Physics Department, Trinity College Dublin.

WHAT IS QUANTUM THEORY ABOUT?

| Introduction | Energy Quantisation | The Uncertainty Principle | Particle-Wave Duality |
| Indeterminacy | Schrödinger's Cat |

Energy Quantisation

You have all seen how hot metal glows, first red, then yellow and then white as its temperature is raised. The details of this spectrum of light emitted by solid objects were not understood when Planck turned his attention to it. He found that the only way to explain the spectrum was to assume that the atoms and molecules in the material could only change energy in discrete units or 'QUANTA'.

As the material is heated, it loses energy in the form of light. Planck suggested that the colour of the light emitted is determined by the size of the quanta of energy lost by the material. The colour of the light is measured by its 'frequency', . The constant of proportionality between the energy of the quantum, E, and the frequency of the light is a new physical constant - Planck's constant.

To see just how strange this idea is by everyday standards, let's imagine an atom as a tiny solar system, with the nucleus taking the place of the sun at the centre, and electrons orbiting it like planets. (This is the model of the atom developed by Bohr and others. The electrons are held by their electrical attraction to the nucleus rather than by the force of gravity).

The electrons move in certain discrete 'allowed' orbits. The further from the nucleus an electron is, the larger its orbital energy. (It takes energy to pull an electron further away from the nucleus). In the diagram above the lower two orbits each contain their allowed quota of 2 electrons, but the third orbit is empty, and there is one electron in the outermost orbit.

If the outer electron moved down to the empty orbit, closer to the nucleus, it would emit energy in the form of light (indicated by the red arrow in the diagram below). But Planck's idea means that the electron can't simply spiral down to the lower orbit. It can't exist in the gaps between allowed orbits. So to lose energy it has to vanish from one orbit and reappear in a lower one!

Planck's constant is very tiny on an everyday scale, and when macroscopic objects such as cars accelerate their energy changes at a rate of an enormous number of tiny energy quanta per second. So it looks as though the energy change is a continuous rather than quantised.


Introduction Another facet of the strange behaviour of quantum particles is
Heisenberg's uncertainty principle .....