A confirmed detection of the echoes would be a “smoking-gun sign of new physics,” said Vitor Cardoso, a physicist at the Instituto Superior Técnico in Lisbon, Portugal, in an email.

“This is a very exciting area of research,” said study co-author Niayesh Afshordi, an astrophysicist at the Perimeter Institute and the University of Waterloo, in a call. “It touches on cornerstones of physics and science as we know it.”

These signals, assuming they exist, would be created by close interactions with black holes, and they could help scientists confirm whether matter that enters black holes is truly gone forever.

A key mystery about black holes might be resolved by eerie “echoes” in spacetime, which could unlock an entirely new branch of exotic physics if they are ever detected. In fact, the echoes may have already been detected, according to a recent study published in the Journal of Cosmology and Astroparticle Physics, though it will take more observations and research to be sure.

“The presence of echoes would certainly be a strong indication that black holes are not the classical perfect-absorber we think they are,” added Cardoso, who was not involved in Afshordi’s study, but has published many papers about gravitational wave echoes.

“If observed, they would change completely our understanding of quantum gravity, and would finally give us clues to when quantum effects become important in gravitational physics.”

Quantum black holes and gravitational waves

The universe is filled with objects and phenomena that defy explanation, but few are as engulfed by speculation as black holes, which are punctures in the fabric of spacetime. Famously, the gravitational force of a black hole is so intense that not even light can escape its borders, the region which is known as the event horizon.

But the more scientists learn about black holes, the more this characterization of them as cosmic dead ends gets complicated. “Event horizons are found at places that you cannot test or experimentally verify, because nothing inside the event horizon can get out, according to the theory of relativity,” said Afshordi. “Exactly what happens at that boundary is really anyone’s guess.”

All of this uncertainty has been amplified in recent decades by the emergence of quantum mechanics. This field aims to understand interactions on the smallest scales of the universe, where physical laws do not cohere with Albert Einstein’s general theory of relativity.

Back in the 1970s, physicist Stephen Hawking was ruminating on how this tension between general relativity and quantum mechanics might manifest in black holes. He proposed a puzzle known as the black hole information paradox, which suggests the event horizon of a black hole might be permeable, which would defy the predictions of relativity.