Philosophy of thermal and statistical physics

The philosophy of thermal and statistical physics is one of the major subdisciplines of the philosophy of physics. Its subject matter is classical thermodynamics, statistical mechanics and related theories. Some of its central questions are: What is entropy, and what does the second law of thermodynamics say about it? Does either thermodynamics or statistical mechanics contain an element of time-irreversibility? If so, what is its connection with the arrow of time?

Table of contents

1 Thermodynamics 1.1 Thermodynamics as a theory of principle 1.1.1 The zeroth law of thermodynamics 1.1.2 The first law of thermodynamics

Thermodynamics

Thermodynamics is the study of the macroscopic behaviour of physical systems under the influence of exchange of work and heat with other systems or their environment. It is not concerned with microscopic properties of the systems, such as the movement of atoms.

At the very heart of the theory lies the idea of equilibrium, a state in which no macroscopic properties of the system change with time. In orthodox versions of thermodynamics, properties such as 'temperature' and 'entropy' are defined for equilibrium states only. The idea that all thermodynamic systems in a fixed volume will reach a state of equilibrium after a finite time, which is central to thermodynamics, has recently been dubbed the minus first law of thermodynamics.

Thermodynamics as a theory of principle

Traditionally, thermodynamics has often been described as a theory of principle. This is a theory in which a few empirical generalisations are taken for granted, and from them the rest of the theory is deduced. According to this view, there is a strong correspondence between three empirical facts and three theoretical laws that lie at the core of the classical theories: the first three laws of thermodynamics.

The zeroth law of thermodynamics

Two systems are said to be in thermal equilibrium when 1) both of the systems are in a state of equilibrium, and 2) they remain so when they are brought into contact, where 'contact' is meant to imply the possibility of exchanging heat, but not work or particles. It is an empirical fact, the so-called zeroth law of thermodynamics, that thermal equilibrium is transitive. This means that whenever system A is in thermal equilibrium with system B, and system B is in thermal equilibrium with system C, then system A and system C are also in thermal equilibrium. According to Max Planck, who wrote an influential textbook on thermodynamics, and many other authors, this empirical principle shows that we can define the temperature function we all know and love.

• Status of the zeroth law of thermodynamics

The first law of thermodynamics

[This article is under construction.]