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In this way it appears that the seventeenth century, which regarded light as mere particles, and the nineteenth century, which regarded it as mere waves, were both wrong—or, if we prefer, both right. Light, and indeed radiation of all kinds, is both particles and waves at the same time. In Professor Compton’s experiments, X-radiation falls on single electrons and behaves like a shower of discrete particles; in the experiments of Laue, Bragg and others, exactly similar radiation falls on a solid crystal and behaves in all respects like a succession of waves. And it is the same throughout nature; the same radiation can simulate both particles and waves at the same time. Now it behaves like particles, now like waves; no general principle yet known can tell us what behaviour it will choose in any particular instance. Clearly we can only preserve our belief in the uniformity of nature by making the supposition that particles and waves are in essence the same thing. And this brings us to the second, and far more exciting, half of our story. The first half, which has just been told, is that radiation can appear now as waves and now as particles; the second is that electrons and protons, the fundamental units of which all matter is composed (p. 62), can also appear now as particles, and now as waves. A duality has recently been discovered in the nature of electrons and protons similar to that already known to exist in the nature of radiation; these also appear to be particles and waves at the same time.
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