On a donut-shaped ring, the theory lives on the surface, while inside hides a curved space — like a saddle widening toward the edge. According to the holographic principle of Juan Maldacena, everything happening inside can be described by data on the boundary.

It turns out that for such a shape, this connection becomes a perfect mirror: a simple measure of one point's influence on another on the surface exactly equals the length of the shortest path through the curved inner space. This is a strict equality, not an approximation, and it works even for the simplest fields, where the most complex calculations unexpectedly simplify down to a single line.

The same holds for quantum entanglement: its measure — entanglement entropy — between two parts of the surface matches the area of a special cross-section inside. This builds an exact dictionary: information on the boundary is a reflection of the geometry in the bulk. Such an approach promises simplified calculations and helps understand quantum gravity near black holes.