How did Einstein get the idea for the theory of relativity?

Answer 1

In his autobiography, Einstein stated that the idea for time dilation due to velocity came when he was riding a trolley away from a clock tower in Bern. He asked himself what time he would see in the clock if that trolley were moving away from the clock at the speed of light. It occurred to him that he would see the clock standing still, because the light that left from the clock one second later would never catch up to the trolley to inform him that any time had elapsed. However, he noted, someone standing at the tower would see time proceed normally. From this idea he first began to think that time itself could actually change depending on one's movement. From this idea Einstein eventually came up with special relativity.

Einstein also recalled that, a few years earlier, he had wondered what he would see if he was moving at the speed of light, next to a beam of light. Since light was a longitudinal wave, for which electrical fields become stronger then weaker then reverse direction, he figured he would see a stationary E-field doing just that. The problem he noted then was that the mathematics for light work ONLY if the wave were moving -- they made no sense if the E-field was simply standing still. He constantly pondered this paradox until he rode a trolley in Bern.