Three Lectures on AdS/CFT

MSU postdoc Steve Avery explains AdS/CFT to non-specialists (i.e., theoretical physicists who do not primarily work on string theory / quantum gravity). Steve is applying for faculty positions this fall -- hire him! :-)

AdS/CFT on this blog. See also Entanglement and fast thermalization in heavy ion collisions: application of AdS/CFT to collisions of heavy ions suggests that rapid thermalization occurs there due to quantum entanglement.

As an example of the versatility of theoreticians, Steve has also been working with me on machine learning and genomic prediction. He just wrote a very fast LASSO implementation in Julia that includes some automated capability to set L1 penalization and detect phase boundaries.

Stringy Bohemian Rhapsody

Wow, this is awesome. Thanks to a former student now managing billions in fixed income assets for the link ;-)

Life on moduli space?

New paper! Although I'm skeptical about the utility of the anthropic principle (see previous discussion on this blog), I couldn't resist pondering a basic question raised by the large number of string vacua, the vast majority of which seem to be supersymmetric.

Is there something special about exact supersymmetry that precludes complex life? It's a simple question, but difficult to answer.

Life on moduli space?

http://arxiv.org/abs/0908.0943

While the number of landscape vacua in string theory is vast, the number of supermoduli vacua which lead to distinct low energy physics is even larger, perhaps infinitely so. From the anthropic perspective it is therefore important to understand whether complex life is possible on moduli space -- i.e., in low energy effective theories with 1. exact supersymmetry and 2. some massless multiplets (moduli). Unless life is essentially impossible on moduli space as a consequence of these characteristics, anthropic reasoning in string theory suggests that the overwhelming majority of sentient beings would observe 1-2. We investigate whether 1 and 2 are by themselves automatically inimical to life and conclude, tentatively, that they are not. In particular, we describe moduli scenarios in which complex life seems possible.

From the paper:

Assuming our current understanding of string theory is correct, the number of distinct vacua with unbroken supersymmetry and exact low-energy moduli (supermoduli) is infinitely larger even than the vast number of string landscape vacua in which supersymmetry is broken and the cosmological constant nonzero \cite{BDG,DK}. For example, in Calabi-Yau compactifications, the continuous parameters determining the shape of the compact space are themselves moduli and result in an infinite set of physically distinct vacua. Indeed, the highly supersymmetric vacua may be on stronger theoretical footing than their non-supersymmetric counterparts \cite{BDG}.

If complex life is possible on even a tiny fraction of points on supermoduli space, it would be difficult to understand, within an anthropic framework, why we do not ourselves observe unbroken supersymmetry and massless moduli fields.

There is thus ample motivation to investigate whether complex life can exist on moduli space -- specifically, in low energy effective theories with 1. exact supersymmetry and 2. some massless multiplets (moduli).

... Below we list some minimal requirements for complex life. In fact, we do not know whether any of these conditions are necessary or sufficient for life, although it seems they are more likely to be necessary (especially B.) than sufficient. These requirements primarily place constraints on low energy physics. As we discuss below, they do not seem to exclude moduli vacua, at least not in any obvious way.

A. structure formation

B. deviation from thermal equilibrium (long lived sources of free energy)

C. stable matter, complex chemistry

Because inflationary dynamics are typically determined by high energy physics, it seems reasonable to assume that the specific properties of any inflationary epoch (including the spectrum of density perturbations) are independent of the low energy properties of a particular vacuum. Therefore, the requirement of an inflationary epoch neither favors nor disfavors properties 1-2. ...

A warning from von Neumann

I can't resist reproducing this quote from John von Neumann, which I think applies well to certain branches of particle theory today. Thank goodness the LHC is coming on line soon...

As a mathematical discipline travels far from its empirical source... it is beset with very grave dangers. It becomes more and more purely aestheticizing, more and more purely l'art pour l'art. ...In other words, at a great distance from its empirical source, or after much "abstract" inbreeding, a mathematical subject is in danger of degeneration.

From the opening material of the book John von Neumann and Modern Economics. I highly recommend the chapter by Paul Samuelson.

Does string theory predict an open universe?

New paper! See here for related discussion.

The first two pragraphs:

If, as suggested by recent results [1], string theory exhibits a landscape of over 10^500 distinct, metastable vacua, its status as a conventional scientific theory is in jeopardy. Scientific theories must make predictions which are falsifiable by experiment. Such a large diversity of vacua means that essentially any low-energy physics might be realizable from string theory. Even ultra high-energy physics experiments may not yield additional information, since scattering at trans-Planckian energies leads to black holes [2] of ever increasing size, whose subsequent behavior (evaporation) is controlled by the low-energy physics of the ambient vacuum state. If recent results are any guide, string theory will be extremely difficult to falsify.

It is therefore important to carefully consider any robust implications of the string landscape. One of these, recently elaborated in [3], is the testable prediction that our universe must be open, with negative curvature. A recent analysis combining WMAP and Sloan Digital Sky Survey data gives Omega =1.003 +- 0.010 [4], but improved future observations could yield a statistically significant central value larger than unity, implying positive curvature. Would this rule out string theory?


hep-th/0610231
Authors: R. Buniy, S. Hsu, A. Zee

It has been claimed that the string landscape predicts an open universe, with negative curvature. The prediction is a consequence of a large number of metastable string vacua, and the properties of the Coleman--De Luccia instanton which describes vacuum tunneling. We examine the robustness of this claim, which is of particular importance since it seems to be string theory's sole claim to falsifiability. We find that, due to subleading tunneling processes, the prediction is sensitive to unknown properties of the landscape. Under plausible assumptions, universes like ours are as likely to be closed as open.

Greetings from Pasadena

I'm at Caltech for a couple of days, giving a talk at the Institute of Quantum Information (slides are here). The weather is beautiful, and, judging by the screaming on campus last night, the tradition of frosh initiation is alive and well.

The Nobel prize in physics was awarded to Mather and Smoot for COBE (measurement of temperature fluctuations in the microwave background, confirming the big bang model of cosmology). When I was a grad student I had an office across the hall from Smoot's group at LBNL. They were moved at some point and some HET postdocs (including Raman Sundrum) inherited the nice view of the bay. Smoot used to drop by all the time to look out the windows and lament his loss. He told me that, contrary to rumor, he was not the Smoot used as a unit of length by MIT students to measure the Harvard bridge. (If you don't know what I'm talking about there is always Google.)

Finally, via Dave Bacon, a hilarious history of string theory by Peter Shor (of quantum factoring fame), which appeared as a review of Smolin's book on Amazon. Now string theorists can complain about how Shor is not smart enough to have an opinion on the subject, or understand what they are doing. (See further down the same page for Lubos' review.) Oh, and as pointed out elsewhere by Wolfgang, it was not Nature scamming the string theorists, but rather Mathematics masquerading as Nature ;-)

The string theorists were scammed!, September 25, 2006
Reviewer: Peter W. Shor (Wellesley, MA USA) - See all my reviews

The part of the book I found most interesting was the part which tells how the string theorists were scammed by Nature (or Mathematics). Of course, Smolin doesn't put it exactly like this, but imagine the following conversation.

String theorists: We've got the Standard Model, and it works great, but it doesn't include gravity, and it doesn't explain lots of other stuff, like why all the elementary particles have the masses they do. We need a new, broader theory.

Nature: Here's a great new theory I can sell you. It combines quantum field theory and gravity, and there's only one adjustable parameter in it, so all you have to do is find the right value of that parameter, and the Standard Model will pop right out.

String theorists: We'll take it.

String theorists (some time later): Wait a minute, Nature, our new theory won't fit into our driveway. String theory has ten dimensions, and our driveway only has four.

Nature: I can sell you a Calabi-Yau manifold. These are really neat gadgets, and they'll fold up string theory into four dimensions, no problem.

String theorists: We'll take one of those as well, please.

Nature: Happy to help.

String theorists (some time later): Wait a minute, Nature, there's too many different ways to fold our Calabi-Yao manifold up. And it keeps trying to come unfolded. And string theory is only compatible with a negative cosmological constant, and we own a positive one.

Nature: No problem. Just let me tie this Calabi-Yao manifold up with some strings and branes, and maybe a little duct tape, and you'll be all set.

String theorists: But our beautiful new theory is so ugly now!

Nature: Ah! But the Anthropic Principle says that all the best theories are ugly.

String theorists: It does?

Nature: It does. And once you make it the fashion to be ugly, you'll ensure that other theories will never beat you in beauty contests.

String theorists: Hooray! Hooray! Look at our beautiful new theory.

Okay, I've taken a few liberties here. But according to Smolin's book, string theory did start out looking like a very promising theory. And, like a scam, as it looks less and less promising, it's hard to resist the temptation to throw good money (or research) after bad in the hope of getting something back for your return. One of the questions Smolin addresses in the rest of the book is why the theoretical physics community has kept with string theory and largely abandoned all the other approaches to quantum gravity. The short answer is that it's hard to admit that you've been scammed. The long answer is much more complicated. Another thing Smolin addresses in the book is other approaches to quantum gravity. And as could be predicted, he gives lots of space to his own approach and too little space to others, especially Alain Connes' non-commutative geometry. But overall, I found it very worthwhile and entertaining, and a good explanation as to how theoretical physics came to be in the state it is today.

Strings in the New Yorker

I think the article, which discusses the new books by Woit and Smolin, is very fair, and it ends with a surprisingly mature recapitulation of the decoupling theorem and the irrelevance of quantum gravity to applied science. Every man, woman and child should read it and then ask their local particle theorist for more clarification.

I've read chunks of Smolin's book and it's quite good, although not without flaws. I have to go back and reread it -- I picked it up in a bookstore and couldn't put it down for at least an hour. I found his discussion of finiteness of string perturbation theory confusing -- he represents Mandelstam as saying one thing in the main text, but the email quoted in the footnotes doesn't seem to back it up. If the analytic continuation is the only problem then it's not on worse footing than many other results in theoretical physics. But, then, what have D'Hoker and Phong (and my grad school colleague Nathan Berkovits) been up to all this time? If I believed in string theory I'd have to spend some time sorting this all out.

...Today, more than a decade after the second revolution, the theory formerly known as strings remains a seductive conjecture rather than an actual set of equations, and the non-uniqueness problem has grown to ridiculous proportions. At the latest count, the number of string theories is estimated to be something like one followed by five hundred zeros. “Why not just take this situation as a reductio ad absurdum?” Smolin asks. But some string theorists are unabashed: each member of this vast ensemble of alternative theories, they observe, describes a different possible universe, one with its own “local weather” and history. What if all these possible universes actually exist? Perhaps every one of them bubbled into being just as our universe did. (Physicists who believe in such a “multiverse” sometimes picture it as a cosmic champagne glass frothing with universe-bubbles.) Most of these universes will not be biofriendly, but a few will have precisely the right conditions for the emergence of intelligent life-forms like us. The fact that our universe appears to be fine-tuned to engender life is not a matter of luck. Rather, it is a consequence of the “anthropic principle”: if our universe weren’t the way it is, we wouldn’t be here to observe it. Partisans of the anthropic principle say that it can be used to weed out all the versions of string theory that are incompatible with our existence, and so rescue string theory from the problem of non-uniqueness.

...Neither Smolin nor Woit calls for the forcible suppression of string theory. They simply ask for a little more diversity. “We are talking about perhaps two dozen theorists,” Smolin says. This is an exceedingly modest request, for theoretical physics is the cheapest of endeavors. Its practitioners require no expensive equipment. All they need is legal pads and pencils and blackboards and chalk to ply their trade, plus room and board and health insurance and a place to park their bikes. Intellectually daunting as the crisis in physics may be, its practical solution would seem to demand little more than the annual interest on the rounding error of a Google founder’s fortune.

“How strange it would be if the final theory were to be discovered in our own lifetimes!” Steven Weinberg wrote some years ago, adding that such a discovery would mark the sharpest discontinuity in intellectual history since the beginning of modern science, in the seventeenth century. Of course, it is possible that a final theory will never be found, that neither string theory nor any of the alternatives mentioned by Smolin and Woit will come to anything. Perhaps the most fundamental truth about nature is simply beyond the human intellect, the way that quantum mechanics is beyond the intellect of a dog. Or perhaps, as Karl Popper believed, there will prove to be no end to the succession of deeper and deeper theories. And, even if a final theory is found, it will leave the questions about nature that most concern us—how the brain gives rise to consciousness, how we are constituted by our genes—untouched. Theoretical physics will be finished, but the rest of science will hardly notice.

Voting and Weighing

There is an old saying in finance: in the short run, the market is a voting machine, but in the long run it's a weighing machine.

That is, the price of a stock at any moment might be determined by sentiment, speculation or mania, and wildly divergent from its "real" value, but in the long run it is hard to hide a lack of profits or revenue at a company. (I guess Amazon holds the world's record, still befuddling investors 10 years down the line :-)

You might think science is a weighing machine, with experiments determining which theories survive and which ones perish. Healthy sciences certainly are weighing machines, and the imminence of weighing forces honesty in the voting. However, in particle physics the timescale over which voting is superseded by weighing has become decades -- the length of a person's entire scientific career. We will very likely (barring something amazing at the LHC, like the discovery of mini-black holes) have the first generation of string theorists retiring soon with absolutely no experimental tests of their *lifetime* of work. Nevertheless, some have been lavishly rewarded by the academic market for their contributions.

String controversy reaches the public

This is the first time I've seen these issues discussed in the popular press (SF Chronicle). (I found this link via Peter Woit's blog; he is quoted in the article.)

I have to say both critics and defenders of string theory make valid points in the article. It is not implausible to me that it could take generations to sort out any theoretical model of quantum gravity. On the other hand, how will we ever know whether a proposed model is right or wrong without experimental input? In order for the subject to qualify as scientific, there must be falsifiable predictions. This doesn't seem to be the case with string theory - in fact, things are moving in the wrong direction with the discovery(?) of more than 10^{100} possible vacua, each with different low-energy physics.

I am not against some number of theorists studying string theory, but by now most of the top departments in the world have string theory groups. If I had to guess I would say resources within theoretical physics are over-allocated to string theory. Even its supporters admit the work is speculative, and unlikely to be subject to experimental test in the near future. This being the case, why are funding levels for string theory research not closer to those in math departments, rather than physics departments? (Physicists, even theorists, generally teach less and get bigger grants than mathematicians. Shouldn't the stringers be lumped with the math guys until they come up with actual testable predictions?)

Nature's guiding hand manifests itself in the form of experimental data. Without this guidance, theoretical physics is in danger of reverting to a subjective field dominated by trends and fashion, like the humanities and social sciences.

Those interested in a sociological analysis of the political and cultural economy of the academic field should read the work of Pierre Bourdieu, who argued that academics mainly fight over the "power to consecrate" - that is, the power to determine which subjects or approaches are deemed worthy or unworthy. This is exactly what I see going on in quantum gravity these days.

My earlier posts on this topic are here and here.

String theory quotes

I'm on record as not being a fan of string theory. My objection is not that it is necessarily wrong, but that we probably won't know one way or another for a very long time.

Recently I've been looking at the blog of Peter Woit, a Columbia math professor who was originally trained as a particle theorist, and a very outspoken critic of string theory. Try his Jan. 11, 2005 post (for some reason the trackback URL doesn't work) for some juicy comments on the current state of string theory. In the interest of fairness, you can also have a look at the blogs of string theorists Lubos Motl or Jacques Distler.

I thought I would list, just for fun, some comments by famous physicists about string theory (all are Nobelists except Woit :-):

Woit: "... string theory ... has given up any claims to being a legitimate science and has taken on the characteristics of a cult. ...I just can't believe the way essentially the entire particle theory establishment, including many people I have the highest respect for, continue to allow this situation to go on without public comment. ..."

Richard Feynman: in Davies and Brown, Superstrings, Cambridge 1988, pp. 194-195:"... I do feel strongly that this is nonsense! ...I think all this superstring stuff is crazy and is in the wrong direction. ... I don't like it that they're not calculating anything. ...why are the masses of the various particles such as quarks what they are? All these numbers ... have no explanations in these string theories - absolutely none! ... "

Sheldon Glashow: "... superstring theory ... is, so far as I can see, totally divorced from experiment or observation. ...string theorists ... will say, "We predicted the existence of gravity." Well, I knew a lot about gravity before there were any string theorists, so I don't take that as a prediction. ... there ain't no experiment that could be done nor is there any observation that could be made that would say, "You guys are wrong." The theory is safe, permanently safe. I ask you, is that a theory of physics or a philosophy? ..."

Phil Anderson:"Is string theory a futile exercise as physics, as I believe it to be? It is an interesting mathematical specialty and has produced and will produce mathematics useful in other contexts, but it seems no more vital as mathematics than other areas of very abstract or specialized math, and doesn't on that basis justify the incredible amount of effort expended on it.

My belief is based on the fact that string theory is the first science in hundreds of years to be pursued in pre-Baconian fashion, without any adequate experimental guidance. It proposes that Nature is the way we would like it to be rather than the way we see it to be; and it is improbable that Nature thinks the same way we do."

String theory and all that

I was asked to give a talk to the physics students here about string theory. Now, I'm not a string theorist, but am probably the closest thing on campus with the possible exception of a guy in the math department.

I emphasized that quantum gravity is perhaps the most conceptually interesting area in all of physics (perhaps all of science). I think I am not exaggerating here, since questions such as Why is there one time direction and three spatial dimensions? Can our universe be multiply-connected on short distances? or What is the endpoint of black hole evaporation? all involve deep and fundamental ideas.

But I also told them, half joking, that I didn't want to work on quantum gravity (at least not all the time) until someone builds a desktop accelerator that can collide particles at Planck energies or at least make small black holes. What I meant by this comment is that physics generally cannot advance by theoretical ideas or mathematics alone. There is no evidence that a single, unique mathematical structure describes our universe. Consequently, we will likely be confronted with more than one theoretical possibility, and only experimental tests can distinguish between them.

We are barely on the threshold of detailed tests of classical general relativity (e.g., using large interferomenters such as LIGO to detect gravity waves). There are no experiments on the drawing board which will test whether these waves are indeed quantized into individual gravitons, and the current generation of particle accelerators are 16 orders of magnitude away from testing the Planck energy. So, I think quantum gravity will not, in a strict sense, be a scientific endeavor for some years to come.