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How did the Michelson-Morley experiment relate to Albert Einstein’s theory of special relativity?

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Portrait of Albert Einstein in a suit and tie, looking slightly to the side with a neutral expression. — Albert Einstein Albert Einstein developed the special and general theory of relativity, which recast gravity as a bending in space-time.

The Michelson-Morley experiment was a pivotal study that failed to detect the luminiferous ether, a hypothetical medium once thought necessary for the transmission of light. By using an interferometer to compare the speed of light in different directions, researchers found no variation, despite Earth’s motion through space.12

This "null result" contradicted classical physics, which assumed that light speeds would add together like other velocities. Albert Einstein reconciled this discrepancy in 1905 by formulating his special theory of relativity. Instead of relying on the ether, Einstein postulated that the speed of light is a universal constant that remains the same for all observers, regardless of their motion. This insight rendered the ether an unnecessary construct and established the constancy of the speed of light as a cornerstone of modern physics.231