Physicist investigates black holes like crime scenes

Black holes have capture the imaginations of scientists and
the public alike. Physicist Nicholas Warner aims to resolve
one of their most enigmatic mysteries. Credit: University
of Southern California

Nicholas Warner wants to figure out how matter behaves at the
most extreme frontier of the universe. Warner, professor of
physics and astronomy and mathematics at USC Dornsife,
recently received a prestigious grant from the European
Research Council to study a problem first revealed by Stephen
Hawking—the black hole information paradox.


So what exactly is this paradox?

Black holes are defined by their immense gravitational
fields—fields that are powerful enough to trap light as well as
matter. This matter carries information, such as its
composition, mass and position. Since black holes collapse upon
themselves, everything gets so compressed that modern
understandings of physics also collapse.

“The simplest way to describe it,” says Warner, “is that one of
the properties of black holes is that once stuff is inside
them, they are featureless from the outside. So when you make a
black hole, you’ve essentially erased from the outside any
information about how you made it.”

According to the current laws of physics, this information must
always be conserved, even when trapped in these black holes. In
the 1980s, Stephen Hawking realized, however, that because
black holes have a temperature, they must evaporate over time
through a process known as Hawking radiation. This process
would destroy the information that the black hole conserved.
This discrepancy is called the
black hole information paradox
.

Warner likens the problem to forensic science—trying to
reconstruct a crime scene from blood spatters and fingerprints.
The formation and evaporation of a black hole erases everything
from the crime scene, but this is forbidden by quantum
mechanics. This problem has been around for about 40 years, and
Warner started working on it in 2007: He uses string theory to
uncover the hidden quantum structure of black holes.

Stop at the horizon

Now, the new five-year ERC Advanced grant will allow Warner to
work at the Institut de Physique Théorique near Paris, puzzling
over the quantum structure of black holes and the recovery of
information. Starting in January 2019, he’ll spend three of the
next five years in France, with the other years at USC
Dornsife.

Warner acknowledges how supportive USC has been of his
work—both in California and in Europe. “I view the fact that I
can go and do this thing as yet another manifestation of the
very strong support they’ve given me,” he says.

The goal of his project is to explore microstate geometries,
which are identical to in their properties, except that
microstate geometries do not have a horizon (the boundary at
which light can no longer escape the black hole’s gravitational
effects). Instead, microstate geometries use string theory and
field theory and stop the inevitable collapse of the black hole
at the horizon, conserving the information stored in the black
hole. Warner intends to study how the trapped matter and
radiation transforms into microstate geometries and is
re-emitted from the black hole’s surface.

Other physicists are working on different parts of the black
hole information paradox, using different approaches. Warner
says it feels a bit like the old parable of the blind men and
the elephant. In the story, six blind men stand next to an
elephant, trying to discern what they are touching. One touches
the side and says it’s a wall; another touches the animal’s
trunk and says it’s a leg.

“Everyone has a piece of the thing, and they extrapolate based
on what they can detect and analyze,” says Warner. That’s not
unlike the various approaches to solving the black hole
paradox. “The other people working on
this are really smart, but I think we have a huge chunk of the
puzzle in our approach,” he says.

Hawking: friend and mentor

Hawking, the renowned theoretical physicist, was Warner’s
thesis adviser at the University of Cambridge, where Warner
earned his Ph.D. in physics and mathematics in 1982. The two
men remained in touch through the years, and Warner was one of
500 people who attended Hawking’s funeral on March 31. He has
also been invited to the public memorial and interment on June
15 in Westminster Abbey, where Hawking will be buried next to
Isaac Newton and Charles Darwin.

As he looked around the room during the funeral, Warner had a
realization. “Basically every scientist there is in some way
working on this particular problem, in one form or another,
that Stephen put out there in 1980.

“It is deeply challenging,” says Warner. “I think that is his
greatest legacy, that he discovered this issue and we are still
trying to figure it out.”

Explore further:

Black holes dissolving like aspirin: How Hawking changed
physics

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