Einstein was fascinated by the nature of light.
In 1905, nearly a decade after this first "thought experiment," Einstein answered these questions with his Special Theory of Relativity. The theory, which revolutionized our understanding of time and space, is based on Einstein's astonishing recognition that light always travels at a constant speed, regardless of how fast you're moving when you measure it. Einstein's explorations into the fundamental properties of light also laid the groundwork for his most impressive achievement, the General Theory of Relativity.
For decades, physicists searched in vain for the ether and proposed elaborate explanations for why they couldn't detect it. Einstein suggested that the long-accepted theory that light moved through ether was simply wrong. He declared that there is no ether to speed up light or slow it down—in other words, the speed of light is constant.
Einstein's crucial breakthrough about the nature of light, made in 1905, can be summed up in a deceptively simple statement: The speed of light is constant.
During the late 1800s and early 1900s, scientists struggled to understand the nature of light.
For centuries, physicists thought there was no limit to how fast an object could travel. But Einstein showed that the universe does, in fact, have a speed limit: the speed of light in a vacuum (that is, empty space). Nothing can travel faster than 300,000 kilometers per second (186,000 miles per second).