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.
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.
“This is a very exciting area of research,” said study co-author Niayesh Afshordi, an astrophysicist at the University of Waterloo, in a call. “It touches on cornerstones of physics and science as we know it.”
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.
“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.
Hawking proposed that a quantum black hole—a black hole model that accounts for quantum mechanics—might eventually leak matter it has swallowed back out of the event horizon. These liberated particles, known as Hawking radiation, would be stripped of all information about their pre-black-hole existence.
This effect would violate established laws of the universe, which do not allow for information about particles to be simply deleted from existence, not even by black holes. But since it is currently impossible to sidle up to a black hole to confirm the existence of Hawking radiation, the idea has lived in the land of theory for decades.
However, that may change now that we have entered the era of gravitational wave astronomy, which has opened an entirely new observational window into the physics of black holes.
What are gravitational wave echoes?
Gravitational waves are disturbances in the curvature of space created by monumental cosmic events, such as the explosion of stars or collisions between massive objects. Scientists snagged the first detection of a wave in 2015 using the Laser Interferometer Gravitational-Wave Observatory (LIGO).
LIGO has since captured several gravitational waves in the years following its initial breakthrough, in tandem with its sister observatory Virgo. One of the most significant events revealed by these detectors was forged by the collision of two neutron stars, which are collapsed cores of dead stars, about 130 million light years from Earth.
The wave, which is called GW170817 because was detected on August 17, 2017, continues to yield tantalizing insights about fundamental problems in physics. For instance, Afshordi and Jahed Abedi, an astrophysicist at the Max Planck Institute for Gravitational Physics who co-authored the recent paper, suggest that the neutron stars were so massive that their merger collapsed into a rapidly spinning black hole.
Assuming that a black hole was born from GW170817, the team aimed to look for signatures of “quantum fuzz” around its event horizon, which might indicate whether Hawking radiation—or anything else—can escape a black hole.
“It's exactly like playing a guitar or a piano inside a room"
“We, and others, have suggested that if you have violent processes such as mergers or collisions of two black holes, you can stimulate Hawking radiation,” Afshordi said. “You could actually put it on steroids, if you will.”
Given that gravitational waves are a product of these violent processes, it’s possible that this hypothetical spike in radiation may be embedded in the wave data as echoes.
This echo effect would be produced by the interplay between the strong gravity of the black hole and the possible quantum effects near the event horizon. For instance, when gravitational waves are produced by the collision of two black holes, they ripple out in all directions like a stone thrown into a pond. In addition to waves traveling outward in the universe, including toward detectors here on Earth, they also bounce back inward at the black hole.
If nothing can escape from a black hole, then any sign of this inward ripple should be annihilated once it passes the event horizon. But if weird quantum particles are constantly escaping the event horizon, as suspected by Hawking, the wave would hit them and get bounced back out again as an echo of the original ripple.
As it travels out again, the wave would hit another barrier, the photon ring, which is a circle of light outside the event horizon. This luminous structure was beautifully captured in the first image ever taken of a black hole, released by the Event Horizon Telescope (EHT) last year.
“Such an echo is trapped between the surface of the quantum black hole and the photon ring,” explained Cardoso. “It's exactly like playing a guitar or a piano inside a room: you will hear echoes because the sound produced by the string is trapped between the walls of the room.”
When the echoes bounce off the photon ring, some of them might leak out into the wider universe. In their paper, Afshordi and Abedi suggest that GW170817 may contain extremely subtle traces of these echoes, though the team said that their findings are still tentative, and will require corroboration from other observations.
What’s next?
Gravitational waves are incredibly elusive, which is why our bodies and our planet (thankfully) aren’t visibly warped when they pass through us. It took decades to design, build, and test observatories sensitive enough to capture the waves, which compress space by as little as one ten-thousandth the width of a proton.
The ghostly nature of the waves means that the tiniest glitch in a detector, or noise from other thermal and quantum sources, can deliver false positives. As a result, LIGO scientists have developed sophisticated models and techniques to weed out false alarms.
The risk of a false detection is even more significant in the case of gravitational wave echoes, which remain hypothetical and therefore difficult to identify. Afshordi and Abedi suggest that the signals they pulled from the GW17081 data could actually be disturbances made by unknown exotic objects or noise in the instruments.
Despite the current uncertainty about the echoes, LIGO continues to improve its sensitivity, and its next batch of detections is expected to be more detailed than before, according to Afshordi. That will help scientists determine if there is a pattern of matching signals in the wave data that hints at echoes from the event horizon.
These observational advances are complemented by the headway scientists have made in modeling the speculative profiles of these echoes. “There was very little theoretical understanding of exactly what these echoes should look like,” back in 2016, Afshordi said. “Now, our group, and many other groups, have studied this much more intensely so we have a much better idea of what we are looking for and better models and strategies.”
Cardoso has likewise spent years researching these echoes, and also hopes to see them confirmed or ruled out in the near future. “We have to start looking now, to develop proper search techniques, and to understand precisely the theory behind such possible echoes,” he said. “For example, what would one learn about quantum gravity if echoes are indeed seen?”
Cardoso noted that the Laser Interferometer Space Antenna (LISA), a space-based gravitational wave detector due for launch in the 2030s, will be able to observe the event horizons of black holes “with an incredible precision” far beyond the capabilities of LIGO and Virgo.
“If quantum gravity, or any other physics, affects the near horizon structure of black holes, LISA will most likely see it,” said Cardoso, who is the co-chair of this mission’s Science Interpretation Package.
For more than a century, people have been grappling with the otherworldly properties and perplexing paradoxes presented by black holes. During that time, Einstein, Hawking, and thousands of other scientists have advanced our conception of these bizarre objects and their implications for fundamental physics.
With the recent advent of stunning black hole photography and gravitational wave detectors, the entire field of black hole physics has gone into overdrive. We are on the precipice of many more discoveries, which could shake up our understanding of the universe.
“Black holes are real objects,” Afshordi emphasized, noting that we now have a picture of one “directly staring at us.”
What will we see in these objects as we continue to stare back?
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