Radio Spectrum 13 Figure 2.3. Constructive and destructive interference Surfing on What? Waves as we know them have to travel through something; sound waves go through the air, water waves through the sea. But what do electromagnetic waves move through? This question greatly bothered scientists at the end of the nineteenth century. Eventually, they invented a medium called the luminiferous aether, which they said filled the Universe. Trouble was, experiments showed that the aether didn't exist. In 1905, Albert Einstein published a theory that explained electromagnetic radiation as tiny particles called photons, which often behaved like waves. The