Who are you calling a wave?

Ever wondered what light is? In 1905, Einstein boldly made the assumption that light came in the form of particles. This caught everyone by surprise, because if there was one thing everyone was certain about, it was that light was a wave.

To be fair, Isaac Newton had thought that light was a stream of particles, but by the start of the twentieth century this idea had been discarded. Thomas Young showed in a beautifully simple experiment in 1801 that light could produce interference patterns when it passed through a pair of narrow slits. Young was pretty versatile: he was a medical doctor, brought the concept of elasticity to engineering, produced mortality tables to help insurance companies to set their premiums and made the first partial translation of Egyptian hieroglyphs.

The mingled beams from Young's slits threw shadings of light and dark onto a screen, corresponding to the addition and subtraction of the ripples in light waves, just as waves interacted on the surface of water.  No other explanation seemed possible.

Scientists of the time could not comprehend how these patterns could be developed by a stream of particles. A particle had to follow a single path from source to screen. Passing particles through a pair of slits should result in two bright areas (one behind each slit) and large swathes of darkness, not the repeating dark and light patterns that everyone could clearly see when they carried out the experiment. Similarly, light, like waves, can bend around corners, a phenomenon called diffraction, while particles are limited to bullet-like straight lines.

So how was science to cope with this new discovery that light behaved as if it were a particle when it interacted with matter? The answer was to say that light had both wave-like and particle-like properties, a solution that is given the label wave/particle duality.

What’s happening here is that scientists are building models. Not literal models like the ball-and-stick molecule models you might have played with at high school, but mental models. Simplified pictures of how something is. When we say light is a wave or it’s a particle, what we really mean is that we’re using the model of a wave or a particle to explain its behaviour. Light is like a wave or particle – but these are both big, human scale world things. In the quantum world, light is just light, but happens to have wave-like or particle-like properties.

Is if to underline the confusion, in 1924, the magnificently named Duke Louis de Broglie thought that if light particles could behave like waves, why not other quantum particles as well? He showed that electrons, normally considered particles, could also behave like waves, producing interference patterns like light through Young’s slits, and being diffracted.

For me, if I need a single model of light it's particles every time. With quantum theory it's possible to explain all the wave-like behaviour in a special kind of particle. More to the point, we have the word of Richard Feynman, something of a hero to me as he is to many physicists. Feynman said:

I want to emphasize that light comes in this form - particles. It is very important to know that light behaves like particles, especially for those of you who have gone to school, where you were probably told something about light behaving like waves. I'm telling you the way it does behave - like particles.