October 1927

Complementarity (physics)

Bohr gives a fuller account of complementarity at the Fifth Solvay Conference

Einstein missed Bohr's first complementarity lecture at Como. He met the interpretation face to face a month later, in Brussels.

Brussels was not the debut

During the Fifth Solvay Conference, held in Brussels from 24 to 29 October 1927, Niels Bohr repeated and expanded his recently developed account of complementarity before physicists including Albert Einstein.

It was not the principle's first public airing. Bohr had spoken about it at Como on 16 September, and the Brussels version was the more developed one.

The consulted proceedings and archival summaries do not establish a single calendar day for Bohr's Solvay presentation.

What complementarity claimed

Bohr argued that a quantum phenomenon can require mutually exclusive experimental arrangements and classical descriptions — wave and particle descriptions being the familiar example — to give a complete account of the available observations.

That is broader than the slogan that something is simply both a wave and a particle: Bohr tied the applicable description to the complete experimental arrangement. His principle is related to Heisenberg's uncertainty relation historically and conceptually, but the two are not interchangeable.

Hendrik Antoon Lorentz presided over the conference, which was officially devoted to electrons and photons.

Still under construction

Surviving records indicate that interaction with colleagues around the Solvay meeting influenced Bohr's final formulation. Archival research characterises the Como account as embryonic and the Brussels account as improved, with the published version continuing to evolve into 1928.

Einstein, who had missed Como, encountered the interpretation at Brussels and challenged the emerging orthodox reading of quantum mechanics. The meeting did not settle the interpretation or convert him.

The two continued the argument at later Solvay meetings and in later publications, including the 1935 EPR exchange.

Sources

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What the sources leave uncertain

The conference dates are secure, but the consulted proceedings and archival summaries do not establish a single calendar day for Bohr's Solvay presentation; the best-supported ISO precision is therefore 1927-10.
Historians dispute how fully developed complementarity was before Brussels. Archival research characterizes the Como account as embryonic and the Brussels account as improved, with the published formulation continuing to evolve into 1928.