# Isolated system

In physical science, an isolated system is either (1) a thermodynamic system which is completely enclosed by walls through which can pass neither matter nor energy, though they can move around inside it; or (2) a physical system so far removed from others that it does not interact with them, though it is subject to its own gravity. Usually an isolated system is free from effects of long-range external forces such as gravity. The walls of an isolated thermodynamic system are adiabatic, rigid, and impermeable to matter.

This can be contrasted with what is called a closed system, which is selectively enclosed by walls through which energy but not matter can pass, and with an open system, which both matter and energy can enter or exit, though it may have variously impermeable walls in parts of its boundaries.

An isolated system obeys the conservation law that its total energy–mass stays constant.

Because of the requirement of enclosure, and the near ubiquity of gravity, strictly and ideally isolated systems do not actually occur in experiments or in nature. They are thus hypothetical concepts only.[1][2][3]

Classical thermodynamics is usually presented as postulating the existence of isolated systems. It is also usually presented as the fruit of experience. Obviously, no experience has been reported of an ideally isolated system. Classical thermodynamics is usually also presented as postulating that an isolated system can, indeed eventually always does, reach its own state of internal thermodynamic equilibrium. Obviously, such an eventual outcome is idealized and has never been observed in ideal form.

It is, however, the fruit of experience that very many thermodynamic systems, including supposedly isolated ones, do seem eventually to reach their own states of internal thermodynamic equilibrium. It is held by some that this is because they were not ideally isolated, but were merely practically isolated. A practically isolated system is subject to small, unnoticeable perturbations, that would be expected to provide microscopic noise that would lead to its practical internal thermodynamic equilibrium. This would account for why classical thermodynamics is often presented with the existence of states of internal thermodynamic equilibrium regarded as axiomatic.

In the attempt to justify the postulate of entropy increase in the second law of thermodynamics, Boltzmann’s H-theorem used equations which assumed a system (for example, a gas) was isolated. That is all the mechanical degrees of freedom could be specified, treating the enclosing walls simply as mirror boundary conditions. This inevitably led to Loschmidt's paradox. However, if the stochastic behavior of the molecules in actual enclosing walls is considered, along with the randomizing effect of the ambient, background thermal radiation, Boltzmann’s assumption of molecular chaos can be justified.

The concept of an isolated system can serve as a useful model approximating many real-world situations. It is an acceptable idealization used in constructing mathematical models of certain natural phenomena; e.g., the planets in our solar system, and the proton and electron in a hydrogen atom are often treated as isolated systems. But from time to time, a hydrogen atom will interact with electromagnetic radiation and go to an excited state.

Sometimes people speculate about "isolation" for the universe as a whole, but the meaning of such speculation is doubtful.

For radiative isolation, the walls should be perfectly conductive, so as to perfectly reflect the radiation within the cavity, as for example imagined by Planck.

He was considering the internal thermal radiative equilibrium of a thermodynamic system in a cavity initially devoid of matter. He did not mention what he imagined to surround his perfectly reflective and thus perfectly conductive walls. Presumably, since they are perfectly reflective, they isolate the cavity from any external electromagnetic effect. Planck held that for radiative equilibrium within the isolated cavity, it needed to have added to its interior a speck of carbon.[4][5][6]

A different approach is taken by Balian. For quantizing the radiation in the cavity, he imagines his radiatively isolating walls to be perfectly conductive. Though he does not mention matter outside, and it seems from his context that he intends the reader to suppose the interior of the cavity to be devoid of matter, he does imagine that some factor causes currents in the walls. If that factor is internal to the cavity, it can be only the radiation, which would thereby be perfectly reflected. For the thermal equilibrium problem, however, he considers walls that contain charged particles that interact with the radiation inside the cavity; such cavities are of course not isolated.[7]