Everyone knows what a wave is. It is a common feature associated with water. Just as waves can be caused by a duck moving over the surface of a pond, so a charged particle, an electron for example, can cause an electromagnetic wave, when it moves through space. The oscillatory motion of the electron disturbs the electric and magnetic fields, causing waves to spread out continuously, like the ripples on the pond. Of course, the analogy is only approximate. There is a fundamental difference between a wave on water and an electromagnetic wave. The latter does not require a continuous medium through which to travel, like water. An electromagnetic oscillation is a periodical disturbance that propagates itself through the electrical structure of matter. However, the comparison may help to make the idea clearer.

The fact that we cannot see these waves does not mean that their presence cannot be detected even in everyday life. We have direct experience of light waves and radio waves, and even X-rays. The only differences between them are their frequency. We know that a wave on water will cause a floating object to bob up and down faster or slower, depending on the intensity of the wave – the ripples caused by the duck, as compared to those provoked by a speedboat. Similarly, the oscillations of the electrons will be proportionate to the intensity of the light wave.

The equations of Maxwell, backed up by the experiments of Hertz and others, provided powerful evidence to support the theory that light consisted of waves, which were electromagnetic in character. However, at the turn of the century, evidence was accumulating which suggested that this theory was wrong. In 1900 Max Planck had shown that the classical wave theory made predictions that were not verified in practice. He suggested that light came in discrete particles or “packets” (quanta). The situation was complicated by the fact that different experiments proved different things. It could be shown that an electron was a particle by letting it strike a fluorescent screen and observing the resulting scintillations; or by watching the tracks made by electrons in a cloud chamber; or by the tiny spot that appeared on a developed photographer’s plate. On the other hand, if two holes are made in a screen, and electrons were allowed to flood in from a single source, they caused an interference pattern, which indicated the presence of a wave.

The most peculiar result of all, however, was obtained in the celebrated two-slot experiment, in which a single electron is fired at a screen containing two slots and a photographer’s plate behind it. Which of the two holes did the electron pass through? The interference pattern on the plate is quite clearly a two-hole pattern. This proves that the electron must have gone through both holes, and then set up an interference pattern. This is against all the laws of common sense, but it has been shown to be irrefutable. The electron behaves both like a particle and a wave. It is in two (or more than two) places at once, and in several states of motion at once! However, as Banesh Hoffmann comments:

Let us not imagine that scientists accepted these new ideas with cries of joy. They fought them and resisted them as much as they could, inventing all sorts of traps and alternative hypotheses in vain attempts to escape them. But the glaring paradoxes were there as early as 1905 in the case of light, and even earlier, and no one had the courage or wit to resolve them until the advent of the new quantum mechanics. The new ideas are so difficult to accept because we still instinctively strive to picture them in terms of the old-fashioned particle, despite Heisenberg’s indeterminacy principle. We still shrink from visualising an electron as something which, having motion, may have no position, and having position, may have no such thing as motion or rest.[1]

Here we see the negation of the negation at work. At first sight, we seem to have come full circle. Newton’s particle theory of light was negated by Maxwell’s wave theory. This, in turn, was negated by the new particle theory, advocated by Planck and Einstein. Yet this does not mean going back to the old Newtonian theory, but a qualitative leap forward, involving a genuine revolution in science. All of science had to be overhauled, including Newton’s law of gravitation.

This revolution did not invalidate Maxwell’s equations, which still remain valid for a vast field of operations. It merely showed that, beyond certain limits, the ideas of classical physics no longer apply. The phenomena of the world of subatomic particles cannot be understood by the methods of classical mechanics. Here the ideas of quantum mechanics and relativity come into play. For most of the present century, physics has been dominated by the theory of relativity and quantum mechanics that in the beginning were rejected out of hand by the scientific establishment, which clung tenaciously to the old views. There is an important lesson here. Any attempt to impose a “final solution” to our view of the universe is doomed to fail.

  1. Hoffmann, B. The Strange Story of the Quantum, p. 147.