<P> Einstein stated that the theory of relativity belongs to a class of "principle - theories". As such, it employs an analytic method, which means that the elements of this theory are not based on hypothesis but on empirical discovery . By observing natural processes, we understand their general characteristics, devise mathematical models to describe what we observed, and by analytical means we deduce the necessary conditions that have to be satisfied . Measurement of separate events must satisfy these conditions and match the theory's conclusions . </P> <P> Relativity is a falsifiable theory: It makes predictions that can be tested by experiment . In the case of special relativity, these include the principle of relativity, the constancy of the speed of light, and time dilation . The predictions of special relativity have been confirmed in numerous tests since Einstein published his paper in 1905, but three experiments conducted between 1881 and 1938 were critical to its validation . These are the Michelson--Morley experiment, the Kennedy--Thorndike experiment, and the Ives--Stilwell experiment . Einstein derived the Lorentz transformations from first principles in 1905, but these three experiments allow the transformations to be induced from experimental evidence . </P> <P> Maxwell's equations--the foundation of classical electromagnetism--describe light as a wave that moves with a characteristic velocity . The modern view is that light needs no medium of transmission, but Maxwell and his contemporaries were convinced that light waves were propagated in a medium, analogous to sound propagating in air, and ripples propagating on the surface of a pond . This hypothetical medium was called the luminiferous aether, at rest relative to the "fixed stars" and through which the Earth moves . Fresnel's partial ether dragging hypothesis ruled out the measurement of first - order (v / c) effects, and although observations of second - order effects (v / c) were possible in principle, Maxwell thought they were too small to be detected with then - current technology . </P> <P> The Michelson--Morley experiment was designed to detect second - order effects of the "aether wind"--the motion of the aether relative to the earth . Michelson designed an instrument called the Michelson interferometer to accomplish this . The apparatus was more than accurate enough to detect the expected effects, but he obtained a null result when the first experiment was conducted in 1881, and again in 1887 . Although the failure to detect an aether wind was a disappointment, the results were accepted by the scientific community . In an attempt to salvage the aether paradigm, FitzGerald and Lorentz independently created an ad hoc hypothesis in which the length of material bodies changes according to their motion through the aether . This was the origin of FitzGerald--Lorentz contraction, and their hypothesis had no theoretical basis . The interpretation of the null result of the Michelson--Morley experiment is that the round - trip travel time for light is isotropic (independent of direction), but the result alone is not enough to discount the theory of the aether or validate the predictions of special relativity . </P>

What is the meaning of the theory of relativity