You could easily mistake Klaus Wacker for a Swiss cabdriver. Leaning against a steel railing in his brown leather jacket, faded jeans, and running shoes, he might be gazing at Mont Blanc and chatting about the weather. In fact, Dr. Wacker is a West German particle physicist - and he's standing 10 stories underground. Around him rises the cylindrical wall of what looks like the inside of a huge grain silo. Behind him, in a cavity carved into the side of the pit, are three floors of densely packed electronic equipment. And beyond the wall beside him is the reason for it all: the four-mile-long circular tunnel of the Super Proton Synchrotron straddling the French-Swiss border here at the European Laboratory for Particle Physics - or CERN, as it is known from its form in French.

He's standing, in other words, at the heart of an investment worth billions of dollars, built with millions of man-hours, spread over thousands of acres, and financed by more than a dozen European nations. And it's all dedicated to the tiniest known thing: the subatomic particle.

He's also standing at the crossroads of scientific history. Behind him are decades of research into quantum mechanics - the science that deals with the fundamental particles and forces of the material world. Around him, from the liquid-crystal display on his digital watch to the computers in his control room, lies the revolutionary technology that has sprung from quantum theory. And ahead of him stretch the mysteries of what it all means - of how to interpret a universe seemingly governed by randomness, almost entirely empty of matter, incapable of being fully measured, and one perhaps shaped by the observer.

For the moment, the world's insight into these deeper meanings grows out of experiments like those done here at CERN's UA-1 (``Underground Area 1''). It is here that a stream of protons - generated in Switzerland, hurtled underground through France, and returned to Switzerland without benefit of customs or immigration - collides with a stream of antiprotons blasted around the track in the opposite direction. It was here, in 1983, that Italian physicist Carlo Rubbia led a team of more than 100 physicists to his Nobel Prize-winning discovery of three particles known as intermediate vector bosons (known as W+, W-, and Z0), which have helped reshape modern theories of particle physics.

To understand the significance of this reshaping, you need to look back into history.

Before quantum, it was a billiard-ball world

A century ago and several hundred miles northeast of here, a young German student stopped in to see one of his teachers for advice about careers. He wanted to become a physicist. He was urged, instead, to become a concert pianist. ``Physics is finished, young man,'' said the teacher. ``It's a dead-end street.''

Fortunately for physics, the young man persisted. His name was Max Planck, and his discovery of the spectral law of black-body radiation - that the energy emitted by a radiating body depends on the frequency of that radiation - was the seminal discovery that motivated the development of quantum theory.

Yet his teacher can be forgiven. As late as the end of the 19th century, physics seemed all but a closed book. The so-called ``classical'' framework built upon Newtonian mechanics had been spectacularly successful. There appeared to be little left for physicists to do. It was a billiard-ball world, with everything determined by the collisions of particles - and with the collisions themselves determined by the initial forces of the universe. Even the most intricate phenomena of chemistry, astronomy, and the other sciences could all be explained - at least in theory - by these collisions.

To be sure, there were a few loose ends to tidy up: black-body radiation, for instance, in which a piece of hot steel was observed to glow cherry red and not (as classical theory said it should) bright blue. But nobody thought such problems could undermine two centuries of physics. Those two centuries had been ushered in by Sir Isaac Newton, the Englishman whose crowning work, the ``Principia'' (1687), traced out the conclusions of his legendary experience with the falling apple.

But the importance of Newton was not, as Edward W. Kolb notes, that he discovered gravity. ``The first person who dropped a rock on his foot discovered gravity,'' says Dr. Kolb, an astronomer at the Fermi National Accelerator Laboratory in Batavia, Ill. ``Newton did something more remarkable. He discovered the universality of gravity - that it's the same force causing the apple to fall on his head and causing the moon to orbit the earth.''

What Newton set forth, in short, was a principle of consistency - the idea that the laws of nature, operating on a tiny scale in a laboratory, also operate throughout intergalactic space. Discover how nature operates in its smallest parts, in other words, and you could explain the action on any other scale. In Newton's conception, these laws were quantifiable - they were subject to measurement and capable of being explained through mathematics. And that, rather naturally, led to the concept that the universe was predictable - however much it was also, in Newton's view, divinely ordained and operated.

How, then, was man to picture the universe? In classical physics, it resembled a great ticking clock. God, in this view, was a kind of supreme watchmaker, setting up all the initial conditions for each particle of matter and then letting those particles take their course. Man ticked right along with the rest of creation.

It was a simple model. Yet it had profound implications, especially when it began to challenge the idea that the atoms themselves were merely inert. ``The view that the ultimate units of matter might be active,'' says Northwestern University historian Stephen Toulmin, ``was generally regarded as subversive. It was associated with the idea that the masses might be politically active instead of doing what they were told.''

Quanta: discrete packets

IN fact, the Newtonian ideas spilled over into social and political consciousness. After all, if the physical laws of the universe were discoverable, why should not social and political laws also be subject to experimentation and proof?

The result, says historian Daniel J. Kevles, author of ``The Physicists: The History of a Scientific Community in Modern America'' and a professor at the California Institute of Technology, was ``a set of social and political ideas [that] were then used as weapons and corrosives to overthrow monarchies, to create democracies, and to celebrate the natural rights of man. There is, in short, a symbiotic and explosive relationship between the scientific revolution of Newtonianism and the social and political developments of the 18th century.''

A century later, in 1900, Planck had resolved the problem of black-body radiation. To do so, he coined the term quanta, using it to describe the way that the energy of radiation was ``partitioned'' or divided - not continuously, but in small, discrete packets.

The world into which Planck quietly dropped this term, however, had been so profoundly shaped by classical physics that even the literature of the day showed its influence. The novels of Thomas Hardy, the plays of Henrik Ibsen, the poetry of Matthew Arnold, all owed major debts to this Newtonian world view.

And so, to a great extent, did the common man-in-the-street views of the universe. What the Greeks thought of as fate and the Calvinists as predestination had taken a new form: mechanical determinism. Why does man act as he does? Ultimately, because of those little billiard balls that constitute his being. Even man's deepest hopes and loves, wrote philosopher Bertrand Russell in a now-famous passage, were ``nearly certain'' to be proved nothing more than ``the outcome of accidental collocations of atoms.''

``Brief and powerless is man's life; on him and all his race the slow, sure doom falls pitiless and dark,'' Russell wrote. ``Blind to good and evil, reckless of destruction, omnipotent matter rolls on its relentless course.''

But what was that ``omnipotent matter'' really made of? The question raised a philosophical sticking point. If atoms could be divided into subatomic particles, how far could that division be carried? Was everything built out of something smaller, or were there fundamental, indivisible particles that could not be reduced further? If so, how could they have any dimension - since everything with size can be conceived of as having smaller constituents? Or were such particles so fundamental that they had no dimensions - leaving unsolved the mystery of why a universe built from such particles should have any discernible size at all?

The classical model had essentially accepted what it called ``reductionism.'' In that view, everything could be reduced to its components. The proper activity for physicists was to try to find the ultimate smallest particles.

The problem with reductionism, however, is illustrated in the tale of the 19th-century lecturer who, asked to explain the cosmos, told his listeners that the earth rested on the back of an elephant. Questioned further, he explained that the elephant stood on the back of a giant tortoise - which, in turn, stood on a still larger tortoise.

``And what does that one stand on?'' asked a woman in the audience.

``Tortoises, madam,'' he replied, ``on and on, nothing but tortoises.''

The story is a favorite of Princeton University professor emeritus John A. Wheeler, who, thinking of it in reverse, uses it to illustrate the futility of trying to account for matter by tracing it back to its smaller substrata. ``You work on down to smaller and smaller [particles],'' he says. ``But you never explain anything if you keep on with turtles. You've got to close the loop somewhere.''

That closing of the loop began just after the turn of the century, with Albert Einstein's first papers on special relativity. But even special relativity theory - the first major adjustment in the Newtonian world view, and one that stretched all sorts of classical assumptions - operated in a world of familiar dimensions. It explained what happened as the objects in that world approached the speed of light - how space and time were interchangeable, and how energy and mass were one. Like classical physics, relativity also had a shaping effect on mankind's world view. The public had only a hazy view of its significance - although Einstein's great statement of the equivalence of mass and energy, E=mc2, became perhaps the world's most famous equation.

`Adrift in a relativistic universe'

BUT before long the word ``relativity'' slipped into public usage. When used apart from physics, it was almost always meant to suggest that there were no absolutes - that everything was ``relative'' to the observer. The view that emerged, as British historian Paul Johnson writes in ``Modern Times: The World from the Twenties to the Eighties,'' was of ``an unguided world adrift in a relativistic universe'' - a world that had ``left its moorings in traditional law and morality.'' That view - along with popular notions of time travel that figure so strongly in contemporary science fiction - constitute the major impact of Einstein's discoveries on popular thought today.

Relativity, however, never really tackled the subatomic world. It left unanswered the central question of the structure of the atom. Did electrons simply whirl around a nucleus made of protons and neutrons, as those once-familiar diagrams of atoms suggested? Could you break up those protons? Or were they the ultimate, basic particles? How far did the tortoises go?

The search for answers took a new direction in 1925. That year the German physicist Werner Heisenberg, summering on the North Sea island of Helgoland, developed the first mathematical statement of quantum mechanics. The following winter, working independently of each other, Austrian physicist Erwin Schr"odinger and English physicist Paul Dirac devised different formulations of the same theory. In the years since these discoveries, quantum mechanics has demanded profound changes in the content of scientific thought. Yet 63 years after its discovery, it has hardly made a dent in mankind's world view. It has had nothing like the impact of either classical physics or relativity.

Why? Dr. Rubbia, who will take over as director general of CERN next year, explains part of the difficulty. ``We are dealing with things that are very far from everyday experience,'' he says. ``We are building a wall up with bricks whose intimate, fundamental identity and structure is somehow no longer evidenced.''

Across the Atlantic, in his office at the University of Texas, physicist Bryce DeWitt adds another reason for the public incomprehension: the highly mathematical nature of the subject. If the public doesn't grasp quantum mechanics, he notes, it is because ``we have to talk about [things like] vectors in the Hilbert space.''

Others list different reasons: that unlike the clockwork classical universe, the quantum world cannot be reduced to a model that is easily visualized; that the news of its discovery in the mid-1920s was buried by the Great Depression and, later, by World War II; that six decades is not enough time for a major discovery to penetrate.

On two points, however, physicists seem to agree. The first has to do with its revolutionary significance. ``When the history of this century is written,'' says Rockefeller University physicist Heinz Pagels, ``we will see that in fact it was the first human contact with the invisible world of the atom that had a greater influence on our civilization than our diplomatic or political history.''

Standing at the bottom of the UA-1 pit at CERN, Dr. Wacker makes the second point: that quantum physics is by no means a closed book, and that its finest discoveries are yet to be made.

``It's clear,'' he notes, ``that this can't be the end of the story.''

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