In the late 19th century, luminiferous aether, æther or ether, meaning light-bearing aether, was the postulated medium for the propagation of light. It was invoked to explain the ability of the apparently wave-based light to propagate through empty space, something that waves should not be able to do.The concept was the topic of considerable debate throughout its history, as it required the existence of an invisible and infinite material with no interaction with physical objects. As the nature of light was explored, especially in the 19th century, the physical qualities required of the aether became increasingly contradictory. By the late 1800s, the existence of the aether was being questioned, although there was no physical theory to replace it.The negative outcome of the Michelson–Morley experiment (MMX) suggested that the aether was non-existent. This led to considerable theoretical work to explain the propagation of light without an aether. A major breakthrough was the theory of relativity, which could explain why the MMX failed to see aether, but was more broadly interpreted to suggest that it wasn't needed. MMX, along with the blackbody radiator and photoelectric effect, were key experiments in the development of modern physics, which includes both relativity and quantum theory, the latter of which explains the apparent wave-like nature of light.