OUTSIDE, the rain sifts steadily downward out of a 38-degree fog. Robert Thicksten, opening his sheepskin jacket as he steps inside the observatory, doesn't need a weather report to make his decision: There will be no ``run'' tonight, no opening of the skyward doors above one of the world's largest telescopes. Between the white, mosquelike dome of the Mt. Palomar observatory and the galaxies and supernovae of interstellar space lies the bane of astronomers: clouds. Mr. Thicksten, a one-time television repairman who is now superintendent of the four giant telescopes operated by the California Institute of Technology here, flicks on a light and shuts the door. But he keeps his coat on: The air inside is motionless, gelid with the cold. On the floor above, crouching in its 530-ton steel supports, sits the Pyrex glass mirror of the 50-year-old 200-inch telescope - the reason, he explains, for the chill inside the building.

When the dome is open, he points out, there's no window between the telescope and the heavens. So the inside and outside temperatures must match: If the mirror were even a few degrees warmer, the cool night air falling on its surface would make it contract unevenly. And even a fraction of a millimeter's buckling would distort the pictures focused by this huge concave disk.

It seems a tiny thing, that fraction of a millimeter. In fact, it's a staggeringly large distance - to a quantum physicist. Quantum calculations deal with distances as small as 10 to the -33 centimeters - a fraction in which one is divided by a one with 33 zeros.

Tiny as they are, the numbers on that scale are slowly changing humanity's world view. As questions pile up about cosmology (the study of the cosmos), they rapidly turn into questions about man's place in the universe. What brought the visible universe into being? Why is it the size it is? Where is it headed? And why is the world we inhabit, where measurements are conveniently made in meters and most things seem conveniently solid, such a rare occurrence in a universe made up almost entirely of emptiness?

These days, answers are coming from a whole new field of study known as quantum cosmology.

Why is the universe like it is? Because, say quantum cosmologists, of the events that occurred during its first few minutes of existence - just after the so-called Big Bang, when all the matter in the cosmos was created.

In those moments, the energies were immense - creating and annihilating particles that have never since been known to exist. To understand those particles and energies, you need quantum mechanics.

Such energies are approximated, however faintly, at the European Laboratory for Particle Physics (CERN) in Geneva, at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., and at a handful of other particle accelerators around the world. Already, laboratory collisions between particles have taken physicists into a range of events that would have happened at 10 to the -12 seconds after the Big Bang. The result has been the steady growth of what they affectionately call the ``particle zoo'' (see box) - the whole collection of subatomic bits of matter bearing such wonderful names as quarks, bosons, leptons, and hadrons.

But 10 to the -12 seconds is a long time - to a cosmologist. That long after the Big Bang, the universe was already showing signs of aging. Yet to match the energies that appeared earlier, some astronomers say, you'd need an accelerator stretching from earth to Alpha Centauri, the brightest star in the Centaurus constellation. And even that wouldn't come close to resolving the most commonplace and mysterious force of all: gravity.

Could utter emptiness produce a universe?

But perhaps, some physicists say, you don't need such an accelerator. You just need to understand the Big Bang - which, they say, was the original quantum physics experiment.

That's what George Helou is trying to do. In the data room of the 200-inch Dome - the only warm room in the building - he sits among video screens and digital counters. Beside him a machine-driven stylus slowly inscribes a wavy red line down the middle of a sheet of graph paper. It's his infrared experiment, he says, which because of the fog is simply ``looking at its navel'' rather than recording impulses from space.

For Dr. Helou, a young Lebanese-born, Cornell-trained researcher at Cal Tech, a quantum mechanical explanation is the only answer to the questions surrounding the Big Bang: Why did it happen, how could utter emptiness and nothingness suddenly produce a material universe, and what happened four minutes before the end of the first three minutes?

Many cosmologists view these as absurd questions - not simply because they don't know the answers, but because, if time itself was created simultaneously with matter, there is no such thing as pre-time to consider. Nevertheless, they say, the reason for the Big Bang is fairly straightforward. Given the quantum nature of matter, everything is driven by probabilities. So there was a small but nonnegligible probability that, since such an explosion could occur, it would occur. And all of a sudden, 10 to 20 billion years ago, it did. And as it did, something was slightly imbalanced. In a rare quantum fluctuation, more matter than anti-matter was created. So the annihilation of matter by anti-matter was not quite complete. The result: A visible universe appears, complete with all its delicately balanced forces.

But the process doesn't stop there. If there is a multitude of parallel universes - as posited by the ``many worlds'' interpretation of quantum mechanics which Helou accepts - there can be a multitude of additional Big Bangs. In fact, he says, ``this kind of event [a Big Bang] is happening all the time. And the only question is, what is the probability of us seeing such an event, or of such an event evolving a universe that is inhabited by an observer?''

Form in space is a construct

Then does matter take form in space only by virtue of an observer's being present? For Helou, that question involves another aspect of quantum mechanics: the difficulty of defining the act of measurement and the role of the observer.

``The whole idea of form in space is really a construct which has more to do with our senses than with the way physics works,'' he says, happy to while away a rainy evening in a discussion some physicists might dismiss as ``merely philosophical.'' ``The reason that when you look around, you see space filled is that you `see' space filled - which means that there is a specific interaction between light and these objects which your eye perceives.''

Probe your world with a photon - the quantum of electromagnetic energy, responsible for light - and you ``see'' all kinds of things: trees, rocks, water, and the fog outside. Probe it with radar, however, and you no longer ``see'' the fog. Probe it with X-rays, and the leaves on the trees also disappear.

But probe it with neutrinos, and you see nothing but a vast subatomic emptiness - inhabited by occasional quarks. If your eyes registered neutrinos rather than photons, the world, to you, would be an uncluttered void.

Matter: almost a void

Then all this matter that we see - where did it come from?

To a cosmologist, that question reshapes itself into one of the most fascinating and puzzling of queries: Why is matter as dense as it is?

At both ends of the scale, after all, the universe is not a very dense place. In interstellar space, and at the tiniest range of subatomic particles, it's an almost complete void.

``Everything's empty,'' muses Herwig Schopper, director general of CERN, looking from his conference-room window across the Swiss border at an apparently very real snow-dusted ridge of the French Alps.

``If you look at the cosmos,'' he explains, ``there are huge distances between the stars - nothing in between.'' The same is true, he says, at the quantum scale.

``So it seems the universe is organized not in a continuous way but at different levels,'' he concludes. And only at the middle level - on a human scale, where distances are measured in meters rather than light-years or subatomic distances - does matter appear to be reasonably substantial.

Yet all the matter now in existence must have been made in the Big Bang - suggesting that, in the first moments, the vast space between particles must not have been there, and that matter must have been almost incredibly dense.

``The whole solar system,'' says Fermilab director Leon M. Lederman, ``and the billions of solar systems that make the Milky Way galaxy, and the billions of galaxies - all of that stuff existed in a domain smaller than a pinhead on a pinhead in the early universe.''

``I have no trouble in squeezing all that matter into a small space,'' he continues, ``unless each object has a radius.''

But what if objects don't have radii? What if the fundamental particles are ``pointlike,'' without any spatial dimension or ``extension''? If that's the case, then the initial conditions of the material universe become more comprehensible. Compress all the empty spaces out of matter, some physicists say, and you'd get something the size of an apple. But if the fundamental particles have no radii, you'd get nothing at all - only forces.

All of which leads Princeton University physicist David Gross to wonder why the universe is as large as it is. If the universe is a self-contained system with nothing added from outside, he says, then ``the natural size for the universe would be 10 to the - -33 centimeters. Anything bigger than that is a mystery.''

A 10-dimension universe?

It certainly is a mystery, agrees David N. Schramm, a University of Chicago cosmologist. His cowboy boots resting on the coffee table, his back to the panoramic view of noon-day sky and Lake Michigan water from his downtown Chicago condominium, he offers an explanation that, for many of his nonscientific colleagues, must seem weirder than the mystery itself.

``We happen to live in three dimensions plus time,'' he says with the patience of a longtime teacher. ``But maybe the universe really has 10 dimensions,'' he adds, using a number that many physicists currently regard as a likely one.

Since we live in only four dimensions, we can't see the six others. We see, instead, the projections of the six others onto our world.

How does that projection work? Hold a book so that the light casts its shadow onto a table. The book has three dimensions (height, width, depth). But the rectangular shadow has only two (height and width). If you rotate the book so that the light hits it squarely on the spine, in fact, the shadow appears as nothing but a thin line - an almost one-dimensional object.

If three dimensions can appear to be only one, Schramm reasons, then surely 8 or 10 dimensions can appear to be only 4.

Michael S. Turner, one of Schramm's colleagues at Fermilab, explains the 10-dimensional universe in a different way. Suppose, he says, holding a tight-rolled piece of paper the size of a drinking straw between his fingers, that you were an ant living on this straw. Your world would be essentially one-dimensional: You could move forward and backward along the straw. But you wouldn't even comprehend the word ``sideways.''

Yet the straw's surface is actually two-dimensional - and, if you unrolled it, the ant would be able to move sideways as well. That would seem like a whole new world of freedom to the ant - although to us, looking down from our three-dimensional world, the ant would still seem highly restricted.

So maybe the unseen dimensions of our universe are simply rolled up within the ones we see.

It's through examples like this, he says, that quantum cosmologists are able to make sense of the behavior of particles that otherwise seems inexplicable. The 10-dimensional universe didn't arise out of any Victorian love of elaborateness, physicists say, but because it was required to understand the known data of the universe.

Related to the question of dimensions, says Mr. Turner, is another spacial issue: the definition of the center of the universe.

`Every point is the center'

The American astronomer Edwin P. Hubble, he notes, discovered 50 years ago that all the galaxies in the universe are moving away from us. ``On the face of it,'' says Turner, ``that seems to say that we're at the center of the universe. But Copernicus taught us a long time ago that the earth is not even the center of the solar system.''

``So how do you resolve it?'' he asks. ``The answer to that paradox is, `Yes indeed, we're at the center of the universe. However, every point is at the center of the universe. All points are equivalent. And if you were to go to any other galaxy, you would see all the other galaxies moving away from it.'''

The reason: The universe is still expanding from its original pinhead beginnings. And as it does, it continues to call into question some of most deeply held convictions of Newtonian mechanics.

Needed: intellectual modesty

``A fundamental assumption we make,'' says Fermilab astronomer Edward W. Kolb, ``is that the laws of physics that we study in a laboratory now are the same laws of physics that operated in the early universe 15 billion years ago, and that operate on stars a billion light-years away.'' Yet even that assumption might turn out to need modification.

All of which leads Princeton University emeritus professor John A. Wheeler to urge a modicum of intellectual modesty.

``There are billions of light-years of time ahead of us, and there are billions upon billions of places yet to be inhabited,'' says the soft-spoken Professor Wheeler, one of the most eloquent commentators on the deeper meanings of quantum mechanics. ``And this business of life and mind and what part it can play in the beginning of things we have no right to judge from our special position at this moment of lonesomeness in the universe.''

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