Langevin to Fokker-planck? Uh oh

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SUMMARY

The discussion focuses on converting the Langevin equation, specifically dy/dt = -dV/dy + η(t) with V(y) = -by^3/3 + ζy, into a Fokker-Planck equation. Participants highlight the relationship between the Langevin and Fokker-Planck equations, emphasizing the continuity equation and the long-time limit known as the Smoluchowski equation. Key references include Brenner and Edwards' "Macrotransport Processes" and Masters' work on time-scale separations, which provide insights into the equivalence of the two approaches in terms of diffusivity.

PREREQUISITES
  • Understanding of Langevin equations and their applications in stochastic processes.
  • Familiarity with Fokker-Planck equations and their role in probability density evolution.
  • Knowledge of the Smoluchowski equation and its significance in statistical mechanics.
  • Basic concepts of statistical dynamics, particularly in relation to particle suspensions.
NEXT STEPS
  • Study the derivation of the Fokker-Planck equation from the Langevin equation.
  • Explore the implications of time-scale separations in statistical mechanics.
  • Investigate Levy distributions and their applications in contrast to Gaussian distributions.
  • Read Brenner and Edwards' "Macrotransport Processes" for a deeper understanding of diffusivity in stochastic systems.
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Researchers and students in statistical mechanics, physicists working with stochastic processes, and anyone interested in the mathematical modeling of particle dynamics.

JVanUW
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This isn't homework but I'm interested.
So I have the langevin equation dy/dt = -dV/dy +η(t)

where V(y) = -by^3/3 + ζy

how can I turn this into a fokker-planck equation?

What I'm getting is

x' = -u(bx^2+ζ) + η(t)

Which I don't know how to solve in closed form.

Any ideas/suggestions?

Thanks!
 
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Yikes... I have some information, but it's not clear how to relate the specific to your question. I have the Langevin equation as m dv/dt = F + f, where f is the randomly fluctuating force. The Fokker-Planck equation is based on the continuity equation and is too messy to write here, but is a time evolution of a probability density. The long-time limit is known as the Smoluchowski equation.

The two equations (Langevin vs. Fokker-Planck/Smoluchowski) differ in a few respects- primarily the relevant timescale, but also the difference in viewing a process as 'diffusive' or as direct modeling of fluctuations in particle velocity.

My reference for this is Brenner and Edwards, "Macrotransport Processes". They begin by defining a timescale that allows for an average velocity, but short enough to allow for fluctuations. This leads to an integral expression for the probability density, and after a few short (but very dense) pages, they show the two approaches yield identical expressions for the diffusivity.

They reference Masters, "Time-scale separations and the validity of the Smoluchowski, Fokker-Planck and Langevin equations as applied to concentrated particle suspensions" Mol. Phys. 57, 303-317 (1986). There may also be some useful material in Balescu's "Statistical Dynamics: Matter out of Equilibrium"- it seems to be an open area of research (by considering Levy distributions instead of Gaussian distributions, for example)
 
I'll check it out thanks!
 

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