Two neutron stars whirl in a close dance, each feeling the tidal field of its partner. It's like the Moon raising oceans on Earth, but here it's spacetime itself and the star's matter that deform. Astrophysicists call this tidal deformation — a measure of how much an object 'bends' under the gravitational pull of its companion. For black holes, this deformation is zero, but for their exotic relatives, so-called ECOs (compact objects without an event horizon)—it exists and carries information about what lies in the dark heart.
In the real universe, compact objects are rarely alone. They are often surrounded by matter falling into the gravitational pit — an accretion disk. Just as water changes how a float responds to waves, a disk can alter the tidal response. The authors of the new study built a model: a thin, self-gravitating gas disk orbits a horizonless object. They took the simplest case, a non-rotating object similar to a black hole but with a reflective surface, and placed it in a spacetime described by a relativistic solution for a thin disk.
The result was elegant. Even in the presence of a disk, the main hallmark of a horizonless object remained: the logarithmic dependence of the response on how compact the object is. In other words, on how close its radius is to the Schwarzschild radius. This dependence is like a fingerprint that physics near the horizon leaves on the tidal response. The disk does not erase it. It only makes the entire response louder — amplifying its overall magnitude. Moreover, the disk's effect can be separated from the effect of the object's nature itself.
The practical takeaway follows. Future gravitational wave detectors, such as LISA or the Einstein Telescope, will be able to pick up signals from mergers of compact objects. If those objects are surrounded by disks, the tidal signal could be brighter than expected, and this must be accounted for when interpreting the data. But the very possibility of distinguishing a black hole from its horizonless counterpart does not vanish. Under suitable conditions, the disk does not mask the fundamental difference. The universe leaves us a key to the mystery of the horizon, even when it veils it in a shroud of gas.
The tidal deformation of the Earth due to the Moon is about 55 centimeters — the solid crust rises and falls twice a day, although we don't notice it.
In Vernor Vinge's novel 'A Fire Upon the Deep', civilizations use neutron stars and black holes as energy sources, and their accretion disks as building material for astroengineering structures.