[It is this definition of simultaneity, coupled with the assumption that all observers, on whatever uniformly moving systems, would obtain the same experimental value for the velocity of light, that leads to the apparent paradoxes of the Special Theory of Relativity. If it be asked why we adopt it, we must in turn ask the inquirer to propose a better system for defining simultaneous events on different moving bodies.]198 [There is nothing in this definition to indicate, directly, whether simultaneity persists for all observers, or whether it is relative, so that events simultaneous to one observer are not so to another. The question must then be investigated; and the answer, of course, will hinge upon the possibility of making proper allowances for the time of transit of the light signals that may be involved. It seems as though this ought to be possible; but a simple experiment will indicate that it is not, unless the observers involved are at rest with respect to one another. AN EINSTEINIAN EXPERIMENT Let us imagine an indefinitely long, straight railroad track, with an observer located somewhere along it at the point $M$. According to the convention suggested above, he has determined points $A$ and $B$ in opposite directions from him along the track, and equally distant from him. We shall imagine, further, than a beneficent Providence supplies two lightning flashes, one striking at $A$ and one at $B$, in such a way that observer $M$ finds them to be simultaneous. While all this is going on, a train is passing--a very long train, amply long enough to overlap the section $AMB$ of the track. Among the passengers there is one, whom we may call $M'$, who is directly opposite $M$ at the instant when, according to $M$, the lightning strikes. Observe he is not opposite $M$ when $M$ sees the flashes, but a brief time earlier--at the instant when, according to $M$'s computation, the simultaneous flashes occurred. At this instant there are definitely determined the points $A'$ and $B'$, on the train; and since we may quite well think of the two systems--train-system and track-system--as in coincidence at this instant, $M'$ is midway between $A'$ and $B'$, and likewise is midway between $A$ and $B$. Now if we think of the train as moving over the track in the direction of the arrow, we see very easily that $M'$ is running away from the light from $A$ and toward that from $B$, and that, despite--or if you prefer because of--the uniform velocity of these light signals, the one from $B$ reaches him, over a slightly shorter course, sooner than the one from $A$, over the slightly longer course. When the light signals reach $M$, $M'$ is no longer abreast of him but has moved along a wee bit, so that at this instant when $M$ has the two signals, one of these has passed $M'$ and the other has yet to reach him. The upshot is that the events which were simultaneous to $M$ are not so to $M'$.