Special theory of Relativity
When the experiments in search of the ether drift failed ,it began to be increasingly realised 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 this Galilian invariance principle were emphasised by French mathematical physicist Henri Poincare when he 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 we look outside by 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 star are actually in motion.
How near,indeed,had poincare thus come to expounding the theory of relativity and yet how far he actually was from it. For, instead pf grasping the implications of 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 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 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 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. We will raise this conjecture to the status of a postulate ,and also introduce another postulate which is only apperently 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 view here to be developed will not require an ansolutely stationary space provided with special properties'.
Thus from negative results of the ether drifts 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 year 1905 as his Special theory of Relativity. And 10 years later in 1916, followed the second and the more complex and difficult part of it in the form of general theory of relativity. The former deals with problem associated with unaccelerated frames of reference i.e those which move with uniform relative velocity with respect to one another and the latter with those associated with accelerated ones.
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