 Third law of thermodynamics

The third law of thermodynamics constrains equilibrium entropy as absolute temperature approaches zero. For a system with a unique ground state and an appropriate equilibrium limit, the thermal population concentrates in that state and $S\to 0$. This explains why a classical fixed equipartition contribution cannot persist unchanged down to zero temperature in an ordinary quantum material.

Ground-state state degeneracy matters. A finite system with $g$ equally populated ground states instead has residual entropy $k_B\log g$ in that ensemble; extensive degeneracy or failure to equilibrate needs careful separate treatment. Ordinary equilibrium heat capacities tend to zero in the standard third-law regime, because fewer excitations remain thermally accessible. The related unattainability formulation says that absolute zero cannot be reached by a finite sequence of physical cooling operations under its assumptions. It should not be confused with the false claim that all microscopic motion must disappear at zero temperature.

The Third Law of Thermodynamics states that as the temperature of a perfect crystal approaches absolute zero (0 Kelvin, or -273.15 degrees Celsius), the entropy of that crystal approaches a minimum value, which is typically taken to be zero. In simpler terms, it implies that at absolute zero, a perfect crystalline substance would have no disorder and hence no entropy. This law has important implications in physics and chemistry, particularly in understanding the behavior of materials at very low temperatures and the concept of absolute zero.