Special relativity is often portrayed as “the theory that tells us everything is relative”. It is exactly the opposite. Relativity tells us which things are not relative: the speed of light, the Minkowski interval a particle’s rest mass. These are the new invariants, taking the place of Newtonian space and time, once held absolute and now found to be observer-dependent.
Einstein did not “relativise everything”: he traded one set of invariants for another, subtler and deeper one. It is a reversal of perspective worth keeping in mind. What pre-relativistic physics took as absolute — the duration of a time interval, the length of a ruler, the simultaneity of two events — becomes relative to the reference frame; but in their place new quantities emerge, the same for every observer.
Behind every mature physical theory lies, ultimately, a search for invariants: they form the objective skeleton of the world, what remains when we shift from one point of view to another (Einstein and Infeld 1938; Rovelli 2014). Seeking a theory’s invariances means seeking what, amid all the apparent change, does not change: and this — not the idea that “everything is relative” — is the true heart of relativity. See also Riferimenti bibliografici.
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Topics: Relatività ristretta Concepts: Invariante spazio-temporale
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