|Physics lecture is Nov. 30|
Clayton Gearhart, Department of Physics, St. John’s University, Collegeville, Minn., will present "The Rotational Specific Heat of Molecular Hydrogen in the Old Quantum Theory," at 4 p.m. Friday, Nov. 30, in 211 Witmer Hall. Coffee and cookies will be served at 3:30 p.m. in 215 Witmer Hall.
“Astonishing successes” and “bitter disappointment”: Thus did the German physicist Fritz Reiche portray the state of quantum theory in his 1921 textbook. As Reiche’s words suggest, the “old quantum theory” — that is, quantum theory up to Heisenberg’s breakthrough in 1925 — was a mélange of inspired guesses and arbitrary assumptions, with many successes, and as many frustrating failures. It was sophisticated and wide-ranging—the impression given by the treatment of the Bohr model in modern physics texts today is thoroughly misleading.
Reiche’s words apply in miniature to the attempts to describe the decrease in the specific heat of hydrogen gas at low temperatures—among the first systems studied in the old quantum theory, and one to which Reiche made important contributions. The first measurements were published early in 1912 by Arnold Eucken in Walther Nemst’s laboratory in Berlin. The theory should have been simple — the rigid rotator, the model for a diatomic molecule, was a standard textbook problem, as it still is today.
Nernst applied a quantum theory of rotators to diatomic gases even before Eucken’s measurements were completed, and that theory figured in the discussions at the first Solvay conference — the meeting that introduced quantum theory to European physicists — late in 1911. Albert Einstein, Paul Ehrenfest, Max Planck, Edwin C. Kemble, Niels Bohr, Erwin Schrodinger, and John Van Vleck, among others, attempted theoretical descriptions of the rotational specific heat, as did Reiche himself in a widely cited 1919 paper.
But for over 15 years, despite persistent and energetic efforts, the problem proved intractable — its solution involves identical particles in ways unsuspected before modern quantum mechanics. By contrast, the old quantum theory worked fairly well to describe infrared spectra of diatomic molecules such as HCI—and in the process, made the specific heat measurements even more puzzling. Later in the 1920s, increasingly detailed measurements of electronic transitions in the spectrum of molecular hydrogen further complicated matters. But those same measurements also helped David Dennison, an American theorist, to come up with a successful theory in 1927 — and in the process, to find persuasive evidence for proton spin! Gearhart will sketch the history of this intriguing problem in early quantum theory.
-- Connie Cicha, Secretary, Physics, firstname.lastname@example.org, 7-2911