In what is arguably his most significant contribution to science, Stephen Hawking suggested that black holes can leak a form of radiation that causes them to gradually ebb away, and eventually end their lives in a massive explosive event.
This radiation , later called “Hawking radiation,” inadvertently causes a problem at the intersection of general relativity and quantum physics — the former being the best description we have of gravity and the universe on cosmically massive scales, while the latter is the most robust model of the physics that governs the very small.
The two theories have been confirmed repeatedly since their distinct inceptions at the start of the 20th century. Yet, they remain frustratingly incompatible.
This incompatibility , which mainly arises from the lack of a theory of “quantum gravity,” 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 a solution thanks to review published in the journal Europhysics Letters last month. In it, University of Sussex physics researchers Xavier Calmet and Stephen D. H. Hsu detail the problem of the Hawking paradox and potential solutions to this cosmological problem.
What’s the Problem With Hawking Radiation?
In a 1974 letter entitled Black hole explosions? published in the journal Nature, a young Hawking proposed that quantum effects, usually ignored in black hole physics, could become significant in the deterioration of mass of a black hole over a period of approximately 10¹⁷ (10 followed by 16 zeroes) seconds.
Black holes are created when massive stars reach the end of their lives and the fuel they use for nuclear fusion is exhausted. The cessation of nuclear fusion ends the outward pressure that supports a star against the inward force of its own gravity.
This results in a core collapse that creates a point in which spacetime is infinitely curved — a central singularity that physics currently can’t explain. At the outer edge of this extreme curvature is the “event horizon” of the black hole, or the point at which not even light is fast enough to escape the gravitational pull of the black hole.
“Hawking investigated quantum effects close to the horizon of black holes realizing that pairs of particles would be spontaneously generated here,” Calmet tells Popular Mechanics. “Looking at a specific pair of particles, he could show that one of the two when produced at the event horizon would fall into the black hole never to be seen again. The other would escape and be in principle visible to an outside observer. This is the famous Hawking radiation.”
When these so-called virtual particles arise, 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 paid back by the particles annihilating each other.
If one particle escapes as Hawking radiation and avoids annihilation, the energy debt that remains has to be paid by the mass of the black hole. This causes it to gradually evaporate as more particles pop into existence and more Hawking radiation is emitted, sapping more mass.
“Hawking radiation is thermal, and thermal radiation is pretty much featureless. This means that it cannot carry information about the object that emitted it,” Calmet says. “This would be a serious issue for black holes.”
He points out that Hawking’s calculation implies that the information about what went into the black hole would be destroyed as the black hole evaporates.
“If true, this would be an issue for physics as one of the key properties of quantum mechanics called ‘unitarity’ implies that it is always possible to watch a movie backward. 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 all the history of the black hole, what went into it,” Calmet says. “If Hawking is right, we would need to accept that one of the well-established theories of physics is wrong. Either we need to modify quantum mechanics or maybe Einstein’s theory of general relativity.”
Fortunately, just this year, the physicists suggested an idea that could do away with the Hawking paradox by using existing mechanisms.
Black Holes May Have Hair After All
Despite being a powerful and mysterious spacetime phenomenon, black holes are fairly easy to describe. This is because they can only have three properties that we are sure of: mass, angular momentum, and electric charge. Theoretical physicist John Wheeler summed this up with the phrase “black holes have no hair.”
Calmet and Hsu suggest that information carried by swallowed matter may be encoded in the gravitational field of a black hole. By calculating corrections to gravity on a quantum level, they showed the potential of the star is sensitive to its internal conditions. This means black holes possess, for lack of a better term, “quantum hair” grown by its progenitor star’s composition.
“When this star collapses to a black hole, the correction remains and black holes thus have a quantum hair,” Calmet explains. “In other words, black holes have some quantum memory of their progenitor star.”
The duo followed this by suggesting that Hawking radiation isn’t entirely thermal in nature. Instead, they believe it has informational quantum hair encoded into it.
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“The very small departures from thermality are enough to explain how the information that is in the black hole remains accessible to an outside observer,” Calmet argues. “This is enough to preserve unitarity and thus, there is no paradox.”
The beauty of Calmet and Hsu’s theory is it requires no adjustments to quantum mechanics or general relativity, or extra mechanisms not already proposed by physics.
“In the end, all the ingredients to solve the problem have been around for quite a while, in a sense Hawking could have solved it himself if he had looked for a simple explanation,” Calmet says. “It is striking to me that solving the information paradox could be done without positing new physics despite what most people have believed for almost five decades.”
Other ideas to solve Hawking’s paradox aren’t nearly as conservative. Indeed, some could change our fundamental concept of the universe–or should that be “universes?”
Black Holes and Baby Universes
The concept of the “multiverse” is the idea that multiple universes exist in addition to our own, but are separated and unable to interact. One new iteration of this idea suggests that the singularity at the heart of a black hole — the infinitely curved point at which all 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, a physics lecturer in the Department of Mathematics and Physics at the University of New Haven, tells Popular Mechanics.
This would mean each universe, like our own, could host billions of black holes, each containing its own baby universe. Poplawski says that this proposition resolves Hawking’s paradox naturally.
“The information does not disappear but goes to the baby universe on the other side of the black hole’s event horizon,” Poplawski continues. “The matter and information that falls into a black hole and emerges from a white hole [the opposite of a black hole which allows exit but not entry] in the baby universe.”
While the theory doesn’t explicitly account for Hawking radiation, much like Einstein’s original theory of general relativity, it doesn’t disallow it. With regard to the eventual evaporation of the black hole, Poplawski says this event would just permanently seal off the infant universe from its parent.
Many other ideas have been put forward to solve Hawking’s paradox, including information remaining in the black hole’s interior and emerging at the end of black hole evaporation. While none have quite wrapped the problem up in a neat bow, Calmet says some of the finest minds in physics are hard at work on the issue.
Hawking was a titan in his field, and his most significant work showed that not even cosmic titans like black holes can last forever. Hawking’s successors are working to ensure this impermanence applies to the paradox that bears his name.
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