The following first appeared in Chemical Heritage 20, no. 1 (Spring 2002), p. 38, and is reprinted here with permission.

A Magnetic Correction?

One of the most important intellectual transformations in the history of modern physics derives from the introduction of the quantum theory in 1900 by the German physicist Max Planck. Much of what scientists know about subatomic behavior originated in Planck's early work and the sophisticated mathematical formalism of quantum mechanics that appeared 25 years later. The increasingly complex picture of nature that these new tools provided was the subject of a talk recently given by Raji Heyrovska. A great deal has been written about the history of quantum mechanics and its impact on physics and other disciplines. Some critics, Heyrovska noted, have claimed that the theory is not a realistic explanation of nature, being just too complicated and obscure. Hoping to have fresh insight into its history, Heyrovska looked back at one of the pivotal episodes in the annals of quantum theory: the studies of atomic hydrogen carried out by the Danish physicist Niels Bohr before World War I. A rigorous examination of the work of Joseph Larmor, an Irish-born theoretical physicist at Cambridge University, she argued, would shed new light on the historical significance of Bohr's work. Hydrogen is the simplest atom, consisting of one proton and an orbiting electron. In a series of seminal papers published in 1913, Bohr merged Max Planck's quantum hypothesis and classical electrodynamics to explain the spectral emission and absorption lines produced by atomic hydrogen. In doing so, Bohr had to make several ad hoc assumptions. In his explanation of the spectral behavior of hydrogen, Bohr adhered to the fundamental tenet of physics "Energy must be conserved in nature" by quantizing the angular momentum of the entire system. According to Heryrovska, Bohr did not attempt to explain his reasoning behind this theoretical construct. Neither did the physicists who used Bohr's results to develop quantum mechanics in the 1920s. Bohr only considered the electrical and mechanical properties of the atom. "The electron and the proton," Heyrovska observed, "have magnetic moments as well." Drawing on Larmor's earlier investigations of magnetic theory and the dynamics of the electron, Heyrovska argued that consideration of magnetic effects helps explain what Bohr missed when he quantized the angular momentum of the hydrogen atom. Though more research on this subject is needed, she concluded that "Larmor's . . . work throw[s] new light on the question of the energy of atomic hydrogen."

Thomas Lassman
Beckman Center