Galileo, describing in 1632 his famous “ship” in the Dialogue Concerning the Two Chief World Systems, had already made the fundamental principle clear: shut inside a ship moving at constant velocity, no mechanical experiment can tell us whether the ship is moving or standing still relative to the harbour. Butterflies fly, drops fall, fish swim the same way, whether the ship is in uniform motion or moored. The laws of mechanics are the same in every system moving in uniform rectilinear motion relative to another.
For nearly three centuries this restriction — “no mechanical experiment” — was accepted without objection. After all, light and electromagnetism seemed to be phenomena of an entirely different nature, to be analysed separately, and nothing ruled out that they might reveal absolute motion relative to a hypothetical ether.
In 1905, Einstein took an apparently small but logically explosive step: he replaced “mechanical” with no experiment. So the laws of electromagnetism too, and hence the speed of light, must be the same in every inertial frame. From this single extension of a word follows everything: time dilation, length contraction, mass–energy equivalence.
The history of physics is full of similar episodes: a small change to an assumption that seemed purely technical turns out to be a conceptual revolution. Teaching relativity, in the end, largely means showing students that Einstein’s truly daring move lies not in the calculations, but in the choice of that one extra word (Einstein and Infeld 1938; Rovelli 2014; Battimelli and Stilli 1999). See also Riferimenti bibliografici.
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Topics: Relatività ristretta
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