Relativity Theory explained by Einstein

The principle of relativity in order to attain the greatest possible clearness , let us return to our example of the railway carriage supposed to be traveling uniformly.We call its motion a uniform translation ( "uniform" because it is of constant velocity and direction, "translation" because although the carriage changes its position relative to the embankment yet it does not rotate in so doing ).

Let us imagine a raven flying through the air in such a manner that its motion , as observed from the embankment , is uniform and in a straight line.If we were to observe the flying raven from the moving railway carriage.We should find that the motion of the raven would be one of different velocity and direction, but that it would still be uniform and in a straight line.
Expressed in an abstract manner we may say : If a mass m is moving uniformly in a straight line with respect to a co-ordinate system K, then it will also be moving uniformly and in a straight line relative to a second co-ordinate system K1 provided that the latter is executing a uniform translatory motion with respect to K. In accordance with the discussion contained in the preceding section , it follows that :If K is a Galileian co-ordinate system , the every other co-ordinate system K' is a Galileian one , when , in relation to K , it is in a condition of uniform motion of translation.Relative to K1 the mechanical laws of Galilei-Newton hold good exactly as they do with respect to K.

We advance a step farther in our generalization when we express the tenet thus: If , relative to K , K1 is a uniformly moving co-ordinate system devoid of rotation , then natural phenomena run their course with respect to K1 according to exactly the same general laws as with respect to K.This statement is called the principle of relativity ( in restricted sense).
As long as one was convinced that all natural phenomena were capable of representation with the help of classical mechanics , there was no need to doubt the validity of this principle of relativity.But in view of the more recent development of electrodynamics and optics it became more and more evident that classical mechanics affords an insufficient foundation for the physical description of all natural phenomena.

Reference : Relativity: the special and the general theory : a popular exposition
De Albert Einstein


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