Relativity begins with a modest question: How does your physics relate to my physics if we are moving relative to each other? Galileo gave one answer: We find exactly the same laws of mechanics if our relative speed is constant. Newton said the same thing but more elaborately by referring all motion—yours, mine, and everyone else's— to an absolute frame of reference in space and time. Nineteenthcentury theorists found Newton's absolute frame a convenient place to locate the hypothetical medium they called the ether, which propagated light and other electromagnetic waves.
Ether physics was a prominent endeavor among Victorian scientists, but it had fatal flaws. For one thing, ether physicists could never agree on a standard model for the mechanical structure of the ether. Also questionable was the concept of motion through an ether anchored in Newton's absolute frame of reference. A series of experiments performed by Albert Michelson and Edward Morley in the 1880s that aimed at detecting Earth's motion relative to an “ether sea” was an impressive failure. The stubborn fact, always observed, is that the speed of light in empty space is the same regardless of the speed and direction of the light source.
A young patent examiner in Bern, Switzerland, named Albert Einstein published a paper in 1905 that resolved the ether problem by simply ignoring it. Einstein postulated two empirical principles that could not be denied: constancy of the speed of light, and a generalization of Galileo's relativity principle to include electromagnetic and optical phenomena. Beginning with these two principles, and without recourse to the ether concept, he proved that, for observers moving relative to each other at constant speeds, length and time measurements are different, perhaps drastically different if the speed is close to the speed of light. For example, if a stationary observer watches a clock moving at high speed he or she sees it ticking more slowly than an observer traveling with the clock. In addition to this “time dilation,” Einstein's 1905 paper insisted that the length dimension of the clock, or of anything else, is contracted in the direction of motion for the stationary observer.
Einstein designed his 1905 “special” theory of relativity with two