Before Einstein’s revolutionary ideas changed the landscape of physics, very little was known about the speed of light. The fact is that, at the time, light was an anomaly; it had characteristics unique to anything else we experienced in the world. For example, was light a particle or a wave? And more importantly, if light is a wave, then what medium does it travel through? While we know the answers to these questions today, they were issues of great debate throughout the scientific community. Rather than answer the first question, we will focus on the supposed medium that light must travel through.
Since all waves known at the time traveled through a known medium (i.e. sound in air), it was proposed that light must also travel through a medium. But this medium must be something completely new and unseen since light also travels throughout space, evident due to the fact that we can see light from the surrounding stars. Therefore, this proposed medium was known as the ether. But, like all scientific theories, the mere development of an idea is not enough; it must be tested and agree with current observations. Along came Albert Michelson and Edward Morley.
Michelson and Morley set out to test whether the ether existed by measuring the speed of light in different directions. The idea was that, since the Earth is moving through space and therefore moving relative to the ether, measuring the speed of light in different directions would yield two different speeds. However, when the experiment was performed, Michelson and Morley found no difference in the speeds. They were forced to conclude that there is no ether for light to move through.
Today, we know their results to have been true. There is no ether acting as the medium for light to travel. Although this experiment did a great job of negating a faulty theory, it did very little to construct something new. However, this would come later when Einstein formulated his theory of special relativity, claiming that light travels at the same speed for all observers.
Sources: The Michelson-Morley Experiment