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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. $\endgroup$ – Ben Crowell Sep 30 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 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 at 14:15
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Many tests of special relativity such as the Michelson–Morley experiment and the Kennedy–Thorndike experiment have shown within tight limits that in an inertial frame the two-way speed of light is isotropic and independent of the closed path considered.

However, the "one-way" speed of light, from a source to a detector, cannot be measured independently of a convention as to how to synchronize the clocks at the source and the detector. What can however be experimentally measured is the round-trip speed (or "two-way" speed of light) from the source to the detector and back again.

In popular and educational books on the theory of relativity, it is frequently claimed that Michelson and Morley experimentally proved the fallacy of the ethereal concept. This is an untrue statement. The outcome of the Michelson-Morley experiment demonstrated the impossibility of detecting ether using an experiment of this type, but not the absence of ether.

One must not lose sight of the fact that the results of the Michelson and Morley experiment did not prevent the notions about the ether from being retained for a good two decades after they obtained their negative measurement result. Michelson himself and Lorentz shared these notions.

Indeed, FitzGerald and Lorentz explained this result by way of the longitudinal shortening of objects moving through the ether, i.e., they explained it within the framework of the ethereal world view. And so it was that in the waning years of his life, in 1952, Einstein wrote in the article “Relativity and the Problem of Space : "Concerning the experiment of Michelson and Morley, H. A. Lorentz showed that the result obtained at least does not contradict the theory of an ether at rest”.

In this regard, the remark of a proponent and popularizer of the theory of relativity, M. von Laue, should also be clear, who wrote: “…it was experimentally impossible to make a choice between this theory (the Lorentz theory) and Einstein’s theory of relativity, and if the Lorentz theory nonetheless took a back seat – even though it still has proponents among physicists – this then undoubtedly occurred due to reasons of a philosophical nature”.

How should M. von Laue’s remark that it is experimentally impossible to make a choice between the Lorentz theory and the Einstein theory be taken? Indeed, according to Lorentz, reference systems at rest in the ether and inertially absolutely moving through the ether are not physically equal. Is this circumstance really in agreement with the fact of the equality of inertial systems in Einstein’s STR?

The equality of inertial systems in the STR is expressed by way of the invariance of the mathematical notation of the laws of nature in these systems, but this form of equality historically migrated to Einstein’s STR from the Lorentz theory. Indeed, the transformations that ensure the immutable form of notation of these same Maxwell equations in different, physically unequal inertial reference systems appear in Lorentz’s work as a consequence of the requirement for such immutability. While Einstein tied the immutability of the form of notation of the physical laws of nature in different reference system to physical equality, Lorentz demonstrated that this requirement can be met even in physically unequal systems absolutely at rest and absolutely in motion. I.e., Lorentz showed that physically unequal inertial reference systems can be transformed into mathematically equal ones by imposing the requirement of invariance on them

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  • $\begingroup$ Brillant reminder of the actual history of the aether hypothesis. $\endgroup$ – Exocytosis Sep 30 at 11:02
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    $\begingroup$ This doesn't answer the question. Very little of the historical material has anything to do with the OP's question. $\endgroup$ – Ben Crowell Sep 30 at 13:23
  • $\begingroup$ It is most convenient reading the text from top to bottom, and not from bottom to top. They key point is that MM experiment say nothing about the one - way speed of light. So as not to create a biased impression, in regard with this question, it is important to emphasize (at least) this circumstance, since this problem is unfairly avoided in many books. $\endgroup$ – Albert Sep 30 at 15:12

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