We considered spontaneous fluctuations of thermodynamic variables in an adiabatically insulated system where nonlinear irreversible processes can take place. We introduced an extended definition of the Onsager-Machlup functional. In terms of this functional, a conditional probability density function was constructed, which describes, generally, non-Gaussian fluctuations. A sufficiently common form of phenomenological laws governing irreversible processes was found. In the case of linear laws, the proposed extension leads to the Onsager-Machlup results.

• Received 1 August 1994

DOI:https://doi.org/10.1103/PhysRevE.51.4190