Special Relativity was Announced in 1904

Relavity historians give 1905 for the theory's origin, but it was really 1904.

Hector Giacomini writes in a new paper, also here:

Henri Poincaré’s Saint Louis lecture, delivered on 24 September 1904 at the International Congress of Arts and Science, occupies a distinctive place in the pre-history of twentieth-century theoretical physics. In this text, Poincaré formulated the principle of relativity in explicit and general terms, not as a narrow empirical rule limited to electrodynamics, but as one of the major guiding principles of mathematical physics. The lecture also offered a principle-based conception of theory centered on invariance, least action, and general theoretical coherence.

Poincare and other great scholars made the trip all the way to St. Louis, Missouri, USA, where the World's Fair was being held. For a month, St. Louis was the center of the world.

The text of the lecture was widely published and distributed in 1904, and an English translation was published in a popular philosophical journal in Jan. 1905. The above paper documents the wide distribution.

Poincare's lecture did not have any formulas, but he clearly had the essence of special relativity.

He wrote two kinds of papers. Technical papers with formulas intended for mathematicians, and papers without formulas intended for a wider audience. He was probably the most widely-read intellectual in Europe. Perhaps some did not appreciate him because he did not attempt to explain the formulas to non-mathematicians.

In explicit and programmatic terms, Poincaré formulated the principle of relativity as follows: the laws of physical phenomena must be the same for an observer at rest and for an observer carried along in uniform translational motion. Consequently, no experiment should allow one to determine whether one is in such uniform motion or not. In the lecture this formulation appears within the canonical list of fundamental principles and is treated on the same conceptual level as energy conservation and the principle of least action.

The relativity principle is not introduced merely as an empirical summary of ether-drift experiments. Rather, Poincaré presents it as a structural requirement increasingly supported by the persistent failure of attempts to detect motion relative to the ether. He discusses in particular the negative results of Michelson-type experiments and emphasizes the remarkable stability of electromagnetic theory under uniform motion. The continued empirical confirmation of null results is interpreted as evidence that the invariance of physical laws under uniform translation may reflect a deep structural property of nature. ...

In this respect, Poincaré’s principle of relativity appears as the explicit crystallization of themes already articulated in his writings [11, 12, 13], and made accessible to German-speaking readers through the 1904 translation of La science et l’hypothèse [14]. Poincaré further analyzes the theoretical devices introduced to preserve this invariance. He discusses Lorentz’s notion of “local time”, obtained by synchronizing clocks through light signals ...

Importantly, Poincaré suggests that the situation may ultimately require a new mechanics in which no velocity could exceed that of light and in which inertia would increase with speed. Such remarks indicate that the principle of relativity is not treated as a peripheral correction within classical mechanics, but as a constraint capable of reshaping its conceptual structure.

Lorentz’s 1904 Theory

An important component of the lecture is Poincaré’s discussion of Hendrik Antoon Lorentz’s recent work. In May 1904 Lorentz had published a major paper [6]. In that work Lorentz presented a refined mathematical formulation of the transformations required to preserve the form of Maxwell’s equations in a moving frame.

Poincaré’s lecture demonstrates that by September 1904 he was fully aware of the structure and implications of Lorentz’s construction. He describes the introduction of local time, the contraction hypothesis — according to which bodies moving through the ether undergo a physical contraction in the direction of motion — and the modification of forces and masses required to reconcile theory with experiment.

The paper makes no mention of Einstein, as he did not write anything on relativity until Summer 1905.

Poincare wrote his great relativity paper in 1905, but his 1904 lecture has the essence: the relativity principle, Lorentz transformations, length contraction, Michelson-Morley, clock synchronization, local time, and a new mechanics where nothing goes faster than light.

Previously I argued that the essence of relativity was the 4D spacetime, Lorentz group, non-euclidean geometry, covariant equations, and extending beyond electromagnetism. Poincare had all these in 1905, and Einstein did not understand them until years later.

Einstein sometimes denied that he knew about Lorentz's 1904 paper and Poincare's 1905 paper, although it is documented that he had access to both before submitting his own 1905 paper. I do not know if he was ever asked about Poincare's St. Louis lecture. It is hard to believe he could have missed it, as it was read by anyone with an interest in Mathematical Physics.

Einstein did not reference Lorentz or Poincare in his famous 1905 relativity paper. Even if he really did not know about these papers, he surely knew about them when he wrote survey papers on relativity a couple of years later.

You could also argue that relativity started in 1895, with Lorentz's paper. He had the approximate Lorentz transformations, Michelson-Morley to second order, local time, length contraction, and relation to Maxwell's equations. Lorentz got the 1902 Nobel Prize for his electromagnetic theory. He did not have the higher order theory he found in 1904, the symmetries as a group, and connecting local time to clock synchronization.