In the last chapter, we presented de Broglie’s proposal that wave–particle duality works for matter in the same way it does for light. That is, although light usually behaves as an electromagnetic wave, it sometimes acts as a particle (photon). At the same time, while electrons mostly behave as particles in CRTs, they sometimes exhibit wave-like behavior. We also saw that recent experiments have demonstrated wave–particle duality in much larger and more massive objects than an electron, such as molecules of a fullerene compound incorporating 60 carbon and 48 fluorine atoms.
Now, this brings up an interesting question: if a sound wave is a vibration of matter, and a photon is a vibration of electric and magnetic fields, what exactly vibrates when matter acts as a wave? The upsetting answer is that there is no directly measurable quantity to correspond to the matter wave itself. This is what physicists call the lack of an experimental observable.
That is, while you can use a microphone to measure the vibrations of air pressure that constitute a sound wave, and you can use a radio receiver to measure the electromagnetic oscillations of light waves (remember our microwave interference experiments using a Gunnplexer?), there is no device that can measure a matter wave directly.
Let’s think back to the single-electron interference experiments performed by Tonomura (Figure 101). At any one time, we can only detect the fluorescence produced ...