If flashbulbs at each end of a long, fast-moving train discharge 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 have to illuminate each of those detectors at some point.

It would not be possible, for example, for the light from each flash to illuminate the detector on the train and not the detector on the ground, or vice versa. When each flash occurs, everything in sight of that flash will be illuminated.

Because these flashes occur simultaneously in the reference frame of the train, the Michelson-Morley findings prove that the light from each flash will illuminate the detector on the train simultaneously, since that detector will be fixed halfway between the two light sources and will not be moving relative to them.

Common sense argues that only one of these detectors could be illuminated simultaneously by those two flashes, since the detectors would be moving apart during the entire experiment and would be in different locations after even the smallest interval of time. However, the laws of light propagation insist that if one of these detectors is illuminated simultaneously, then so must the other.

This website was designed to show that if this experiment were actually carried out in a laboratory, and if each flash occurred when each detector was located exactly halfway between the two light sources (zero lateral separation), then the light from each flash would reach each detector simultaneously, even though that behavior of light would be counterintuitive.