New Theoretical Study Sheds Light on How Information Escapes from Evaporating Black Hole

by johnsmith

An international team of physicists from RIKEN, Cornell University and the University of California, Santa Barbara, has used a new spacetime geometry with a wormhole-like structure to show that information is not necessarily irretrievably lost from evaporating black holes.

A traversable wormhole in four space-time dimensions. Image credit: NASA / G. Bacon, STScI.

Einstein’s theory of general relativity predicted that once an object falls inside a black hole’s event horizon, it ends up at the center of the black hole called a singularity where it is completely crushed.

In the 1970s, Stephen Hawking calculated that black holes should emit radiation when quantum mechanics is considered.

“This is called black hole evaporation because the black hole shrinks, just like an evaporating water droplet,” said Dr. Kanato Goto, a researcher with the RIKEN Interdisciplinary Theoretical and Mathematical Sciences and the Department of Physics at Cornell University.

This, however, led to a paradox. Eventually, the black hole will evaporate entirely — and so too will any information about its swallowed contents.

But this contradicts a fundamental dictum of quantum physics: that information cannot vanish from the Universe.

“This suggests that general relativity and quantum mechanics as they currently stand are inconsistent with each other. We have to find a unified framework for quantum gravity,” Dr. Goto said.

“Many physicists suspect that the information that escapes is encoded somehow in the radiation.”

“To investigate, they compute the entropy of the radiation, which measures how much information is lost from the perspective of someone outside the black hole.”

“In 1993, physicist Don Page calculated that if no information is lost, the entropy will initially grow, but will drop to zero as the black hole disappears.”

“When physicists simply combine quantum mechanics with the standard description of a black hole in general relativity, Page appears to be wrong — the entropy continually grows as the black hole shrinks, indicating information is lost.”

But recently, physicists have explored how black holes mimic wormholes — providing an escape route for information.

“This is not a wormhole in the real world, but a way of mathematically computing the entropy of the radiation,” Dr. Goto said.

“A wormhole connects the interior of the black hole and the radiation outside, like a bridge.”

When Dr. Goto and his colleagues performed a detailed analysis combining both the standard description and a wormhole picture, their result matched Page’s prediction, suggesting that physicists are right to suspect that information is preserved even after the black hole’s demise.

“We discovered a new spacetime geometry with a wormhole-like structure that had been overlooked in conventional computations,” Dr. Goto said.

“Entropy computed using this new geometry gives a completely different result.”

“But this raises new questions. We still don’t know the basic mechanism of how information is carried away by the radiation. We need a theory of quantum gravity.”

The team’s paper appears in the Journal of High Energy Physics.


K. Goto et al. 2021. Replica wormholes for an evaporating 2D black hole. J. High Energ. Phys 289; doi: 10.1007/JHEP04(2021)289

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