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Dr. Albert Einstein: Theoretical Physics, Relativity and the Photoelectric Effect, 1935

Special Relativity

Einstein's theory of special relativity is fundamentally a theory of measurement. He qualified the theory as "special" because it refers only to uniform velocities (meaning to objects either at rest or moving at a constant speed). In formulating his theory, Einstein dismissed the concept of the "ether," and with it the "idea of absolute rest." Prior to the generation of Einstein's theory of special relativity, physicists had understood motion to occur against a backdrop of absolute rest (the "ether"), with this backdrop acting as a reference point for all motion. In dismissing the concept of this backdrop, Einstein called for a reconsideration of all motion. According to his theory, all motion is relative and every concept that incorporates space and time must be considered in relative terms. This means that there is no constant point of reference against which to measure motion. Measurement of motion is never absolute, but relative to a given position in space and time. Returning to Galileo's cannonball, Einstein considered this: the cannonball falling from the mast of the ship would appear to an observer standing on the deck of that ship as though it dropped straight down; however, to an observer standing on the shore, the cannonball would appear to follow a curved trajectory on its way to the base of the mast. Which trajectory did the ball actually follow? According to Einstein's theory of special relativity, the answer is, both—and neither. Each observer's observation is valid in its own reference frame, yet each is no more than an artifact of the measurement, or observation, undertaken by the observer.