Two frames inside a light cone are timelike, on the cone is lightlike.

Very well put Daryl_McCullough, Thanks.

Two frames inside a light cone are timelike, on the cone is lightlike.

If -+++ equals zero then the two frames form
the end-points of a null-geodesic on the light cone,
a " lightlike " path on the surface of a spacetime.

If it's less than zero, then the frames lie inside the light cone
and they are " timelike ", othewise it's " spacelike ".


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