Great Physicists: The Life and Times of Leading Physicists from Galileo to Hawking

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Paul Dirac has offered the opinion that his fellow theorists are guided not only by their hopes, but just as importantly, by their fears. Theoretical researchers find it hard, he says, to ignore fears that their work contains hidden, possibly disastrous, flaws; and their thoughts, influenced by this worry, are not so logical as they might be: “You might think a good research worker would review the situation quite calmly and unemotionally and with a completely logical mind, and proceed to develop whatever ideas he has in an entirely rational way. This is far from being the case. The research worker is only human and, if he has great hopes, he also has great fears…. As a result, his course of action is very much disturbed. He is not able to fix his attention on the correct line of development.”

If there was a fundamental fear threatening the development of quantum theory during its first two decades, it was the concept of wave-particle duality, demanded because light can appear to be wavelike in certain experiments and particle-like in others. Einstein was among the first to face the duality mystery. In spite of long-established experimental and theoretical evidence for light as waves, Einstein proposed a particle theory of light to explain puzzling features of the photoelectric effect. Einstein's equation E = hv for the energy E of a light particle or photon casually introduces the duality theme: the equation combines E, a property of light as a particle, with the frequency v, a property of light as a wave.

From the logical viewpoint, this was a paradox, which hardly any theoretician but Einstein had the courage to confront. How could light be two essentially different things, wave and particle, at the same time? The duality seemed to be a threat, a “fundamental blemish” that might, if pushed too far, bring the entire theoretical edifice crashing down.

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