What is String Theory?

Yesterday Joe Polchinski gave a lunch-time talk at the KITP on the topic of What is String Theory? No answer to the question, but he provided an outline of three topics being discussed at the current KITP workshop program that have something to do with it.

  • String field theory: he wrote down the Witten open-string action and advertised that as the best candidate for a definition of string theory that could go on a t-shirt. He noted some of the problems with this, especially how to understand closed strings, which are somehow “emergent”, “hidden in the measure” on string field space, which one doesn’t really understand.
  • The Berkovits pure spinor formalism for quantizing the superstring: if you want a consistent theory, you need supersymmetry, and Polchinski explained that the quantization of both supergravity and the superstring are ferociously complicated subjects. He hopes that the Berkovits formalism will provide a more lucid (perturbative) quantization of the superstring, one allowing a proof of finiteness at higher loops. This topic doesn’t really address the “what is string theory?” question, since it is supposed to be equivalent to other ways of quantizing the superstring, and only valid perturbatively.
  • AdS/CFT and integrability: here there’s an answer to the “what is string theory?” question, but it’s in some ways a disappointing one for the idea of a single string theory that unifies everything and goes beyond QFT. If you believe the full gauge/string duality speculative framework, there are lots of string theories, each of which is defined by fiat to be a certain QFT. If this is right, perturbative string theory is just a tool useful in the study of some strongly-coupled QFTs, and non-perturbative string theory isn’t really a subject distinct from QFT. If you want to unify physics starting from thinking about the SM, at short distances you have a weakly-coupled QFT, with no role for string theory. And, in this picture, there are lots of string theories…
  • At the end, someone asked about the LHC and supersymmetry, Polchinski responded that string theory didn’t require LHC-scale supersymmetry, but if supersymmetry was discovered at the LHC then there would be a “sociological” effect encouraging to string theorists. I also noticed recently that Polchinski has a web-page On some criticisms of string theory.

    In his discussion of the pure spinor formalism, he noted that supersymmetry doesn’t seem to “resonate” with mathematicians, but that pure spinors are more something they recognize. This is certainly true, with supersymmetry something frustratingly close to some standard mathematical constructions, but quite different in other ways. Pure spinors occur naturally when one tries to construct spinors geometrically. Projectively, the space of pure spinors is SO(2n)/U(n), a space which has some quite beautiful properties. In the Borel-Weil geometric construction of representations, spinors are holomorphic sections of a line bundle over this space (for details of this, see the chapter on spinors in Loop Groups, the book by Pressley and Segal).

    For the superstring, one is interested in the case of n=5, and a certain sigma model with target space the space of pure spinors. There’s a more general class of sigma models of which this is a special case, and for more about some of the interesting connections of this to other subjects, see the recent KITP talks by Nekrasov and Frenkel. The Frenkel talk is especially interesting, since it involves several other quite beautiful related ideas. He describes one motivation for studying some of these sigma models that comes from geometric Langlands. While he was at Santa Barbara, Frenkel also gave two nice survey talks about geometric Langlands, see here.

    Update: Clifford Johnson explains here that not only do we not know what string theory is, but we can’t even say anything useful about what it isn’t, other than “it is not a theory of strings”. The problem with this situation, according to him is:

    people who don’t know what they’re talking about, and sometimes with an axe to grind, shouting loudly (and sometimes deliberately misleadingly) about it.

    Update: More thoughts from Clifford on the question of how to deal with string theory critics.

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    49 Responses to What is String Theory?

    1. Sumar Ongi says:

      He hopes that the Berkovits formalism will provide a more lucid (perturbative) quantization of the superstring, one allowing a proof of finiteness at higher loops.

      Can one infer from this that, in Polchinski’s view, there’s no complete proof of finiteness yet?

    2. Peter Woit says:


      He was specifically asked about this, and did not claim that a complete proof of finiteness exists.

    3. Alberto G.P. says:

      Like some banks, do you think string theory is in crisis, today? What is the opinion of the main theoretical high energy physicists?

    4. Peter Woit says:


      I think the analogy with some banks is apt. One part of the U.S. financial crisis seems to be the fact that some of the largest banks are “zombie” institutions, insolvent but unwilling to acknowledge this and draw the necessary conclusions. The idea of string theory unification is also bankrupt at this point, but being propped up because it would be very inconvenient for many people to acknowledge this and they don’t see any palatable alternative to soldiering on. One difference is that the banks could (and probably will) have their insolvency problem fixed by pouring in lots of government money. This won’t work with string theory.

      Among “main theoretical high energy physicists” you’ll find a wide spectrum of opinion about string theory, from strong defenders to those even more unhappy about its state than I am. One thing that I think is undeniable is that the spectrum of opinions has shifted in recent years toward a much more skeptical take on string theory’s prospects.

    5. Coin says:

      Is String Field Theory still an area of active work and development? It seems when I try to look up material on string field theory I mostly find stuff from the 80s to late 90s (Witten’s work in the area seems to come from this time period as well if I’m not mistaken?).

    6. chapieau says:

      Recently I have got the feeling that string theory is going to become a purely mathematical subject. It could be condensed in two “Theorems” to be proved:
      1) Under very general assumptions every reasonable field theory can be embedded in a string theory which makes its UV (high energy) dynamics consistent.
      2) Maldacena conjecture shows that both the problem of “solving” string theory and the problem of strong coupling of gauge theories are equivalently complicated. Solving one solves the other.
      Waiting for some brilliant mathematician to demonstrate the theorems, phycicists are going back to (LHC) phenomenology and toward bottom-up approaches

    7. Kea says:

      One assumes that Clifford’s people who don’t know what they are talking about includes everybody that is not a respectable string theorist, which is to say that it includes nobody who disagrees with the basic stringy party line. This includes a rapidly increasing number of very respectable physicists from other areas of physics, most notably experimental. But of course, they don’t really know what they’re talking about because they have not really studied string theory. Heh, maybe if they did they would appreciate the necessity of forgetting about contact with experiment, and then appreciate the beauty of the mathematics, and be thankful for their now superior understanding of the fundamentals of physics. Yeah, right.

    8. Tony Smith says:

      Peter, you said:
      “… Projectively, the space of pure spinors is SO(2n)/U(n) …
      For the superstring, one is interested in the case of n=5 …”,
      which sort of confuses me.

      As to the math (not so confusing), Reese Harvey (in his book Spinors and Calibrations) says
      “… the set Cpx(n) of orthogonal complex structures on R^2n has two connected components Cpx+(n) and Cpx-(n), with
      Cpx(n) = O(2n)/U(n)
      Cpx+/-(n) = SO(2n)/U(n)

      Harvey calls O(2n)/U(n) “the twistor space (at a point on a manifold), i.e., the twistor fiber”,
      and says(I am using “n” instead of Harvey’s “p” here):
      “… Let PURE(n) .. denote the set of all pure spinors …
      consider the complex case …
      PURE_C / C* = Cpx(2n) = O(2n)/U(n)

      The square of a pure spinor represents the associated null plane in
      /\R(n,n) …”.

      In the relevant case of n=5, it seems to me:

      O(10)/U(5) which has 45 – 25 = 20 real dimensions is the twistor space,
      and it has two components (for the two mirror image half-spinors of Spin(10)) that each are SO(10)/U(5) with 45-25 = 20 real dimensions.

      Since PURE_C /C* = O(10)/U(5) has 45-25 = 20 real dimensions,
      it has 10 complex dimensions.
      Since the C* of complex scalars is 1-complex-dimensional,
      PURE_C has 10+1 = 11 complex dimensions.

      Since Spin(10) half-spinors are 16-dimensional,
      11 of the 16 dimensions of Spin(10) half-spinors are PURE (in the real context).

      That is consistent with the table in vol. 2 of the book Spinors and Spacetime, by Penrose and Rindler, that lists in table B.65 on page 453 for the even dimensional vector space n=10
      the dimension d_10 of the space of pure spinors as 11
      and the dimension of half-spinors as 16.

      If my understanding stated above is roughly correct,
      then it seems to me that the math/geometry is quite beautiful,
      what really confuses me is how it is applied by superstring people:

      Do they use the SU(5) as a GUT ?
      If so, how do they deal with proton decay experimental results ?

      Do they use the twistor space as generalized Penrose twistors ?
      If so, how do they deal with massive fundamental particles ?

      Do they just ignore concrete stuff like proton decay and particle mass,
      taking the Clifford Johnson attitude “We’re still working on it” so that their highly trained minds won’t be corrupted by such messy detail stuff ?
      (That reminds me of the HHGG where Vroomfondel said
      “… our brains must be too highly trained …”.)

      Tony Smith

    9. Walter Mondale says:

      Dude, you’re obsessed with string theory. Ok, we get your point. It’s unprovable. But man, do you have to spend day after day, week after week, year after year, going on and on and on about this? I mean I can see a scientist being really into his research, but this is a scientist being really into “unresearch”. You are just gushing about how a theory is not satisfactory. I’m not even saying you’re wrong… but.. don’t you ever get tired of this and think, maybe I’ll go take up a new hobby like sailing or something….

    10. anon. says:

      ‘… don’t you ever get tired of this and think, maybe I’ll go take up a new hobby like sailing or something …’

      Sailing is a good way to be alone with chicks for a few hours, but you’re missing the basic string theory point. The flaw in string theory is not just ‘it’s unprovable’ as you claim to grasp. The flaw is that it’s not falsifiable. You can’t prove models true in general, they are just approximations. E.g. quantum electrodynamics can’t be proved, instead the theory survives experimental checks which have the capacity to falsify theories. The crisis in string theory is caused by the fact that it has been hyped as a self-consistent quantum gravity and unification theory without even any experimental support at all, never mind proof.

    11. somebody says:

      Alberto, look at the title of this blog, man. What do YOU think Peter thinks about the “crisis” in string theory?

      Coin, in the last decade, there were two developments in string field theory that I consider “major”. One was from Ashoke Sen’s beautifully simple idea of tachyon condensation, which gave us the basic conceptual understanding of SFT that we have now. The second is Schnabl’s solution of the closed string theory vacuum, which is more technical, but has created some powerful new tools and has made previously intractable questions within reach.

      anon., Walter wasn’t talking about string theory, he was talking about Peter’s obsession with it. So it is not fair to say that he is “missing the basic string theory point”, whatever that might be.

      I have one quibble about Polchinski’s talk – it seemed to focus entirely on the big-machines of formalism. Perhaps he was outlining the topics in the KITP program schedule.

      Either way, he is looking for a way to answer the real question “what is string theory?”, and not give the audience an exposition about “what is string theory (currently)?”. This seems to have been lost on some of Peter’s fans.

      One interesting point I was reminded of in his talk was that there seems to be no simple way to derive 4D supergravity.

    12. Peter Woit says:


      You have the geometry right. The twistors here are different than Penrose’s, they’re the 10d analog of what he is doing in 4d.

      The SU(5) isn’t the SU(5) in GUTs. This story is all about the basic question of quantizing the 10d superstring. To get GUTs or any particle physics you need to add more structure (compactification, for one thing).

    13. Walter Mondale says:

      Um, duh yes I know it’s not disprovable either. This is given a subtle hint in the title “Not Even Wrong.” Don’t talk down to me, you spoon. The point is that spending years blogging and writing books about this subject is excessive. At this point in time, is there really anything else to add? Sure, there’s some new conference or lecture by some renowned genius. But does it really truly add something new to the debate to point out for the 3,209th time how the latest development is still not satisfactory, because this too can’t be proven or disproven via experiment? I mean he lives in NEW YORK CITY for crying out loud, there’s so much to do. He could be getting laid every night. But no, instead he does this….

    14. Peter Woit says:


      Odd, there’s nothing in this posting about the proving or disproving string theory by experiment. You seem to be quite obsessed by this.

      Also curious is how many people are concerned that my writing critical blog entries about string theory is interfering with my sex life (for instance, I recall the immortal words of Nima Arkani-Hamed as reported by Lubos “What’s wrong with these people? Why don’t they choose f***ing instead of writing about things that they don’t like and they don’t understand?”). Thanks for your concern, but rest assured, your worries about this are misplaced.

      One not-so-obvious thing I should perhaps point out is that, especially after you’ve been doing it for a while, writing blog postings takes very little time. This one took less than a half an hour. Significantly more time was taken up by watching Polchinski’s talk on-line, but it was an interesting and clear summary of the latest thinking on some quite technical issues, so definitely worth the hour or two I spent on it.

      I’d be very glad if someone else would take over the job of providing a counterweight to the huge amount of hype and misleading promotion of string theory going on, which is doing continuing and serious damage to particle theory. For now it seems worthwhile to devote at least a few hours a week to this project, since the damage to my sex life is minimal. Much more serious actually is my recent fascination with reading financial news and following the ongoing economic crisis. That I’ve really got to stop….

    15. Syksy Rasanen says:

      The comic posted by Clifford is amazing. The fact that it is promoted by someone who identifies with the ‘bitch-slapper’ is symptomatic. Peter, nothing you write about ‘string theory partisans’ could possibly give a worse impression than this – this is self-parody of the highest order.

    16. yoyoq says:

      walter, what is worse ? :
      the guy with the blog complaining about
      string theory or the guy who writes into the blog complaining
      about complaining about string theory

    17. Thomas R Love says:

      Joseph Polchinski has an article on arXiv.org with the title “What is String Theory?”


      154 pages! From 1994!

    18. db says:

      yoyoq> or the guy who writes into the blog complaining
      about complaining about complaining about string theory?


    19. Coin says:

      “Somebody”, thanks for the explanation.

      I had not realized tachyon condensation was an idea from String Field Theory. Is everyone who works with tachyon condensation doing String Field Theory then, or is that just where the idea originated?

    20. Alberto G.P. says:


      Thanks for the response!


      As I am a pedestrian, and my English is limited, I can only formulate very simple questions, questions that even a child would understand. As you seem a smart guy, I also have a question for you. In the last decade, what do you think has been the progress to obtain the electron mass using string theory?


    21. Peter Woit says:


      “tachyon condensation” just means that the usual perturbative vacuum state of fluctuations about zero field is unstable (there are tachyons) with a stable vacuum state given by fluctuations about some non-zero field (condensation of the tachyon). This is a general phenomenon in field theory, not specific to string field theory.

      Alberto G.P.,

      In the last decade there has been no progress towards obtaining the electron mass in string theory. There has however been a great deal of progress towards showing that you can get any electron mass you want in string theory.

    22. somebody says:

      Peter, nice try. Yes, tachyon condensation exists outside of string field theory. So does gauge invariance, BRST symmetry, action functionals, … , addition, multiplication, and a zillion other things. 🙂

      “Coin”: the point was that the tachyon condensation I meant was specifically the tachyon condensation that is relevant in string field theory. The one that takes you from a spacefilling D-brane to the closed string vaccum.

      Alberto, I will ignore your sarcasm and asssume it is the arrogance of the beginner. I recommend that you read my comments on this thread if you really want an answer:


      The short answer is this: QFT doesn’t predict the elctron mass either, but that hardly makes it non-preditive. I will ignore Peter’s rhetoric, this has been discussed before, he has been told and knows full well whats up, and he just want to insinuate to his readers that predicting the electron mass is necessary for a theory to be predictive.

    23. Coin says:

      woit: OK. Would it still be correct though to say that in order to apply tachyon condensation to String Theory, one must be using the String Field tools?

    24. Peter Woit says:


      It’s a field theoretical concept, so you need some sort of field. I don’t know what it would mean in perturbative string theory.

    25. Jack Lothian says:


      While I agree with many points made on Peter’s site & I often enjoy reading some of the discussions, I too feel that at times the discussions become repetitive & circular. Whenever I feel like this, I tend to stay away from the site for a few weeks or a month. When I come back I can usually pick up the threads with a renewed interest.

      While I understand some of the points you are making, I do not understant the anger or hostility. Peter is not forcing you come to his site. It is your free choice, so why are you angry with him? He is just doing something that he enjoys doing & the world is free to ignore it or read it.

    26. Walter Mondale says:

      I am not angry at him, I don’t really know why you are saying that. I was annoyed at that other guy who talked down to me, but not Woit. I’m just kind of amazed at the spectacle of the blog, but I agree, if that’s what he wants to do, that’s what he wants to do. And he says his sex life is satisfactory, presumably from all the blog-related groupie activity. What more can one ask for?

    27. Alberto G.P. says:


      Do you know if there is a solution of string theory that explains the Bhabha scattering, like QED?, Do you think I’m arrogant for asking? Maybe, ‘the new science’ doesn’t need basic questions.

      I think the problem is not with the string theory, but, with the philosophy of science.

      BTW, I am not a professional physicist. I’m only a physics buff. I try to study physics as a hobby, but in a level much lower than it would be needed for studying string theory, because I’m completely unable to understand its math background. At the future, maybe, string theory will explain the other physics theories, and all the experiments, that the more conventional theories have already explained, then, I’ll ask for forgiveness, even though, there were no new predictions. Meanwhile, I will come back to my Greiner’s books.

      Good luck.

    28. David B. says:

      Dear Alberto:

      To the extent that string theory is compatible with gauge invariant interactions, and that the low energy effective field theory of string processes is indistinguishable from ordinary quantum field theoretical models, the Bhabha scattering is predicted with the same shape as it has in quantum electrodynamics.

      The hard issue is not to say that one has quantum field theories in string theory that look sufficiently similar to what we see, but to what extent one can predict parameters that in the standard model are numbers that one measures (theoretical inputs) and for which there is no theory.

    29. Thomas R Love says:


      You have mentioned this before, but it bears repeating since it echoes Polinski’s title. In a 2005 article in Nature:


      Witten wrote:

      We still don’t know where all these ideas are coming from — or heading to.
      One day we may understand what string theory really is.

      Witten doesn’t know what string theory is.
      Polenski asked the question, but did not answer it, so he doesn’t know either.

      I have observed that string theory’s PR (Public Relations)
      Is better than string theory’s PR (Physical Review).

      “Walter Mondale” wrote:

      Dude, you’re obsessed with string theory. Ok, we get your point. It’s unprovable. But man, do you have to spend day after day, week after week, year after year, going on and on and on about this?

      Walter: Peter is like the boy who said: “The Emperor has no clothes”, he just has to keep saying it until the world gets the message. There are “string theorists” who treat the subject more like a religion. They have to, it is not a science.

      I read Peter’s book “Not Even Wrong” soon after it came out. It is a must read for all physicists. So, Walter, read the book!

    30. st says:

      I am baffled by the often use of analogy between string theory and religion. The two could not be more different. One is supposed to be a (correct/incorrect/not even wrong) theory about the physical world, and the other is about the supernatural, which as such, can never be proved or disproved by the scientific method.

    31. someody says:

      Thomas Love says : “Witten doesn’t know what string theory is. Polenski [sic] asked the question, but did not answer it, so he doesn’t know either.”

      What is your point? If we understood string theory (at least at the level we understand QFT), then we would know immediately whether it was right or wrong and that would be the end of the string “controversy” nonsense. The point precisely is that string theory as it stands is incompletely understood. And no, this is not a secret known only to Witten and Polchinski.

    32. Alberto G.P. says:


      After the response by David, and the response that a known string theorist gave me, I have realised that string theory is a model that can explain both QFT (Y-M theories) and general relativity in a consistent mathematical framework. Namely, we can explain all the laws of the fundamental physics/nature, although we can’t explain the fundamental parameters, using string theory. So, I have to apologise for my ignorance, for me the polemic is over.

    33. Shantanu says:

      somebody and other string theory enthusiasts, could you provide answers to the following questions ?

      o Why hasn’t string theory solved the cosmological constant problem?

      o What does string theory predict for the value of Theta_13, a quantity which is soon going to measured?

      o Does string theory violate equivalence principle? (I have heard both yes
      and no to this answer)


    34. someody says:

      Alberto, just saying that the parameters of low energy physics are not fixed is perhaps a bit too generous a view of the current state of string theory. It is true, but the real problem is that we don’t really understand the true nature of string theory. The situation is very much like that of the proverbial blind men and the elephant. We are stuck to certain partial descriptions and specific tools, and because of this model-building is extremely hard. This is one of the reasons why “fixing all the parameters” in a useful way hasn’t happened.

    35. Peter Woit says:


      You have been misled. There’s an active campaign on to mislead people in this manner. The whole point of Polchinski’s talk is that there is no known mathematically consistent version of string theory that does what they hope (provide a unification of the SM and gravity).


      The answers to this are well-known:

      1. the anthropic landscape “explains” the CC (and if you don’t like this, well, no one else can explain it).

      2. string theory does not predict SM parameters.

      3. light moduli fields will violate the equivalence principle. Like everything else, you can get whatever you want in string theory, have these or not have these…


      You are being misleading. The problem is not that you can’t “fix all the parameters”, the problem is that you can’t fix ANYTHING at all. Nothing, zero, zip nada. The excuse that “we don’t really understand the true nature of string theory” is getting very old. Everything you do learn about string theory provides more and more evidence that it can never predict anything about particle physics.

    36. someody says:

      Shantanu, my current view on string theory is that it is really a paradigm for building UV complete models. But if this paradigm is going to be useful, it should have a better (non-perturbative) definition, where we have the freedom to build models following ideas instead of being stuck with specific technical tools. The existence of such a definition is what made QFT useful for UV-INcomplete model building.

      It is because it is a paradigm, that you can have many possibilities (“anything you want” in Peter’s pejorative description) in string theory. But note that QFT can also have anything you want, because it is also a paradigm. But because of the non-perturbative definition available in QFT, model-building was still fruitful.

      One problem with not having a non-perturbative definition is that we can solve many problems (including the ones you point out) in string theory individually, but putting them all together becomes a huge challenge. This is the reason why more specific predictions at this stage are perhaps not very meaningful.

      To Peter: I was talking about stabilizing (fixing) moduli in a specific vacuum. You are again confusing this with fixing fundamental constants dynamically, and accusing me of being misleading.

      Peter also says: “The excuse that “we don’t really understand the true nature of string theory” is getting very old.”

      The “excuse”? Should we apologize to you, Peter? 🙂

      Some of these problems are truly hard, just wishing otherwise is not a particularly useful strategy for progress. Many smart people have been battling quantum gravity for 50 or so years, and it is not so unreasonable that the problem is genuinely hard. In particular, the “what is string theory?” we ask now is certainly not the same “what is sstring theory?”

    37. Peter Woit says:


      Feynman said it well: “String theorists don’t make predictions, they make excuses”. This has nothing to do with apologizing, it has to do with being willing to admit that an idea doesn’t work and move on to something else. In science this happens all the time and requires no apology. Most ideas don’t work out.

      The reason string theory predicts nothing, and the Standard Model is the most highly predictive physical theory known to man has nothing to do with the different statuses of their non-perturbative formulations. You can in principle write down arbitrarily complicated QFTs, and get, more or less “anything you want”. If you did that, no one would take you seriously. Instead, it turns out that a quite simple gauge theory works precisely, on the nose, passing literally thousands of non-trivial experimental tests. In string theory on the other hand, all versions of the theory that are simple enough to analyze don’t come even close to looking like physics. Instead, one has to invoke more and more complicated “string theory backgrounds” in order to avoid contradiction with experiment, never actually getting a calculation of anything you can compare to experiment.

      “more specific predictions at this stage are perhaps not very meaningful”.

      You really insist on saying things in the most misleading way possible. More specific than what? Again, string theory predicts absolutely zero about particle physics. The problem is not that it is not specific enough, only giving you rough information about particle physics, it is that it gives you NO information about particle physics at all. Nothing.

    38. somebody says:

      Peter, I will ignore your appeals to authority and vitriolic comments. Lets stay focussed on the science.

      Peter: “You can in principle write down arbitrarily complicated QFTs, and get, more or less “anything you want”…. Instead, it turns out that a quite simple gauge theory works precisely, on the nose, passing literally thousands of non-trivial experimental tests. etc.”

      How do you even call a QFT “simple” if you only had a bunch of rules for writing down messy scattering amplitudes? The non-abelian gauge symmetry of the non-perturbative description is what gives any sense to the statement that the theory is simple because the gauge groups are! “Simplicity” is essentially a meaningless concept when there is no non-perturbative definition (like in string theory, currently). This is one reason why it is worthwhile trying to answer the question “what is string theory?” and come up with a non-perturbative definition.

      Peter: “You really insist on saying things in the most misleading way possible. More specific than what? Again, string theory predicts absolutely zero about particle physics. The problem is not that it is not specific enough, only giving you rough information about particle physics, it is that it gives you NO information about particle physics at all. Nothing. ”

      You are attacking a paradigm for not being a model. As a paradigm, non-abelain gauge invariance, chiral fermions, multiple generations etc. (all in the context of a theory that also includes gravity!) are all to be found in string theory. Your argument is like saying that QFT (as a paradigm) predicts nothing. But standard model, a specific QFT, is indeed predictive. So sttempts to gain model-building powers in string theory are well justified.

      Last but not the least, you have a tendency to repeat claims many times hoping that this will turn them into facts. I just wanted to point out that this is not the case. 🙂

    39. Tumbledried says:

      Dear Peter, Somebody,

      Many apologies to interject, but I would like to suggest that we suspend for the time being this increasingly vitriolic exchange and focus on some other matters instead.

      Dear Somebody,

      I am quite interested to hear your opinion as to whether you consider it reasonable, considering the status of string “theory” as a definite work in progress which even the world experts do not have a good intuitive feel for, as to look at other alternatives to string theory.

      There are certainly many other models and theories out there that people are working on, and I assume a fair proportion of these may contain valuable insights about the nature of reality. I would be quite interested to hear from you whether it would be worth the time and energy to take a serious look at some of these, even though they are not The One True Theory that has been the focus of tens of thousands of person hours since the late 1970s.

      For instance theology and pure philosophy has consumed much more of human time and effort over the centuries than string theory has for the last 30 years. Many extremely brilliant people, far more gifted than you or I, spent time on such things, debating endless metaphysical questions that were doubtless considered most profound at the time. Yet arguably we are no further than we were in such matters since the time of the ancient Greeks.

      Dear Peter,

      I greatly admire your tenacity in these matters. It is unfortunate, but it seems in these times that sometimes a point needs to be repeated over and over again in order for people to finally understand it.

      Best Regards,

    40. Peter Woit says:

      Thanks Tumbledried,

      I confess that I’m infuriated by these arguments claiming that something that predicts nothing is the same as something that predicts just about everything we understand about fundamental physics. Once a science is taken over by people claiming that black is white since they’re both kind of like grey it’s pretty much all over. It’s very sad to see what has happened to particle theory over the last 25 years.

    41. Luca says:

      I am an Italian student of Philosophy at the University of Studies of Milan. Recently I have read the book Not Even Wrong in the Italian translation and I found it very stimulating. In my university there are not many professors that study these themes and when they do it, they seem more worried to talk of old questions of early 20th century, i.e. of the paradigmatic change in physics due to the introduction of quanta. To be honest, I find more interesting the modern physics, because it seems to be in a critical phase, as the book suggests.
      Reflecting on these themes, I have reached a personal opinion about the actual problem of theoretical physics. For me the problem is this: which form should a theory of everything have? There are two ways to answer this question: 1) it should be able to predict a lot of new phenomena; 2) it should be able to describe the world for what it is, without justifying it (this is the thesis I support). The problem is if a theory of everything would be scientific in accord with our normal scientific codes. I suspect not. A theory of everything does nothing but say that what it is there is because there must be or there would not be. Probably it seems a play on words, but it is not. A theory of everything, in my opinion, would not be predictive, but descriptive, and it would be a strange sort of theory in which the physical parameters have those values only because if they had not them, our universe would not exist. Hence the anthropic principle is a consequence of these assumptions. From this theoretical point of view, the most coherent string theory would not be a theory that provides us with results, but a theory that describes those results were already found. I personally regard a theory of everything as a natural language. It is only a logical connection of sentences, words and punctuations, and it cannot make predictions. However, in all circumstances (experienced before or completely new), we are able to adapt the language in order to make a detailed description of the situation, even if we have no idea why the situation is the way it is. Our linguistic code provides us with a system that can face every circumstance and also accept new terms in it without this fact implies the collapse of the whole theory (in this case, language). This is probably the form a theory of everything should have: for every new observation there is a way to accommodate the syntax of the theory without compromising the theory itself.
      The problem is if this way to proceed is real science or not, and I am really dubious of it.

      Kind Regards

      Luca Ferrari

    42. somebody says:

      Tumbledried, thanks for providing the necessary distraction and saving Peter from himself. 🙂

      I don’t have much to say about your comments equating string theory to theology because it is in fact true that for an outsider, the difference is hard to tell. All the elaborate formalism just seems like pointless crap. To see that there is indeed direction (and a very clear one) you need a bit of expertise, I am afraid.

      But, your comments regarding alterantives, I can make a few comments. I think there is a widespread misconception that there are many ignored alternatives. I think that most good string theorists would be eager to look into alternatives IF it had even *some* of the good features of a fundamental theory. But most people don’t seem to realize how difficult it is to come up with even a mildly promising alternative. It is not like there are tons of alternatives out there and theorists are ignoring them. Quantum gravity is a difficult problem and despite half a century of work, there are only very few ideas which are not stillborn. The self-interest of theorists motivates them to consider new promising ideas very seriously. I don’t even know one promising alternative idea out there which is not getting the attention it deserves. Another interesting fact is that almost all the ideas that seemed promising, seem to arise naturally in string theory. Supergravity, kaluza-klein theories, etc. were separate ideas initially, but they arise automatically in string theory.

    43. Shantanu says:

      Somebody, many people whom I know who used to work on string theory
      have left string theory and switched to astrophysics, gravitational waves, LQG for precisely the same reasons Peter is talking about.

    44. Tumbledried says:

      Dear Peter,

      You are most welcome.

      Dear Somebody,

      Thanks for your polite and considered response. I must admit that I was perhaps being a bit too honest with you, but you are quite correct, I am an outsider, or at least not a string theorist.

      I am reassured by your response that, at least the part of the stringy community you frequent, are quite open to new ideas, providing that they are reasonably promising. This seems acceptable to me. It is of course disheartening to hear that there do not appear to be any promising alternative directions to string at the moment. Nonetheless I remain optimistic that the open problem of further unification is not impossible and some bright academic may find some different way of approaching it.

      Best Regards,

    45. klaus says:

      shantanu: people say a lot of things about why they leave a subject, partly to protect their own egos. most people I know, who left string theory, left it because the job market is brutal, and not because they got fascinated by something else. i am sure there are some people who do it because of the reasons you suggest, but the overwhelming majority that i have had first hand experience with, do not belong to that category, and leave because staying on is tough.

    46. chris says:

      dear somebody,

      the argument of paradigm vs. model you invoke is rather interesting. however, there is one small but in my opinion rather essential point that you are missing in your comparison to early quantum field theory. you say:
      “How do you even call a QFT “simple” if you only had a bunch of rules for writing down messy scattering amplitudes?”
      which is partly true (because scattering amplitudes display the same sorts of symmetries just in a more hidden way) but not quite captures what early (<1970) qft looked like. keep in mind, that in qft the lagrangean was there right in the beginning. and step by step people came to grasp with how to evaluate path integrals with it. that’s the story. and if you look at paradigms: there were a few. s-matrix theory, bootstrap, hadronic strings… all of them failed. advancement came about through better methods to do the path integral and that was it.

      judging string theory is not up to me – future generations of experimentalists will certainly do that – but i am sceptic towards claims of paradigm shift without the paradigm fully visible. it leaves the same taste in my mouth than forward references like ‘we will show this in one of our next publications’. my gut feeling is that anounced revolutions rarely happen.

    47. Daniel de França MTd2 says:

      This is certainly offtopic, but worth posting.


      This is a video of Levni Yilmaz talking about his private life with his father in a very creative bud sad and nostalgic way.

      He is one of Yilmaz sons(that guy of the theory of gravitaion). He is a huge hit on Youtube with several million of visits, and talks about thing of daily life, no science.

    48. somebody says:

      Chris, I was busy and didn’t see your comment. Anyway, the point was not how history unfolded in the case of quantum field theory. The point was that the simplicity of a theory is not necessarily visible when you look at scattering amplitudes. A Lagrangian type description is where that is manifest, which we do not have yet in string theory.

      Also I am using the word “paradigm” as just another word for “framework”. One of my suspicions about string theory is NOT that it is too much of a paradigm shift, but that it is probably not enough of one.

      Finally, wanting a non-perturbative definition is not the advertisement for a coming revolution, it is the statement of an actual open problem.

    49. chris says:

      dear somebody,

      thanks for your reply and clarifications. what you say sounds very reasonable to me.

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