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The Postulates of Special Relativity

Inertial Coordinate Systems

      From the first postulate, it follows that there is no coordinate system which is in absolute rest. All motion with constant speed is relative and any coordinate system moving with constant speed (relative to the "fixed stars") is called an inertial coordinate system (or inertial frame [of reference]).

Two inertial frames A and B are moving with constant speed  relative to each other. An observer at rest in A will say that objects at rest in B are moving with respect to A. On the other hand, an observer at rest in B will say that it is the objects at rest in A that are moving with respect to B. Motion is relative!

       
 
 

Actually, Einstein was not influenced so much by the Michelson-Morley experiment at the time when he wrote down The Postulates of Special Relativity as he was by his so-called "Gedankenexperimenten" (imaginary "experiments" in his head) and by Ernst Mach and his principle, Mach's principle1, as well as by Poincaré and his book La Science et l'Hypothèse.

1Mach's principle: The inertial forces experienced by a body in nonuniform motion are determined by the quantity and distribution of matter in the Universe.

 

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