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The wave nature of matter

Question: When does a particle not behave like a particle?

Answer: When it behaves like a wave!

In the first quarter of the twentieth century physicists began to realise that particle did not always behave like particles they could behave like waves. They called this Wave-Particle Duality.

This theory suggests that there is no really basic distinction between a particle and a wave. The differences that we observe arise simply from the particular experiment that we are doing at the time.

Some important steps in the development of the wave-particle theory were as follows:

1923 discovery of the Compton effect
1923 de Broglie's matter-wave theory
1927 the diffraction of electrons and the Uncertainty Principle of Heisenberg

An interesting idea concerning waves and particles was put forward by G.I. Taylor in 1924. He reasoned that if light only behaved like a particle, then in an interference experiment such as Young's slits it would be possible to reduce the intensity to a point where only one photon was passing through the apparatus at a particular time and you would therefore expect there to be no interference - the photon could not interfere with itself. This does not happen, however - the interference pattern is always visible although very faint, suggesting some kind of wave property of the photon.

Diffraction appears to be some kind of statistical behaviour of each individual photon, and not a reaction between photons. If we attempt to determine which slit a given photon has passed through we destroy the diffraction pattern, because we have altered the nature of the experiment.

Student investigation
The Planck constant is actually a very small number but imagine that we lived in a world where the planck constant was 1.
Write an essay discussing how different this world might be.

© Keith Gibbs