It would have been awesome to watch Dragon Princes in battle (from a safe hiding place), but I’d almost rather have witnessed “The Tussles in Brussels,” the two most prominent confrontations between Albert Einstein and Niels Bohr.

The Tussles would be the Fifth (1927) and Seventh (1933) Solvay Conferences.  Each conference was to center on a particular Quantum Mechanics application (“Electrons and Photons” and “The Atomic Nucleus,” respectively).  However, the Einstein-Bohr discussions went right to the fundamentals — exactly what does a QM calculation tell us?

Einstein’s strength was in his physical intuition.  By all accounts he was a good mathematician but not a great one.  However, he was very good indeed at identifying important problems and guiding excellent mathematicians as he and they attacked those problems together.

Like Newton, Einstein was a particle guy.  He based his famous thought experiments on what his intuition told him about how particles would behave in a given situation.  That intuition and that orientation led him to paradoxes such as entanglement, the EPR Paradox, and the instantaneously collapsing spherical lightwave we discussed earlier.  Einstein was convinced that the particles QM workers think about (photons, electrons, etc.) must in fact be manifestations of some deeper, more fine-grained reality.

Bohr was six years younger than Einstein.  Both Bohr and Einstein had attained Directorship of an Institute at age 35, but Bohr’s has his name on it.  He started out as a particle guy — his first splash was a trio of papers that treated the hydrogen atom like a one-planet solar system.  But that model ran into serious difficulties for many-electron atoms so Bohr switched his allegiance from particles to Schrödinger’s wave theory.  Solve a Schrödinger equation and you can calculate statistics like average value and estimated spread around the average for a given property (position, momentum, spin, etc).

Here’s where Ludwig Wittgenstein may have come into the picture.  Wittgenstein is famous for his telegraphically opaque writing style and for the fact that he spent much of his later life disagreeing with his earlier writings.  His 1921 book, Tractatus Logico-Philosophicus (in German despite the Latin title) was a primary impetus to the Logical Positivist school of philosophy.  I’m stripping out much detail here, but the book’s long-lasting impact on QM may have come from its Proposition 7: “Whereof one cannot speak, thereof one must be silent.“

I suspect that Bohr was deeply influenced by the LP movement, which was all the rage in the mid-1920s while he was developing the Copenhagen Interpretation of QM.

An enormous literature, including quite a lot of twaddle, has grown up around the question, “Once you’ve derived the Schrödinger wave function for a given system, how do you interpret what you have?”  Bohr’s Copenhagen Interpretation was that the function can only describe relative probabilities for the results of a measurement.  It might tell you, for instance, that there’s a 50% chance that a particle will show up between here and here but only a 5% chance of finding it beyond there.

Following Logical Positivism all the way to the bank, Bohr denounced as nonsensical or even dangerously misleading any attachment of further meaning to a QM result.  He went so far as to deny the very existence of a particle prior to a measurement that detects it.  That’s serious Proposition 7 there.

I’ve read several accounts of the Solvay Conference debates between Einstein and Bohr.  All of them agree that the conversation was inconclusive but decisive.  Einstein steadfastly maintained that QM could not be a complete description of reality whilst Bohr refused to even consider anything other than inscrutable randomness beneath the statistics.  The audience consensus went to Bohr.

None of the accounts, even the very complete one that I found in George Musser’s book Spooky Action at A Distance, provide a satisfactory explanation for why Bohr’s interpretation dominates today.  Einstein described multiple situations where QM’s logic appeared to contradict itself or firmly established experimental results.  However, at each challenge Bohr deflected the argument from Einstein’s central point to argue a subsidiary issue such as whether Einstein was denying the Heisenberg Uncertainty Principle.

Albert still stood at the end of the bouts, but Niels got the spectators’ decision on points.  Did the ref make the difference?

~~ Rich Olcott