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 within the carrier medium of air in the 17th century. Light, however, presents a puzzle for physics\: Is it a wave? If so, 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?