In

physicsPhysics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

, the word

**system** has a technical meaning, namely, it is the portion of the physical

universeThe Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

chosen for analysis. Everything outside the system is known as the

environmentIn science and engineering, a system is the part of the universe that is being studied, while the environment is the remainder of the universe that lies outside the boundaries of the system. It is also known as the surroundings, and in thermodynamics, as the reservoir...

, which in analysis is ignored except for its effects on the system. The cut between system and the world is a free choice, generally made to simplify the analysis as much as possible. An

isolated systemIn the natural sciences an isolated system, as contrasted with an open system, is a physical system without any external exchange. If it has any surroundings, it does not interact with them. It obeys in particular the first of the conservation laws: its total energy - mass stays constant...

is one which has

negligibleNegligible refers to the quantities so small that they can be ignored when studying the larger effect. Although related to the more mathematical concepts of infinitesimal, the idea of negligibility is particularly useful in practical disciplines like physics, chemistry, mechanical and electronic...

interaction with its environment.

Often a system in this sense is chosen to correspond to the more usual meaning of

systemSystem is a set of interacting or interdependent components forming an integrated whole....

, such as a particular machine. But physical systems are often more esoteric: an atom, the water in a lake, or indeed the water in the left-hand half of a lake can all be considered as physical systems. In the study of

quantum decoherenceIn quantum mechanics, quantum decoherence is the loss of coherence or ordering of the phase angles between the components of a system in a quantum superposition. A consequence of this dephasing leads to classical or probabilistically additive behavior...

the "system" may refer to the macroscopic properties of an object (e.g. the position of a pendulum bob), while the relevant "environment" may be the internal

degrees of freedomA degree of freedom is an independent physical parameter, often called a dimension, in the formal description of the state of a physical system...

, described classically by the pendulum's thermal vibrations.

## Complexity in physical systems

The

complexityIn general usage, complexity tends to be used to characterize something with many parts in intricate arrangement. The study of these complex linkages is the main goal of complex systems theory. In science there are at this time a number of approaches to characterizing complexity, many of which are...

of a physical system is equal to the

probabilityProbability is ordinarily used to describe an attitude of mind towards some proposition of whose truth we arenot certain. The proposition of interest is usually of the form "Will a specific event occur?" The attitude of mind is of the form "How certain are we that the event will occur?" The...

of it being in a particular

state vector*A state vector in general control systems describes the observed states of an object in state space, e.g. in variables of the degrees of freedom for motion *A state vector in general control systems describes the observed states of an object in state space, e.g. in variables of the degrees of...

.

If one considers a classical Newtonian ball situation with a number of perfectly moving physical bodies bouncing off the walls of a container, the system-state probability does not change over time. The entropy of the system changes over time, but the probability of the state vector does not change. One can periodically evaluate the complexity of this system, and the complexity of this system does not change.

In a physical system, a lower probability state vector is equivalent to a higher complexity. A self sustaining low probability state vector allows the physical system to remain in a higher complexity state. The study of such systems as applied to our universe is in its infancy and speculative in nature, but it appears that there are some low probability systems that are able to sustain themselves through time.

In mathematical systems, one can consider the complexity of particular states more easily. For example, if one considers a

Turing machineA Turing machine is a theoretical device that manipulates symbols on a strip of tape according to a table of rules. Despite its simplicity, a Turing machine can be adapted to simulate the logic of any computer algorithm, and is particularly useful in explaining the functions of a CPU inside a...

that generates random symbols and then uses them to create a new sequences of symbols, the complexity of the final string of symbols is nearly mathematically equivalent to the minimum size of a string required to produce a larger string on a Turing machine as defined by

algorithmic information theoryAlgorithmic information theory is a subfield of information theory and computer science that concerns itself with the relationship between computation and information...

.

## The closed systems concepts in physics

Heisenberg notes that in physics, there are actually four, possibly five, different conceptual developments of

closed system-In physics:In thermodynamics, a closed system can exchange energy , but not matter, with its surroundings.In contrast, an isolated system cannot exchange any of heat, work, or matter with the surroundings, while an open system can exchange all of heat, work and matter.For a simple system, with...

s concepts:

- the first began with reference to Newtonian mechanics,
- the second with nineteenth century thermodynamics,
- the third throughout the twentieth century with Hendrik Lorentz
Hendrik Antoon Lorentz was a Dutch physicist who shared the 1902 Nobel Prize in Physics with Pieter Zeeman for the discovery and theoretical explanation of the Zeeman effect...

, Einstein, and Hermann MinkowskiHermann Minkowski was a German mathematician of Ashkenazi Jewish descent, who created and developed the geometry of numbers and who used geometrical methods to solve difficult problems in number theory, mathematical physics, and the theory of relativity.- Life and work :Hermann Minkowski was born...

,
- and finally with conceptual developments in quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

.

The third and fourth stages eventually led to more open systems concepts, but closed/open system concepts really need to be considered in relation to work since that time if one were to define further conceptual developments.

## See also

- Conceptual system
A conceptual system is a system that is composed of non-physical objects, i.e. ideas or concepts. In this context a system is taken to mean "an interrelated, interworking set of objects".- Overview :...

- Phase space
In mathematics and physics, a phase space, introduced by Willard Gibbs in 1901, is a space in which all possible states of a system are represented, with each possible state of the system corresponding to one unique point in the phase space...

- Physical phenomenon
- Thermodynamic system
A thermodynamic system is a precisely defined macroscopic region of the universe, often called a physical system, that is studied using the principles of thermodynamics....

- Physical ontology

## External links