Henry Poincaré was trying to save the Old classical theory of Physics by Suitable Adjustments & Modifications in it.

When the experiments, like Michelson Morley Experiment, in search of the ether drift failed, it began to be increasingly realized that there was no such thing as an absolute or privileged frame of reference and that the basic laws of physics took the same form in all inertial frames of reference. The implications of the Galilean Invariance principle were emphasized by the French mathematical physicist Henry Poincaré when if stated that

. . . . . . . .the laws of physical phenomena [are] the same, whether for an observer fixed or for an observer carried along in a uniform movement of translation, so that we have not and could not have any means of discerning whether or not we are carried along in such a motion.

This simply means that if we are drifting with uniform speed in a spaceship, with all the windows closed, we shall not be able to say, with the help of any experiments we might choose to perform, whether we are at rest or in motion. If, however, we look out of a window we shall be able to say merely that we are in motion with respect to the fixed stars, but not whether we or the stars are actually in motion.
How near, indeed, had Poincaré thus came to expounding the theory of relativity and yet how far he actually was from it. For instead of grasping the implications of the failure of all ether-drift experiments, and building up a new theory on its basis, discarding older notions of space and time, he concerned himself with trying to somehow save the old (& cold 🙂 ) classical theory by suitable adjustments and modifications in it.

The real import of the negative results of the ether-drift experiments was clearly seen and understood by Albert Einstein.

For, discussing the reciprocal electrodynamic action of a magnet and a conductor where the experimentally observable phenomenon depends only on the relative motion of the conductor and the magnet, he goes on to say:

Examples of this sort, together with the unsuccessful attempts to discover any motion of the earth relative to the light medium, suggest that the phenomena of the electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest. They suggest rather that .. . . . .the same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good [i.e., inertial frames of reference] . We will raise this conjecture {the purport of which will hereafter be called the Principle of Relativity} to the status of a postulate, and also introduce another postulate which is only apparently irreconcilable with the former, namely, that light is always propagated in empty space with a definite velocity $c$ which is independent of the state of motion of the emitting body. . . . . . . . . . . . The introduction of a luminiferous ether will prove to be superfluous in as much as the they here to be developed will not require an absolutely stationary space provided with special properties. . .. . . .

from Electrodynamics of Moving Bodies

Thus, from the negative results of the ether-drift experiments and from his own reasoning, Einstein felt fully convinced that there was no such thing as an absolute or fixed frame of reference. He examined the physical consequences of the absence of such a frame of reference and had the boldness to break away from old and traditional concepts of space and time. He knit his conclusions and revolutionary ideas into a cogent theory which he announced to an unsuspecting world in the year 1905 as his Special theory of relativity. And, ten years later , in 1915, followed the second and the more complex and difficult part of it in the form of the General Theory of Relativity.

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