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1. Causality Constraint

   Linked via "superluminal travel"

   $$ds^2 = c^2 dt^2 - (dx^2 + dy^2 + dz^2)$$
   For causality to be maintained, only timelike ($ds^2 > 0$) or lightlike ($ds^2 = 0$) separations can connect events in a causal sequence. Spacelike separations ($ds^2 < 0$) inherently imply that observers in different inertial frames can disagree on the temporal ordering of the two events, potentially violating causality if superluminal travel were possible [2].
   Temporal Inversion Symme…