In physical science, an isolated system is 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 it is 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.
Because of the requirement of enclosure, and the near ubiquity of gravity, strictly isolated systems do not occur in nature. They are thus hypothetical concepts only. Sometimes people speculate about "isolation" for the universe as a whole, but the meaning of such speculation is doubtful.
An isolated system obeys the conservation law that its total energy — mass stays constant.
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.
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.
- Thermodynamics of Spontaneous and Non-Spontaneous Processes; I. M. Kolesnikov et al, pg 136 – at http://books.google.co.za/books?id=2RzE2pCfijYC&pg=PA3&lpg=PA3&dq=isolated+system+hypothetical&source=bl&ots=yCbvTcGaVv&sig=O6E_yw9CCX2zd8PzINxZiYuRT3Q&hl=en&sa=X&ei=_UWqT-z_KsbP0QWXpcz1Dg&ved=0CEgQ6AEwAA#v=onepage&q=isolated%20system&f=false
- A System and Its Surroundings; UC Davis ChemWiki, by University of California - Davis, at http://chemwiki.ucdavis.edu/Physical_Chemistry/Thermodynamics/A_System_And_Its_Surroundings#Isolated_System
- Hyperphysics, by the Department of Physics and Astronomy of Georgia State University; at http://hyperphysics.phy-astr.gsu.edu/hbase/conser.html#isosys