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Sky and Telescope - Stringy Holes: Hawking Concedes Defeat

By Robert Naeye

July 22, 2004 | Famed Cambridge University physicist Stephen
Hawking has finally come around to accepting what many of his
colleagues have thought for decades: black holes preserve
information about the material that falls into them. In a
lecture delivered on July 21st at a physics conference in
Dublin, Ireland, Hawking outlined his reasoning and conceded
defeat in a bet he made in 1997 with Caltech physicist John
Preskill. Hawking bought a baseball encyclopedia for Preskill
and had it shipped across the Atlantic.

Although many physicists agree with Hawking's conclusion, they
don't necessarily buy his argument. Preskill himself says he
does not understand it, and Caltech physicist Kip Thorne, who
sided with Hawking in the bet, is not ready to concede defeat.
Mainstream media outlets have reported this story largely
because of Hawking's celebrity status, not because his Dublin
lecture broke new ground. "I think it is a bit overhyped,"
says physicist Greg Landsberg (Brown University).

What, exactly, is going on here?

In 1974 Hawking published a landmark paper titled "Black Holes
Ain't So Black." Contrary to the assumption of other
scientists, Hawking showed that due to quantum effects, black
holes slowly radiate matter into the surrounding space - a
nearly infinitesimal trickle of particles later termed
"Hawking radiation." At the very end of a black hole's life,
as its size has been whittled down to that of an atomic
nucleus, it evaporates much more rapidly in a flood of Hawking
radiation.

Most physicists quickly embraced Hawking's "black holes ain't
black" idea, but Hawking himself noted a problem that has
since come to be known as the "information paradox." Hawking's
calculations indicated that the radiation streaming from a
black hole would be featureless and seemingly random, and thus
would carry no information about the material that originally
formed the black hole or that later fell into it. In other
words, black holes would not preserve any record of the
material they swallow.

But that conclusion violated a central tenet of quantum
mechanics, the extraordinarily successful theory that explains
the interactions of matter and energy at subatomic scales. In
quantum mechanics, it should always be possible theoretically
to trace back the initial conditions of a physical system to
its origin. Because this tenet plays such an important role in
physical law, and has been experimentally corroborated many
times, most physicists have long thought that black holes must
somehow retain a memory of the material from which they
formed, even if it might be impossible in practice to extract
that information. As Landsberg says, "It is a basic principle
of quantum mechanics that information can't be destroyed."

A few experts in general relativity, such as Hawking and
Thorne, argued that the extreme gravitational forces in a
black hole would literally scrunch the information out of
existence. In the view of these relativists, all of the matter
in a black hole falls to the center, forming a point of zero
volume and infinite density known as a singularity. The
infinite gravity at the singularity destroys all information.
The outer boundary of the black hole, known as the event
horizon, is the region from within which light cannot escape.
According to Hawking, Thorne, and others, the region between
the singularity and the event horizon was empty space.

Physicists have long tried to find a way out of the
information paradox. In his Dublin lecture, Hawking used a
concept known as imaginary time, in which the three known
dimensions of space and one of time are instead modeled as
four dimensions of space, with time becoming one of them.
Hawking argued that in imaginary time, black holes preserve
information. But many physicists do not think the paradox can
be resolved this way. "We should stick with real time, not
imaginary time," says physicist Samir Mathur (Ohio State
University). "The original problem gets sidetracked when we go
to imaginary time."

Earlier this year, Mathur and his colleagues used string
theory to show how black holes can indeed preserve
information. In the March 1st issue of Nuclear Physics B, they
modeled black holes as large balls of tangled strings - tiny
fundamental strands of energy that constitute all matter and
energy in the universe. In string theory, the universe is a
symphony of strings vibrating in 10 dimensions, and the mode
of each string's vibration (like the note on a violin)
determines whether it is an electron, quark, photon, or some
other type of particle.

Mathur's calculations show that when black holes are modeled
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