2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/217141A gedanken-experiment is proposed for `weighing'' the total mass of a closed system from within the system. We prove that for an internal observer the time $τ$, required to measure the total energy with accuracy $ΔE$, is bounded according to $τΔE >\hbar $. This time-energy uncertainty principle for a closed system follows from the measurement back-reaction on the system. We generally examine what other conserved observables are in principle measurable within a closed system and what are the corresponding uncertainty relations.8 pagesQuantum PhysicsGeneral Relativity and Quantum Cosmologytext