  For n+1 dimensional asymptotically AdS spacetimes which have holographic
duals on their n dimensional conformal boundaries, we show that the imposition
of causality on the boundary theory is sufficient to prove positivity of mass
for the spacetime when n > 2, without the assumption of any local energy
condition. We make crucial use of a generalization of the time-delay formula
calculated in gr-qc/9404019, which relates the time delay of a bulk null curve
with respect to a boundary null geodesic to the Ashtekar-Magnon mass of the
spacetime. We also discuss holographic causality for the negative mass AdS
soliton and its implications for the positive energy conjecture of Horowitz and
Myers.
