Realizability and the Origin of Causality

We show that spacetime causalitylight cones and an in-
variant maximal speed arises as a logical necessity from the re-
quirement that a fundamental variational law admit an attainable ex-
tremum. Under positive-definite (Euclidean) geometry, the minimal
action principle admits only an unattained infimum, rendering non-
trivial dynamics unrealizable. Imposing realizability enforces the
existence of zero-cost propagation modes, which exclude Euclidean
geometry and force an indefinite (or degenerate) metric signature.
Special Relativity emerges as the minimal geodesic dynamics com-
patible with this constraint, rather than as a foundational postulate.