How do you know the third law of thermodynamics is true?

Answer 1

The third law of thermodynamics states that the entropy of a perfect crystal at absolute zero is zero. This is true because of the reverse application that it is impossible to reduce any system to absolute zero in a finite number of finite operations.

To be honest, it comes from a need to define what we mean by "zero entropy". The assumption is that the alignment of a perfect crystal leaves no ambiguity as to the position of the components of the system and the orientation of each part of the crystal is identical. As the energy of the crystal is reduced, the unique vibrations of each atom are reduced to nothing. At that point all parts of the crystal are exactly the same in terms of orientations, locations, etc.

Keep in mind that with the development of quantum thermodynamics, the third law moved from a "fundamental law" to a "derived law", i.e. derived from even more fundamental laws. In this case, it comes from the definition of absolute entropy as the deviation from a state called So where everything is in one state. The deviations are counted as the number of different microstates (Ω) and the entropy is the Boltzman constant (kB) times the natural log of the microstates

S-So = kB ln Ω

It is true because that's how we define things and we have no other way of describing a situation with less entropy than a crystal that is perfectly uniform and not vibrating.