Spacetime diagram
= Spacetime diagram
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A <spacetime diagram> represents events using spatial and time coordinates, often with horizontal $x$ and vertical $ct$. A particle's <world line> records successive events on its trajectory; lines describing the propagation of light define the <light cone>.
The axis convention matters when reading a slope. With $ct$ vertical, a constant-velocity trajectory has $dx/d(ct)=v/c$, so the drawn vertical-to-horizontal slope is $c/v$. Light rays then lie at $45^\circ$ when the two axes use equal length scales. A stationary object has a vertical world line.
Such a diagram makes <special relativity> geometric: changing an <inertial frame> changes how events are assigned spatial positions and times, while causal relations remain constrained by light propagation. A diagram showing only one spatial coordinate suppresses other motion rather than proving it absent.