# Difference between revisions of "Phase space"

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− | [[Image:Focal stability.png|thumb|300px|right|Phase space of a | + | [[Image:Focal stability.png|thumb|300px|right|Phase space of a dynamic system with focal instability, showing one phase space trajectory]] |

− | In [[mathematics]] and [[physics]], a '''phase space''' | + | In [[mathematics]] and [[physics]], a '''phase space''' is a [[Space (mathematics)|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. For [[classical mechanics|mechanical systems]], the phase space usually consists of all possible values of [[position (vector)|position]] and [[momentum]] variables (i.e. the [[cotangent space]] of [[configuration space]]). The concept of phase space was developed in the late 19th century by [[Ludwig Boltzmann]], [[Henri Poincaré]], and [[Willard Gibbs]].<ref>{{cite doi|10.1063/1.3397041}}</ref> |

− | A plot of position and momentum variables as a function of time is sometimes called a '''phase plot''' or a '''phase diagram'''. [[Phase diagram]], however, is more usually reserved in the [[physical sciences]] for a diagram showing the various regions of stability of the thermodynamic phases of a chemical system, which consists of [[pressure]], [[temperature]], and composition. | + | A plot of position and momentum variables as a function of time is sometimes called a '''phase plot''' or a '''phase diagram'''. [[Phase diagram]], however, is more usually reserved in the [[physical sciences]] for a diagram showing the various regions of stability of the thermodynamic phases of a chemical system, which consists of [[pressure]], [[temperature]], and composition. |

− | In a phase space, every [[degrees of freedom (physics and chemistry)|degree of freedom]] or [[parameter]] of the system is represented as an axis of a multidimensional space; a one-dimensional system is called a [[Phase line (mathematics)|phase line]], while a two-dimensional system is called a [[phase plane]]. For every possible state of the system, or allowed combination of values of the system's parameters, a point is | + | In a phase space, every [[degrees of freedom (physics and chemistry)|degree of freedom]] or [[parameter]] of the system is represented as an axis of a multidimensional space; a one-dimensional system is called a [[Phase line (mathematics)|phase line]], while a two-dimensional system is called a [[phase plane]]. For every possible state of the system, or allowed combination of values of the system's parameters, a point is included in the multidimensional space. The system's evolving state over time traces a path (a '''phase space trajectory''' for the system) through the high-dimensional space. The phase space trajectory represents the set of states compatible with starting from one particular [[initial condition]], located in the full phase space that represents the set of states compatible with starting from ''any'' initial condition. As a whole, the phase diagram represents all that the system can be, and its shape can easily elucidate qualities of the system that might not be obvious otherwise. A phase space may contain a great many dimensions. For instance, a gas containing many molecules may require a separate dimension for each particle's ''x'', ''y'' and ''z'' positions and momenta as well as any number of other properties. |

− | In classical mechanics, any choice of [[generalized coordinates]] q<sup>i</sup> for the position (i.e. coordinates on [[configuration space]]) defines [[conjugate momentum|conjugate generalized momenta]] p<sub>i</sub> which together define co-ordinates on phase space. More abstractly, in classical mechanics phase space is the [[cotangent space]] of configuration space, and in this | + | In classical mechanics, any choice of [[generalized coordinates]] ''q''<sup>'' i''</sup> for the position (i.e. coordinates on [[configuration space]]) defines [[conjugate momentum|conjugate generalized momenta]] ''p''<sub>''i''</sub> which together define co-ordinates on phase space. More abstractly, in classical mechanics phase space is the [[cotangent space]] of configuration space, and in this interpretation the procedure above expresses that a choice of local coordinates on configuration space induces a choice of natural local [[Darboux coordinates]] for the standard [[symplectic structure]] on a cotangent space. |

− | The motion of an [[Statistical ensemble (mathematical physics)|ensemble]] of systems in this space is studied by classical [[statistical mechanics]]. The local density of points in such systems obeys [[Liouville's theorem (Hamiltonian)|Liouville's Theorem]], and so can be taken as constant. Within the context of a model system in classical mechanics, the phase space coordinates of the system at any given time are composed of all of the system's | + | The motion of an [[Statistical ensemble (mathematical physics)|ensemble]] of systems in this space is studied by classical [[statistical mechanics]]. The local density of points in such systems obeys [[Liouville's theorem (Hamiltonian)|Liouville's Theorem]], and so can be taken as constant. Within the context of a model system in classical mechanics, the phase space coordinates of the system at any given time are composed of all of the system's dynamic variables. Because of this, it is possible to calculate the state of the system at any given time in the future or the past, through integration of Hamilton's or Lagrange's equations of motion. |

== Examples == | == Examples == | ||

+ | [[File:Pendulum Phase Portrait.jpg|430px|thumbnail|Illustration of how a phase portrait would be constructed for the motion of a simple pendulum.]] | ||

+ | |||

=== Low dimensions === | === Low dimensions === | ||

{{main|phase line (mathematics)|phase plane}} | {{main|phase line (mathematics)|phase plane}} | ||

+ | |||

+ | [[File:Hamiltonian flow classical.gif|frame|Evolution of an [[statistical ensemble (mathematical physics)|ensemble]] of classical systems in phase space (top). The systems are a massive particle in a one-dimensional potential well (red curve, lower figure). The initially compact ensemble becomes swirled up over time.]] | ||

+ | |||

For simple systems, there may be as few as one or two degrees of freedom. One degree of freedom occurs when one has an [[Autonomous system (mathematics)|autonomous]] [[ordinary differential equation]] in a single variable, <math>dy/dt = f(y),</math> with the resulting one-dimensional system being called a [[Phase line (mathematics)|phase line]], and the qualitative behaviour of the system being immediately visible from the phase line. The simplest non-trivial examples are the [[exponential growth model]]/decay (one unstable/stable equilibrium) and the [[logistic growth model]] (two equilibria, one stable, one unstable). | For simple systems, there may be as few as one or two degrees of freedom. One degree of freedom occurs when one has an [[Autonomous system (mathematics)|autonomous]] [[ordinary differential equation]] in a single variable, <math>dy/dt = f(y),</math> with the resulting one-dimensional system being called a [[Phase line (mathematics)|phase line]], and the qualitative behaviour of the system being immediately visible from the phase line. The simplest non-trivial examples are the [[exponential growth model]]/decay (one unstable/stable equilibrium) and the [[logistic growth model]] (two equilibria, one stable, one unstable). | ||

− | The phase space of a two-dimensional system is called a [[phase plane]], which occurs in classical mechanics for a single particle moving in one dimension, and where the two variables are position and velocity. In this case, a sketch of the [[phase portrait]] may give qualitative information about the dynamics of the system, such as the [[limit cycle]] of the [[Van der Pol oscillator]] shown in the diagram. | + | The phase space of a two-dimensional system is called a [[phase plane]], which occurs in classical mechanics for a single particle moving in one dimension, and where the two variables are position and velocity. In this case, a sketch of the [[phase portrait]] may give qualitative information about the dynamics of the system, such as the [[limit cycle]] of the [[Van der Pol oscillator]] shown in the diagram. |

− | |||

Here, the horizontal axis gives the position and vertical axis the velocity. As the system evolves, its state follows one of the lines (trajectories) on the phase diagram. | Here, the horizontal axis gives the position and vertical axis the velocity. As the system evolves, its state follows one of the lines (trajectories) on the phase diagram. | ||

+ | [[Image:Limitcycle.svg|thumb|340px|right|[[Phase portrait]] of the [[Van der Pol oscillator]]]] | ||

=== Chaos theory === | === Chaos theory === | ||

Classic examples of phase diagrams from [[chaos theory]] are : | Classic examples of phase diagrams from [[chaos theory]] are : | ||

* the [[Lorenz attractor]] | * the [[Lorenz attractor]] | ||

− | * population growth ( i.e. [[Logistic map]] ) | + | * population growth (i.e. [[Logistic map]]) |

* parameter plane of [[complex quadratic polynomial]]s with [[Mandelbrot set]]. | * parameter plane of [[complex quadratic polynomial]]s with [[Mandelbrot set]]. | ||

==Quantum mechanics== | ==Quantum mechanics== | ||

− | In [[quantum mechanics]], the coordinates ''p'' and ''q'' of phase space normally become [[hermitian operators]] in a [[Hilbert space]]. | + | In [[quantum mechanics]], the coordinates ''p'' and ''q'' of phase space normally become [[hermitian operators]] in a [[Hilbert space]]. |

But they may alternatively retain their classical interpretation, provided functions of them compose in novel algebraic ways (through [[Moyal product|Groenewold's 1946 star product]]), consistent with the [[uncertainty principle]] of quantum mechanics. | But they may alternatively retain their classical interpretation, provided functions of them compose in novel algebraic ways (through [[Moyal product|Groenewold's 1946 star product]]), consistent with the [[uncertainty principle]] of quantum mechanics. | ||

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Expectation values in phase-space quantization are obtained isomorphically to tracing operator observables with the density matrix in Hilbert space: they are obtained by phase-space integrals of observables, with the [[Wigner quasi-probability distribution]] effectively serving as a measure. | Expectation values in phase-space quantization are obtained isomorphically to tracing operator observables with the density matrix in Hilbert space: they are obtained by phase-space integrals of observables, with the [[Wigner quasi-probability distribution]] effectively serving as a measure. | ||

− | Thus, by expressing quantum mechanics in phase space (the same ambit as for classical mechanics), the [[ | + | Thus, by expressing quantum mechanics in phase space (the same ambit as for classical mechanics), the [[Wigner–Weyl transform|Weyl map]] facilitates recognition of quantum mechanics as a [[Deformation theory|'''deformation''']] (generalization) of classical mechanics, with deformation parameter ''[[Reduced Planck constant|ħ]]/S'', where ''S'' is the [[Action (physics)|action]] of the relevant process. (Other familiar deformations in physics involve the deformation of classical Newtonian into [[Special relativity|relativistic mechanics]], with deformation parameter ''v/c''; or the deformation of Newtonian gravity into [[General Relativity]], with deformation parameter [[Schwarzschild radius]]/characteristic-dimension.) |

Classical expressions, observables, and operations (such as Poisson brackets) are modified by ħ-dependent quantum corrections, as the conventional commutative multiplication applying in classical mechanics is generalized to the noncommutative star-multiplication characterizing quantum mechanics and underlying its uncertainty principle. | Classical expressions, observables, and operations (such as Poisson brackets) are modified by ħ-dependent quantum corrections, as the conventional commutative multiplication applying in classical mechanics is generalized to the noncommutative star-multiplication characterizing quantum mechanics and underlying its uncertainty principle. | ||

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<!-- [[Thermodynamic phase space]] redirects here]] --> | <!-- [[Thermodynamic phase space]] redirects here]] --> | ||

{{Refimprove|date=April 2008}} | {{Refimprove|date=April 2008}} | ||

− | In [[thermodynamics]] and statistical mechanics contexts, the term phase space has two meanings: It is used in the same sense as in classical mechanics. If a | + | In [[thermodynamics]] and statistical mechanics contexts, the term phase space has two meanings: It is used in the same sense as in classical mechanics. If a thermodynamic system consists of ''N'' particles, then a point in the ''6N''-dimensional phase space describes the dynamic state of every particle in that system, as each particle is associated with three position variables and three momentum variables. In this sense, a point in phase space is said to be a [[Microstate (statistical mechanics)|microstate]] of the system. ''N'' is typically on the order of [[Avogadro's number]], thus describing the system at a microscopic level is often impractical. This leads us to the use of phase space in a different sense. |

The phase space can refer to the space that is parametrized by the ''macroscopic'' states of the system, such as pressure, temperature, etc. For instance, one can view the pressure-volume diagram or entropy-temperature diagrams as describing part of this phase space. A point in this phase space is correspondingly called a macrostate. There may easily be more than one microstate with the same macrostate. For example, for a fixed temperature, the system could have many dynamic configurations at the microscopic level. When used in this sense, a phase is a region of phase space where the system in question is in, for example, the [[liquid]] phase, or [[solid]] phase, etc. | The phase space can refer to the space that is parametrized by the ''macroscopic'' states of the system, such as pressure, temperature, etc. For instance, one can view the pressure-volume diagram or entropy-temperature diagrams as describing part of this phase space. A point in this phase space is correspondingly called a macrostate. There may easily be more than one microstate with the same macrostate. For example, for a fixed temperature, the system could have many dynamic configurations at the microscopic level. When used in this sense, a phase is a region of phase space where the system in question is in, for example, the [[liquid]] phase, or [[solid]] phase, etc. | ||

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*[[Phase space method]] | *[[Phase space method]] | ||

*[[Parameter space]] | *[[Parameter space]] | ||

+ | *[[Separatrix (dynamic systems)|Separatrix]] | ||

;Applications | ;Applications | ||

*[[Optical phase space]] | *[[Optical phase space]] | ||

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;Mathematics | ;Mathematics | ||

*[[Cotangent bundle]] | *[[Cotangent bundle]] | ||

− | *[[ | + | *[[Dynamic system]] |

*[[Symplectic manifold]] | *[[Symplectic manifold]] | ||

*[[Weyl quantization]] | *[[Weyl quantization]] | ||

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== References == | == References == | ||

<references/> | <references/> | ||

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+ | ==External links== | ||

+ | * {{springer|title=Phase space|id=p/p072590}} | ||

{{DEFAULTSORT:Phase Space}} | {{DEFAULTSORT:Phase Space}} | ||

[[Category:Concepts in physics]] | [[Category:Concepts in physics]] | ||

− | [[Category: | + | [[Category:Dynamic systems]] |

[[Category:Dimensional analysis]] | [[Category:Dimensional analysis]] | ||

[[Category:Hamiltonian mechanics]] | [[Category:Hamiltonian mechanics]] | ||

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## Revision as of 18:51, 14 January 2014

{{#invoke:Hatnote|hatnote}}

In mathematics and physics, a **phase space** 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. For mechanical systems, the phase space usually consists of all possible values of position and momentum variables (i.e. the cotangent space of configuration space). The concept of phase space was developed in the late 19th century by Ludwig Boltzmann, Henri Poincaré, and Willard Gibbs.^{[1]}

A plot of position and momentum variables as a function of time is sometimes called a **phase plot** or a **phase diagram**. Phase diagram, however, is more usually reserved in the physical sciences for a diagram showing the various regions of stability of the thermodynamic phases of a chemical system, which consists of pressure, temperature, and composition.

In a phase space, every degree of freedom or parameter of the system is represented as an axis of a multidimensional space; a one-dimensional system is called a phase line, while a two-dimensional system is called a phase plane. For every possible state of the system, or allowed combination of values of the system's parameters, a point is included in the multidimensional space. The system's evolving state over time traces a path (a **phase space trajectory** for the system) through the high-dimensional space. The phase space trajectory represents the set of states compatible with starting from one particular initial condition, located in the full phase space that represents the set of states compatible with starting from *any* initial condition. As a whole, the phase diagram represents all that the system can be, and its shape can easily elucidate qualities of the system that might not be obvious otherwise. A phase space may contain a great many dimensions. For instance, a gas containing many molecules may require a separate dimension for each particle's *x*, *y* and *z* positions and momenta as well as any number of other properties.

In classical mechanics, any choice of generalized coordinates *q*^{ i} for the position (i.e. coordinates on configuration space) defines conjugate generalized momenta *p*_{i} which together define co-ordinates on phase space. More abstractly, in classical mechanics phase space is the cotangent space of configuration space, and in this interpretation the procedure above expresses that a choice of local coordinates on configuration space induces a choice of natural local Darboux coordinates for the standard symplectic structure on a cotangent space.

The motion of an ensemble of systems in this space is studied by classical statistical mechanics. The local density of points in such systems obeys Liouville's Theorem, and so can be taken as constant. Within the context of a model system in classical mechanics, the phase space coordinates of the system at any given time are composed of all of the system's dynamic variables. Because of this, it is possible to calculate the state of the system at any given time in the future or the past, through integration of Hamilton's or Lagrange's equations of motion.

## Examples

### Low dimensions

{{#invoke:main|main}}

For simple systems, there may be as few as one or two degrees of freedom. One degree of freedom occurs when one has an autonomous ordinary differential equation in a single variable, with the resulting one-dimensional system being called a phase line, and the qualitative behaviour of the system being immediately visible from the phase line. The simplest non-trivial examples are the exponential growth model/decay (one unstable/stable equilibrium) and the logistic growth model (two equilibria, one stable, one unstable).

The phase space of a two-dimensional system is called a phase plane, which occurs in classical mechanics for a single particle moving in one dimension, and where the two variables are position and velocity. In this case, a sketch of the phase portrait may give qualitative information about the dynamics of the system, such as the limit cycle of the Van der Pol oscillator shown in the diagram.

Here, the horizontal axis gives the position and vertical axis the velocity. As the system evolves, its state follows one of the lines (trajectories) on the phase diagram.

### Chaos theory

Classic examples of phase diagrams from chaos theory are :

- the Lorenz attractor
- population growth (i.e. Logistic map)
- parameter plane of complex quadratic polynomials with Mandelbrot set.

## Quantum mechanics

In quantum mechanics, the coordinates *p* and *q* of phase space normally become hermitian operators in a Hilbert space.

But they may alternatively retain their classical interpretation, provided functions of them compose in novel algebraic ways (through Groenewold's 1946 star product), consistent with the uncertainty principle of quantum mechanics.
Every quantum mechanical observable corresponds to a unique function or distribution on phase space, and vice versa, as specified by Hermann Weyl (1927) and supplemented by John von Neumann (1931); Eugene Wigner (1932); and, in a grand synthesis, by H J Groenewold (1946).
With J E Moyal (1949), these completed the foundations of the **phase space formulation** of quantum mechanics, a complete and logically autonomous reformulation of quantum mechanics.^{[2]} (Its modern abstractions include deformation quantization and geometric quantization.)

Expectation values in phase-space quantization are obtained isomorphically to tracing operator observables with the density matrix in Hilbert space: they are obtained by phase-space integrals of observables, with the Wigner quasi-probability distribution effectively serving as a measure.

Thus, by expressing quantum mechanics in phase space (the same ambit as for classical mechanics), the Weyl map facilitates recognition of quantum mechanics as a **deformation** (generalization) of classical mechanics, with deformation parameter *ħ/S*, where *S* is the action of the relevant process. (Other familiar deformations in physics involve the deformation of classical Newtonian into relativistic mechanics, with deformation parameter *v/c*; or the deformation of Newtonian gravity into General Relativity, with deformation parameter Schwarzschild radius/characteristic-dimension.)

Classical expressions, observables, and operations (such as Poisson brackets) are modified by ħ-dependent quantum corrections, as the conventional commutative multiplication applying in classical mechanics is generalized to the noncommutative star-multiplication characterizing quantum mechanics and underlying its uncertainty principle.

## Thermodynamics and statistical mechanics

{{ safesubst:#invoke:Unsubst||$N=Refimprove |date=__DATE__ |$B=
{{#invoke:Message box|ambox}}
}}
In thermodynamics and statistical mechanics contexts, the term phase space has two meanings: It is used in the same sense as in classical mechanics. If a thermodynamic system consists of *N* particles, then a point in the *6N*-dimensional phase space describes the dynamic state of every particle in that system, as each particle is associated with three position variables and three momentum variables. In this sense, a point in phase space is said to be a microstate of the system. *N* is typically on the order of Avogadro's number, thus describing the system at a microscopic level is often impractical. This leads us to the use of phase space in a different sense.

The phase space can refer to the space that is parametrized by the *macroscopic* states of the system, such as pressure, temperature, etc. For instance, one can view the pressure-volume diagram or entropy-temperature diagrams as describing part of this phase space. A point in this phase space is correspondingly called a macrostate. There may easily be more than one microstate with the same macrostate. For example, for a fixed temperature, the system could have many dynamic configurations at the microscopic level. When used in this sense, a phase is a region of phase space where the system in question is in, for example, the liquid phase, or solid phase, etc.

Since there are many more microstates than macrostates, the phase space in the first sense is usually a manifold of much larger dimensions than the second sense. Clearly, many more parameters are required to register every detail of the system down to the molecular or atomic scale than to simply specify, say, the temperature or the pressure of the system.

## Phase Integral

In classical statistical mechanics (continuous energies) the concept of
phase space provides a classical analog to the partition function (sum
over states) known as the phase integral.^{[3]} Instead of summing the
Boltzmann factor over discretely spaced energy states (defined by
appropriate integer quantum numbers for each degree of freedom) one may
integrate over continuous phase space. Such integration essentially
consists of two parts: integration of the momentum component of all
degrees of freedom (momentum space) and integration of the position
component of all degrees of freedom (configuration space). Once the
phase integral is known, it may be related to the classical partition
function by multiplication of a normalization constant representing the
number of quantum energy states per unit phase space. As shown in detail
in,^{[4]} this normalization constant
is simply the inverse of Planck's constant raised to a power equal to the
number of degrees of freedom for the system.

## See also

- Phase line, 1-dimensional case
- Phase plane, 2-dimensional case
- Phase portrait
- Phase space method
- Parameter space
- Separatrix

- Applications

- Optical phase space
- State space (controls) for information about state space (similar to phase state) in control engineering.
- State space for information about state space with discrete states in computer science.
- Molecular dynamics

- Mathematics

- Physics

- Classical mechanics
- Hamiltonian mechanics
- Lagrangian mechanics
- State space (physics) for information about state space in physics

Template:Colend

## References

- ↑ Template:Cite doi
- ↑ Template:Cite doi
- ↑ Laurendeau, Normand M. Statistical Thermodynamics: Fundamentals and Applications. New York: Cambridge University Press, 2005. 164-66. Print.
- ↑ Template:Cite web

## External links

- {{#invoke:citation/CS1|citation

|CitationClass=citation }}