People have long been creating and believing theories about nature. Some theories accurately represented nature but most did not and were eventually replaced by better theories. This is a tale of two novel, ingenious, and controversial theories that are similar in fundamental ways. The first, Ptolemy's theory of the motions of heavenly bodies, lasted for 1500 years until it eventually became clear to most that it was inherently flawed and misleading. It was created starting with a false assumption. The second, Albert Einstein's theory of relativity has endured for about 100 years and has basic characteristics in common with Ptolemy's theory.

     In their primes, both theories were very useful and were consistent with the available evidence. (We are especially aware of relativity theory's tremendous help in advancing science.) This contributed greatly to their believability. However, each was predicated on strange and inexplicable observed phenomena. Ptolemy's theory was based on observations of heavenly bodies circling Earth. No one knew what caused this circular motion around Earth. (Aristarcus was on the right track with his heliocentric theory, but Ptolemy's ideas were easier to understand and more popular.) It was concluded that circular motion is an innate characteristic of heavenly bodies. This conclusion seems absurd today but very intelligent and capable people believed it. The observed motions of heavenly bodies could be accounted for by this innate circular motion, and Ptolemy's theory eventually became a complete mathematical model of the heavens. This mathematical model, which appeared to explain the perplexing motions of heavenly bodies, obscured the hidden underlying phenomena responsible for the strange observations. It inhibited the search for these hidden phenomena that were eventually found by Copernicus, Galileo, Kepler, Newton and others.

     Similarly, relativity theory is a complete mathematical model of observed phenomena involving systems moving relative to one another. It is the result of assuming that the speed of light is constant through every system (when the light is not impeded by matter). When Einstein was creating relativity theory there was no plausible explanation for the speed of light always appearing to be the same through all systems, regardless of the systems' velocities. Therefore, he relied on the experimental evidence indicating that the speed of light is constant in all inertial systems, just as Ptolemy relied on the evidence that heavenly bodies move in circular paths.* Today most physicists believe in this constant speed of light, just like our ancestors believed in the circular motion of heavenly bodies.

     This website shows why the speed of light appears constant in all inertial frames and why a belief in this constant speed of light is misleading. It shows how relativity theory, like Ptolemy's theory, can be fundamentally misleading. It explains physical causes for the apparent slowing of moving clocks and the other observed phenomena predicted by relativity theory and attributed to relative motion. It also explains a variety of other phenomena such as the "mass" and "inertia" of bodies. This ability to explain significant other phenomena in addition to relativistic phenomena is further evidence of the theory's soundness -- much as the ability of the Copernican theory to explain logically a variety of important phenomena added to its plausibility.

     The fervor with which Ptolemy's theory was believed made it difficult to introduce a better theory. The same is true of relativity theory where many proponents are certain the theory is correct because it agrees with the experimental evidence.** Eventually Ptolemy's theory was replaced by a better way of thinking. Relativity theory may experience the same fate when the theory's shortcomings become more widely recognized and it is seen that the apparent constant speed of light is probably an illusion (like the apparent circular motion of heavenly bodies is an illusion) and that there are logical physical causes for the phenomena that relativity theory attributes to relative motion.

* In his 1905 paper in which he introduced special relativity theory, Einstein assumes without apparent reservations that the speed of light is constant (in vacuum in all inertial frames). In his book, Relativity, The Special and the General Theory (Crown Publishers, New York, 1961, p. 17) Einstein writes, "There is hardly a simpler law in physics than that according to which light is propagated in empty space. Every child at school knows, or believes he knows, that this propagation takes place in straight lines with a velocity c=300,000 km/sec." The book goes on to say that experiments show that the speed of light is independent of the color of the light and “the velocity of motion of the body emitting the light” and that it is improbable that the velocity of propagation of light in space depends on the direction of propagation. This leads to the following sentence. "In short, let us assume that the simple law of the constancy of the velocity of light c (in vacuum) is justifiably believed by the child at school." Today students of relativity theory become aware of the many strange consequences of assuming that the speed of light is constant. They learn to accept that there can be no absolute distance or time or mass -- much as students of Ptolemy's theory learned to accept that circular motion is a natural characteristic of heavenly bodies.

** The soundness of relativity theory is often equated to its agreement with experimental evidence. What is not generally realized is that a theory can be in complete agreement with the experimental evidence and still be wrong. Ptolemy and Copernicus showed us that theories need to be logical and explain the physical causes for observed phenomena, in addition to being in agreement with experimental evidence. In the past a variety of alternatives to relativity theory were proposed that are not in agreement with experimental evidence or are based on assumptions as implausible as the constant speed of light assumption of relativity theory. This lack of a viable alternative to relativity theory contributed to the certainty with which many believe relativity theory is sound. Further, physics texts teach that experiments proved that light is not propagated through the all-pervading medium assumed by Thomas Young, James Maxwell, Hendrik Lorentz, Albert Michelson, and many others. The physics texts are wrong about this. They are wrong because many of the very interesting and far-reaching consequences of the medium have not been understood (as this website shows).