David Peat

8 From Certainty to UncertaintyComplementarityJust as relativity taught that clocks can run at different rates, lengthscan contract, and twins on different journeys age at different rates, sotoo quantum theory brought with it a number of curious and bizarrenew concepts. One is called wave-particle duality. In some situationsan electron can only be understood if it is behaving like a wave delocalizedover all space. In other situations an electron is detected as a particleconfined within a tiny region of space. But how can something beeverywhere and at the same time also be located at a unique point inspace?Niels Bohr elevated duality to a universal principle he termed“complementarity.” A single description “this is a wave” or “this is aparticle,” he argued, is never enough to exhaust the richness of a quantumsystem. Quantum systems demand the overlapping of severalcomplementary descriptions that when taken together appear paradoxicaland even contradictory. Quantum theory was opening the doorto a new type of logic about the world.Bohr believed that complementarity was far more general than justa description of the nature of electrons. Complementarity, he felt, wasbasic to human consciousness and to the way the mind works. Untilthe twentieth century, science had dealt in the certainties of Aristotelianlogic: “A thing is either A or not-A.” Now it was entering a world inwhich something can be “both A and not-A.” Rather than creating exhaustivedescriptions of the world or drawing a single map that correspondsin all its features to the external world, science was having toproduce a series of maps showing different features, maps that neverquite overlap.Chance and the Irrational in NatureIf complementarity shook our naive belief in the uniqueness of scientificphysical objects, certainty was to receive yet another shock in theform of the new role taken by chance. Think, for example, of MarieCurie’s discovery of radium. This element is radioactive, which means