Michelson and Morley's experiment drawing
Michelson and Morley mounted their apparatus on a stone block floating in a pool of mercury, and rotated it to seek changes in relation to the motion of the earth in its orbit around the sun. They arranged one set of light beams to travel parallel to the direction of the earth's motion through space, another set to travel crosswise to the motion.

No difference could be found. Other experiments on the speed of light also produced results contrary to what physicists expected.

These experiments, like most important new science, were done at the very limit of available techniques. The results were long in dispute. It was only after the invention of lasers that it became easy to show beyond reasonable doubt that the speed of light is invariable.

You can EXIT to a full explanation from Georgia State University, or
EXIT to read the Michelson-Morley paper

Image: A. A. Michelson and E. W. Morley, "On the Relative Motion of the Earth and the Luminiferous Ether," American Journal of Science, 34, 333-345 (1887).
Previous: E=mc²