When one perceives something, the distance between the sense organ and source is bridged in some way.

There are two possible explanations: either the source emits matter, which travels across space, or a short-term change propagates within the microworld between the source and the sense organ. This, for example, is how sound was first explained as a wave within the carrier medium of air in ancient times.

Light, however, presents a puzzle for physics: Is it a wave? If it is, what is it that oscillates? Unlike sound, light can also pass through a vacuum. Is it a stream of particles then? If it is, why doesn’t the source of light become lighter?