Simultaneity Relativity of simultaneity Of all of the ramifications of the constancy of the speed of light, few are more startling than its implications for the concept of simultaneity. We say that two events are simultaneous if, according to our system of synchronized clocks, they take place at the same time . Consider the following situation, often referred to as Einstein's Train Paradox. A high speed train is moving along some tracks when a pair of lightning flashes strike the ground, one at the front of the train and one at the back. According to an observer on the ground, O g , the two strikes are simultaneous.
Lightning strikes the ground at both ends of a moving train.
According to a ground based observer, O g , the strikes are simultaneous. Now consider what an observer, O t , who is on the train, midway between the front and back, will see. The light radiating from each lightning flash travels in a single wavefront. Since both wavefronts reach O g at the same time, after having started from the front and back of the train, and since O t is moving into the light from the front and away from the light from the back, O t must see the light from the front arrive first. To a physicist of the 19th century, the answer would simply be that the light from the front of the train travels at a greater velocity relative to O t than light from the back.