Two houses with identical facades. Windows, doors, wall color — everything matches. But inside, one is empty, while the other buzzes with life. How can you tell without stepping inside? In space, there's a similar puzzle: a black hole and a gravastar look the same from the outside, but they are built differently. A black hole is emptiness within, while a gravastar is a thin shell with a dense core.

The authors calculated how a particle is affected by its own field — the so-called self-force. It's like an echo: the particle "shouts," and space answers back. For a scalar charge (a simplified model) near a black hole, the self-force vanishes, but near a gravastar it pushes the particle. At large distances, the force falls off as the inverse fifth power of distance, but with a correction that depends on the gravastar's radius. An electric charge also feels the difference: a correction is added to the usual attraction, revealing the internal structure.

Inside the gravastar, the picture is even more intriguing. The particle is drawn toward the center, but the force vanishes at the very middle. Harmonic oscillations arise, like a ball in a pit. This is a direct consequence of the "soft" internal geometry, where there is no singularity. So, the self-force acts as a probe that reaches where telescopes cannot.