Logical test of Einstein's theory of relative simultaneity
Einstein claims events may be simultaneous in one frame of reference relative to one observer and not simultaneous to another observer. This is supposed to follow from the belief that the speed of light is always constant.
Read about The train-and-platform thought experiment here: Relativity of simultaneity - Wikipedia, the free encyclopedia
Now add to this two stop watches, one at each end of the train car, ie. front and back. Have electronic circuits arranged to stop each stop watch when it senses a photon hitting the wall of the train car (ie. using a photocell). Start each stop watch while the train car is at rest relative to the observer on the outside of the train car, start the train car and do the experiment as described. After the experiment, stop the train car and bring the two observers and the two stop watches together. If the observers saw the photons hitting the front and back of the train car at different times during the experiment (when the train car was in motion relative to the outside observer), then they should both see different times on the stop watches, after the experiment even while both observers and both stop watches are in the same frame of reference. This is considered to be impossible in Special relativity. Recall that in the twin or clock paradox the explanation used by the Einsteinian proponents to get assymetric time dilation in the final result. The twin and/or clock that is accelerated is supposed to be the twin and/or clock that undergoes time dilation. If this weren't the case then the readings on the clock that was moved or accelerated and the one left behind would be inconsistant as observed by all observers.
You can use stopwatches to show events are always simultaneous to all observers anywhere showing that an instant in time is the same instant in time everywhere. In the pre-Einsteinian aether theories, the speed of light would be expected to have a particular value in the aether frame of reference which would be the same in any direction. Experiments that result in a constant observed speed of light can probably be explained by the effects of the Lorentz transformations on the instruments and observers and can be argued to be consistant with the pre-Einsteinian aether theories (ie. Larmor, Lorentz, Poncare).