The Landscape - For Real This Time
Clifford at 1:51 am, August 14th, 2005A couple of weeks ago I used the phrase “The Landscape” in the title of a post but I was really referring to my garden, and I went on to mention there that I had deliberately chosen a misleading title for fun. Several people would not have known why this was misleading. I’d like to explain what I had in mind. This is also a continuation of the story I began in another earlier post concerning approaches to cosmology in string theory, the subject of the workshop I’m attending at the Aspen Center for Physics.
(Cautionary note: I won’t be able to include lots of details. This is meant to be a light sketch of some of the activity going on in the field, and some of the questions that have arisen, aimed at non-experts. Of course, I invite useful discussion of all levels in the comments.)
I’m going to assume that you’ll recall the discussion from the earlier post, and the sketch to the right. The curve is a very simplified illustration of a very important point. It is the potential energy curve (I’ll often just say “potential”) for one of the scalar fields in the underlying theory, and it is hoped that almost all the scalar fields produced by string theory (showing up as one of the modes of the string, like all particles do in this approach to string theory) have a potential that is a bit like that. (I’ll give you a geometrical picture of what a scalar field is, in a while, if you’re not sure what that is.) There are two key features. 1) It has a nice well in the middle. This is where the scalar will like to settle, if anywhere near the well. 2) The value of the potential at the well (where the particle will settle) is positive.
This positivity is important. Such positive contributions to the total energy of the system will break the underlying “supersymmetry” of the string theory, and give a postitive value for the cosmological constant. (This potential energy of the system is referred to as the “vacuum energy”, being the “ground state” energy associated to universe thus constructed - this is the same as what a cosmological constant is, classically anyway.) We care about both of these because we know that the world is not supersymmetric (see the earlier post for what supersymmetry is) and because it is currently believed (and this may well turn out to be wrong (!) see Mark’s recent post) that our world does have a positive cosmological constant.
I should emphasize at this point that until recently, string theory studies have been mostly focused on models which were supersymmetric, in which case they have vanishing or negative vacuum energy (cosmological constant). A huge amount of knowledge and computational technques have been developed to study such cases. The possibility (and it is still just a possibility) that our universe might have a positive cosmological constant started a big discussion within the field about whether such vacua (solutions of the theory) could be reliably constructed within string theory, because it is very hard to do. I’ve already mentioned in the previous post that various scenarios (such as those of KKLT) were eventually presented for how such vacua could be constructed. The key ingredients are well-known. They are the “branes” (extended objects) of various sorts, and I also talked about those in that post.
Let’s move on a bit. What people are doing in the field now is exploring these constructions using the string theory technology we have available. The problem is that the computational technology is right at the edge of what we can do, and it is not easy to control these vacua. This means that people are still confused as to the reliability of the solutions that have been found, but - as far as is known - the basic scenarios which generate these sorts of solutions are very plausible indeed. Allied to that fact is the realization (pointed out first in this paper and explored and developed further in this paper) that there are very many vacua spanning rather closely spaced values of the cosmological constant. So there’s a lot of choices, basically, and they look a lot like each other. So you might ask “why choose one over the other?”. We’ll come back to that in a bit.
Within the limitations of the techniques that we have for exploring the contruction of these vacua it is now understood that there are vast numbers of these vacua, and a huge amount of them may have characteristics (such as the value of the cosmological constant) comparable to our world.
So I promised in my most recent post that this would have something to do with mountains. Let’s have a closer look at the picture I took up near the Maroon Lake earlier today:
Lets’s think of height as representing potential energy, just like on our earlier sketch. Let’s think of the valley (the surface of the lake, say) as being at zero energy. Then all higher elevations are positive energy, and you see that there are several interesting features. Staying in the valley for a moment, notice that there are several positions which have the same vanishing energy and are neighbours of each other. In other words, you can move around on the surface of the lake and stay at the same height. These are the supersymmetric vacua that we understand best. The degrees of freedom to move around on the surface of the lake and visit different supersymmetric vacua without changing height (no cost in energy) are those pesky massless scalar fields (”moduli”) that I mentioned in the earlier post. Horizontal position is the value of the field - a number. We need need to break supersymmetry and fix the scalars to specific values since we know that must be so from observation of our world.
So we are somewhere at higher elevation. In fact, we want to find a solution that is like a well in our original sketch: it’s the bottom of an isolated valley somewhere up in the mountains. If you squint at the photo, you can see some of them up there in the craggy shoulders of the Maroon Bells mountains. And there are lots of hidden ones. They all have interesting properties, and some of those properties match those of our world. The new results in the field - exploring the Landscape of possibilities (Ah! Now you see where the name comes from!) suggest that there are vast numbers of these which are all very close in characteristics to our world.
This was disappointing to some poeple. This is because there was a hope that string theory might produce a single vacuum which corresponds to our world, with all properties of our world determined by this single solution. In the setup visualized in this hope, all we had to do was study hard and find this solution and thereby understand once and for all why our world is the way it is: The world would be string theoretic, as would follow from the fact that it had popped out as the unique solution to string theory. This was the old “Theory of Everything” story, which I’ve already pointed out on this blog as being frightfully naive, in my opinion (see beginning of next paragraph for why I think so). (It must be said that when Green and Schwarz pointed out the results of certain computational miracles in the early days of string theory showing that these theories -potentially containing both quantum gravity and particle physics ! - were consistent, the enthusiasm which ensued about the prospects of the theory is quite understandable. But we’re a bit more sober these days, and rightly so.)
Instead, a new movement began. The idea began to arise that maybe not everything about our universe is fundamentally computable in string theory. As I’ve said before on this blog, in view of the history of the way science has always worked (new theories take over and extend the range of applicabiltiy the old, again and again) this is not an entirely unreasonable characteristic of any physical theory. I would go as far as to say that it is perfectly fine for us to accept that this might true about string theory while still remaining extremely enthusiastic about it, given its remarkable properties.
But it did not stop there. Still hanging on to the idea that string theory is some sort of “final theory”, some people - most famously, Lenny Susskind at Stanford (who I’ve just noticed has a Wikipedia entry!) - began to combine that idea of lots of solutions (with apparently no dynamical way (physics reason) to choose just one) with the idea that our world would have to be explained by “Anthropic” reasoning (an idea which had been brought into particle physics discussions earlier by Steven Weinberg). Something along the lines of “we live in this particular solution of string theory because it has characteristics conducive to us living in this particular solution of string theory”. Many people just don’t like this approach, and say that it is no longer science. We can talk about that at length, but that’s not really the point of this post. All you need to know is that this is the origin of the Big Discussion that people will tell you is going on in string theory right now.
Sure, it’s a big discussion, but it is entirely overstated that this is the one thing occupying the minds of all string theorists, and that the fate of the entire field depends upon the outcome of this argument. (This is the impression given in publications such as the New York Times, who seldom tell you what’s going on in a field unless there’s a good controversy to present, and also on blogs such as this one.)
So why, you ask, are all these people to whom I alluded not worrying about the issue? Are they just: 1) Out of the loop? 2) Careless? 3) Begging to get their grants cut?
The answer is simple. The discussion is an interesting one to have, and it is good that there are people having it, but frankly it is far too premature to conclude much about anything about these issues. There are several things that we simply do not know about the theory, and several things over which we have little computational control. First, we do not really have a proper non-perturbative formulation of the theory. What I mean by this is that we already know that string theory is not a theory of strings. It is only describable in terms of strings when a certain parameter is rather small. This parameter (called the “string coupling” in this regime) measures how likely strings are to split into other strings or join together to make other strings, processes which can change the physics. We can compute relably when this parameter is small. In 1995 or so it was realised that when this parameter is large, we lose the nice description in terms of strings and other things happen. One of the things that can happen is that extended objects -the branes- become important, and we have not yet developed the techniques for handling all that these branes can do. And that may just be the begining of the story of the interesting physics that can happen. It is this interesting physics that has allowed us to construct these scenarios for constructing the very vacua that have begun this exciting discussion. We must not forget that we have not finished the job that we began - to understand the theory.
One key thing to note about the landscape picture is the lack of control we still have for studying the whole picture. As Steve Giddings pointed out in a seminar yesterday (here at the workshop) that he was giving on some new results in the subject, it is as though the mountains are partly shrouded in thick clouds, and so we cannot see the whole picture. (Greg Moore, in the audience, pointed out that we don’t know if we can understand how to move from one valley to another without going into the clouds either, which, to my mind, further illustrates our current limitations.) Worse than that, there is still the possibility that knowing the entire landscape using current approaches may be of limited use if there is an as yet undiscovered dynamical mechanism which favours one region of the landscape over another. What I’m saying is that by all means we should explore the Landscape and learn as much as we can, but we should remember the limitations of the techniques that we have, and so not use what we find to conclude things too hastily.
Lots of things can happen to make the Big Discussion/Argument all yesterday’s news, and most of them I’m sure I don’t know - nobody knows: that’s why it’s called “Research”. As I said, it is still possible that there is a dynamical vacuum selection mechanism out there which might reduce the huge number of apparent solutions down to a few, or even one. More conservatively, it might turn out that we learn new physics for exploring the solutions (better control of D-branes and other “R-R backgrounds”, as the terminology goes) and rule out several of them, reducing the number to something considerably less dramatic, which might help put the Anthropic philosophy to rest. This would need to be accompanied by a giving up of that need to elevate the theory to being a “Final theory”. If one just thinks of it as just a physical theory, then there is no pressure to make it explain everything.
I suspect that when we understand the theory better we will see that there are good reasons not to expect that the it need be the Final Theory. It is has anything to do with nature, it’s probably just the Next Theory, albeit a very elegant and powerful one which will change the way we think about spacetime and the entire universe. This will still be something to be extremely excited about, even though it won’t be the end of the story.
-cvj
nobody knows: that’s why it’s called “Researchâ€.
I do not follow… the name “re-search” seems to imply to look again into a previous search, so it is already known
Is it because of this that other languages use “in-vestig-ation” ( that translates to looking “into the small traces”, more or less)?
… albeit a very elegant and powerful one which will change the way we think about spacetime and the entire universe
Exactly: we can aim to a final theory given a way to thing about spacetime and the entire universe. Langrange/Hamilton Mechanics was the Final Theory in terms of absolute position and momenta. I would be very happy if we had a Final Theory restricted to thinking in terms of force and matter fields.
I realise it may be asking a lot, but can you give us an idea about how a vacuum selection principle might work? I’m not asking you to propose one :-), but maybe you could give us an idea, even a totally oversimplified idea, about what such a thing might look like in principle? Has anyone made a definite proposal, even an oversimplified one?
Hi Amanda, good question. Well, the examples we know best are in field theory. You have a dynamical principle based on fields. You write kinetic and potential energy terms where the fields are the variables and the principle of extremiizing the action picks us out (classical) vacua. Roughly (there is more to it, but this is enough for this discussion) quantum field theory uses the same tools - say, in terms of the path integral. This is all done on the background of a given spacetime. Now this works - for the vacuum selection issue, and of course things are extremely interesting in field theory too: We’re still learning a lot about both perturbative and non-perturbative physics in that context too, but that’s maybe a different issue from the one you raised.
So you might ask why don’t we just treat string theory like a clever way of writing a field theory of an infinite number of fields (this might also resonate with Alejandro Rivero’s comment above), and so write a field theory for it? Well, this is what is called “string field theory”. This is an interesting and hard area of research. My own view is that string field theory is extremely useful for studying particular vacua of the theory, but still is incomplete as an approach, even in principle. But we don’t have a lot of examples - they are hard to construct- so I could just be plain wrong.
(It turns out that even when you can write Lagrangians in field theory, you miss interesting non-perturbative physics such as strong-weak coupling duality (sometimes to a very different field theory). We know that such dualities exist in string theory, and they even exchange the basic background spacetimes and so relying on string field theory might well be obscuring a huge amount of physics.)
Another reason to suppose that we have to do more than just field-theory-like approaches using the fields of the string is that we’ve defined those fields (either implicitly or explicitly) on a given background spacetime. But string theory contains the dynamics of spaceteime too, and so we need to find a way of expressing the dynamics of all that it describes without referring to it as a string in a particular spacetime with a particular set of modes of vibration (the fields). Otherwise we’ll never be able to see everything it has to say about how spacetimes works in conjunction with everything else. This is the “background independence” problem that Lee Smolin likes to emphasize. (I understand that there are interesting remarks about Lee’s essay by Lubos Motl here.) Lee is right, it is a problem and we’d like to get to grips with it. String theorists are not ignoring it. It’s just a hard problem, and so progress there is slow, but we’ve learned a lot in recent times, such as the holographic principle, which may be useful in that endeavour.
I don’t know if it is clear that we can’t make a lot of progress toward making contact with Nature (and real experiments) without background independent formulations. We might still be able to say some interesting things about our world. (I note that James Graber in the comment thread of the “Two Cheers…” post made an interesting remark along this line.) Actually, I think this is the spirit of what people are hoping to do when the do string phenomenology, or even when they search the Landscape for interesting models. They are using what we already know about how the world looks to do some of the searching by hand, not worrying about whether or not there is a dynamical mechanism for doing the searching that we have not already found. We can’t know in advance whether this is a useful approach or not, and so effert should be put into it, as is indeed what is happening.
Best.
-cvj
Hi Clifford,
You give an excellent and very clear description of the landscape issue, and what I would characterize as the most sensible attitude one can take towards it while still believing string theory has something to do with a unified theory. I’m on vacation in Maine, so won’t take much time to comment here, but a couple remarks:
1. You claim that my blog gives the impression that the Landscape controversy “is the one thing occupying the minds of all string theorists, and that the fate of the entire field depends on the outcome of this argument.” Perhaps it gives that impression, but for the record I agree with people on both sides of this controversy: with Susskind that many string theorists are in denial about the implications of the infinite variety of conjectural vacuum states of the theory (e.g. I think it is not occupying their minds enough), with his opponents that what he is doing is pseudo-science. As for the fate of the field, I do think that if Susskind’s point of view takes over, it’s no longer a science, basically because you’ll never scientifically explain anything that way.
2. One problem with believing that a non-perturbative formulation of the theory will save the day was pointed out by Steve Shenker at the Toronto panel discussion. The best non-perturbative formulation you have is the duality with QFTs a la AdS/CFT, and if this is really a general phenomenon, there’s an infinite variety of string theories, even non-perturbatively.
Hi Peter,
Thanks for the comments. I hope that your time in Maine is good. (What is this word “va-ca-tion”? I must look it up some time.
)
I appreciate the points (1) and (2). About (2), I would say that it is going a bit far to say that it is a problem. That would be like looking in a library for a particular book or type of book, and then if the first good book you find is from the wrong genre, you conclude that since this is the best example you have so far, the whole search is doomed.
Cheers!
-cvj
Good morning, I see the flame wars have not yet started… there seems to be an interesting issue in Amanda’s and Peter’s comments above. We are used to background independence and vaccuum selection in the context of field theory. We have control there over the configuration space, we know then which configuration is a stable classical solution, how states transform into each other, and which is the preferred state in a certain physical situation (though probably not in a cosmological situation).
All this brings about a desire to have similar structures in quantum gravity, including the famous background independence (by now I have worried-looking strangers in the street stopping me to ask about this). But, as Tom Banks (among others) emphasizes, it is not clear we have any right to expect this in quantum gravity, string theory or not. For example in AdS/CFT, it is pretty clear that SU(N) theories for different N are not states of the same theory. There is no physical mechanism for them to explore each other. Tom gives other interesting examples in which the intuition from field theory fails, based on semi-classical gravity only (so it is not string theory specific).
Now, suppose the conjectured vacua are actual solutions of string theory, there are many of them and they are physically disconnected in the above sense, what do you do then?
Also, very nice post Clifford, I enjoyed reading it.
best,
Moshe
Good food for thought, Moshe. Thanks. And thanks for the overall comment about the post.
I should stress again that I’m not pinning my hopes on a vacuum selection principle, by the way. I just don’t think we’ve explored the possibility fully.
-cvj
Moshe, when worried-looking strangers stop me in the street to ask about background independence, can I send them to you?
I think the landscape should be taken seriously, because it might be true. Some people are upset by the possibility, but that seems to be quite beside the point. String theory remains the leading quantum-gravity game in town, whether or not predicts a unique ground state. But as Clifford emphasizes, we’re a long way from understanding the complete picture, so apocalyptic predictions on either side of the landscape debate seem premature.
Sean,
By all means, send them to me, I guarentee I will confuse them even more…
Lots of those issues are not specific to string theory, just that string theory is now developed enough to attempt an answer. It goes without saying that one should see what the theory says without any prior prejudice. Not sure what you mean by “apocalyptic predictions”, that was not the intention of my previous comment.
best,
Moshe
From the point of view of QFT, doesnt the question of
the anthropic principle
wear thin, given a constructible
background from theoretical predictions. In principle,
the exactly solvable model of gravity/standard model,
including additional KK modes can produce we hope
to measure. A finite number of obervables can be used
in a quantum action to produce the background, including extensions as the MSSM, intstantons, and the GR background, which certainly exists. The point being
that instead of the anthropic principle, why not
compute the action and find out the naturalness, rather than spending years debating it.
From Sean:
Someone should make clear (again!) what a deeply problematic remark this is. Here’s why: If the landscape is true, then it appears to dictate a mode of explanation of features of the observed universe that badly undercuts or even demolishes the testability of the entire theoretical framework out of which the landscape came. If you can’t test the theory, ie, if you can’t say under what conditions it could be found to be false, then what scientific substance remains in the assertion that the theory is (or might be) true? At best, it seems to me, one might hope that there are some core truths mirrored in the theoretical framework that gave rise to the landscape, and these truths might someday be embodied in a theory that is truly testable. Clifford appears to be emphasizing this latter point of view.
The apparently eminently sensible statement that nature is just the way it is, and not the way we would like it to be, can be taken too far. Logically, this statement implies that the world could be constructed in such a way that its sentient inhabitants can never find testable explanations for what they observe. It seems to me inescapable that science is based on the faith that nature is not constructed in that way. Perhaps this is what Einstein meant when he said that “she is subtle, but she is not malicious”. Of course one has the right to repudiate this faith, but one should just say so, and not wrap the repudiation in a theoretical superstructure that obscures what is really being said.
It actually behooves me sometimes, when the calculations are straightforward to the extent that only months are involved to achieve the demise of the ‘anthropic principle.’ All to the better.
Chris W said: “If the landscape is true, then it appears to dictate….”
I think you mean, “If the landscape is true, and if the totally unjustified Susskindian presumption that the Universe is *equally likely* to select any one of its elements as a way to be born is also true, then….”
Sean said:
\”I think the landscape should be taken seriously, because it might be true. Some people are upset by the possibility, but that seems to be quite beside the point. String theory remains the leading quantum-gravity game in town, whether or not predicts a unique ground state.\”
What do you mean \”IT\” \”MIGHT\” be true. Can you elaborate what exactly is \”it\”, and what do you mean \”might\”?
The correct wording is the landscape(s), you missed a s, a very big S, since there are 10^500 different ones of them. I do not think it is the correct English syntax to use \”it\” to reference to a group of objects. Maybe by using \”it\”, you mean one particular one, out of the 10^500, that SSTers may be able to uniquely lock down, out of all the 10^500?
There is only one kind of vacuum as far as our observation goes. For any particular one out any of the 10^500 vacuas, there is never a \”might\”. IT either matches our observed vacuum, or doesn\’t. There is no might. There is only two possible outcomes, either all 10^500 vacuas are all wrong, or all 10^500 - 1 are wrong, except for one. And the chance that any unique math rules which keeps only one vacua and eliminate all others, also happen to give the correct vacuum, such odd is diminishingly small.
Now, I know SST already made one prediction which is wrong. SST predicted the existance of gravitons. But gravitons can NOT exist since it breaks the equivalence principle of GR!!!
Just imagine that gravity exists between ANY inertial mass, even the tinest one, even the one attributed to pure energy and not rest mass, like photons. Let\’s say there is one photon of low energy and another one at the edge of the universe. These two photos STILL gravitates! According to the graviton theory they still exchange a certain number of gravitons despite the distance.
What it means is each photons needs to emit a virtually infinite number of gravitons per second, to be able to gravitate with all massed in the universe. How could energy still be conserved if there are infinite number of gravitons? And not to meantion not a single graviton has been observed.
Einstein taught us we really should not consider gravity as a conventional force. But rather, it is a pure geometry effect. The geometrization interpretation is the only correct interpretation of GR. There is no need of graviton to mediate gravity. Therefore any theory that predicts existence of graviton is wrong.
Quantoken
Any theoretical physicist worth the bread and butter he earns knows full well that GR and QM are incompatible and their attempt is to reconcile the two.
So why do they took graviton for so granted, as the graviton, quantized gravity, is exactly where the incompatibility lies! Quantization of gravity field is NOT renormalizable! This is the SAME statement as saying that graviton, if exist, would have to be infinite in number. No particle can have an infinite count, so therefore if you accept the equivalence principle you have to accept that graviton does not exist, Gravity is not a force, but simply the curved geometry of spacetime.
Quantoken
Dear Clifford,
Thanks for your piece on the landscape, which I agree is in a certain sense a very sensible view. At the same time, your stance is deeply puzzling to me. I’d like to take this opportunity to explain why, because I believe this is the core of the disagreement between those who consider themselves “string theorists” and those who like myself, remain outside the “string community”, in spite of doing some technical reserach on string theory.
You seem to reason from an unstated premise, which is that, whatever happens, string theory will turn out to be relevent for the description of nature. Even in your closing, when you contemplate different possibilities, including that string theory is just “The Next Theory” you don’t mention the possibility that string theory will just be not relevent for nature. This is also evident in the rasoning in your paramgraph: “The idea began to arise that maybe not everything about our universe is fundamentally computable in string theory… I would go as far as to say that it is perfectly fine for us to accept that this might true about string theory while still remaining extremely enthusiastic about it, given its remarkable properties.” You don’t consier the possibility that nothing will be computable in string theory because it is not the right theory.
There are two logically possible styles of reasoning about string thoery.
Method A: ASSUME 1) that there is a real non-perturbative theory behind all the approximate caclulations and 2) that it is relevent for nature. Then interpret various results, having to do with dualities, the landscape etc given these asumptions.
Method B: Look for evidence that the two assumptions of method A are true.
One evaluates results very differently, depending on whether one uses method A or method B. There is nothing wrong with using Method A from time to time, so long as the assumptions are made explicit, and the risks that are thereby taken on explicitly acknowledged. One can learn things that will turn out be true about the theory, if 1) is true, or about nature, if 2) is true. But one cannot do science only or even mostly by Method A, no matter how promsing an idea may seem. What I find disturbing in your essay, and in many conversations with string theorists is that they reason by Method A but they do not state expliclty their assumptions. This puts me often in the uncomfortable situation, when discussing with a string theorist, of having to add, “but there is one more possibility, the theory might be wrong.”
Many of us who seem fated to remain outside the “string community” are there becase we approach string theory by method B. We may, as I do, work sometimes on technical problems in string theory, motivated by our hope that evidence be uncovered that will show us whether assumptionss 1) and 2) are true or not. This leads to a different evaluations of results. For example, from the point of view of Method B work aimed to demonstrate the assumptions, such as attempts to prove conjectures like finiteness or the different dualities, is more highly valued than it seems to be by people whose work seems to grounded on the assumption that those conjectures are true.
I should emphasize that we are not being unfair here. Most of those who work on other approaches to quantum gravity and particle phsyics approach our own theories through method B. If you come to the loops05 meeting-and you are very sincerely invited-you will find that we are at least as hard on our own approaches as we are on string theory. Observing both communities, what I see is an overemphasis on self-criticism and in the non-string communities, and too much reasoninng with method A in the string community.
Nowhere is the difference stronger than in the evaluation of the landscape results. From the point of view of method A, we are just following the theory to see where it leads. Since we assume beforehand that the theory is right, this is a worthy project.
But from the point of view of method B, the failure to come up with any method to make falsiable predictions, coupled with the failure to find a fundamental, fully non-perturbative formulation of string theory, both after many years of work by many smart people, count as evidence against asssumptions 1) and 2).
I myself am drawn to the ideas of string theory, and I would be happy if they turn out to be true. But I believe an objective scientist must appraoch an untested theory by Method B rather than by Method A. The reason is that reasoning by Method A can lead to a situationi where a large group of people come to irrationally believe ini the existence of a theory they can neither construct nor test.
Another way to say this is that it is more scientific to work on problems, presented by nature, rather than theories. If we commit ourselves too strongly to theories before they are confirmed by having survivred many attempts to falsify them, we risk wasting lots of time and careers on ideas that turn out, beautiful as they are, to be false.
Another consequence of Method A seems to be a lack of interest in other directions. Someone, perhaps Moshe, said on a blog recently that if there were good results on quantum gravity people would get excited and work on them. If by “people” was meant “string theorists” this is just not the case. There have been a continuous stream of significant, non-trivial results on several background independent approaches to quantum gravity over the last 20 years and the community of people who works on such approaches is growing fast. But we see very few string theorists taking an active interest in any of these approaches. If you think I exaggerate the significance of the results, come to loops05, or look at recent papers by the speakers there. Or just talk to someone in the field.
The problem is that if you reason from Method A, you are bound to over-evaluate results in string theory, and under-evaluate results in alternative approaches, becuase you are already committed to one view being right.
Perhaps you think I am being unfair in characterizing your reasoning in terms of methd A. So let me pose a question, “What would make you give up string theory? Is there a theoretical result, an experimental discovery, or the lack of such, that woudl make you put your considerable talents in other directions?”
Lest you think this is unfair, I know the answer for myself, for each of the several theories I work on, and can happily answer the same question, if needed.
Thanks,
Lee
Hi Moshe,
I think Sean was not referring to anything you said, but the general chatter going around that things are in dire straits. (As mentioned in the main post).
Are any of the strangers that stop you and ask about background independence not worried-looking? Might there be any happy-looking ones? Might be a clue, you see.
Cheers,
-cvj
Hi Lee,
Thanks for your comments. And thanks for your valuable and insightful psychoanalysis of me and “the string community”.
As a member of the scientific community, I tend to use the customs of my people: We confront scientific ideas -when they are ready to be tested- with observations made about Nature, and when they are found wanting we discard them. I thought it was clear to everyone reading this article (and the one it is the sequel to) that this is one such scientific idea. Perhaps I was wrong.
I used the phrase “If it has anything to do with Nature…” in the last paragraph. Furthermore I thought the whole tone of the article was about research into what the correct theory of Nature is, and that this is one approach I was describing - in the second of a series of articles.
In my mentioning one approach to a problem, I did not realize that I have to stop and mention all the other approaches all the time. I just took it for granted that the intelligent reader (having seen the word “research” and other such phrases) can conclude that we don’t know the answers yet, and so we cannot know if we’re looking in the right place.
I’m so sorry if I was not clear.
Let me say it clearly for you and those people out there who you think might be confused:
“Research” is here taken to mean that we don’t yet know the answers. By examining a set of ideas to see if they contain the answers, we are allowing for the possibility that those ideas do not contain the answers.
I do apologize to anyone out there who was misled by my articles into thinking that the act of my describing a particular area of research implied that we already knew that it was the answer - before having completed the research.
I thank you, Lee.
-cvj
Hi again Lee,
I forgot one thing. You’re reminding us in your comments in several places that we are not as honest and hard on ourselves as those excellent people in your community. You mention “observing” our community.
Where exactly do you make these observations? The last time I saw you at the kind of workshop where the really hard work on string theory is taking place -where the kind of self-questioning and examination of the whole endeavour is put into practice- was very very many years ago in the middle to late 90’s in Santa Barbara.
Now, I’m not asking for an attendance sheet from you. Let me be the first to say that I’ve not been to all the workshops. Not even most of them. But then, neither have you. So are you sure that you should be making such pronouncements about the motivations, character, and scientific honesty of an entire community of people based on your “observations”? If so, your clarity of vision is ….. remarkable.
Cheers!
-cvj
Clifford,
Not sure what you mean, but I’ll be sure to check next time when someone asks me about the issue, but cannot explain what they mean exactly.
Lee,
I am glad you correctly remember what I wrote, the context was the claim by Peter Woit that string theorists are not interested in QFT. I was puzzled about this claim, since I think that exisiting research into formal aspects of string theory or of QFT are so interwoven, the two are practically indistinguishable. More generally, I think there is a long history of outside ideas having deep influence on the string community, I hope this trend continues. I will be really happy to have a chat about this when the opportunity arises.
best,
Moshe
Hi Moshe,
Gosh, I meant nothing deep. Just a poor attempt at a joke before my morning oatmeal!
-cvj
I’d like to thank Clifford on the approach here.
I do not want to denigrate the fine conversation being developed here, so providing link on my website from laymen perspective, to hone my understanding with a better model apprehension. Like those being shown here.
I bet it is a real slow process for some like myself.
Lee was, most recently, at Strings 2005, and he does hang out with his stringy colleagues at Perimeter.
I don’t know whether that counts for you, but it does for me.
I also don’t know what the contours of the ultimate theory of quantum gravity will look like. But I do think that it is very likely that, whatever it turns out to be, it will have classical solutions about-which it is well-approximated by a perturbative string theory. (Which is not, necessarily, to say that our world is likely to be well-approximated by a perturbative string theory.)
I could explain why I think that, but, instead, I will throw out what I see as the possible alternatives:
1) quantum gravity is never perturbative, about any of its vacua
2) it does possess perturbative vacua, but the short-distance behaviour is totally-different (non-stringy).
Discuss and enjoy …
Sorry Clifford, tone and any kind of subtelty are lost in this medium, I guess there may be some purpose in those smiley faces after all…
Jacques,
Strings meetings are not the best measures of the activity that goes on in the field. They are reports on results. There’s never much time to have open discussions about what we’re doing, where we’re going, etc. This year’s strings did (admirably and unusually) try to have an official discussion, and that is good. But panel discussions are not what I’m talking about, I’m talking about workshops and smaller meetings, where the real work gets done.
So if it “counts” for you, that’s fine. But I don’t think that going to a few meetings, (big or small) and having some colleauges that do string theory occupy the same building qualifies one to make broad comments about the scientific integrity and motivations of an entire scientific field. Not even most of the “insiders” of the string community would do that!
Cheers,
-cvj
Jacques,
I should say however: Thanks so much for trying to bring actual science back to the discussion, - the actual central intent of my post- rather than sociology and psychoanalysis.
By (1), are you saying that a possibility is that while gravity surely does have a perturbative regime (we’re in it most of the time when we do particle physics -so far-) you’re saying that this perturbative regime is somehow neccessarily classical? That when the theory is fully quantum, it is also neccessarily non-perturbative.
That’s interesting. But I’m not sure how this is to work. Is it really possible to consistently treat weak gravity as classical and not worry (at least in principle, if not in practice*) about the apparent internal contradicition? Or do you have some dynamial means by which the cross-over to strong coupling and switching on quantum mechanics can be done together. I can’t quite make it work, at first thought.
Oh….maybe one could formulate it as a sort of phase transition. Hmmmm….
Cheers,
-cvj
*But maybe that’s your point.
Hi Plato,
Thanks for the commets. And the interesting drawing and comments on your own blog.
Cheers,
-cvj
P.S. (Um, having “borrowed” my own landscape picture which I struggled so mightily to bring back from the wilderness (
) could you please more explicitly credit your source for it on your blog? If all this string theory stuff turns out to be wrong, and the gates are shut at the other “community”, I might try for a second career as a photographer 
)
Clifford, I don’t think you understood Lee’s point. His point was not that you didn’t acknowledge that the assumptions in A could be false but that you reasoned from the assumptions in A whithout explecitly saying so. Reasoning from B about the Landscape brings one to the conclusion that it is evidence that string theory may be a waste of time. This was his point in a nutshell, now could you please address his actual point and perhaps even his direct question to you,“What would make you give up string theory?”
I see I have been unclear.
In the regime familiar to particle physicists, gravitational effects are suppressed by a small parameter, ε=E2/Mpl2. This is uncontroversial.
The question is: what happens when we try to probe the theory in a regime where ε is not small? Is the theory (in some instances) still weakly-coupled, or are we always in a strong-coupling (”nonperturbative”) regime?
I threw out a few possible answers to that question.
I see that <sub> and <sup> work in the Preview, but are wiped-out on posting.
This is irritating.
But, perhaps, by judiciously inserting the missing elements into my comment, you can make sense of what I wrote.
To respond briefly to Lee’s post: to most string theorists, the evidence that string theory is a consistent theory of quantum gravity is overwhelmingly stronger than the evidence in favor of any other approach leading to such a theory. We can debate whether that judgement is true or not (and it’s a worthy debate to have), but if it is true, we should hardly be surprised that most of their effort is spent elucidating properties of the theory, rather than fishing around for alternatives. Fortunately for the field, not everyone makes the same judgements, so work on alternatives does continue.
It does raise the question whether there are any results that would make people suspect that string theory was wrong. Of course it’s a hypothetical question, but I can imagine several results which would convince (or would have convinced, had they been true) most people to look elsewhere. For example, you could imagine showing that there really weren’t any non-supersymmetric vacuum states that didn’t decay in a Planck time. Or, reaching back, Green and Schwarz could have found that the anomalies didn’t cancel. Basically, one can imagine all sorts of no-go theorems for matching string theory to the real world; but our current understanding doesn’t imply any such results, so we should keep plugging.
Even if string theory has nothing to say about quantum gravity in the real world, results like AdS/CFT would still make it worthy of study from a physics point of view.
HI Scott,
The first thing I did (after thanking Lee for his comments) was answer his question. Please read my comment.
Thanks!
-cvj
Hi Aaron,
Yes, this is a point that I (and several others) brought up on the comment stream in the “Two Cheers….” post. It is worth bringing up here again so thanks. String theory seems to be trying to tell us something profound about things that we already know are important in our descriptions of Nature: Gauge theories. So as I’ve said before, that alone is reason enough to explore it. Also as I’ve said before, not to do so can at best be described as negligent.
Cheers,
-cvj
Jacques - I see now! Thanks!
-cvj
Isn’t Method A saying String Theory is like QCD - lots of details to work out, but essentially correct? Perhaps Method B leads to “String Theory is like a lot of the toy QFTs - topological, exactly solvable, in not 3+1 dimensions, etc. - that give us insight into theory, but don’t apply to the real world” ?
I would say that a significant number of scientists (string theorists or not) are using Method C.
Method C: Use both Methods A and B in your daily research.
In other words, I don’t see why these things always have to be boiled down to stark and oversimplified choices.
And I repeat again: In attempting to explain to the non-experts what sort of work is being done in the field I am not required to remind the reader explicitly at every turn that the people involved are using a combination of methods A and B, just like a scientist in any other field does.
Cheers,
-cvj
Could an administrator of this blog add a link (in the sidebar) to a help page on the use of HTML markup in comments? It would nice to know what’s available, and in particular to have help in avoiding use of markup that is unsupported although it is properly rendered at preview time. Thanks.
[Since this is off-topic, feel free to read and delete.]
Jacques, Chris W. We will try. Thanks for your patience.
-cvj
Moshe,
I didn’t claim that “string theorists are not interested in QFT”, (or actually claim anything about what they’re interested in or not interested in). What I did say is that those not yet understood aspects of QFT that don’t have something to do with a putative string dual aren’t studied by string theorists, and that these may actually be the ones that hold the answer to how to get beyond the standard model.
Clifford and Sean,
Here’s a more specific version of Lee’s final question which I don’t think your answers to him address, and which may be related to what what he had in mind.
As long as no one can answer the question of what exactly string theory is, it seems unlikely that one can show that it is either internally inconsistent, or inconsistent with observed features of nature. If string theory is wrong (as an idea about unification), it quite possibly is wrong not because it predicts something that disagrees with experiment, but because it is essentially vacuous (as an idea about unification), consistent with virtually anything. What I’ve found shocking about recent statements by Susskind and Douglas is their apparent unwillingness to give up on string theory (as an idea about unification) even if they don’t have a plausible way to use it to ever predict anything. So, if an infinite landscape of string theory vacua really exists, so vast that it is compatible with anything that any particle experimentalist has ever measured or ever will measure, would a conclusive demonstration of this existence cause you to abandon work on string theory (as a theory ounification)?
Thanks Peter. I did answer that. In summary - You and Lee may have issues with the approach, beliefs and motivations of some string theorists. All I am saying is that there are several others who have different approaches, motivations and beliefs. To ascribe a set of beliefs that you don’t agree with to an entire field and then go on to condemn (in your view) all activity taking place under that banner seems to me to be a bit strong.
As regards the specific physics question: I repeat my answer to that too: I would be very happy with a stringy (or related) model -vacuum if you like- which reproduced everything we already know about Nature and then told me something specific about the results of all known experiments. Assuming those results are confirmed by the experiments, that model then becomes my framework for describing Nature until I get to a new regime where I need a new model. Forget discussions about whether there are other isolated vacua or not. I’m happy to take them one at a time! I already emphasised in my post that I don’t require the theory to give me just one vacuum. It would be nice, but it is not neccessary. You don’t require gauge theory to give you just the Standard Model of particle physics, do you? Do you question the validity of gauge theory because I can construct an SU(17) Lagrangian, whereas nature seems only to care about SU(3), SU(2), and U(1)? I hope not.
What is the problem with this? Has science not always proceeded this way? Why is there this knee-jerk reaction against treating string theory the same way as any other model of the universe?
Now if someone could prove that it is impossible to find within string theory that there is no way that it can connect to Nature….that it does not even have a chance to describe even a small and useful regime of it, then I will drop working on it -at least that application of it…there are others- immediately. But right now, most of the the field is engaged on developing the theory to the point where we can firmly honestly answer that question one way or the other - maybe formulate that proof - rather than subjecting the question to our prejudices, as seems to be more common.
Let’s try to devote our energies to understanding the theory well enough so that we can answer the question. This is what research is about. Also: This does not forbid others from looking at alternatives. If a better one comes along, I guarantee that everybody (more or less) will drop what they’re doing and work on that.
Cheers,
-cvj
CVJ, I really appreciate your patience. I’m hard pressed, however, to think of any branch of science outside of particle physics that is in the String Theory Bind - i.e., there is a huge, plausible theoretical structure with no experimental guidance. So I think Method C applies differently everywhere else in science. I’m imagining writing a little booklet for prospective physics students “How to do Science without Experiments”.
Arun,
Thanks for your comment, but I don’t understand. We are not working in a closed box here. The experimental guidance is a follows:
(1) Every experiment that has been done since the dawn of time.
(2) Every observation that we have made about Nature since the dawn of time.
That’s an awful lot of guidance to be getting on with. It would be nice to have more, but to say “no experimental guidance” is just….. a bit strong, shall we say.
cheers,
-cvj
I tend to think that the group theory generating
the known masses and the possibly detectable
other masses will indicate naturalness of the ‘landscape’.
There seems to be an E_8 there.
now that doesn’t really answer his question especially in the very plausible scenario that string theory can never be tested. You seem to be simply to imply that the question is premature, but what would make you try studying a different path absent string theory eventually becoming testable and then falsified. Ah but I see peter has already phrased this more specific question and you have answered that you would only abandon researching string theory if a better theory comes along absent experimental or theoretical(really old experimentall) falsification. So even if it becomes completely nonpredictive( beyond any doubt) you still wouldn’t try a different way.
Thanks
Dear Clifford,
I should start by apologizing if I gave the impression that I was attacking the integrity of you or anyone else. I have enormous respect for you and in fact for most string theorists. That is why I am driven to try to understand the sources of persistent disagreement, which to me have been painful exactly because I find myself disagreeing so often with people I respect so much.
I do not think that anyone in string theory is reasoning dishonestly, or there is any issue of motive or character. But I do see that there are big differences between how different people evaluate the same set of results. For me, at least, its important to try to understand why we disagree so strongly. This is all I was trying to do.
Let’s take Sean’s statement that: “the evidence that string theory is a consistent theory of quantum gravity is overwhelmingly stronger than the evidence in favor of any other approach leading to such a theory.” To believe this you have 1) to not be among the people who are convinced that any quantum theory of gravity must be background independent and 2) believe that the true facts about string theory are given by so far unproven conjectures such as finiteness, S duality and the Maldacena conjecture. Conversely, if you believe background independence is paramount, and suspect that the failure to prove those conjectures after much work by good people might be because perhaps they are not true (at least in the strong forms needed to make string theory a quantum theory of gravity), then you disagree with Sean’s statement.
There is no reason why we should all agree about everything. But we should try as hard as we can to reach agreement where we can, that is part of being scientists. We are all in the bad situation in which each of us is surrounded by friends who easily agree with us, but faced with skeptics who are very hard to convince. This is not a good situation because we will never have definitive progress until we all agree. Everything I do is done with the hope of reaching that goal. Again, I apologize if I appeared to attack you or anyone else personally.
Thanks,
Lee
Peter,
As anticipated, we are back to that… so I dug up this quote
“One of the main reasons I’ve been on such an anti-string theory campaign is the attitude of string theorists that thinking about field theory is something completely understood, that only fools still think about it, real men do strings, etc…. It seems to me that this attitude has made it very hard for people to work seriously on QFT.”
I remember being puzzled by this, as I have been surrounded for many years by people doing serious work on QFT. I remember giving lots of examples of work done by string theorists on formal aspects of QFT, including but not limited to the AdS/CFT duality. All this work has the common property that it can be formulated (usually after the fact) in field theory language, but the string theory language seems to be the most efficient way of organizing and generalizing this knowledge.
What drew me into that conversation to start with is the claim that there are other interesting corners of QFT which are worth exploring, I am still extremely interested in hearing about them. Ultimately, that kind of discussion would be so much more interesting…
best,
Moshe
Um, having “borrowed†my own landscape picture which I struggled so mightily to bring back from the wilderness (
could you please more explicitly credit your source for it on your blog? If all this string theory stuff turns out to be wrong, and the gates are shut at the other “communityâ€, I might try for a second career as a photographer )
Yes of course I always direct link these images much as I do paragraphs, as I recognize this ownership. I have corrected this particular image as Yours by accrediting it in an update.
I have many pictures like this too, going through Banff.
There is always something nice to be said about the mountain views. “Hills and Valleys” are nice too. Wayne Hu has a nice interpretation of this somewhere:)
Bloggery time seems slow to update, so it will show later.
Scott,
What is this!?
You’re just making up stuff I did not say. Please don’t do that. It is particularly odd (adn kind of funny) to do that in the same comment thread, and within 6 comments of what I said to boot! Please re-read carefully my comment 41. My criteria for evaluating the worth of string theory are pretty standard ones for evaluating any scientific theory.
I think you and some of the other folks might have more fun arguing with someone a much more unconventional or whacky world view. My position is just good ol’ fashioned scientific practice! Sorry about that!
Cheers,
-cvj
Lee,
Apology accepted. Frankly, I’m reacting firmly (but politely, I hope) because I’m a bit tired of this business of people taking a few examples of a given position (taken by a minority of the field) and extrapolating that everyone who works on the subject must have the same view. It’s just not productive.
Yes, common ground upon which we can agree that progress is being made is good to seek and to find. Hopefully, the best measure on which we can agree will not be cleverly written essays from either side of the debate, but actual scientific results. There are several concrete results representing significant and unambiguous advances in understanding string theory, and its possible relation to Nature, and I’m sure there are such results in different approaches - such as yours - too. Let us celebrate that and move on with our programs, learning from each other when we can.
Best,
-cvj
I have never really understood the logical path from “landscape” to “can’t predict anything, isn’t really science.” As discussed before in the “Two Cheers” thread, I think the proper analogy is string theory:vacuum state::quantum field theory:standard model. If the landscape exists, then we have to actually go out and measure the parameters of the low-energy effective theory, rather than calculating them from first principles. Who cares? That’s exactly the situation we’ve always been in with field theory. Of course it makes the “theory of unification” promises hard to keep, but I never cared about those promises anyway, I cared about quantizing gravity.
Let’s imagine we did hard work on string theory and found two things: (1) a complete, background-independent non-perturbative formulation of the theory, and (2) a jillion vacuum states, one of which looked exactly like the Standard Model (or the MSSM, or whatever we knew about particle physics at the time). I would call the combination of those two things “a scientific theory that fit all the data”; how could anyone disagree?
Of course, if we have another complete quantum theory of gravity that also fits in with particle physics, but makes different predictions for Planck-scale phenomena, we would want to think of ways to distinguish between them experimentally. We should be so lucky.
Yep, I agree, Sean. See my comment 41.
It does seem that people run along that path pretty rapidly, and then start yelling. I don’t get it. The point is (in my mind, at least) that the possible* existence and validity of a Landscape of vacua has nothing to do with the Anthropic debate….that is a separate issue. See comment 41 folks.
-cvj
*possible: too early to tell. More research needed. See my original post/essay at the top of the page.
Lee– you’re completely right about the attitudes toward what is a promising theory of quantum gravity, of course. Different people disagree in good faith about which problems are fundamental and which clues are worth paying attention to, which is a good kind of disagreement to have. I would just add that one can be fully convinced that background independence is a good thing to have, without thinking that starting from background independence is the most fruitful route to pursue. Personally, I think those features you mentioned — finiteness being the most obvious — are incredibly powerful evidence that string theory is on the right track. (Even if they haven’t been proven rigorously, which things rarely are in field theory.)
As I explained in this post (attributing the remarks to Howard Georgi), it is a nontrivial statement that a putative “theory of quantum gravity” can predict anything about Planck-scale physics.
A further elaboration of the difficult hurdles faced by alternative “theories of quantum gravity” is sketched, but not elucidated in comments 23,29 above.
Moshe,
I hope my later comments clarified that in what you quote by QFT I was referring to those aspects of QFT not accessible to study via a string dual. As an example of what I have in mind, in 2d much is known about how to understand certain QFTs in terms of representation theory of infinite dimensional groups, little is known about whether anything like this works for 3d or 4d theories. But going on about one’s favorite ideas in a comment thread to a posting on a very different topic is not a good idea….
Clifford,
In my comment I was just trying to ask you a very specific question, not to ascribe beliefs to or condemn a whole field. My reference to Susskind and Douglas was specifically to my own interpretation of certain statements of theirs, and maybe that’s even wrong.
I don’t seem to have made clear the specific question that I’m trying to get an answer to. What I’m trying to understand is whether you see the same danger in the existence of the landscape that I (and many others) see, and what your reaction to it is. What if the landscape is so vast that it contains an infinite class of vacua that agree with the standard model, and this infinite class is so vast that no falsifiable prediction about beyond standard model physics can ever be extracted from the string theory framework because of it? Do you see a danger that string theory will end up being a theory that is not falsifiable in the conventional way that physical theories have been up until now, because the landscape can describe anything? If this turns out to be the case, would you abandon string theory (as an idea about unification)?
As for the analogy with gauge theory: the standard model gauge theory is a rigid structure specified by a relatively small number of discrete choices and continuous parameters. Once this small number of things is specified, an infinite number of predictions follow and the theory is highly falsifiable: if an experiment finds a result that disagrees with the standard model, the only way to save the gauge theory framework is generally to dramatically increase the complexity of the theory, and you can’t do very much of this before you have to admit to defeat.
String theory is different in that simple compactifications don’t work, and one seems to have to use a very complicated compactification scheme even to reproduce the small number of discrete choices that partially specify the standard model. The danger of working with such complex schemes is that you may be able to get anything you want by just making things ever and ever more complex. One could imagine an alternate universe in which this was what happened when you tried to use gauge theory to explain particle physics, but if it had happened I think people would have quickly abandoned gauge theory. Will people abandon string theory if it becomes clear this is the way things are going?
My last comment crossed Sean’s, but I think it explains exactly what is wrong with the
string theory is to vacuum state
as
quantum field theory is to standar model analogy.
I also don’t think the “I only care about quantum gravity, not unification with particle physics” point of view is viable. I’m less optimistic than Lee that any of us will ever see any experimental evidence relevant to quantum gravity in our lifetimes. By the current “consistent theory of quantum gravity” standards of string theory, it is looking like there’s an infinite number of them. If these theories don’t say something about particle physics, there is no way for us to tell which if any of them have anything to do with the real world.
Peter,
Duality a la AdS/CFT is just one of the directions in understanding QFT, others are studies of holomorphic quantities in SUSY gauge theories (which Seiberg-Witten is the most famous example), relation to matrix models and topological strings, twistors and perturbative YM, and the list goes on. I would really characterize the study of (mostly SUSY) gauge theories as one of the major themes in string theory in the last decade.
As for your example, in case you are not already aware of this, there is some talk about infinite dimensional algebras in N=4 SYM (both at weak coupling and via AdS/CFT at strong coupling). One example (apologies to all the others…) of an interesting paper on this is hep-th/0308089.
best,
Moshe
Peter’s comments reveal the crucial difference of viewpoints. If you think of string theory as primarily an attempt at unifying the forces and going beyond the Standard Model, of course you will think the landscape is a disaster. I don’t think of it that way; I’ve always thought of it as a quantum theory of gravity. As Clifford points out, there is plenty of experimental evidence that we need quantum gravity; e.g., the fact that the Earth goes around the sun.
Per Jacques’s comment, one can replace “theory of quantum gravity” with “ultraviolet completion of the Standard Model coupled to general relativity” as one wishes.
Clifford, You answered peter’s more specific version of Lee’s question
So you will only start researching something else if string theory is somehow proven wrong or somebody makes up a theory you like better, but( unless you simply forgot to mention this third thing) if it is at some point shown to be completely unpredictive you don’t say you will start looking for another avenue of research. So either you failed to answer peter and my question about whether you would abandon string theory if it became apparent that it was completely vacuous or my previous comment was not out of place and was an accurate depiction of what you said. So if I am wrong please feel free to actually answer the question of what you would do if it string theory does end up to be essentially vacuous.
Hi Peter,
Thanks. I would still say that the criteria for determining whether to stop working on the theory or not are as stated in comment 41. I’m having trouble imagining how the scenario you outline is qualitatively differerent from the example of gauge theory and the standard model. It seems to me that it is just a matter of degree. But I allow for something odd happening, in my final paragraphs below.
First, let me state the “matter of degree” scenario. Let us say that the Landscape picture is true and you have no way of ruling out lots of solutions: there is no nonperturbative principle, say. (I’m obliged to say: This is a hypothesis for this paragraph, it may not be true!) So you find a model (stringy or otherwise) which fits all current experiments, possibly using measured data to fix some uncalculable quantities (the horror!). Or lets say you find N such models.
Then one day you do an experiment that gives some new data. If M of them don’t fit the data, you have N-M candidate models left. You refine your predictions for the next generation of experiments. If all N of them do not fit the experimental data, you throw them all out. You’ve falsified the theory. Find a new framework.
In this program, I don’t care how big N is. Even if it is infinite (in some enumerable sense) that’s ok. I can write down an infinite variety of Standard models right now, within the gauge theory framework. It does not invalidate the process of studying gauge theories and making predictions about the results of scattering experiements.
Ok, like I said, if its not a matter of degree, then I guess it is something like this. If the structure of the landscape (if it exists) is so perverse that we can’t unambiguously carry out the above scheme (I guess that’s equivalent to saying that N is uncountably infinite in some horrible sense? Whatever.), then it is indeed not a workable theory. I would then not* work on it. Big deal. Most people have no trouble saying this. It’s not like you’re getting me to admit to some horrible attrocity or something.
The point is that until we’ve done the research, we don’t know. None of us have a clue, I stress. We’ve none of us seen a theory like this before, that’s what is so exciting! Sure, we can imagine lots of horrible, unexpected, and contrived things taking place in this world, and get ourselves scared by them. Most of them don’t happen.
You mustn’t condemn a scientific endeavour on the basis of your fears about the things that might go wrong to spoil the outcome.
So let’s do the research and see what really happens.
Cheers,
-cvj
(*original version, amusingly, had the “not” missing…. but it is clear from the rest of the paragraph that I meant it to be there. Thanks Scott!)
Scott.
I’m puzzled again by your definitons. My mistake. In my world, to prove that a theory is unpredictive is the same as proving that it is “wrong” in some broad, but clear sense. So I did not include that possibility explicitly since I did not think of it as distinct enough to mention. My mistake. I thought that again this is standard in science. (God, I’m so old-fashioned it seems!)
Ok here goes, in short. I’m throwing down the gauntlet:
You nay-sayers out there - Here is the challenge: Work carefully and hard on string theory (please! we need the help! it’s hard!) and unambiguously prove that it has nothing to do with Nature, either because it predicts wrong answers, or because it can predict nothing. This is the best way to stop people working on it, if it disturbs you so terribly that people are working on it.
This is the way science has always proceeded.
Otherwise, please do not stand in our way because you have some idealogical reason to dislike the theory.
Best,
-cvj
Sean,
Obviously the mechanics of the world are expected to be consistant, and we need to find a new theory which encompasses both QM and GR but that does not automatically mean that the way to do this is by quantizing gravity and so even though that is the most common approach tried the need for consistancy does not demand quantum gravity.
Clifford, I don’t know if I am being dence or not, but you tell me that the completely vacuous scenario would be included in you proven wrong scenario by some strange twist of english while you tell peter.
Did you mean to type “would then not work on it” or do you somehow consider this scenario to not be the equivalent of unpredictive.
thanks in advance for the clarification
I don’t think I will accept your challenge though as I figure the other path of trying, and most likely failing, to make a better theory would be more fun.
Whoops! Typo!
Thanks….. (”Freudian slip!” They’ll all shout!)
-cvj
P.S. “strange twist of english”. We’re doing science here, not english. The sense is clear. I know of no “correct” theories that are inherently unpredictive. Do you? Or would you like to use Pauli’s “not even wrong” category, which inspired Peter’s blog’s name? Ok. Go ahead.
Moshe,
My mistake to try to say something simple about the issue of the work done at the string theory/QFT interface. It seems to me an extremely interesting, but very complicated subject, much worth discussing, but highly off-topic here. You’re right much of this is about SUSY gauge theories, and one quick comment about them. I’ve always found the subject extremely frustrating, there’s something very right about these theories (for one thing, the mathematics is just fantastic), but there’s also something very wrong about them (the MSSM is hideous). Something very interesting is going on there, but I don’t know what. Thanks for the reference, that’s a topic I’ve never looked into, but should.
Somebody please answer the question “Do gravitons gravitate amoungst themselves“?
According to conventional wisdom, gravitons certainly carry energy. And energy is mass, and therefore gravitons must gravitate amongst themselves, if the Equivalence Principle is still correct.
So gravitons must emit gravitons to exchange gravity force amongst themselves. Then the gravitons emitted by gravitons must also gravitate and therefore emit their own graviton, and it goes on and on, leading to a picture of infinitely many gravitons, many with incredibly small energy. But there are infinitely many gravitons nevertheless.
It then contradicts the Bekenstein Bound, if you consider the entropy represented by all the possible states all those gravitons can sit at.
To resolve the paradox, you must thus conclude that gravitons do NOT exist, and gravity is NOT a force exchanged by bosons, but merely spacetime geometry effects. And certainly, SST gets it all wrong by predicting graviton, something that does not exist.
Again, answer the damn question: “”Do gravitons gravitate amoungst themselves“?!
Quantoken
Clifford,
Sorry for harassing you into stating the obvious that once one has shown a theory is unpredictive, it’s wrong (or not even wrong…) and one has to give up on it, but I think this discussion was worthwhile, it certainly helped me clarify some things for myself. And it’s helpful to see that we share fundamental criteria for evaluating science. I’m afraid that I sometimes share what I take to be Lee’s perception that for some string theorists, the possibility that the idea of string-based unification is just wrong seems to be something they won’t even admit to be a possibility.
No, I’m not going to take you up on your suggestion and devote myself to working on string theory. There are already many, many smart people doing this, and they appear to me to be doing a good job of slowly accumulating evidence that the string theory unification idea doesn’t work. I don’t think I could significantly speed that process up. I’ll stick to pointing out what other people have already found, and trying to develop what seem to me to be more promising ideas.
The main thing this clarified for me is the whole issue of falsifiability. You and Sean are right that it’s a good idea to think about the analogy between the gauge theory and string theory frameworks, although I draw different conclusions from this analogy. I guess I do think that the difference is one of degree, but that differences of degree are crucial. Whatever theoretical framework one has, one can generally find some way of making it fit the facts. If it’s a good theoretical framework it’s easy, if it’s not you have to engage in all sorts of ugly contortions. Thus, in evaluating theoretical frameworks, a sense of aesthetics is crucial, and claims like those that Susskind is making that it doesn’t matter if things are really ugly are dangerous. I’ve been thinking a lot in recent years about this kind of “aesthetic” issue, and the connection to falsifiability is something I hadn’t thought about before.
Peter,
I think this thread has just been fantastic! We’ve actually had a really productive discussion, and agreed that we’re all still doing science, and that several things are worth doing. We did it without all getting on our high horses, or getting abusive…..
For the non-experts, I think that our discussion may have served to clarify a lot of the mixed messages that have been put out there about what string theorists are doing, what they hope to do and also served to show why they’re just part of the whole tapestry of good science being done.
This is all just great! Let’s continue to talk!
Thanks all.
-cvj
Yes.
Gravitational waves scatter (”gravitationally”) off each other in the classical theory, and that