Entanglement

BOHR-EINSTEIN DEBATE

Quantum mechanics is probabilistic. While it’s mathematical framework is unquestionably rock-solid, its implications can be paradoxical. Einstein was dissatisfied not with the mathematical framework of quantum theory, but with its philosophical implications. Einstein believed that any physical theory which is devoid of an observer-independent reality “denied the most basic goal of rational science,” an attitude that led him to his famous debates with Niels Bohr. We now know that those debates were much more than just a mere sideshow in physics.The fifth Solvay Conference was held in October 1927. This is where the famous debates between Einstein and Bohr on the interpretation of quantum mechanics began.

solvay-conference
Figure 5: The Solvay Conference, founded by the Belgian industrialist Ernest Solvay in 1912, was where the world’s most notable physicists met to discuss the newly formulated quantum theory. 17 of the 29 attendees were already or later became Nobel Prize winners.

Warner Heisenberg had the following recollection of the 1930 Solvay Conference:

The discussions were soon focused upon a dual between Einstein and Bohr on the question as to what extent atomic theory in its present form could be considered to be the final solution of the difficulties which had been discussed for several decades. We generally met already at breakfast in the hotel, and Einstein began to describe an ideal [Gedanken] experiment in which he thought the inner contradictions of the Copenhagen interpretation were especially clearly visible. Einstein, Bohr, and I walked together from the hotel to the conference building, and I listened to the lively discussion between those two people whose philosophical attitudes were so different, and from time to time I added a remark on the structure of the mathematical formalism. During the meeting and particularly in the pauses we younger people, mostly Pauli and I, tried to analyze Einstein’s experiment, and at lunchtime the discussion between Bohr and others from Copenhagen. Bohr had usually finished the complete analysis of the ideal experiment by late afternoon and show it to Einstein at the supper table. Einstein had no good objection to this analysis, but in his heart he was not convinced.

Einstein instinctively believed in the “shortcomings” of quantum theory. But he was not able to identify exactly its inadequacies. The consensus view today is that Bohr had prevailed in these debates. But the quantum mechanics fashioned through the works of Planck, Bohr, Heisenberg, Schrodinger, Born, Dirac, Pauli, and many others—last but not the least, Einstein himself—has been found to endure more than 80 years of scrutiny by experiments. Not once was its scope found inadequate “having encompassed numerous previously inexplicable phenomena and resulting in many extraordinary predictions”.

It was during the 1933 Solvay conference that Einstein first sowed the seed that would later become the famous EPR argument, his bomb-shell against quantum theory (we will talk about the EPR paradox in the next page 12). Bohr was dumbfounded. His rebuttal came weeks later in the form of a paper in Physical Review where he refuted the EPR results, claiming an ambiguity in the assumptions made by EPR regarding the criterion of physical reality “contains an ambiguity as regards the meaning of the expression ‘without in any way disturbing a system’.”

Regarding Einstein’s position, the British mathematician and physicist Sir Roger Penrose asked “Can it really be true that Einstein, in any significant sense, was profoundly ‘wrong’ as the followers of Bohr might maintain?” He later adds “I do not believe so. I would, myself, side strongly with Einstein in his belief in submicroscopic reality, and with his conviction that present-day quantum mechanics is fundamentally incomplete.”

So, Einstein’s challenge had a profound impact as it encouraged next generation of physicists such as David Bohm, John Bell, and Hugh Everett to probe and reevaluate Bohr’s Copenhagen Interpretation. It inspired Bell’s theorem and the subsequent testing of Bell’s inequalities. It spawned brand-new research into areas such as quantum information theory, quantum cryptography, quantum teleportation, and quantum computing. Those stories and much more will follow in the subsequent pages.

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