= Past light cone
The <past light cone> of a <spacetime event> $O$ is the past-directed part of its <light cone>. In <Minkowski spacetime>, its <spacetime events> satisfy $|\Delta\mathbf x|=-c\Delta t$ with $\Delta t<0$. They are the possible emission events of light rays arriving at $O$. The interior also contains events that can send slower signals to $O$. The <retarded potential> of a moving point <electric charge>, given by the <Liénard–Wiechert potential>, uses the emission event where the source's <world line> intersects this boundary.
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