Guest Blogger: Joe Polchinski on the String Debates

By Sean Carroll | December 7, 2006 12:31 am

You may have read here and there about the genteel discussions concerning the status of string theory within contemporary theoretical physics. We’ve discussed it on CV here, here, and even way back here, and Clifford has hosted a multipart discussion at Asymptotia (I, II, III, IV, V, VI).

We are now very happy to host a guest post by the man who wrote the book, as it were, on string theory — Joe Polchinski of the Kavli Institute for Theoretical Physics at UC Santa Barbara. Joe was asked by American Scientist to review Peter Woit’s Not Even Wrong and Lee Smolin’s The Trouble With Physics. Here is a slightly-modified version of the review, enhanced by footnotes that expand on some more technical points.


This is a review/response, written some time ago, that has just appeared in American Scientist. A few notes: 1) I did not choose the title, but at least insisted on the question mark so as to invoke Hinchliffe’s rule (if the title is a question, the answer is `no’). 2) Am. Sci. edited my review for style, I have reverted figures of speech that I did not care for. 3) I have added footnotes on some key points. I look forward to comments, unfortunately I will be incommunicado on Dec. 8 and 9.

All Strung Out?

Joe Polchinski

The Trouble with Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next. Lee Smolin. xxiv + 392 pp. Houghton Mifflin, 2006. $26.

Not Even Wrong: The Failure of String Theory and the Search for Unity in Physical Law. xxi + 291 pp. Basic Books, 2006. $26.95.

The 1970’s were an exhilarating time in particle physics. After decades of effort, theoretical physicists had come to understand the weak and strong nuclear forces and had combined them with the electromagnetic force in the so-called Standard Model. Fresh from this success, they turned to the problem of finding a unified theory, a single principle that would account for all three of these forces and the properties of the various subatomic particles. Some investigators even sought to unify gravity with the other three forces and to resolve the problems that arise when gravity is combined with quantum theory.

The Standard Model is a quantum field theory, in which particles behave as mathematical points, but a small group of theorists explored the possibility that under enough magnification, particles would prove to be oscillating loops or strands of “string.” Although this seemingly odd idea attracted little attention at first, by 1984 it had become apparent that this approach was able to solve some key problems that otherwise seemed insurmountable. Rather suddenly, the attention of many of those working on unification shifted to string theory, and there it has stayed since.

Today, after more than 20 years of concentrated effort, what has been accomplished? What has string theory predicted? Lee Smolin, in The Trouble With Physics, and Peter Woit, in Not Even Wrong, argue that string theory has largely failed. What is worse, they contend, too many theorists continue to focus their efforts on this idea, monopolizing valuable scientific resources that should be shifted in more promising directions.

Smolin presents the rise and fall of string theory as a morality play. He accurately captures the excitement that theorists felt at the discovery of this unexpected and powerful new idea. But this story, however grippingly told, is more a work of drama than of history. Even the turning point, the first crack in the facade, is based on a myth: Smolin claims that string theorists had predicted that the energy of the vacuum — something often called dark energy — could not be positive and that the surprising 1998 discovery of the accelerating expansion of the universe (which implies the existence of positive dark energy) caused a hasty retreat. There was, in fact, no such prediction [1]. Although his book is for the most part thoroughly referenced, Smolin cites no source on this point. He quotes Edward Witten, but Witten made his comments in a very different context — and three years after the discovery of accelerating expansion. Indeed, the quotation is doubly taken out of context, because at the same meeting at which Witten spoke, his former student Eva Silverstein gave a solution to the problem about which he was so pessimistic. (Contrary to another myth, young string theorists are not so intimidated by their elders.)

As Smolin charts the fall of string theory, he presents further misconceptions. For example, he asserts that a certain key idea of string theory — something called Maldacena duality, the conjectured equivalence between a string theory defined on one space and a quantum field theory defined on the boundary of that space — makes no precise mathematical statements. It certainly does. These statements have been verified by a variety of methods, including computer simulations [2]. He also asserts that the evidence supports only a weak form of this conjecture, without quantum mechanics. In fact, Juan Maldacena’s theory is fully quantum mechanical [3].

A crucial principle, according to Smolin, is background independence — roughly speaking, consistency with Einstein’s insight that the shape of spacetime is dynamical — and Smolin repeatedly criticizes string theory for not having this property. Here he is mistaking an aspect of the mathematical language being used for one of the physics being described. New physical theories are often discovered using a mathematical language that is not the most suitable for them. This mismatch is not surprising, because one is trying to describe something that is different from anything in previous experience. For example, Einstein originally formulated special relativity in language that now seems clumsy, and it was mathematician Hermann Minkowski’s introduction of four-vectors and spacetime that made further progress possible.

In string theory it has always been clear that the physics is background-independent even if the language being used is not, and the search for a more suitable language continues. Indeed (as Smolin belatedly notes), Maldacena duality provides a solution to this problem, one that is unexpected and powerful. The solution is still not complete: One must pin down spacetime on the edges, but in the middle it is free to twist and even tear as it will, and black holes can form and then decay. This need to constrain the edges is connected with a property known as the holographic principle, which appears to be an essential feature of quantum gravity. Extending this principle to spaces with the edges free will require a major new insight. It is possible that the solution to this problem already exists among the alternative approaches that Smolin favors. But his principal candidate (loop quantum gravity) is, as yet, much more background-dependent than the current form of string theory [4].

Much of Smolin’s criticism of string theory deals with its lack of mathematical rigor. But physics is not mathematics. Physicists work by calculation, physical reasoning, modeling and cross-checking more than by proof, and what they can understand is generally much greater than what can be rigorously demonstrated. For example, quantum field theory, which underlies the Standard Model and much else in physics, is notoriously difficult to put on a rigorous foundation. Indeed, much of the interest that mathematicians have in physics, and in string theory in particular, arises not from its rigor but from the opposite: Physicists by their methods can obtain new results whose mathematical underpinning is not obvious. String theorists have a strong sense that they are discovering something, not inventing it. The process is sometimes messy, with unexpected twists and turns (not least the strings themselves!), and rigor is not the main tool.

Woit covers some of the same ground, although his interests are more centered on particle physics and on the connection with mathematics than on the nature of spacetime. His telling is more direct, but it is rather stuffed with detail and jargon, and his criticisms of string theory are simpler and somewhat repetitious.

A major point for Woit is that no one knows exactly what string theory is, because it is specified only through an infinite mathematical series whose sum is ill-defined. This assertion is partly true: With new physical theories there is often a long period between the first insight and the final mathematical form. For quantum field theory, the state of affairs that Woit describes lasted for half a century [5]. In string theory the situation is much better than he suggests, because for 10 years we have had tools (dualities) that give us in many cases a precise definition of the theory. These have led in turn to many new applications of string theory, such as to the quantum mechanics of black holes, and there are hints to a more complete understanding.

But what about the lack of predictions? This is the key question, for Woit, for Smolin and for string theory. Why have the last 20 years been a time of unusually little contact between theory and experiment? The problem is partly on the experimental side: The Standard Model works too well. It takes great time, ingenuity and resources to try to look beyond it, and often what is found is still the Standard Model.

A second challenge was set forth by Max Planck more than a century ago. When one combines the fundamental constants of special relativity, general relativity and quantum mechanics, one finds that they determine a distance scale at which these theories appear to come together: the Planck length of 10-33 centimeters. To put this number in perspective, one would have to magnify an atom a billion times to make it the size of a coffee cup, and one would have to magnify the Planck length a trillion trillion times to make it the size of an atom. If we could probe the Planck length directly, we would be able to see the strings and extra dimensions, or whatever else is lurking there, and be done with it. But we cannot do that, and so instead we must look for indirect evidence. And, as was the case with atomic theory, one cannot predict how long such a leap will take.

Smolin addresses the problem of the Planck length (“It is a lie,” he says). Indeed, Planck’s calculation applies to a worst-case scenario. String theorists have identified at least half a dozen ways that new physics might arise at accessible scales [6], and Smolin points to another in the theories that he favors [7], but each of these is a long shot. As far as experiment yet shows, Planck’s challenge stands.

Or it may be that string theory has already made a connection with observation — one of immense significance. Positive dark energy is the greatest experimental discovery of the past 30 years regarding the basic laws of physics. Its existence came as a surprise to almost everyone in physics and astronomy, except for a small number, including, in particular, Steven Weinberg.

In the 1980s, Weinberg had been trying to solve the long-standing puzzle of why the density of dark energy is not actually much greater. He argued that if the underlying theory had multiple vacua describing an enormous number of potential universes, it would not only explain why the density of dark energy is not high, but would also predict that it is not zero. Weinberg’s reasoning was contrary to all conventional wisdom, but remarkably his prediction was borne out by observation a decade later.

The connection between string theory and dark energy is still a subject of much controversy, and it may be that Weinberg got the right answer for the wrong reason. However, it may well turn out that he got the right answer for the right reason. If so, it will be one of the great insights in the history of physics, and the multivacuum property of string theory, seemingly one of its main challenges, will, in fact, be just what nature requires.

A second unexpected connection comes from studies carried out using the Relativistic Heavy Ion Collider, a particle accelerator at Brookhaven National Laboratory. This machine smashes together nuclei at high energy to produce a hot, strongly interacting plasma. Physicists have found that some of the properties of this plasma are better modeled (via duality) as a tiny black hole in a space with extra dimensions than as the expected clump of elementary particles in the usual four dimensions of spacetime. The prediction here is again not a sharp one, as the string model works much better than expected. String-theory skeptics could take the point of view that it is just a mathematical spinoff. However, one of the repeated lessons of physics is unity — nature uses a small number of principles in diverse ways. And so the quantum gravity that is manifesting itself in dual form at Brookhaven is likely to be the same one that operates everywhere else in the universe.

A further development over the past few years, as our understanding has deepened, has been the extensive study of the experimental consequences of specific kinds of string theory. Many of these make distinctive predictions for particle physics and cosmology. Most or all of these may well be falsified by experiment (which is, after all, the fate of most new models). The conclusive test of string theory may still be far off, but in the meantime, science proceeds through many small steps.

A central question for both Smolin and Woit is why so many very good scientists continue to work on an idea that has allegedly failed so badly. Both books offer explanations in terms of the sociology of science and the psychology of scientists. These forces do exist, and it is worth reflecting on their possible negative effects, but such influences are not as strong as these authors posit. String theorists include mavericks and contrarians, strong-willed individuals who have made major contributions — not just in string theory but in other parts of physics as well. The borders between string theory and other areas of physics are not closed, and theorists would emigrate if they did not believe that this was the most promising direction in which to invest their time and energies.

In fact, the flow of intellectual talent has been in the other direction: In recent years, leading scientists in particle phenomenology, inflationary cosmology and other fields have found ideas generated by string theory to be useful in their disciplines, just as mathematicians have long done. Many have begun to work with string theorists and have in turn contributed their perspectives to the subject and expanded the view of how string theory relates to nature.

This convergence on an unproven idea is remarkable. Again, it is worth taking a step back and reflecting on whether the net result is the best way to move science forward, and in particular whether young scientists are sufficiently encouraged to think about the big questions of science in new ways. These are important issues — and not simple ones. However, much of what Smolin and Woit attribute to sociology is really a difference of scientific judgment.

In the end, these books fail to capture much of the spirit and logic of string theory. For that, Brian Greene’s The Elegant Universe (first published in 1999) or Leonard Susskind’s The Cosmic Landscape (2005) do a better job. The interested reader might also look to particle-phenomenologist Lisa Randall’s Warped Passages (2005) and cosmologist Alexander Vilenkin’s Many Worlds in One (2006) for accounts by two scientists from other fields who have seen a growing convergence between string theory and their ideas about how the cosmos is put together.

Joseph Polchinski is a professor of physics at the University of California, Santa Barbara, and a permanent member of the Kavli Institute for Theoretical Physics. He is the author of the two-volume text String Theory (Cambridge University Press, 1998).


[1] It is obvious that there could have been no such prediction. From 1995-98, string theorists were discovering a host of new nonperturbative tools: dualities, branes, black hole entropy counting, matrix theory, and AdS/CFT duality. These were at the time studied almost exclusively in the context of supersymmetry. The problem of moduli stabilization, necessary for any nonsupersymmetric compactification (and positive energy density states are necessarily nonsupersymmetric) was left for the future; there were no general results or predictions. Page 154 refers to no-go theorems. There was a prominent no-go theorem two years later due to Maldacena and Nunez. However, not only the timing but also the physics is misstated. This paper makes several restrictive assumptions, and gives a long list of well-known papers, some as early as 1986, to which its results simply don’t apply. So this was never a broad constraint on string theory.

[2] On the string theory side, all calculations of anomalous dimensions and correlators represent precise statements about the strong coupling behavior of the gauge theory. However, it is argued on page 282 that the gauge theory is not known to exist. For the purpose of this discussion it is sharpest to focus on the gauge theories in 1+1 and 2+1 dimensions, which were shown by Itzhaki, Maldacena, Sonnenschein, and Yankielowicz to also give background-independent constructions of quantum gravity. These theories are superrenormalizable – their couplings go to zero as powers at short distance — so they are even better-defined than QCD, and one can calculate to arbitrary accuracy on the lattice. Even the supersymmetry is no problem: the lattice breaks it, but because of the superrenormalizability one can calculate explicitly the counterterms needed to restore the symmetry in the continuum limit, and so all the predictions of AdS/CFT can be checked algorithmically.

This has already been done, not by Monte Carlo but by using discrete light-cone quantization, which has the nice property of preserving SUSY and also not paying an extra numerical penalty for large N. The present results of Hiller, Pinsky, Salwen, and Trittman are notable. The error bars are still large (but again, the issue is whether there are predictions in principle, not what can be done with today’s technology) but it does appear that the gauge theory Hilbert space, truncated to 3 x 1012 states, is in fact describing a graviton moving in a curved spacetime. Possibly less algorithmic, but numerically impressive, is the four-loop calculation of Bern, Czakon, Dixon, Kosower, and Smirnov: the Pade extrapolation to strong coupling agrees with the prediction of AdS/CFT to one or two percent.

[3] The gauge theory is a consistent and fully quantum mechanical theory, so if it contains classical gravity then it is by definition a solution to the problem of unifying Einstein’s theory with quantum mechanics. Moreover, the gravitational field must itself be quantized, because the duality relates gauge theory states to correctly quantized graviton states.

It is very difficult to define a `weak form’ of the duality which accounts for all the successful tests and is not actually the strong form. I am taking the definition here from page 144, which refers to classical supergravity as the lowest approximation, and talks about the duality being true only at this lowest order.

However, to get more background I have looked at the relevant papers by Arnsdorf and Smolin and by Smolin. The central arguments of these papers are wrong. One argument is that AdS/CFT duality cannot describe the bending of light by a gravitational field because there is a dual description with a fixed causal structure. If true, of course, this would invalidate the duality, but it is not. The gauge theory has a fixed causal structure, but signals do not move on null geodesics: there is refraction, so signals slow down and bend, and it is this that is dual to the bending of light by a gravitational field. Indeed, this duality between ordinary refraction and gravitational lensing is one of the fascinating maps between gravitation and nongravitational physics that are implied by the duality.

The second argument is that the tests of AdS/CFT duality are consistent with a weaker notion of `conformal induction,’ whereby a boundary theory can be defined from any field theory in AdS space by taking the limit as the correlators approach the boundary. This misses an important point. In general this procedure does not actually define a self-contained field theory on the boundary. Consider a signal in the bulk, which at time t is moving toward the boundary so as to reach it at a later time t’. According to the definition of conformal induction, the existence of this signal is not encoded in the boundary theory at time t, so that theory has no time evolution operator: the state at time t does not determine the state at time t’. In AdS/CFT the boundary is a true QFT, with a time evolution operator, and the signal is encoded even at time t. As a rough model of how this can work, imagine that every one-particle state in the bulk maps to a two-particle state in the boundary, where the separation of the particles plays the role of the radial coordinate: as they come close together the bulk particle move to the boundary, as they separate it moves away. Something like this happens even in real QCD, in the contexts of color transparency and BFKL diffusion.

[4] I am referring here to the problem of the constraints. Until these are solved, one does not really have background independence: there is an enormous Hilbert space, most of which is unphysical. In AdS/CFT, not only the bulk spacetime but also the bulk diffeomorphism group are emergent: the CFT fields are completely invariant under the bulk diffeomorphisms (this is also what happens in the much more common phenomenon of emergent gauge symmetry). In effect the constraints are already solved. One of the lessons of duality is that only the physical information is common to the different descriptions, while the extra gauge structure is not, it is an artifact of language not physics. (The CFT has its own SU(N) gauge invariance, but here it is straightforward to write down invariant objects.)

[5] I am counting from the mid-20’s, when the commutation relations for the electromagnetic field were first written down, to the mid-70’s when lattice gauge theory gave the first reasonably complete definition of a QFT, and when nonperturbative effects began to be understood systematically.

[6] The ones that came to mind were modifications of the gravitational force law on laboratory scales, strings, black holes, and extra dimensions at particle accelerators, cosmic superstrings, and trans-Planckian corrections to the CMB. One might also count more specific cosmic scenarios like DBI inflation, pre-Big-Bang cosmology, the ekpyrotic universe, and brane gas cosmologies.

[7] I have a question about violation of Lorentz invariance, perhaps this is the place to ask it. In the case of the four-Fermi theory of the weak interaction, one could have solved the UV problem in many ways by violating Lorentz invariance, but preservation of Lorentz invariance led almost uniquely to spontaneously broken Yang-Mills theory. Why weren’t Lorentz-breaking cutoffs tried? Because they would have spoiled the success of Lorentz invariance at low energies, through virtual effects. Now, the Standard Model has of order 25 renormalizable parameters, but it would have roughly as many more if Lorentz invariance were not imposed; most of the new LV parameters are known to be zero to high accuracy. So, if your UV theory of gravity violates Lorentz invariance, this should feed down into these low energy LV parameters through virtual effects. Does there exist a framework to calculate this effect? Has it been done?

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  • Aaron Bergman

    The link to Smolin in footnote [3] is busted.

  • Sean

    Fixed; thanks.

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  • Abi

    I just wanted to pass on the links to e-mail interviews of two Indian string theorists: Ashoke Sen and Sunil Mukhi.

    Both the interviews appeared just three days ago in the blog of a freelance journalist.

  • Thomas Larsson

    A crucial principle, according to Smolin, is background independence

    Background independence is a fundamental principle, but not the only one. The key lessons from GR are background independence and locality, and the lessons from QFT are locality and QM in the sense of Fock. It is hence natural to assume that QG is based on three pillars:

    * Background independence
    * Locality
    * Quantum theory

    All of the major QG contenders fail to satisfy some of these desiderata:

    * Perturbative string theory is not background independent.

    * Holographic theories, e.g. AdS/CFT, are not local.

    * LQG is neither local nor quantum in the sense of Fock quantization.

    * ‘t Hooft’s Planck-scale determinism is obviously not quantum. However, it is remarkable that ‘t Hooft seems to be so concerned with locality that he is even willing to consider hidden variables.

    A common objection is that local observables do not exist in quantum gravity. This can obviously not be correct, since 25 out of 26 consistent quantum gravities in 2D are local rather than holographic (no-ghost theorem).

  • a

    Since 1 prediction would be better than 1000 words, let me focus on footnote [6].
    It is a list of issues that allowed some contact with phenomenology, and maybe with cosmology. Surely there was a flow of leading phenomenologists and cosmologists towards string theory. It also caused a flow of young string theorists in the opposite direction. But the main effect of this contact was, in my view, that phenomenologists and cosmologists and maybe experimentalists could directly see what string theory can do and what cannot do, and started considering the possibility that “not even wrong” might turn out to be its epitaph.

  • invcit

    In this never-ending and oftentimes heated debate, it is truly refreshing to read a review that actually focuses on the physics.

  • nc

    A very nicely balanced review, particularly at the end where the failure of both books to inspire the reader to study strings is deplored! Two brief points, though.

    1. On Woit’s problem with the lack of rigor in the mathematics of string theory: when does abject speculation become physics? According to Heisenberg, ‘learned trash’ becomes ‘discovery’ at the time it is experimentally confirmed, and not before that time. The beautiful Einstein-Hilbert field equation of GR (1915) was widely promoted only in 1919 after being tested and having empirical evidence! In the same way, the beautiful Dirac equation was dismissed viciously in 1929 because it had one unphysical solution (antimatter) as well as predicting the electron! Heisenberg wrote:

    “The saddest chapter of modern physics is and remains the Dirac theory. … I regard the Dirac theory … as learned trash which no one can take seriously.”

    (M. Kaku, Einstein’s Cosmos, Phoenix, 2005, p 123.)

    Yet after experimental confirmation he responded:

    “I think that this discovery of anti-matter was perhaps the biggest jump of all the big jumps in our century.”

    (Ibid, p124.)

    Popper is wrong about falsifiability because Archimedes didn’t make falsifiable predictions when he came up with a proof of the law of buoyancy (the facts were already known). Falsifiability is an incomplete criterion for science. You can also prove things by rigorous logic, even if you don’t make checkable predictions. The problem for Woit is that the rigour is missing from string theory.

    2. Smolin’s point about loop quantum gravity in his actual detailed Perimeter Institute lectures (has Polchinski seen them?) is that loop quantum gravity is a bridge building exercise between well established QFT methods (path integrals) and the field equation of general relativity. By Ockham’s razor, if there is a way of getting quantum gravity introducing a lot of needless, unpredictive complexity (M-theory), then that science should choose the simplest theory which fits the empirical facts. The celebration of M-theory is way premature, and is drowning out every alternative with noise, particularly where there are alleged factual predictions (Tony Smith was censored off arXiv for one, and I’m censored for something completely different). I may be wrong over my ideas, but the evidence stands and won’t be investigated or checked until M-theory is defended less rigorously than now!

  • Arun

    “5] I am counting from the mid-20’s, when the commutation relations for the electromagnetic field were first written down, to the mid-70’s when lattice gauge theory gave the first reasonably complete definition of a QFT, and when nonperturbative effects began to be understood systematically.”

    The problem with this historical comparison is that QFT had a great number of experimental successes, verifications, whatever you want to call them, along the way.

    While there are certainly experimental signatures accessible to us in the near future which would require string theory as the underlying explanation, there is no experiment that can rule out string theory. It is the string theorist who might decide to give up the quest on her own, there is nothing Nature can tell her that would convince her. I think this is a situation with no precedent in science.

    I think Woit might even withdraw his book if there was even one clear answer to the question – what experiment with such and such results would convince one that string theory does not apply to nature? Of course, the same question needs to be applied to all the other theories out there as well.

  • Arun

    Also, my dumb question of the day – in the AdS/CFT correspondence, can the classical limit be taken on each side of the correspondence, while preserving the correspondence?

    The question is “inspired” by the thoughts that
    a. we undeniably live in a world with a classical limit, with classical gravity.
    b. the AdS side, with gravity, to be relevant to anything, should have a limit with classical gravity.
    c. what is the classical limit of the CFT side if it is QCD-like?

  • anon

    Arun, the classical limit on the gauge theory side is the N_c (number of colors) goes to infinity limit. This is “classical” in a sense (quantum loops are suppressed), but it’s not the same limit one would ordinarily think of as the classical Yang-Mills theory. A quantum theory can have different classical limits.

  • Alejandro Rivero

    In recent years, leading scientists in particle phenomenology, inflationary cosmology and other fields have found ideas generated by string theory to be useful in their disciplines,


    This parragraph clarifies the sociology and confirms the partition of the ArXiV (cond-mat, hep-th, math-ph, gr-qc, hep-ph, quant-ph, etc.). hep-th is not about particle theory; if it were, it should be hep-ph. The criticisms from Smolin and Woit comes frome the belief on a relationship between hep-th and hep-ph, and perhaps even with gr-qc. That relationship could to be claimed back in the seventies, even if at these ages another disciplines (cond-mat for instance) have already decided to have their own theoretical teams.

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  • Blake Stacey

    Wow. I “invented” brane gas cosmology for a science-fiction story based on what I had read in Zwiebach’s book (and learned in the class from which the book was born). That would have been early 2005, at the latest. I guess I was a little too late and didn’t read widely enough first.

    Now, two challenges remain: work in a few hints about “winding modes” to exaggerate my competence even more, and find a publisher daft enough to put the thing on bookshelves.

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  • Robert

    I would like to contribute another argument (or another way of stating the same argument) against this “AdS/CFT cannot describe bending of light because of fixed causal background in CFT” argument which I trace back to St. Carlip (although in other circumstances): If that were true, you would get a different theory by doing classical GR but with the field redefinition g_mn = eta_mn + dg_mn. Obviously, this is just a change of names (if you keep all orders of dg_mn) but it looks like a theory propagating in the flat background metric eta_mn.

    The resolution comes from the fact, that you should only require causality for gauge invariant observables. And those propagate according to the full metric while dg_mn propagates in the background but is not observable. A similar example is electromagnetism in Coulomb (A_0=0) gauge: There, the gauge field seems to propagate with infinite speed but this is of course an artefact of the gauge choice.

    Regarding background independence of the formulation of a theory, I would like to mention that usually we do not require this (or the analogue thing) for gauge theories: There purists would require that the theory is expressed only in terms of gauge invariant observables (without mentioning a background i.e. a gauge around which A_m is the expansion): -1/4 tr(F^2) is no good as F is not gauge invariant. The ‘proper’ way of doing it is in terms of Wilson lines but this does not make the theory any prettier. The other advantage of the A’s is that they come form a linear (actually: affine) space while the gauge invariant, background independent observables come from a much more complicated space.

    The theory is the same, it is just so much more convenient to use the language of the A’s. So why not allow the similar thing in the case of gravity?

  • Peter Fred

    There seems to be an unending debate on this topic. The trouble may be that we can not base gravitational phenomena on mass because we have no idea of the inherent, essential characteristics of mass that would cause it to either attract other mass or to warp space. Similarly, the Scholastics who endlessly debated among themselves about epicycles etc had no idea of the inherent, essential properties that the earth possessed that would cause all the objects in the sky to rotate around it in a 24-hour period. So can gravitational phenomena be put on a more sound footing as was done in the past with a theory built on an unsound premise? Check out Check out.

  • Brett

    I wanted to answer the question the question posed in [7].

    In short, this is a significant problem for any theory that predicts Lorentz violation. There is no known phenomenon that could take strong Lorentz violations at high energies (i.e. a Lorentz-violating cutoff) and weaken them at low energies enough to be compatible with experimental bounds. Virtual particles with momenta near the cutoff make large contributions to low-scale Lorentz violation, which are suppressed only by powers of the coupling constant and possibly logarithms of ratios of scales.

    The most explicit calculation of this that has been published is, I believe, in Collins, et al. Phys. Rev. Lett. 93, 191301 (2004). They take a Lorentz-violating cutoff and show how it affects one low-energy function. No one has published a more general analysis of how this works. (I myself have thought about doing it–taking a theory with no Lorentz violation in the Lagrangian but a Lorentz-violating regulator and seeing how the Lorentz violation shows up in the Lagrangian of the low-energy effective field theory–however, I have not gotten around to it.)

  • Jacques Distler

    There is no known phenomenon that could take strong Lorentz violations at high energies (i.e. a Lorentz-violating cutoff) and weaken them at low energies enough to be compatible with experimental bounds.

    That statement needs some caveats.

    Lattice gauge theory is a Lorentz-violating cutoff. However, the residual discrete symmetry group, which is unbroken, is large enough to guarantee that all Lorenz-violating effects are in irrelevant operators that disappear in the continuum limit.

    So you are, presumably, talking about Lorentz-violation severe enough that it can creep into relevant or marginal operators.

    In any case, LQG doesn’t “predict” Lorentz violation in 4 dimensions. There’s a naive (and totally misguided) hope that something like the 3D results of Freidel and Livine might carry over to 4D.

    Their result is that gravity coupled to matter in 3D is equivalent (upon integrating out gravity) to a matter theory on a non-commutative spacetime. The “trick” of integrating out gravity in 3D, where the gravitational field has no local degrees of freedom, does not (of course) carry over to 4D, where the gravitational field has massless local degrees of freedom.

    Nevertheless, hope springs eternal …

  • Brett

    Ah, yes, there is that caveat. Obviously, the low-energy effective theory must contain a renormalizable operator with the same symmetries as the the Lorentz violation in the high energy theory. Otherwise, there’s nowhere for the Lorentz violation to go in the low energy theory. Roughly speaking, in four dimensions there are no renormalizable Lorentz-violating operators at low energy with more symmetry than a two-index symmetry tensor (roughly speaking, I say…), while I believe that a lattice regulator has the symmetries of a three-index tensor (plus higher order tensors).

    Collins, et al. actually argue that, because no forms of Lorentz violation that correspond to renormalizable operators at low energy are suppressed, we should only be looking experimentally at nonrenormalizable Lorentz-violating operators. Those operators are suppressed at low energies, but this irrelevance has nothing to do with their Lorentz-violating character; it’s just a product of their nonrenormalizability.

  • Tony Smith

    Back in 1983, Joe Polchinski (with Wise and Alvarez-Gaume in Nuc. Phys. B221 495-523) found, in the context of “Minimal Low-Energy Supergravity”, that
    “… The renormalization group equation … tends to attract the top quark mass toward a fixed point of about 125 GeV
    It also puts an upper bound of 195 GeV on the mass …”.

    This was indeed a prediction of a heavy T-quark, and was in fact NOT in line with then-conventional expectations.

    Then-conventional expectations were exemplified by the announcement in 1984 by Carlo Rubbia at CERN that CERN had discovered the T-quark and its mass was about 40 GeV (see for example Nature 310 (12 Jul 84) 97).

    It was not until 1987 or so that experimental data began to indicate that the T-quark mass might be over 100 GeV, when ARGUS B-Bbar experiments showed an unexpectedly large mixing parameter.
    When the T-quark was observed by Fermilab a few years later, it was found to be in the 125 – 195 GeV mass range predicted by Joe Polchinski.

    What puzzles me is that Joe Polchinski did not embrace his prediction as an indication that supergravity renormalization group models must contain important elements of truth, and then embark on a program of studying and modifying such models,
    instead, he became a member of the herd that has been (and still is) working on conventional superstring theory, which AFAIK has not produced anything like as dramatic a prediction as his T-quark mass prediction.

    Of course, it is possible that Joe Polchinski may have been discouraged by difficulties in showing finiteness of supergravity, but it is interesting that such finiteness is still an open question (for example, a UCLA workshop next week is about “Is N=8 Supergravity Finite?”).

    It is also possible to contend that supergravity is just a part of superstring theory if it turns out to be a low-energy limit of something superstring-related like M-theory,
    that position seems to me to be a disingenuous effort to claim for superstring theory the successes of a possibly competing theory, especially since the successful prediction that Joe Polchnski made back in 1983 was based on supergravity structures that were then not thought to be related to superstring-type structures, so superstring theory played no role even in inspiring the ideas used in the successful prediction.

    Tony Smith

  • George Musser

    I have a few scientific questions for Joe, Sean, or another of the string gurus here….

    1. Why are positive energy density states necessarily nonsupersymmetric?

    2. If Lorentz invariance is exact, then the fine-scale structure of spacetime cannot be latticelike, so what does happen at the Planck scale in theories that preserve this symmetry all the way down?

    3. In the book, Lee argues that Weinberg’s anthropic prediction for lambda was way off, if all parameters (not just lambda) are allowed to vary over the ensemble. Is that a fair objection?


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  • Peter Woit

    I’ve posted something about this at my blog, but it seems like it would be best for discussion of this to be hosted here, so I’ve turned off comments there, and what follows is the bulk of my posting.

    First of all I should say that I was quite pleased to see Polchinski’s review. While I disagree with much of it, it’s a serious and reasonable response to the two books, the kind of response I was hoping that they would get, opening the possibility of a fruitful discussion.

    Much of Polchinski’s review refers specifically to Smolin’s arguments; some of it deals with the endless debate over “background independence”, and the “emergent” nature of space-time in string theory vs. loop quantum gravity. I’ll leave that argument to others.

    Polchinski notes that I make an important point out of the lack of a non-perturbative formulation of string theory and criticizes this, referring to the existence of non-perturbative definitions based on dualities in certain special backgrounds. The most well-known example of this is AdS/CFT, where it appears that one can simply define string theory in terms of the dual QFT. This gives a string theory with the wrong number of large space-time dimensions (5), and with all sorts of unphysical properties (e.g. exact supersymmetry). If it really works, you’ve got a precisely well-defined string theory, but one that has a low-energy limit completely different than the standard model in 4d that we want. This kind of string theory is well-worth investigating since it may be a useful tool in better understanding QCD, but it just does not and can not give the standard model. The claim of my book is not that string theories are not interesting or sometimes useful, just that they have failed in the main use for which they are being sold, as a unified theory of particle physics and gravity.

    The lack of any progress towards this goal of a unified theory over the past 32 years (counting from the first proposal to use strings to do unification back in 1974) has led string theorists to come up with various dubious historical analogies to justify claiming that 32 years is not an unusual amount of time to investigate a theory and see if it is going to work. In this case Polchinski argues that it took about 50 years to get from the first formulation of QED to a potentially rigorous non-perturbative version of the theory (using lattice gauge theory). The problem with this analogy is of course that in QED non-perturbative effects are pretty much irrelevant, with perturbation theory describing precisely the physics you want to describe and can measure, whereas with string theory the perturbative theory doesn’t connect to the real world. When QED was first written down as a perturbative theory, the first-order terms agreed precisely with experimental results, and if anything like this were true of string theory, we wouldn’t be having this discussion. For the one theory where non-perturbative effects are important, QCD, the time lag between when people figured out what the right theory was, and when its non-perturbative formulation was written down, was just a few months (Wilson was lecturing on lattice gauge theory in the summer of 1973, having taken up the problem earlier in the year after the discovery of asymptotic freedom).

    Polchinski agrees that the key problem for string theory is its inability to come up with predictions about physics at observable energies. He attributes this simply to the fact that the Planck energy is so large, but I think this is misleading. The source of the problem is not really difficulties in extrapolating from the Planck scale down to low energy, but in not even knowing what the theory at the Planck scale is supposed to be (back to that problem about non-perturbative string theory…).

    Weinberg’s anthropic argument for the size of the cosmological constant is described by Polchinski as a possible “prediction” of string theory, and he recommends Susskind’s book as a good description of the latest views of string theorists. I’ve been far too rude to Polchinski in the past in expressing my views about this “anthropic landscape” philosophy, so I won’t go on about it here. He neglects to mention in his review that many of his most prominent colleagues in the string theory community are probably closer in their views on this subject to mine and Smolin’s than to his, and that our books are the only ones I know of that explain the extremely serious problems with the landscape philosophy.

    Recently string theorists have taken to pointing to attempts to use AdS/CFT to say something about heavy-ion physics as a major success of string theory, and Polchinski also does this. I’m no expert on this subject, but those who are like Larry McLerran have recently been extremely publicly critical of claims like the one here that “Physicists have found that some of the properties of this plasma are better modeled (via duality) as a tiny black hole in a space with extra dimensions than as the predicted clump of elementary particles in the usual four dimensions of spacetime.” My impression is that many experts in this subject would take strong exception to the “better” in Polchinski’s claim.

    Finally, about the “sociological” issues, Polchinski disagrees about their importance, believing they are less important than scientific judgments, but I’m pleased to see that he does to some extent acknowledge that there’s a serious question being raised that deserves discussion in the theoretical physics community: “This convergence on an unproven idea is remarkable. Again, it is worth taking a step back and reflecting on whether the net result is the best way to move science forward, and in particular whether young scientists are sufficiently encouraged to think about the big questions of science in new ways. These are important issues — and not simple ones.”

    Again, my thanks to him for his serious and highly reasonable response to the two books.

  • Alejandro Rivero

    I am not so worried about the issue of having the right or the wrong dimension; what I *really* would like to see is, say, a prediction of 6 flavours in QCD with one of them so masive that it can not bind into mesons. That should be realistic enough to me even if it were in any random number of dimensions.

  • Peter Woit


    I don’t want to claim to be a “string guru”, but about one of your questions:

    1. In a supersymmetric theory, basically the Hamiltonian operator (which gives the energy), is the square of operators that generate the supersymmetry. If a state is supersymmetric, the supersymmetry generators applied to it will give zero, and thus so will their square, the Hamiltonian.

  • Robert

    Peter’s answer to George is only the first half of the answer. N.B. that besides zero cosmological constant, negative (as in anti-de-Sitter space) is compatible with supersymmetry. The no-go can be traced back to the fact that there is no globally timelike future directed Killing vector field in de Sitter space (the one that is timelike futuredirected over here is not on ‘the other side of dS’) and the supercharges would have to comute into the Hamiltonian which generates a flow along this field.

  • Robert

    IIRC the way Lorentz violation is supposed to show up in loopy physics is that the dispersion relation is violated and the speed of light depends on energy (showing up in early or late arrival of ultra high energy gamma ray burst photons compared to ones of lower energy). The idea is that even if the relative effect is quite small the absolute size could be measurable as these photons have traveled across half the universe. Does anybody have an understanding of how this effect arises? Furthermore, the point seems to be (I think I read this in some abstract) that the loop prediction for this to happen is with a smaller power of (E/M_pl) than string theory making the loop prediction observable with the next generation of instruments while the stringy version is many orders of magnitude smaller.

    Which calculation this referes to? What do I have to compute to get this energy dependent speed of light?

  • George Musser

    Peter, is this related to what you say in the book about supersymmetry being a sort of square root of translation? How does this argument hold given that supersymmetry must be broken?

    Robert, forgive my laggardly brain, but might you be able to unpack your explanation? All I got is that the issue is related to the causal structure of spacetime.


  • Jeff Harvey


    Thanks for taking the time to write such a thoughtful review.


    Have a look at nucl-th/0604032 and you will find that Larry McLerran
    (or at least his collaborators) describes the calculation of the viscosity
    to entropy ration of the Quark Gluon Plasma via AdS/CFT as
    “An amazing theoretical discovery…”

  • Joseph Smidt

    I’ve promised myself I would take a class on String Theory. I have Polchinski’s books and worked through a few problems from the first few chapters and find it interesting. Thanks Headrick for that very helpful solutions manual!!!

    But I am still unsure what to think about string theory since it has been around so long with nothing concrete. However, I want to take a course so I can see for myself all the details be fore I judge it. Posts like this keep my spirits up. Thanks for sharing it. :)

  • Arun


    Arun, the classical limit on the gauge theory side is the N_c (number of colors) goes to infinity limit. This is “classical” in a sense (quantum loops are suppressed), but it’s not the same limit one would ordinarily think of as the classical Yang-Mills theory. A quantum theory can have different classical limits.

    Is holography preserved in some classical limit?
    Is the bulk theory (though in 5 dimensions) anything like our classical world in this limit?

  • Peter Woit


    I was referring to McLerran’s summary talk at a recent conference

    where he awarded Brian Greene a “Pinocchio Award” for a statement that seems to me very similar to Polchinski’s.

  • Joseph Smidt

    Does anybody know some good research papers on this mentioned above:

    “A second unexpected connection comes from studies carried out using the Relativistic Heavy Ion Collider, a particle accelerator at Brookhaven National Laboratory. This machine smashes together nuclei at high energy to produce a hot, strongly interacting plasma. Physicists have found that some of the properties of this plasma are better modeled (via duality) as a tiny black hole in a space with extra dimensions than as the expected clump of elementary particles in the usual four dimensions of spacetime.”

    I would love to read the details of this. Thanks.

  • Haelfix

    Robert, afaicr, there is some hope of seeing qg stringy inspired signatures with Planck satelite. The debate I seem to recall was whether it scaled like Mpl^-2 or Mpl^-4. The former, depending on the scale could in principle be seen, the latter otoh would never be seen.

    If LQG predicts a Mpl ^-1, that would be great in the sense that we would have falsifiable material.

  • Sean

    Joseph– have a look at this post at Backreaction, which has some references.

    George [24]– some attempts at brief answers.

    1. Keep in mind that almost every state is non-supersymmetric. Being supersymmetric is a very special property, just like being rotationally invariant. There’s a hand-wavy argument that works in flat spacetime: namely, that contributions to the energy from bosons and fermions exactly cancel, and the (vacuum) energy is zero. That’s not quite right in the presence of gravity, as you can also have negative-energy supersymmetric states.

    2. I think nobody knows what happens at the Planck scale. But it would be surprising if it were something simple like a lattice.

    3. That depends on how other things are allowed to vary; it’s certainly a sticky situation. On the one hand, the subspace of parameters in which we could live is certainly a small one. On the other, we don’t really know how big it is, nor what the measure on the space should be. I’m on the side of people who think we have no reason to believe that Weinberg’s assumptions describe the real world, and we need to understand much more before we can claim to have “understood” the value of the cosmological constant, even from anthropic arguments.

  • Carl Brannen

    There is no known phenomenon that could take strong Lorentz violations at high energies (i.e. a Lorentz-violating cutoff) and weaken them at low energies enough to be compatible with experimental bounds.

    The Feynman checkerboard model of the Dirac propagator in 1+1 dimensions does this, in a sense. There are various generalizations to 3+1 dimensions. Look for “Feynman checkerboard” on google for more.

  • GP1

    There is no doubt that String Theory is an experimentally verified theory. In String Theory if it can be reproduced as special effects it must be true. As you all know, most of the predictions of String theory, such as time travel, extradordinary dimensions, (which in string theory simply means the depth of analytical epicycles) are proved by the most famous Doctor of Philosophy Doctor Greene the string theory evangelist in his movies. By experimentally duplicating the predictions made by String Theorists in his movies by state of the art special effects the most famous Doctor Greene experimentally proved the predictions of string theory. (Note also Polchinsky’s reference to computer graphics as proof of stringy scenarios). It is a total ignorance of the elegance of the string to state that string theory makes no experimental predictions.

  • George Musser

    Sean, in flat space, does the cancellation still occur if supersymmetry is broken? Or to flip the question around, given the degree of supersymmetry-breaking we know must have taken place (or else we’d have seen the sparticles already), does the observed value of lambda make sense?

    Does the flat-space argument beg the question? I.e. if we’re already talking about flat space, doesn’t lambda have to be identically zero, or else space wouldn’t be flat?


  • Sean

    George– by “flat space” I meant “with gravity turned off,” sorry for being unclear. The cancellation does not occur once supersymmetry is broken; breaking susy introduces a new, unambiguously positive contribution to the vacuum energy, roughly the susy-breaking scale to the 4th power. Which is wrong by at least 60 orders of magnitude, given that susy is broken at a TeV or above. If it were really true that unbroken supersymmetry implied zero vacuum energy, that would be a flat-out disaster. But we can imagine starting with a supersymmetric state with a large negative vacuum energy, and then breaking susy to contribute a large positive vacuum energy, so that they just about (but not quite) cancel. In the string landscape picture, that is purportedly the kind of state we find ourselves in today.

  • Joe Polchinski

    Thanks to everyone for your comments; most questions seem already to have been ably answered. Just a few remarks:

    Brett #20,22: Thanks for the reference, this is certainly what I would expect. I understand that there is the hope for a `deformed algebra’ rather than a simple violation, but to an outsider it seems that what is being done in LQG is to return to pre-covariant methods of QFT, cut things off in that form, and hope for the best. It would be good to see some calculations.

    George #24: 1) As several people have noted, the supersymmetry algebra is H = sum_i Q_i^2 so the energy is nonnegative and vanishes precisely for supersmmetric states for which all the Q_i annihilate the vacuum. In supergravity there is an additional term -|W|^2 on the RHS, so depending on the value of W supersymmetric states have zero or negative energy but not positive. Robert #29 gives an alternate explanation. In our world, H must be near zero as a result of a near perfect cancellation between +Q^2 and -|W|^2, because SUSY is badly broken.

    2) An example that many people have pointed to is quantum mechanics, which cuts off the classical phase space at a scale hbar, but not by introducing any sort of rigid lattice.

    3) Weinberg was clever, in that you can vary lambda alone without changing anything in the early universe, because Lambda has no effect until recently. Thus he could formulate a well-posed question. When you vary anything else, like the density perturbations, then you also vary things like the amount of inflation and so you need to know the probability measure. There are many people with ideas about this, notably Linde, Vilenkin, Aguirre, Bousso, Easther et al. Different measures give different results. Indeed, even Weinberg’s original assumptions may be wrong. I expect that there is a meaningful probability measure (we already assume such for the inflationary perturbations) and that we need to figure it out: maybe there is some dual form in which it becomes an ordinary QM measure. Anyway, we cannot declare final victory over the c.c. until we have a framework for answering this question.

    Peter #26,35 Joseph #36: I was basing my comments largely on Rajagopal’s talk which seemed quite sober. I also note the very recent comments in Sabine Hossenfelder’s blog, where Bill Zajc, spokesperson for the PHENIX detector, has a great deal to say about the impact of stringy ideas.

    What is the moral? Strong coupling field theory is hard. Nonequilibrium field theory is hard. Nonequilibrium strong coupling field theory is hard^2, and yet here we have one we can solve exactly. It’s not the one we want, but it is not so completely different either, since supersymmetry and conformal invariance are broken at finite temperature. So it should be useful at least as a model, and possibly as a a quantitative guide.

  • Kris Krogh


    “I was basing my comments largely on Rajagopal’s talk which seemed quite sober.”

    Is that enough to cite an unrefereed talk as evidence, without checking it?

  • Eugene

    Sean :

    That depends on how other things are allowed to vary; it’s certainly a sticky situation. On the one hand, the subspace of parameters in which we could live is certainly a small one…..

    I am not even sure that I agree that the subspace of parameters in which we could live in is small. Honestly, I don’t know if it is infinitely small, or infinitely big. The problem is, we don’t know what are the allowed range of values that these parameters can vary given some theory (as opposed to what range we can live in), so how do we measure “smallness” or “bigness”?

    More in general about Weinberg’s lambda prediction :

    I don’t think anthropic arguments ala Weinberg is needed to be invoked when deciding which theories/framework/potential/racehorse is “better” (in fact, I think the whole obsession with Weinberg’s anthropic “prediction” is too narrow a viewpoint). Each of these must make predictions on the probability distributions of fundamental parameters, I am sure we can use observations and statistics to decide which is “better”.

    I am not saying anthropic arguments are bad. However I am saying that anthropic arguments are not needed to make sense of probability distributions. We can happily make observations and rule out models/theories based on confidence levels.

  • Alejandro Rivero

    Small rant about sociology: a thing that enerves me is the concept of “free market of ideas”, when it happens that theoretical physics research (and HEP of course; but well, even education in general) is almost the quintaessential example of a subsidized sector.
    People takes “competition=free market” as a definition. Hey, but also every individual player in a bureaucratic economy is involved in a competition. Even the imperium-wide examinations of the old Chinese system were a kind of competition.
    (I was going to claim that a free market of ideas could be possible under Kropotkinian conditions, but even the almost-Kropotninian economical system pictured by Le Guin in “The Dispossesed” showed inflexibilities).

  • WeemaWhopper

    Very civilized and helpful response, Prof. Polchinski.

    Have to differ on your sentence `Postitive dark energy is the greatest experimental discovery of the past 30 years regarding the basic laws of physics.’

    I’d say the observation of the W and Z in 1980 or so are greater; how easy it is to forget the confusion of that time, with atomic parity violation and weak magnetism confusions casting doubt on the electroweak unification. Non-zero neutrino mass difference is up there too, as it really is not a Standard Model effect.

    Also, dark energy is an observational discovery… no experiment done there. A subtle point, perhaps…

    But those are small potatoes. Even if all the stringy ideas are totally irrelevant to the LHC as I believe to be most probable, string theory is a useful extrapolation of earlier ideas.

  • nc


    By ‘positive dark energy’ Prof Polchinski presumably means the secure experimental evidence from supernovae redshifts which show no slowing down in expansion. But there are two : (1) the gravitational retardation of distant galaxies etc is being offset by acceleration due to dark energy, and (2) there is simply no gravitational slowing down mechanism.

    Explanation (1) is mainstream (the lambda-CDM general relativity cosmology), but explanation (2) is championed by Nobel Laureate Philip Anderson, who wrote: ‘the flat universe is just not decelerating, it isn’t really accelerating’ – Philip Anderson,

    Explanation (2) suggests Standard Model type (Yang-Mills) quantum field theory is the theory of gravity, because you’d expect a weakening in gravitational attraction in any situation when the gravity charges (masses) are rapidly receding from one another, due the “graviton” redshift. Ie, where the visible light from a galaxy is seriously redshifted by recession of the galaxy, the gravitons being exchanged with it will also be severely redshifted (weakening the gravity coupling constant between the two charges), which is a mechanism totally omitted in general relativity. This was predicted ahead of Perlmutter’s observations, unlike explanation (1) which relies on the ad hoc invention of dark energy.

  • WeemaWhopper

    nc, there is a subtle difference between an experiment and an observation. Experiments allow repeatability and some control over conditions. Unfortunately for the big bang, we’re stuck merely observing the one we’ve got, which of course is not Permutter’s or Polchinski’s or anyone else’s fault. Experiment and observation are both empirical. and both bring crucial evidence to the table. But I’d not call the evidence for the dark energy experimental in origin… it is observational in origin.

  • James

    You all forget the true nature of the universe can only be found in the Holy Bible.

  • Christine Dantas

    Joe Polchinski wrote:

    Positive dark energy is the greatest experimental discovery of the past 30 years regarding the basic laws of physics.

    Indeed, there are several evidences indicating that the expansion of the Universe is accelerating.

    However, I would be more confident when some details are completely understood. The devil is in the details. I cite here two papers as examples of what I mean.

    – The type Ia supernova SNLS-03D3bb from a super-Chandrasekhar-mass white dwarf star — Nature 443, 308-311 (21 September 2006) — Although this remarkable SN can be easily seen as an outlier, an hence promptly removed from cosmological investigations, there could be contamination from other “super-Chandra” SNs in the current SN Type Ia data, although less evident (because of lower mass) than SNLS-03D3bb. A thorough study on these objects and their possible contamination in the SN data used for cosmology must be clearly understood.

    – The Uncorrelated Universe: Statistical Anisotropy and the Vanishing Angular Correlation Function in WMAP Years 1-3 (astro-ph/0605135). Or see their homepage. I quote from their paper: “If indeed the observed l=2 and 3 CMB fluctuations are not cosmological, there are important consequences. Certainly, one must reconsider all CMB results that rely on low ls (…) Moreover, the CMB-galaxy cross-correlation, which has been used to provide evidence for the Integrated Sachs-Wolfe effect and hence the existence of dark energy, also gets contributions from the lowest multipoles (…)”

    Details like these might turn out to be just mere “details” that will not affect the current evidences. Or not. Some would rather adopt a radically skeptical position, some others would bet on the current evidences with no worries. That is a personal choice.

    In any case, all independent evidences must eventually fit into the same picture, and the gaps in our knowledge hopefully will be erased with further data. For that matter, careful and independent astrophysical investigations (like, e.g., the current efforts to test the evidence for dark energy from clusters of galaxies at high z) will be very interesting.

    [Some references on this are:

    A Cooray et al. 2004 Growth rate of large-scale structure as a powerful probe of dark energy Phys. Rev. D 69 027301

    J Weller et al. 2002 Constraining dark energy with Sunyaev-Zel’dovich cluster surveys Phys. Rev. Lett. 88 231301]

    The connection between string theory and dark energy is still a subject of much controversy, and it may be that Weinberg got the right answer for the wrong reason. However, it may well turn out that he got the right answer for the right reason. If so, it will be one of the great insights in the history of physics, and the multivacuum property of string theory, seemingly one of its main challenges, will, in fact, be just what nature requires.

    I do not understand the logic in the above paragraph, I believe it would be interesting to further elaborate. What if the details that I have mentioned previously turn out to be important, or otherwise, if future investigations with new and better data indicate something else? Will it mean to be a clear evidence to falsify string theory? (Or for that matter, any theory based on a multivacuum scenario?) What are the specific predictions of these theories?

    Or, alternatively, if it turns out that indeed there is an acceleration caused by a dark energy component with clear and specific numbers at hand, why would this imply univocally a signature of a multivacuum? What about other proposals? How are we going to be able to discern “what nature requires” among them except with the aid of clear experimental/observational predictions from these theories?

    I do not mean here to flame this already naturally hot thread, but just to indicate some of my concerns, which I believe are also shared by some.

    Best regards,

  • Arun

    What is the probability that given we exist and given that our existence is equally probable in any spiral galaxy, that we’d actually exist in a rather small grouping of galaxies instead of being in the thick of a supercluster?

  • Sean

    Christine, the fact that the universe is accelerating is by now extremely well-established. Even if you don’t believe in the supernova data at all (and there’s no reason not to), the CMB plus some very weak constraint on the Hubble constant implies acceleration — and that doesn’t depend on the low-l multipoles at all. The CMB plus constraints on the matter density strongly implies the existence of dark energy, or perhaps modified gravity. And independent observations of large-scale structure, cluster counts, gamma-ray bursts, and baryon acoustic oscillations all provide additional evidence.

    It makes all the sense in the world to keep an open mind about any particular explanation for the acceleration, but the universe is definitely accelerating.

  • Christine Dantas

    Dear Sean,

    Thanks for the comment. Yes, as I wrote previously, there are several evidences, and if it was not clear from my previous comment, let me add that I see these evidences as very interesting results. It is not the case that I do or do not believe in one dataset or another, in any particular sense, but thanks for pointing out the low-l results as having no effect on deriving the acceleration. I’ll review that. In any case, the fact that I very much would prefer the situation in which all details were completely understood still makes sense to me. Perhaps that is a too much conservative position, I don’t know. For instance, I do place a considerable distinction between the two sentences: “there are several evidences for the acceleration” and “the universe is definitely accelerating” (for instance, you seem to use both sentences interchangeably). Maybe that is not the point of view of many, and I respect that.

    Best regards,

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  • http://www.anthropic-principle.ORG island

    I’ve got an even better one, Christine:

    Hey, Joe. The balance points that define the anthropic coincidences appear to be self-regulating in areas where we can make this distintion, so the implication is… what you see, is what you’ve got, and that doesn’t change with expansion. I wonder how that could happen, unless accelerating matter generation counterbalances accelerating expansion…

    Anthropic reasoning also indicates that characteristics/traits/asymmetries… are inherent, and are evolved to higher orders of the same basic structure, which also supports the first implication:

    In their Quantum McSilly rebuttal of the anthropic principle, Starkman and Trotta decided that, “in order to live and thus view the universe, humans need to collect and expend energy, so humans should prefer a universe that is flying apart as slowly as possible, making it easier to go out and collect energy to expend. In such a universe, the cosmological constant should be as low as possible, even lower than the value seen now.”

    The valuable connection to energy consumption that they’ve made is arrogantly maligned by Wheeler’s interpretation, because we’re here to work, not watch, (necessity being the mother of invention, and all that), and “pound-for-pound”, we are magnitudes more energy-efficient at generating matter/antimatter pairs than Black Holes or Supernovae… speaking of counterbalancing effects.

    we cannot declare final victory over the c.c. until we can define the stability mechanism with real first principles, instead of excuses.

  • nc

    “Even if you don’t believe in the supernova data at all (and there’s no reason not to), the CMB plus some very weak constraint on the Hubble constant implies acceleration … It makes all the sense in the world to keep an open mind about any particular explanation for the acceleration, but the universe is definitely accelerating.” – Sean

    “…the flat universe is just not decelerating, it isn’t really accelerating…” – Philip Anderson,

    The fact that it is Philip Anderson in the last quote should not matter. Just concentrate on the science:

    GR is not the final theory of gravity, which will have to take account of quantum effects. GR predicts that expansion there is a departure from Hubble’s law at extreme redshift due to deceleration caused by gravity.

    The supposed “acceleration of the universe” observation that there is no gravitational retardation in evidence, not that there is acceleration.

    You can explain this either by dismissing long-range gravity or you say there is an acceleration due to dark energy which cancels out the effect of long range gravity.

    Any Yang-Mills quantum gravity however predicts the lack of gravitational acceleration precisely so there is simply no room for any significant amount of ad hoc dark energy in explaining the result: the quantum gravity coupling (effective charge) for gravity falls off due to the redshift of the gauge bosons being exchanged when the masses are receding at relativistic velocities. There are different analyses to this problem, but all lead to the same conclusions!

  • Plato
  • nc

    “This picture of dark energy is consistent with Albert Einstein’s prediction of nearly a century ago that a repulsive form of gravity emanates from empty space.” – Plato

    Einstein was falsifying a steady state cosmology by adding a cosmological constant to general relativity without mechanism, and he used a much greater amount of dark energy (enough to cancel gravity effects at a distance equal to the average separation of galaxies in the universe). What needs today isn’t a reversion of prejudiced ad hoc “explanation” using “dark energy”. What is needed is quantum gravity that predicts what is seen.

  • nc

    Einstein was falsifying [the facts by inventing] a steady state cosmology… What is needed today isn’t a reversion to prejudiced ad hoc “explanation” using dark energy. Sorry, I’ll call it a day.

  • B

    Dear Joe:

    Regarding your question in footnote [7], it seems to me there is some confusion here about violations of Lorentz invariance (which single out a preferred restframe) and deformations of Lorentz-transformations (which are observer independent, but have a modified functional dependence on the boost parameter). I believe Lee Smolin referred to the latter, since he has worked on the topic for quite some while. You find a nice introduction e.g. in

    I have put together some more references (that also address Robert’s question from above) on a post on my blog (my comment got too long):

    Deformed Special Relativity

    I sincerely hope that this clarifies at least some points.

    I should admit though that the status of a quantum field theory which successfully includes these deformations of special relativity is presently very unsatisfactory. Many publications on the topic use arguments based on a couple of equations, with a consistent framework still lacking, which is very frustrating. I am currently working on cleanly formulating a quantum field theory with DSR, and I know that others are as well. I am very optimistic that there will be some progress soon.

    It is however possible to arrive at some general predictions by making use of the deformed transformations, or by using kinematical arguments. Many examples of this can be found in papers by Giovanni Amelino-Camelia et at, see e.g. gr-qc/0412136.

    Best regards,


  • Ari Heikkinen

    About the “suggested reading” at the end of the review, there’s a huge problem with atleast Greene’s book (it’s the only one of them I’ve read so I can only comment it), because it has only claims (such as particles being tiny vibrating strings of which amplitude and wavelength corresponds to different masses and force charges of them and that 11 dimensions of which 7 of them are curled up in Calabi-Yau shapes are required, etc.) and absolutely nothing to back these claims up, other than “according to string theory…”. Even something obviously as important to the theory as Calabi-Yau shapes, the only explanation I can recall from the book of it was a picture of how they might look like.

    Add to that, there’s not even a single equation in the book and that the author happily concludes that it could be all wrong.

    I mean, a reader basicly has to take his word for it and based on his credentials hoping he’s not making it all up. Although nicely written, it could have aswell been written by a science fiction writer and there would be no difference if the author did his homework on basics of physics.

    So my question is, is there any book out there that would explain string theory with actual equations, from the beginning, building up with any previous insights, to the present and how it actually connects to real world physics?

  • Sean

    Ari, the difference between The Elegant Universe and science fiction is that the former is backed up by large books full of equations — one of them is linked right at the top of this review! Here’s another, and another, and another.

  • Ari Heikkinen

    Sean, thanks for the links! Which one would you consider to be the most self-contained and up-to-date? According to Woit, Zwiebach’s book would only cover a small part of the string theory story not taking the reader very far into the issue of how to connect strings to real world physics.

  • Sean

    You should look at the books for yourself; Zwiebach’s is aimed at undergraduates, so goes more slowly and doesn’t get as far, whereas the others are for graduate students.

  • Joseph Smidt

    Hey, Great links to:
    String Theory and M-Theory: A Modern Introduction
    Supersymmetry and String Theory: Beyond the Standard Model

    These books do not seemed to be released yet. Does anybody know how these will be different then Polchinski’s books? Are they just more up to date or do they cover more ground or different topics? Also, any other good books on Supersymmetry besides Weinberg’s and Wess and Bagger’s? Thanks.

  • Plato
  • Plato

    For the lay people here.

    What is Dark Energy?

    Richard Feynman and others who developed the quantum theory of matter realized that empty space is filled with “virtual” particles continually forming and destroying themselves. These particles create a negative pressure that pulls space outward. No one, however, could predict this energy’s magnitude.

    Some “thing” had to cause it? We needed a microscopic theory?

    If superfluid anomalies are considered, then relativistic realization of a “flat space” at such an “energy extreme” could have encouraged, the change to the universe speeding up? Of course I speculate.

    The “curvature parameters” had to be encouraged some how?

  • Clay Aiken

    I’d like to be your next guest blogger, ok thx.

    Guest Blogger: Clay Aiken

    I just wanted to wish all my friends here a truly blessed holiday season!! And here’s hoping to more good times ahead!! What a rollercoaster ride we’ve been on together, haven’t we?? But I believe that it is only as we view the earth from high above, and then come crashing down to earth, with the dualistic winds of betrayal and good cheer blowing upon our faces, that we truly understand that our lives are but a glimmer of hope, a ray of sunshine, in a dark world of destruction. Ah, but a glimpse!! And so I ask you to pause, and to consider for one brief moment the true meaning of Christmas, which was revealed to me in a dream, a vision I shall now share with you.

    Waking up in the middle of the night and startled by a chill that overcame me completely, I sought to nestle closer to my dog Jane. In vain I grasped at her torso. Where had she gone?? Oh, travesty, oh perplexity divine!! And so I left my room, only to see that she had escaped through the open back door. Distressed, I chased after her, following the fresh foot prints down a hill into a gully, a gully that was the scene of many toboggan rides, where Clay and his dog shared an unadulterated joy together. Orgasms of the mind!!

    As I called out her name, I heard a voice from a tall tree, beckoning me to approach. This tree spoke to me in a deep, soothing voice, saying “To be a tree is to be what I am!! I would not have it any other way. It is only as we accept our lot that we can be truly free, and to enjoy all the bounty that is given to us. Clay, I’ve seen you grow up before my eyes, and I am amazed at the man you have become!! So full of life, and an instrument of love and faithful devotion to reveal and ascertain all that the Creator of the Universe has bestowed upon us all!! And so in this holiday season, understand that the following is what Christmas is really all about: Sometimes a smile is all that is needed to bring peace and harmony to the earth, a firm handshake and a well-timed wink can topple totalitarian governments, and giggle of glee can wrest Lucifer’s control of this sad world, and give it to the spirit of a small child!! Spread this message Clay, this trifecta of joy and dazzling delights, for it is as we seek to understand, that we truly know completely. This I truly and honestly believe!!”

    And so, on that night, I learned the true meaning of Christmas, and I ask that you will now pause and consider what you can do. Whether it be a smile, giggle of glee, or a handshake and wink, understand that everyone is needed in the battle!! What will you do to save humanity in its darkest hour??


  • invcit

    Joseph Smidt,

    About SUSY: it kind of depends on what level you want and what aspects you are most interested in. There are many reviews online, for example the 2001 BUSSTEPP Lectures on SUSY by Figueroa-O’Farrill, and the more comprehensive An Introduction to Global Supersymmetry by Argyres. If you’re just starting out, my advice is to just *pick a convention* (I like Wess & Bagger’s) and try to work out as much as you can yourself following any and all notes.

  • Joseph Smidt


    Thanks, I am currently taking quantum field theory and wanted to have a full selection of texts to learn SUSY from since often one text helps with some things better than others. Thanks for the online review tips too.

  • Damysus

    Having read all the above comments on Joseph Polchinski’s reviews (and referred back to various parts of his two volumes on string theory and Weinberg’s Quantum Theory of Fields) it seems that one needs to step back a little to consider the fast-moving sub-quantum universe as a rather small correction to our manifestly positive reality. Who will list the micro-subtleties, in plain English, which have led to this? Of course, this goes to those most macroscopic of phenomena, dark energy and the cosmological constant…

  • nc

    “Richard Feynman and others who developed the quantum theory of matter realized that empty space is filled with “virtual” particles continually forming and destroying themselves. These particles create a negative pressure that pulls space outward. No one, however, could predict this energy’s magnitude.” – Plato

    No, pair production in only occurs above the IR cutoff. (Collision energies of 0.511 Mev/particle.) Space is thus only filled with particle creation-annihilation loops at distances closer than 1 fm to a unit charge, where the electric field strength exceeds 10^20 v/m. This is the basis of renormalization of electric charge, which has empirical evidence. is a recent analysis of quantum field theory progress that contains useful information on pair production and polarization around pages 70-80 if I recall correctly. For an earlier review of the subject, see

  • Plato

    Glast and high energy photons, or LHC?

    What use “Calorimetric designs” if one does not record where that “extra energy” is going? Is it insignificant?

    Of course I could be mistaken, but in seeing “Gran Sasso” such developements one has to wonder what was missed? Maybe a “bulk perspective” where “the medium” is the message?:)

  • Elliot

    I would say its about time for that new policy 😉


  • Sean

    Yeah. Bizarre rambling comment deleted.

  • mclaren

    Alas, while I do have a degree in physics I don’t know anywhere near enough about string theory to comment on the physics. The math sails far above my head — and is likely to do so for well over 99.999% of the world’s population.

    That said, it does seem that Dr. Polchinski follows the tried and true path of the typical string theorist in defending their turf against Smolin et al. To wit: zero in on some minutia involving the nitty gritty of who said what, or which theorist allegedly predicted what, or which term is or is not defined correctly by Smolin et al. All well and good.

    But that stuff remains irrelevant to Smolin’s and Woit’s basic point. Namely — if you never provide a specific falsifiable experimentally testable hypothesis, you’re not doing science. Every single string theorist who reviews Smolin’s and Woit’s books seems to ignore this issue. To put it bluntly, Smolin’s and Woit’s essential criticism remains valid even if they don’t know a proton from a neutron or the difference between kinetic and potential energy. Still doesn’t change the basic fact that string theorists have failed to produce testable numbers from their theory. To put it bluntly, it’s a game of “shoot the messenger” and that can’t erase the message that without experimentally testable numbers, you’re not doing science.

    Another even larger issue looms. Specifically, how long?

    How long do we wait before we conclude that string theory is “a degenerating research program” as Hubert Dreyfus famously said of good old fashioned top-down AI? The fact that ferocious debate still rages about Dreyfus’ characterization of AI after 50 years of work in the field suggests we may have a long ways to go before the debate over string theory gets settled.

    There must be _some_ point at which lack of experimentally testable number dooms string theory. Are we there yet? Probably not…but it’s hard to be sure. 30 years is a long time. On the other hand, string theory remains so much more mathematically complex than previous theoretical physics models that there’s a good argument to be made that we need more time to work it out. The analogy here is QCD and the infinities that finally got purged courtesy of renormalization. Is there a renormalization-like trick that will let us squeeze testable numbers from M theory and collapse the landscape? I don’t know. No one does at this point.

    I can’t think of any previous model from theoretical physics in which the chief practitioner (Ed Witten in this case) was awarded a Fields Medal just for the mathematics involved. So the math is clearly extremely formidable.

    Withal, some point of no return exists. If we get 50 years out, 75 years out, 100 years out, at some point a failure to produce experimentally testable numbers will become fatal to string theory. The analogy there is with unfortunate historical cul de sacs like phlogiston or alchemy.

    The real hope at this point is that at somewhat higher energies (of the kind we might get in the LHC), albeit not nearly high enough for unification of all 4 basic forces, some minor side effects might show up that would point indirectly to testable predictions from string theory. The analogy here is the Casimir Effect in quantum chromodynamics (though that didn’t require an accelerator to observe).

    If the LHC fails to produce tangential confirmation, cosmological data from satellites might conceivably fill the gap.

    If, however, after another 20 years or so we get no indirect confirmation from LHC or cosmological data from satellites, and still no hard numbers from string theorists that can be directly tested, my sense is that this debate is going to start to shift conclusively in Woit’s and Smolin’s direction. Until then, haggling and squabbling and quibbling about Woit’s and Smolin’s putative lack of familiarity with this or that microscopic detail of some obscure aspect of string theory math or terminology seems pointless, since that’s irrelevant to the basic issue they raise.

  • Robert

    How long are will there still be experimental physics? The whole point of experiements is testing theories and as long as experimentalists are not doing that it’s just senseless encyclopedic, unmotivated measuring (like counting the hairs on the heads of as many people as possible) and not science. We have told experimentalists over and over again to study Planck energy collisions and they keep ignoring this good advice.

    What they measure with all their semiconductors and quantum hall effects and what not might have applications in other areas and might help to build faster computers in the end but is not fundamental science.

    Still they fail to deliver a single expriment that studies matter at the Planck scale. How much longer is this supposed to go on?

  • Vince

    Of course string theory is science. Its hypothesis is that all elementary particles are actually loops of string. Just because this hypothesis will be proven by scientific experiments to be true or false in 50-100 years doesn’t mean it’s not science.

  • Kepler

    The books discuss the ‘landscapes’ presented by string theory. Specifically, string theory offers a ridiculous number like 10^500 ways of inserting the little strings into the curled dimensions. These multifarious possibilities correspond then to as many different predictions of particle masses and thereby render the theory devoid of predictive power, a theory of anything amounting to a theory of nothing. Or so I read. But I have not seen this objection to string theory countered or discussed in these comments.

    What would Eddington have thought of string theory?

  • Zelah

    What is the definition of Science?

    The definative hallmark of Science is the Testing of Theories.

    Unfortunately, Mr Woit does not understand Popper’s Theories.

    Popper was primarily attacking communism’s claim of being a science. The problem with the communists was that THEY WERE NOT EVEN TRYING find criteria for which communism would be falsified.

    Popper would have understood that QFT was the established theory and would have said that QFT was the THEORY IN NEED OF FALSIFICATION. Also, Popper understood that too rigid a proscription would destroy creativity in real science.

    Mr Woit is correct that the hype around String Theory is embarrassing.

    However, Mr Woit claim that String Theory is not Science is also embarrassing.

    Is Mathematics not a science?

    Is not Phenomenology not a science?

    The correct evaluation of String theory is to LOOK AT THE COMPETITORS!
    String Theory has its problems but the competition is in a worst state!


  • nc

    Zelah, maths is an art or a tool, not a science:

    ‘Science n. The observation, identification, description, experimental investigation, and theoretical explanation of phenomena.’ –

    ‘Science is the belief in the ignorance of [the speculative consensus of] experts.’ – R. P. Feynman, The Pleasure of Finding Things Out, 1999, p187.

  • Joe Polchinski

    Hi all, I’m sorry it’s so hard for me to keep up….

    Christine #51: If the constants of nature are environmental (and Weinberg of course is not the first to suggest this, just the first to make such a quantitative argument), then it is very difficult to be certain one way or the other. In the long run I am optimistic that somehow we will figure things out. There have been various challenges to the landscape, e.g. hep-th/0309170 as well as the issues I mentioned in post #43. Sean is right that for now we should keep our minds open. For myself, the most useful direction for now is to search for the full nonperturbative form of string theory and see what it tells up.

    Sabine #61: Thanks for the info. Again, given the importance of Lorentz invariance e.g. in identifying the correct theory of the weak interaction, I would argue that any candidate theory which does not have manifest Lorentz- or deformed-Lorentz invariance has a large burden of proof in showing that it can reproduce Lorentz-invariant physics where it is known to hold to high precision.

  • Christine Dantas

    Dear Joe Polchinski,

    Thanks for a honest comment. I also look forward to see new developments towards a nonperturbative string theory as much as to other approaches to quantum gravity. These are all fascinating developments to me. I hope I will be able to see a good level of resolution/understanding about these problems in my lifetime.

    Best wishes,

  • Plato

    mcLarenif you never provide a specific falsifiable experimentally testable hypothesis, you’re not doing science.Every single string theorist who reviews Smolin’s and Woit’s books seems to ignore this issue.

    I do not know how “any scientist” even those which include string theorist could have ever failed to understand what it takes?

    IN order for Smolin to debate another competitor he needed to understand what that competitor is doing. So he may even take a “philosophical view” of why one position is better then another. He may devise a philosophical basis of why “symmetry,” versus, “against symmetry” may lead to his conclusions.

    Let’s call it, “Two Traditions in the search for Fundamental Physics.”

    For some reason people do not think that if they adopt one view according their philosophical thinking, that there is no other, or vice versa?

    So all things grow from this? Media, fictional stories on the Simpson’s, or the String King versions :)

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  • Alex Leibowitz

    As long as string theory produces interesting results, from an intellectual standpoint, and further drives development in other areas of thought (such as mathematics) — then why worry about whether or not it is ‘science’? It isn’t as if ‘string theory’ has proved impossible to think, so why not keep on thinking about it? At worst, we are at least mapping out the physics of some possible world, which if nothing else is a very interesting thought experiment.

  • Ron Macnaughton

    To me, the most significant objection by Dr. Joe Polchinski against Smolin’s book is the issue of a positive cosmological constant. Lee Smolin seemed to say string theory accomodates a zero or negative constant but not a positive one. Was he wrong about that? Has someone since shown a string theory consistent with a positive cosmological constant?

    Also the website for the Gravity Probe B satellite (lecture notes by Everett) mentioned that some predictions of string theories might be confirmed. But I thought a problem with string theory is that it makes no predictions.

  • Steve Bryan

    I would be completely out of my depth making any nontrivial comment about this string theory controlversy but I think the issue of “it makes no predictions” is simply a misunderstanding of the context where statements like that are made. The physical scale and energy levels involved are so extreme compared to the physics that has been previously explored that experimentalists face an issue of feasibility. They might be able to design an experiment that tests a prediction of string theory but the resources required are beyond any budget they could ever obtain. In fact with current technology and wealth such experiments are simply beyond our reach.

    We live with an implicit assumption of exponential growth so what is beyond our grasp today may not remain that way in the future. Also we can hope that more subtle ideas might make useful experiments less “costly” than a brute force attempt. The items mentioned in footnote [6] are some possible candidates (some involve the possibility that nature is “kind” and less brute force than the worst case scenerio is sufficient).

  • B

    Hi Joe,

    I would argue that any candidate theory which does not have manifest Lorentz- or deformed-Lorentz invariance has a large burden of proof in showing that it can reproduce Lorentz-invariant physics where it is known to hold to high precision.

    Yes. I do absolutely agree. That requires a theoretical framework which is actually capable of reproducing the standard model qft limit. The absence of which in the DSR approach is currently a huge frustration for me, but I’m optimistic it can – and will be – done in the soon future.



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  • String Skeptic

    Thanks to all for an interesting set of comments. What I’m surprised to see, though, is so little comment about the fact that Dr. Polchinski’s remarks are clearly self-serving (after all, he has heavily invested himself in String Theory) and actually quite biased. In his own words: But what about the lack of predictions? While Dr. Polchinski asks the question, he fails to answer it, waving his hands with such ferocity that I believe he may have achieved the first human-powered flight (perhaps String Theory predicted that?). Dr. P seems to be quite pleased with a field that has yielded no predictive power or verifiable insight into the universe; without this, String Theory is a lot of things (an infinite revenue generator for physicists, an interesting mathematical construct, a religion), but it is certainly not science.

  • String Fan

    String Skeptic on Apr 20th, 2007 at 9:26 pm
    Thanks to all for an interesting set of comments. What I’m surprised to see, though, is so little comment about the fact that Dr. Polchinski’s remarks are clearly self-serving …

    Whereas your own interests are hidden by your anonymity. Perhaps you are Lee Smolin, he has not yet made a public appearance on this thread.

  • B

    @ String Fan

    Perhaps you are Lee Smolin, he has not yet made a public appearance on this thread.


  • String Fan

    @ B

    I followed the link to, but Smolin doesn’t really respond to Polchinski’s review there, he just makes excuses why he won’t.

  • Plato

    The same can be said of anyone who has a niche in science with regards to what they are looking at?

    They will be “biased” by their work?

    Why “loop quantum gravity” if you had already made up your mind on quantum gravity? Why any other aspect of quantum gravity, if you see only promise in “loop quantum?”

  • Lee Smolin


    Since i have been asked repeatedly, here is a response to Polchnski’s review of TTWP:


  • Moshe

    Hi Lee, two questions about technical points you raise in your response. Both are tired old issues by now, but despite discussing this previously with you I still do not have a clear idea what you have in mind, so I’ll try again:

    1. you say “Supersymmetry, however, appears necessary in perturbative string theories to cancel the tachyonic instabilities”. There are many well known examples of non-supersymmetric string theories that have no tachyons at tree level. In previous conversations I gave you a few examples, can you please clarify the sense in which this statement is correct.

    2. Regarding the “weak form” of ads/cft correspondence, I never was able to understand what precisely can be a weak form of the duality which is consistent with all the evidence but in fact does not coincide with what you call the “strong form”. Can you clarify please?



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  • Gina

    Joe Polchinski’s review was one of the highlights of the debate, and Lee’s respond is also of interest. There is one point, which seems to be central in Lee’s argument and rhetoric, that I find hard to understand:

    “It is indeed, characteristic of the style of research I criticize in TTWP, that a difficult problem whose solution was absolutely necessary to the success of string theory as a physical theory-moduli stabilization-could be happily left for the future. I do not recall any string theory talk or paper stating “string theory will be an interesting candidate for a physical theory if the very difficult problem of moduli stabilization can be successfully solved.” But this is what they ought to have said, (here is my ethics coming in here) had they given a true account of the situation.”

    To the extent that Lee is correct on the factual matters, what Lee describes (and refers to in the book as accessive “optimism” ), is a scientific methodology. Perhaps it is a bold scientific methodology. Parhaps an ingenious scientific methodology (or both), perhaps it has flaws. But why to relate it with ethics? How is it related with ethics? Lee describes himself in the book as “the most optimistic” (in terms of scientific style) in the LQG community. Does it means he is the least ethical?

    I do not claim there is no connection at all between scientific methodology and ethics, but, in any case, referring to ethics in such a context requires a much more detailed and explicit explanation.

  • Lee Smolin

    Dear Moshe,

    As you say we have talked about these things before, so I apologize if I repeat myself.

    1) There are some cases, but my understanding is that they are far from generic. Some were described, if I recall right, but Silverstein et al, which however required that for each fermion there is a boson of the same mass. This is a bit weaker than supersymmetry but it is still the strong. Furthermore, it was an open question whether cancellation of the tachyon survives past one loop.

    Here I think you are switching the burden of proof. The burden of proof is on those who think that all the theories that satisfy the semiclassical conditions used to define the landscape by an effective field theory argument, define real theories, to show that there are real quantum theories defined to arbitary order, for example. by fluctuations of worldsheets in these backgrounds. This has not been done. We can argue about special cases on each side, but the fact remains that we do not know how to compute quantum corrections in any of the landscape theories with positive cc.

    2) Again you are switching the burden of proof. My point is that with the present state of knowledge you cannot even formulate a precise conjecture for the strong equivalence, because there is no precise, non-perturbative definition of either side of the conjectured equivalence. In the absence of this, what we have are 1) a lot of evicdence for a correspondence at the classical supergravity level and 2) evidence about special cases that go beyond that which are extremal in one way or another, i.e involve BPS states. The minimimal conjecture that includes all of these is that the semiclassical results are true because of conformal induction while the special cases work because of the power of the extended supersymmetry algebra.

    There are many cases in string theory where conjectures are only strongly supported in extremal or near extremal limits, including the black hole results and S-duality. It seems to me very plausible that in these cases, as well as in AdS/CFT, the extended SUSY algebra is so powerful that it implies equivalences between states of different theories in their BPS and near BPS sectors, that are not true generally. This would be true if, for example, the BPS condition together with the extended SUSY algebra were sufficient to compute spectra and degeneracies. I suspect this is true, but I have not tried to prove it. But, here again, the burden of proof is on someone who believes a result demonstrated in special cases with lots of symmetery extends to cases with no symmetry to prove it, not the other way around.

    Dear Gina,

    Optimism is not the same thing as not telling the whole truth about what is known presently. The great optimists, for example, Feynman, were completely open about what they did not know and had not shown. Feynman used to put a sentence in the abstract of his papers listing “What is not shown here is…”. He complained that the journal editors edited them out.

    It is very easy to say, “My personal belief is that X is true, but presently we have only weak evidence for that, and it has definitely not yet been shown.” Or “I find this theory intriguing but for it to be a theory of nature X, Y and Z will have to be shown.”



  • anon.

    Lee wrote:

    In the absence of this, what we have are 1) a lot of evicdence for a correspondence at the classical supergravity level and 2) evidence about special cases that go beyond that which are extremal in one way or another, i.e involve BPS states.

    This is not an accurate summary of the status of checks on AdS/CFT. It ignores the general conceptual understanding of why a quantum gravity theory on AdS should always be dual to a conformal field theory, the detailed checks of anomalous dimensions (which, increasingly, it seems are understood in the complete range from weak to strong coupling), and any number of other issues. It’s probably not worth elaborating here since this discussion has occurred numerous times before in various blogs and it appears that Lee is content to repeat the same highly misleading claims.

  • Lee Smolin

    Dear Anon,

    This is exactly the point, there is a missing assumption in your argument. I do agree there is a decent, heuristic (not rigorous) argument that ‘a quantum gravity theory on AdS should always be dual to a conformal field theory’. In my paper on the subject with Arnsdorf we called this argument conformal induction. But there is no non-perturbative definition of string theory with asymptotically AdS boundary conditions, and indeed, except for certain extremal limits, nor is there any perturbative definition. (I agree there is a definition of supergravity and agree that the argument of conformal induction establishes a relationship between it and a CFT which is a limit of N=4 SyM.) So you cannot use this argument to reason from a theory that so far has not been shown to exist.

    One can reason in several ways:

    1) one can state that one is making an additional assumption

    A) there is a full non-perturbative definition of string theory with asymptotically AdS boundary conditions.

    If you assume A you can use this assumption to learn things that will be true about this theory, if it exists. But because you made this assumption you cannot use the same argument to demonstrate A, because you are then assuming what needs to be proving. This is the fallacy in your argument.


    2) You can evaluate the results on AdS/CFT as to how strong evidence they give for A). In that case you also cannot use A to strengthen the case for A.


    3) You can try to actually formulate the theory that A) asserts exists. This means give a genuine non-perturbative formulation of string theory. This is what I think we should try harder to do.


    4) You can try to state a weaker conjecture, which is that string theory is DEFINED by it being dual to N=4 SYM. This is different from the conjecture that there are two theories which are equivalent. To go this route however, one needs 1) a full non-perturbative definition of N=4 SYM, which doesn’t exist, because its hard to make lattice regularizations of SUSY theories. 2) a precise description of the observables of the bulk quantum gravity theory in the precise Hilbert space of N=4 SYM.

    Let me emphasize that I don’t rule out the possibility that 3) or 4) are doable and that there really is a precise non-perturbative string theory in this context. A real demonstration of 3) or 4) would be tremendous progress.

    All I am asking for is that we be straightforward about what has and has not been shown and we do not reason in fallacious ways that assume the existence of the theory we are aiming to construct. I would claim that, among other things, to do so slows down progress because it makes people complacent about what still is not known, so they don’t try so hard to do the big steps that real progress requires.

    Finally I would hope that our taking the time to sort out and clarify the actual logic behind this complicated situation so as to make clear what remains to do would be taken as friendly and supportive of string theory. If people were not so defensive they would see this as helpful. This was the spirit that my work on this with Arnsdorf was done.



  • Moshe

    OK Lee, I am sure neither one of us wants to get into another prolonged discussion, maybe in person sometime. Let me just say that I dispute some of the statements you mention as facts about string theory, as well as the relevance of the arguments you give to the questions I asked. I’ll leave it at that.

  • Aaron Bergman

    Repeated without comment:

    Finally I would hope that our taking the time to sort out and clarify the actual logic behind this complicated situation so as to make clear what remains to do would be taken as friendly and supportive of string theory.

  • Lucci

    Re Lee Smolin’s remark to Moshe that the cases in which nonsupersymmetric string theories don’t yield tachyons are not “generic”, it seems to me that Smolin has failed to properly appreciate Joe Polchinski’s statement, in his review of Smolin and Woit, that during 1995-98 “there were no general results or predictions” in string theory. (Smolin himself quotes this statement in his response to Polchinski.) The clear implication of Polchinski’s statement, taken in context, is that string theorists were not even trying to obtain general results and theories, but were concerned instead with applying new (nonperturbative) tools to various particular cases. And given this focus on particular cases (a focus that seems natural as a way of acquiring familiarity with the tools in question), it’s unclear why string theorists should be criticized – as Smolin criticizes them – for not pointing out that their efforts did not yield generic results or solve certain general problems (such as moduli stabilization). After all, one doesn’t normally expect that the study of a particular case will solve a general problem.

  • Gina

    Lee: “Feynman used to put a sentence in the abstract of his papers listing ‘What is not shown here is…’. He complained that the journal editors edited them out.”

    Dear Lee, This description shows that the convention is not to state or emphasize what was not proved but rather what was proved. Professional ethics is largely based on conventions. According to your own description, even Feynman was unable to change this convention. (In general physics journals, not string theory journals.) This only supports my opinion that your critique regarding string theory’s methodology has little to do with ethics and maybe it has little to do with string theory as well.

    (As we saw again and again the questions of what was proved and what was not proved and what is the interpretation are complicated and disputed; be that in the complicated issue of finiteness of string theory, or the less complicated question if the “free lunch theorem” has anything to do with evolution. It looks that the editors and not Feynman were correct in their approach. It is hard enough for authors to tell clearly what was shown in their paper and what is the interpretation. It is OK to leave the “what was not shown” part for others (or for blogs.))

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  • Zephir

    By Aether Wave Theory both string theory, both LQG theory are dual theories, both describing the nested spongy density fluctuations of the hypthetical inertial environment, forming the interior of black hole, which we are living in. These fluctuations are conceptually simmilar to density fluctuations inside of dense condensing supercritical vapor, so they’re fulfilling the common Newtonian dynamics on the background.

  • gvdl

    Not being a physicist, I can’t comment on the science of this thread. Surely the details of the string theory are not really all that significant to his overall argument that the ‘sociology’ of the academy is stifling originality. Please tell me that Lee is wrong and that original science IS really being done and innovative young scientists do not have to conform to received opinion.

    As a mature age Ph.D. student myself I had already resigned myself to leaving the academy, it being unwelcoming to the unexpected. I had hped that was just understandable age prejudice and had was not endemic.

    Perhaps I’m a hopeless romantic but Lee’s indictment of modern science is scary and I have yet to see a single rebuttal to the meat of the main thesis presented in TTWP.

  • forest

    They will be “biased” by their work?


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Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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