The most famous failed experiment in history
In 1887, physicists Albert Michelson and Edward Morley set out to prove the existence of the "luminiferous aether," an invisible medium thought to carry light waves through space. They built an ultra-sensitive instrument to measure differences in the speed of light. To their shock, they found absolutely no difference, disproving the aether entirely. This brilliant "failure" cleared the way for Albert Einstein to formulate his revolutionary theory of special relativity.
The Search for the Carrier of Light
Throughout the nineteenth century, physicists operated under a seemingly unbreakable rule of mechanics: waves require a medium to travel through. Ocean waves disturb water, sound waves compress air, and seismic waves propagate through the crust of the Earth. When James Clerk Maxwell synthesized electricity and magnetism into a unified set of equations in the 1860s, he demonstrated that light itself is a transverse electromagnetic wave traveling at a characteristic speed. Naturally, natural philosophers concluded that light must also move through an underlying physical substance, which they termed the luminiferous aether.
This hypothetical aether was imagined as an invisible, massless, and entirely frictionless substance that permeated all of space, filling the voids between atoms and the vast expanses between stars. Because celestial bodies moved through it without experiencing measurable orbital decay, the aether had to be extraordinarily rarefied. Yet, because light waves propagate at an immense speed, the aether also had to possess immense rigidity. Most critically for experimental physics, the stationary aether provided an absolute reference frame against which all motion in the universe could theoretically be measured.
If space were saturated with a stationary aether, the Earth's movement around the Sun—traveling at roughly thirty kilometers per second—would inevitably create an 'aether wind' streaming across the planet's surface. Just as a runner feels a headwind on a calm day, an observer on Earth should detect light moving slower when fired upstream into the aether wind and faster when moving across or downstream. Measuring this slight velocity difference offered physicists a way to confirm the existence of the aether once and for all.