Details
CASIMIR, Hendrik B.G. (1909-2000). 2 Autograph letters to the physicist Samuel Goudsmit, signed ("Henk Casimir"). N.p. [Copenhagen], 27 September 1930 and Zurich, June 22, 1933. 9½ pages, 4o, in Dutch, creased where previously folded, minor marginal chipping to one sheet. With English translation. Excellent scientific letters to Goudsmit from the discoverer of the "Casimir effect" (a quantum force that pulls two metal plates, placed a short distance apart, towards one another) and the "Casimir trick" for simplifying calculations with the Dirac equation involving spin value. Both of Casimir's letters discuss spectroscopy, electron spin, matrices, etc.., as the following extract from the Sept. 1930 letter shows:
"... it has become clear to me, especially because of some remarks from [Lev Davidovich] Landau, that application of Dirac's or Pauli's theory to a light-emitting electron (although giving the right order of magnitude for the doublet splitting) just isn't very useful. It seems preferable to treat the problem by introducing a current for both orbit and spin. Thus both spin and orbit each give rise to a magnetic field. When writing down the equations one has to take into account that J is a diagonal matrix. But for coupling between s and l no assumption is made.
This converges also for s states and then gives the well-known result. One can be sure that one simply has to get the same result as with the Dirac theory, if one neglects relativity effects..."
This letter also contains Casimir's entertaining account of an automobile trip in the eastern United States, as well as news about his fellow members at the Niels Bohr Institute in Copenhagen: "Here we have Gamov[sic], Rosenfelt and Landau. Landau, a young Russian, is astoundingly smart. Grossly unmannered but not un-entertaining. Also Bohr likes him very much. . . . Of course we have again some pain-in-the-neck assignments. A Faraday lecture which has to be written is the most urgent. Tomorrow Bohr and I will escape and see if we can get it done." (2)
"... it has become clear to me, especially because of some remarks from [Lev Davidovich] Landau, that application of Dirac's or Pauli's theory to a light-emitting electron (although giving the right order of magnitude for the doublet splitting) just isn't very useful. It seems preferable to treat the problem by introducing a current for both orbit and spin. Thus both spin and orbit each give rise to a magnetic field. When writing down the equations one has to take into account that J is a diagonal matrix. But for coupling between s and l no assumption is made.
This converges also for s states and then gives the well-known result. One can be sure that one simply has to get the same result as with the Dirac theory, if one neglects relativity effects..."
This letter also contains Casimir's entertaining account of an automobile trip in the eastern United States, as well as news about his fellow members at the Niels Bohr Institute in Copenhagen: "Here we have Gamov[sic], Rosenfelt and Landau. Landau, a young Russian, is astoundingly smart. Grossly unmannered but not un-entertaining. Also Bohr likes him very much. . . . Of course we have again some pain-in-the-neck assignments. A Faraday lecture which has to be written is the most urgent. Tomorrow Bohr and I will escape and see if we can get it done." (2)