Bohr’s reply to EPR (Part I)

80 years ago today, on October 15, 1935, Niels Bohr’s reply to “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?” was published in Physical Review.

It is a deeply puzzling document.

One puzzle is why it exists at all.  EPR’s argument is addressed to physicists who regard quantum mechanics as yielding, in principle, a complete description of physical reality.  That’s not Bohr.  For Bohr, QM doesn’t provide a complete description of physical reality because it doesn’t provide a description of physical reality at all.  It was an integral part of Bohr’s philosophy of quantum mechanics that any description of reality had to be in classical terms.  This meant, for instance, that, because, classically, electrons are particles and light is a wave, any talk of matter waves or  light quanta is only a symbolic expedient, not to be taken literally (see, e.g., Bohr 1929, p. 17). The lesson of quantum mechanics, according to Bohr, it that we must give up the quest for a complete description of reality, and settle for partial descriptions in terms of complementary classical concepts.  EPR conclude, in their last paragraph, that the “wave function does not provide a complete description of the physical reality.”  This is something that Bohr takes for granted; for him wave-functions are not descriptions of physical reality at all.

Another source of puzzlement is why, if a reply were needed, a single sentence would not have sufficed.

In his contribution to the volume, Albert Einstein: Philosopher-Scientist, Bohr wrote, in connection with a precursor to the EPR argument,

In my opinion, there could be no other way to deem a logically consistent mathematical formalism as inadequate than by demonstrating the departure of its consequences from experience or by proving that its predictions did not exhaust the possibilities of observation, and Einstein’s argument could be directed to neither of these ends (p. 229).

The same goes for the EPR argument. EPR did not attempt to show that the mathematical formalism of QM is inconsistent.  They did not attempt to show that its predictions depart from what is observed, and they did not attempt to show that there are predictions that can be made about the results of observation that go beyond what one can get from QM.   That is, they did not attempt to show that QM was inadequate, in any sense recognized by Bohr. Why, then, would the above-quoted sentence not suffice as a reply to EPR?

Here’s my conjecture about what troubled Bohr about the EPR paper.  Though he takes for granted what they strive to argue for, that QM cannot yield a complete description of physical reality, where he departs from EPR is in the last line of their paper.  They write, in conclusion,

While we have thus shown that the wave function does not provide a complete description of the physical reality, we left open the question of whether or not such a description exists. We believe, however, that such a theory is possible.

That’s where Bohr and EPR part ways.  Bohr accepts the conclusion that EPR argue for, that QM cannot yield a complete description of physical reality; what he does not accept is the suggestion (not argued for by EPR) that a more complete description is possible.

References

Bohr, Niels (1929).  Introductory Survey.  In Atomic Theory and the Description of Nature (Cambridge University Press, 1934), 

(1935). Can Quantum-Mechanical Description of Physical Reality Be Considered Complete? Physical Review 48, 696-702.

——— (1949).  Discussions with Einstein on Epistemological Problems in Atomic Physics, in P.A. Schilpp, ed., Albert Einstein: Philosopher-Scientist (Chicago: Open Court Press), 199–241.

Einstein, Albert, Boris Podolsky, and Nathan Rosen (1935). Can Quantum-Mechanical Description of Physical Reality Be Considered Complete? Physical Review 47, 777-780.