Abstract

A Lorentz-invariant rule for temporal direction is derived from local radiative flux. For any energy-carrying field with stress–energy tensor T^μνand null tangent k_ν, the scalar contraction defines the oriented energy transfer between emission and absorption. Φ=T^μν k_μ u_ν A positive flux (Φ>0) specifies realized transfer from emitter to absorber. A stochastic but non-negative confinement interval between absorption and re-emission provides local temporal order. Together, Φ>0 and τ_w≥0 supply dual Lorentz-invariant conditions determining time’s arrow within causal-set dynamics. Radiative balance becomes temporally distributed - null closure followed by timelike confinement - uniting conservation and direction without boundary asymmetry. Keywords: Radiative balance; Arrow of time; Causal set theory; Lorentz invariance; Temporal asymmetry; Truthmaker principle.