If large flashbulbs are attached at each end of a long, fast-moving train and if each flashbulb discharges when a photon detector at the center of the train is directly opposite a similar detector on the ground, light waves will radiate out from each flash in all directions and will illuminate each of those detectors at some point.

The Michelson-Morley findings prove that the wave front from each flash will reach the central detector on the train simultaneously, and laboratory findings from binary stars and other moving light sources prove that those same light waves will also reach the detector on the ground simultaneously. That conclusion is counterintuitive but it is based on laboratory findings that have been replicated and confirmed.