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John Preskill Wars of the holographic world

erally, my impression is that Suss- kind’s colourful stories about his interactions with colleagues are hon- estly reported, take only excusable dramatic licence and only rarely veer toward self-aggrandizement. Physicists love paradoxes, because a deeply vexing paradox can expose in stark relief where our understanding of the laws of physics is deficient, and point the way towards pathbreaking

Kevin Rauch/Allan Davis/Science Photo Library new insights. No paradox has stirred more passionate debate among physi- cists in recent decades than the “black- hole information-loss paradox” – the theme of The Black Hole War. Classical gravitation theory states that nothing can escape from a black hole; however, Stephen Hawking discovered in the mid-1970s that black holes actually emit radiation due to quantum effects. This emission is very weak for a large black hole, but it gets stronger and stronger as an evaporating black hole gets smaller and smaller, until, if we Battle of wits In the popular imagination, the iconic symmetries of elementary particles wait long enough, the black hole even- Information loss American theoretical physicist is are broken, persuaded me to aban- tually explodes and disappears. in black holes has Richard Feynman, in all his safe-crack- don the fruitful work I was doing on But Hawking also argued that the been a source ing, bongo-thumping, woman-chasing phase transitions in the very early uni- emitted radiation is featureless, de- of controversy. glory. I suspect that many physicists, verse to focus full-bore on techni- void of any information about what if asked to name a living colleague color, which became the topic of my the black hole swallowed to attain its The Black Hole War: who best captures the spirit of Feyn- PhD dissertation. initial mass. Thus, Hawking asserted, My Battle with man, would give the same answer as Yet by the summer of 1980, Lenny, the formation and subsequent com- Stephen Hawking to me: Leonard Susskind. As far as I his T-shirt still soaked from a just- plete evaporation of a black hole is a Make the World Safe know, Susskind does not crack safes, completed jog, stood in my office at truly irreversible process, in which for Quantum thump bongos, or chase women, yet he Harvard loudly insisting that his some information about the past state Mechanics shares Feynman’s brash cockiness earlier ideas about technicolor were of the universe is permanently erased. Leonard Susskind (which in Susskind’s case is leavened deeply flawed and should be rejected. While physicists are accustomed to 2008 Little, Brown by occasional redeeming flashes of I resisted for many months before the idea that information can become £27.99/$27.99hb self-deprecation) and Feynman’s gift concluding that Susskind was prob- inaccessible in practice, the claim that 480pp for spinning fascinating anecdotes. If ably right about having been wrong. information can be irretrievably lost you are having a group of physicists It was an unforgettable lesson in in principle was highly inflammatory. over for dinner and want to be sure to scientific integrity. What seemed paradoxical was that, have a good time, invite Susskind. During the 1990s I had some mem- by following the accepted laws of Like Feynman, Susskind is a born orable discussions with Susskind physics to an apparently unassailable teacher who longs to connect with his about the quantum physics of black conclusion, Hawking had overturned audience and often succeeds. Best of holes. So I was more than a little curi- a tenet at the heart of quantum me- all, Susskind channels Feynman’s un- ous to see how well his version of the chanics. This precipitated a crisis, and canny intuition, and can penetrate to evolution of the subject in The Black although awareness of the crisis the heart of a deep scientific puzzle. Hole War would match my own recol- spread only gradually, by 1990 there These estimable qualities are on dis- lections, especially given that three was widespread appreciation among play in Susskind’s impressive new of the central events in his narrative physicists that information loss in book, The Black Hole War. are conferences we both attended: at black-hole evaporation is a deep and I should make some disclaimers. I Aspen in 1992, Santa Barbara in 1993 important problem. have known and admired Susskind and Cambridge in 1994. After an engaging introductory for 30 years, and his publications and I was gratified to be described as a explanation of some of the relevant our many discussions have strongly “wiry man” in Susskind’s account of features of black holes and quantum influenced my scientific develop- a lunch-table conversation at the physics, The Black Hole War recounts ment. To give but one example, his Santa Barbara conference, which may Susskind’s own 20-year struggle to elegant 1979 paper on “technicolor”, have been a better description of my pinpoint flaws in Hawking’s reason- which proposed a new model of how physique in 1993 than it is today. Gen- ing and, as he puts it, “make the world

Physics World December 2008 41 Christmas books physicsworld.com

safe for quantum mechanics”. He holographic principle can be justified spices his account with tongue-in- Is the black-hole only in mathematical models that are cheek military metaphors (not just the far from realistic. Third, even with book’s title, but also chapter headings war really finished? holographic models one hungers for such as “The Dutch resistance,” deeper insight into the meaning of “Skirmish at Aspen,” and “The battle Perhaps not holography, and a better grasp of of Santa Barbara”), which, though a where Hawking’s argument failed. bit forced, are just about witty enough The battle to achieve a wholly satisfy- to be forgiven. I trust that most read- This is the most ambitious part of the ing reconciliation of quantum theory ers will recognize that disagreements book: five chapters that build toward with gravitation is still very much a among theoretical physicists are not an explanation of how string theorists work in progress. really so hostile as perhaps the belli- learned to count the microscopic The Black Hole War succeeds on cose language implies. quantum states of black holes, and to two levels: as an engaging memoir The pivotal event in the saga oc- construct explicit models of quantum by a skilled story teller; and as a com- curred in 1994, when Susskind, eluci- gravity that exhibit the holographic pelling introduction to some elusive dating ideas originated by Dutch duality between 3D and 2D descrip- but captivating scientific ideas. physicist Gerard ’t Hooft, formulated tions of the same phenomenon. Though some other stars in the the- the “holographic principle”. This out- When it applies, this duality has oretical physics firmament make landishly unintuitive idea suggests great power, because it connects the enjoyable cameo appearances (Feyn- that our 3D world is illusory, in that evaporating black hole with a comple- man, Hawking and ’t Hooft among all of the information residing deep mentary physical system in which con- others), this is really a story with two inside a 3D volume can actually be servation of information is manifest. protagonists: Susskind and the black encoded on its 2D surface. An import- Finally, in 2004, Hawking concluded hole. Fortunately, both characters are ant implication is that when informa- that information is really conserved engrossing enough to keep the pages tion seems to tumble into a black hole, after all, and Susskind’s defence of turning late into the night. it actually remains accessible outside quantum mechanics was vindicated. the black hole’s surface. Is the black-hole war really fin- John Preskill is a theoretical physicist and Susskind goes on to explain the ished? Perhaps not. First, the case for director of the Institute for Quantum subsequent developments in string string theory as a description of na- Information at the California Institute of theory that allowed this outrageous ture is far from airtight. Second, even Technology, US, e-mail preskill@ proposal to gain wide acceptance. if string theory is correct, so far the theory.caltech.edu

Superstition Belief in the Age of Science Robert L. Park

“Bob Park has done it again. His lucid, humorous, style—the envy of those of us who fancy themselves writers—gets through the pervasive nonsense that he finds everywhere, from the ‘afterlife’ delusion to intelligent design.” —James Randi, president of the James Randi Educational Foundation Cloth $24.95 £14.95 978-0-691-13355-3

The Atom and the Apple Twelve Tales from Contemporary Physics Sébastien Balibar

“Sébastien Balibar offers a refreshing romp through physics rooted in experimental reality, and even, in everyday reality. As a theorist, I am enthralled.”—A. Zee, author of Fearful Symmetry and An Old Man’s Toy Cloth $24.95 £14.95 978-0-691-13108-5

(0800) 243407 UK 800.777.4726 U.S. press.princeton.edu

42 Physics World December 2008