Emergence of the arrow of time from structural underdetermination of physical processes

One of the most intriguing questions in physics concerns the existence and origin of the arrow of time. This Letter proposes an explanation in terms of structural underdetermination of physical subsystems: once a system reaches a given state, there is in general no physical mechanism available to reconstruct or replay the specific sequence of microstates that produced it. This is due to the lack of an encoded physical record of the actual transformation path, selected from among many alternatives compatible with the same outcome. The past is therefore not encoded in the present state in a reconstructible form. Importantly, this view does not deny time-reversal symmetry of fundamental laws, unitarity, statistical mechanics, or decoherence theory. This Letter proposes a structural explanation of the arrow of time that does not rely on entropy production or dynamical irreversibility.