Final answer:
The Michelson-Morley experiment concluded that the speed of light is constant and not affected by the Earth's motion, contradicting the then-popular ether theory.
Step-by-step explanation:
In the Michelson-Morley experiment, two light beams were raced at right angles to each other to detect a possible slight difference in their speeds. The results of this experiment showed that the speed of light in a vacuum is independent of the motion of the Earth about the Sun. The experiment, aiming to measure the Earth's motion through the supposed 'ether', expected to observe an interference pattern shift, suggesting varying light speeds. However, no such shift was observed, leading to two critical conclusions: the non-existence of ether and the constancy of the speed of light irrespective of the relative motion of the source and the observer.