شنبه، شهریور ۰۷، ۱۳۸۸

Amazing trip

...
That puzzle was still unresolved when, in December 1925, Niels Bohr arrived in Leiden to attend the festivities for the golden jubilee of Lorentz's doctorate. Late one evening in 1946, Bohr told me in his home in Gamle Carlsberg what happened to him on that trip.

Bohr's train trip

Bohr's train to Leiden made a stop in Hamburg, where he was met by Wolfgang Pauli and Otto Stern, who had come to the station to ask him what he thought about spin. Bohr must have said that it was very interesting (his favorite way of expressing his belief that something was wrong) but he could not see how an electron moving in the electric field of the nucleus could experience the magnetic field necessary for producing fine structure. (As Uhlenbeck admitted later, "I must say in retrospect that Sem and I in our euphoria had not really appreciated [this] basic difficulty.") On his arrival in Leiden, Bohr was met at the train by Ehrenfest and Albert Einstein, who asked him what he thought about spin. Bohr must have said that it was very, very interesting but what about the magnetic field? Ehrenfest replied that Einstein had resolved that. The electron in its rest frame sees a rotating electric field; hence by elementary relativity it also sees a magnetic field. The net result is an effective spin-orbit coupling. Bohr was at once convinced. When told of the factor of 2 he expressed confidence that this problem would find a natural resolution. He urged Sem and George to write a more detailed note on their work. They did; Bohr added an approving comment.

After Leiden, Bohr traveled to Gottingen. There he was met at the station by Heisenberg and Pascual Jordan, who asked what he thought about spin. Bohr replied that it was a great advance and explained about the spin-orbit coupling. Heisenberg remarked that he had heard this remark before but that he could not remember who made it and when. (I will return to this point shortly.) On Bohr's way home the train stopped at Berlin, where he was met at the station by Pauli, who had made the trip from Hamburg for the sole purpose of asking Bohr what he now thought about spin. Bohr said it was a great advance, to which Pauli replied, "Eine neue Kopenhagener Irrlehre" (a new Copenhagen heresy). After his return home Bohr wrote to Ehrenfest that he had become "a prophet of the electron magnet gospel."

- GEORGE UHLENDECK AND THE DISCOVERY OF ELECTRON SPIN, Abraham Pais, Physics Today, Dec. 1989

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