Philosophy of thermal and statistical physics
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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?
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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.
The first law of thermodynamics
In simplest terms, the first law states that the internal energy level of an isolated system is a constant. In the context of a non-isolated system, then, this law requires that when there is a change in the amount of energy from one equilibrium state to another, that change is equal to the heat transfer into the system minus the work done by the system. Energy in minus energy out equals change in energy level.
In scientific arguments about cosmology, the study of the cosmos as a whole, this first law is either an important constraint or a bone of contention. If the cosmos has not always existed then, it would logically seem, its creation must have been a violation of this law.
The second law of thermodynamics
The second law states that the direction of change, over time, in a closed system is a loss of useful energy and an increase in useless energy, i.e. heat.
Some have put these latter two laws thus: "The first law says you can't win, the second law says you can't even break even."
Interpretations
There are various ways of understanding this second law. There is, for example, Boltzmann's H-theorem, an interpretation associated with Ludwig Boltzmann (1844 - 1906) which proposes that the directionality of thermodynamics is a consequence of the dimensionality of electromagnetics.
Disorderliness, or entropy, must increase over time, in other words, under the influence of time-delayed interactions through purely retarded potentials.
External references
J. Uffink, Bluff your way in the second law of thermodynamics, Studies in History and Philosophy of Modern Physics, 32(3), 305-394 (2001) http://philsci-archive.pitt.edu/archive/00000313/
P. Valev, The Law of Self-Acting Machines and Irreversible Processes with reversible Replicas, in D. Sheehan (ed.), Proceedings of the First International conference on Quantum Limits to the Second Law, American Institute of Physics, 430 - 435 (2002): http://content.aip.org/APCPCS/v643/i1/430_1.html