The Universe_Leading Scientists Explore the Origin, Mysteries, and Future of the Cosmos

by John Brockman

  Murray says to me, “What do you do?”

  So I said, “I’m working on this theory that hadrons are like rubber bands, these one-dimensional stringy things.”

  And he starts to laugh . . . and laugh. And I start to feel like, well, as my grandmother used to say, Poopwasser. I was so crushed by the great man’s comments that I couldn’t continue the conversation, so I said, “What are you working on, Murray?” And of course he said, “Didn’t you hear my lecture?” Fortunately at that point the elevator started to go.

  I didn’t see Murray again for two years. Then there was a big conference at Fermilab, and a thousand people were there. And me, I’m still a relative nobody. And Murray is in constant competition with his colleague Richard Feynman over who is the world’s greatest physicist.

  As I’m standing there talking to a group of my friends, Murray walks by and in an instant turns my career and my life around. He interrupts the conversation, and in front of all my friends and closest colleagues, says, “I want to apologize to you.”

  I didn’t know he remembered me, so I said, “What for?” He said, “For laughing at you in the elevator that time. The stuff you’re doing is the greatest stuff in the world. It’s just absolutely fantastic, and in my concluding talk at the conference I’m going to talk about nothing but your stuff. We’ve got to sit down during the conference and talk about it. You’ve got to explain it to me carefully, so that I get it right.”

  Something unimaginable had just happened to me, and I was suddenly on a cloud. So for the next three or four days at the conference, I trailed Murray around, and I would say, “Now, Murray?” And Murray would say, “No, I have to talk to somebody important.”

  At some point, there was a long line at the conference for people trying to talk to the travel agent. I was going to go to Israel and I had to change my ticket. It took about forty-five minutes to get to the front of the line, and when I’m two people from the front of the line, you can imagine what happens. Murray comes over and plucks me out of the line and says, “Now I want to talk. Let’s talk now.” Of course, I was not going to turn Murray down, so I say, “OK, let’s talk,” and he says, “I have fifteen minutes. Can you explain to me in fifteen minutes what this is all about?” I said OK, and we sat down, and for fourteen minutes we played a little game: He says to me, “Can you explain it to me in terms of quantum field theory?” And I say, “OK, I’ll try. I’ll explain it to you in terms of partons.” Around 1968, Feynman had proposed that protons, neutrons, and hadrons were made of little point particles. He didn’t know very much about them, but he could see in the data, correctly, that there were elements that made you think that a proton was made up out of little point particles. When you scatter protons off electrons, electrons come out. When you look at the rubbish that comes out, it tends to look as if you’ve struck a whole bunch of little tiny dots. Those he called partons. He didn’t know what they were, that was just his name for them. Parts of protons.

  Now, you have to understand how competitive Murray and Dick Feynman were. So Murray says to me, “Partons? Partons? Put-ons! Put-ons! You’re putting me on!” And I thought, “What’s going on here?” I had really said the wrong word. And finally he says, “What do these partons have?” I said, “Well, they have momentum. They have an electric charge.” And he says, “Do they have SU(3)?” SU(3) was just a property of particles, like the electric charge is a property, or like their spin. Another property was their SU(3)-ness, which is a property that distinguishes proton from neutron. It’s the thing that distinguished different particles which are otherwise very similar. Murray Gell-Mann and Yuval Ne’eman had discovered it in the early ’60s, and it was what Murray became most famous for, and it led directly to the quark idea. I said, “Yeah, they can have SU-3,” and he says, “Oh, you mean a quark!” So for fourteen minutes he had played this power game with me. He wanted me to say “quark,” which was his idea, and not “partons,” which was Dick’s idea. Fourteen of the fifteen minutes had gone by and he lets me start talking, and I explained to him everything in one minute, and he looks at his watch and says, “Excuse me, but I have to talk to somebody important.”

  So I’m on a rollercoaster. I had gone up, down, up, down, and now I’m really down. I thought to myself, “Murray didn’t understand a word I said. He’s not interested. He’s not going to spend his time in his lecture talking about my work,” and then off in a corner somewhere I hear Murray holding forth to about fifteen people, and he’s just spouting everything I told him and giving me all the credit I could hope for: “Susskind says this. Susskind says that. We have to listen to Susskind.” And indeed, his talk at the end of the conference was all “Susskind this, Susskind that.” And that was the start of my career. I owe Murray a lot. He’s a man of tremendous integrity, and he cares about the truth, and he certainly has an interesting personality.

  That jump start to my career happened around 1971. I was teaching at the Belfer Graduate School of Science, which was part of Yeshiva University, way uptown. It was an extraordinary place for a brief period of time, and it had some of the greatest theoretical physics in the world; it was outstanding. The place closed up. It went broke and I had to move to Stanford. When I went to Stanford I was an elementary-particle physicist. I was only interested in the mathematical structure of this thing. I became interested in elementary particles through it. Other people began to recognize that this was not the exactly right theory of hadrons, although it’s closely related to the right theory.

  I should go back a step. There were many things wrong with this theory—not wrong with the mathematical theory, but wrong in trying to compare it with nature, and to compare it with hadrons. Some of them were fixed up very beautifully by John Schwarz and Andre Neveu and a whole group of very mathematically minded string theorists, who concocted all kinds of new versions of it, and these new versions were incidentally the start of the process of discovering this incredible diversity. Each of the new versions was a little bit different, and it was always hoped that one of the new versions would look exactly like protons, neutrons, mesons, and so forth. It never happened. There were some fatal flaws.

  The first was that the theory only made sense in a ridiculous number of dimensions—ten dimensions. That’s not a good thing for people who live in four dimensions. That got fixed and turned out not to be so bad. The other problem was that when the theory was solved it included forces between particles that were like gravitational forces. This theory was not behaving like nuclear physics—like it was supposed to behave. It was behaving like Newtonian gravity. Particles were having forces between them that were not the kind of forces that hold a proton and neutron together but the kind of forces that hold the solar system together.

  I lost a little bit of interest in it, because I was not interested at that time in gravity. John Schwarz and a number of others, including Joel Sherk, realized that this was a great opportunity. They said, “Don’t think of it as a theory of hadrons, think of it as a theory of gravity.” So out of a debacle, they turned it into a theory of gravitation instead of a theory of protons and neutrons. I wasn’t interested at that point in gravity; I didn’t know very much about gravity, and so I continued doing elementary-particle physics. Elementary-particle physicists at that time were not interested in gravity. They had no interest in gravity at all. There were people who were interested in gravity but they had no interest in string theory. So a small, isolated group of people—John Schwarz, Michael Green, Pierre Ramond and few others—carried the field on.

  I became interested in it again because I became interested in black holes. Hawking had studied black holes, discovered that they radiate, that they have a temperature, that they glow, and that they give off light. I met Hawking and Gerard ’t Hooft in the attic of Werner Erhard’s house in San Francisco. Erhard was a fan of Sidney Coleman. Dick Feynman, myself, and David Finkelstein were his gurus. And of course we didn’t give a damn about his silly business, but we loved his cigars, we loved h
is liquor, we loved the food that we got from him, and he was fun. He was very, very smart.

  Hawking came and told us his ideas about black holes, and one of the things he told us was that things which fall into the black hole disappear from the universe completely and can never be returned, even in some scrambled form. Now, information is not supposed to be lost. It’s a dictum of physics that information is preserved. What that means is that in principle you can always take a sufficiently precise look at things and figure out what happened in the past—infinitely accurately—by running them backwards.

  Hawking was saying that when things fall into a black hole they’re truly lost and you can never reconstruct what fell in. This violated a number of basic principles of quantum mechanics, and ’t Hooft and I were stunned. Nobody else paid any attention, but we were both really stunned. I remember ’t Hooft and myself were standing, glaring at the blackboard. We must have stood there for fifteen minutes without saying a word when Hawking told us these things. I was sure that Hawking was wrong. ’t Hooft was sure that Hawking was wrong. And Hawking was absolutely sure that he was right in saying that information was lost inside black holes.

  For thirteen years I thought about this—continuously, pretty much—and at the end of that thirteen years I began to suspect that string theory had in its guts a solution to this problem. And so I became interested again in string theory. I didn’t remember anything about it. I had to go back and read my own papers, because I tried reading other people’s papers and I couldn’t understand them.

  In the intervening years, powerful mathematics was brought to bear on the theory. I found it rather dry, since it was rather completely mathematics with very little of an intuitive, physical picture. The main things that happened were that, first of all, five versions of it were discovered. Tricks were discovered about how to get rid of the extra dimensions. You don’t actually get rid of them, you curl them up into little dimensions. You can read all about that in Brian Greene’s book, The Elegant Universe. That turned out to be a good thing.

  John Schwarz and Michael Green, and a few other people, worked out the very difficult mathematics in great detail and demonstrated that the theory was not inconsistent in the ways that people thought it might be. When they showed that the mathematics was firm, Ed Witten got very excited, and once Ed Witten walked into it, well, he’s a real mathematical powerhouse and dominated the field very strongly. Witten’s written many famous papers, but one of his key papers, which may have been the most important one, was written in about 1990. He and collaborators around him worked out the beginnings of a mathematics of these Calabi-Yau manifolds, which are tiny, curled-up spaces that are very well explained in Brian Greene’s book.

  Ed is also a physicist, and he had a lot of interest in trying to make this into a real theory of elementary particles. He never quite succeeded but discovered a lot of beautiful mathematics about it. I found a lot of it rather dry, because it was not addressing physics questions the way I enjoy addressing them. It was just a little too mathematical for my taste. My taste leans less toward the mathematical and more toward the pictorial. I think in terms of pictures.

  I wasn’t really following the subject too closely at that point. I was still interested in black holes, and it wasn’t until about 1993 that I began to suspect that there were ingredients in string theory that could resolve this puzzle of Hawking’s. So at that point I really got into it. I started to think about the connection between string theory and black holes.

  String theory was a theory of gravity. When you have gravity, you can have black holes, and so string theory had to have black holes in it, and it should have a resolution of this problem. Over a period of a couple of years, it did have a resolution. It did, in fact, turn out that Hawking was wrong. That is to say, he was wrong in a great way. When a person puts a finger on a problem of that magnitude, independently of whether they got it right or they got it wrong, they have a tremendous impact on the subject. And he has had a tremendous impact.

  I developed some simplified ways of thinking about it that demonstrated that black holes did not lose information, that things don’t fall into the black hole and disappear, they eventually come back out. They are all scrambled up, but nevertheless they come back out. I began writing papers on that, and my paper, which said that stuff does not get lost inside a black hole in string theory, stimulated the string community to start thinking about black holes. There was an eruption of papers—mine, Joe Polchinski’s, Andy Strominger’s, Cumrun Vafa’s—that really nailed that problem down. And black holes have been solved. Black holes have been understood. To this day the only real physics problem that has been solved by string theory is the problem of black holes. It led to some extremely revolutionary and strange ideas.

  Up to now string theory has had nothing to say about cosmology. Nobody has understood the relationship between string theory and the Big Bang, inflation, and other aspects of cosmology. I frequently go to conferences that often have string theorists and cosmologists, and usually the string theory talks consist of apologizing for the fact that they haven’t got anything interesting to tell the cosmologists. This is going to change very rapidly now, because people have recognized the enormous diversity of the theory.

  People have been trying to do business the old way. With string theory, they were trying to do the things that they would have done with the earlier theories and it didn’t make a lot of sense for them to do so. They should have been looking at what’s really unique and different about string theory, not what looks similar to the old kind of theories. And the thing which is really unique and very, very special is that it has this diversity, that it gives rise to an incredibly wild number of different kinds of environments that physics can take place in.

  11

  Smolin vs. Susskind: The Anthropic Principle

  INTRODUCTION by John Brockman

  In the summer of 2004, I received a copy of an email sent by Leonard Susskind to a group of physicists which included an attached file entitled “Answer to Smolin.” This was the opening salvo of an intense email exchange between Susskind and Smolin concerning Smolin’s argument that “the Anthropic Principle (AP) cannot yield any falsifiable predictions, and therefore cannot be a part of science.”

  After reading several postings by both physicists, I asked them if (a) they would consider posting some of the comments on Edge.org, and (b) if they would each write a new, and final, “letter.”

  Both agreed, but only after a negotiation: (1) No more than one letter each; (2) neither sees the other’s letter in advance; (3) no changes after the fact. A physics shoot-out.

  While this is a conversation conducted by physicists for physicists, it should nonetheless be of interest to Edge readers, as it’s in the context of previous Edge features with the authors, it’s instructive as to how science is done, and it’s a debate that clarifies, not obfuscates. And finally it’s a good example of what Edge is all about, where contributors share the boundaries of their knowledge and experience with each other and respond to challenges, comments, criticisms, and insights. The constant shifting of metaphors, the intensity with which we advance our ideas to each other—this is what intellectuals do. Edge draws attention to the larger context of intellectual life.

  On July 26, 2004, Lee Smolin published a paper (hep-th/0407213, “Scientific alternatives to the anthropic principle”). He emailed Leonard Susskind, asking for a comment. Not having had a chance to read the paper, Susskind asked Smolin if he would summarize the arguments. Here is Smolin’s email summary:

  Dear Lenny,

  Thanks. Glad to. I’ll start with one of the main arguments I make.

  I show that the argument of Weinberg and others [Garriga and Vilenkin] are incorrect. The subtle point is that their arguments have embedded in them correct arguments having to do only with what we observe. To an already correct argument is then added mention of the anthropic principle. As it is added to an already correct argument, the anthropic princi
ple plays no role in the actual scientific argument.

  Here is how it goes: We start with a theory of structure formation that tells us “Too large positive Lambda interferes with galaxy formation.”

  We do observe that galaxies have formed. Therefore we predict that the cosmological constant could not have been too large. This is correct reasoning, and it agrees with observation.

  What Weinberg and others have done is to make the error of embedding this argument into one that first mentions some version of the anthropic principle. The mistake is not to notice that because it has been added to an argument that is already correct, the mention of the anthropic principle [or the principle of mediocrity, or life] plays absolutely no role in the argument.

  The logic of their arguments is: A implies B; B is observed; B, together with theory C, implies D.

  Here A is any form of the anthropic principle or principle of mediocrity, together with assumptions about priors, probability distributions on universes etc., plus our own existence, that leads to the conclusion that we should observe B.

  B is that galaxies have formed.

  C is the theory of structure formation.

  D is that the cosmological constant is not too large.

  The fallacy is not to recognize that the first line plays no role in the argument, and the prediction of D is equally strong if it is dropped. One can prove this by noting that if D were not seen, one would have to question the theory C [assuming the observation is correct, as it certainly is here]. One would have no reason to question either A or the assertion that A implies B.

  This is the same fallacy involved in Hoyle’s argument about carbon. He reasoned simply from an observation that carbon is plentiful in our universe to a prediction that, as it must have been formed in stars, there must be a resonance at a particular energy. This was correct and the resonance was observed. But he fallaciously attributed the argument to the existence of life, which was a non sequitur.