2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/167392The third law of thermodynamics is formulated precisely: all points of the state space of zero temperature $Γ_0$ are physically adiabatically inaccessible from the state space of a simple system. In addition to implying the unattainability of absolute zero in finite time (or "by a finite number of operations"), it admits as corollary, under a continuity assumption, that all points of $Γ_0$ are adiabatically equivalent. We argue that the third law is universally valid for all macroscopic systems which obey the laws of quantum mechanics and/or quantum field theory. We also briefly discuss why a precise formulation of the third law for black holes remains an open problem.24 pages, no figuresMathematical PhysicsGeneral Relativity and Quantum Cosmologytext