THE SPEED OF LIGHT IS CONSTANT

Einstein rejected the notion of luminiferous ether. Instead, he proposed an axiom: In a vacuum, the speed at which light, or any other EM field, travels is an absolute constant. This is the case regardless of the motion of the observer with respect to the source. (In media other than a vacuum, such as glass, this axiom does not apply.) Armed with this axiom, Einstein set out to deduce what logically follows.
Einstein did all his work by using a combination of mathematics and daydreaming that he called “mind journeys.” He wasn’t an experimentalist but a theorist. There is a saying in physics: “One experimentalist can keep a dozen theorists busy.” Einstein turned that inside out. His theories have kept thousands of experimentalists occupied.