A soap bubble hangs in the air, and its shape already contains everything needed to calculate the internal tension. A similar principle worked in a new study on quantum gravity. Physicists took a closed surface, an analog of such a bubble, and specified a few numbers on it: for example, the electric field strength and the curvature. That turned out to be enough for a quantum object to spontaneously emerge inside. Any compact volume with such data on its boundary prepares a specific quantum state, just as a film stretched over a hoop takes the only possible shape.
The recipe is simple and elegant. Take two “bubbles” with fields specified on their walls, glue them along the boundary, and compute a special integral over all possible geometries inside. This integral gives the transition probability between states, as if we were asking how similar two soap bubbles are when looking only at their edges. No infinities or distant observers are needed — everything happens in a finite volume.
The method was immediately applied to entropy — a measure of quantum state entanglement. For a part of the boundary, they computed the Rényi entropy and confirmed that it behaves reasonably: it is non-negative and decreases with the growth of the parameter. Continuing the result analytically, they obtained the von Neumann entropy — the very quantity that in quantum gravity is often related to the area of a minimal surface. And indeed, the answer matched the known formula where entropy equals the area of the partition dividing the volume in half.
This approach connects geometry with quantum information directly, without unnecessary intermediaries. It could simplify calculations in theories where spacetime itself emerges from quantum bits, and hint at how the Universe's memory is structured at the deepest level. And at the heart of it all is the simple image of a soap film, whose shape is dictated only by the hoop and tension.
The von Neumann entropy in this work was obtained by analytic continuation — the same trick physicists use to turn imaginary temperatures into real ones when studying phase transitions.
In Greg Egan's novel “The City of Permutations,” quantum states are created inside a simulation with specified boundary conditions — almost like in this paper.