Testing Absolute vs Relative Simultaneity with the Spin-orbit Interaction and the Sagnac Effect

All the experiments supporting special relativity (SR) formulated with Einstein synchronization support as well SR with absolute synchronization, if the corresponding coordinate transformations foresee time dilation and length contraction. We first test absolute vs relative simultaneity with a non-relativistic model of the spin-orbit interaction by taking into account either the effect of the electron hidden momentum or the relativistic effect of the Thomas precession, based on
non-conservation of simultaneity. As second test, we consider a thought experiment equivalent to the Sagnac effect, where a clock measures the time taken by a counter-propagating light signal to perform a round trip on a closed path. While these experiments are coherently described with absolute simultaneity, the result of our tests points out inconsistencies in the case of relative simultaneity, thus favoring the formulation of SR with absolute synchronization, while advocating that
further research and tests on simultaneity are needed for the comprehension of relativistic theories.