The second postulate of special relativity deals with constancy of light in inertial reference frames. But, how did Einstein come to this conclusion? Did he know about the Michelson-Morley experiment?
Special Relativity – Did Einstein Know About the Michelson-Morley Experiment?
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There is an alternate formulation to special relativity, Lorentz Ether Theory. This alternate theory allows the ether frame to still exist. Nobody teaches it. Why?
Special relativity makes two very simple assumptions, that the laws of physics are the same in all inertial frames, and that the speed of light is the same to all inertial observers. The Lorentz transformation and everything implied by it follow from these simple assumptions.
Lorentz Ether Theory on the other hand posits a special frame, the ether frame, where Maxwell's laws truly do hold. Per this theory, this is the only frame in which the one-way speed of light is Maxwell's c. Lorentz Ether Theory also posits time dilation and length contraction as axiomatic. These lead to the Lorentz transformation, and to the round trip speed of light being Maxwell's c.
The only way to distinguish these two theories is to find a way to measure the one-way speed of light. That's not possible, and thus there is no way to experimentally distinguish the two theories. Yet physics instructors only teach special relativity. You have to dig deep, very deep, to find proponents of Lorentz Ether Theory.
One reason is that the assumptions of time dilation and length contraction as axiomatic seem rather ad hoc (and that's putting it nicely). An even bigger reason is that Lorentz Ether Theory introduces a key untestable hypothesis, the existence of the ether frame. This frame cannot be detected. Time dilation and length contraction conspire to hide it from view. A bigger reason yet is general relativity. The axioms of Lorentz Ether Theory are inconsistent with general relativity. The final nail in the coffin is quantum mechanics, which eliminates the need for a medium through which light propagates. Without that very self-contradictory medium (the luminiferous aether), what's the point of having an ether frame?
The modern geometrical perspective of special relativity isn't so much that the speed of light is constant but rather that there exists some finite speed that is the same to all observers, and that light necessarily moves at this speed because it is carried by massless particles.
What motivates the existence of this finite universally agreed upon speed is geometry. What geometries yield a universe in which Newtonian mechanics appears to hold in the limit of zero velocity, and what do these geometries say about a speed that is the same to all observers?
The answer is that there are two cases: This universally agreed upon speed is infinite or finite. An infinite universally agreed upon speed results in Newton's universe. A finite speed results in Minkowski space-time describing the geometry of special relativity. Experimentally, the finite speed of light appears to be the same to all observers, thus falsifying the notion of a Newtonian universe with Euclidean space and time as the independent variable.
As far as I know, the only clue at the time that the speed of light would be invariant were Maxwell's Equations where "something" shows up as a constant. However, speed of light being invariant in all inertial reference frames is very counter-intuitive. One might rather expect physics to be slightly different in different frames, which is what the MM experiment was looking for.
Best Answer
He just knew the name of the experiment not any details. The experiment didn't play any role in the formulation of STR by Albert Einstein.
The context is taken from the book:
Special Theory of Relativity by V. A.; Atanov, Yuri (Trans.) Ugarov (Author)
Art: Was Michelson's experiment "decisive" for the creation οΙ the special theory οΙ relativity?
An article by R. Shankland, published in 1963, the following excerpt from his interview with Einstein dating back to 1950:
A letter written by Albert Einstein: