A seemingly intractable black hole paradox first proposed by physicist Stephen Hawking could finally be resolved — by wormholes through space-time.
The "black hole information paradox" refers to the fact that information cannot be destroyed in the universe, and yet when a black hole eventually evaporates, whatever information was gobbled up by this cosmic vacuum cleaner should have long since vanished. The new study proposes that the paradox could be resolved by nature's ultimate cheat code: wormholes, or passages through space-time.
"A wormhole connects the interior of the black hole and the radiation outside, like a bridge," Kanato Goto, a theoretical physicist at the RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program in Japan, said in a statement.
Under Goto's theory, a second surface appears inside the event horizon of a black hole, the boundary beyond which nothing can escape. Threads from a wormhole connect that surface to the outside world, entangling information between the interior of the black hole and the radiation leaks at its edges.
Black hole information paradox
In the 1970s, Hawking discovered that black holes aren't exactly black, but at first, he didn't realize the giant problem he had created. Before his discovery, physicists had assumed that black holes were exceedingly simple. Sure, all sorts of complicated stuff fell into them, but the black holes locked all that information away, never to be seen again.
But Hawking found that black holes release radiation, and can eventually evaporate entirely, in a process now known as Hawking radiation But that radiation didn't carry any information itself. Indeed, it couldn't; by definition, the event horizon of a black hole prevents information from leaving. So, when a black hole finally evaporates and disappears from the universe, where did all its locked-up information go?
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This is the black hole information paradox. One possibility is that information can be destroyed, which seems to violate everything we know about physics. (For instance, if information can be lost, then you can't reconstruct the past from present events, or predict future events.) Instead, most physicists try to solve the paradox by finding some way — any way — for the information inside the black hole to leak out through the Hawking radiation. That way, when the black hole disappears, the information is still present in the universe.
Either way, describing this process requires new physics.
"This suggests that general relativity and quantum mechanics as they currently stand are inconsistent with each other," Goto said. "We have to find a unified framework for quantum gravity."
A tale of two entropies
In 1992, physicist Don Page, a former graduate student of Hawking, viewed the information paradox problem another way. He started by looking at quantum entanglement, which is when distant particles have their fates linked. This entanglement acts as the quantum mechanical connection between the Hawking radiation and the black hole itself. Page measured the amount of entanglement by calculating the "entanglement entropy," which is a measure of the amount of information contained in the entangled Hawking radiation.
In Hawking's original calculation, no information escapes, and the entanglement entropy always increases until the black hole finally disappears. But Page found that if black holes do indeed release information, the entanglement entropy initially grows; then, halfway through the black hole's lifetime, it decreases before finally reaching zero, when the black hole evaporates (meaning all the information inside the black hole has finally escaped).
If Page's calculations are correct, this suggests that if black holes do allow information to escape, then something special has to happen around the halfway point of their lives. While Page's work didn't solve the information paradox, it did give physicists something juicy to work on. If they could give black holes a midlife crisis, then that solution might just resolve the paradox.
Through the wormhole
More recently, several teams of theorists have been applying mathematical techniques borrowed from string theory — one approach to unifying Einstein's relativity with quantum mechanics — to examine this problem. They were examining how space-time near an event horizon might be more complex than scientists initially thought. How complex? As complex as possible, allowing any sort of curving and bending at the microscopic scale.
Their work led to two surprising features. One was the appearance of a "quantum extremal surface" just below the event horizon. This interior surface moderates the amount of information leaving the black hole. Initially, it doesn't do much. But when the black hole is halfway through its life, it begins to dominate the entanglement, reducing the amount of information released) so that the entanglement entropy follows Page's predictions.
Secondly, the calculations revealed the presence of wormholes — a lot of them. These wormholes appeared to connect the quantum extremal surface to the exterior of the black hole, allowing the information to bypass the event horizon and be released as Hawking radiation.
But that previous work was only applied to highly simplified "toy" models (such as one-dimensional versions of black holes). With Goto's work, that same result has now been applied to more realistic scenarios — a major advance that brings this work closer to explaining reality.
Still, there are a lot of questions. For one, it's not clear yet if the wormholes that appear in the mathematics are the same wormholes that we think of as shortcuts in time and space.
They are so deeply buried in the math that it's difficult to determine their physical meaning. On one hand, it could mean that literal wormholes thread in and out of an evaporating black hole. Or it could just be a sign that space-time near a black hole is nonlocal, which is a hallmark of entanglement — two entangled particles do not need to be in causal contact in order to influence each other.
One of the other major issues is that, while physicists have identified a possible mechanism to relieve the paradox, they don't know how it actually works. There's no known process that actually performs the work of taking the information that's inside a black hole and encoding it in the Hawking radiation. In other words, physicists have built a possible road to solving the information paradox, but they haven't found any way to build the trucks that travel down that road.
"We still don't know the basic mechanism of how information is carried away by the radiation," Goto said. "We need a theory of quantum gravity."
Originally published on Live Science.
