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[old] principles will not come forth from out of the fray, victorious and intact.

36 These transformations were derived first in 1887 by Woldemar Voigt, who noted that under them the wave equation retains its form.

These ideas were discussed also in his essays La science et l'hypothese (1901) and La valuer de la science (1905). English translations in PoincarĂ© (1951) and PoincarĂ© (1958), respectively. Quoted from Poincare (1958).

Poincare continued to work on the relativity principle and, in 1905, practically simultaneously with Einstein, created much the same mathematical framework. What Poincare, like Lorentz before him, did not do was abandon the ether and base the whole of physics on new kinematical principles.40 FitzGerald and Lorentz had appreciated that a new postulate was required to reconcile the classical principle of relativity with observed electromagnetic phenomena, but their contraction hypothesis was of a dynamic origin. Einstein (ignorant of Lorentz's work since 1895) took a different way out.41 Rather than give the ether new mechanical properties, or revise the foundations of electrodynamics, he changed the structure of the underlying concepts of space and time by postulating that

(i) The laws of physics are of the same form in all inertial frames.

(ii) In any given inertial frame, the speed of light is the same whether the source is at rest or in uniform motion.

Put simply, special relativity is the application of these two principles, which

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