Albert Einstein is known throughout the world for his theory of relativity. He was only 16 years old when he made his first “rather childish” thought experiments relating to it, as he later recalled. “If a person could run after a light wave with the same speed as light, you would have a wave arrangement which could be completely independent of time. Of course, such a thing is impossible.”

Over the next two decades of intense thought and imagination, Einstein developed relativity theory in two distinct steps. The first was special relativity, published during his “miracle year” of 1905. General relativity came a decade later.

Special relativity is so called because it is restricted to the special case in which observers in different reference frames move in uniform linear motion with respect to each other. General relativity expanded the theory to include non-uniform motion.

According to the special theory of relativity, time and space are not absolute; they are relative to one’s frame of reference. What is constant or invariant throughout the universe, however, is the speed of light.

Upon generalizing his theory, Einstein discovered the equivalence between acceleration and gravity.  According to general relativity, gravity is not a mysterious, instantaneous force between bodies, as portrayed by Isaac Newton. Rather, gravity is an object’s acceleration as it travels without any external force through the curved space and time that mass creates. What we feel as gravity is simply a consequence of geometry. The American theoretical physicist John Wheeler observed, “Mass tells space-time how to curve, and space-time tells mass how to move.”

Einstein was awarded the Nobel Prize in Physics in 1922. Ironically, it was not for his greatest achievement, the general theory of relativity, which was poorly understood at the time. The award was for his explanation of the photoelectric effect, in which he showed that light behaves as if it consists of discrete particles or quanta. This work, published during his “miracle year”, was also revolutionary. Einstein’s quantum theory of light gave birth to the new field of quantum mechanics. Virtually single-handedly, Albert Einstein brought about the transition from classical to modern physics.

To promote a better understanding of relativity among the public, Einstein wrote a popular book in 1920 entitled Relativity: The Special and The General Theory. When Einstein published this book, the difficulty in understanding relativity theory had become the subject of humor. One apocryphal story is that when the British astronomer Arthur Eddington was asked how it felt to be one of only three people in the world to comprehend relativity, he paused in responding, leading his interviewer to ask, “What is wrong, Mr. Eddington?” Eddington replied: “I’m sorry, I was just wondering who the third person is.”