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John Wheeler's Allegorical
Justification of the Copenhagen
Interpretation of Quantum Theory

The Copenhagen Interpretation of Quantum Theory holds that particles do not exist as such until they are subjected of some interaction through measurement. The noted physicist John Wheeler gives a rational for the interrelationship of particles physical existence and the process of measurement. It comes from an actual incident he was involved in concerning the parlor game Twenty Questions.

Then my turn came, the fourth to be sent from the room so that Lothar Nordheim's other fifteen after-dinner guests could consult in secret and agree on a difficult work. I was locked out an unbielvably long time. On finally being readmitted, I found a smile on everyone's face, a sign of a joke or a plot. I nevertheless started my attempt to find the word.

"Is it animal?"
"No."

"Is it a mineral?"
"Yes."

"Is it green?"
"No."

"Is it white?"
"Yes."

These answers came quickly. Then the questions took longer in the answering. It was strange. All I wanted from my friends was a simple 'yes' or 'no'. Yet the one queried would think and think. "yes or no", "no or yes", before responding. Finally I felt I was getting hot on the trail, that the word might be cloud. I knew I was allowed only one chance at the final word. I ventured it:

"Is it cloud?" "Yes," came the reply, and everyone in the room burst out laughing. They explained to me that there had been no word in the room. They had agreed not to agree on a word. Each one questioned could answer as he pleased -- with one requirement that he should have a word in mind compatible with his own response and all that had gone before. Otherwise, if I challenged, he lost. The surprise version of the game of Twenty Questions was therefore as difficult for my colleagues as it was for me.

What is the symbolism of the story? The world, we once believed, exists out there indepedent of any act of observation. The electron in the atom we once considered to have at each momenta definite position and a definite momentum. I, entering, thought the room contained a definite word. In actuality the word was developed step by step through the questions I raised, as the information about the electron into being by the experiment that the experimenter chooses to make: that is, by the kind of registering equipment that he puts into place. Had I asked different questions or the same questions in a different order I would have ended up with a different word as the experimenter would have ended with a different story for the doings of the electron.

However, the power I had in bringing the particular word cloud into being was partial only. A major part of the selection lay in the 'yes' or 'no' replies of the colleagues around the room. Similarly the experimenter h as some substantial influence on what will happen to the electron by the choice of experiments he will do on it, 'questions he will put to nature'; but he knows there is a certain unpredictability about what any given one of his measurements will disclose, about what "answers nature will give", about what will happen when 'God plays dice'. This comparison between the world of quantum observation and the surprise version of the game of Twenty Questions misses much but it makes the central point. In the game, no word is a word until that word is promoted reality by the choice of questions asked and answers given. In the real world of quantum physics, no elementary phenomenon is a phenomenon until it is a recorded phenomenon.

Source:

P.C.W. Davies and J.R. Brown, The ghost in the atom, Cambridge University Press, 1986.

(To be continued.)

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