Shadow and percolation I: discrete landscapes with independence

Abstract

Let X be a planar random field on Z^2 which we interpret as a random height function describing some landscape of montains. We consider a source of light (a sun) located at infinity in a direction parallel with an axis od Z^2 and emitting rays which are all parallel and make a slope l with the horizontal plane. Given the value of l some montains of the landscape will be lit by the sun and other will be in the shadow of some higher mountain. Under some assumptions on X, including and independence assumption, we prove that this model may present two different phases depending on l. When l>0 is small enough then, almost surely, there exists an unbounded cluster of points in the shadow. However, if l is big enough then, almost surely, there exists an unbounded cluster of points lit by the sun. We reformulate this problem in terms of percolation of a field alpha which has a simple definition (in terms of X) but that does not present many of the nice properties usually found in percolation models such as FKG inequality, invariance by rotation or finite range correlations.