David Garfinkle

At the edge of spacetime, where all light rays end, live symmetries that physicists have debated for half a century. They are called superrotations, transformations of the celestial sphere that diverge at isolated points, like vortices in superfluid helium.

A stone falls into a pond — near the shore, the ripples are barely visible, while the water churns at the point of impact. The electric field around a charge behaves similarly: close in, it's sharp and tangled, but at a distance it stretches into a smooth wave that carries energy away. Physicists call this distant region the radiation zone.

A tightrope walker steps along a taut wire. Each step is a search for balance, an instant calculation of forces. In much the same way, Einstein's equations describe how matter and energy cause spacetime to curve.