Interlude: The BKS Proposal
Sie haben sich heiss und innig geliebt. Helen Dukas
In January 1924, Niels Bohr, Hendrik Anton Kramers, and John Clarke Slater submitted to the Philosophical Magazine an article [B1] that contained drastic theoretical proposals concerning the interaction of light and matter. It was written after Compton's discovery, yet it rejected the photon. It was also written after Einstein and Bohr had met. This chapter on the BKS proposal serves a twofold purpose. It is a postscript to the story of the photon and a prelude to the Bohr- Einstein dialogue which will occupy us more fully later on.
I have already mentioned that Einstein was immediately and strongly impressed by Bohr's work of 1913. The two men did not yet know each other at that time. A number of years were to pass before their first encounter; meanwhile, they followed each other's published work. Also, Ehrenfest kept Einstein informed of the progress of Bohr's thinking. ' Ehrenfest tells me many details from Niels Bohr's Gedankenküche [thought kitchen]; his must be a first-rate mind, extremely critical and far-seeing, which never loses track of the grand design' [E1]. Einstein remained forever deeply respectful of Bohr's pioneering work. When he was nearly seventy, he wrote 'That this insecure and contradictory foundation [of physics in the years from 1910 to 1920] was sufficient to enable a man of Bohr's unique instinct and tact to discover the major laws of the spectral lines and of the electron shells of the atoms together with their significance for chemistry appeared to me like a miracle--and appears to me as a miracle even today. This is the highest form of musicality in the sphere of thought' [E2].
Einstein and Bohr finally met in the spring of 1920, in Berlin. At that time, they both had already been widely recognized as men of destiny who would leave their indelible marks on the physics of the twentieth century. The impact of their encounter was intense and went well beyond a meeting of minds only. Shortly after his visit, Einstein wrote to Bohr, 'Not often in life has a human being caused me such joy by his mere presence as you did' [E3]. Two days later, he wrote to Ehrenfest, ' Bohr was here, and I am as much in love with him as you are. He is