If you lurch into the event horizon of a black hole, will you ever come out? According to a series of new calculations which just solved a 50-year old problem even Stephen Hawking couldn’t figure out, the answer is yes.
It’s being described as a landmark calculation—the biggest thing to happen in the field since the work of the famous British physicist established the problem in the first place.
Since the 1970s, physicists have been grappling with a logical contradiction in calculations surrounding a black hole called the “black hole information paradox.”
Hawking would use his “semiclassical” quantum/general-relativity hybrid-explanation of the physics of a black hole to describe what happens to matter in and around it.
He discovered that quantum uncertainty causes small amounts of radiation to emanate from a black hole called “Hawking radiation.” This eventually causes it to lose mass and evaporate into nothingness. If the black hole loses mass and eventually disappears, then what falls in must appear again somewhere. The question is: where/how/why does the information escape?
The authors of the new calculations, including scientists from UC Santa Barbara, have uncovered additional effects permitted by general relativity but that Hawking didn’t include, which describe a strange situation in which information that falls into a black hole will eventually come out, and that this phenomenon happens at the same time, and is partially to blame for the evaporation of a black hole.
Quantum entanglement
The way in which it works is through quantum entanglement, a phenomenon which simply means that particles of matter can be linked on the quantum level, and display patterns and reactivity to each other even though they could be separated by thousands of miles.
Don Page, a physicist at the University of Alberta, was a graduate student whose studies of black holes were key in helping his advisor, Stephen Hawking, make his realization that black holes emit radiation. In 1980, Page broke with Hawking and argued that information must be released or preserved in black holes, causing a schism among physicists at the time.
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Page would go on to establish a timeline of a black hole’s lifespan—shaped like an upside-down V known as “Page time” or the “Page curve”—it described how information which fell into the black hole would escape through emitted Hawking radiation until the black hole was no more. This was called “entanglement entropy,” and set up physicists for a 30-year lay up to make a slam dunk calculation.
The V-shaped decline
“Over the past two years, physicists have shown that the entanglement entropy of black holes really does follow the Page curve, indicating that information gets out,” explains George Musser writing for Quanta Magazine.
The slam dunk was started in October 2018 by Ahmed Almheiri at the Institute for Advanced Study when he used quantum computing to create a universe in which a simple black hole system located at the center of space began emitting radiation as per Hawking’s theory.
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The system begins to radiate as one entangled particle enters and another one leaves. This process continues, and the number of entangled particles increases, increasing the level of entanglement entropy.
If one imagines the black hole as the contents of a snow globe, and the glass of the globe as the event horizon (the edge of the black hole where the laws of physics begin to break down), Almheiri found that as the entangled entropy grew within the system, a “quantum extremal surface,” formed on the glass of the snow globe, just inside the event horizon.
Everything inside the quantum extremal surface is not part of the black hole, but rather like a collection of entangled particles which no longer contribute to the entropy in the system. Furthermore, the innermost particles in the simulated black hole became likewise detached from the black hole, forming something which Almheiri called “the island.”
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At this point, non-entangled radiation begins to be emitted, and the black hole breathes itself out of existence.
On to the next one
In demonstrating that entanglement entropy of black holes followed the Page curve, Almheiri and his friends confirmed that black holes do in fact release information, though it comes out in such disorder as to appear like an encrypted password.
By now if one’s brain is still working after all this, Almheiri’s research amazingly includes theoretical tools that would allow researchers to “decrypt” the scrambled entangled particles in the quantum extremal surface, and figure out what they are and where they came from.
Last year, having just solved a 50-year puzzle and proved Page’s life’s work, the team decided to focus on the mysterious “island” of particles that were in—but not “of” the black hole. The island is part of the radiation, but hasn’t flown out or been transferred to the extremal surface.
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This disconnect is theorized as being part of the reason why black holes go down the other side of the Page curve, and if solving the black hole information paradox seemed hard, Musser described the issue of the mysterious island as causing the team to “look off into the distance, momentarily lost for words.”
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