3 8 2 D O C . 4 2 0 W H AT I S T H E T H E O R Y O F R E L AT I V I T Y ? 420. “What Is the Theory of Relativity?” [Berlin, after 2 March 1929] Youth Tribune What Is the Theory of Relativity? The whole matter turns on the question, Is there a physically privileged state of motion at any arbitrarily chosen point? Allegory: A fish in a river asks himself this question. In the first instance, he will arrive at the answer, “Yes.” For if he stops moving his fins, then the resulting state of motion is distinguished with respect to all others. Let us try the same thing with our space. The most varied experiences appear to demonstrate that all states of motion are equivalent, and that no one of them is dis- tinguished by nature. This proposition (the principle of relativity) is raised by the theory of relativity to the status of a postulate, and that theory attempts to adapt all of physics to this principle. Thus, the relativization of simultaneity was arrived at by adapting the law of the propagation of light to this principle. Likewise, the theory of gravitation was obtained by interpreting the concepts of inertia and the weight of bodies in the sense of the same principle. In terms of methodology, the principle of relativity takes on a place within phys- ical science similar to that of the law of the impossibility of a perpetuum mobile of the second kind (the impossibility of converting all of the heat in seawater into me- chanical energy) in the theory of heat.