Physicists Believe They Have Solved a Black Hole Paradox

Physicists Believe They Have Solved a Black Hole Paradox

November 1, 2020 – A group of physicists believe they may have solved a decades-old paradox about black holes. In a series of groundbreaking calculations, they showed that black holes can shed information, which for so long seemed impossible. This paradox was first pointed out by Epstein pal Stephen Hawking five decades ago.

The story goes like this, according to Quanta Magazine article “Stephen Hawking’s Black Hole Paradox Keeps Physicists Puzzled“:

In 1991, Hawking and Kip Thorne bet John Preskill that information that falls into a black hole gets destroyed and can never be retrieved. Called the black hole information paradox, this prospect follows from Hawking’s landmark 1974 discovery about black holes — regions of inescapable gravity, where space-time curves steeply toward a central point known as the singularity. Hawking had shown that black holes are not truly black. Quantum uncertainty causes them to radiate a small amount of heat, dubbed ‘Hawking radiation.’ They lose mass in the process and ultimately evaporate away. This evaporation leads to a paradox: Anything that falls into a black hole will seemingly be lost forever, violating ‘unitarity’ — a central principle of quantum mechanics that says the present always preserves information about the past.

Hawking and Thorne argued that the radiation emitted by a black hole would be too hopelessly scrambled to retrieve any useful information about what fell into it, even in principle. Preskill bet that information somehow escapes black holes, even though physicists would presumably need a complete theory of quantum gravity to understand the mechanism behind how this could happen.

In 2012, a new twist on the paradox emerged. Nobody had explained precisely how information would get out of a black hole, and that lack of a specific mechanism inspired Joseph Polchinski and three colleagues to revisit the problem. Conventional wisdom had long held that once someone passed the event horizon, they would slowly be pulled apart by the extreme gravity as they fell toward the singularity. Polchinski and his co-authors argued that instead, in-falling observers would encounter a literal wall of fire at the event horizon, burning up before ever getting near the singularity.

At the heart of the firewall puzzle lies a conflict between three fundamental postulates. The first is the equivalence principle of Albert Einstein’s general theory of relativity: Because there’s no difference between acceleration due to gravity and the acceleration of a rocket, an astronaut named Alice shouldn’t feel anything amiss as she crosses a black hole horizon. The second is unitarity, which implies that information cannot be destroyed. Lastly, there’s locality, which holds that events happening at a particular point in space can only influence nearby points. This means that the laws of physics should work as expected far away from a black hole, even if they break down at some point within the black hole — either at the singularity or at the event horizon. – Quanta Magazine

There are multiple facets to this paradox, and as you can see this is something that has kept physicists and scientists puzzled for many years, trying to reconcile it and come up with a conclusion. One theory that seemed to get close was regarding quantum entanglement, suggests that perhaps microscopic wormholes were created, invisible to the naked eye, that connect these seemingly distant points in space and time.

Accretion Disk

“Material, such as gas, dust and other stellar debris that has come close to a black hole but not quite fallen into it, forms a flattened band of spinning matter around the event horizon called the accretion disk (or disc). Although no-one has ever actually seen a black hole or even its event horizon, this accretion disk can be seen, because the spinning particles are accelerated to tremendous speeds by the huge gravity of the black hole, releasing heat and powerful x-rays and gamma rays out into the universe as they smash into each other.”Physics of the Universe, Event Horizon and Accertion Disk 

The latest discovery attempts to reconcile these paradoxes and present a unified theory behind this black hole paradox. According to George Musser of Quanta Magazine, in an expose entitled “The Black Hole Information Paradox Comes to an End,” he writes:

In a series of breakthrough papers, theoretical physicists have come tantalizingly close to resolving the black hole information paradox that has entranced and bedeviled them for nearly 50 years. Information, they now say with confidence, does escape a black hole. If you jump into one, you will not be gone for good. Particle by particle, the information needed to reconstitute your body will reemerge. Most physicists have long assumed it would; that was the upshot of string theory, their leading candidate for a unified theory of nature. But the new calculations, though inspired by string theory, stand on their own, with nary a string in sight. Information gets out through the workings of gravity itself — just ordinary gravity with a single layer of quantum effects.

This is a peculiar role reversal for gravity. According to Einstein’s general theory of relativity, the gravity of a black hole is so intense that nothing can escape it. The more sophisticated understanding of black holes developed by Stephen Hawking and his colleagues in the 1970s did not question this principle. Hawking and others sought to describe matter in and around black holes using quantum theory, but they continued to describe gravity using Einstein’s classical theory — a hybrid approach that physicists call ‘semiclassical.’ Although the approach predicted new effects at the perimeter of the hole, the interior remained strictly sealed off. Physicists figured that Hawking had nailed the semiclassical calculation. Any further progress would have to treat gravity, too, as quantum.

That is what the authors of the new studies dispute. They have found additional semiclassical effects — new gravitational configurations that Einstein’s theory permits, but that Hawking did not include. Muted at first, these effects come to dominate when the black hole gets to be extremely old. The hole transforms from a hermit kingdom to a vigorously open system. Not only does information spill out, anything new that falls in is regurgitated almost immediately. The revised semiclassical theory has yet to explain how exactly the information gets out, but such has been the pace of discovery in the past two years that theorists already have hints of the escape mechanism.

‘That is the most exciting thing that has happened in this subject, I think, since Hawking,’ said one of the co-authors, Donald Marolf of the University of California, Santa Barbara.

‘It’s a landmark calculation,’ said Eva Silverstein of Stanford University, a leading theoretical physicist who was not directly involved.

You might expect the authors to celebrate, but they say they also feel let down. Had the calculation involved deep features of quantum gravity rather than a light dusting, it might have been even harder to pull off, but once that was accomplished, it would have illuminated those depths. So they worry they may have solved this one problem without achieving the broader closure they sought. ‘The hope was, if we could answer this question — if we could see the information coming out — in order to do that we would have had to learn about the microscopic theory,’ said Geoff Penington of the University of California, Berkeley, alluding to a fully quantum theory of gravity.

What it all means is being intensely debated in Zoom calls and webinars. The work is highly mathematical and has a Rube Goldberg quality to it, stringing together one calculational trick after another in a way that is hard to interpret. Wormholes, the holographic principle, emergent space-time, quantum entanglement, quantum computers: Nearly every concept in fundamental physics these days makes an appearance, making the subject both captivating and confounding. – Quanta Magazine

Not everyone agrees with this new calculation and what it means. Some physicists believe that Hawking was correct and some think that this new calculation is only partly correct. However, one area of agreement is the nature of space-time inside a black hole which they say goes away. They believe that space-time is linked with gravity and that within the event horizon of the black hole, this dissolves which is why information can escape.

Don Page, physicist at the University of Alberta in Canada says that the nature of black holes themselves are in fact paradoxical. They appear to be rotting from the outside, in. These structures appear to defy the laws of physics as they are currently understood, although our understanding is constantly changing as more information comes to light. Page, who worked under Stephen Hawking at one point, believed he discovered a fundamental point everyone else was missing in their calculations:

He considered an aspect of the process that had been relatively neglected: quantum entanglement. The emitted radiation maintains a quantum mechanical link to its place of origin. If you measure either the radiation or the black hole on its own, it looks random, but if you consider them jointly, they exhibit a pattern. It’s like encrypting your data with a password. The data without the password is gibberish. The password, if you have chosen a good one, is meaningless too. But together they unlock the information. Maybe, thought Page, information can come out of the black hole in a similarly encrypted form.

Page calculated what that would mean for the total amount of entanglement between the black hole and the radiation, a quantity known as the entanglement entropy. At the start of the whole process, the entanglement entropy is zero, since the black hole has not yet emitted any radiation to be entangled with. At the end of the process, if information is preserved, the entanglement entropy should be zero again, since there is no longer a black hole. ‘I got curious how the radiation entropy would change in between,’ Page said.

Initially, as radiation trickles out, the entanglement entropy grows. Page reasoned that this trend has to reverse. The entropy has to stop rising and start dropping if it is to hit zero by the endpoint. Over time, the entanglement entropy should follow a curve shaped like an inverted V. – Quanta Magazine

This is now known as the “Page Curve” and is essential for reconciling the paradox and getting closer to a unified theory on Black Holes.

Matthew S. Williams of Interesting Engineering writes in “Unraveling the Mysteries of Black Holes” that three scientists have won the Nobel Prize this year for their work on Black Holes:

Black holes are among the most enigmatic objects in the Universe. Stellar black holes are, to put it briefly, the remnants of massive stars that have reached the end of their life cycles and collapsed into a region of spacetime that is incredibly dense. The gravitational force is so strong that nothing  – not even light – can escape its surface, and the laws of time and space become warped.

Ever since they were predicted about a century ago, astronomers and astrophysicists have struggled to learn more about these strange and mysterious objects. This has been no small feat, and has required the dedication and efforts of generations of scientists. More questions remain than answers. But what they have learned so far has already revealed much about the Universe we live in and the laws that govern it.

After theorizing their existence, scientists began to see indications of these objects by the latter half of the 20th century. By studying their effects on the space surrounding them, they were able to indirectly identify where black holes reside. Today, many astronomers believe that most galaxies have a supermassive black hole (SMBH) at their core, and that includes the Milky Way. 

This year, the Nobel Prize in Physics has been awarded to three scientists who have had a particularly large impact in the field of black hole research. One half of the prize was awarded to famed British mathematical physicist Roger Penrose who – in the words of Caltech scientist Kip Thorne – ‘revolutionized the mathematical tools that we use to analyze the properties of spacetime.’ 

The other half of the prize was awarded to astronomers Andrea Ghez and Reinhard Genzel, both of whom have led groups since the 1990s which mapped the orbits of stars close to the Galactic Center. These studies led Ghez and Genzel to conclude that an extremely massive object – known as Sagittarius A* (Sgr A*) – was dictating the stars’ movements, thus providing the most compelling proof for the existence of an SMBH at the core of our galaxy.

The Nobel Prize Committee has recognized all three individuals for achievements that go back many decades and have revolutionized our understanding of the Universe.

In recognition of his immense contributions, the Royal Swedish Academy of Sciences has chosen to award half of the 2020 Nobel Prize in Physics to Robert Penrose. When Penrose began studying black holes, there were several unresolved theoretical issues that were causing astronomers no shortage of headaches! 

For one, a key assumption was that black holes had to be spherically symmetrical, otherwise they shouldn’t be able to collapse to a single point and create a singularity. In response, Penrose developed a theory that did away with the assumption of spherical symmetry and assumed only that the collapsing matter had a positive energy density. In order to do this, he had to invent new mathematical methods and find a way to describe the geometry of black holes. 

For starters, Penrose developed the concept of a trapped surface, a closed two-dimensional surface where all light rays traveling perpendicular to the surface converge towards the future. This was contrary to what happens with a spherical surface in flat space, where light-rays traveling outwards diverge.

A consequence of this theory is that within a black hole’s Event Horizon, time and space switch roles, to the point where traveling out of a black hole would be tantamount to traveling backward in time. An even more dramatic consequence of the trapped surface is that all the matter that collapsed to form the black hole will be frozen in time. 

To visualize space-time, Penrose introduced what would come to be known as the ‘Penrose diagram.’ This technique uses conformal transformations, where points infinitely far away in space (and events in the infinite past or future) can be brought in from infinity to fit inside a framework of finite size – aka. a Penrose diagram. – Interesting Engineering

Because of all these recent insights, 2020 could be dubbed the “Year of the Black Hole.” Scientists and physicists seem to be converging on a unified theory of black holes and getting closer to solving the long-standing problems associated with them.

Almost all the images we have of black holes are actually artistic renditions and representations of what scientists and physicists think they look like. However, Interesting Engineering has put together a video of how the first image of a real black hole was captured:

This image is of the supermassive black hole found within Virgo A.

Adrian Cho with Science Magazine also has recently published an article going into some of the weird aspects of black holes, and what makes them so fascinating to physicists:

Less than 5 years ago, physicists rocked the scientific world when they first spotted gravitational waves—fleeting ripples in space and time—set off when two gargantuan black holes billions of light-years away swirled into each other. Since then, scientists have detected a scad of similar events, mostly reported event by event. Today, however, researchers with a global network of gravitational wave detectors announced the first major statistical analyses of their data so far, 50 events in all. Posted online in four papers, the analyses show that black holes—ghostly ultra intense gravitational fields left behind when massive stars collapse—are both more common and stranger than expected. They also shed light on mysteries such as how such black holes pair up before merging.

The new studies, posted on the physics preprint server arXiv, ‘are super-important,’ says Carl Rodriguez, an astrophysicist at Carnegie Mellon University who was not involved in the work. ‘With an individual event, there’s only so much you can do in comparing to astrophysics models. But with a catalog you can not only begin to constrain the theory, you can start to understand the landscape.’ Selma de Mink, an astrophysicist at Harvard University, says she and her colleagues have been waiting to do their own analyses of the data trove. ‘There will definitely be a flurry of papers that are rushing to take the first stabs at the data.’

The observations come from three huge L-shaped optical instruments called interferometers that can measure the infinitesimal stretching of space itself by a passing gravitational wave. Two of those detectors belong to the Laser Interferometer Gravitational-Wave Observatory (LIGO), a pair of detectors with arms 4 kilometers long in Louisiana and Washington that spotted the first gravitational waves in 2015. The third detector is Virgo, an interferometer near Pisa, Italy, that has 3-kilometer-long arms and joined the hunt for gravitational waves in 2017.

LIGO and Virgo had already spotted 11 events, including one merger of neutron stars, an event that may shed light on how the universe forges heavy elements. Now, the team has cataloged 37 additional black hole mergers, one likely neutron star merger, and one possible merger of a black hole and neutron star from the first half of its third observing run, from April through September 2019.

Analyses of all 50 events show that when it comes to black holes, ‘the diversity is surprisingly large,’ says Frank Ohme, a gravitational wave astronomer at the Max Planck Institute for Gravitational Physics. From details of the mergers’ chirplike signals, scientists can calculate the masses of the colliding black holes. They expected to find a ‘mass gap’ between about 45 and 135 solar masses—the result of particle physics processes that should blow apart stars within a certain mass range before they can collapse into black holes.

However, LIGO and Virgo have now spotted mergers involving black holes squarely within the gap, including one with a mass of roughly 85 solar masses. De Mink, who models the evolution of black hole pairs from binary star systems, says accounting for the interlopers will be challenging. The mass gap is ‘such a clear prediction from the models that it’s hard to believe that there’s not a feature there’ in the mass spectrum, she says.

Similarly, scientists expected another forbidden range below five solar masses, based on previous observations of individual black holes peacefully orbiting normal stars. But at least one hole in the catalog appears to fall below that limit. ‘How do you describe the boundaries of this population?’ Ohme asks. ‘It’s not such a clear picture anymore.’

Their new ability to take a census of black holes has also enabled researchers to probe whether black holes in a merging pair point in the same direction as they orbit each other—a potential clue to how the pair came together in the first place. If the spins align with the orbital axis, the black holes might have formed from a pair of stars that were born together, naturally acquired matching spins, and remained companions after they collapsed. If the spins point in different directions, the black holes might have formed first and then somehow paired later. Which formation channel dominates is a subject of intense debate. – Science Magazine

Perhaps scientists and physicists all working on the same kind of calculations and problems, has created some kind of quantum entanglement event where, hundredth monkey style, the creative consciousness has brought fourth the understanding to get us that much closer to solving all the riddles of these structures. Researchers who have been studying black holes for decades are hopeful, and filled with renewed optimism that one day we will find the answers we are looking for. They also believe that this is actually just the beginning of science itself. They say they have found answers to questions they didn’t even know that they had. Often, this is how science works, sometimes the answer comes to you before you even knew you needed it. In that sense, the end is just the beginning and the beginning is the end.

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