137 - Arthur I. Miller [54]
Bohr’s complementarity and beyond
Bohr, however, was furious. He refused to let Heisenberg publish his paper on the subject, saying that Heisenberg had not provided any firm foundation for his argument. Furthermore, Heisenberg had based the argument entirely on the assumption that light and electrons behaved like particles.
Bohr insisted that electrons and light be understood as both wave and particle, even though this could not be imagined. One could visualize electrons and light as either a wave or a particle so long as one remembered the restrictions required by quantum mechanics, among them Heisenberg’s uncertainty principle understood within the larger context of waves and particles.
This meant that electrons in experiments could exhibit one aspect or the other, but not both at once. If one experimented on an electron as if it were a wave, that was what it would be for the duration of the experiment, and similarly if one treated it as a particle. Bohr called this “complementarity.”
Bohr was convinced that complementarity was relevant not only to physics but also to psychology and to life itself. Its basic idea, he wrote, “bears a deep-going analogy to the general difficulty in the formation of human ideas, inherent in the distinction between subject and object.” As in the Chinese concept of yin and yang, complementary pairs of concepts defined reality. There is nothing paradoxical about an electron having the characteristics of both a wave and a particle until an experiment is performed on it. It dawned on Bohr that in the weird quantum world there need not be only yes and no, an electron need not actually be either particle or wave. There could be in-betweens as well as ambiguities. An electron’s wave and particle aspects complement each other, and their totality makes up what the electron is. Thus the electron is made up of complementary pairs—wave and particle, and position and momentum. Similarly it is the tension between complementary pairs—love and hate, life and death, light and darkness—that shapes our everyday existence.
Bohr sent the manuscript of his article on complementarity to Pauli for corrections and critical remarks. Pauli replied immediately. Apart from certain comments on details, he entirely agreed with Bohr’s thesis.
Only the more philosophically inclined scientists took complementarity seriously. Pauli was one. He began to look to complementarity as another way to study consciousness as in the various ways of “knowing” practiced in the East and West. He was growing more and more interested in the conscious and the unconscious, the rational and the irrational, and in how physics could be used to understand these complementarities. He was beginning to suspect that this was to be his life’s work. The only problem was how to approach it.
Paul Dirac and quantum electrodynamics
The previous autumn the eccentric twenty-five-year-old English physicist Paul Dirac had visited the Bohr Institute. Dirac had already made important contributions to atomic physics and was eager to rub shoulders with other physicists of his generation whose papers he had studied in detail, Heisenberg and Pauli among them.
Dirac had been privy to the intense conversations between Bohr and Heisenberg on the issue of whether light and matter could be both wave and particle. In 1927 he was able to provide the vital clarification through a mathematical method he had developed for moving between the two and thus brought about “complete harmony between the wave and light quantum descriptions.” Dirac’s mathematical method ultimately concerned the way in which electrons and light interact. It formed the basis for a whole new subject, which scientists dubbed quantum electrodynamics. Pauli and Heisenberg worked enthusiastically to develop this new field.
Dirac’s equation
The following year Dirac came up with a crucial equation—the Dirac equation. It described how electrons interacted with light and also agreed with relativity theory. The equations in Heisenberg’s and Schrödinger’s theories did not agree with relativity,