The history of quantum mechanics

 

The history of quantum mechanics

We now know that all matter is composed of atoms. Each atom is in turn made up of electrons ‘orbiting’ a nucleus consisting of protons and neutrons. Atoms are discrete. They are ‘localised’: ‘here’ or ‘there’.

But towards the end of the 19th Century, atoms were really rather controversial. In fact, it was a determination to refute the existence of atoms that led the German physicist Max Planck to study the properties and behaviour of so-called ‘black-body’ radiation.

What he found in an ‘act of desperation’ in late-1900 turned him into a committed atomist, but it took a few more years for the real significance of his discovery to sink in.

Planck had concluded that radiation is absorbed and emitted as though it is composed of discrete bits which he called quanta. In 1905, Albert Einstein went further. He speculated that the quanta are real – radiation itself comes in discrete lumps of light-energy. Today we call these lumps photons.

Is light a wave or a particle?

Einstein’s hypothesis posed a bit of a problem. There was an already well-established body of evidence in favour of a wave theory of light. The key observation is called the 'double slit experiment'.

Push light through a narrow aperture or slit and it will squeeze through, bend around at the edges and spread out beyond. It ‘diffracts’.

Cut two slits side-by-side and we get interference. Waves diffracted by the two slits produce an alternating pattern of light and dark bands called interference fringes. This kind of behaviour is not limited to light – such wave interference is easily demonstrated using water waves.