Schroedinger’s cat has become a sort of diagnostic tool for testing all kinds of interpretations of quantum physics. It wasn’t the last such Gedankenexperiment, nor was it the first. It was preceded by a set of lively debates between the chief architect of the Copenhagen interpretation, Niels Bohr, and Albert Einstein, its fiercest critic.
These began in 1927, at the Fifth Conference of the International Solvay Institutes for Physics and Chemistry in Brussels. Here, Bohr, Heisenberg, and Born rolled out the elements of their recently assembled Copenhagen interpretation of quantum physics. Provocatively, Born and Heisenberg made the case that quantum theory was now a complete theory, “whose fundamental physical and mathematical assumptions are no longer susceptible of any modification.”
The 29 attendees of the Fifth Solvay Conference, including Einstein, Bohr, Heisenberg, Born, Schroedinger, Planck, Pauli, Dirac, and de Broglie.
This didn’t sit well with Einstein, who felt that the probabilities inherent to the Copenhagen interpretation implied that something must be missing. When Einstein listened to his colleagues make their case, he sat quietly. Instead he saved his first counterstrike until the conference-goers were assembled in the dining hall. Then, and again and again over the next half decade, Einstein tried to come up with hypothetical experiments that could beat the Heisenberg uncertainty principle. Doing so would disprove the duality pillar and bring the Copenhagen interpretation—and all of its probabilities—crashing down.
In each of his responses to Einstein’s challenges, Bohr would carefully work through the exact process by which the quantum entity would ultimately be detected by the macroscopic observer. This allowed him to identify, in every single case, an error in Einstein’s logic. On one occasion, Bohr even managed to turn Einstein’s own theory of relativity against him!
QUANTUM QUOTE
[T]he unambiguous account of proper quantum phenomena must, in principle, include a description of all relevant features of the experimental arrangement.
—Niels Bohr
Unfortunately, we don’t have enough space to describe the many different thought experiments that were proposed during this time, though you’ll find some very good accounts referenced in Appendix D. The important point is that Bohr successfully defended the Copenhagen interpretation against its toughest critic, who also happened to be one of the most esteemed physicists of all time. In each case, Bohr revealed just how important the process of measurement was to quantum physics. Moreover, he shed light on the fact that our view of events in the quantum realm is necessarily filtered through the macroscopic instruments we use to generate signals that are large enough for us to perceive.
While Einstein conceded that Bohr had responded to all of his challenges, he continued to reject the Copenhageners’ claim that quantum physics as they knew it was a complete description of the microscopic world. We consider Einstein’s side of the story while exploring the completeness of quantum physics.
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