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Context: Our state (NSW, Australia) recently got a new syllabus for the year 12 physics course, and as such we are the first year going through with the new course.

One of the things we need to learn is evidence for Einstein's Special Theory of Relativity. Throughout the year, my physics teacher has said that the Michelson-Morley experiment does not provide evidence for SR as it was a null result - it could not conclude the aether existed, and it didn't set out to prove that light was constant regardless of the frame of reference.

However, during my course of studying for the final exams, I have been finding many people and school papers claiming that Michelson-Morley does support SR as it implied that light travelled at a constant speed regardless of the frame of reference.

I do not know which view is correct, and would like someone to clarify.

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Null result is a measurement, and it excludes the hypothesis that light needed a medium to propagate. The solutions of Maxwell's equations had to be interpreted in a different way than the solutions of other wave equations: it is not a medium that carries the energy and momentum of the light beams.

When one's modeling stops depending on a medium the other consequences of the model can be explored.

Lorenz and Larmor:

looked for the transformation under which Maxwell's equations are invariant when transformed from the aether to a moving frame. They extended the FitzGerald–Lorentz contraction hypothesis and found out that the time coordinate has to be modified as well ("local time"). Henri Poincaré gave a physical interpretation to local time (to first order in v/c, the relative velocity of the two reference frames normalized to the speed of light) as the consequence of clock synchronization, under the assumption that the speed of light is constant in moving frames. Larmor is credited to have been the first to understand the crucial time dilation property inherent in his equations.

They still believed the aether at the time. When the MM experiment demonstrated there was no aether, the mathematics of the Lorenz transfrmations is left to describe the motion of magnetic and electric field solutions of the Maxwell equations. Lorenz transformations have c constant , the velocity of light in the medium , inherent. As there was no medium, c became the velocity of light in vacuum.

So it is a deduced proof, but aren't most proofs in physics deduced?

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The Michelson-Morley (MM) experiment is influential because it falsified the aether theories of the 19th century and thus lead to the development of the Special Theory of Relativity (STR).

Up until the 1970s, the common view has been that the MM experiment provided the measurements on which STR is based. And if STR is based on these measurements, then they can also serve as evidence for the theory.

The null result of the experiment has two important implications:

  • First, it implies that the earth is not moving through space, as only observers at rest with respect to the vacuum of space should measure an isotropic speed of light.
  • Next, provided that the earth is in fact moving through space, the null result implies that the speed of light is independent of the motion of the observer, as apparently even observers in motion with respect to the vacuum of space measure an isotropic speed of light.

The first of these implications has been interpreted to mean that the motion of the earth through space is undetectable - and more generally, that observers cannot detect their own motion. Hence, motion in STR is strictly relative, meaning that you always need a reference point for motion; without a reference point, you can't tell whether you are moving or not (as suggested by the null result of the MM experiment).

The second implication is the basis for the "principle of the constancy of the velocity of light", as defined by STR.

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The null result of M-M experiment actually tells you that there is some problem with Galilean transformation of Maxwell's equations for electrodynamics. For further details refer to Tests of special relativity . For a much detailed discussion on relativistic electrodynamics refer to Liénard–Wiechert potential and Feynman lectures.

PS. If you even do a bit of research you will find out that there are even no direct tests for length contraction.

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    $\begingroup$ What does this have to do with the Liénard–Wiechert potential? "Relativistic electrodynamics" sounds impressive, but there isn't any nonrelativistic electrodynamics. That's sort of the point of the Michelson-Morley experiment: that Maxwell's equations are relativistic. $\endgroup$
    – user4552
    Sep 30, 2019 at 13:25
  • $\begingroup$ 1. I hope this clarifies the implications of null result in the MM experiment. 2. Relativistic electrodynamics refers to electromagnetism incorporating SR. 3. Chronologically speaking the Lorentz transformation comes out from the Liénard–Wiechert potential so I think its always good to read it before asking any questions on electromagnetism and SR. $\endgroup$
    – Debobrata
    Sep 30, 2019 at 14:11
  • $\begingroup$ "That's sort of the point of the Michelson-Morley experiment: that Maxwell's equations are relativistic". It doesn't tell that Maxwell's equations are relativistic. It just gives the second postulate of SR. $\endgroup$
    – Debobrata
    Sep 30, 2019 at 14:15

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