Quantum vacuum is not absolute emptiness. It is more like a taut membrane that constantly ripples from invisible particles popping in and out of existence. Even in total darkness, this membrane trembles. Physicists decided to see how it behaves around a thin spherical shell, inside which is ordinary flat space, like in our room.

To do this, they took massless fields (the simplest kind of trembling) and honestly computed two things: how the vacuum changes the electrical properties of space, this is called vacuum polarization, and how it presses on the shell itself, i.e., the energy-momentum tensor. They performed calculations in two ways: approximately, when the shell weakly curves space, and exactly — using a computer. At the shell's center, an unexpected effect emerged: the vacuum creates pressure there, similar to the Casimir effect — the force that brings two plates together in a vacuum. And this pressure remains finite even if the shell is compressed almost to the size of a black hole.

The most intriguing result awaited outside. When the shell was squeezed more and more, the vacuum around it quickly became indistinguishable from the vacuum around a real black hole. As if space 'forgets' that the inside of the shell is empty and begins to behave as if an event horizon had already formed there. This hints at universality: perhaps any superdense object without a horizon — for example, a hypothetical 'black star'—looks like a black hole from outside not only to light but also to the quantum vacuum. The authors emphasize that their work is a foundation for future calculations, where one can account for the backreaction of the vacuum on the shell itself and test how stable such objects are.