Einstein's reinterpretation of the Fizeau experiment: How it turned out to be crucial for special relativity

In this article, we aim at clarifying the role played by Fizeau’s 1851 experiment, both in the context of discovery and in the context of justification of the special theory of relativity. In 1907 Laue proved that Fresnel's formula was a consequence of the relativistic composition of velocities; since then, Einstein regarded Fizeau's experiment as confirmatory evidence for his theory, and even as a crucial experiment in favor of the relativistic addition of velocities. On the other hand, in the 1920's Einstein stated that this experiment was decisive in the path that led him to the discovery of his theory before 1905, but he did not explain why. We survey all the available evidence on this subject and conclude that the original ether-drag experiment was reinterpreted within a new conceptual framework in which the meaning of the very concept of velocity undergoes a radical change.