Quantum theory is unsettling. Nobel laureate Richard Feynman admitted that it "appears peculiar and mysterious to everyone--both to the novice and to the experienced physicist." Niels Bohr, one of its founders, told a young colleague, "If it does not boggle your mind, you understand nothing." Physicists have been quarreling over its interpretation since the legendary arguments between Bohr and Einstein in the 1920s. So have philosophers, who agree that it has profound implications but cannot agree on what they are. Even the man on the street has heard strange rumors about the Heisenberg Uncertainty Principle, of reality changing when we try to observe it, and of paradoxes where cats are neither alive nor dead till someone looks at them.
Quantum strangeness, as it is sometimes called, has been a boon to New Age quackery. Books such as The Tao of Physics (1975) and The Dancing Wu Li Masters (1979) popularized the idea that quantum theory has something to do with eastern mysticism. These books seem almost sober today when we hear of "quantum telepathy," "quantum ESP," and, more recently, "quantum healing," a fad spawned by Deepak Chopra's 1990 book of that name. There is a flood of such quantum flapdoodle (as the physicist Murray Gell-Mann called it). What, if anything, does it all mean? Amid all the flapdoodle, what are the serious philosophical ideas? And what of the many authors who claim that quantum theory has implications favorable to religious belief? Are they on to something, or have they been taken in by fuzzy thinking and New Age nonsense?
It all began with a puzzle called wave-particle duality. This puzzle first appeared in the study of light. Light was understood by the end of the nineteenth century to consist of waves in the electromagnetic field that fills all of space. The idea of fields goes back to Michael Faraday, who thought of magnetic and electrical forces as being caused by invisible "lines of force" stretching between objects. He envisioned space as being permeated by such force fields. In 1864, James Clerk Maxwell wrote down the complete set of equations that govern electromagnetic fields and showed that waves propagate in them, just as sound waves propagate in air.
This understanding of light is correct, but it turned out there was more to the story. Strange things began to turn up. In 1900, Max Planck found that a certain theoretical conundrum could be resolved only by assuming that the energy in light waves comes in discrete, indivisible chunks, which he called quanta. In other words, light acts in some ways like it is made up of little particles. Planck's idea seemed absurd, for a wave is something spread out and continuous, while a particle is something pointlike and discrete. How can something be both one and the other?
And yet, in 1905, Einstein found that Planck's idea was needed to explain another puzzling behavior of light, called the photoelectric effect. These developments led Louis de Broglie to make an inspired guess: If waves (such as light) can act like particles, then perhaps particles (such as electrons) can act like waves. And, indeed, this proved to be the case. It took a generation of brilliant physicists (including Bohr, Heisenberg, Schrodinger, Born, Dirac, and Pauli) to develop a mathematically consistent and coherent theory that described and made some sense out of wave-particle duality. Their quantum theory has been spectacularly successful. It has been applied to a vast range of phenomena, and hundreds of thousands of its predictions about all sorts of physical systems have been confirmed with astonishing accuracy.
Great theoretical advances in physics typically result in profound unifications of our understanding of nature. Newton's theories gave a unified account of celestial and terrestrial phenomena; Maxwell's equations unified electricity, magnetism, and optics; and the theory of relativity unified space and time. Among …