But actually, the existence of gravitational waves traveling at the speed of light was clearly predicted by Henri Poincaré as early as June 5, 1905, as a strict requirement of relativistic space-time geometry. Poincaré made this requirement explicit in his academic note Sur la dynamique de l'électron (On electron dynamics, June 5, 1905) published by the French Académie des Sciences. After explicitly formulating special relativity in this fundamental article, Poincaré further develops the requirement emitted by Hendrik Antoon Lorentz that the new space-time transformation leading to special relativity should apply to all existing forces and not just to the electromagnetic interaction.
Henri Poincaré concludes that, as a consequence of the new space-time geometry, gravitation must generate waves traveling at the speed of light in a similar way to electromagnetism.
Following the propaganda contained in the press releases of scientific collaborations and institutions, almost all medias attribute to Albert Einstein the original prediction of gravitational waves.
The Physical Review Letters article by LIGO and VIRGO Observation of Gravitational Waves from a Binary Black Hole Merger, PRL 116, 061102 (11 February 2016), explicitly sates https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.116.061102 : "In 1916, the year after the final formulation of the field equations of general relativity, Albert Einstein predicted the existence of gravitational waves". What, then, about the work done by Henri Poincaré 11 years before the Einstein finding ?
Actually, the situation seems quite clear. In his short article of 5 June 1905 Sur la dynamique de l'électron, C.R. T.140 (1905) 1504-1508 (Comptes Rendus de l'Académie des Sciences, France), http://www.academie-sciences.fr/pdf/dossiers/Poincare/Poincare_pdf/Poincare_CR1905.pdf , the French mathematician and physicist Henri Poincaré explicitly formulated special relativity upgrading the space-time transformations that he called "Lorentz transformations" and to which he referred as the "Lorentz group". After having worked out and discussed the new space-time geometry, Poincaré writes:
Mais ce n’est pas tout: Lorentz, dans l’Ouvrage cité, a jugé nécessaire de compléter son hypothèse en supposant que toutes les forces, quelle qu’en soit l’origine, soient affectées, par une translation [a change of inertial frame in Poincaré's language], de la même manière que les forces électromagnétiques, et que, par conséquent, l’effet produit sur leurs composantes par la transformation de Lorentz est encore défini par les équations (4).
Il importait d’examiner cette hypothèse de plus près et en particulier de rechercher quelles modifications elle nous obligerait à apporter aux lois de la gravitation. C’est ce que j’ai cherché à déterminer; j’ai été d’abord conduit à supposer que la propagation de la gravitation n’est pas instantanée, mais se fait avec la vitesse de la lumière. (...)
Quand nous parlerons donc de la position ou de la vitesse du corps attirant, il s’agira de cette position ou de cette vitesse à l’instant où l’onde gravifique est partie de ce corps; quand nous parlerons de la position ou de la vitesse du corps attiré, il s’agira de cette position ou de cette vitesse à l’instant où ce corps attiré a été atteint par l’onde gravifique émanée de l’autre corps; il est clair que le premier instant est antérieur au second.
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Gravitational waves were thus explicitly predicted by Henri Poincaré in his 5 june 1905 article formulating special relativity.
In special relativity, such as already defined by Poincaré and Lorentz, the speed of light c is not just the speed of a specific object (light) but a universal parameter defining the space-time geometry. As a consequence, no physical object, signal, correlation... can travel faster than c .
According to Poincaré in his article of 5 June 1905, the requirement of a universal space-time geometry with the speed of light c as the critical speed implies that the gravitational force must be propagated by gravitational waves with a speed equal to c , just as electromagnetic waves carry the electromagnetic interaction.
As Henri Poincaré explicitly underlines, the space-time geometry defined by Lorentz tranformations applies to all existing forces including the gravitational ones. Thus, gravitation cannot propagate instantaneously and must instead propagate at the speed of light. The same argument clearly applies to any object associated to gravitation.
Considering as a simple example the gravitational interaction between two bodies, Poincaré introduces a "gravific wave" leaving the first body, traveling at the speed of light and reaching the second body at a later time. This was the original formulation of the prediction of gravitational waves in a context where its general scope was obvious. Poincaré had been working for years on electromagnetism, and knew perfectly well that more sophisticated scenarios than the example he was providing could be imagined without altering the role of c as the critical speed.
A decade later, with general relativity, Albert Einstein considered in detail more involved scenarios than the one made explicit by Poincaré, incorporating in particular an effective space-time curvature generated by gravitation in a static universe. But this does not invalidate the basic principle formulated by Henri Poincaré in 1905.
In his article, Poincaré also refers to the previous work by Pierre-Simon de Laplace, Count of Laplace (1749-1827), one of the main French scientists of the period of Napoléon Bonaparte. Laplace had already considered the possibility that gravitation propagates at some finite speed, but he did not question the basic space-time geometry.
An abridged version of this article has been posted to this address.