Principle of Equivalence
Special relativity applies only to cases in which objects are moving at a uniform velocity. General relativity, however, is applicable to all forms of accelerated motion. This theory of general relativity arose from Einstein's principle of equivalence. Einstein formulated this principle by examining a given mass in two different states. The first state occurs when the mass in question is acted on by gravity, and the second when the mass is in a state of inertia (when it resists forces and accelerations). According to Einstein's principle of equivalence, the given mass is equivalent in both states. Take, for example, a spinning top. According to the principle of equivalence, the top has the same mass whether it is falling off a desk (being acted on by gravity) or whether it is spinning atop a desk (in a state of inertia). This principle may seem obvious, and in fact people since Newton's time had simply assumed it to be true. However, the implications of the principle of equivalence are far from obvious, and Einstein was the first to realize those implications.
The Franklin Institute realized the magnitude of Einstein's achievement and was honored by his presence at it annual Medal Day ceremonies. The gratitude of The Franklin Institute is conveyed in the following notes, thanking Einstein for his attendance and stating: "We feel that you added much to the dignity of the occasion by your presence." You can read the full text of thank you letters by clicking on the thumbnails at right.