to be a quantum object, but the cat cannot be ignored anymore. In the last three decades, the gedanken experiments envisioned by Einstein and his friends have become "ungedankened," bringing the issues of their meaning back to the fore.

Last summer, two teams of physicists managed to make currents go in two directions at once around tiny superconducting loops of wire — a feat they compared to Schrödinger's cat. Such feats, said Wojciech Zurek, a theorist at Los Alamos National Laboratory, raise the question of why we live in a classical world at all, rather than in a quantum blur. 

Bohr postulated a border between the quantum and classical worlds, but theorists prefer that there be only one world that can somehow supply its own solidity. That is the idea behind a new concept called decoherence, in which the interaction of wave functions with the environment upsets the delicate balance of quantum states and makes a cat alive or dead but not in between.

"We don't need an observer, just some `thing' watching," Dr. Zurek explained. When we look at something, he said, we take advantage of photons, the carriers of light, which contain information that has been extracted from the object. It is this loss of information into the environment that is enough to crash the wave function, Dr. Zurek says.

Decoherence, as Dr. Zurek notes, takes the observer off a pedestal and relieves quantum theory of some of its mysticism, but there is plenty of weirdness left. Take the quantum computer, which Dr. Lederman refers to as "a kinder, gentler interpretation of quantum spookiness."

Ordinary computers store data and perform computations as a series of "bits," switches that are either on or off, but in a quantum computer, due to the principle of superposition, so-called qubits can be on and off at the same time, enabling them to calculate and store myriads of numbers at a time.

In principle, according to David Deutsch, an Oxford University researcher who is one of quantum computing's more outspoken pioneers, a vast number of computations, "potentially more than there are atoms in the universe," could be superposed inside a quantum computer to solve problems that would take a classical computer longer than the age of the universe.

In the minds of many experts, this kind of computing illuminates the nature of reality itself.

Dr. Deutsch claims that the very theory of a quantum computer forces physicists to take seriously the many-worlds interpretation of quantum theory. The amount of information being processed in these parallel computations, he explains, is more than the universe can hold. Therefore, they must be happening in other parallel universes out in the "multiverse," as it is sometimes called.

"There is no other theory of what is happening," he said. The world is much bigger than it looks, a realization that he thinks will have a psychological impact equivalent to the first photographs of atoms. Indeed, for Dr. Deutsch there seems to be a deep connection between physics and computation. The structure of the quantum computer, he says, consists of many things going on at once, lots or parallel computations. "Any physical process in quantum mechanics," he said. "consists of classical computations going on in parallel." 

"The quantum theory of computation is quantum theory," he said.

The Roots of Weirdness

Quantum mechanics is the language in which physicists describe all the phenomena of nature save one, namely gravity, which is explained by Einstein's general theory of relativity. The two theories — one describing a discontinuous "quantized" reality and the other a smoothly curving space-time continuum — are mathematically incompatible, but physicists look to their eventual marriage, a so-called quantum gravity.

"There are different views as to whether quantum theory will encompass gravity or whether both quantum theory and general relativity will have to be modified," said Lee Smolin, a theorist at Penn State.

Some groundwork was laid as far back as the 1960's by Dr. Wheeler, 89, who has argued quantum theory with both Einstein and Bohr. Even space and time, Dr. Wheeler has pointed out, must ultimately pay their dues to the uncertainty principle and become discontinuous, breaking down at very small distances or in the compressed throes of the big bang into a space-time "foam."

Most physicists today put their hope for such a theory in super- strings, an ongoing and mathematically dense effort to understand nature as consisting of tiny strings vibrating in 10-dimensional space.

In a sort of missive from the front, Edward Witten of the Institute for Advanced Study in Princeton, N.J., said recently that so far quantum mechanics appeared to hold up in string land exactly as it was described in textbooks. But, he said in an e-mail message, "Quantum mechanics is somehow integrated with geometry in a way that we don't really understand yet."

The quantum is mysterious, he went on, because it goes against intuition. "I am one of those who believes that the quantum will remain mysterious in the sense that if the future brings any changes in the basic formulation of quantum mechanics, I suspect our ordinary intuition will be left even farther behind."

Intuition notwithstanding, some thinkers wonder whether or not quantum weirdness might, in fact, be the simplest way to make a universe. After all, without the uncertainty principle to fuzz the locations of its buzzing inhabitants, the atom would collapse in an electromagnetic heap. Without quantum fluctuations to roil the unholy smoothness of the big bang, there would be no galaxies, stars or friendly warm planets. Without the uncertainty principle to forbid nothingness, there might not even be a universe.

"We will first recognize how simple the universe is," Dr. Wheeler has often said, "when we recognize how strange it is." Einstein often said that the question that really consumed him was whether God had any choice in creating the world. It may be in the end that we find out that for God, the only game in town was a dice game.

Copyright 2001 The New York Times Company