This post is a response to a question posed by a visitor to this site. A significant part of Einstein’s groundbreaking discoveries drew from his gedanken (thought) experiments. When most people believed that time is absolute, i.e., it unfolds at a uniform rate and is measured to be the same by all observers, Einstein realized that concepts such as time and motion are relative to the observer. He imagined sitting on a train at the station and suddenly observing a train on a nearby track that appears to be moving. To determine which of the two trains is actually in motion, he would have to look out the opposite window to see whether his train was moving relative to the station. These thought experiments played a role in his conception of his Special Theory of Relativity in which he postulated that the speed of light must be measured the same for all observers in all inertial frames of reference. A consequence of this bold postulate was the conclusion that time is not absolute; it depends on the speed of the observer relative to the location of an event. Similarly, in conceptualizing his General Theory of Relativity, Einstein imagined accelerating upward in an elevator and comparing this to the feeling of a gravitational pull.
In his famous quote, “Imagination is more important than knowledge. Knowledge is limited. Imagination encircles the world,” Einstein was expressing the idea that knowledge is the set of things we know, but imagination is our means of discovering things that we don’t yet know, and in this way, is unlimited.
