THERE'S nothing space-age about Drasko Jovanovic's laboratory. A makeshift clutch of portable buildings on a wind-swept prairie, it looks like something left over from a down-at-heel school district. On the coldest winter days, coffee spilled on the linoleum floor freezes solid. Even the video screens ranked along the walls of his homemade control room are a mismatch of styles, shapes, and sizes. But there's nothing down-home about the experiments this mustachioed Yugoslav conducts here. As he sits at a computer keyboard, his face aglow in the light from several screens, the enthusiasm is almost palpable. Once again, he's on the trail of the neutrino. What troubles Dr. Jovanovic is not that these subatomic bits of matter have no electric charge: That's what their Italian name (``the little neutral ones'') means. Nor is he particularly bothered by the fact that they have no size - that they are, in physicists' language, ``pointlike,'' lacking all dimension. He even accepts the fact that neutrinos, naturally produced by nuclear reactions on the sun, rain down onto the earth in massive amounts - and pass harmlessly right through plants, animals, people, and the entire globe, emerging untouched out the other side.

Instead, what really bothers him is his conviction that these particles - for particles they surely are - have no mass at all.

``How can it be massless,'' he muses, ``and yet have these properties?''

The ``properties'' they do have, - such as ``left-handed spin'' and a velocity equal to the speed of light - show up in the vast, elaborate experiment Jovanovic has set up here at the Fermi National Accelerator Laboratory. Whenever his experiment is running, the world's largest atom smasher (or ``particle accelerator,'' as it is properly known) blasts a five-foot-wide beam of neutrinos down an earth-covered tunnel toward his buildings.

The beam arrives in bursts, 10 billion particles every couple of seconds. He targets the bursts onto 60,000 tons of high-grade steel plates - left over, he says, from American military shipbuilding operations during World War II. Between them is sandwiched layer upon layer of sensitive detectors. The neutrinos glide harmlessly through the steel and the detectors - and through the back wall of the laboratory as well. And then, Jovanovic says with a chuckle, his arm extended eastward, they just keep going - ``through the bedrooms of West Chicago and out across Lake Michigan,'' before they vanish on their trajectory into space.

But now and then, deep within the vast, empty spaces that make up the molecules of the steel plates, something happens. A lone neutrino collides head-on with a quark - a fundamental particle inside a proton at the center of an atom. The smashup produces a bevy of new particles. And some of these leave their mark for the human world to trace - like the long-tailed muon he suddenly exclaims over, stabbing his finger at the telltale green track arcing across his computer screen. It's as close as you can come, Jovanovic explains, to ``seeing'' a subatomic particle.

Baryons, mesons, and other odd stuff

Just what are these particles? For thousands of physicists around the world, answering such questions is a life's work. On the one hand, the answer is simple. Neutrinos, along with electrons, belong to a family of elementary particles called leptons - which, along with baryons, mesons, and other oddly named bits of stuff, are thought to be the fundamental constituents of matter. They are what lies at the end of a centuries-long search for the basic, indivisible building blocks of the universe - for the ``solid, massy, hard, impenetrable, movable Particles'' that Sir Isaac Newton thought made up all matter.

On the other hand, even the word ``particle'' is misleading. It suggests that these objects, like billiard balls, bang into one another and come away unscathed. But sometimes they don't. Ram them together hard enough - with the 1 trillion electron volts of energy available in Fermilab's new superconducting Tevatron, for example - and they produce other particles.

What's especially puzzling is that one of these basic particles - the quark, three of which combine to make up the proton at the center of an atom - doesn't seem to exist all by itself. ``It's as if you said buildings are made of bricks,'' says Edward W. Kolb, a blue-jeaned and T-shirted cosmologist working at Fermilab, ``but you couldn't have a brick unless it was in a building.''

It's not simply a question of size - that the quarks are too small to see. It's that they are inescapably confined within the proton. ``The tricky thing is that they're fundamental building blocks, but you can't isolate them,'' says cosmologist Kolb.

What's more, these fundamental particles frequently behave more like waves than particles - more like ripples on a pond than BBs shot from a gun. If you're trying to envision how the universe is put together, that causes serious problems. You can imagine holding a BB, however tiny, in your hand. Glue enough of them together, and you can imagine making a brick. With enough bricks, you can make an entire material universe.

But how can you hold a wave in your hand? How can what physicists call a ``field'' - technically defined as the region or space through which a wave pattern operates - produce something as substantial as the chair in which you're sitting? Is all that we call ``matter'' really just fields?

That's the conclusion many practicing physicists are coming to. ``When I think of matter,'' says theoretical physicist Freeman Dyson of the Institute for Advanced Study at Princeton, N.J., ``I like to think mostly of fields.''

So are we walking around on fields rather than on particles?

``We are fields rather than particles,'' he says simply.

``The world is made of waves,'' agrees Nick Herbert, author of ``Quantum Reality: Beyond the New Physics,'' and a former research physicist. ``And then when you look at the world, it turns into real particles.''

When you look at the world? ``The act of looking is what turns it into particles,'' he explains. Then, with a shrug, he observes that ``this is the way the theory works. Whenever you don't look, you have to describe the world as this half-real waviness. The reason we know this is that the math tells us. If you don't treat it this way, you get the wrong answers.''

An observer-created reality?

So important is the presence of the observer in this quantum world, in fact, that some physicists speak of ``an observer-created reality.'' How can that be? The explanation lies in perhaps the most famous of the principles of quantum mechanics - the so-called Heisenberg uncertainty principle, named for its discoverer, German physicist Werner Heisenberg.

This principle is often taken to mean that the act of measurement changes the character of the thing measured. In itself, that idea is not surprising. Bird watchers recognize the importance of that concept when they hide from the birds they're studying, and chemists recognize it when they change the temperature of a test tube of hot liquid by sticking in a cold thermometer.

But the Heisenberg principle says much more. It tells us that two complementary attributes of a basic particle - such as its position and its momentum - cannot both be measured accurately. The more precisely you chart one, the fuzzier the answer will be on the other.

Taken to extremes, it says that if you know exactly where a particle is, you have no idea about its motion - or, if you measure its momentum perfectly, you can't ever know where to look for it. It says, in other words, that at the heart of our ability to measure lies an irremovable uncertainty.

That may sound esoteric. But it leads to a conception of the universe profoundly different from anything Newton conceived. For, according to quantum mechanics, an elementary particle such as an electron - one of those basic bricks out of which everything is built - has only a statistical probability of being somewhere.

The best that can be said of such particles is that, if you consider enough of them together, you can predict their location very accurately. Physicists liken the situation to that of a coin which, on a single flip, comes up randomly either heads or tails - but whose successive flips, if you take enough of them, always generate something approaching a 50-50 split between heads and tails.

A weird world

Newton, of course, knew that he couldn't measure the precise location and momentum of each particle in the universe. But for several centuries, that difficulty was thought of as merely a result of cumbersome and inaccurate measuring apparatus - as though you had sent out a lad with a yardstick to check the breadth of a cat hair.

Quantum mechanics showed that the problem was far more complex than that. What hinders measurement, it said, is not bad machinery. It's the basic nature of the world. The particles simply aren't ``there'' to be located in ways we might expect.

Welcome to the weird world of quantum physics. It's a world that is, in Nick Herbert's words, ``odder than anything we could have invented ourselves. It's as though we've broken through into an entirely new world that we didn't create. Physicists have tried, for the past 50 years, to make it seem ordinary - and failed.''

Undergirding the ordinary

Heinz R. Pagels, a Rockefeller University physicist and author of ``The Cosmic Code: Quantum Physics as the Language of Nature,'' puts it another way. ``The quantum world,'' he says, ``isn't there the way the ordinary world of our experience around us is.''

Yet on one thing the physicists agree: that whatever it is, this quantum world is what undergirds the ordinary world we see around us. If this quantum world really is so extraordinary - if reality somehow depends on whether or not you are looking, and if the highly ordered universe is constructed out of mere probabilities, and if all that seems substantial is built up of things that have no dimension of their own - what implications does that hold for mankind's world view?

To frame answers, we need first to look back at why quantum mechanics was developed. We'll see how the once-dependable clockwork-like physics of Newton began to be inadequate to explain the workings of subatomic particles.

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