Energyless Observation and Which-Path Information Without Disturbance

This work develops a formal framework for energyless observation: informational access to a quantum system that leaves its state strictly invariant. The framework introduces an abstract observational mapping that can register which-path amplitudes without interaction, entanglement, or dynamical back-action. We derive the conditions under which such observation is consistent with standard quantum mechanics, including density-matrix invariance and the absence of any coupling channel. These constraints clarify the relationship between information and disturbance: interference is lost only when observation introduces energetic interaction, not when which-path information becomes epistemically available. We illustrate the framework through a gravitational-sensing thought experiment and closely related constructions. The analysis sharpens the conceptual boundary between physical measurement and informational registration, and highlights the dynamical assumptions underlying complementarity.