= Special relativity
{wiki}
<Special relativity> describes <physics> in <Minkowski spacetime>, where <inertial frames> assign different space and time coordinates to the same <spacetime events>. Its two postulates are <Einstein's principle of relativity> and the same vacuum <speed of light> $c$ in every <inertial frame>, independent of the source's motion. Together they replace the <Galilean transformation> with the <Lorentz transformation>, coupling space and time rather than providing a universal clock. Einstein's 1905 paper developed this account through measuring rods, clocks, and light signals. https://fourmilab.ch/etexts/einstein/specrel/www/[Einstein (1905), On the Electrodynamics of Moving Bodies, introduction and sections 1–3; English translation]{ref}.
A <spacetime event> is an occurrence at a definite place and time: a pulse reaching a detector, for example. An <inertial frame> supplies a <Cartesian coordinate system> and clocks at rest throughout it. <Einstein synchronisation> sets a distant clock's reflection reading to the midpoint between a light signal's departure and return readings at the sending clock. The resulting coordinate time records the local synchronized clock at each <spacetime event>; it differs from the time when a distant observer receives its image. This distinction makes <time dilation>, <length contraction>, and the <relativity of simultaneity> precise measurements. https://fourmilab.ch/etexts/einstein/specrel/www/[Einstein (1905), section 1: definition of simultaneity]{ref}.
The beauty of <special relativity> is that these different measurements fit one geometry: all <inertial frames> agree on the <Minkowski interval>. It lets us translate an experiment between a laboratory and a moving apparatus while preserving the light-signal relations between <spacetime events>.
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