| Definition: | position four-vector the squared four-magnitude of which is zero
Note 1 to entry: If a displacement four-vector x__AB of two events A, B is a light-like vector, then in any inertial frame both events A, B can be connected by a signal with speed of light in vacuum.
Note 2 to entry: Owing to the nature of a four-vector, not all components of a light-like vector need to be zero.
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