Eighty Years of Quantum Mechanics

By Don Howard

A high point among the Forum events at the April APS meeting was the session titled “80 Years of Quantum Mechanics: A New International Project,” reporting on the work of a major new collaborative project on the history of quantum mechanics. With a distinguished advisory board, it includes a wide array of participating institutions such as the Einstein Papers Project, the Niels Bohr Archive, the Perimeter Institute, and the Chinese Academy of Sciences. This project is led by six primary institutions: the Max Planck Institute for History of Science and the Fritz Haber Institute (both in Berlin), The Johns Hopkins University, University of Notre Dame, University of Minnesota, and University of Pittsburgh. The overall project goal is to develop a deeper understanding of the genesis and development of quantum physics; it will be achieved by sponsoring individual and collaborative scholarship; through organizing conferences, workshops, and symposia; and by the design and establishment of new, mainly online, research tools and information sources. Two major international conferences have occurred to date, one in Berlin in summer of 2007 and one in Utrecht this past July. The third such conference will be hosted by the University of Minnesota in 2010.

The April APS session featured presentations by two of the project directors, Michel Janssen of the University of Minnesota, and Christoph Lehner from the Max Planck Institute for History of Science, each reporting on major research initiatives it has sponsored, and one by Alexei Kojevnikov of the University of British Columbia.

Janssen spoke on “Van Vleck and Slater: Two Americans on the Road to Matrix Mechanics.” These theorists played a prominent role in the development of American physics starting in the mid-1920s. Not well appreciated is the role they played early that decade in recasting classical dispersion theory into a form adapted to the new quantum theory. Van Vleck and Max Born provided the first explicit derivation of Kramers’ new dispersion formula employing the (Born) correspondence rule, while Slater introduced the idea of a virtual oscillator between each two electron orbits. Also underappreciated is how this work stimulated the genesis of matrix mechanics, for Slater’s oscillators survived the demise of the Bohr-Kramers-Slater theory and became associated with the transition amplitudes that form the matrix elements in Heisenberg’s matrix mechanics.

Under the title “Creative Confusion: Quantum Theory on the Way to Wave Mechanics,” Lehner spoke about complementary work on the genesis of Schrödinger’s wave mechanics being carried out in Berlin. Interesting and novel in this work is the emphasis on the role of continuing worries after 1910 about the proper physical interpretation of particle indistinguishability in quantum statistics. This problem engaged the attention of Debye, Ehrenfest and Einstein, among others. But it was Schrödinger who achieved the crucial insight in the summer of 1925 during the immediate run-up to the development of wave mechanics—an insight reflected in the fact that N-particle Schrödinger wave functions live not in physical space but in 3N-dimensional configuration space.

Finally Kojevnikov spoke about “‘Knabenphysik’: The Birth of Quantum Mechanics from a Postdoctoral Viewpoint.” While the concept of a “postdoc” hadn’t really crystallized in a formal sense by the mid-1920s, the comparative professional youth of the founders of the new quantum mechanics is a striking sociological fact. Of the 80 authors of the 200 most important papers published from mid-1925 through early 1927, the majority were under 30 years of age, and that subgroup authored 65 percent of the papers. Moreover, 60 percent of these authors had received their PhDs after 1920. Heisenberg is typical of these young physicists. Their marginal professional status raises important questions about intellectual independence, and restrictions on where they could use their funding helps explain why Bohr’s institute in Copenhagen became such an important center for research in the field.