3.9   Where this leaves us

• Looks ahead to how what we have learned will open the door to quantum theory.

So now we have what we set out to find, when we started our adventure with special relativity: an equation that relates the energy of a photon to a particle property: the momentum that it rather surprisingly possesses. This will provide the key link to wave particle duality, since we can also relate photon energy to its wave properties, such as the wavelength of the light. Putting the two together is going to be our jumping-off point for exploring the matter waves we hypothesised in Chapter 1. Before we take that great leap, though, we need to shore up our understanding of waves, so that’s where we’re going next. We’ll be back, in Chapter 5, to make use of the key results from this section and, in the mean time, you can dazzle people at parties by showing them how to derive the most famous equation in science.