Neither particle nor wave

The information about the particle behaviour and the wave behaviour of light, presented in the preceding chapter, forms an extremely peculiar combination when taken together. And yet both are true: the experiments have been done as described, with the outcome as stated. There are experiments that tell us clearly that light consists of waves; there are others that tell us, equally clearly, that light consists of particles. Somehow we must find a description of light that incorporates both kinds of behaviour.

This is a formidable undertaking, but there is one cheerful fact: in absolutely none of the experiments that have ever been done did the particle and the wave behaviours occur simultaneously. Because this is a fact that covers the entire history of physics, we feel justified in assuming that this is a general feature of Nature, and that there cannot ever be an experiment in which the wave and particle behaviours both occur at the same time. This is a generalization, for we can never expect to do all possible experiments. This generalizing hypothesis was first announced by Bohr, who had the wonderful combination of daring and insight to postulate that the observation of certain quantities excludes the simultaneous observation of certain others.