Relativity and quantum mechanics rank among the greatest achievements of 20th-century science, constituting the sum of all fundamental physical knowledge. The former describes the world of the very large, including black holes and the expanding universe. The latter explains the world of the very small, the microscopic realm of atoms and subatomic particles. One man—Albert Einstein—was the undisputed father of the first theory and the godfather of the second.
What was the secret of Einstein’s success? He once said that if a physical theory cannot be explained to a child, it is probably worthless. In other words, he thought in terms of simple physical pictures. His greatness lay in his ability to use such pictures to solve fundamental problems, such as the conflict between Isaac Newton’s theory of mechanics and James Clerk Maxwell’s theory of light.
The Newtonian system was based on common sense—that a second on Earth is the same as a second throughout the solar system. We could synchronize our watches anywhere in the universe because time beats uniformly. Likewise, a foot or a pound on Earth is the same as a foot or a pound in every other location.
Using mental images of clocks and trains, light beams and speeding bicycles, Einstein realized that Newton’s system could not be right because it contradicted Maxwell’s theory of light. Einstein showed that the speed of light must be constant, no matter how fast you move. For that to be true, time must get slower the faster you move. Stranger still, lengths contract and masses must increase as you approach the speed of light. Space and time became relative in his new theory. This pivotal insight overthrew 250 years of Newtonian physics.
