Since 2015, gravitational waves have allowed scientists to find several previously undetected black holes and determine their masses and distances from Earth. However, two UK physicists think it’s feasible to do better. In a recent publication, the researchers suggest that gravitational waves might reveal how black holes devour items as they expand and assist resolve Hawking radiation’s information conundrum.

Anything that goes past the event boundaries of black holes is eaten.

Any item that crosses black holes’ event horizons is devoured. Meanwhile, Hawking radiation leaks energy to space. Black-body radiation shrinks and finally disappears black holes, as proposed by Stephen Hawking in 1974. These leaking photons’ energy is governed by the black hole’s mass. The black hole loses all the knowledge it had on the items it grabbed, defying quantum physics’ non-destruction of information.

Many physicists have proposed answers to this conundrum, most of which include Hawking radiation encoding information. However, Louis Hamaide and Theo Torres of King’s College London believe gravitational waves may offer a more natural solution. They found that by monitoring the gravitational radiation emitted by an item dragged into a black hole, practically all of its information may be recovered.

Black hole gravitational waves have been observed by LIGO–Virgo. This kilometre-sized laser interferometer detects black hole signals as they spiral in on each other and combine. After traveling millions of light-years to Earth, these black holes’ gravitational radiation is still visible.

Falling item

In their new study, Hamaide and Torres examine the radiation emitted by extremely small particles falling into Schwarzschild black holes, which do not spin and have no electrical charge. Perturbation theory, which corrects a black hole’s characteristics by the infalling item, is used in their computations. Unlike numerical simulations and curve fitting, this method gives an accurate, analytical formula for the emitted radiation.

Researchers determined that an in-falling object’s signature is startlingly simple. The amplitude of gravitational waves encodes the trapped object’s mass, whereas the frequency encodes the black hole’s mass. By viewing the emission from several angles, the capture’s course and phase may be determined.

These data would be easier to collect and comprehend than Hawking radiation’s “very spread out” information, according to Hamaide. He adds, “We are seeing that the information comes in very nice packets.”

These gravitational-wave signals are questioned by other researchers. The University of Waterloo’s Robert Mann believes that what counts is what produced the black hole, not what falls into it. He also agrees with the authors that a black hole is “basically an open quantum system,” but they do little quantum or semi-classical analysis.

Deficit quantum

The object’s wavefunction is quantum-mechanical, yet Hamaide and Torres concede that the fingerprints are classical. They estimate that classical information would make up well over 99.9% of the total, but only 100% will resolve the information conundrum. In other words, no matter how good their measurements are, their analysis can never recover all black hole information.

Vitor Cardoso of the University of Lisbon in Portugal and the Niels Bohr Institute in Copenhagen believes that it would not be able to quantify classical information in all instances because matter falling with full spherical symmetry would not emit gravitational waves. Cardoso also believes that any meaningful measurements could be achieved due to the necessity for several, infinitely sensitive detectors around the source.

Aspects of data retrieval that are worth considering

Jorge Pullin of Louisiana State University in the US doubts the current work’s practicality but applauds the authors’ “interesting points about information retrieval”. He emphasizes that gravitational wave measurements cannot clarify the colliding particles’ mass and spin (including sign). He adds, “This is not likely to change too much in the near future.”

Hamaide concedes that no detector can detect the pertubative system’s small impulses. He believes astrophysicists should take comfort in one part of their profession. This is because it rules out the theoretical possibility (known as degeneracy) that as gravitational wave detectors get more sensitive, it would become harder to pinpoint black hole masses and other features. He responds, “No way.”

The study is reported in Classical and Quantum Gravity.

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