ether — if light is a wave, what is it waving? This was the problem physicists confronted when they saw the wave-like properties of light but also saw that light propagated through a vacuum. They speculated that there was something called the “ether” which was a quasimaterial substance that permeated the universe and formed the medium through which light traveled. The wave-like or particle-like nature of light and the nature of this “ether” were preoccupations of physicists until the early 20th century, when the ether was banished as unnecessary and the wave/particle dual nature of light was established.