Susskind: String theory not a complete picture of how quantum gravity works

For the latest on quantum gravity, readers might want to look at talks from some events of the last couple weeks. At the new ICTP-SAIFR theoretical physics institute in Sao Paulo, a school on quantum gravity has talks available here, with a follow-up workshop here. At Stanford last week the topic was Frontiers of Quantum Gravity and Cosmology, in honor of Renata Kallosh and Stephen Shenker.

Matt Strassler was at the Stanford conference, and he blogs about it here, describing most of the speakers as “string theorists” who are no longer working on string theory, and most of the quantum gravity talks as not being about string theory (this is also true of the ICTP-SAIFR workshop). I don’t really understand his comment

Why has the controversy gone on so long? It is because the mathematics required to study these problems is simply too hard — no one has figured out how to simplify it enough to understand precisely what happens when black holes form, radiate particles, and evaporate.

since the problem isn’t “too hard” mathematics, but the lack of a consistent theory (which he makes clear later in the posting).

Most remarkably, he described the talk by Lenny Susskind, one of the leading promoters of string theory, as follows:

Susskind stated clearly his view that string theory, as currently understood, does not appear to provide a complete picture of how quantum gravity works. Well, various people have been saying this about string theory for a long time (including ‘t Hooft, and including string theory/gravity experts like Steve Giddings, not to mention various experts on quantum gravity who viscerally hate string theory). I’m not enough of an expert on quantum gravity that you should weight my opinion highly, but progress has been so slow that I’ve been worried about this since around 2003 or so. It’s remarkable to hear Susskind, who helped invent string theory over 40 years ago, say this so forcefully. What it tells you is that the firewall puzzle was the loose end that, when you pulled on it, took down an entire intellectual program, a hope that the puzzles of black holes would soon be resolved. We need new insights — perhaps into quantum gravity in general, or perhaps into string theory in particular — without which these hard problems won’t get solved.

For many years now, the most influential figures in string theory have given up on the idea of using it to say anything about particle physics, and results from the LHC have put nails in that coffin, removing the small remaining hope that SUSY or extra dimensions would be seen at the TeV scale. The “firewall” paradox seems to have made it clear that string theory-inspired AdS/CFT doesn’t resolve the problem of non-perturbative quantum gravity, leading to renewed interest in other approaches. This leaves string theory now as just a “tool” to be used to study topics like heavy-ion physics. Things don’t seem to be working out very well there either.

This entry was posted in Uncategorized. Bookmark the permalink.

16 Responses to Susskind: String theory not a complete picture of how quantum gravity works

  1. P says:

    Per usual, overly negative and pessimistic.

    Matt’s treatment is quite fair, though, also per usual. I’d recommend readers of your blog to look at his for a more balanced approach.

  2. lun says:

    As an orthogonal issue, but one related to something you discuss on this blog often, by the calibre of speakers and topics discussed, and the _level_ of the discussion, this conference could have been at the IAS, yet it is nearly entirely organized and financed by a “BRIC” country.
    Employment in an institute as prestigious as the ICTP (or, say, the Tata institute in India) is likewise becoming equivalent to employment in a large research level university in the US and Europe.
    Could this be the answer to the abysmal situation in the “first world” job market in theoretical physics? Moving to a country such as India or Brazil could become a realistic alternative to leaving physics for young people interested in basic science.
    Since employment in these institutions follows somewhat different administrative rules w.r.t. the US system, this could also lead to a research program that is less dependent on senior-initiated fads a la landscapes and firewalls.
    Could BRIC countries rescue theoretical physics?

  3. Bernhard says:


    the “somewhat different administrative rules w.r.t. the US system” these institutions follow is a complete nightmare. If you want a job in a public institution in Brazil you have to have abundant time to waste to follow the endless and also saturated process of the so-called “concursos”. Ah, and be prepared to speak fluent Portuguese by the time you apply, you won´t go far there with just English. I think we are slightly off-topic so let me stop here.

  4. Pete says:

    Background: the concurso is a long (all day, occasionally two) exam, usually written in Portuguese and then read out by the candidate to the committee, set by the university to which you apply on (usually fairly general) topics of their choice. If you apply lots of places, this takes a lot of time…

    The institutes (as opposed to the public universities) often don’t do the concurso, rather their hiring tends to be very much US-style. You’ll be expected to teach in Portuguese at some point, but you probably have at least a year to learn, and it’s not hard (especially if you know Spanish, of which it’s pretty much a dialect).

    Also the public universities (especially the better ones) are starting to let people do the concurso in English. You’ll still need to be competent to pass, and you’ll need to commit to learning Portuguese fairly soon. But the system isn’t especially overloaded. If you know stuff outside your specialisation reasonably well, you can pass the concurso. If you only know stuff outside your specialisation at second-year undergraduate level though, either you have to cram once the topics are released or you will not pass.

    On the other hand, at a university the teaching load isn’t especially nice, unless you like teaching evening classes for the part-time students (of course, if you really get into the culture then the class ends about the time you want to go out, get a meal and go on the town; and you won’t be teaching before lunch the next day). And you probably don’t want to have children there otherwise life gets too complicated.

  5. Bernhard says:


    “But the system isn’t especially overloaded.”

    It is not specially overloaded if you compare with the normal situation in HEP. And specially if you want to be in a center – the research situation outside the centers is not as nearly as decent. And about the “concurso”, the form varies from university to university with places like CPBF having much more specific exams. There, by the way, it is a terrific place to work, with zero teaching obligations. But a piece of this cake is in my opinion as hard (or as easy) as getting a position in Europe. The rest is “easier”, but not as nearly as interesting.

    All in all, if you look at Brazilian faculties, there are usually 99% Brazilians working there. The situation might change a bit, but I doubt it in any drastic way in the near future.

    Peter, sorry for highjacking the discussion – my last comment about this…

  6. Hernán Meier says:


    Well this is my first post, but if you are considering to do a Ph.D in an underdeveloped country at the same level of top institutions in developed countries you can consider the Balseiro Insititut in Argentina ( it´s considered the best Physic and Nuclear related institute in Argentina and maybe the entire Latinoamerica.
    Moreover, with the plus to be located in Bariloche city (Tahoe is awful compared to that place)
    The main problem is the mathematical level they ask you to have the entry.

  7. Arjun says:

    “Since employment in these institutions follows somewhat different administrative rules w.r.t. the US system, this could also lead to a research program that is less dependent on senior-initiated fads a la landscapes and firewalls.
    Could BRIC countries rescue theoretical physics?”

    I am from India and got my PhD at an US institution in theoretical physics. I can say with confidence that this would be a very bad career move—to try to move to India—even if you get a faculty position at TIFR or IISC or HRI. For one, you will be paid much much lower than what your colleagues pursuing private sector jobs will be earning in India, in Indian rupees. And unlike in the US where even as a physics graduate student you have access to pretty good standards of living, you will find yourself struggling to provide yourself and your family with basic luxuries. While you will be allocated a govt. built apartment within the campus itself, it will be of very dismal quality.

    Not to mention the hierarchy nightmare over there in India. Indian culture is traditionally hierarchical. This coupled with the colonial history has resulted in a deeply hierarchical society. So your seniors will drive your research agenda. Moreover to get a faculty position over there you have to be “known” by these seniors and also be “liked” by them.

    A few colleagues made such moves tempted by the recent “boom” in Indian economy and the promises of the govt. to put more money into research. One colleague told me to never, never come back to India; another said that he’s decided to settle for it even though he knows he’ll not do world-class research.

    And given the fact that the economy is now tanking (much talked about in both Indian and international media these days), it remains to be seen if the funding promises will be fulfilled or not. Given my experience with India, most probably not!

  8. Peter Woit says:

    Well, at least string theory has given us this:

  9. Marcus says:

    Tim Blair! McGill, MS advisor was string theorist Alex Maloney. Totally awesome you must watch this, only 8 minutes. Thanks, P.W.

  10. Marcus says:

    correction: Tim Blais (aka “acapella science”)

  11. abg says:

    [off-topic] The best Brazilian universities (USP and Unicamp) can still hire without “concurso” and knowledge of Portuguese (which is not a Spanish dialect!), but only English. However it is rare.

    Also, there exists some kind of “intellectual communism” in Brazil, such that all professors are treated equally, in every aspect, independent of the quality of the work which they do. Once someone is approved in a “concurso”, he or she can do absolutely nothing and still maintain his or her position. It is very difficult to fire a public worker in Brazil. And the result is a bunch of incompetent people occupying jobs positions, while the stimulated and talented scientist must stop his research to study for a “concurso”.

  12. NumCracker says:


    Actually, it is a “pseudo intellectual communism” once you forget about the probatory time before getting tenured. On the other hand, research grants are attributed on the basis of productivity as anywhere in the developed world. Also, I recommend people interested in moving to South to write to Nathan Bercovits @IFT … he is an american, with high qualification standards in ST, which decided to do such a wise move some of guys here are describing. Nathan seems happy at IFT/ICTP and is still very productive here in Brazil. Would I start a PhD/Posdoc in HEP in South Hemisfere it would be there.


  13. abg says:

    [Off-topic] I do not think that the probative stage helps very much. Indeed, once someone is approved in it he or she can do practically *no research* at all and have a guaranteed position (recall that the duration of a probative stage is around 2 years, and most academicians will work for 2 or 3 decades!). Even worse, a docent can be continuously promoted in Brazilian public universities but can never recede.

    And about the research grants, it is true that it is based in productivity, but it is not necessarily a good thing since it is extremely normative! For example, the article in which Cesar Lattes changed completely the field of particle physics (he was one of the discovers of the meson pi) would have the same value as any article published in any magazine independent of the quality of the work; and it would have 1/2 of the value a published textbook, again independently of the its quality.

    It would be better if the universities here could negotiate the grants and even in the academic position freely, that is, without any rigid governmental bureaucrat legislation, like many universities around the developed world (this is the case of USP and Unicamp, but is is an exception).

    Let me finish by saying that, *however*, there exists brilliant scientists working at now in Brazil, and they are very product in a broader sense. See, for instance,

  14. abg says:

    Another remark: Nathan Jacob Berkovits works at UNESP, which is a state university of São Paulo, like USP and UNICAMP. I would say that these 3 SP’s State Universities are more or less an exception to my comments above.

  15. NumCracker says:


    While our exchange is completey off-topic it may be helpfull for some enthusiasts wanting to definetely move, or just doing a pos-doc, bellow the equator 😉 So I intend to finish my collaboration by just saying that other interesting (exceptional) federal institutions are: UFRGS, ITA, UFABC, UFRJ, UFMG, UNB, UFPE, CBPF. In addition, researchers have decent computational infrastructure provided by SINAPAD/MCT and free-access to a huge national base of scientific periodics (by CAPES.)
    The really accurate numbers are 4 years before getting tenured, 35 years of service before retiring. The average salary is about US$ 50k up to 100k depending on the hierarchy. Even after tenured, one just progress on such carrer by commitment and effective productiveness during decades. One would expect to spend at least 8h/w by lecturing, but more realistically one would consider about 12h/w. It is not a quite dynamic scientific environment as (*part of*) Europe or USA, but with that salary (and a properly chosen place to live) life is surely nicer.


  16. abg says:

    I agree with the information in the latter post, which are indeed accurate. Except a detail: it is in fact possible to be promoted in a public university in Brazil without (a high) productivity, but just with sufficient “time of service”, that is, being associated in the university for a time long enough.

    Finally, if some academician is interested in coming to Brazil to work in any (public) university, I recommend the reading of the following blog, maintained by a Brazilian logician, who wrote (and still write) a lot of things about the life in science here:

Comments are closed.