‘Reality’ in modern physics

The Copenhagen interpretation According to modern physics, the world is made up from small bits of energy, called ‘quanta’. Each object in the world is a quantum system, made up of individual quanta acting together. In effect, this is like saying that the world and its objects are made up of interacting particles, except that quanta are not quite particles. The trouble is that quanta sometimes behave like particles, and they sometimes behave like waves. Broadly speaking, when we consider a quantum system evolving on its own, we describe its behaviour as though it were a system of waves. But, if we consider a quantum system interacting with something else, the interaction is described as though the quantum system were made up of uncertain particles. For example, if we consider a beam of light, it travels through space as though it were made up from waves. It gets bent or ‘refracted’ when it passes through regions of space where its speed changes; and this refraction gives rise to the colours of the rainbow, as different wavelengths of light get differently bent. But, if the same beam of light strikes the surface of a photo-electric metal, this light now behaves like a stream of particles, which strike against orbiting electrons in the metal atoms. Some electrons are thus knocked out of orbit, right out of the metal surface. The results are a build-up of electrical charge and a flow of electric current. This dual nature of quanta (acting sometimes like particles, sometimes like waves) has some strange implications. As a further example, suppose that a screen with two neighbouring slits is placed in between a source of light and a photographic plate. An illustration is given in figure 1 (overleaf). Next, suppose that just one quantum of light travels from the source, through the slits, to the photographic plate. This single quantum of light, which is called a ‘photon’, travels like a wave. From its source, the wave