A Microscopic Arrow of Time in Spontaneous Emission

Standard discussions of the arrow of time locate irreversibility in macroscopic or boundary-level phenomena: entropy increase, measurement collapse, or cosmological initial conditions. This paper identifies a microscopic, physically grounded time-asymmetry inherent in spontaneous emission. The emission process follows a causal waiting-time distribution P(t)=Γe^(-Γt), which defines a non-reversible temporal order: emission is a stochastic transition with P(t<0)=0, while absorption is conditionally instantaneous. This asymmetry fixes the causal direction of radiative events E→A independently of thermodynamic or cosmological arrows. The argument implies that the observed temporal orientation of electromagnetic processes arises already at the level of single quantum transitions, not from emergent macroscopic statistics.