The history of relativity is quite interesting as a few people had similar ideas around the same time. While Einstein without a doubt is responsible for the theory and interpretation of relativity, Lorentz and Poincaré had similar ideas.

In 1904 in “Electromagnetic phenomena in a system moving with any velocity smaller than that of light” Lorentz ( Citation: Lorentz, 1937 Lorentz, H. (1937). Electromagnetic phenomena in a system moving with any velocity smaller than that of light. In Collected papers: Volume v. (pp. 172–197). Springer. Retrieved from https://isidore.co/misc/Physics%20papers%20and%20books/Classic%20Papers/Electromagnetic%20phenomena%20in%20a%20system%20moving%20with%20any%20velocity%20smaller%20than%20that%20of%20light%20(H.%20A.%20Lorentz%201904).pdf ) proposed that electrons change shape while moving through the ether . To be able to match the results of the Michelson-Morley and Kaufmann experiments, a set of transformations were proposed that are now known as the Lorentz transformations. Poincaré formalized this idea a bit more, by making the derivations more rigorous. In 1905 he published a note to announce some of his results ( Citation: Poincare, 1905 Poincare, H. (1905). Sur la dynamique de l’electron’, comptes rendus 140 june 5th, 1504-08. In fact, poincaré’s anticipated einstein’s by more than three weeks. Relativité Restraimte SR. Retrieved from https://fr.wikisource.org/wiki/Sur_la_dynamique_de_l’électron_(juin) ):

• Formally deriving the Lorentz transformations from the principle of least action.
• A dismissal of Langevin’s idea that the volume of an electron stays constant while moving through the ether.
• The force of gravity acts with the speed of light

Interestingly, it was Poincaré who first proposed that the force of gravity should act with the speed of light. Already in 1805, Laplace had proposed that gravity is not instantaneous (though postulated that it was much faster than light), but here Poincare is the first to propose that gravity and light have the same speed. The note itself omits a lot of details, but a few months later he published his full results in ( Citation: Poincaré, 1913 Poincaré, H. (1913). La dynamique de l’électron. A. Dumas. Retrieved from https://fr.wikisource.org/wiki/Sur_la_dynamique_de_l’électron ) under the same title. It was a bit unfortunate that this paper did not receive a lot of attention until Minkowski’s work, because it already shows some very fundamental results:

• The invarients $x^2 + y^2 + z^2 - c^2t^2$, $E^2-B^2$, & $\bb{E} \cdot \bb{B}$
• The “Wick rotation” of time to imaginary time using the coordinates $x, y, z, i c t$
• 4-vectors for the current and their transformation rules
• Lie algebras for the Lorentz group

Bibliography⌗