  Penrose et al. investigated the physical incoherence of the spacetime with
negative mass via the bending of light. Precise estimates of time-delay of null
geodesics were needed and played a pivotal role in their proof. In this paper,
we construct an intermediate diagonal metric and make a reduction of this
problem to a causality comparison in the compactified spacetimes regarding
timelike connectedness near the conformal infinities. This different approach
allows us to avoid encountering the difficulties and subtle issues Penrose et
al. met. It provides a new, substantially simple, and physically natural
non-PDE viewpoint to understand the positive mass theorem. This elementary
argument modestly applies to asymptotically flat solutions which are vacuum and
stationary near infinity.
