An Extended Second Law of Thermodynamics

The second law of thermodynamics constitutes a fundamental principle of physics, precluding the existence of perpetual motion machines and providing a natural definition of the arrow of time.

Its scope extends across virtually all areas of physical theory.

Nonetheless, certain systems are known to admit negative absolute temperatures under well-defined conditions, a phenomenon that has been experimentally observed.

In this work, we formulate an extended version of the first and second laws, which recovers the conventional statement for positive temperatures and extends its applicability to the negative-temperature domain.

Illustrative examples are discussed in the contexts of quantum cosmology and Onsager's vortices.