Who did It First?
Poincare and relativity
28 November 2011, 18:00 PM
Einstein and PoincareIN June of 1905 the theory of special relativity was explained in two papers, one written by Henri Poincare, and the other by Albert Einstein. Poincare sent his paper "On the Dynamics of the Electron" to the Academy of Science in Paris on June 5, and Einstein submitted his paper "On the Electrodynamics of Moving Bodies" to the Annalen der Physik on June 30. Poincare followed up with a more detailed paper (now often referred to as the Palermo paper) in July, and Einstein followed with his paper "Does the Inertia of a Body Depend on Its Energy Content?" in September.
Admittedly it is controversial to claim that Poincare's papers in the summer of 1905 described "special relativity", since that name is now so closely associated with the particular point of view introduced by Einstein. Nevertheless, Poincare's two 1905 papers, together with his previous writings, clearly described a theory of relativity, even if it was not identical to Einstein's theory in all of its philosophical commitments. As early as 1898 Poincare had explicitly denied any justification for the idea of absolute simultaneity, and had proposed the operational definition of simultaneity based on light signals. By 1905 he had the full Lorentz transformations for electrodynamics, including the group property, and had corrected Lorentz's expression for the current density.
He goes on to say that Lorentz had accounted for the negative results of Michelson and Morley by means of the Fitzgerald contraction, but that this by itself was not sufficient to ensure complete relativity, because the Fitzgerald contraction had been justified only for electromagnetic formations, not for whatever forces are responsible for holding atoms and sub-atomic particles together, nor for gravitational forces. Indeed, Poincare shows that the entire inertia of a charged particle could be attributed to electromagnetic forces only if the charge was zero.
Thus the Lorentz-covariance of electromagnetism does not entitle us to assume that all of physics (particularly the mechanics of material objects) is Lorentz-covariant. Since Poincare did not know (and we still do not know today) the origin of inertial mass, he realized that it was necessary to simply infer the transformation properties of the forces responsible for inertial mass from the principle of relativity, i.e., to identify the conditions that these forces or bonds must satisfy for electron equilibrium to be Lorentz covariant. This had also been recognized by Lorentz.
In summary, Lorentz had shown two things, first, that the laws of electromagnetism (Maxwell's equations) are covariant under Lorentz transformations, and second, somewhat tautologically, that if all physics (including the inertia of material bodies) is reducible to electromagnetism, then all physics is covariant under Lorentz transformations, and therefore the principle of relativity applies. Of course, this would entail a revision of the laws of mechanics, which had previously been thought to be covariant with respect to Galilean, not Lorentzian, transformations. However, no one knew how to show that mechanical inertia is reducible entirely to electromagnetic forces so it was not possible to derive the principle of relativity in such a constructive way. What Lorentz and Poincare were obliged to do is simply to assume the principle of relativity is valid and then to infer the consequences. One of these consequences is that, obviously, variations in all mass less forces must propagate at the speed of light. At this point we begin to see one aspect of what Poincare has in mind when he talks about working out the consequences.
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