Final answer:
The ether hypothesis was based on the understanding that waves, like sound, require a medium; thus, light was presumed to need one too. The Michelson-Morley experiment debunked the existence of ether by not detecting Earth's motion relative to it, leading to a shift in the understanding of light's propagation and aiding in the development of special relativity.
Step-by-step explanation:
Why Was It Once Believed That Light Must Travel Through a Medium?
The belief that light must travel through a medium was based on the fact that all other known types of waves required a material medium to propagate. In the 19th century, the understanding of light as a wave led to the assumption that it also needed a medium for its propagation. This hypothetical medium was named ether, but its existence was never proven. Investigations like Young's double-slit experiment and the foundations laid by Maxwell's equations supported the wave nature of light and the need for a medium.
However, the turning point came with the famous Michelson-Morley experiment. The experiment aimed to detect the relative motion of Earth through the ether, which was presumed to be the carrier of light waves. The startling outcome was that no such motion was detected, suggesting that ether did not exist and that light could indeed propagate through empty space without any medium. This led to a major paradigm shift in physics, contributing to the development of special relativity by Albert Einstein.