In the 19th century, scientists believed that all waves required a medium through which to travel. Sound waves move through air, and water waves through water, so it seemed logical that light waves must also need a medium. This hypothetical substance was called the luminiferous ether – thought to fill all of space, invisible yet essential for the transmission of light and electromagnetic forces.
The ether was imagined as an elastic and weightless substance that permeated everything, even solid matter. According to this view, light waves were vibrations in the ether, and the Earth moved through it as it orbited the Sun. If that were true, scientists reasoned, there should be a measurable difference in the speed of light depending on whether the Earth was moving with or against the ether flow – similar to how wind affects the speed of sound.
The most famous attempt to detect this effect was made by Albert A. Michelson and Edward W. Morley in 1887. Using a highly sensitive interferometer, they measured the speed of light in perpendicular directions, expecting to find small variations caused by Earth’s motion through the ether. To their surprise, they detected no difference at all. The experiment’s “null result” implied that either the Earth dragged the ether with it, or, more radically, that the ether did not exist at all.
Physicists tried to explain the result without abandoning the ether. George FitzGerald and Hendrik Lorentz proposed that moving bodies contract in length along the direction of motion – an idea known as length contraction – to account for the missing effect. Yet these explanations felt artificial and raised further questions.
The Michelson–Morley experiment became one of the most significant results in physics, forcing scientists to reconsider the nature of space, time, and motion. It hinted that the speed of light might be the same for all observers, regardless of their motion through space. This puzzling conclusion undermined the foundations of classical physics and set the stage for Einstein’s revolutionary theory of relativity, which would discard the ether entirely.