  A point source on a plane constantly emits particles which rapidly diffuse
and then stick to a growing cluster. The growth probability of a cluster is
presented as a sum over all possible scenarios leading to the same final shape.
The classical point for the action, defined as a minus logarithm of the growth
probability, describes the most probable scenario and reproduces the Laplacian
growth equation, which embraces numerous fundamental free boundary dynamics in
non-equilibrium physics. For non-classical scenarios we introduce virtual point
sources, in which presence the action becomes the Kullback-Leibler entropy.
Strikingly, this entropy is shown to be the sum of electrostatic energies of
layers grown per elementary time unit. Hence the growth probability of the
presented non-equilibrium process obeys the Gibbs-Boltzmann statistics, which,
as a rule, is not applied out from equilibrium. Each layer's probability is
expressed as a product of simple factors in an auxiliary complex plane after a
properly chosen conformal map. The action at this plane is a sum of Robin
functions, which solve the Liouville equation. At the end we establish
connections of our theory with the tau-function of the integrable Toda
hierarchy and with the Liouville theory for non-critical quantum strings.
