This article will summarize some old achievements of the great physicist but also discuss some of his recent opinions about string theory.
Murray Gell-Mann was born on September 15th, 1929 in Lower East Side of New York to a family of Western Ukrainian Jewish immigrants. When he was fifteen, he joined Yale. ;-) See some pictures from his early life.
In the 1950s, when he was in his 20s, he studied cosmic rays and discovered/invented the strangeness in order to make sense out of the isospin, other quantum numbers, and their relationships (e.g. using the key Gell-Mann-Nishijima formula).
I wrote his biography one year ago, in Oskar Klein and Murray Gell-Mann: birthdays. So I won't write everything again. Let me just say that Murray Gell-Mann was the most important one among the first pioneers who realized that there were quarks inside hadrons which is what earned him the 1969 physics Nobel prize. Note that all these things, including the award, had been completed years before the discovery of QCD.
Together with Richard Feynman, his occasional rival, they (FG) described the vector-pseudovector tensor structure of the weak interactions (a refinement of Fermi's four-fermion theory of the beta decay) soon after the parity violation was appreciated. The FG effective theory was fully good (FG), even though some famous experimenters have reached the opposite conclusion (recall the well-known story about their scalar-tensor conclusion that only depended on the last point of their graph).
The lesson is that theorists are often right even if the experimenters say No. The only experimenters you can uncritically trust are infallible Gods - but there are not too many of them around. ;-)
Among Gell-Mann's more recent discoveries, the interpretation of quantum mechanics via the Consistent Histories is a favorite one on this blog because it is my interpretation of choice. Gell-Mann has also coined his own version of Feynman's "Shut Up And Calculate" interpretation of quantum mechanics. His is called "quantum flapdoodle", as in a section of his book "The Quark and the Jaguar". At any rate, his point is that no problem exists and needless "New Age" mystification (such as Einstein's "spooky telepathic" nonlocality or "pre-cognitive remote viewing") shouldn't be introduced. By the way, Stephen Hawking's way of saying the same thing is: "When I hear about Schrödinger's Cat, I reach for my gun."
Gell-Mann has spent some energy with and invested some organization skills into the issue of complexity in Santa Fe.
Murray Gell-Mann is a keen linguist - who studies the common ancestry of languages (he thinks that their common origin, labeled as the Borean language, must go more than those 20,000 years into the past!) - and a collector of East Asian antiquities. His fascination with languages has been manifesting itself for decades. After all, he had to borrow the wonderful name of "quarks" from an otherwise incomprehensible book by James Joyce. The related term of the "eightfold way" was inspired by Buddhism: click the picture to learn about the essence. ;-)
Finally, Gell-Mann has also coined the term "squalid state physics" to describe the lack of a desire of some physics disciplines to find the pure truth. Of course, particle physics has learned something from "dirt physics", too. But that doesn't mean that Gell-Mann's observation wasn't precious in general.
At Harvard, I have talked to Murray Gell-Mann during the Sidneyfest in 2005 and it was a lot of fun. Of course, I couldn't resist to ask him about things such as Feynman's non-existent teethbrushes and the commercial for Enron. :-) Murray Gell-Mann has also had some pretty famous students, including Ken Wilson, Sidney Coleman, James Hartle, and Barton Zwiebach.
The last name of the previous paragraph leads me to another fundamental role that Gell-Mann has played: a holy patron of string theory. It was the main topic of a recent interview and I will dedicate the rest of this article to this issue, too.
On Friday 9/11, Science News published a very interesting
But there are much more nontrivial and much more important statements where I fully agree with Gell-Mann.
Gell-Mann is asked about his attitude to string theory. He says that it's promising and recalls the history of string theory - a very accurate and detailed account for an 80-year-old linguist! :-) - which includes his role of a "conservationist" who founded the Caltech "natural reserve" for the "endangered superstring species". :-)
Gell-Mann's protection was clearly crucial for the subsequent emergence of the discoveries and Murray morally owns something like 10% of the "stocks" in the AdS/CFT correspondence, Matrix theory, and many other huge discoveries that would be unlikely to have occurred without his divine hand of a patron. All fans of fundamental physics are immensely grateful to Gell-Mann for these "sociological" contributions and his excellent intuition about the work of others.
Murray Gell-Mann says a lot of important things that I completely agree with. He discusses the cosmological constant and notices that an unbroken supersymmetry would make it vanish. It's the only known symmetry that may circumvent an implication of Gell-Mann's totalitarian principle (he modestly doesn't use the term!), namely that the term has to be nonzero. A broken supersymmetry - and the breaking is needed - only solves it partially. Much like your humble correspondent, he seems to view supersymmetry at the LHC to be more likely than not.
He is also told by the reporter that "battles of new ideas against conventional wisdom are common in science". Gell-Mann thinks it's very interesting how these negative principles get embedded in science sometimes. Instead, he emphasizes that most challenges to scientific orthodoxy are wrong. Lots of their authors are cranks. Sometimes, the challenges are right. Their authors face a lot of disbelief - and Gell-Mann offers some examples, e.g. the continental drift.
What's important in string theory
There are many other cool things in the interview, showing that Gell-Mann's brain is alive and kicking. ;-) But I decided to repost the following two paragraphs - about the key understudied research in string theory - as the most important lesson of the interview:
I am puzzled by what seems to me the paucity of effort to find the underlying principle of superstring theory-based unified theory. Einstein didn’t just cobble together his general relativistic theory of gravitation. Instead he found the principle, which was general relativity, general invariance under change of coordinate system. Very deep result. And all that was necessary then to write down the equation was to contact Einstein’s classmate Marcel Grossmann, who knew about Riemannian geometry and ask him what was the equation, and he gave Einstein the formula. Once you find the principle, the theory is not that far behind. And that principle is in some sense a symmetry principle always.You have heard the holy word. Let me tell you that I am dedicating a few hours every day exactly to the topics described by Gell-Mann. And I also feel as a member of an endangered species. It's just bad that there seem to be such a small number of people in this world who are doing these things.
Well, why isn’t there more effort on the part of theorists in this field to uncover that principle? Also, back in the days when the superstring theory was thought to be connected with hadrons rather than all the particles and all the forces, back in that day the underlying theory for hadrons was thought to be capable of being formulated as a bootstrap theory, where all the hadrons were made up of one another in a self-consistent bootstrap scheme. And that’s where superstring theory originated, in that bootstrap situation. Well, why not investigate that further? Why not look further into the notion of the bootstrap and see if there is some sort of modern symmetry principle that would underlie the superstring-based theory of all the forces and all the particles. Some modern equivalent of the bootstrap idea, perhaps related to something that they call modular invariance. Whenever I talk with wonderful brilliant people who work on this stuff, I ask what don’t you look more at the bootstrap and why don’t you look more at the underlying principle...
It's clear that most people - including most theoretical physicists - simply don't have the capacity to work on such things. You know, if Gell-Mann has seen further than others, it is because he was surrounded by dwarfs (as he said himself). For example, I don't expect those loud cranks who still haven't understood that they must learn string theory to say anything new about quantum gravity to be able to help us. Like dogs, they just can't ever be helpful. But many of the theoretical physicists surely do have the tools, and they still don't seem to think about it.
You know, string theory, much like QCD, has worked in a very "constructive" way so far, by describing the set of the degrees of freedom explicitly, from the scratch. The bootstrap hasn't been "needed", or at least, it hasn't been "usefully applied" yet. But I find it extremely likely that the ultimate principle that will cover all of string theory won't have this form. The allowed vacua and their degrees of freedom will have to be "dynamically" derived from a set of principles and consistency criteria - a future version of the bootstrap program.
It also seems extremely likely that some UV/IR links - modeled by the modular invariance in the context of perturbative closed strings - will be important for the formulation of the ultimate principle. Non-perturbatively, it seems obvious that such a link will have to constrain the black hole microstates, i.e. the generic high-mass particle species in any theory of quantum gravity. The spectrum and detailed structure of the black hole microstates must be linked to low-energy fields and all of their higher-order interactions. These conditions will admit a limited number of solutions that will coincide with the allowed configurations of string/M-theory.
Worldsheet, spacetime, M, exceptional symmetries
Moreover, it's conceivable that we won't be able to work "fully on the worldsheet" or "fully in the spacetime". I feel that the ultimate set of consistency rules for quantum gravity will work "simultaneously" for the generalized worldvolumes as well as spacetime. So I am spending a lot of time by attempts to import some lessons - and methods to derive or generate new degrees of freedom - from spacetimes to the worldvolumes, and vice versa.
Modular invariance, mutual locality of operators, Dirac quantization rules, similar conditions, and their generalizations play an important role. But it remains to be seen whether there is a concise, ultimate principle or set of principles, why it generalizes the conformal symmetry (and modular invariance) in the perturbative limit, and why it admits old perturbative solutions as well as new, non-perturbative solutions such as the 11-dimensional vacuum of M-theory.
Of course, one of the most obvious testing grounds for such new sets of ideas is the exceptional U-duality group of M-theory on tori - i.e. the maximally supersymmetric supergravity. The exceptional groups are pretty and they must have a pretty cool explanation in terms of a structure we still don't fully know.
Let me remind you that the exceptional U-duality groups - the exact discrete symmetries of the theory - get extended to the full continuous exceptional non-compact symmetries in the low-energy, SUGRA limit. Originally, these symmetries were deduced simply by the constraint of supersymmetry requiring that the scalars had to span a quotient of Lie groups and the exceptional groups had the right dimensions. We can also deduce the exceptional groups (by the method of group enhancement) from their classical subgroups that appear in various limits. But there should be a more direct way that reveals all these symmetries.
Of course, this question is probably equivalent to the question "What is M?". In other words, how do we derive the right degrees of freedom that superseded the degrees of freedom on a string at a stronger coupling? While "M" sometimes behaves as a membrane, it must be much more flexible if it can be used to derive the S-matrix of M-theory. It must be able to behave as an M5-brane and other things, too.
I think that some kind of bootstrap is needed to determine what "M" and its structure of symmetries really is. Is there a third person in the world who cares about this possibly most important question of science? These core topics of string theory are currently understudied at least by two orders of magnitude.
Hat tip for the interview: Andrew Zimmerman