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How did the Michelson-Morley experiment challenge the idea of an ether?

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Brian Greene sits in a living room, holding a pen, with handwritten equations m = E/c^2 and E = mc^2 on a whiteboard beside him, highlighting c^2 as the speed of light. — relativity Brian Greene kicking off his *Your Daily Equation* video series with Albert Einstein's famous equation E = mc².

The Michelson-Morley experiment challenged the existence of the luminiferous ether—a hypothetical medium once thought to permeate space and support the transmission of light—by failing to detect the Earth's motion through it.12

Physicists hypothesized that if the ether existed, an observer moving through it would measure different speeds of light depending on their direction of travel relative to the medium. Using a sensitive optical interferometer, A.A. Michelson and Edward W. Morley compared the speed of light in two mutually perpendicular directions as the Earth orbited the Sun.21

Despite the instrument's high precision, the experiment yielded a null result: no difference in the speed of light was observed. This outcome discredited ether theories, as it suggested that the speed of light is independent of the observer's motion. This finding was later reconciled by Albert Einstein in his theory of special relativity, which established the speed of light as a universal constant, rendering the ether an unnecessary construct.12