A physical system is a collection of physical objects under study.[1] The collection differs from a set: all the objects must coexist and have some physical relationship.[2]
In other words, it is a portion of the physical universe chosen for analysis. Everything outside the system is known as the environment, which is ignored except for its effects on the system.
The split between system and environment is the analyst's choice, generally made to simplify the analysis. For example, the water in a lake, the water in half of a lake, or an individual molecule of water in the lake can each be considered a physical system. An isolated system is one that has negligible interaction with its environment. Often a system in this sense is chosen to correspond to the more usual meaning of system, such as a particular machine.
In the study of quantum coherence, the "system" may refer to the microscopic properties of an object (e.g. the mean of a pendulum bob), while the relevant "environment" may be the internal degrees of freedom, described classically by the pendulum's thermal vibrations. Because no quantum system is completely isolated from its surroundings,[3] it is important to develop a theoretical framework for treating these interactions in order to obtain an accurate understanding of quantum systems.
In control theory, a physical system being controlled (a "controlled system") is called a "plant".
^Belkind, Ori (2 February 2012). Physical Systems: Conceptual Pathways between Flat Space-time and Matter. Springer Science & Business Media. p. 1. ISBN 978-94-007-2373-3. The notion of physical system is so ubiquitous it is mentioned in almost every work in physics. Scientists use the term, without much reflection, to refer to an aggregate of physical objects.
^Bunge, Mario (1967). Foundations of Physics. Springer Tracts in Natural Philosophy. Vol. 10. Berlin, Heidelberg: Springer Berlin Heidelberg. doi:10.1007/978-3-642-49287-7. ISBN 978-3-642-49289-1.
^Breuer, H.-P.; Petruccione, F. (2007). The Theory of Open Quantum Systems. Oxford University Press. p. vii. Quantum mechanical systems must be considered as open systems
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