On Hawking’s Concession
John Preskill
Last Wednesday (
The black hole information loss puzzle
The bet concerns what happens to
information that is hidden behind the event horizon of a black hole: Is the
information destroyed and lost forever, or might it in principle be
recovered from the radiation that is emitted as the black hole evaporates? This
question was first raised by Hawking in this very important
paper (written in 1975 and published over a year later). Hawking pointed
out that the process of black hole evaporation (which he had discovered
earlier) could not be reconciled with the principles of quantum physics and
gravitational physics that were then generally accepted. This was a brilliant
insight, and for much of the theoretical physics community it took many years
to appreciate fully the depth of the problem that Hawking had posed.
Hawking had precipitated a genuine
crisis in fundamental physics, and it seemed that we would have to give up
at least one of our cherished beliefs. Hawking's
radical suggestion was that the foundations of quantum theory needed to be
revised. According to quantum mechanics, although physical processes can
transform the information that is encoded in a physical system into a form that
is inaccessible in practice, in principle the information can always be
recovered. If I burn volume A of the encyclopedia or volume B of the
encyclopedia, the flames and the ashes look pretty much alike in both cases,
but actually there are subtle differences --- with sufficient cleverness and
sufficiently advanced technology, I ought to be able to decipher the content of
the volume by observing the flames and the ashes.
If black holes respect the usual rules of quantum physics, then if we toss
volume A into a black hole, eventually we ought to be able (in principle) to
read the encyclopedia by closely observing the light emitted as the black hole
evaporates. But Hawking argued that black hole evaporation is fundamentally
different that ordinary physical processes, that information that falls behind
the event horizon of a black hole will be lost forever, remaining concealed
even after the black hole has evaporated completely and disappeared.
The controversy
Many physicists, especially those whose background
(like Hawking's) is in gravitational physics,
accepted Hawking conclusion (and still do). But there is another possible point
of view that has been favored by many physicists whose background (like mine)
is in elementary particle physics: Perhaps information really can escape from a
black hole, but we will not be able to understand just how the information
escapes without deep new insights into the physics of processes in which both
quantum effects and gravitational effects are important.
Two physicists who argued persuasively for this latter point of view were Gerard 't Hooft (
Hawking’s reversal
Hawking has followed this work by the string theorists with great interest; I
think he has been especially impressed by Maldacena's
contributions, which suggest (as ’t Hooft, Susskind, and others had
anticipated) that information is encoded in black hole spacetimes
in a very subtle way. This past year he has been thinking a lot about how his
earlier conclusions about information loss might be evaded, and in his talk in
Stephen’s change of heart surprised me; it isn't yet clear to me why he
finds his new argument so compelling that he is willing to reverse a position
that he has held adamantly for nearly 30 years. And it saddens me a little in a
way --- Stephen and I have had many discussions about the black hole
information puzzle over the years, and I have always enjoyed those discussions
and have learned from them, even though we disagreed. If we are really on the
same side now, our future discussions won't be as much fun. We'll find other
things to disagree about, I suppose, but probably nothing else as deep and
engrossing.
Settling the bet
There were quite a few print, television, and radio reporters present at
Stephen’s GR17 talk, and at a press conference that immediately followed.
Stephen had requested that Kip and I participate in the press conference, so we
were actually on the stage during the talk. (Kip and I were both in
A lot of people ask me why I requested a baseball encyclopedia. Well, the
timing was good --- the 8th edition of Total Baseball: the
Ultimate Baseball Encyclopedia was published this month. This edition is
the first to compile Win Shares for every major league season in baseball
history. That's a new statistic invented by my hero, Bill James, so I'm glad to
have the book. (It was a schlep getting it home, though; it’s 2,688
pages.) My favorite article about the bet is Joe
Friesen’s story for the Globe and Mail, describing the reactions of
the author and publisher to the news. Incidentally, I wondered whether I should
ask for a CD-ROM encyclopedia instead of a print encyclopedia. But print is the
right choice --- it’s heavy like a black hole. And it takes much longer
to get the information out --- that's like a black hole, too.
Stephen, Kip, and I make bets for fun,
but the scientific question at issue in this case is an important one that we
(and many other physicists) deeply care about. I don’t think any of us
could have imagined the attention this bet would receive (I know I
didn’t). It’s a bit humbling to think that no matter what else I do
in life, my claim to fame will be that I won a couple of bets, but I figure I
should just enjoy it.
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