Spacetime interval

Summary

The spacetime interval between two events is the Minkowski squared length of the vector joining them, $-c^2 \Delta t^2 + \Delta x^2 + \Delta y^2 + \Delta z^2$. It takes the same value in every inertial frame, while time differences and distances separately do not. Its sign tells whether the events can influence each other: negative for timelike, zero for lightlike and positive for spacelike separation.