In 1905 Einstein generalized the Galelian relativity principle [ applicable only to Newtons laws ] to the whole of Physics by postulating that:
All inertial frames are equivalent for all experiments i.e. no experiment can measure absolute velocity.
Maxwell's equations and the speed of light must be the same for all observers.
Figure 1.4: In the light of knowledge attained, the happy achievement seems almost a matter of course, and any intelligent student can grasp it without too much trouble. But the years of anxious searching in the dark, with their intense longing, their alternations of confidence and exhaustion, and the final emergence into the light- only those who have themselves experienced it can understand that. Albert Einstein.
Einstein's motivation was to avoid inconsistencies between Maxwell's equations and Galelian relativity. The Lorentz transformations must relate to actual space and time measurements.
Special Relativity abolishes the idea of absolute space [ for example the ether ] and absolute time, but it leaves unexplained the origin of inertial frames. Mach's principle says that the inertial frames are determined be the rest of the matter in the universe [ i.e. those that are non- accelerating with respect to the rest of the universe ]. Einstein tried later to incorporate this idea into General Relativity.n