Philosophically speaking, the observation that time passes at different rates for different observers suggested it wasn't external to the universe. Somehow the universe had to include time itself in a way that explained its dilation at rapid speeds and the limiting value of the speed of light. This was accomplished by the German-Lithuanian mathematician Hermann Minkowski in his four-dimensional space-time. In Minkowski's four-dimensional world every particle, every body, every thing that existed in the universe traced out its own path, and the time it experienced was measured along this path. For a ray of light that time was zero. It was as if from the point of view of light itself, travel was instantaneous, and you can't go faster than that.

This gave a fine mathematical basis for the special theory of relativity that Einstein had put forward in 1905 during his years working at the Swiss patent office in Bern. In his journeys to and from work he continued thinking deeply about these things, and asked whether the force you feel under a sharp acceleration, or under a change of direction in the line of motion, was any different from the force of gravity. He concluded it wasn't, and this led him to his 1915 general theory of relativity, which gave an intrinsic curvature to Minkowski's space-time in the presence of gravitational fields.

Four years later, during the solar eclipse of 1919, light rays were seen to curve as they passed the sun, so Einstein's new theory was vindicated and he achieved undying fame. His results went beyond Newtonian mechanics to explain observed anomalies in planetary motion, and more besides. But the elephant in the room was quantum mechanics, which seemed to operate on quite different principles.