Relativity

• Einstein talking—> It is not clear what is to be understood as “position” and “space”. I stand at the window of a railway carriage which is traveling uniformly, and to drop a stone on the embankment, without throwing it. Then, disregarding the influence of the air resistance, I see the stone descend in a straight line. A pedestrian who observes the misdeed from the footpath notices that the stone falls to the Earth in a parabolic curve {lol how?!} I now ask: “Do the “positions” traversed by the stone lie “in reality” on a straight line or a curve [a parabola]? Moreover, what is meant here by motion “in space”? In the first place, we entirely shun the vague word “s[ace”, of which we must honestly acknowledge that we cannot form the slightest conception, and so we replace it by “motion relative to a practically rigid body of reference.” the positions relative to the body of reference [railway carriage or embankment] have already been defined in detail; if instead of “body of reference” we insert “system of coordinates”, which is a useful idea for mathematical description, we are in a position to say: The stone traverses a straight line relative to a system of coordinates rigidly attached to the carriage, but relative to a system of coordinates rigidly attached to the ground [embankment] it describes a parabola. With the aid of this example it is clearly seen that there is no such thing as an independently existing trajectory [the path-curve], but only a trajectory relative to a particular body of reference.