A weaker assumption finally proves a 50-year-old mass inequality

The Penrose inequality says a black hole can't be heavier than its size alone would suggest — a bound Penrose conjectured in 1973 but which has resisted proof in full generality. By introducing a minimal late-time condition on how a black hole settles down (it just needs to stabilize, not fully relax), the author constructs a new geometric object — a 'tangentially maximal' hypersurface — and reduces the spacetime problem to a known Riemannian result. The approach sidesteps decades of technical obstacles and brings the full spacetime inequality within reach.