Virtual constant speed of light, c
In 1887 the now-famous Michelson-Morley experiment was conducted to search for a light-propagating medium. The experiment used Michelson's interferometer concept shown at left. The idea was to take a light beam L from source S and divide it using a half-silvered mirror X so that part continues to mirror M1 and part is reflected to mirror M2. Mirrors M1 and M2 reflect the two parts back to X where the light is combined into light beam L' which goes to instrument I. If the time for light to travel from S to I via M1 is the same as the time via M2 then the two parts of the light in L' are in phase and this is detected at I. When the travel times are slightly different, so that the two parts are out of phase, this also is apparent at I.

It was reasoned that if light is propagated through a medium through which Earth is moving, the travel time for the two paths will be affected differently (like the travel time for a round-trip radar signal over a given absolute distance in frame B moving through the medium takes longer along the x axis than along the y axis). The interferometer was oriented in different directions to search for the light-propagating medium, but it always appeared that the speed of the light along the two paths of the apparatus was the same. This experiment, and similar experiments since then, contributed to the widespread belief in the constant speed of light, c. Physics textbooks still incorrectly cite this experiment as proof that light is not propagated through a medium.

Not long after the experiment, people suggested reasons for the inability to detect the medium. Some thought that Earth might drag the medium along with it so that on Earth the medium is always stationary. Lorentz, Fitzgerald, and probably others suggested that the motion of the apparatus through the medium might cause a foreshortening of the apparatus in the direction of motion through the medium just enough to make the travel times for the two light paths the same. This suggestion was generally regarded as unrealistic because there seemed to be no logical reason for the apparatus to contract. It was not known at the time that the apparatus was comprised of atoms in constant dynamic interaction and comprised of much smaller-scale constituents that are constantly exchanging energy with one another over huge distances relative to their size. We now know that the contraction of a spaceframe or atomic structure is a natural consequence of the qm, and that the contraction will exactly offset the slowing of a round-trip light signal in the direction of absolute motion because the slowing and the contraction have the same cause.

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