Light passing near a massive body slightly changes its path — gravity pulls even on rays. In ordinary situations, a beam of light from a distant star is squeezed as if drawn together by an invisible rubber band. This property is called the null convergence condition: gravity makes light trajectories converge rather than diverge.

But when a black hole turns into a white hole, something strange happens. First, the region from which nothing can escape, the black hole, shrinks and disappears. Then a region that expels everything outward, a white hole, emerges and expands. Roy Kerr and other physicists have long suspected that such a transition breaks the standard rules. The new work shows that for light rays to start diverging in a white hole, the convergence condition must be violated. Otherwise, the transformation simply cannot occur.

The authors constructed concrete transition models using Painlevé–Gullstrand coordinates — something like a map that shows how space flows through time. In one model with a Bardeen-type mass function, the violation of the convergence condition turned out to be localized: it lasts only a brief moment while the black hole evaporates and the white hole forms. It is as if the rubber band pulling the rays together momentarily loosens, allowing them to fly apart.

The sharpest scenario, an instantaneous transition, requires an infinite violation of the condition. At that moment, the usual description of spacetime as a smooth surface ceases to work. A full quantum theory of gravity is needed, where spacetime curvature is replaced by something more fundamental. Without it, we only see a hint that the classical picture of the world is incomplete.