SPECIAL THEORY OF RELATIVITY
In the nineteenth century space itself was thought to be an absolute frame of reference relative to which absolute motions could be measured. Space was also imagined to be filled with a stationary invisible medium, the ether, which carried electromagnetic waves (as air transports sound waves). If light must move through such a medium, it was argued that its speed should differ depending on its direction relative to a moving observer. Thus the earth's absolute velocity could be measured against this stationary medium by timing the speed of light in various directions.
In 1887 the American physicists Albert Michelson (1852-1931) and Edward Morley (1838-1923) sought to detect a difference in the speed of light beams propagated over the same distance, one parallel to the earth's motion around the sun and one simultaneously transmitted perpendicular to it. The time lighwould spend moving across the earth's path, back anc forth, was calculated to be root of (1 - (y2tc2)) shorter that the time it would take light to move parallel to the earth's path in one direction and then in the opposite direction; here y is the absolute velocity of the earth s motion and c is the velocity of light. It was reasoned that the anticipated minute difference in the relative sueeds of the light beams would lead to an evaluation : the earth's absolute motion in space. But no matter which direction measurements were made or at at time of year the experiment was performed, the -esult never changed; There was no measu rable difference in the times of the light beams. This led to the conclusion that we cannot measure earth's absolute velocity in space.
The Michelson-Morley experiments were bewildering to the scientific community at the time Einstein began work on his special theory. Einstein rejected the notion of ether and concluded that the solution to the problem lay in recognizing the inadequacy of the Newtonian concepts of space and time. To develop the mathematical formulation of his theory, he laid two postulates;
FIRST POSTULATE: The laws of physics are the same for 'servers in uniformly moving frames of refer-
SECOND POSTULATE: The velocity of light (299,792 kilometers per second) is the same for all observers in space regardless of the motion of the source or of observer.
Let us amplify these two postulates. Suppose two observers are moving uniformly relative to each other (not accelerating). They are said to occupy inertial frames of reference. Neither frame of reference is preferred by the laws of physics; there is no way to distinguish' one from another. They have equal physical status; and for that matter all uniformly moving frames of reference have equal status. Neither observer is able to determine the uniform motion of his own reference frame by any experiments conducted within that frame of reference. For example, a person occupying an inside cabin (no portholes) of a ship cruising at uniform speed in calm seas could not conduct any kind of experiment to indicate whether the ship was moving uniformly at sea or still tied to the dock. That is, experiments like throwing a ball, flipping a coin, or playing a game of billiards would not distinguish between the two situations. If the person performed another experiment, using a frame of reference different from the reference frame provided by the cabin, say by looking out of a porthole at the ocean, he or she could determine the state of motion relative to that other frame of reference. But exclusive of that possibility, the only means of detecting motion would be under acceleration as when the ocean became stormy-then the deviation from uniform motion would be painfully evident.
Einstein's second postulate is obviously consistent with the Michelson-Morley experiment: The speed of the light beam in the experiments did not depend on the direction of the light; it did not depend on the light source; it was not affected by the earth's motion. In other words the speed of light is independent of the relative velocity of the source and the observer. Regardless of whether we measure the velocity of light in our reference frame or in some other, we find that the velocity of light always has the same value. Einstein reasoned that what is different between frames of reference moving relative to each other is the measurement of space and time.
