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At the Introduction, PoR-a:

{...the same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good.}

At the beginning of paragraph 2, PoR-b:

{1. The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems of co-ordinates in uniform translatory motion.}

Taking into account the writing epoch, it is clear that "the equation of mechanics" that Einstein is referring are the Newtonian ones. To be more precise in this point and to put out any reasonable doubt, I will put here the beginning of paragraph1:

{Let us take a system of co-ordinates in which the equations of Newtonian mechanics hold good. In order to render our presentation more precise and to distinguish this system of co-ordinates verbally from others which will be introduced hereafter, we call it the "stationary system".

If a material point is at rest relatively to this system of co-ordinates, its position can be defined relatively thereto by the employment of rigid standards of measurements and the methods of Euclidean geometry, and can be expressed in Cartesian co-ordinates.}

As we see, the words "Euclidean", "Cartesian" and "Newtonian" are written directly by Einstein, corresponding to the 1905 epoch knowledge. But what is the new basic idea that Einstein is introducing in his paper? We can find it very clearly expressed in the Introduction:

{The introduction of a "luminiferous ether" will prove to be superfluous inasmuch as the view here to be developed will not require an "absolute stationary space" provided with special properties, nor assign a velocity-vector to a point of the empty space in which electromagnetic processes take place.

The theory to be developed is based -like all electrodynamics- on the kinetics of the rigid body, since the assertions of any such theory have to do with the relationships between rigid bodies (system of co-ordinates), clocks and electromagnetic processes. Insufficient consideration of this circumstance lies at the root of the difficulties which the electrodynamics of moving bodies at present encounters.}

I want to put emphasis on the identification that Einstein makes between "rigid bodies" and "systems of co-ordinates". Once the ether is taken out as not existent, only the bodies themselves can be used to determine an inertial system of co-ordinates. In my opinion, this is the new basic idea that Einstein is introducing here. As a result, the separation of electrodynamics from the rest of Physics is eliminated, extending Galileo’s Principle of Relativity to all Physics. This is particularly clear in version b) of the PoR, where ALL laws of Physics are included, without any exception.

This last assertion is far to be trivial, if we take into account that after the development of General Relativity in 1916, gravity is totally excluded from Einstein’s 1905 PoR. Another important point is the mix of electrodynamics with Newtonian mechanics in the a) version of the PoR. We can’t forget that Einstein expended the rest of his live trying (without success) to unite gravity with electrodynamics in the context of General Relativity.

The principal motivation of this post is to make clear in what extend Einstein’s 1905 PoR is considered today still valid, if the development of General Relativity (GR) in 1916 excludes or not the possible development in the future of a 1905 Relativity gravity theory with the PoR maintaining its original validity. As a final remark I want to remember that the universal mass-energy relationship fruit of 1905 Relativity continue being valid in the context of GR.

RVHG(R.Valls)