Conversations on Quantum Gravity

Things for many years now have been going badly for string theory on the public relations front. Today the Economist has Physics seeks the future: Bye, bye, little Susy, where one finds out that:

But, no Susy, no string theory. And, 13 years after the LHC opened, no sparticles have shown up. Even two as-yet-unexplained results announced earlier this year (one from the LHC and one from a smaller machine) offer no evidence directly supporting Susy. Many physicists thus worry they have been on a wild-goose chase…
Without Susy, string theory thus looks pretty-much dead as a theory of everything. Which, if true, clears the field for non-string theories of everything.

Unfortunately for the public understanding of science, this is followed by

But at the moment the bookies’ favourite for unifying relativity and the Standard Model is something called “entropic gravity”… in the past five years, Brian Swingle of Harvard University and Sean Carroll of the California Institute of Technology have begun building models of what Dr Verlinde’s ideas might mean in practice, using ideas from quantum information theory.

For something much more anecdotal, on Saturday night I was having dinner outside in a hut during a rainstorm on the Upper East Side (having fled an aborted Central Park concert), and started talking to a couple seated nearby. When informed I taught math and did physics, one of them recommended Carlo Rovelli’s new book to me, and said he hoped I wasn’t doing string theory. Luckily I could reassure him about that.

This morning I found out about Conversations on Quantum Gravity, a fascinating book published by Cambridge that appeared online today, hard copies for sale in November. It consists of interviews about quantum gravity put together by Dutch string theorist Jay Armas, starting in 2011. The scale of this project is immense: there are 37 interviews, most of them rather long and detailed, making up a book of 716 pages. What I’m writing here is based on a day’s worth skimming of the book. I’ll likely go back again and look more carefully at parts of it.

Roughly half the interviewees are string theorists, with the author making a concerted effort to also include non-string theory approaches to quantum gravity. I made the mistake of starting off by reading some of the string theorist interviews, which was rather depressing. By the end of the day, after making my way through about 20 long interviews with string theorists, with few exceptions the story they were telling was one I’m all too familiar with. It’s roughly

We don’t actually know what string theory is, just that it’s a “framework” that encompasses QFT and much more. We can’t predict anything with it now and don’t see any plausible way of predicting anything in the future, but the theory is a successful theory of quantum gravity, unlike our competition. There is no good reason for people to be working on anything else.

For example, here’s Cumrun Vafa:

If a young student asks you what approach to quantum gravity they should work on, what would your answer be?

There is no question that string theory is the right framework to understand quantum gravity. By this I mean that it is closer to the truth than any other existent theory.

Is it worth exploring other approaches?

Well . . . certainly being close-minded is not good. We should be open to other developments. But the fact that there exist other subjects does not justify exploring them if they are not on equal footing with string theory.

and here’s Edward Witten:

Due to the lack of experimental data, there exist a plethora of different approaches to quantising gravity. Which of these approaches, in your opinion, is closer to a true description of nature and why?

I would say your premise is a little misleading. String theory is the only idea about quantum gravity with any substance. One sign is that where critics have had interesting ideas (non-commutative geometry, black hole entropy, twistor theory) they have tended to be absorbed as part of string theory.

and David Gross:

So you don’t think that other approaches like loop quantum gravity have . . .

Loop quantum gravity is total BS. I mean, it’s really not worth discussing it. Don’t put that in the book. But, it really isn’t.

Luckily Armas doesn’t take up Gross on the suggestion that loop quantum gravity is not worth discussing, interviewing quite a few people who are working on research programs that have grown out of it. I got much more out of these interviews, which were very different in tone and content than the ones with string theorists. Many of them gave a very clear account of the technical problems these approaches have encountered, referring to very specific well-defined models and calculations. Instead of the triumphalist claims and vague speculation of the string theorists there was a careful explanation of exactly what they were trying to do and the problems they were trying to overcome.

There’s a huge amount worth reading in these interviews, perhaps I’ll later add some more pointers. A couple specific examples that occur to me right now are Steve Carlip’s careful discussion of the quantization of the toy model of 2+1 dimensional gravity, and Lee Smolin’s very personal account of his frustration at the reception of his book “The Trouble With Physics”.

If your institution is paying Cambridge for access, you should take advantage of this now and take a look. Congratulations to Jay Armas for bringing us this material.

Update: There’s a new preprint out by historian of science Sophie Ritson, Constraints and Divergent Assessments of Fertility in Non-empirical Physics in the History of the String Theory Controversy, which examines in detail the arguments of the string wars and later over how to evaluate string theory. While I don’t think there’s a single reference in the 716 page Armas book to anything I’ve written, my views do make an appearance in this article.

Update: There’s a linked editorial in the Economist Fundamental physics is humanity’s most extraordinary achievement, which (rather optimistically) sees the current state of affairs as:

Supersymmetry is a stalking horse for a yet-deeper idea, string theory, which posits that everything is ultimately made of infinitesimally small objects that are most easily conceptualised by those without the maths to understand them properly as taut, vibrating strings.

So sure were most physicists that these ideas would turn out to be true that they were prepared to move hubristically forward with their theorising without experimental backup—because, for the first decades of Supersymmetry’s existence, no machine powerful enough to test its predictions existed. But now, in the form of the Large Hadron Collider, near Geneva, one does. And hubris is turning rapidly to nemesis, for of the particles predicted by Supersymmetry there is no sign.

Suddenly, the subject looks wide open again. The Supersymmetricians have their tails between their legs as new theories of everything to fill the vacuum left by string theory’s implosion are coming in left, right and centre.

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40 Responses to Conversations on Quantum Gravity

  1. Jackiw–Teitelboim says:

    One might not be so unwise in conceiving the possibility that realistic progress in fundamental physics in near future, in the reductionist sense well articulated by Weinberg in his many writings (which ironically, many “string theorists” nowadays working on subjects such as “celestial” holography, “it from qubit,” whatever, clearly gave up on superstring unification), might not be the problem of quantum gravity at all.

    As I student to whom reading older Proceedings and Reviews, around the time when the current SM was established, is an enjoyable weekend hobby, it seems to me that at some time, researches decided that the search for a better understanding of the fundamental interaction should necessarily include quantum gravity.

    Now it might be the time to revisit this issue of what happened when “grand unification,” of SM interactions, should necessarily be a “theory of everything.”

  2. How stringent a test of string theory is the non-detection of supersymmetry at the LHC? I thought that string theory would still be consistent if supersymmetry were found at higher energies, its just that this wouldn’t solve the hierarchy problem.

  3. Peter Woit says:

    Laurence Lurio,
    The accurate statement is that string theory predicts nothing about anything, including nothing about whether the LHC would see superpartners. Unfortunately for string theorists, pre-LHC this is not what they were saying publicly. For some more details about this, see for instance this blog posting
    https://www.math.columbia.edu/~woit/wordpress/?p=3864
    written in 2011 when the LHC results had started to come in and string theorists had started back-pedaling.

    For another example, from a 2006 highly negative review of my book in the LA Times:

    “As for Woit’s claim that string theory has “absolutely zero connection with experiment,” experiments already planned for a new European particle accelerator will look for the existence of extra dimensions and extra families of particles — both predicted by string theory.”

    So, pre-LHC, string theorists were happy to mislead the public with “string theory predicts superpartners at the LHC” when confronted with the lack of predictivity of the theory, and this has led to stories like the Economist’s. What’s really remarkable, and recurs throughout the interviews, is that they are now perfectly fine with the idea that the theory they work on can’t predict anything, while at the same time claiming that a theory in this situation is much better than any other theoretical approach.

  4. DrDave says:

    ” When informed I taught math and did physics, one of them recommended Carlo Rovelli’s new book to me, and said he hoped I wasn’t doing string theory. Luckily I could reassure him about that.”
    First smile of a long day.

  5. Topologist Guy says:

    Peter,

    I’ve always found the claim that “string theory predicts that the LHC will find extra dimensions” to be highly misleading, more so than the claim that superpartners will be discovered at the LHC. The “prediction” that extra dimensions will be accessible at ~ 10 TeV energy scales came not from “string theory” per se, but rather from some very specific model building that used ingredients from string/M-theory (for example, the Randall-Sundrum model https://arxiv.org/pdf/hep-ph/9905221.pdf). There is no sense in which a fundamental string theory calculation “predicted” the Randall-Sundrum braneworld scenario.

    Jackiw-Teitelboim,

    I agree that there are avenues in fundamental physics research other than quantum gravity. As a mathematician, I have noticed a deep difference in the attitudes towards quantum field theory by mathematicians and high-energy theorists. Most high-energy theorists view the four-dimensional QFTs of the standard model as entirely settled science. As a mathematician, however, I feel like great progress could be made in making the quantization of four-dimensional field theories rigorous. While theorists dismiss the question of mathematical rigour, it seems clear to me that a rigorous understanding of 4D interacting field quantization would illuminate the complex mathematical structure of these QFTs. The problem of “Yang-Mills and mass gap” appeals to me, and I think there are great opportunities available in more deeply understanding the mathematical structure of the standard model QFTs, with a distant goal possibly being the quantization of Yang-Mills by cohomological pushforward or an appropriate generalization thereof.

  6. Peter Woit says:

    Topologist Guy,

    When people tried to use “the LHC might see extra dimensions or produce black holes” as a “prediction of string theory” I always thought this was so ridiculous I didn’t even know how to argue with it. Just about no one ever really took that seriously.

    For mathematicians it’s important to point out that most of the QFT they’ve been looking at is TQFT. Amazingly that leads to dramatic topological results in 4d, but the underlying QFTs (supersymmetric Yang-Mills with twisted differentials) used to build TQFTs like the one that gives Donaldson invariants have all sorts of different and interesting mathematical structure. The fundamental confinement/mass gap problem is just part of it. The standard model QFT is a chiral gauge theory, and these have additional poorly understood problems that go much beyond the problems of phi^4 or pure Yang-Mills. I’ve spent most of my career frustrated by the extent to which theorists have lost any interest in those problems in favor of moving into extremely complicated and ill-defined speculation about string/brane theories in high dimensions.

    In what I’ve been working on recently I’ve been struck by how little is really understood about the relation between 4d Minkowski and Euclidean signature theories. For quantum gravity as far as I can tell this remains a complete mystery.

  7. Gavin says:

    It seems to me that loop quantum gravity (or any other approach to quantum gravity) is really no better than string theory in terms of producing useful predictions that can be tested. While LQG predicts violations of Lorentz invariance, given there isn’t a clear scale at which this would happen, this kind of prediction seems on part with saying “supersymmetry at some scale that may or may not be in reach of experiments” is a prediction of string theory. Furthermore, as I understand, it is currently not known whether GR and quantum field theory emerge from loop quantum gravity in an appropriate semiclassical limit.

    I would have thought this blog would advocate for theoretical physicists spending more time understanding the mathematics behind established theories, than expressing a preference for one speculative approach over another.

  8. Peter Woit says:

    Gavin,
    If you actually read my posting I think you’ll see nothing promoting dubious claims about predictivity of one speculative approach over another. What I am pointing to is that the unprofessional “We rule, the other side is hopeless BS” behavior is mainly coming from one side, at least in the interviews in this book.

  9. Peter Shor says:

    Gavin,

    Does loop quantum gravity really predict violations of Lorentz invariance, or is this the same kind of non-rigorous hand-waving that theoretical physicists are prone to? I don’t see any reason why a theory that’s constructed from non-Lorentz invariant elements couldn’t give entirely Lorentz-invariant predictions.

  10. Peter Woit says:

    Peter Shor,
    There have been various claims about this made in the past, and from what I remember, they’re discussed in some of the interviews in the book. I don’t think there’s any point in reviving arguments over those. From reading the many interviews in this book, the current situation about “predictions” is actually much more reasonable on all sides than it was in the past: neither LQG nor string theory people are any more claiming such things.

  11. Shantanu says:

    Peter : doesn’t string theory and also certain BSM models such as Kostelecky’s SME extension also allow for violation of Lorentz invariance?

  12. Peter Woit says:

    Shantanu,
    The situation with string theory is always the same. Since you don’t really know what the theory is, it is consistent with just about everything, and predicts nothing. That string theory doesn’t predict anything used to be a claim that would generate some argument, but reading through the interviews in this book, it now seems to be the consensus even among string theorists.

  13. Peter,
    I agree with you that the relation between Euclidean and Minkowski signature is of fundamental importance for quantum gravity, and, like you, I cannot understand why people do not pay more attention to this issue. Actually understanding what the signatures mean, rather than complexifying them out of existence, is surely one of the most important issues here.

  14. Alex says:

    “… One sign is that where critics have had interesting ideas (non-commutative geometry, black hole entropy, twistor theory) they have tended to be absorbed as part of string theory…”

    I wonder what Connes and Penrose think about that (actually, I know: they think it’s a BS claim).

    “…Loop quantum gravity is total BS. I mean, it’s really not worth discussing it. Don’t put that in the book. But, it really isn’t…”

    Just wow. Is this the proposed level of the debate for this Nobel prize winner? And it seems he even doesn’t have the guts to say it in public, I think it’s good that they did put it in the book. Now we know who is who.

    Evidently, confrontation didn’t work with these people. So, I think it’s time to just ignore and alienate them. They did good work, they enjoyed the deserved praise, but it’s time to move on, they are doing damage to the field.

    Now, if you ask me what to do in relation to research in QG, it’s really an interesting moment. A lot has been investigated, many theories have been developed, we did learn some things. But, also, all of this didn’t live to its many promises. I think it’s time to step back, make a deep breath, open the window. But not for more new ideas and theories. This method of proposing theories because of mere aesthetics, or because they make some supposedly falsifiable predictions is not good.

    I think that, rather than new theories, it would be better to do research about the deep conceptual problems posed by the very notion of quantizing gravity, or to investigate phenomenological models based on the basic qualities that a QG theory must supposedly have (say, superposition of geometries, discretization of geometry, etc) in order to see if we can get some input from actual experiments. And, if we must do theory, to do it closely to the methods, principles and notions from the actual theories that we know they work (GR and QFT), and not start throwing everything in the kitchen sink (as in string theory, ending in that pathological quote from Witten). And I know that even these basic proposals are debatable, but at least they try to mark some criterion before launching to speculate.

  15. Peter Woit says:

    Alex,
    I just realized that there’s a pattern here: Vafa is Witten’s student and Witten is Gross’s student. Vafa is 61, Witten 70, and Gross 80. I suspect it would not be hard to find 50 year old students of Vafa, their 40 year old students, and so on, all of whom would, at least in private, argue that string theory rules/LQG sux. To a large degree, the whole subject of quantum gravity has sunk into a depressing anti-scientific tribalism, with the string theory tribe now rallying around ill-defined ideas that somehow entanglement/quantum information theory/machine learning etc. are the way forward. You can try and ignore this tribe and its behavior, but it remains the dominant one, at least in US academia.

    My current weird relationship to quantum gravity is that, after a long career of mostly ignoring it since I thought the way forward was likely to be to find a unification with the parts of fundamental theory we do understand (the Standard Model) and I didn’t have a good idea about that. Recently though I’ve become more and more convinced that the ideas about twistors and unification I’ve been thinking about do say something about quantum gravity, and I’m trying to write up a revised version of my paper that emphasizes that. Hope to have that done soon, doubt that it will attract any interest from the string theory tribe, but perhaps another tribe will see things differently.

  16. DKepler says:

    I agree with Alex.

    A Nobel laureate using such a language in response to a request for a scientific opinion is simply disgusting, and indicates the insecurity String theorists are facing. Talking of unscientific biases, when entropic gravity was trending, I recall someone asking a string theorist, a Nobel laureate, about his opinion on related emergent gravity works (Jacobson, Padmanabhan, and others), and his response was he is more in favor of what Verlinde does because <>. Guess who this laureate was?

  17. Dimitris Papadimitriou says:

    Nothing in theoretical physics can goes on for long (and remain healthy) without, at least, some hints of experimental evidence.
    In the case of QG, there is almost nothing known with confidence (with the exception of some strongly negative results about potential Lorentz- Invariance violations from astrophysical observations). A minimum of observational evidence is necessary for people to have some solid background for their QGT candidates, otherwise we’ll have again and again the usual (mainly among string theorists, but not exclusively) situation:
    Guesses and Conjectures built upon other Conjectures without an end, as is the case with Holography and the attempted “solutions” of the black hole information problem, where String people tried to cure an inconsistency with rather far fetched hypotheses (based on the pompously dubbed “Central Dogma”), that quickly led to even more serious inconsistencies.
    They, eventually, throw out of the window almost all established physics (QFT, locality/Causality, the Equivalence Principle- is there anything left?), and these are not mild or subtle violations…
    Ok, I don’t mean that everything we know must remain intact, but as the situation is now, anyone can hypothesize anything without any consequences.

  18. Peter Woit says:

    DKepler,
    I just tried re-reading the Verlinde interview in the book, it’s quite remarkable. It seems that he believes there is some more fundamental theory underlying string theory
    “I really think that you can try to explain string theory as an emergent framework from something else underlying it.”
    “In general, I think that string theory is a cleverer idea and better motivated than any of these other ideas, which I think are just guesses. When I wrote my paper [26] people sent me emails saying that they knew what the microscopic model for what I was advocating was. I didn’t believe it and I didn’t even have to look at it because the possibility that we just know what it is like is really small. The probability is just one in a googolplex, so I don’t think we have any chance of finding it. I think we can find general principles but not the microscopic model. Also, I think that LQG is too contrived and it doesn’t explain anything.”
    So, he has given up not only on previous hopes for some connection to experiment, but also on any hope of actually having a well-defined fundamental theory. Elsewhere he argues string theory is too beautiful, truth is something unknown and uglier.
    Honestly, none of this makes any sense to me, and the Economist take that this is now our leading theory is disturbing.

    On top of this, there’s
    “I think that people who follow other approaches should learn more about string theory as there is no other competing formulation. There’s no comparison between string theory and other formulations.”
    Having no well-defined theory coupled with this kind of critical attitude towards others who are trying to work with well-defined theories is kind of breathtaking.

  19. Nogo says:

    I really don’t understand the “human story” behind all this.
    Why talented, respected people with real accomplishments act in such a way as to attract disrespect and even ridicule? Gordon Kane with his predictions (and now postdictions?) of SuSy discovery, string theorists just redefining “arrogance”, Mochizuki with his treatment of critics… I don’t get it.

  20. Peter Woit says:

    Nogo,
    I think the “human story” is that often very smart people are not good at realizing they were wrong about something and admitting it. Perhaps being smart even makes things worse: you lack experience at being wrong.

  21. Dimitris Papadimitriou says:

    Peter Woit, Nogo,
    Maybe it’s something like a “Majority Syndrome”: If someone is a leading personality of a research field that is popular and risk-free ( in the sense that it’s not gonna be falsified any time soon), and also has a “natural” confidence that is quite common among smart and relatively famous people, maybe he/she isn’t immune to this exaggerated kind of arrogance.

    There are many other smart theorists, not only in various QG research programs, but also mathematical physisists, GR experts, and others that work in equally important areas that don’t have (not even closely) such type of behaviour.
    They’re just doing their hard work without much publicity and far out claims.
    Needless to say: If the situation was reversed, and these other research areas were more popular (and overhyped), maybe they would exhibit the same kind of
    unrestrained arrogance.

    In my opinion, only a few string theorists have such a behaviour, though.
    There are many young people that doing their thing, and avoid exaggerated claims, even if they, sometimes, adopt some pompous phrases (like the “Central Dogma” and the like) that others have invent, to make things sound more important that they really are.

  22. Peter Woit says:

    Dimitris Papadimitriou,
    While most string theorists don’t behave like this, in reading the 20 or so string theorist interviews it was disturbing how many of them do, and the overall attitude of not acknowledging failures.
    String theory is a very unusual subject though, since it has gone from having a very positive public perception to a much more skeptical and negative one. Some of the bad behavior surely has to do with a feeling that string theory is under unfair attack, so the thing to do is fight back, fairness be damned.

  23. DKepler says:

    Thanks Peter for those excerpts from Verlinde’s interview. There is a great irony here which perhaps also applies on a broader level to the entire String community. The key ideas on which Verlinde based his work came from Jacobson (and later work by Padmanabhan), and Ted’s earlier research was in topics closer to canonical gravity, LQG etc. which are being called “BS” by Gross. There seems to be some kind of a zealot like attitude towards absorbing anything not coming from string theory as part of string theory and then continue abusing other ideas. LQG may be useless as a physical theory, just like string theory. How can we say that the mathematical tools it has introduced can not be useful, while using the same argument to favor string theory?

  24. Andrew Thomas says:

    I’m no fan of string theory, but regarding the David Gross quote, if you give an interview to a journalist “off the record” then the rules of confidentiality apply and should be respected by the journalist. That then gives the interviewee freedom to speak openly and say things he or she would not normally say, which might include highly-inflammatory opinions. If that off-the-record protocol is not respected by a journalist then that freedom is lost.

  25. Peter Woit says:

    Andrew Thomas,
    This clearly was an on the record interview. One can argue that after saying something on the record, Gross changed his mind and asked that it be off the record. All of the interviews were edited afterwards with the collaboration of the interviewee. Armas explains
    “The interviewees had the opportunity to review and modify the edited transcripts as they pleased which, in many cases, contained additional questions that I added a posteriori over the years and to which the interviewees provided answers. Some of the interviewees spent a considerable amount of time reviewing and editing their transcripts, while some performed only minimal edits or none. Given the long time span between the date of some of the interviews and the publishing date of this book, some interviewees decided to update part of their answers to better reflect their current understanding of the topics discussed, while others decided to keep their original answers.”
    From this I think it’s pretty clear that Gross approved leaving the “BS” statement in.

  26. Anonymous says:

    I know this topic is getting a little old but Gross, Witten, and a number of other string theorists were asked again about LQG at the recent strings conference in this 3 min video: https://www.youtube.com/watch?v=Wh9jqAUavNo

    Anyway, I don’t think it’s fair for people to get too upset about the “BS” comment. Scientists are still people and most work on things where there are competing explanations and ideas. There will be deriding comments occurring occasionally in group meeting and in private seminars everywhere.

  27. Peter Woit says:

    Anonymous,
    Sure, in private scientists will often refer to competing research programs as “BS”. It’s not though a usual thing to do for publication or in an outreach program for the public (that’s where that video came from). The public perception of string theory has become very negative (for good reason), and I think this has led string theorists to feel embattled and justified in lashing out at others in this unusual way.

    The substantive critique in those video comments is that LQG is basically an attempt to quantize gravity emulating the success with the Standard Model as a quantization of connections (using for gravity the chiral spin connection and vierbein Ashtekar variables). All those commenting in the video make much the same claim, that they believe the reason you run into technical problems with this is that it is “not radical enough”. The problem though is that since no one knows what “string theory” is right now, they don’t have a well-defined proposal for what fundamental objects should replace connections/vierbeins. For a while this was supersymmetric versions of loops in 10 dimensions, now it’s maybe a quantum error correcting code, hard to tell.

    It may very well be that connections/vierbeins are the right variables, we just haven’t found the right way of quantizing the theory. Gross and Veneziano make the analogy as the Fermi theory vs. Glashow-Weinberg-Salam. Maybe it’s more accurate that the LQG starting point is like trying to describe the weak interactions using massive gauge fields. This has renormalizability problems, but is almost right, you just need to add the Higgs mechanism.

  28. Justin Glick says:

    Peter,

    Regarding your last comment, are you suggesting that your latest thinking about twistors may be incorporated into LQG, perhaps providing key insights leading to a consistent theory?

  29. Shantanu says:

    Peter, there are other faculty at UCSB who have worked on LQG (and non-string approaches). Wonder what Gross thinks about this ? Does anyone have any data on funding in string theory vs LQG?
    Some univs like Penn State and LSU have invested heavily in LQG. Someone should have asked this question how/why they did this.

  30. Nogo says:

    I guess “not radical enough” is in the eye of the beholder – I remember Lee Smolin arguing in his book that String theory was not radical enough, because it was not background-independent…

  31. Peter Woit says:

    Justin Glick,
    More details to come soon in a revised version of the paper, but one idea in the older version is that gravity is a gauge theory of one of the SU(2) factors of Spin(4), which is much the same as saying you should do GR in terms of Ashtekar variables. This is the historical starting point of the LQG program. The context I’m giving for this has new features (the other SU(2) is an internal symmetry, imaginary time direction is the Higgs), perhaps these will lead to some different possibilities to pursue, we’ll see.

  32. Peter Woit says:

    Shantanu,
    The conflict over resources between LQG and string theory has been going on for over 30 years and is a long, complicated and evolving story. The unusually aggressive attacks on LQG you sometimes see from string theorists I think have something to do with that conflict.

  33. Peter Woit says:

    Nogo,
    The funny thing is that the conventional wisdom in “string theory” these days is that Smolin was right, that string theory (a theory of strings) is not radical enough. Of course what people have done to avoid acknowledging this is to redefine the words “string theory” (nowadays, pretty much all string theorists agree that they don’t know what it is, but it’s not a theory of strings).

  34. off topic says:

    Lawrence Krauss had a long interview with Gross in July that mostly covered Gross’ life in physics. At the end, Gross used the recent black hole information paradox work as an example of string theory’s utility. Krauss was skeptical and said that he was not optimistic but Gross pushed back pretty strongly saying that the burden of proof was now on those that think there is a paradox. Nothing much else was said about string theory.

    One funny fact that is only slightly relevant because Witten and Gross have been mentioned here was that Gross wrote a one line recommendation letter for Witten’s application to the Harvard Society of Fellows: “he’s smarter than me and probably smarter than you so accept him”.

  35. DV says:

    It is indeed quite breathtaking to realise the level of contempt the string theory stalwarts have for loop quantum gravity. Especially when one considers the fact that not one of them has actually they tried to sit down and understand what LQG actually is. If they did, they would realise that it is in fact the most conservative possible approach towards quantising gravity, which does not require any extra dimensions or symmetries, and which actually succeeds (for the most part) in its original goal.

    Further, re Gavin, Lorentz invariance is perfectly obeyed by LQG, unlike the situation in string theory where whether or not Lorentz invariance is broken is something that is almost dependent on the choices the theorist makes. Moreover general relativity does emerge as the long wavelength, “low energy” limit of LQG. The question of QFT is still somewhat more open but rapid progress is being made towards answering it.

    It is exhausting advocating for LQG for more than 10 years now in the face of what seems like an iron curtain of willful ignorance which string theorists have put up. I take some consolation in Planck’s words:

    “A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.”

  36. Peter Woit says:

    DV,
    I think there are string theorists who have some understanding of LQG, and the fact that it’s a conservative approach to quantizing gravity is exactly what they don’t like about it (if you watch the youtube video someone linked to, they are very explicit about this).

    What’s happened with string theory over the years is that it has evolved into ever more radical, speculative and ill-defined formulations. Starting with the quantized string, then branes, then an unknown 11 dimensional “Master/Mother/Membrane” theory, nowadays it’s “It from Qubit”, with no one having any idea how the qubits are going to be put together to get “It” (or even what “It” is). The people who have been doing this for nearly forty years (as well as the younger ones they have trained, no the field is reproducing, can’t die off) find it inconceivable that this might all be a mistake.

  37. anon2 says:

    The field is reproducing, to some extent, but it seems to me that the number of students interested in string theory or high-energy physics in general has dropped significantly during the last 10-15 years.

  38. Suomynona says:

    DV:
    Last I checked it wasn’t possible to recover GR as a low energy limit of LQG. The existence of something which can be identified with a massless spin-2 excitation is, I thought, still an open question. Do you have a reference?

    It’s also not obvious why SU(2) should be the gauge group of quantum gravity.

  39. Peter Woit says:

    All,
    This really isn’t the place for this kind of technical argument over LQG, which, honestly, gets tedious real fast when most people involved are trying to score points, not clarify things. The interviews in this book with people who have worked on LQG, are working on LQG, or various ideas that have grown out of it contain a lot of detailed arguments of this kind, with references. If you want to understand things, those are great sources.

    About SU(2) and quantum gravity, the answer is independent of LQG, going back to the story of Ashtekar variables, explained a bit in the latest version of my paper on unification using twistors and Euclidean QFT. It’s simplest in Euclidean signature, where the space-time rotation group is Spin(4)=SU(2)xSU(2). The remarkable fact is that in 4d you can write down the condition for the curvature of a connection to solve Einstein’s equations just using one Lie (SU(2)) component of the spin connection, so you have an SU(2) gauge theory (you also have vierbeins though, which are a different story).

    Minkowski space-time makes this more problematic, since there Spin(3,1)=SL(2,C).

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