Wednesday, February 10, 2010

The Black Hole War

I just finished a really good popular physics book, the best I can remember reading for a long time. It is "The Black Hole War", by Lenny Susskind, an eminent Stanford professor of physics. Among other major achievements, Susskind has a strong claim to be the inventor of string theory, and - unlike with some other current popular authors - everything he says can be taken extremely seriously.

Susskind's topic is one that is close to my heart, indeed I did my dissertation on it, more or less. I was part of the Santa Barbara group of string theory physicists - a.k.a. "the enemy" in Susskind's book, at least as far as the "black hole war" goes. My vote was counted in the tally shown on p. 262 of the book; unfortunately, I'm pretty sure I voted for the "wrong" side, along with the rest of the Santa Barbara crew.

The problem, and the subject of the "War" which Susskind recounts, is simple: what happens to matter swallowed up by a black hole? One possibility is that it just vanishes forever, and this was the general belief until Hawking - in one of the most beautiful computations ever carried out, and the first to combine general relativity and quantum mechanics in any substantial way - showed that black holes have a temperature and they radiate energy like every other warm object. Eventually, they "evaporate" completely and vanish.

But Hawking's calculation opened a huge can of worms because it indicated no connection at all between the matter which went in and that which came out. In other words, the evaporating black hole creates "something from nothing". Energy is conserved, to be sure, but everything else about the matter - all of its "information" - is erased, in a mathematically complete sense, and replaced by a featureless, memoryless, random collection of particles.

Now, this is not how physics has ever worked. In physics, the situation now comes from the situation before, through a one-to-one connection. The situation now does not just arise spontaneously from nothing, in some random state. That just sounds wrong, and it seems mathematically impossible to implement.

However, wrong as this consequence seemed, Hawking's calculation seemed right, and most physicists didn't see the big deal since there were no black holes handy to test with anyway.

But a few physicists, most notably Susskind and 'tHooft, recognized the problem as a critical matter of principle that should be resolved. And they felt quite strongly that Hawking's picture was wrong, and that proving it would teach us profound things about gravity and universe.

In 1994, the paradox seemed completely impenetrable; but by 1997 it had been resolved, more or less, and Susskind and 'tHooft proved right.

History will record these three years as among the most momentous in science. Below I present their chronology, with some introductory years added for context, to give the reader some feeling for the times, which were a strange admixture of excitement and despair. People were waiting for something big to happen, not really believing that it would - and then it did. There's a lesson in there, not least for yours truly, who quit the field just before it exploded. I was at Santa Barbara from 1989-94, a student of Steve Giddings.

March, 1991

Witten discovers a simplified, 2-dimensional black hole solution in string theory. It is exciting both because it is simple, and because it exists within string theory, a partial theory of quantum gravity, suggesting that it might illuminate the paradox of Hawking.

November, 1991

Callan, Giddings, Harvey, and Strominger propose the "CGHS" model of black hole formation and evaporation, based on Witten's black hole.


The "black hole information problem" takes the string theory community by storm, sparked by the string-inspired CGHS model, and helped by a lull in progress in string theory itself. I began working with Giddings and we wrote a followup to the CGHS paper.


The Santa Barbara Black Hole Conference, a.k.a "The Battle of Santa Barbara", in Susskind's dramatic rendition. Heated debate, fascinating ideas - but no resolutions.

In fact the most important result, by far, to be announced during the conference is the proof of Fermat's Last Theorem.

Meanwhile, in a major blow to the particle physics community, the SSC accelerator is canceled by Congress. My thesis advisor Giddings is quoted in a major news magazine saying that, had he known that would happen, he would have gone to law school.


The calm before the storm. Black hole work mushrooms in string theory, and the ideas remain tantalizing, but true solutions seem wholly out of reach. Many, including yours truly, are very discouraged.

March, 1995. University of Southern California.

At the Strings '95 conference, Witten informs a stunned audience that string theory, previously thought to reside in 10 dimensions, actually has a hidden, 11th dimension. The most famous scientific talk in recent memory, it sparks a revolution in string theory.

The significance of it all was still pretty unclear though. At the conference final dinner, I listened to Susskind's wrapup speech, in which he described the whole field as "angels dancing on the head of a pin". I am sure he never really believed that, and if you read his book you won't believe it either - but it still might be true!

October, 1995

String theory expands yet again, as Joe Polchinski of Santa Barbara discovers 10 additional types of matter hidden within it, the "D-branes". D-brane theory is so beautiful and compelling that once you study it, you can't believe that string theory could not be right.

Polchinski wrote me a letter of recommendation upon my graduation; however, I suspect that it was not a very good letter! At any rate, I left the field several months before his historic discovery.

January, 1996

Vafa and Strominger use D-branes to build a model black hole for which they can identify the internal states directly and see that information is not lost. The problem is unraveling.

November, 1997

Juan Maldacena, using D-branes as well as most other major ideas of the previous two decades of theoretical physics research, conjectures that string theory in 4-dimensional spaces equivalent to "dual", non-string theory in 3 dimensions. It is both mind blowing and arcane, but it has over 6000 references and appears to solve problems even in the previously-fossilized field of nuclear physics.

Shortly thereafter, Witten shows that creating a black hole in the 4-dimensional space is the same as adding temperature to the dual 3-dimensional theory.

The veil of the Black Hole is lifted, at least in part, and nobody believes any more that information is sucked into a hole, never to return. The "war" described in Susskind's book is over.


Suhail Manzoor said...

But is String theory right?

Will Nelson said...

Who can say? But in my opinion it's the only proposal that feels right. String theory has a depth to it that one doesn't find in any other theories. With other approaches one gets out more or less what one puts in, whereas with string theory the more one looks, the more one finds. Also, with strings it isn't necessary to make any further assumptions beyond the original one of a string model; the whole theory is determined from that simple idea. So it's a very beautiful, unified structure, to my mind the most amazing mathematical structure ever discovered, and hence perhaps the most suitable to be the "theory of everything".