Speed of Light:

Einstein discarded the idea of luminiferous ether. Rather, he proposed an axiom: In a vacuum, the speed at which the light, or any other EM field, travels is a fixed constant. This is the situation regardless of the motion of the observer in respect to the source. (In media other than a vacuum, like glass, this axiom does not exert). Armed with this axiom, Einstein set out to assume what logically obeys.

Einstein did all his work by using a grouping of mathematics and day-dreaming which he termed as "mind journeys." He wasn't an experimentalist however a theorist. There is a saying in physics: "One experimentalist can remain a dozen theorists busy." Einstein turned that within out. His theories have reserved thousands of experimentalists busy.