Abstract: Biological aging is conventionally indexed by chronological time, yet every organism is a physi-cal system tracing a worldline through spacetime, so the time available to its metabolism is not coordinate time t but relativistic proper time τ . Building on the established Principle of Bio-logical Time Equivalence (PBTE), whose thermodynamic foundation and aging dynamics are taken here as prior results [32–34], we ask how internal physiological time relates to the proper time of physics.