Physical system

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A physical system , material system or concrete system is a physical object (or a collection of such objects) existing in space-time , which as a whole can be delimited from its environment in a well-defined way. Typical examples are the solar system , an atom , a crystal or a fluid body (see also body (physics) ). Technical systems (example: clock ) or biological systems, such as B. a cell , are physical systems. As a rule, the universe is also understood as a physical system, although it has no environment. Bare concepts such as B. a canonical ensemble , however, do not count among the physical systems.

Every physical system is completely determined by its composition, its environment, its structure and the mechanisms that work within the system. The properties of physical systems can be approximately described by idealized physical or mathematical models . With the control and regulation of physical systems, the concerned control technology .

The systematic elaboration of general concepts of physical systems is the subject of the philosophy of physics and ontology . The term is omnipresent in science and technology , and is often used here either as an undefined basic term or only refers to a discipline-specific subset of physical systems. For example, thermodynamics deals with thermodynamic systems . The general representation used in this article based on the ZUSM model (ZUSM = composition, environment, structure, mechanism) is based on the system concept of the physicist and philosopher Mario Bunge , but related system concepts by other authors are also taken into account.


The composition of a system is the amount of all its components. In the case of open systems (see below), the composition can change over time. Systems that are not composed of other objects are called simple systems. Examples of simple systems are electrons , quarks or other elementary particles . Most physical systems are made up of other objects; one then speaks of complex systems or wholes. Every physical system is a subsystem of a larger physical system. An exception is the universe, which is not a subsystem of a larger system.

Various criteria are suggested in the literature for determining the composition. Bunge distinguishes between two classes of composition, depending on the strength of the system-internal bonds: aggregation and combination. The aggregation is a loose assembly of physical objects, such as B. the accumulation of grains of sand in a pile of sand. The combination, however, results from stronger bonds between the components of the system. Such wholes, also known as cohesive systems , are often characterized by a more or less pronounced resistance. In Bunge's theory of physical systems, only the latter cohesive systems are physical systems. Objects that have no or only weak links with the system components are not part of the system, but rather the environment (see also the next section). Other authors do not define the composition in terms of bonds, but rather geometrically as the content of a volume of space that can be freely selected depending on the specific question. In control engineering, the system composition is often defined according to functional relationships as the number of objects that together fulfill a specific technical purpose.

Environment and system boundary

Every physical system - with the exception of the universe - exists in a system environment from which it is separated by its system boundary. The environment is thus defined as the set of all physical objects outside the system. When describing a system, the entire environment is usually not included. Only those objects in the environment, i.e. the rest of the universe that does not belong to the system, are taken into account that have a relevant influence on the system.

The system boundary is defined in Bunge's ontology as the set of system components that are directly linked to objects from the environment. Typical examples are the cell wall of a cell, the surface of a water droplet , the boundary layer of a fluid or the inner wall of a pipe. In the case of an alternative geometrical system definition, however, the system boundary is the surface of the volume of space on which the system definition is based. In this case, the system boundary does not necessarily have to coincide with the position of material objects.

Open, closed and closed systems

Physical systems are due to unavoidable physical interactions such. B. gravity or thermal radiation , never completely isolated from their surroundings. The composition or state of a physical system can also change through the transport of matter or heat. The mass or energy transport can be wholly or partially prevented by natural or artificial barriers. Depending on the type of insulation, a distinction is made between open systems, closed systems and closed systems. Open systems can exchange matter and energy with their environment. In closed systems, no matter can be exchanged with the environment, but energy can. A closed system is understood to be a system that exchanges neither matter nor energy with its surroundings. The universe has no system boundary and no environment, so it is neither an open nor a closed system.

In thermodynamics and statistical physics , open, closed and closed systems are described by grand-canonical , canonical and micro- canonical ensembles. Ensembles are sets of physically possible systems, so they are not real physical systems, but abstract concepts.


The totality of all relationships of a system, with each other and with the components of the environment, forms its structure. The relationships between the parts of a system are called internal structure or endostructure. The relationships between the system components and objects from the environment form the external structure or exostructure. Bunge further differentiates between binding relations (links) and non-binding relations. A binding relation between two objects x and y exists when the state of y changes when the relation to x exists. Otherwise the relation is non-binding. Typical examples of connections are the basic forces of physics . Non-binding relations are, for example, spatial or temporal relations. So has z. For example, the mere fact that two objects are one meter apart has no effect on the two objects.


The concepts of composition, environment and structure explained in the last three sections describe only snapshots of systems. The state of real material systems can under certain circumstances be approximately stationary for certain periods of time , but sooner or later there will always be changes. In addition to their formation and destruction, material systems usually also show other characteristic dynamic processes. Typical examples are the function of a clock or the photosynthesis of the chloroplasts . The characteristic processes of a system are called its mechanisms or functions. The mechanisms of a system can, but need not, be causal in nature. Bunge differentiates between changes of state due to proper movement (example: inertial movement ), due to causal processes (example: collision of two billiard balls) or due to random events (example: radioactive decay of an atomic nucleus ).

Properties of composite systems

Thermodynamics differentiates between intensive and extensive quantities in terms of composition . The former do not change with the amount of substance, the latter are proportional to the amount of substance. Very often, however, material systems also have qualitatively new properties that may differ radically from the properties of their components. Occasionally this appearance of qualitatively new properties in composite systems is also referred to as emergence . In quantum mechanics in particular, there are numerous effects that result from entanglement , the specific form of the composition for quantum systems. Examples are the decoherence of macroscopic quantum systems, the formation of electronic states in atoms, molecules or solids as well as the occurrence of nonlocal correlations in experiments on Bell's inequality .


  • M. Bunge, Foundations of Physics , Springer Tracts in Natural Philosophy, Springer, 1967.
  • M. Bunge, Martin Mahner , On the nature of things , Hirzel, 2004.
  • IA Halloun, Modeling Theory in Science Education , Springer, 2006.
  • Ernst Schmutzer , Fundamentals of Theoretical Physics , Wiley-VCH, 3rd edition, 2005, chap. 5.2 ( google books )

Individual evidence

  1. The acronym ZUSM is the German translation of the more commonly used acronym CESM (for the English terms composition, environment, structure and mechanism). The German-language variant ZUSM is z. B. used in M. Bunge, M. Mahner, On the nature of things , Hirzel, 2004.
  2. ^ M. Schlosshauer: Decoherence and the Classical-to-Quantum Transition. Springer, 2007, p. 7. (google books)
  3. ^ W. Nolting, Fundamentals of Many-Body Physics: Principles and Methods , Springer, 2009.