Geometrical optics of constrained Brownian motion: three short stories

• Published in: Journal of physics. A, Mathematical and theoretical Mathematical and theoretical, 2019-10, Vol.52 (41), p.415001
• Main Authors: Meerson, Baruch, Smith, Naftali R
• Format: Article
• Language: English
• Subjects: Brownian motion; dynamical phase transitions; large deviations; optimal fluctuation method
• ISSN: 1751-8113; 1751-8121
• DOI: 10.1088/1751-8121/ab3f0f

The optimal fluctuation method-essentially geometrical optics-gives a deep insight into large deviations of Brownian motion. Here we illustrate this point by telling three short stories about Brownian motions, 'pushed' into a large-deviation regime by constraints. In story 1 we compute the short-time large deviation function (LDF) of the winding angle of a Brownian particle wandering around a reflecting disk in the plane. Story 2 addresses a stretched Brownian motion above absorbing obstacles in the plane. We compute the short-time LDF of the position of the surviving Brownian particle at an intermediate point. Story 3 deals with survival of a Brownian particle in 1  +  1 dimension against absorption by a wall which advances according to a power law , where . We also calculate the LDF of the particle position at an earlier time, conditional on the survival by a later time. In all three stories we uncover singularities of the LDFs which have a simple geometric origin and can be interpreted as dynamical phase transitions. We also use the small-deviation limit of the geometrical optics to reconstruct the distribution of typical fluctuations. We argue that, in stories 2 and 3, this is the Ferrari-Spohn distribution.