In
statistical mechanics, the Fokker–Planck equation is a
partial differential equation that describes the
time evolution
Time evolution is the change of state brought about by the passage of time, applicable to systems with internal state (also called ''stateful systems''). In this formulation, ''time'' is not required to be a continuous parameter, but may be disc ...
of the
probability density function
In probability theory, a probability density function (PDF), or density of a continuous random variable, is a function whose value at any given sample (or point) in the sample space (the set of possible values taken by the random variable) ca ...
of the velocity of a particle under the influence of
drag forces and random forces, as in
Brownian motion. The equation can be generalized to other observables as well.
It is named after
Adriaan Fokker and
Max Planck, who described it in 1914 and 1917. It is also known as the Kolmogorov forward equation, after
Andrey Kolmogorov
Andrey Nikolaevich Kolmogorov ( rus, Андре́й Никола́евич Колмого́ров, p=ɐnˈdrʲej nʲɪkɐˈlajɪvʲɪtɕ kəlmɐˈɡorəf, a=Ru-Andrey Nikolaevich Kolmogorov.ogg, 25 April 1903 – 20 October 1987) was a Sovi ...
, who independently discovered it in 1931. When applied to particle position distributions, it is better known as the Smoluchowski equation (after
Marian Smoluchowski), and in this context it is equivalent to the
convection–diffusion equation
The convection–diffusion equation is a combination of the diffusion and convection (advection) equations, and describes physical phenomena where particles, energy, or other physical quantities are transferred inside a physical system due to two ...
. The case with zero
diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical ...
is the
continuity equation. The Fokker–Planck equation is obtained from the
master equation through
Kramers–Moyal expansion.
The first consistent microscopic derivation of the Fokker–Planck equation in the single scheme of
classical and
quantum mechanics
Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
was performed by
Nikolay Bogoliubov and
Nikolay Krylov.
One dimension
In one spatial dimension ''x'', for an
Itô process driven by the standard
Wiener process
In mathematics, the Wiener process is a real-valued continuous-time stochastic process named in honor of American mathematician Norbert Wiener for his investigations on the mathematical properties of the one-dimensional Brownian motion. It is ...
and described by the
stochastic differential equation (SDE)
with
drift
Drift or Drifts may refer to:
Geography
* Drift or ford (crossing) of a river
* Drift, Kentucky, unincorporated community in the United States
* In Cornwall, England:
** Drift, Cornwall, village
** Drift Reservoir, associated with the village
...
and
diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical ...
coefficient
, the Fokker–Planck equation for the probability density
of the random variable
is
In the following, use
.
Define the
infinitesimal generator (the following can be found in Ref.
):
The ''transition probability''
, the probability of going from
to
, is introduced here; the expectation can be written as
Now we replace in the definition of
, multiply by
and integrate over
. The limit is taken on
Note now that
which is the Chapman–Kolmogorov theorem. Changing the dummy variable
to
, one gets
which is a time derivative. Finally we arrive to
From here, the Kolmogorov backward equation can be deduced. If we instead use the adjoint operator of
,
, defined such that
then we arrive to the Kolmogorov forward equation, or Fokker–Planck equation, which, simplifying the notation
, in its differential form reads
Remains the issue of defining explicitly
. This can be done taking the expectation from the integral form of the
Itô's lemma:
The part that depends on
vanished because of the martingale property.
Then, for a particle subject to an Itô equation, using
it can be easily calculated, using integration by parts, that
which bring us to the Fokker–Planck equation:
While the Fokker–Planck equation is used with problems where the initial distribution is known, if the problem is to know the distribution at previous times, the
Feynman–Kac formula can be used, which is a consequence of the Kolmogorov backward equation.
The stochastic process defined above in the Itô sense can be rewritten within the
Stratonovich convention as a Stratonovich SDE:
It includes an added noise-induced drift term due to diffusion gradient effects if the noise is state-dependent. This convention is more often used in physical applications. Indeed, it is well known that any solution to the Stratonovich SDE is a solution to the Itô SDE.
The zero-drift equation with constant diffusion can be considered as a model of classical
Brownian motion:
This model has discrete spectrum of solutions if the condition of fixed boundaries is added for
:
It has been shown
that in this case an analytical spectrum of solutions allows deriving a local uncertainty relation for the coordinate-velocity phase volume:
Here
is a minimal value of a corresponding diffusion spectrum
, while
and
represent the uncertainty of coordinate–velocity definition.
Higher dimensions
More generally, if
where
and
are -dimensional random
vectors,
is an
matrix and
is an ''M''-dimensional standard
Wiener process
In mathematics, the Wiener process is a real-valued continuous-time stochastic process named in honor of American mathematician Norbert Wiener for his investigations on the mathematical properties of the one-dimensional Brownian motion. It is ...
, the probability density
for
satisfies the Fokker–Planck equation
with drift vector
and diffusion
tensor
In mathematics, a tensor is an algebraic object that describes a multilinear relationship between sets of algebraic objects related to a vector space. Tensors may map between different objects such as vectors, scalars, and even other tensor ...
, i.e.
If instead of an Itô SDE, a
Stratonovich SDE is considered,
the Fokker–Planck equation will read:
[
]
Examples
Wiener process
A standard scalar Wiener process
In mathematics, the Wiener process is a real-valued continuous-time stochastic process named in honor of American mathematician Norbert Wiener for his investigations on the mathematical properties of the one-dimensional Brownian motion. It is ...
is generated by the stochastic differential equation
Here the drift term is zero and the diffusion coefficient is 1/2. Thus the corresponding Fokker–Planck equation is
which is the simplest form of a diffusion equation. If the initial condition is , the solution is
Ornstein–Uhlenbeck process
The Ornstein–Uhlenbeck process is a process defined as
with . Physically, this equation can be motivated as follows: a particle of mass with velocity moving in a medium, e.g., a fluid, will experience a friction force which resists motion whose magnitude can be approximated as being proportional to particle's velocity with . Other particles in the medium will randomly kick the particle as they collide with it and this effect can be approximated by a white noise term; . Newton's second law is written as
Taking for simplicity and changing the notation as leads to the familiar form .
The corresponding Fokker–Planck equation is
The stationary solution () is
Plasma physics
In plasma physics, the distribution function for a particle species , , takes the place of the probability density function
In probability theory, a probability density function (PDF), or density of a continuous random variable, is a function whose value at any given sample (or point) in the sample space (the set of possible values taken by the random variable) ca ...
. The corresponding Boltzmann equation is given by
where the third term includes the particle acceleration due to the Lorentz force and the Fokker–Planck term at the right-hand side represents the effects of particle collisions. The quantities and are the average change in velocity a particle of type experiences due to collisions with all other particle species in unit time. Expressions for these quantities are given elsewhere. If collisions are ignored, the Boltzmann equation reduces to the Vlasov equation The Vlasov equation is a differential equation describing time evolution of the distribution function of plasma consisting of charged particles with long-range interaction, e.g. Coulomb. The equation was first suggested for description of plasma ...
.
Smoluchowski Diffusion Equation
The Smoluchowski Diffusion equation is the Fokker–Planck equation restricted to Brownian particles affected by an external force .
Where is the diffusion constant and . The importance of this equation is it allows for both the inclusion of the effect of temperature on the system of particles and a spatially dependent diffusion constant.
Starting with the Langevin Equation
In physics, a Langevin equation (named after Paul Langevin) is a stochastic differential equation describing how a system evolves when subjected to a combination of deterministic and fluctuating ("random") forces. The dependent variables in a Lang ...
of a Brownian particle in external field , where is the friction term, is a fluctuating force on the particle, and is the amplitude of the fluctuation.
At equilibrium the frictional force is much greater than the inertial force, . Therefore, the Langevin equation becomes,
Which generates the following Fokker–Planck equation,
Rearranging the Fokker–Planck equation,
Where . Note, the diffusion coefficient may not necessarily be spatially independent if or are spatially dependent.
Next, the total number of particles in any particular volume is given by,
Therefore, the flux of particles can be determined by taking the time derivative of the number of particles in a given volume, plugging in the Fokker–Planck equation, and then applying Gauss's Theorem
In vector calculus, the divergence theorem, also known as Gauss's theorem or Ostrogradsky's theorem, reprinted in is a theorem which relates the ''flux'' of a vector field through a closed surface to the ''divergence'' of the field in the vol ...
.
In equilibrium, it is assumed that the flux goes to zero. Therefore, Boltzmann statistics can be applied for the probability of a particles location at equilibrium, where is a conservative force and the probability of a particle being in a state is given as .
This relation is a realization of the fluctuation–dissipation theorem. Now applying to and using the Fluctuation-dissipation theorem,
Rearranging,
Therefore, the Fokker–Planck equation becomes the Smoluchowski equation,
for an arbitrary force .
Computational considerations
Brownian motion follows the Langevin equation
In physics, a Langevin equation (named after Paul Langevin) is a stochastic differential equation describing how a system evolves when subjected to a combination of deterministic and fluctuating ("random") forces. The dependent variables in a Lang ...
, which can be solved for many different stochastic forcings with results being averaged (canonical ensemble in molecular dynamics
Molecular dynamics (MD) is a computer simulation method for analyzing the physical movements of atoms and molecules. The atoms and molecules are allowed to interact for a fixed period of time, giving a view of the dynamic "evolution" of th ...
). However, instead of this computationally intensive approach, one can use the Fokker–Planck equation and consider the probability of the particle having a velocity in the interval when it starts its motion with at time 0.
1-D Linear Potential Example
Theory
Starting with a linear potential of the form the corresponding Smoluchowski equation becomes,
Where the diffusion constant, , is constant over space and time. The boundary conditions are such that the probability vanishes at with an initial condition of the ensemble of particles starting in the same place, .
Defining and and applying the coordinate transformation,
With the Smoluchowki equation becomes,
Which is the free diffusion equation with solution,
And after transforming back to the original coordinates,
Simulation
The simulation on the right was completed using a Brownian dynamics
Brownian dynamics (BD) can be used to describe the motion of molecules for example in molecular simulations or in reality. It is a simplified version of Langevin dynamics and corresponds to the limit where no average acceleration takes place. Thi ...
simulation. Starting with a Langevin equation for the system,
where is the friction term, is a fluctuating force on the particle, and is the amplitude of the fluctuation. At equilibrium the frictional force is much greater than the inertial force, . Therefore, the Langevin equation becomes,
For the Brownian dynamic simulation the fluctuation force is assumed to be Gaussian with the amplitude being dependent of the temperature of the system . Rewriting the Langevin equation,
where is the Einstein relation. The integration of this equation was done using the Euler–Maruyama method to numerically approximate the path of this Brownian particle.
Solution
Being a partial differential equation, the Fokker–Planck equation can be solved analytically only in special cases. A formal analogy of the Fokker–Planck equation with the Schrödinger equation
The Schrödinger equation is a linear partial differential equation that governs the wave function of a quantum-mechanical system. It is a key result in quantum mechanics, and its discovery was a significant landmark in the development of th ...
allows the use of advanced operator techniques known from quantum mechanics for its solution in a number of cases. Furthermore, in the case of overdamped dynamics when the Fokker–Planck equation contains second partial derivatives with respect to all spatial variables, the equation can be written in the form of a master equation that can easily be solved numerically.
In many applications, one is only interested in the steady-state probability distribution , which can be found from .
The computation of mean first passage times and splitting probabilities can be reduced to the solution of an ordinary differential equation which is intimately related to the Fokker–Planck equation.
Particular cases with known solution and inversion
In mathematical finance for volatility smile modeling of options via local volatility, one has the problem of deriving a diffusion coefficient consistent with a probability density obtained from market option quotes. The problem is therefore an inversion of the Fokker–Planck equation: Given the density f(x,t) of the option underlying ''X'' deduced from the option market, one aims at finding the local volatility consistent with ''f''. This is an inverse problem that has been solved in general by Dupire (1994, 1997) with a non-parametric solution. Brigo and Mercurio (2002, 2003) propose a solution in parametric form via a particular local volatility consistent with a solution of the Fokker–Planck equation given by a mixture model
In statistics, a mixture model is a probabilistic model for representing the presence of subpopulations within an overall population, without requiring that an observed data set should identify the sub-population to which an individual observati ...
. More information is available also in Fengler (2008), Gatheral (2008), and Musiela and Rutkowski (2008).
Fokker–Planck equation and path integral
Every Fokker–Planck equation is equivalent to a path integral. The path integral formulation is an excellent starting point for the application of field theory methods. This is used, for instance, in critical dynamics.
A derivation of the path integral is possible in a similar way as in quantum mechanics. The derivation for a Fokker–Planck equation with one variable is as follows. Start by inserting a delta function and then integrating by parts:
The -derivatives here only act on the -function, not on . Integrate over a time interval ,
Insert the Fourier integral
for the -function,
This equation expresses as functional of . Iterating times and performing the limit gives a path integral with action
The variables conjugate to are called "response variables".
Although formally equivalent, different problems may be solved more easily in the Fokker–Planck equation or the path integral formulation. The equilibrium distribution for instance may be obtained more directly from the Fokker–Planck equation.
See also
* Kolmogorov backward equation In probability theory, Kolmogorov equations, including Kolmogorov forward equations and Kolmogorov backward equations, characterize continuous-time Markov processes. In particular, they describe how the probability that a continuous-time Markov pr ...
* Boltzmann equation
* Vlasov equation The Vlasov equation is a differential equation describing time evolution of the distribution function of plasma consisting of charged particles with long-range interaction, e.g. Coulomb. The equation was first suggested for description of plasma ...
* Master equation
* Mean-field game theory Mean-field game theory is the study of strategic decision making by small interacting agents in very large populations. It lies at the intersection of game theory with stochastic analysis and control theory. The use of the term "mean field" is insp ...
* Bogoliubov–Born–Green–Kirkwood–Yvon hierarchy of equations
* Ornstein–Uhlenbeck process
* Convection–diffusion equation
The convection–diffusion equation is a combination of the diffusion and convection (advection) equations, and describes physical phenomena where particles, energy, or other physical quantities are transferred inside a physical system due to two ...
* Klein–Kramers equation
Notes and references
Further reading
*
*
*
*
{{DEFAULTSORT:Fokker-Planck Equation
Stochastic processes
Equations
Parabolic partial differential equations
Max Planck
Stochastic calculus
Mathematical finance
Transport phenomena