Wave scattering by black holes has been described for decades within a stationary framework: a monochromatic wave is partially absorbed and partially reflected. Complete absorption was considered impossible due to the centrifugal barrier and spacetime curvature. However, in materials science, coherent perfect absorption was recently discovered: a specially shaped wave packet can be completely absorbed by a medium without reflection. This work transfers the idea to gravitational systems. Nearby in the archive there are already works on quasinormal modes, horizons without singularities, and gravitational atoms, but no one has considered time-controlled excitation of resonances in the complex plane to achieve perfect absorption.
The authors modeled the scattering of wave packets on potentials describing rectangular barriers, ultracompact stars, and black holes. The time evolution of the wave equation was solved numerically, and initial data were chosen so that their frequency and shape matched virtual absorption modes—complex solutions with zero reflection. To find these modes in higher-dimensional spacetimes, they used a combination of a spectral method based on Chebyshev collocation, a continued fraction method, and a large-dimension analytic approximation. The energy inside and outside the potential barrier was tracked over time to confirm accumulation and subsequent release.
It is shown that when the system is irradiated with a wave packet whose frequency matches a virtual absorption mode, reflection vanishes for the entire duration of the signal. Energy accumulates in the potential region with efficiency close to 100%, and after the signal ends it is released exponentially, like the ringing of quasinormal modes. For Schwarzschild and Bardeen black holes, two-sided virtual absorption is demonstrated when excited by complex-conjugate quasinormal modes. Virtual absorption modes for Tangherlini black holes in dimensions d ≥ 10 are calculated for the first time: for a scalar field in d = 400, a mode with frequency ω $r_h$ = 195.1821 + 10.6846i is found. It is discovered that in the four-dimensional case only purely imaginary algebraically special modes exist, while in higher dimensions complex modes with a rich structure appear.
The work expands the view of black holes as dissipative systems. Unlike neighboring studies that focused on stationary modes or Hawking radiation, here it is shown for the first time that temporal modulation of the signal allows perfect absorption in realistic astrophysical scenarios. This connects black hole physics with the actively developing field of coherent perfect absorption in optics and acoustics. Furthermore, the discovery of complex virtual modes in higher dimensions points to hidden symmetries in the perturbation equations.
The natural next step is to search for virtual absorption modes for rotating Kerr black holes and for more realistic stellar models. It may be possible to find configurations where the accumulated energy leads to nonlinear effects, up to the collapse of a star into a black hole. It is also interesting to investigate whether virtual absorption can be used to amplify gravitational waves or to test quantum effects near the horizon.
The results may affect the understanding of accretion processes and black hole dynamics, as well as the development of wave control methods in metamaterials.
The plan is to extend the analysis to rotating black holes and to investigate the stability of virtual modes in the presence of nonlinearities.
The work is directly related to the black hole information paradox: if a black hole can temporarily absorb radiation without reflection, this may shed light on how information is preserved and released. Virtual absorption may also be key to understanding the mechanism of black hole formation from ultracompact objects.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The paper explores time-controlled excitation of virtual absorption modes to achieve complete absorption of wave packets on black hole and star potentials. The authors numerically simulated the evolution, choosing initial data in accordance with complex resonances, and for the first time calculated such modes for Tangherlini black holes in higher dimensions.
Where its strength is
The strength of the work lies in transferring the concept of coherent perfect absorption to gravitational systems and demonstrating that virtual modes, previously considered unattainable, can be achieved with precisely tailored wave packets.
Where it could go next
Worth trying
Why this may matter
If virtual absorption can be realized in astrophysical conditions, it would provide a new way to accumulate energy in strong gravitational fields and possibly shed light on processes near the horizon.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.60, against a usual 0.64. It looks like few have drilled nearby.
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.