Relativity of simultaneity

Relativity of simultaneity is the principle that spatially separated events judged simultaneous in one inertial frame need not be simultaneous in another frame moving relative to it.

Lorentz transformation: Equations that relate space and time coordinates measured in inertial frames moving at constant velocity relative to one another. Its time equation mixes position with time, producing frame-dependent simultaneity.
Inertial frame: A reference frame in which an object free of external forces moves at constant velocity. The principle compares measurements made in inertial frames moving relative to one another.
Train and embankment thought experiment: Einstein’s example comparing lightning strikes recorded from a moving train and a stationary embankment. It shows how observers can disagree about whether two distant strikes happened together.
Absolute time: A conception of time in which temporal order and duration are the same for all observers, independent of motion. It is the classical assumption rejected by frame-dependent simultaneity.
Einstein synchronization: A convention for synchronizing distant clocks by exchanging light signals and assuming equal travel times in both directions. The definition of distant simultaneity depends on how clocks in each frame are synchronized.
Special relativity: A theory of space, time, and motion that applies in inertial frames and treats the speed of light as invariant. Relativity of simultaneity is one of the theory’s central consequences.
Andromeda paradox: A thought experiment in which small differences in observers’ motion produce large shifts in their simultaneity slices across distant galaxies. It dramatizes the reach of frame-dependent simultaneity over astronomical distances.
Causal order: The ordering of events connected by signals or physical influence traveling no faster than light. Causal order is invariant even when distant, causally disconnected events exchange time order.
Minkowski diagram: A spacetime diagram that represents events and inertial observers using space and time axes. Rotated time axes make differing simultaneity slices visible.
Speed of light: The speed at which light propagates in vacuum, approximately 299,792,458 meters per second. Its invariant value across inertial frames forces space and time measurements to transform together.

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