What happens when black holes die?


In what is arguably his most significant contribution to science, Stephen Hawking has suggested that black holes can let out a form of radiation that causes them to gradually recoil and eventually end their lives in a massive explosive event.

This radiation, later called “Hawking radiation”, inadvertently poses a problem at the intersection of general relativity and quantum physics – the former being the best description we have of gravity and the universe at cosmically massive scales, while the latter is the most robust model of physics that governs the tiny.

Both theories have been repeatedly confirmed since their separate beginnings in the early 20th century. Yet they remain frustratingly incompatible.

This incompatibility, which stems primarily from the lack of a “quantum gravity” theory, was compounded in the mid-1970s when Hawking took the principles of quantum physics and applied them to the edge of black holes. A paradox was born that physicists have been working for 50 years to solve.

We may finally be on the verge of finding a solution thanks to a review published in the journal Europhysics Letters last month. In it, University of Sussex physics researchers Xavier Calmet and Stephen DH Hsu detail the problem of Hawking’s paradox and potential solutions to this cosmological problem.

What’s wrong with Hawking radiation?

In a 1974 letter titled Black hole explosions? published in the journal Naturea young Hawking proposed that quantum effects, usually ignored in black hole physics, could become significant in the deterioration of a black hole’s mass over a period of about 10¹⁷ (10 followed by 16 zeros) seconds.

Black holes are created when massive stars reach the end of their life and the fuel they use for nuclear fusion runs out. Stopping nuclear fusion ends the outer pressure that sustains a star against the inner force of its own gravity.

This results in a core collapse that creates a point in which spacetime is infinitely bent – a central singularity that physics cannot currently explain. At the outer edge of this extreme curvature is the black hole’s “event horizon”, or the point at which even light is not fast enough to escape the black hole’s gravitational pull.

“Hawking investigated quantum effects near the horizon of black holes realizing that pairs of particles would be generated spontaneously here,” Calmet said. Popular mechanics. “By looking at a specific pair of particles, he was able to show that one of them, when produced at the event horizon, would fall into the black hole never to be seen again. The other would escape and would in principle be visible to an outside observer.This is the famous Hawking radiation.

When these so-called virtual particles appear, they do so with equal and opposite charges to avoid violating the law of conservation of energy, which states that energy can neither be created nor destroyed. Like a bank, the vacuum of space has an overdraft facility, but this debt is usually quickly repaid by the particles annihilating each other.

If a particle escapes as Hawking radiation and avoids annihilation, the remaining energy debt must be paid by the mass of the black hole. This causes it to gradually evaporate as more particles appear and more Hawking radiation is emitted, sapping more mass.

“Either we have to modify quantum mechanics or Einstein’s theory of general relativity.”

“Hawking radiation is thermal, and thermal radiation is pretty much featureless. This means that it cannot contain information about the object that emitted it,” explains Calmet. “That would be a serious problem for black holes.”

He points out that Hawking’s calculation implies that information about what entered the black hole would be destroyed as the black hole evaporates.

“If true, that would be a problem for physics because one of the key properties of quantum mechanics called ‘unity’ implies that it’s always possible to watch a movie upside down. In other words , from the observation of the radiation emitted by a black hole, quantum mechanics tells us that we should be able to reconstruct the entire history of the black hole, what happened there, “says Calmet. “If Hawking is right, we should accept that one of the well-established theories of physics is wrong. Either we have to modify quantum mechanics or Einstein’s theory of general relativity.

Fortunately, again this year, physicists have come up with an idea that could eliminate Hawking’s paradox using existing mechanisms.

Black holes may have hair after all

Aaron HorowitzGetty Images

Although a powerful and mysterious phenomenon in space, black holes are quite easy to describe. Indeed, they can only have three properties of which we are sure: mass, angular momentum and electric charge. Theoretical physicist John Wheeler summed it up with the phrase “black holes don’t have hair”.

Calmet and Hsu suggest that information carried by swallowed matter can be encoded in the gravitational field of a black hole. By calculating gravity corrections at a quantum level, they showed that the star’s potential is sensitive to its internal conditions. This means that black holes possess, for lack of a better term, “quantum hair” grown by the composition of their progenitor star.

“When that star collapses into a black hole, the correction remains, and so black holes have a quantum hair,” Calmet says. “In other words, black holes have a quantum memory of their progenitor star.”

The duo followed this up by suggesting that Hawking radiation is not entirely thermal in nature. Instead, they believe it contains encoded informational quantum hair.


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    “The very small deviations from the thermality are enough to explain how the information that is in the black hole remains accessible to an outside observer,” explains Calmet. “It is sufficient to preserve unitarity and therefore, there is no paradox.”

    The beauty of Calmet and Hsu’s theory is that it requires no adjustments to quantum mechanics or general relativity, or additional mechanisms not yet offered by physics.

    “At the end of the day, all the ingredients to solve the problem have been there for a while, in a sense Hawking could have solved it himself if he had looked for a simple explanation,” says Calmet. “It is striking to me that solving the information paradox can be done without coming up with new physics despite what most people have believed for almost five decades.”

    Other ideas for solving Hawking’s paradox are not so conservative. Indeed, some might change our fundamental concept of the universe – or should it be “universe?”

    Black holes and baby universes

    The concept of “multiverse” is the idea that multiple universes exist in addition to our own, but are separate and unable to interact. A new iteration of this idea suggests that the singularity at the heart of a black hole – the infinitely curved point where all the laws of physics break down – is actually a separate and distinct infant universe.

    “In my theory, every black hole is actually a wormhole or an ‘Einstein-Rosen bridge’ to a new universe on the other side of the black hole’s event horizon,” Nikodem Poplawski, professor of physics at Department of Mathematics and Physics at the University of New Haven, tells Popular mechanics.

    This would mean that each universe, like ours, could host billions of black holes, each containing its own baby universe. Poplawski says that this proposition naturally resolves Hawking’s paradox.

    “Information does not disappear but goes to the baby universe on the other side of the black hole’s event horizon,” Poplawski continues. “Matter and information that falls into a black hole and comes out of a white hole [the opposite of a black hole which allows exit but not entry] in the world of baby.

    Although the theory does not explicitly account for Hawking radiation, much like Einstein’s original theory of general relativity, it does not forbid it. Regarding the eventual evaporation of the black hole, Poplawski says that this event would permanently isolate the nascent universe from its parent.

    Many other ideas have been put forward to solve Hawking’s paradox, including information remaining inside the black hole and emerging at the end of the black hole’s evaporation. While none have quite solved the problem in a neat arc, Calmet says some of the greatest minds in physics are hard at work on the matter.

    Hawking was a titan in his field, and his most important work showed that even cosmic titans like black holes can’t last forever. Hawking’s successors work to apply this impermanence to the paradox that bears his name.

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