appeared in June 1905 in On the Electrodynamics of Moving Bodies.

The Lorentz transformations are derived easily from Einstein's postulates. Consider two frames x, y, z, t and x y', z', t ' with the latter moving with speed v in the x direction relative to the first, and assume that their origins coincide at time t = t' = 0. The first postulate implies that the relations between the coordinates are linear, and in particular that x' = y (x - vt), x = y (x' + vt'), for some constant y . Now consider a ray of light emitted from the common origin at t = t' = 0. Since the speed of light is c in both frames, we must have c = x/t = x '/t '. Combining these equations yields y = (1 - v2/c2)-1/2.43

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