## Abstract

Since its inception in 1907, the Ehrenfest urn model (EUM) has served as a test bed of key concepts of statistical mechanics. Here we employ this model to study large deviations of a time-additive quantity. We consider two continuous-time versions of the EUM with K urns and N balls: with and without interactions between the balls in the same urn. We evaluate the probability distribution P_{T} (n = aN) that the average number of balls in one urn over time T, n, takes any specified value aN, where 0 ≤ a ≤ 1. For long observation time, T → ∞, a Donsker-Varadhan large deviation principle holds: -ln P_{T} (n = aN) ≃ TI(a,N,K, ⋯), where ⋯ denote additional parameters of the model. We calculate the rate function I(a,N,K, ⋯) exactly by two different methods due to Donsker and Varadhan and compare the exact results with those obtained with a variant of WKB approximation (after Wentzel, Kramers and Brillouin). In the absence of interactions the WKB prediction for I(a,N,K, ⋯) is exact for any N. In the presence of interactions the WKB method gives asymptotically exact results for N ≥ 1. The WKB method also uncovers the (very simple) time history of the system which dominates the contribution of different time histories to P_{T}(N = aN).

Original language | American English |
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Article number | 053202 |

Journal | Journal of Statistical Mechanics: Theory and Experiment |

Volume | 2018 |

Issue number | 5 |

DOIs | |

State | Published - 4 May 2018 |

### Bibliographical note

Publisher Copyright:© 2018 IOP Publishing Ltd and SISSA Medialab srl.

## Keywords

- fluctuation phenomena
- large deviations in non-equilibrium systems
- stochastic particle dynamics
- zero-range processes