Science or Sociology?
Joseph Polchinski, 5/20/07
This is a continuation of the on-line discussion between Lee Smolin and myself, which began with my review of his book and has now continued with his response. A copy of this exchange (without the associated comment threads) is here.
Dear Lee,
Thank you for your recent response to my review. It will certainly be helpful in clarifying the issues. Let me start with your wish that I do more to address the broader issues in your book. When I accepted the offer to review these two books, I made two resolutions. The first was to stick to the physics, because this is our ultimate goal, and because it is an area where I can contribute expertise. Also, keeping my first resolution would help me to keep the second, which was to stay positive. I am happy that my review has been well-received. Your response raises some issues of physics, and these are the most interesting things to discuss, but I will also address some of the broader issues you raise, including the process of physics, ethics, and the question in the title. Let me emphasize that I have no desire to criticize you personally, but in order to present my point of view I must take serious issue both with your facts and with the way that they are presented.
Regarding your points:
The fictitious prediction of a non-positive cosmological constant. This is a key point in your book, and the explanation that you now give makes no logical sense. In your book you say (A) “… it [a non-positive cosmological constant] was widely understood to be a consequence of string theory.” You now justify this by the argument that a non-positive cosmological constant is a consequence of unbroken supersymmetry (true), so A would follow from (B) Unbroken supersymmetry was widely understood to be a consequence of string theory. But even if this were true, it would not support your story about the observation of the dark energy leading to a “genuine crisis, … a clear disagreement between observation and a prediction of string theory.” There would already have been a crisis, since supersymmetry must obviously be broken in nature; seeing the dark energy would not add to this. But in fact the true situation, as you can find in my book or in many review articles, was closer to the opposite of B than to B: (B’) Supersymmetry is broken in almost all Calabi-Yau vacua of heterotic string theory. We have no controlled examples because at least one modulus rolls off, usually to a regime where we cannot calculate. The solution to this problem may have to wait until we have a non-perturbative formulation of gravity, or even a solution to the cosmological constant problem.
In your response you largely raise issues surrounding B’, including the Witten quote, but I want to return to what you have actually written in your book. It is a compelling story, which leads into your discussion of “a group of experts doing what they can to save a cherished theory in the face of data that seem to contradict it.” It surely made a big impression on every reader; it was mentioned in several blogs, and in Peter Shor’s Amazon review. And it never happened. It is an example of something that that happens all too often in your book: you have a story that you believe, or want to believe, and you ignore the facts.
You go on to challenge the ethics of string theorists in regard to how they presented the issue of moduli stabilization in their talks and papers. I am quite sure that in every colloquium that I gave I said something that could be summarized as “We do not understand the vacuum in string theory. The cosmological constant problem is telling us that there is something that we do not understand about our own vacuum. And, we do not know the underlying principle of string theory. These various problems may be related.” The cosmological constant and the nature of string theory seemed much more critical than the moduli stabilization problem, and these are certainly what I and most other string theorists emphasized.
This scientific judgment has largely been borne out in time. In 1995-98 these incredible new nonperturbative tools were developed, and over the next few years many string theorists worked on the problem of applying them to less and less supersymmetric situations, culminating in the construction of stabilized vacua. Obviously many questions remain, and these are widely and openly debated. It seems like a successful scientific process: people knew what the important problems were, worked in various directions (a fair number did work on moduli stabilization over the years), and when the right tools became available the problem was solved. As you point out, the stabilization problem is nearly one hundred years old, and now string theorists (primarily the younger generation, I might add) have solved it. You are portraying a crisis where there is actually a major success, and you are creating an ethical issue where there is none.
AdS/CFT duality. You raise the issue of the existence of the gauge theory. There are two points here. First, Wilson’s construction of quantum field theory has been used successfully for 40 years. It is used in a controlled way by condensed matter physicists, lattice gauge theorists, constructive quantum field theorists, and many others. To argue that a technique that is so well understood does not apply to the case at hand, the scientific ethic requires that you do more than just say Not proven! Sociology! as you have done. You need to give an argument, ideally pointing to a calculation that one could do, or at least discuss, in which one would get the wrong answer.
I have given a specific argument why we are well within the domain of applicability of Wilson: there are 1+1 and 2+1 dimensional versions of AdS/CFT, which are also constructions of quantum gravity, and for which the gauge theory is super-renormalizable (and there are no chiral fermions): the counterterms needed to reach the supersymmetric continuum limit can be calculated in closed form – thus there is an algorithmic definition of the gauge theory side of the duality. You could perhaps argue that there will be a breaking of supersymmetry that will survive in the continuum limit, and we could sit down and do the calculation. But I know what this answer is, because I have done this kind of calculation many times (it is basically just dimensional analysis). Similar calculations, for rotational invariance and chiral symmetry, are routine in lattice gauge theory.
As a further ethical point, in your book you state that it is astounding that Gary Horowitz and I ignore the question of the existence of the gauge theory, and you then use this to make a point about groupthink (this is in your chapter on sociology). While you were writing your book, you and I discussed the above points in detail, so you knew that we had not ignored the issue but had thought about it deeply. You do not even acknowledge the existence of a scientific counterargument to your statement, and in saying that Gary and I ignore the issue you are omitting facts that are known to you in order to turn an issue of science into one of sociology. Again you impose your own beliefs on the facts; thus I am reluctant to accept as accurate the various statements that you attribute elsewhere to anonymous string theorists and others.
You raise again the issue of a weak form of Maldacena duality. As you know, it is very difficult to find a sensible weak form that is consistent with all the evidence and yet not the strong form. In my review I have gone through your book and papers and identified three proposals, and explained why each is wrong. Again, you have not acknowledged the existence of scientific counterarguments, but have just reasserted your original point. If your arguments had been made in a serious way, I would expect that you would have given some deep thought to them and be ready to defend them.
There are some interesting points, one of which I will turn to next.
The role of rigor and calculation. Here we disagree. Let me give some arguments in support of my point of view. A nice example is provided by your paper `The Maldacena conjecture and Rehren duality’ with Arnsdorf, hep-th/0106073.
You argue that strong forms of the Maldacena duality are ruled out because Rehren duality implies that the bulk causal structure is always the fixed causal structure of AdS_5, and so there cannot be gravitational bending of light. But this would in turn imply that there cannot be refraction in the CFT, because the causal structure in the bulk projects to the boundary: null geodesics that travel from boundary to boundary, through the AdS_5 bulk, connect points that lie on null boundary geodesics. Now, the gauge theory certainly does have refraction: there are interactions, so in any state of finite density the speed of propagation will be less than 1. (Since Rehren duality does not refer to the value of the coupling, this argument would hold even at weak coupling, where the refraction can be calculated explicitly.)
You have emphasized that Rehren duality is rigorous, so apparently the problem is that you have assumed that it implies more than it does. Generally, rigorous results have very specific assumptions and very precise consequences. In physics, which is a process of discovery, this can make them worse than useless, since one tends to assume that their assumptions, and their implications, are broader than they actually are. Further, as this example shows, a chain of reasoning is only as strong as its weakest step. Rigor generally makes the strongest steps stronger still – to prove something it is necessary to understand the physics very well first – and so it is often not the critical point where the most effort should be applied. Your paper illustrates another problem with rigor: it is hard to get it right. If one makes one error the whole thing breaks, whereas a good physical argument is more robust. Thus, your paper gives the appearance of rigor, yet reaches a conclusion that is physically nonsensical.
This question of calculation deserves further discussion, and your paper with Arnsdorf makes for an interesting case study, in comparison with mine with Susskind and Toumbas, hep-th/9903228. (I apologize for picking so much on this one paper, but it really does address many of the points at issue, and it is central to the discussion of AdS/CFT in your various reviews.) You argue that there are two difficulties with AdS/CFT: that strong forms of it are inconsistent with the bending of light by gravitational fields, and that the evidence supports a weaker relation that you call conformal induction. We also present two apparent paradoxes: that the duality seems to require acausal behavior, and negative energy densities, in the CFT. The papers differ in that yours contains a handful of very short equations, while ours contains several detailed calculations. What we do is to translate our argument from the imprecise language of words to the precise language of equations.
We then find that the amount of negative energy that must be `borrowed’ is exactly consistent with earlier bounds of Ford and Roman, gr-qc/9901074, and that a simple quantum mechanical model shows that an apparent acausality in the classical variables is in fact fully causal when one looks at the full quantum state. Along the way we learn something interesting about how AdS/CFT works.
This process of translation of an idea from words to calculation will be familiar to any theoretical physicist. It is often the hardest part of a problem, and the point where the greatest creativity enters. Many word-ideas die quickly at this point, or are transmuted or sharpened. Had you applied it to your word-ideas, you would probably have quickly recognized their falsehood. Further, over-reliance on the imprecise language of words is surely correlated with the tendency to confuse scientific arguments with sociological ones.
Finally, I have recently attended a number of talks by leading workers in LQG, at a KITP workshop and the April APS meeting. I am quite certain that the standard of rigor was not higher than in string theory or other areas of physics. In fact, there were quite a number of uncontrolled approximations. This is not necessarily bad – I will also use such approximations when this is all that is available – but it is not rigor. So your insistence on rigor does not actually describe how science is done even in your own field.
Background independence. I think people are a bit tired of the who-is-more-background-independent argument, since it seems to come down to definitions. Let me put things in physical terms. As you say, suppose that the strong form of Maldacena duality is true. This would mean that we can consider a box as large as we want – a light-year, 106 light-years, with an arbitrarily small negative cosmological constant, and AdS/CFT provides a complete construction of quantum gravity within that space. This would include: the formation and decay of (nonsupersymmetric) black holes; graviton scattering at hyper-Planckian energies; physically continuous transitions from one topology, through a quantum state with no geometric interpretation, to a different topology; states where a submanifold of spacetime has a noncommutative geometry; states with a variety of apparent geometric singularities, where the physics is nonsingular. All of these, and many others with a variety of geometries and topologies (you can put a lot in an AdS box), and arbitrary quantum superpositions of them, can be identified in the gauge theory, and so are described algorithmically by the duality. It may not include spaces with interesting cosmologies, or with an effective positive cosmological constant. You call this a very weak and limited form of background independence.
Even here you are blowing things out of proportion: your reply refers five times to the “global symmetry algebra,” but almost immediately after the original work of Maldacena, the duality was extended to systems with reduced symmetry, or none. Your own PI colleagues, Alex Buchel and Rob Myers, have made important contributions to this subject, and I note also the series of papers by Hertog and Horowitz on strongly time-dependent boundary conditions.
A second physics point concerns the constraints. It is not that I am ignorant of the conventional wisdom here, I am challenging it. You believe that the large Hilbert space in which the constraints act is necessary in order to describe all possible backgrounds of quantum gravity. No, only the much smaller set of states that satisfy the constraints is needed. The larger space may play a useful auxiliary role, but it is not physical: the universe cannot be in such a state, and observables must keep the system within the physical subspace. So what are these larger spaces for? One thing we have learned, from emergent gauge theory, is that they are not necessary: one can start from a system with no constraints, only physical variables, and the constraints are needed only to describe the classical limit efficiently. We have learned a similar lesson from dualities such as AdS/CFT: these larger spaces are very different in different classical limits, they are not intrinsic to the quantum theory. Thus, all this focus on constraints is putting effort into something that is unphysical and actually intrinsic to a certain classical limit.
Cosmology. I have agreed that we may be far from sharp prediction. However, string theory has played a valuable role in suggesting new ideas. Moreover, the variety of kinds of models being explored phenomenologically is large; it is clear that some of these arise easily in the landscape (e.g. a pure cosmological constant), while others may be rigorously excluded (the constraints of Arkani-Hamed et al, and others).
Regarding the atomic analogy, the long period that I was referring to was the hundred years between the first scientific argument for atoms (Dalton) and the confirmation (Brownian motion). I agree, however, that one should not get too caught up in analogies. No analogy is perfect: in the 19th century there was a wealth of unexplained phenomena from the natural world, while our current era is historically exceptional in that phenomena beyond the Standard Model are so few.
RHIC. You say that quantum gravity is not being used here. But the QCD plasma entropy is being related to the Bekenstein-Hawking entropy, which depends on hbar, and the ideal viscosity (a concept discovered through AdS/CFT, which is now a standard idea in heavy ion physics) is quantum mechanical.
Moreover, I am puzzled by your repeated statement that the evidence supports AdS/CFT describing only classical supergravity. The gauge theory is fully quantum mechanical, so if it contains classical gravity why is this not exactly what we are all looking for: a theory that unites Einstein’s theory and quantum mechanics? This should be of great interest to anyone working on quantum gravity – how does the gauge theory manage to do this? So we look closer, and we find that it’s… string theory! It is clear why you have trouble with this: according to your book, gravity is a theory of principle, which must be understood by seers, while gauge theory is a constructive theory, which can be built by craftsmen. AdS/CFT would then imply that craftsmen are dual to seers.
But seriously, duality does erase distinctions that we make with our classical experiences and vocabularies, because one quantum theory has many classical limits. Thus, quantum mechanics first erased the distinction between particles and waves. QFT dualities erased the distinction between quanta and solitons, which once seemed absolute. Maldacena duality erases at least much of the distinction between gauge theory and gravity. Unexpected perhaps, but those who ignore this lesson are likely to end up as backward-seers rather than forward-seers.
Other physics. Sean has suggested that I comment on the understanding of string theory in time-dependent backgrounds. Here I will give my own way of thinking about this, which is rather particle-physicsy; other string theorists might emphasize different things. If you have the flat spacetime S-matrix, you actually know a lot about curved spacetime, since you can form a very complicated geometry by throwing together a lot of gravitons in a coherent state. From a particle physics perspective, where the goal is to measure the underlying Lagrangian, this is enough: the S-matrix encodes all local physics in curved spacetime. Further, with this effective Lagrangian one can study processes in a fully curved spacetime, as long as the curvature stays below the string scale. One can then list things that are not covered by this: first, cosmological questions like initial conditions and spacetime singularities, and these are indeed open questions and the subject of active research; second, the possibility of an intrinsic non-locality in physics, so that local measurements do not capture everything. The second possibility has been widely discussed: the black hole information paradox gives a strong indication that such nonlocality exists; the black hole complementarity principle, and the holographic principle, are general statements of the nature of the nonlocality; and, the BFSS matrix theory and AdS/CFT duality are very concrete realizations of locality emerging from a nonlocal starting point. Certainly deep questions about the nature of time remain, and I expect that the solutions will build on our current understanding of the holographic principle.
On the UV finiteness of string perturbation theory, the one-line physics proof is that the regions of world-sheet moduli space that would correspond to UV divergences in field theory actually turn out to describe IR physics. The decomposition of moduli space that Zwiebach uses to formulate closed-string field theory is probably the best for seeing this. The IR divergences are described by low energy effective field theory, so the finiteness problem is reduced to the already-solved problem of IR divergences in quantum field theory. This may seem awfully simple, but I have done enough calculational checks of different parts of it to take it seriously.
Ethics and sociology. Coming back to ethics, the principal scientific ethic is that scientists take responsibility for what they say: When a statement is made, to what extent has it been thought through, and appropriate checks and counterarguments considered (and, yes, the appropriate calculations done)? To what extent are known difficulties acknowledged? When a new counterargument is given, is it addressed, and the original assertion modified if necessary? Are facts presented in a clear and direct manner? This is howscientists judge one another. It is clear why this is necessary: science works by the parallel activity of many minds, and it is necessary that information be exchanged in as accurate a way as possible. Given the above discussion, I find your claim to the ethical high ground to be ironic.
Regarding group-think: you interpret the reaction of string theorists to your book as more evidence for your point of view. Rather, I think that much of this is a natural reaction to what many see as a distorted presentation of the facts. Regarding the personal insults, I think that you set the tone here with characterizations such as that quoted in the New Yorker, so it seems like posturing for you to claim the high road when a few string theorists respond in kind. However, I hope that those contributing to this discussion will try to keep to the same reasoned attitude that I have tried for.
Overwhelmingly the concentration on string theory is a scientific judgement, made by a very diverse group of theorists. Look at any of the several dozen most well-known string theorists: my own scientific experiences and tastes, both inside and outside string theory, are very different from any of theirs, just as they are from each other. I think of myself as a theoretical physicist first, and cross over the boundaries between string theory and several other fields depending on what looks important and interesting, as do many others. String theorists can be rather focussed, but they are not as closed to new ideas as you portray. For example, such ideas as holography and eternal inflation were developed outside of string theory, and might have become `alternative ideas.’ Instead they were recognized as likely parts of the big picture.
There is a reasonable concern that younger string theorists, educated in string theory rather than in other fields, might find it harder to cross these lines. Indeed, during the first and second string revolutions, there was inevitably more concentration, as these new ideas opened up a whole range of new concepts and methods. It is a very positive development that new connections between string theory and other areas have developed – heavy ion physics, low energy hadronic physics, LHC physics, cosmology, mathematics, general relativity, and many areas of quantum field theory – and that many young people are taking advantage of the opportunity to cross these lines, and in both directions. This broadening of perspective should be, and I think is, strongly encouraged.
Coming back to the question in the title, I have agreed that sociological effects exist; they must, since science is a human activity. However, when I read your book, knowing the facts, the case actually seems quite weak. To make the case for a strong sociological effect, at each turn you are forced to stretch the facts beyond recognition. On the other hand, when you discuss the science, your overemphasis on the usefulness and applicability of rigor ignores the kind of physical reasoning that physicists actually use in practice with great success, so your are leaving out at least 95% of what makes physics really work.
Thomas D:
You name ’em I read ’em. Not just popular books but papers. Going back 35 years I might add. I don’t demand anything from anybody. Prof Polchinsky is fully able to decide what he wants to do without you defending him using yet another group-think insult. Prof Polchinsky responses in CV are to Smolin book on ‘troubles’, not on LQG. Clearly he feels further debate on this already well-debated subject is useful. The logical next step in rising to the challenge is a book. After all, Prof Susskind wrote a book to promote the delusional illusion of string-inspired cosmic landscape and argued that the next frontier of science is anthropic worship, which Prof Smolin and others disagree. He responded with TTWP book. Interesting that we haven’t heard too much about this aspect of string nonsense since. So if Prof Polchinsky boss at KITP presented the question “Is string theory bullshit?” in a recent conference on ST, a credible response could be a book of the same class as above. Surely, after so many years of string work, so much energy and money invested, a semi-popular book presenting the great discoveries of ST, summarizing the results and presenting a bright future can be written? With thousands of researchers in the field, consuming hundreds of millions of dollars of public money, cannot one authoritative figure stand up to face accountability? (I don’t regard Greene and Randall books as string books.)
Pingback: yet anotherblog » Blog Archive » ΣκατολογικÎÏ‚ αντιδÏάσεις
Dear Amos,
Exactly who thought what and how many worried or were confident is an interesting question for an historian of science. However, I have a strong memory of worrying about this after conversations with string theorists and after reading Witten’s paper hep-th/0106109,written in 2001, roughly halfway through the period in question, 1998-2003 . I can recall lots of people I talked to were worried. In the same period I was giving talks and writing papers about the implications of positive lambda in LQG, partly because they are beautiful and partly to show that LQG could incorporate positive Lambda, while string theory could not. I always made that point in talks on the subject and was never challenged.
But, to answer your question with an example, lets use Witten, whose writing is pretty unambiguous. Here is more of the paragraphs I quoted in TTWP:
“However, an important no go theorem [6,7] says that there is no classical compacti cation of ten- or eleven-dimensional supergravity to de Sitter space of any dimension. This means that there is no classical way to get de Sitter space from string theory or M-theory. By a “classical” compacti cation, I would mean a family of compacti cations in which G (nô€€€2)=n becomes arbitrarily small and a supergravity or string theory description becomes arbitrarily good.
The no go theorem means that this does not exist.
In fact, classical or not, I don’t know any clear-cut way to get de Sitter space from string theory or M-theory. This last statement is not very surprising given the classical no go theorem. For, in view of the usual problems in stabilizing moduli, it is hard to get de Sitter space in a reliable fashion at the quantum level given that it does not arise classically. (For an analysis of a situation in which most moduli can be stabilized, leading in the large volume limit to a nonsupersymmetric vacuum with Lambda= 0, see [8].)
The absence of a classical de Sitter limit suggests that the possible values of N in string/M-theory are sporadic, rather than arising from in nite families, and that there might be only finitely many choices. If the number of choices is finite, I would not personally expect it to be possible to get N > 10^10^100. But de Sitter space with such large N is needed to agree with the most obvious interpretation of recent astronomical data.”
Now it turns out that at least at the semiclassical approximation KKLT work, Witten’s expectations appear to have been at least partly wrong, because N >> 10^500 seems to exist and to be sufficient. Of course, we don’t know whether there is a real consistent perturbative string theory past the semiclassical level of that calculation, so he could turn out to be right in the end.
I don’t know why so much is made of this, as to the extent to which this was a crisis for string theory, it was, at least at leading approximation solved by 2003, by the KKLT paper building on ideas and techniques Joe and others introduced. So this is a strory that comes out positively for string theory and the optimists among the string theorists. As I said before, I told the story to emphasize the implications of how the problem was so far solved.
Lets put it another way. The positive lambda string theory landscape was found in 2003. It has seriously puzzling implications. Were there another way to get positive lambda in string theory the crisis of the landscape might be avoided. But there is none known. Therefore there was no string theory compatible with positive lambda before 2003. But lambda was measured to be positive in 1998. Therefore there should have been a crisis.
The right thing to say,which is not what Joe is saying is, ‘we were very worried that the theory might be wrong, because it disagreed with data, and so we worked hard and solved the problem and saved the theory from otherwise having to be abandoned.” To the extent that string theorists want to claim there was no crisis in 1998-2003 they are implicitly claiming that disagreement with data is not a reason to suspect a theory is not true.
“Most embarrassing observation in physics — that’s the only quick thing I can say about dark energy that’s also true.”
– Edward Witten, String Phenomenology 2005, Strings 2005, quoted in: http://www.itp.ac.cn/Download/spac2005/Kallosh-Beijing.ppt
(The observational evidence for a small positive CC is ambiguous because all that is seen is a lack of expected gravitational slowing of distant supernovae. According to GR, they should be slowing down at great distances, just as a bullet fired upwards will be slowed down by gravity. However, as Prof. Phil Anderson pointed out in a blog comment on Cosmic Variance, ‘… the flat universe is just not decelerating, it isn’t really accelerating …’ – in other words, the simplest explanation is that there ain’t any gravity force between receding masses over great distances. The reason for this lack of gravity is easy to explain with quantum gravity. The exchange of gravitons of some sort causes gravity. When the masses are receding, the received gravitons are redshifted, so they have less energy than the normally do, E = hf.
Lee,
It seems to me that we are still not entirely sure that the dark energy we observe is in fact a cosmologicgal constant with w = -1. It certainly looks like it is today, however in the period you mention this was far from clear. It could easily have been (and still might be) some form of quntiessence, which is why there was no ‘crisis’ in string theory over how to get a postive cc at that time.
Also, it should be mentioned that the basis of the existence of dark energy prior to 2002-2003 was the supernova observations. It was not really on completely firm ground until WMAP reported its results.
Dear Eric,
When you say, “It could easily have been (and still might be) some form of quntiessence, which is why there was no ‘crisis’ in string theory over how to get a postive cc at that time’ you seem to be implying that while there was no version of string theory before 2003 compatible with a constant positive vacuum energy, there were versions of string theory compatible with quintessence, which is a slowly changing but still positive vacuum energy. Is this right? If so, was there a prediction or expectation from string theory before 2003 for positive vacuum energy with some value of w besides -1? If so, then wouldn’t this still give an alternative to the landscape problem?
Thanks,
Lee
Lee,
My main point is that until the WMAP first year results, the problem of addressing the problem of dark energy was not seriously addressed. The KKLT solution came shortly thereafter. The difficulty in obtaining a cosmological constant in critical string theory was well-known and can be understood by considering the graviton beta function, which is just the Ricci tensor to lowest order. Conformal invariance requires that this beta function vanish, which implies a Ricci-flat solution to the vacuum Einstein equations. Thus, a cosmological constant is inconsistent with conformal invariance in strings.
However, there are linear dilaton backgrounds where problem of obtaining postive vacuum energy was solved. See for example, Antoniadis, Bachas, Ellis, and Nanopoulos:
1. Phys.Lett.B211:393,1988
2. Nucl.Phys.B328:117-139,1989
3. Phys.Lett.B257:278-284,1991
Best,
Eric
It was not really on completely firm ground until WMAP reported its results.
I would say that some people are somewhat more skeptic (?)/ critical (?)/ pessimistic (?) than average. See the recent paper by Lieu:
http://arxiv.org/abs/0705.2462
(specially the tables).
Christine
Dear all, There was an interesting discussion on the topic of “A chain of reasoning is only as strong as its weakest step” over the “reference frame”, where LM offered a detailed critique on my point of view (also on other issues), but some comments there were more positive to it. (Of course, I am encouraged by Joe’s rather positive reaction to my point of view on this specific point.)
Dear Lee,
You keep claiming that there was no version of string theory before KKLT compatible with a constant positive vacuum energy.
I beg to differ:
hep-th/0106209 [84 HEP citations]
Silverstein: (A)Ds Backgrounds From Asymmetric Orientifolds
hep-th/0205316 [87 HEP citations]
Maloney: De Sitter Space In Noncritical String Theory
Eric:
So what new assumptions had to be made in ST in order for a positive CC to be viable?? Please try to be specific in lay terms. Thank you.
Dear Eric,
Please tell me if I recall correctly that linear dilaton backgrounds are conformally flat spacetimes, with a special choice of dilaton field linearly related to the time coordinate? If so, then these are very special backgrounds, and the fact that a positive vacuum energy in one frame may be possible did not lead to a solution to the real physical problem of finding a background that could be our universe. If I recall right there were also potential issues with instabilities also in these solutions. So the existence of a a few special and highly unphysical cases like these did not change the situation with regard to the problem of finding a potentially realistic vacuum.
Dear c,
I have basically the same question; I don’t have time to study again these papers, I did read them once, but 1) non-critical string theories, while interesting toys are unphysical and if I recall right there were also unphysical features of Silverstein’s construction.
Again, one would need an historian to do careful research to straighten out everyone’s recollections, but my memory is that untill the KKLT paper there was not believed to be a possibly viable string background that could describe a universe like ours, with a tiny but positive vacuum energy. But given that I don’t have now the time to study these papers, please tell us either that the solutions in these papers were and still are potentially realistic vacuum or tell us what unphysical feature they have.
Thanks,
Lee
Lee, here is my recollection, I don’t think we disagree on the facts, but probably on the phrasing. When the evidence for positive CC became overwhelming (though they were strong hints before) the situation in string theory was the following:
1. They were no known examples of string compactifications with positive CC, where everything was under computational control.
2. BUT, there was no statement (or reason) that there CANNOT be such vacua. In fact such vacua were constructed shortly thereafter by KKLT and others.
3. There was a no-go theorem (Maldacena-Nunez) showing that starting with higher dimensional theory, one cannot obtain deSitter vacua without invoking elements unique to string theory (e.g orientifolds). This statement is still correct…
In addition, I don’t remember anybody in that period (yourself included) claiming that string theory predicts non-positive CC, in papers or books or conferences or in person, quite possibly because that was untrue…
Lee,
I believe that linear dilaton backgrounds and non-critical string theory are one and the same. Sometimes this is called Liouville string theory or, more recently, supercritical string theory. The issue of whether or not any specific realistic worked out vacua have been discussed in the literature is not really relevant to the main point, which is that it was known before KKLT how to get a postive vacuum energy in string theory, which I believe now firmly established. I think that most of this work was in regards to inflation, rather than a solution to dark energy, as the existence of dark energy was not known at that time. More recent work has also been done.
Leaving the question, whether string theory was assumed to predict a non positive CC, to the historians, I as an outsider would be more interested, if the since then constructed string vacua with positive CC today look simple and natural or if they rather look artificial and far fetched. And here I would be most interested to have the opinion of Professor Polchinski first.
YES I would second Mike’s suggestion and add: What new assumptions had to be applied to ST in achieve a positive CC?. In lay terms would be very, very helpful. Thanks.
String theory never made a prediction that there couldn’t be a positive cosmological constant. The correct statement is simply that, for a long time, string theorists didn’t know how to compactify from ten dimensions down to four and obtain a positive cosmological constant. That could have been because there was no such way — and some people speculated about that, and wondered whether quintessence (which is not hard to get in string theory) could be responsible for the dark energy. Or it could simply have been because it was a hard problem, and nobody had put a lot of work into it, since most people thought the cosmological constant was zero.
As it turns out, it was the latter, as many people would have guessed. It’s a difficult problem, because getting a positive cosmological constant requires that we break supersymmetry, and that makes everything trickier, including stabilizing the moduli (pinning down the shape and size of the extra dimensions). But, faced with an unexpected experimental result, people did what scientists are supposed to do, which was to see whether it could be fit into their understanding, or whether they would have to change their theories in some way. As it happens, no alteration in the basic tenets of string theory was required; there are lots of ways to get a positive cosmological constant, sadly all too many.
If there is any deep lesson to be learned here, it’s not that string theorists refused to face up to the failure of their ideas — the ideas were fine, they just hadn’t been developed enough. The lesson is that it’s really really hard to fully understand the physical implications of a rich theory in the absence of direct experimental clues.
“If there is any deep lesson to be learned here, it’s not that string theorists refused to face up to the failure of their ideas — the ideas were fine, they just hadn’t been developed enough. The lesson is that it’s really really hard to fully understand the physical implications of a rich theory in the absence of direct experimental clues.”
The definitive refusal to face up to failure occurred not before, but after KKLT, as some leading string theorists abandoned conventional scientific ethics, which require admitting failure when you are forced to keep making your theory more and more complicated in order to avoid contradiction with experiment, getting farther and farther from making a real prediction. Going on about how this is a success story, that the “the ideas are fine” or making excuses about a “rich theory” is just absurd. This is not in any sense an edifying story about scientific progress, but rather one that most particle theorists recognize as a deeply embarassing one. The effort being made on this blog to put lipstick on this pig is really misguided.
Peter,
At least questions such as the cc and particle physics CAN be addressed within string theory. That by itself is sufficient reason to study it. I do not believe that there is, at present, any other viable framework to study these problems (although I’m sure Lee will argue for LQG). If there is any failure, it is on the part of string theorists to completely understand the theory, not on string theory itself. As Sean points out, it is a very deep and rich theory, and the challenge to understand it completely is a diffiuclt one. However, my guess is that in the end, it will be worth the effort. Of course, we could just follow your path and not even try.
Eric,
I’ve never anywhere argued that people should stop trying to understand string theory. I am arguing that they should stop promoting as a success story something (KKLT and other moduli stablilization schemes) that is obviously a scientific failure and dead end. By doing this they are doing a lot of damage to the public perception of science. I really don’t think Sean, Joe, or the many varieties of Landscape (anthropic or otherwise) proponents have a clue as to how much damage this is causing. Going after Lee over this particular issue is a big mistake, since you’re ending up misguidedly promoting as a scientific success something which has been a huge failure and embarassment.
If you want to defend string theory against its critics, you should really think twice before taking your stand on the issue of the Landscape. It’s a completely indefensible position.
Peter, I doubt that anyone reading this far hasn’t heard our respective positions on this before, but nevertheless: I don’t really care whether the Landscape is a success story or not. I care whether it is true or false as a description of the world.
Gravity exists, and quantum mechanics exists. The correct theory of nature must be compatible with both of them. In the judgment of many people, string theory has a good chance to be that theory, or at least be a step toward the right theory. String theory seems to predict a landscape. If so, it may not be possible to make unique predictions for low-energy particle physics. Or it may be possible, via some correlations in the properties of different vacua or some cosmological selection principle. Right now we don’t know. One way or another, we are going to have to live with whatever is true. The universe doesn’t care whether we can predict the mass of the muon or not.
I’m not interested in defending string theory, telling happy stories, or forcing the universe to conform to my wishes. I just want to do my best to understand nature and tell the truth about it. If the public is interested in following what we’re doing, whatever tentative state our understanding might be in, I’m happy to do my best to explain it. Whether our current ideas are right or wrong, time will tell.
Peter,
In addition to agreeing with what Sean said, I don’t think that I have made any arguments in favor of the landscape. My personal opinion on this is that there will be discovered some vacuum selection principle once we have a fully non-perturbative formulation. Right now, we just don’t have the tools to answer this question, and we won’t unless people work to develop them.
Dear Eric,
The problem i have with your statement, ‘My personal opinion on this is that there will be discovered some vacuum selection principle once we have a fully non-perturbative formulation.” is that people have been saying this since it was clear there was a vast landscape of solutions in 1986. Then came the “second revolution” based on duality and branes which give, if not a fully non-perturbative formulation, at least the ability to calculate in some cases non-perturbative effects. The result was that things did not get better, they got much worse. First, because we could argue from S duality that many of the supersymmetric vacua that are stable at weak coupling are also stable at strong coupling. Prior to that there were many hopes that non-perturbative effects would destabalize the supersymmetric vacua. Now we know that whatever else happens, there is a vast landscape of supersymmetric vacua stable at both strong and weak coupling. This greatly limits the scope of any dynamical vacuum selection principle, because it will not explain why the world is not in one of the vast number of stable supersymmetric vacua.
Then, when fluxes and branes played the key role in stabilizing the moduli and getting us positive lambda, the role of non-perturbative effects was to make the landscape problem worse again. The no go results say that there are no compactifications down to deSitter classically, but when non-perturbative effects are added there are now another vast catelogue of solutions.
So, the trend is that non-perturbative effects make the landscape problem worse, not better. So is there a way out? One is to put a bit more effort into other ways to quantize gravity and unify the interations that may not have a landscape problem. One is to try to reinvent string theory a fresh, completely non-perturbatively. One is to invent approaches to the landscape that do lead to falsifiable predictions-and the only one ever proposed is cosmological natural selection.
Would you not agree that either or all of these three options are more likely to succeed than to continue straight ahead applying to the landscape strategies based on the anthropic principle that have been shown by straightforward logical reasoning to have no chance of leading to falsifiable predictions? If so, then why are string theorists putting much more effort into this than into any of the three options i mentioned?
Thanks,
Lee
Lee,
My statement was that we’ll probably only understood why a particular vacuum is selected when there is a completely non-perturbative formulation, not just adding non-perturbative effects. One could always try to formulate such a theory from first principles; however, I believe the approach that is taken is to work backwards from perturbative string theory. If it’s possible to find a vacuum (or vacua) that reproduces known particle physics, has a positive vacuum energy, and all moduli are stabilized, then this may provide clues to the completely nonperturbative theory. The problem with trying to do this from first principles is that we don’t really know what the principles should be or what this theory should look like. We just know it’s perturbative corners. Perhaps other approaches such as LQG will eventually shed light on this, but in all liklihood string theory will still be part of the picture. So, people should work on what they find interesting and where they can make progress. I don’t know why there needs to be this polarization with respect to string theory and other approaches to quantum gravity.
Eric