Wave Particle Duality For Beginners

I started off trying to do a post about determinism, and it's religious equivalent, predestination. But I soon found I was having to spend too many words trying to describe wave-particle duality and quantum uncertainty, so I'll try hiving that off into this post. So here we go, one of the deepest, most mysterious and least understood areas of modern physics, explained for the intelligent non-scientist in a little under 750 words! And a jolly nice picture of a t-shirt to boot.

So, what's wave-particle duality all about? Essentially it just says that things (more later about what 'things') sometimes act like a waves - spreading out, reflecting, refracting, diffracting, interfering with other waves, that sort of thing - and sometimes act like particles, travelling in straight, narrow lines, bouncing off one another, and so on, rather like billiard balls (or pool balls).

Victorian-era scientists 'knew' that light was a wave and that atoms and electrons were particles. They also 'knew' that the job of scientists was pretty much over, as all scientific principles had been discovered ... this was before electronics, before the nuclear atom, before relativity, before television. It's amazing what people 'know' sometimes. Around about a century ago several (mostly young) scientists spoilt the party by discovering that the world is a lot stranger than anyone had realised.

A guy called Max Planck threw the first spanner in the works by showing that when a body (such as the filament of a light bulb) emits light, it does so in discrete lumps, known as photons, whose energy depends only on the frequency of the light. Then Albert Einstein showed that when light was absorbed by metal, kicking out an electron in the photoelectric effect, it was also absorbed in photons. Essentially light is created as particles, travels in waves, and is absorbed as particles again. This was revolutionary and took a long time to be accepted. Nevertheless, when the experiments were carried out that is what they showed: light behaves as waves in some circumstances but as particles in others.

Particles rule okay? Well, no, not really. A few years later, Neils Bohr - who didn't actually believe in photons - showed that electrons in an atom orbit around a dense, positively charged, central nucleus, but they can only do so in certain discreet orbits. So the electron can't zoom around any old where. Years later Louis de Broglie showed that these allowed orbits could be explained if the electrons behaved like waves which were only stable if their orbit was a whole number of wavelengths. I.e. at the end of one orbit around the nucleus the wave function was exactly back where it started. He generalised this to say that all moving particles can be described as matter waves, a hypothesis which was later demonstrated to be true when electrons were fired through a diffraction grating, resulting in a wave interference pattern. So, electrons, amongst other things, behave as particles in some circumstances but as waves in others.

All very well and good, but where does determinism come into it? Well, the essence of determinism is that the laws of nature are completely predictable: if you could know the exact properties of every entity in the universe at any one time, you could predict their properties at every other time. In other words, determinism says: "everything is fixed and you can't change it".

Erwin Schrödinger and Werner Heisenberg extended de Broglie's work and made it more precise. In doing so they had to deal with a difficulty in the transition from wave-like behaviour to particle-like. A wave is spread out whereas a particle is at a single location. Somehow the wave function of a matter wave must 'collapse' into a particle at a point. They showed that the matter wave acted as a probability distribution: the actual position in which a particle appeared was random, more likely where the matter wave was biggest, less likely where it was smaller, but still essentially indeterminate.

Einstein and de Broglie both hated this randomness; Einstein famously said that "God does not play dice with the universe". The trouble is that as the years go on and as quantum theory becomes more refined and more tested, that fundamental randomness remains. The transition from wave behaviour to particle behaviour is inherently unpredictable.

Our best scientific description of the universe is that its behaviour is not predetermined; there is room for free will.