A black hole is not just the universe's trash can; it is more like a safe with a secret code. For decades, it was thought that any signal sent toward the event horizon would be partially reflected back: curved spacetime acts like a funhouse mirror, and some energy always escapes. Complete absorption seemed impossible because of the centrifugal barrier, which throws the wave back like a revolving door repelling an unwary visitor. But physicists have found the code to this safe: if you shape the incoming signal correctly in time, the black hole will absorb it entirely, not returning a single quantum.
The secret lies in so-called virtual absorption modes—complex frequencies at which reflection disappears. This is not just a mathematical abstraction: the authors calculated such modes for Schwarzschild, Bardeen, and Tangherlini black holes in higher-dimensional spacetimes. For example, for a scalar field in 400-dimensional space, they found a mode with frequency ω $r_h$ = 195.1821 + 10.6846i. When a wave packet is tuned to this frequency, it slips inside like a key into a well-oiled lock, and energy accumulates near the horizon with up to 100% efficiency. After the signal ends, the safe opens by itself: energy is released exponentially, like the dying ring of a bell—these are the same quasinormal modes that describe the "sound" of a perturbed black hole.
The idea came from materials science, where coherent perfect absorption was recently discovered: a specially shaped light pulse can be completely absorbed by a medium without reflection. The authors transferred this technique to gravitational systems and showed that a black hole is not a passive object but a dissipative system that can be controlled. Two-sided virtual absorption occurs when exciting complex-conjugate quasinormal modes: a signal enters from one side, and its "mirror twin" from the other, balancing each other out.
In the long run, this approach could shed light on the black hole information paradox: if an object can temporarily absorb radiation without reflection, it hints at how information is stored and released. Perhaps virtual absorption even plays a role in the collapse of ultracompact stars into black holes: the accumulated energy could serve as a trigger. And in a more down-to-earth area—metamaterials—the same principles could help create perfect absorbers for light and sound. The safe with the secret code turned out to be not so impregnable after all.
In four-dimensional spacetime, black holes have only purely imaginary perfect absorption modes, which cannot be excited by a real signal. But in spacetimes with more than four dimensions, complex modes appear, and the effect becomes achievable.
The idea of a perfect absorber that stores energy and then releases it evokes the black holes in Greg Egan's novel 'Schild's Ladder', where objects can manipulate radiation for their own evolution.