Dandelion. Cello. Read those two words, and your brain instantly conjures a stream of associations, the most prominent of which have to do with vibrations. Our mental category of “dandelion-ness” is strongly linked to the color of light waves that vibrate about half a million billion times a second: yellow. The velvety throb of a cello has as its most obvious characteristic a relatively low musical pitch --- the note you're spontaneously imagining right now might be one whose sound vibrations repeat at a rate of a hundred times a second.
Evolution seems to have designed our two most important senses around the assumption that our environment is made of waves, whereas up until now, we've mostly taken the view that Nature can be understood by breaking her down into smaller and smaller parts, ending up with particles as her most fundamental building blocks. Does that work for light and sound? Sound waves are disturbances in air, which is made of atoms, but light, on the other hand, isn't a vibration of atoms. Light, unlike sound, can travel through a vacuum: if you're reading this by sunlight, you're taking advantage of light that had to make it through millions of miles of vacuum to get to you. Waves, then, are not just a trick that vibrating atoms can do. Waves are one of the basic phenomena of the universe. At the end of this book, we'll even see that the things we've been calling particles, such as electrons, are really waves!1