5 4 D O C . 4 4 T H E T H E O R Y O F R E L A T I V I T Y 44. “The Theory of Relativity” [Einstein 1927p] Dated 7 September 1927 In: Reclam Praktisches Wissen, 1927, pp. 5–8 Every motion can, conceptually, be considered only as a “relative” motion i.e., in order to describe the motion of an object, I must ask the question in relation to what other object is the object in question moving. If, for example, an express train is moving along a track, then I can relate the motion of the train to the track as a “frame of reference”: The cars are then moving relative to the track. I could how- ever also use the cars as the frame of reference then the track is moving relative to the cars. This has always been known it is self-evident and has nothing to do with the much more extensive statement that we call the principle of relativity, and with which we shall be concerned in the following. As a “frame of reference,” analogously to the train track or the railroad car, we shall henceforth use the “Cartesian coordinate system,” which consists of three rig- id, planar walls, mutually perpendicular and connected to each other to form a rigid body. The position of any event in relation to this coordinate system is described (essentially) by giving the lengths of the three perpendicular lines or “coordinates” (x, y, z) that can be drawn from the point where the event occurs to each of those three walls. A “Galilean coordinate system” is one in which the laws of Galilean-Newtonian mechanics hold. Its fundamental law, which is known as the law of inertia, is well known (I): A body that is sufficiently distant from all other bodies persists at rest or in a uniform and rectilinear motion. Such “sufficiently distant” bodies, to which the law of inertia can be applied with high accuracy, are the visible fixed stars. If we now connect our coordinate system rigidly to the Earth, then each fixed star de- scribes a circle with an enormous radius in the course of every day, in contradiction to the law of inertia, according to which they should remain at rest or in a state of constant linear velocity. If we adhere to the law, then we can refer motions only to coordinate systems with respect to which—“relative” to which—the fixed stars [p. 5]
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