As God's dice fall; was Einstein wrong and Bohr right? Experiment goes against the EPR paradox - Albert Einstein and Niels Bohr

Science News,  Jan 11, 1986  by Dietrick E. Thomsen

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As God's Dice Fall

"I can't believe that. That's much too concrete to be real." So Martin Klein of Yale University quotes Niels Bohr. Quantum theory, of which Bohr was one of the main progenitors, is anything but too concrete. This attitude of Bohr's may reflect both what he saw in quantum theory and what he gave to it, and it could be a basis for the famous controversy between him and Albert Einstein that lasted more than three decades.

In their lifetimes neither made the other budge. They died good friends but conceptually unreconciled. Their colleagues, pupils and followers have ramified and continued the argument. Many of them and others gathered recently at the American Academy of Arts and Sciences in Cambridge, Mass., for "A Symposium Commemorating the Centennial of Niels Bohr." A series of experiments done near Paris has put a definite advantage in Bohr's court with respect to a very important part of the argument, the challenge historically known as the Einstein-Podolsky-Rosen (EPR) paradox.

Einstein is famous for remarks about God not throwing dice. These epigrams led to a general belief that his disagreement with problems of determinism and causality, an unhappiness with the uncertain, statistical quality of quantum mechanical predictions. Not so, said Einstein himself (in his correspondence with Max Born, cited by N. David Mermin of Cornell University in Itahaca, N.Y., in the April 1985 PHYSICS TODAY). The most basic unhappiness, prior to Einstein's admitted dislike of the statistical aspects, was over reality, the reality of physical attributes and properties.

Quantum theory comes with built-in ontological difficulties. Contradictory states of being are linked together: An object seems to be both a wave and a particle. Certain pairs of properties of objects, such as position and momentum, are linked by an uncertainty principle that says the better you know one of them the worse you know the other.

These dualities, ambiguities and uncertainties reopened the question of what is real, an issue which had long been decided in classical physics (by a more or less Aristotelian consensus). In classical physics a wave is a wave; a particle is a particle. Position and momentum have precise meanings and values, and they are quite independent of one another and of any observer. They exist objectively.

This doesn't seem to be so in quantum mechanics. The uncertainties and the linkages seem to damage or destroy the independence and objectivity of these attributes. Yet, when an experimenter measures a position or a momentum, the datum comes up such and so with no apparent difference from a measurement in classical physics. It seems quite real.

Where do precision and actuality enter? It seems to be somewhare connected with the act of measurement. And so arises the long and agonized debate over the effect of an act of measurement on reality in quantum mechanics, a question that simply doesn't exist in classical physics. A variety of positions have been taken and are hotly argued, but the one associated with Bohr's Copenhagen school goes roughly like this: The act of measurement has a very important effect on the reality of things. The physical attributes in question (and some have gone so far as to say the objects themselves) are at most potentially real. The act of measurement makes them actual.

Einstein could not put up with any of this. He insisted that objects must have physical attributes that are always actual and real, quite independently of any observer or act of measurement. Quantum mechanics has all these uncertainties because it is an incomplete theory. It does not tell enough because it does not know enough. There are aspects of the situation that we do not see, the famous "hidden variables." If we could know these hidden variables, the problems would drop away, and the quantum world would reveal itself to be as precise and objective as the classical world.

Bohr's response to this was that quantum mechanics is all the theory we are going to get, and we had better content ourselves with dealing on its terms.

Instead of looking for hidden variables directly--how do you look for something when you don't know what it is you are looking for?--Einstein and his followers devised challenges for quantum theory by which they hoped to drive it into paradox on its own terms. One problem here is that you have to be careful of your paradox. Quantum theory contains built-in paradoxes, which its supporters tend to accept as part of nature depending on their philosophical predilections, and if you present them with a certain paradox, they may say, "So what?"

In 1935, Einstein, with Boris Podolsky and Nathan Rosen, published a description of a hypothetical situation they thought would confound quantum mechanics. It is known as the Einstein-Podolsky-Rosen paradox. Now, on its 50th anniversary, the EPR paradox seems finally to have been "refuted," according to Mermin.

The EPR paradox takes off from the phenomenon of correlation. Suppose an atom emits two photons of light in a single process and they go off in opposite directions. These two photons are correlated with each other by the terms of their origin. Let's suppose their polarizations are opposite: If you measure the polarization of photon A to be left at any instant, photon B's has to be right at the same instant.