A RIKEN physicist and two colleagues have found that a wormhole—a bridge
connecting distant regions of the Universe—helps to shed light on the
mystery of what happens to information about matter consumed by black holes.

Einstein's theory of general relativity predicts that nothing that falls
into a black hole can escape its clutches. But in the 1970s, Stephen Hawking
calculated that black holes should emit radiation when quantum mechanics,
the theory governing the microscopic realm, is considered. "This is called
black hole evaporation because the black hole shrinks, just like an
evaporating water droplet," explains Kanato Goto of the RIKEN
Interdisciplinary Theoretical and Mathematical Sciences.

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,"
says Goto. "We have to find a unified framework for quantum gravity."

Many physicists suspect that the information escapes, 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, notes Goto. "A wormhole connects the interior of the black hole
and the radiation outside, like a bridge."

When Goto and his two 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," says Goto. "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," Goto says. "We need a
theory of quantum gravity."

## Reference:

Kanato Goto et al, Replica wormholes for an evaporating 2D black hole,
Journal of High Energy Physics (2021).
DOI: 10.1007/JHEP04(2021)289

Tags:
Physics