Princeton, N.J. — OVER lunch at the Princeton University Faculty Club, one of the nation's most respected younger physicists explains in four words the real difficulty with quantum physics. ``Nobody truly understands it,'' says David Gross. ``The basic philosophical or metaphysical issues that quantum mechanics raised haven't been solved,'' Professor Gross notes. How worthwhile is the pursuit of such issues?
The physics community is divided. Some go so far as to assert that philosophical questions are essentially irrelevant to today's physics.
``The old physicists [in the early decades of this century] were steeped in a tradition of philosophy, and they would always interpret what they did in some profoundly philosophical language,'' says Nobel laureate Sheldon Glashow, a theoretical physicist at Harvard University. These days, he adds, such ``philosophical meanderings are not the point any longer.''
But others, like Princeton University physicist John Archibald Wheeler, constantly press toward broader significance. Behind the ``strange'' world of the quantum, he wrote in a 1986 article , ``is surely an idea so simple, so beautiful, so compelling that when - in a decade, a century, or a millennium - we grasp it, we will all say to each other, how could it have been otherwise? How could we have been so stupid for so long?''
How are the fundamental ideas of quantum mechanics to be interpreted? Here, too, respectable physicists hold widely divergent views. Among them:
The Copenhagen interpretation
Danish physicist Niels Bohr, one of the founders of quantum mechanics, argued that there was no such thing as a deep reality. ``There is no quantum world,'' he wrote. ``There is only an abstract quantum description.''
The only way that an objective world ever seems to appear, he held, is through observation: Without an observer, nothing can be thought of as real.
Why is this so? Because, according to this interpretation, the universe is not (as Newtonian physics says) a vast ticking clock where all action is predetermined.
Instead, it's a collection of probabilities.
An electron may be either here or there. But until someone looks and makes a measurement, it cannot be said to be in either place. It exists in all possible states at once - or, in technical language, in a superposition of a number of states. Only when a measurement is actually performed does the electron declare itself.
``In the measurement process,'' says John Schwarz of the California Institute of Technology, ``you've forced it into a particular choice of what previously existed as a superposition of different possibilities.''
Without measurement, no choice has been made. And without making that choice, there is no deep reality. The many-worlds interpretation
Those who feel uneasy with a lack of ``deep reality'' can turn to its opposite: that there are an endless number of realities, existing in an endless number of parallel universes. This theory posits that, every time a probability is reduced to a certainty by means of a measurement, the universe ``branches.'' In other words, it divides into as many other universes as are needed to accommodate all the other possible outcomes of that measurement. Put simply, everything that can possibly happen, happens - though not in your own universe.
``You create your own reality by the act of measurement,'' says University of Texas professor Bryce DeWitt, one of the foremost proponents of this interpretation. ``But there are copies of you in all the other worlds that have created alternate realities.''
It's a view that has many proponents among practicing physicists - but that leaves many others cold. John Ellis, from his paper-cluttered office at the European Laboratory for Particle Physics (CERN) in Geneva, describes it as ``so baroque or rococo as to be ridiculous.''
Professor DeWitt, however, compares the nay-sayers to those who pooh-poohed Copernicus' theory of the motion of the earth on the grounds that nobody could feel it moving. The fact that we are unaware of any branching process, he says, means nothing more than that we inhabit only one of the possible universes. He concedes, however, that the theory is difficult to verify.
The undivided wholeness of reality
One way to resolve the difficulties of either an observer-created reality or a multitude of universes is to propose a universe in which there is no distinction between the observer and the observed. This interpretation, which makes consciousness a central feature of the universe, is a favorite of those who claim that modern physics supports the views of Eastern mysticism. Yet even so nonmystical a theoretical physicist as Freeman Dyson sees the attractiveness of this view. ``You won't really understand quantum mechanics deeply,'' he says, ``unless you also understand the nature of mind.''
One leading proponent of this interpretation, British physicist David Bohm, sees it as an antidote for the age-old fascination that scientists have for breaking up the universe into tiny, analyzable pieces. If man thinks of the universe as ``constituted of independent fragments,'' he writes in ``Wholeness and the Implicate Order,'' ``then that is how his mind will tend to operate.'' If, however, he dissolves the barrier between the observer and the observed - if ``he can include everything coherently and harmoniously in an overall whole that is undivided, unbroken, and without a border'' - then the mind will ``move in a similar way.''
CERN physicist John Bell puts it another way. ``The fundamental known fact is consciousness - I think, therefore I am. And at some point the laws of science will have to incorporate that.''