The dual problem of optimal transportation in Lorentz-Finsler geometry is studied. It is shown that in general no solution exists even in the presence of an optimal coupling. Under natural assumptions dual solutions are established. It is further shown that the existence of a dual solution implies that the optimal transport is timelike on a set of full measure. In the second part the persistence of absolute continuity along an optimal transportation under obvious assumptions is proven and a solution to the relativistic Monge problem is provided.
resultsLet M be a smooth manifold. Throughout the article one fixes a complete Riemannian metric h on M , though local changes to the metric will be allowed. Consider a continuous function L :denotes the zero section in T M ) and positive homogenous of degree 2 such that the second fiber derivative is non-degenerate with index dim M − 1. Let C be a causal structure of (M, L), see [21], and define the Lagrangian L on T M by settingThe pair (M, L) is referred to as a Lorentz-Finsler manifold.One calls an absolutely continuous curve γ : I → M (C-)causal ifγ ∈ C for almost all t ∈ I. Note that this condition already implies that the tangent vector is contained in C whenever it exists.Denote with J + (x) the set of points y ∈ M such that there exists a causal curve γ : [a, b] → M with γ(a) = x and γ(b) = y. Two points x and y will be called causally related if y ∈ J + (x). Note that this relation is in general asymmetric. Define the setand J − (y) : {x ∈ M | y ∈ J + (x)}. A Lorentz-Finsler manifold is said to be causal if it does not admit a closed causal curve, i.e. J + ∩ △ = ∅ for △ := {(x, x)| x ∈ M }. Definition 2.1. A causal Lorentz-Finsler manifold (M, L) is globally hyperbolic if the sets J + (x) ∩ J − (y) are compact for all x, y ∈ M .