The movies, The questions, The errors, The guiding thread, Comments

Map of the sky changing during the year, for c = 300 km/s

The light of two apparent images interferes
 

Rain drops show aberration, rain fronts don't
 

When wave fronts show aberration, simultaneity relates to the observer

The Einstein train registered on a strip: Minkowski geometry

• Which was Michelson's aim?
  To show that he is able to measure the motion of the earth through its superposition on the velocity of light. Such a superposition was known to take place since Bradley's observation of the aberration of starlight (movie).
• How did he perform it?
  He constucted his interferometer as a set of mirrors which produce two close images of a source. The light from these apparent images could interfere (movie), and the interference pattern is extremely sensitive to changes in distance of the apparent images. Such changes would be prodiced by the expected anisotropy of the velocity of light. Apparently the motion of the earth does not show up in the room, just as Galilae expected. Light could be suggested to propagate in some medium (which came into the name aether) contained in the room.
• What did he really find?
  In his room, he did not find any anisotropy of the velocity of light. Light could be suggested to propagate in some medium (which came into the name aether) contained in the room like the air and in relative rest to it.
• What did he conclude?
  1. The set is not good enough (it was good enough, after all).
  2. The result of the experiment is at variance with the observation of aberration (movie), as it was explained by addition of velocities. In the end, this turned out to be a water-tight proof of a contradiction. It is the geometric clue after many attempts to explain the contradiction by the mechanics of light propagation (properties of the aether which Michelson found contained).
• Where the rat was hidden?
  In the elementary picture, wave fronts do not show aberration, but it was sufficient for the time being to refer to wave crests which show the observed aberration. If one accepts that the wave fronts too have to show the aberration (movie), one ends with the relativity of simultaneity. The question how to control simultaneity led Einstein to start with the axiom that the speed of light (the scalar value devoid of any direction) is not object of any composition with other velocities (movie). In geometric terms: the speed of reflected light is independent on the motion of the mirror, in contrast to tennis experience. This originates the Minkowski geometry on a registration strip (movie).

• The experiment does not prove Einstein's axiom that the speed of light does not change in superpositions. Einstein's axiom is an axiom, not to be proven, but to be checked and backed in its consequences. Its applicability could be disproven only, if one finds an essential conclusion to be wrong.
• The experiment does not prove that the velocity of light is isotropic everywhere, or the same everywhere. "Always" and "everwhere" can never be proven, because we test only here and now. "Alwas" and "everwhere" can be disproven, when our experiment here and now fails.
• The experiment proves nothing about the speed of light in the universe. In addition, we need an etalon for velocity that far out. Using the Planck's constant combined with the elementary charge as etalon, we actually find a constant speed through observation of the hyperfine structure of atomic spectra.
• The experiment does not disprove the existence of an aether. In its consequences, the aether only reaffirms its uselessness for understanding the propagation of light.
• The experiment was sensitive enough when one considers our present knowledge of the motion with respect to the cosmic radiation background (about 500 km/s).

• Does the earth really move instead of the sun? First argument: the order in the retrograde motion of planets! (It is observed near opposition or inner conjunction only.)
• Second argument (2000 years later): The aberration of star light. The motion of the earth and Newton's emanation hypothesis for light support each other. The velocity of light is composed with that of the earth by simple (vectorial) addition as well.
• Only when light is a wave, interference can be understood. However, wave fronts do not show aberration in a Galilean theory (where all velocities are to be composed additively). Fresnel called this problem the most difficult problem of the wave theory.
• Fresnel argued for a telescope, which cuts out by its aperture lens a wave group that moved again like a particle. However, this excuse required that material objects would not disturb the propagation of light. Aberration seems to measure motion against the isotropy system inside a closed laboratory in contrast to the expectation expressed by Galileo (the Galileo relativity principle). To save the principle, the isotropy system had to have a material carrier, the aether, and this aether had to be all-pervading. This aether replaces both speed trap and alley tree as outer reference for velocity. Reluctantly, the hypothesis of an all-pervading ("stationary") aether was accepted.
• Michelson found that -- inside the room -- the motion of the earth produces no anisotropy of the propagation. When you expect an aether to determine the propagation, it must be contained in the room. The aether cannot be all-pervading, but is confined and the more is dragged along with the earth - just as one would expect from a material substance. The question of the aberration is open again. Fresnel's explanation collapsed. Michelson's text:
  The interpretation of these results is that there is no displacement of the interference bands. The result of the hypothesis of a stationary aether is thus shown to be incorrect, and the necessary conclusion follows that the hypothesis is erroneous.
  This conclusion directly contradicts the explanation of the phenomenon of aberration which has been hitherto generally accepted, and which presupposes that the earth moves through the aether, the latter remaining at rest.
  
• The 19th century discussion clustered about the mechanics of wave propagation, the properties of the aether. It ended in Lorentz' problematic length contraction hypothesis, shown to be wrong by Einstein's theory.
• Galileo' principle of relativity appeared to be at stake because it required first an all-pervading aether to include aberration, but second to be contained in rooms to answer Michelson's result. It was discussed that Galieo's principle would be valid for mechanics only.
• Lorentz and Drude finally admitted the geometric argument that wave fronts itself must show aberration. The found the motion-dependent form of simultaneity. The question of controlling simultaneity led Einstein to his findings.
• Einstein's theory of relativity solved the problem of simultaneity on a purely geometric way without referring to any properties of the aether and the aether in general. The requirement that the aberration of particles coincides with the aberration of wave fronts yields the relativity of simultaneity and SRT in consequence. SRT reinstates Galiileo's principle.

• Einstein was not particularly aware of the Michelson experiment. He intended to solve the problem of controlling simultaneity.
• From the point of the resulting SRT, the result of Michelson's experiment is trivial. Again from the point of SRT, the Michelson experiment backs the Einstein axiom at a very fundamental level. (Michelson did not agree with this point of view. He always understood his result as backing the hypothesis of a material aether, dragged along with the earth and confined in the basement.)
  When Michelson won the Nobel price for his technics, the experiment became tightly bound to the foundation of the theory. It helped decisively to overcome the conservative resistance against the new concepts of space and time.
• Of course, the Michelson experiment does not prove any theory, just as no particular experiment or particular observation can prove a theory. A theory can be backed at best, and disproven in the negative case. A theory cannot be deduced from experiment or observation, it must be seen, be found. The best backing is the technical use of a theory. Our confidence in SRT is backed best by the prediction of antiparticles and the explanation of the spin. These are structural issues, not depending on small effects. Prescise measurements of the properties of light propagation may find small discrepancies, which would then show that the Maxwell equation have to be corrected.
• The Michelson experiment was first performed in 1881 in a cellar of the Astrophysical Observatory Potsdam. It was improved many times, in particular by Michelson. His interferometer made a triumphal procession through the physical technology.
• The Wikipedia article contains some questionable phrasing which reflects the many "didactic simplifications" of the logical steps. For instance,
  • The Michelson–Morley experiment was not an attempt to detect the existence of aether, but to demonstrate the superposition of the motion of the earth with the propagation of light, in aether terms, of to demonstrate an all-pervading aether.
  • The article does not make clear, that the consideration of aether properties shifts the attention to mechanics, while the aberration poses a geometric problem.
  • In the part about Length contraction and Lorentz transformation, the Lorentz contraction and the relativistic length contraction are mixed. The difference is nowhere explained. The Lorentz contraction is assumed to be a material effect of the aether wind. The relativisitc length contraction is a purely geometic effect producedd by the relative perspective of two observers moving relative to each other. Both effects lead to the same formula (with different velocities, however), therefore the confusion. The SRT shows, of course, that a Lorentz contraction does not exist.
  • The Lorentz transformation is presented as arising by accounting for some time dilation required by different experiments except aberration. Again, these experiments refer to the motion of the supposed aether, but never cite two observers in relative motion. The LT were found through accounting for aberration and the relativity of simultaneity. The relativistic time dilation again is an affect of perspective between two observers in relative motion.

References