At this juncture the theory of relativity entered the arena.  As a result of an analysis of the physical conceptions of time and space, it became evident that in realily there is not the least incompatibilitiy between the principle of relativity and the law of propagation of light, and that by systematically holding fast to both these laws a logically rigid theory could be arrived at.  This theory has been called the special theory of relativity to distinguish it from the extended theory, with which we shall deal later.  In the following pages we shall present the fundamental ideas of the special theory of relativity.

ON THE IDEA OF TIME IN PHYSICS

Lightning has struck the rails on our railway embankment at two places A and B far distant from each other.  I make the additional assertion that these two lightning flashes occurred simultaneously.  If I ask you whether there is sense in this statement, you will answer my question with a decided “Yes.”  But if I now approach you with the request to explain to me the sense of the statement more precisely, you find after some consideration that the answer to this question is not so easy as it appears at first sight.

After some time perhaps the following answer would occur to you:  “The significance of the statement is clear in itself and needs no further explanation; of course it would require some consideration if I were to be commissioned to determine by observations whether in the actual case the two events took place simultaneously or not.”  I cannot be satisfied with this answer for the following reason.  Supposing that as a result of ingenious considerations an able meteorologist were to discover that the lightning must always strike the places A and B simultaneously, then we should be faced with the task of testing whether or not this theoretical result is in accordance with the reality.  We encounter the same difficulty with all physical statements in which the conception " simultaneous " plays a part.  The concept does not exist for the physicist until he has the possibility of discovering whether or not it is fulfilled in an actual case.  We thus require a definition of simultaneity such that this definition supplies us with the method by means of which, in the present case, he can decide by experiment whether or not both the lightning strokes occurred simultaneously.  As long as this requirement is not satisfied, I allow myself to be deceived as a physicist (and of course the same applies if I am not a physicist), when I imagine that I am able to attach a meaning to the statement of simultaneity. (I would ask the reader not to proceed farther until he is fully convinced on this point.)

After thinking the matter over for some time you then offer the following suggestion with which to test simultaneity.  By measuring along the rails, the connecting line ab should be measured up and an observer placed at the mid-point M of the distance ab.  This observer should be supplied with an arrangement (e.g. two mirrors inclined at 90^0) which allows him visually to observe both places A and B at the same time.  If the observer perceives the two flashes of lightning at the same time, then they are simultaneous.