Confusion concerning the extrapolated endpoint. When will it ever end?

Abstract

In a paper on ``the Brownian motion analog of the well-known Milne problem in radiative transfer theory'' [\textit{J Stat Phys} 25 (1981) 569--82], Burschka and Titulaer reported: ``The value we find for this `Milne extrapolation length' is, in the appropriate dimensionless units, approximately twice the value found in the radiative transfer problem.'' A study by Ziff [\textit{J Stat Phys} 65 (1991) 1217--33], concerned with the absorption of particles executing a Rayleigh flight (randomly directed displacements of equal length $l$) by a black sphere of radius $R$, led to a value for the extrapolation length $\gamma$ about half as small as the benchmark result (for $R\gg l)$. The first discrepancy is shown to result from the disparity of the two length scales; the second, from the zero variance of the jump lengths. Ziff's finding that $\gamma$ is independent of $R$ for $0<l\leq 2R$, cannot be reconciled with studies based on the Lorentz-Boltzmann equation and the Klein-Kramers equation.