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