Friday, December 07, 2012

What is background independence and how important is it?

Bugmenot asked:
  1. What is background independence and how important is it?

  2. In order to be a theory of everything, will the final string-theory/m-theory have to be background independent?

  3. Does the current lack of background independence show string theory is currently NOT a theory of everything?
My understanding from Wikipedia is that the ADS/CFT shows hopeful hints. Are there any recent papers that have made progress in this direction?

I've tried google but get haven't been able to get a definitive answer to this question.

I found this interesting post by Lubos Motl, but it is from 2005.

Before I answer, let me mention some news: Sean Carroll is leaving Discover Blogs (and his largely inactive co-bloggers at Cosmic Variance), returning back to his personal Preposterous Universe.

Lumo answers:
  1. Background independence is generally the independence of the equations defining a theory on all the allowed values of its degrees of freedom, especially values of spacetime fields, especially the metric tensor. However, this concept has various levels that are inequivalent and the differences are often important to answer questions about the "necessity" of background independence, see below.

  2. We don't know. The [manifest, see below] background independence is an aesthetic expectation, one could say a prejudice, that we cannot prove in any scientific way, so the progress in science may show that it has been a good guide or it was a misleading excessive constraint. For several centuries, we have known that science can't systematically make progress by imposing arbitrary philosophical dogmas and stubbornly defending them. Science often finds out that some philosophical expectations, however "beautiful" or "convincing", have been invalid. Expectations about the "background independence" aren't an exception. Again, it is unknown whether the final "best" form of a theory of everything (if there exists "one best form" at all, which is another albeit related "if") will be [manifestly] background-independent.

  3. No, there's no known way to show that the lack of background independence already implies that a theory isn't a complete theory of all interactions and types of matter. Some necessary conditions for consistency may be understood in the future but at this moment, it's a speculation whether they exist.
Now, the subtleties. You implicitly wrote that string theory is background-dependent. This is a very delicate question. Some formulations (particular sets of equations used to define the theory, at least for a subclass of situations) such as AdS/CFT or Matrix theory are background-dependent. For example, AdS/CFT is formulated as a theory with a preferred background, the empty space \(AdS_d\times M\), and all other states are built "on top of that". Similarly, matrix theory defines the theory for the flat space times some simple manifold (torus, K3, etc.). There is no way to see "completely" different backgrounds in this picture and even the equivalence with other nearby shapes of spacetime is far from obvious. In Matrix theory, one has to construct a new matrix model for a new background (this fact is a part of the light-cone gauge package).

However, these are just observations about what the equations "look like". Invariant statements about a theory clearly shouldn't depend on the way how equations "look like", about some possibly misleading coating on the surface: they should only depend on the actual mathematical and physical properties of the theory that may be measured. When we are asking questions about the validity or completeness of a theory, we should really be talking not about "background independence seen in the equations" but rather "background independence of the dynamics".

The dynamics of string theory is demonstrably background-independent.

This point may be shown in most formulations we know. Perturbative string theory (which requires the string coupling to remain weak and uses the weakness to organize all the around "fundamental strings" as the only elementary objects while everything else is a "soliton" or "composite") is a power-law expansion around a predetermined background but we may easily show that if we define perturbative string theory as an expansion around a different background, we get an equivalent theory. One background may be obtained from another background by adding actual physical excitations (a coherent state of gravitons and moduli) allowed by this "another background". There's only one perturbative superstring theory in this sense – whose spacetime fields may be divided to "background" and "excitations" in various ways. But the freedom to divide the fields into "background" and "excitations around it" in many ways isn't a vice in any sense. It is a virtue and, one could say, a necessity because a preferred background (identified with the vacuum ground state) is needed to describe the Hilbert space in an explicit way, approximately as a Fock space.

There is a related question whether the "space of possible backgrounds" is connected. Much of it is connected by dualities and various transitions: T-dualities, S-dualities, U-dualities, conifold and flop transitions, and various related ones that are more fancy and understood by fewer people. It's much more connected than people would be imagining in the 1980s. When we look at simple and symmetric enough vacua, they really seem to be connected: there's just one component of string/M-theory. On the other hand, the total connectedness isn't a dogma. It's a scientific – and mathematical – question whose both answers are conceivable until proved otherwise. The same equations may admit solutions that can't be deformed to one another at all. My ex-adviser Tom Banks is a defender of the viewpoint that sufficiently different backgrounds in string/M-theory should be considered disconnected although his quantum-gravity-based reasoning isn't quite comprehensible to anyone else.

When we talk about background independence, there is one more technical question, namely whether we want the theory to have the same form for all backgrounds including those that change the spacetime at infinity, or just backgrounds that preserve the fields in the asymptotic region. AdS/CFT is background-dependent in one sense because it requires the fields at infinity to converge to the \(AdS_d\times M\) geometry with all the fields at their expected values (usually zero). Generally, configurations that change the asymptotic region are "heavily infinite-energy" states that can't really be constructed reliably in the original CFT. However, if you only consider backgrounds that differ in the "bulk", one could still say that even AdS/CFT (and similarly Matrix theory) is background-independent although not manifestly so.

There's one sense in which the AdS/CFT has always been background-independent: the detailed places in the "bulk" aren't visible in the CFT description because the CFT has a smaller spacetime dimensionality. One spacetime dimension (the radial one, plus the compact ones if they exist) is totally emergent. Because the regions of the AdS space aren't visible in the formulation (CFT) at all, we may say that the CFT (holographically) produces the background out of nothing.

Now, the big elephant is "manifest background independence", a form of equations that don't try to show you any preferred background at all and that are as easy (or difficult) to be applied to one background as any other, arbitrarily faraway background. All the backgrounds should emerge as solutions and they should emerge "with the same ease". This is the "manifest background independence". Some people always mean "manifest background independence" when they talk about "background independence": it should be really easy to see that all the backgrounds follow from the same equations, they think. Again, it's an aesthetic expectation that can't be shown "necessary" for anything in physics, not even the "completeness" of a theory as a final TOE.

There are limited successes. For example, the cubic Witten's open string field theory (of the Chern-Simons type) may be written in the background-independent way so that the cubic term is the only term in the action that is left. It's elegant but in reality, we always solve the equations so that we find a background-like solution and expand around it, to get back to the quadratic plus cubic (Chern-Simons-like) form of the action. While the purely cubic starting point is elegant, we are not learning too much from the first step: we're just reformulating the consistency conditions for the backgrounds as the fact that they solve some (somewhat formal) equations.

String field theory is only good to study perturbative stringy physics (and for some technical reasos, it's actually fully working for processes with internal open strings only although all closed string states may be seen as poles in the scattering amplitudes). Nonperturbatively (at strong coupling), background independence becomes harder because it should make all S-dualities (equivalence between strongly coupled string theory of one type and weakly coupled string theory of another type or the same type) manifest. Despite the overwhelming evidence supporting dualities, there's no known formulation that makes all of them manifest.

There's no way to convincingly argue that there's something wrong about this situation. In fact, one could go further. One could say that physicists have accumulated circumstantial evidence that "the formulation making all symmetries and relationships manifest" is a chimera, whether we like the flavor of these results or not. It's quite a typical situation that formulations making some features of the theory manifest make other features of the theory "hard to see" and vice versa. Because it's so typical, it could even be a "law" – a new kind of "complementarity" which goes directly against "background independence" – although we would have to formulate the law rigorously and no one knows how to do so.

For example, ordinary perturbative string theory in spaces asymptoting the 10-dimensional Minkowski space may be written down using "covariant" equations. That's the word for a description that makes the spacetime Lorentz symmetry manifest. But when we do so, the unitarity – especially the absence of negative-norm "ghost" states in the spectrum – becomes hard to prove. And vice versa. The light-cone gauge formulations make the unitarity manifest but they obscure the symmetry under some generators of the Lorentz symmetry. It's sort of inevitable.

Also, the covariant approaches (RNS) make the spacetime supersymmetry somewhat hard to prove. This "complementarity" may not be inevitable; Nathan Berkovits' pure spinor formalism, if it works and I bet it does, makes both the Lorentz symmetry and the supersymmetry manifest. It's also close to a light-cone gauge Green-Schwarz description so the "unitarity" isn't too hard, either. However, it has an infinite number of world sheet ghosts (and ghosts for ghosts, and so on, indefinitely) and one could argue that the absence of various problems connected with them is non-manifest.

The landscape of string/M-theory, as we know it today, is rather complicated and has lots of structure. We must sharpen our tools if we want to study some transitions in this landscape, a region of it. The tools needed for distinct questions seem to be inequivalent. A manifest background-independent formulation of string theory would make all these transitions equally accessible – all the tools would really be "one tool" used in many ways. In some sense, this desired construction would have to unify "all branches of maths" that become relevant for the research of separated questions in various corners of string theory (and believe me, it does look like different corners of string theory force you to learn functions and algebraic and geometric structures that are really different, studied by very different mathematicians etc.). It would be a formulation that stands "well above" this whole landscape "manifold". Such a "one size fits all" formulation is intriguing but it is in no way guaranteed to exist and failures of attempts to find it over the years provide us with some evidence (although not a proof) that it doesn't exist.

Instead, many people are imagining that string theory's landscape is a sort of a manifold that must inevitably be described by "patches" that are smoothly glued to their neighbors. Each patch requires somewhat different maths. Just like manifolds may be described in terms of an atlas of patches, the same thing could be true for the landscape of string/M-theory. We also have more unified, less fragmented ways to think about the manifolds. It's not clear whether the counterpart of these ways is possible for the stringy landscape and if it is possible, whether the human mind is capable of finding it.

So nothing is guaranteed. The transitions in the landscape and the dualities and duality groups are so mathematically diverse and rich that a formulation that "spits out" all of them as solutions to some universal equations or conditions is an ambitious goal, indeed. It may be impossible to find it.

I also want to mention one simple point about non-stringy theories. The background independence is sometimes used as a "marketing slogan" for some non-stringy proposals but the slogan is extremely misleading because instead of explaining all the duality groups in the whole landscape, including e.g. the \(E_{7(7)}(\ZZ)\) U-duality group of M-theory on a seven-torus (these exceptional Lie groups are rather complicated by themselves, and they should appear as one of the solutions to some conditions among many), these alternative theories rather tell you that no spacetime and no transitions and no interesting dualities exist at all. While their proponents try to convince you that you should like this answer, this answer is obviously wrong because the transitions, dualities, and especially the spacetime itself does exist. This version of "background-independent theories" should be called "backgrounds-prohibiting theories" or "spacetime-prohibiting theories" and of course, the fact that one can't derive any realistic spacetimes out of them is a reason to immediately abandon them, not to consider them viable competitors of string/M-theory. This version of "background independence" has absolutely nothing to do with the ambitious goal of finding rules that allow us to derive "all dualities and transitions we know in physics (not only the new, purely stringy ones but also the older ones that have been known in physics before string theory)" as solutions. Instead, this marketing type of "background independence" is a sleight of hand to argue that we should forget all the physics and there's nothing to explain, no dualities, no transitions, no moduli spaces, no spacetime. And when we believe there's nothing out there, no relevant maths etc., a theory of everything becomes equivalent to a theory of nothing and it's easy to write it down. That's a wrong and intellectually vacuous answer that should be refused, not explained or adopted.

To summarize, background independence is generally an attempt to find as universal, all-encompassing, and elegant formulations of theories, especially string/M-theory, as possible, but it is an emotional expectation, not a solid condition that theories have to obey, and we must actually listen to the evidence if we want to know whether the expectation is right, to what extent it is right, and what new related issues we have to learn even though we had no idea they could matter. It's also possible that the background-independent equations are actually "conditions of consistency of quantum gravity" (which may be written by some quantitative conditions whose precise form is only partially known): when we try to find all the solutions, we find the whole landscape of string/M-theory. Such a formulation of string/M-theory would be extremely non-constructive but after all, that's what "background independence" always wanted. Maybe we don't want too much of background independence.


  1. Ha ha :-D

    I had already spotted that question over there (darn why can I not just stop clicking Physics SE :-/ ...) and planned to read Lumo's answer during my lunch break.

    Now it has become a TRF article and I can read it here, thanks Lumo :-)

  2. So until we have a background-independent ST formulation it will be impossible to take the landscape picture seriously, right?

  3. Well, no, one of my points was exactly the opposite. Such a formulation could make certain claims on the landscape "truly rigorous" but it's in no way guaranteed that it must exist, maybe it doesn't, but even if it doesn't exist, it doesn't mean that one can't study string theory reliably. We've been doing so increasingly accurately for 40+ years.

    I think that the existence of the landscape of possible solutions is pretty much settled by now. That of course doesn't mean that these possibilities are realized "somewhere" in the real world. The existence of a multiverse is a different, less guaranteed to be true hypothesis than the existence of the landscape. And even if the multiverse exists, it's very far from knowing that any method used by the anthropic people is actually valid.

  4. Dear Dilaton, I hope you have calmed down because of the SE politics. In fact, I believe that even Ron will return. It's a part of his identity. He will just accept that they had the power to suspend him. In 1 month, all the punishment will "completely evaporate". It's not something that is meant to have permanent consequences.

  5. I have no background (:D) in string theory so I hope my question is not totally stupid...

    Is this a similar situation to what is happening with e.g. spin measurement along different axis? That in priciple there might be properties of the space-time and matter that cannot be observed at the same time?

  6. Thanks for your story, Ron, but come on. There is no religion with commandments you sketched. Your service for the community is making impact on many people who enjoy it - and there are forces that you simply cannot influence.

    Be sure that some people may be begging you to return there in one month but I calmly ask you to return without begging because this is what a mature man realizing he can't also win often does and because the religious fundamentalist behavior is simply inappropriate in such a situation. It's a server that belongs to none of us, there is a lot of freedom for physics-oriented activities and fun, but there are some limits given by the server's not living in the middle of the vacuum.

    If I knew about some realistic picture with new rules that would protect you/us from similar situations, I would support it, but I know that de facto changing the ownership rights of a domain that may have value vastly exceeding what we could pay is simply not a plausible - or fair - goal. It would look like an attempt to "nationalize" i.e. steal the domain or server, and that's something we know from 1948 and we don't have good experience with, I think.

  7. Lubos, of course I don't mean taking anything away from anybody. I mean a new site, like math.overflow, with a new host and a new domain (and run on available free q/a forum software). As for the religion, there is such a religion, one which you unconsciously and instinctively follow, which was founded in the 1920s by Wolfgang Pauli. It's this religion that comanded you to criticise loops, even at great personal and professional cost.

  8. Free stackexchange clones exist, they are reasonable. I do not have any attachment to the site anymore, I only liked it because I could tell it was free for sure (and I could tell because I wasn't banned). I am not interested in "role playing games", I hate online games. My addiction is to doing justice for the underrecognized marginalized folks, and for advertizing their results and for exposing the charlatans and frauds, and the fraudulent ideas, that have high positions in the field today. This doesn't take a lot of effort if you know what you're talking about. Lubos has been doing it here for years. We disagree on some of the technical content, but not on the need for honest criticism of dogmatic claims.

    You are wrong about the financial aspects--- the reason this happened now is because I came to a bunch of other sites (like stackoverflow) and annoyed them with asshole behavior, complaining about the moderation on skeptics, and saying that they need to tolerate assholes, like the physicists do. This is a goad to them to try to get the physicists to stop doing it. If the site is capable of being taken over by shog9, then I'm going to get banned, and I don't want to waste too much time, get it over with early, because the longer you're there, the harder it is to migrate your text to the next forum.

    The previous forum was wikipedia, they were totally free until around 2008. The one before was usenet.

    I only used stackexchange because it was science, and it was open (or at least, it quickly turned open with a little bit of prodding), so it gives a way to say correct things that can't be said elsewhere:

    1. Large extra dimensions are bunk

    2. Cold fusion is not.

    3. Loop quantum gravity has degenerated into obviously false nonsense (although the ideas are still annoyingly mathematically interesting).

    4. S-matrix theory was suppressed unfairly.

    and so on and so on. I tried to mix answers to well known stuff, some new calculations that didn't take more than a day or two to do, and proselyzing for discarded stuff. There are also smaller things, like recognizing Chew and Mandelstam for their contribution to string theory, and Candlin for Grassman integration.

    But mostly it serves as a publishing avenue for original stuff that is suppressed elsewhere, that you are just not allowed to say because of the politics of science. This is the purpose the internet serves. It also served the purpose of publically refereeing some wrong stuff in the literature. It was easy enough to do both things when anyone is free to criticise and argue, because nobody could come and claim stupid things with impugnity (as some nuclear physicist just came and did on my last answer, with no fear of getting called out or exposed for charlatanry), because they would get _called out on it publically_, by knowledgable people. Those people who didn't know their stuff either learned it, or got intimidated and left, or else stuck to answering really simple questions.

    That's the way it should be. That's the way math.overflow works, and this is why math is having a reneissance the likes of which we haven't seen since, well, the reneissance. (I am sorry Lubos for spamming your site, I'll stop now. If it's too much unrelated crap, I understand if you throw this into the dustbin).

  9. Hi Ron ;-)

    I said to Ron in chat he can come here to ping me if he feels like doing it :-D
    Yeah, I strongly suspected that you would not come back ...
    What annoys me is that it is not our physics community that has an issue with you being there speaking your opinion, being blatantly honest, etc but people who should neithe be concerned about it (since they are not interested in physics and in the site) nor should they constantly negtively interfer with our business.

    I've heard that MathOverflow is an independent mathematics research level site, I am right in this? And if so could we seal a deal with them to establish an analogue PhysicsOverflow site of the kind, level, purpose, etc as you suggest? I dont know any people at MathOverflow ...

    I was thinking about asking a question in Physics SE meta about if we can and want to make a new site outside the SE network since what happened to you really annoyed me.

    But now I have to think about an important comment Eugene has adressed at me. I see he is here to and I will have to ask him something about it ...

    Maybe you can peak in here on TRF from time to time, just dont ask about CF here ;-P
    (i am joking ...)


  10. Hi Eugene,
    I wanted to mail you tomorrow but now I see you here.

    Thanks for your enlightening comments in the vigorous discussions, they were valuable at least to me.
    You are right and I am much too addicted to the site, letting me carry away to far.

    Since this election business started I was permanently worried about things going bad and the shooting down of Ron almost stole my sleep ... :-/. The shoking thing is even though it was not fun at all about the last 2-3 weeks I could not let go and it started do affect my "real world business".
    Do you (or Lumo) have an advice what I should do now in my situation? I'm not sure if I should completely stay away from it (but for how long a time), just not log in, only reduce the times I visit it or ...?

    I am now quite shoked by my addiction :-(

  11. Dear Lumo, what about a Physics or even better TheoreticalPhysicsOverflow addition / addition or spinoff to this MathOverflow site I have heard of? Do you or Ron have contact with some of these guys?

  12. Dear Ron, thank you for your reply.

    If the site is capable of being taken over by shog9

    Unless I have seriously misunderstood, "shog9" (interesting choice of screen name: the Shoggoths are the hideously deformed, murderous beings created by the Old Ones to do their bidding), the individual who suspended you until next month, is not (only) a volunteer moderator on another SE site but a full-time salaried employee of Stackexchange, thus he is not "taking over" anything that his boss doesn't already own. But even if he's not a paid employee, he is doing the owners' bidding.

    Free stackexchange clones exist, they are reasonable.

    Great! What are you waiting for then, why haven't you started a competing website already? I'll tell you why, it's because it's a lot of hard work with no certainty of success. The founder of Stackexchange worked like an animal for years, almost not sleeping at all, working around the clock, ruining his health, completely neglecting his family and personal life. All without any certainty that his huge investment of time and health would ever pay off. That`s what being an entrepreneur means, you take immense risks and break your back for years. You profited from his labor because it afforded you a podium and a microphone and that's fine, because he's also profited from you. That he is immensely richer than you now is also fair, it's how our capitalist system rewards successful entrepreneurs, and no better system that provides the greatest good to the greatest number of people has yet been found.

  13. Dont worry Ron, Lumo would kindly ask you to stop being a canonical spammer if he were annoyed with you posting here ... ;-)

  14. @Ron Maimon

    Joe Fitzsimons (who proposed theoretical physics.SE) tried to start an independent site after theoretical physics.SE was closed down. But I don't think it gained enough support. It might be interesting to see if he is still willing to pursue the project. He established a discussion page, see here .

  15. Dear Dilaton, honestly I do not feel that I can counsel you on what to do, nor even that you need it.

    I have a very high opinion of your character and intellect and I am one hundred percent certain that you will quickly choose what is best for you :)

  16. Lubos was stunningly fair to the cold fusion comments I made on a thread here on TRF, despite more than usual assholish behavior from me, despite me spamming the site with something like 4 comments in a row, and despite the fact that he considers these cold fusion ideas "crackpot rants". This only cemented my certainty that he is an honest to goodness scientist, with actual unwavering integrity (something which one usually could infer from the quality of his science commentary, but you never know, sometimes people sell out when not protected by an academic umbrella--- I know the pressure). I hope he could read the cold fusion explanation I put on stackexchange, and look into the details of deuteron hole banding and 3-body fusion (although this stuff is more condensed matter than he usually studies, there are really no experts here, and the qualitative agreement with otherwise completely inexplicable Iwamura mass-spec transmutation data makes me pretty confident that this is the right explanation), because he might have an idea for calculating the lifetime and energy width of the KeV bands, seeing what density is required for 3-body fusion rates, estimating cross sections, or perhaps rule it out entirely (not with bogus back of the envelope estimates, but using actual cross section estimates at 20KeV) or find the expected x-ray intensity. These were things I was supposed to calculate over thanksgiving, but instead my daughter had an ear infection, and I was busy programming some bio stuff.

    The only issue with math.overflow is that I think they licensed the stackexchange software, and this is expensive. The free clones are not as full featured, but close (I looked at some of them), but branching off of math.overflow might be a good idea anyway. I know one of the folks who runs it, but not very well (he was an undergrad prof. of mine, and he is very good). If that doesn't work, the only expensive thing is the hosting, and there might be academic tricks around that (for example, qmechanic and Arnold are profs, and might be able to persuade someone to host a site on a university server). The software issues are really next to nil, even if you don't license the software, and even if you want eye candy, the amount of work required to get a site online isn't so large (the math.overflow people did it themselves with a full research and teaching load.

    Also, since the entire content of SE is licensed under a free license, one can borrow many nice questions without asking, the theoretical physics ones and research-level ones for sure.

    But I hope one agrees that one will not censor ideas, just downvote them (if they deserve it). I really don't want to suppress anything of potential value, and that means tolerating a certain amount of downvoted junk.

  17. Go to stackoverflow, and tell shog9 to try to reproduce with himself. End of problem.

  18. It's not that much work--- you overestimate the work this internet entrepreneur put in. Stackexchange is piggybacking on usenet, everyone involved in usenet looked to do something to keep the spirit alive. So you got Wikipedia, and MySpace, and blogs, and so on. One stackexchange clone written as masters project for CS folks, it takes a few weeks at the worst. The big barrier is hosting, which I don't have, and I am working at a university I hesitate to name, because I don't want their reputation to suffer by association.

  19. Dear Lubos,

    in the perturbative quantisation of gravity, one splits the metrics into a background metrics and into a small perturbation, which becomes the graviton field after the quantisation. In perturbative string theory, the graviton is one of the many vibration modes of the string.

    From this I suspect that simply:

    background-dependent = perturbative definition

    background-independent = nonperturbative definition

    If this is true than a nonperturbative definition is clearly needed in the end, because the perturbation series is not convergent, so I can only use it for crude estimates.

    Am I right?

  20. @facebook-500289561:disqus ---I don't always follow your explanations on physics SE, and I don't always agree with them, but I enjoy them and the energy you bring to the site. I haven't done theoretical physics since grad school, but I try to keep up when I can. I got frustrated by the, what I saw, as rather arrogant and arbitrary rulings by some of the moderators and stopped posting there, but still visit. I also enjoy this blog immensely when it discusses physics and math topics, and I agree that mathoverflow is much better than physics SE. It would be nice if you could create a blog of your own. I do know and appreciate how much work Lubos puts into his blog posts, and also that he can be insulting when I am being stupid, but I defer to superior knowledge and the interaction can be invigorating even :) Best regs, GW

    I don't know why you support cold fusion and haven't looked into it, but

    I know that Julian Schwinger thought it was something and felt insulted that he was ignored and called a crackpot (in the last few years before he died.)

  21. @facebook-500289561:disqus Ron I would like to host such a site. I have hosting with no restrictions. I think OSQA is the best clone. I will gladly give you FTP access. I would set up the software but otherwise be completely hands off. I'll even chip in to register a domain if you guarantee you'll put your knowledge and effort into it. Let me know your email if you are interested.

  22. Good luck, Dilaton. But I am simply not into this nomadic lifestyle, permanently escaping something and going somewhere else where one has absolutely no good and rational reasons to think that things will be rosier.

  23. No, the background independence isn't the same thing as nonperturbative definition.

    The ability of going to strong coupling without making the definition less relevant is a part of the "complete" background independence. But it's not the only hard task. Background independence must also allow you to change the shape of the spacetime, not just the coupling constants such as the string coupling. For example, matrix string theory is a nonperturbative definition of type IIA string theory and it exactly defines the theory for any finite value of "g" but it's still background-dependent because you can't use the theory for backgrounds with additional dimensions compactified on a Calabi-Yau, and most other similar things.

    The converse implication doesn't necessarily hold, either. Conventional perturbative string theory is surely perturbative but it is "partly" background-independent because aside from the restriction on weak coupling, one may change the spacetime in any way. In the usual path integrals with Riemann surfaces, this independence isn't manifest but in Witten's string field theory, it may be made manifest. Moreover, it used to be believed that string field theory would be a good definition for the strong coupling as well. We no longer think so - string field theory is just another way to study perturbative string theory (plus the D-brane extensions). But it's not really a proof that there can't be any perturbative definition such that the formalism from which the perturbative series is derived also admits the calculation of all strongly coupled things "exactly" despite the fact that it is based on perturbative strings.

    But yes, due to the divergences of the perturbative series, these series just don't determine everything about the finite-g value of the amplitudes.

  24. I'd like to see a website where anna v and Arnold Neumaier feel nurtured, welcome, and protected. Both are sweet souls who give generously of their knowledge. With all due respect to you, Ron (and I have thanked you on Stackexchange for all the good that you did there, plus I feel a kinship with your rebellious nature), my support for you is riddled with caveats and to some extent hinges on the fact that anna, Arnold and others have spoken out so eloquently in your favor and, more generally, in favor of a governance that better reflects the actual culture among professional physicists. Lubos, of course, does not require my support or anyone's, he carries his own force field wherever he goes and would be comfortable posting via Telnet with mittens on his hands from a tent in the Antarctic and still gain #1 rep :)

  25. From what I've read, what we truly want is to be able to define string theory on the background of our own universe. That would give us the complete TOE as outlined in

  26. Hi Ron,

    I do not now enough about the moment about these CF to say something meaningful, would have to take some time to read more about it ... But it would be great if you could discuss it with Lumo :-).
    I was asking myself too if a University with a nice (theoretical) physics department would not be happy to host a new physics site ... I plan to write a new (last ...?) meta post in the lunch time telling them that you do not intend to come back, asking if it is time to leave SE and set up a new site and gather in the answer suggestions how we could do it, and telling Shog9 he can reproduce with himself ;-) :-D :-)... Well, the last I should better formulate a bit carefully if I want my post to have an effect ...

    BTW I have voted now for Qmechanic, Chris Gerig, and CrazyBody ... I felt uneasy with Manishearths nomination right from the start, knowining that he is too loyel to the SE policy etc. In the meantime, I have noted that mods from other sites and SE team memers are reassuring Manishearth to be there if he needs them because of the rough discussion going on in the community (look into the hbar). Sweet, isnt it (trying to not start vomiting ...)?

    BTW do you know who Qmechanic is? I like him/her very much and I am so curious to know who he/she is ... I could bet Lumo must have reported about cool papers he/she has written here on TRF ...

    Ok, now I have some work to do ...

    See you ;-)

  27. The paragraph, below, by Lubos, made me jumped to a vague conclusion (i.e. one not insanely set in stone, I hope!) that mathematical physicists who are in some extreme pursuit of a TOE will run up against an absolute requirement of learning how to "make do" similar to how the only sensible thing to do in respect of wave-particle duality is to learn to accept it.

    (For me, with my unrealistic - more so in the past than in the present - philosophizing ambitions, the solution was to make a deliberate self-controlling Tolerance Principled effort to 'accEPT' certain unavoidable 'cognitive dissonances'. %-})

    There's no way to convincingly argue that there's something wrong about this situation. In fact, one could go further. One could say that physicists have accumulated circumstantial evidence that "the formulation making all symmetries and relationships manifest" is a chimera, whether we like the flavor of these results or not. It's quite a typical situation that formulations making some features of the theory manifest make other features of the theory "hard to see" and vice versa. Because it's so typical, it could even be a "law" – a new kind of "complementarity" which goes directly against "background independence" – although we would have to formulate the law rigorously and no one knows how to do so.

  28. Hi Dilaton. Please wait a bit before speaking for Ron on META. If Ron does not believe that it is a money plan, but it is just a control freak plan maybe we can win through and regain the independence in content of physics.SE.

  29. Well, hence supergravity is the low-energy background-independent theory whose UV completion leads to M-Theory (from whom ST derives) I can't see how background independence should be an "emergent" property (instead of a foundation for physics). Could you please enlight this point?

  30. Hi Anna,

    I had just poste an appropriat question on meta suggesting to leave and start our own site... But now I have deleted it and copied my test into a word document. It was only up for about 7 minutes. It would certainly have been my dead at Physics SE too.
    But at present there are enough people with the approriate amount of reputation to see deleted posts, so I expect them to ban me anyway ...
    But it would be good if you, Arnold, Twistor, and other nice people could achieve that we get more freedom such that maybe even Ron reconsiders his decision. I will just lurk as a silent observer now.

  31. This is what I had posted, I can still repost it if needed ... :

    Ron Maimon does no longer wish to return to Physics SE.

    He told me this yesterday in the comments below this
    TRF article. In his opinion, the ever increasing negative impact on our
    site and community of people, who are not interested in physics and
    (the well being) of our site and community, but only care about the
    enforment of their rules and policy by all means, has led to a phase
    transition of physics SE into a state where it is no longer possible to
    freely interact and learn and discuss physics as would have been needed
    in a physics community.

    I mostly agree with this. Allowance of an open discourse whererin
    mistakes and wrong ideas can (sometimes blatant) honestly and
    explicitely be critisized is of paramount importance to make progress in
    science. Interested non experts looking at physics SE too profit and
    learn most if it is clearly stated and distinguished what is correct and
    what is wrong. In my opinion, our community of physicists and physics
    friends would need granted from the SE team a bit more freedom and
    independence to interpret and apply the policy, conventions, and rules
    in accordance with the needs of our group of grown up physics students,
    researchers, professers, and other people interested in physics, who is
    perfectly capable of self regulating many things. In the last days it
    has become crystal clear that we will never get any leeway to run our
    business in a more self dependent manner.

    So maybe it would be best to start our own physics site outside the
    SE network? Maybe the answers to this post could be used by people who
    have (more or less concrete) ideas about how this could be done, to make
    some proposals about how we could start our own site (as long as it
    does not get closed ...)?

    I personlly have heard only about MathOverflow which seemignly does quite well. Or in the comment discussion on TRF,
    somebody seems to know where people from the Theoretical Physics site
    have gone and that they are trying to run their own site. The most
    difficult issue could be to find a host for a new site, but maybe some
    universities with a strong physics departement would be happy to host
    such a physics question and answer site?

  32. I have high enough reputation to be able to see deleted posts and I do not see yours. Also I can see the result of the elections up to now, and maybe Ron was effective, or maybe things will have played out as they are doing now anyway. still, not to worry about the results.

  33. Hi Ron, why not hold your horses a bit. Do you really need the hassle of running such a site? anyway I can see the result of the elections up to now, and maybe you were
    effective, or maybe things will have played out as they are doing now
    anyway. still, not to worry about the results.

  34. Hm strange that you cant see it, or did I delete it too fast?
    Arnold Neumaier has just written in chat that he will be gone, onless they implement his suggested new features to undo bans by high rep users. Zephyr's account, he agreed with us and was banned for one day too, is no deleted. I dont know if he asked for being removed or if they just did it ...
    Can you come to chat?

  35. Dear Ron, I think you have pledged to write a guest blog, probably about cold fusion, and now you have more spacetime for that.

  36. There's something I don't quite understand about all this. SE is asking for something very simple: civility. I personally don't think that's an unreasonable request.

    SE has no interest whatsoever in squelching ideas; in fact, one of the basic principles is that moderators do not evaluate questions and answers for technical merit.

    SE is a platform for the exchange of ideas, not personal attacks. You should be in favor of that, not against it.

    By attempting to make it a civil rights issue, you've all taken your eyes off the ball. How much time and effort in the past few days has been spent navel-gazing and hand-wringing over SE policies, rather than discussing Physics?

  37. Apologies to Dilaton, Ron etc., but I agree with Robert, and frankly speaking, I have spent virtually no time with these battles in recent days. Well, I haven't even read all the comments about this story on my own blog because they're just among the less relevant and interesting in my optics.

    There isn't any human right that on a private server, a user with an arbitrarily hostile behavior towards the establishment of the server has to be tolerate. I don't want such human rights because they contradict my understanding of private ownership, the right of every person to make his or her living room a pleasant place according to his taste.

  38. I have not read I physics question since Lumo pointed his answer to the one discusse here out and I was not able to get a single thing done in the "real world" either ... :-/

  39. Yeah, that's ok Lumo,

    it seems to be game over anyway now and we are clearly the losers. But so what, that's life ...

    I can still spam TRF with physics questions, about dual spinor models and other things, if I need to ... ;-P :-D ;-)


  40. The word "civility" on internet forums means something different than what you think. It is simply a trick of moderators. The "incivility" is not presented with examples of you cussing at people (which no one ever does), it is based on the feeling that the user "creating a disruptive atmosphere" with "percieved hostility toward other users", and this is the feeling most people get when you tell them that they are factually wrong. So you end up banning any knowledge that contradicts anybody.

    What it amounts to is disallowing criticism of any authoritative statement using first principles reasoning, since the reasoning by its nature is unsourced and it "rudely" challenges safe conventions. It makes people extremely uncomfortable when something they believe is challenged, and it feels like a direct and most personal insult. Any scientist will know the feeling, it's what you felt when you first heard the outlines of BFSS theory, before it was accepted.

    You will never get people on public forums to see the difference, therefore, for these forums to function as scientific sources containing factual knowledge, you must get rid of all the civility rules altogether. On these sites, the politics come straight from the pits of hell, the moderators are playing a social game, they never contradict each other, and they behave as a one-mind herd. Any moderator that disagrees with another one is stripped of moderatorship quickly. Conflict is always resolved in the moderator's favor, which means get rid of all conflict. This makes a strong disincentive to criticism, since moderators are politicians and they never criticize anything.
    This is an internet fact of life--- disagreement on technical content, especially unsourced solid arguments contradicting published claims, will lead you to be classified as unbearably, obnoxiously, rude, and it will be used as an excuse to get rid of you.

    This is why civility requirements are incompatible with science, and there is no way to explain the nuances, so you just have to ask the site to tolerate rudeness" and if they say no, or at least if they enforce the no, then you leave (it's easy, you're blocked).

    The problem of cussing cranks and people who argue incessantly insulting each other is essentially nonexistent on forums with voting. I have not seen a single instance.

    For Lubos, nobody is demanding anything from a private server, this is not something one can demand. It is within stackexchange's power, as a private entity, to do whatever the heck they want to do. This is why physicists need an independently run forum. If the mathematicians give their forum to area53, they'll need a one too.

  41. This person was scamming in some way--- I emailed, but the information was vague, no details, not academic, and I told him I couldn't entrust the personal information on such a website to a stranger, who would be able to scan IP's and so on. He even linked me to this page: which was full of facebook friends, but is now deleted. It was paranoia inducing. This is why hosting is a serious problem, you need to place trust in the host. I suppose commercial hosting can be trusted, and it's not expensive. Lubos, either post this, or delete the insincere and frightening offer above.

  42. Hey Ron,

    I am deeply mourning your death at physics se :-(
    Everythig is so unfair.
    And thanks for warning the mathematicians to not go into the se network.

    Do you think I should post my meta question asking if we should start a new site and how to do it? Maibe I should chance some detailsto increase the chance that it does not immediately get closed or even deleted.

    Manyshearth is now controlling everything. Is soon as somebody mentionions what has happend to you he is there. If people say they disagree, are sad about it etc he immediately disagrees, cites the pafapraphs of the faq or the user agreement etc. This is horrible,the same way it must have been in eastern germany before reunification. the communists had their ears everywhere, strict adherence of the leaders to the official party line, and you could not even privately express your opinion or they would get you.
    Even our own mods being nice does not help in important situations, as we have just seen.

  43. Dear Lubos,

    thanks for the answer. I thought that a nonperturbative definition must be background independent, because the large quantum fluctuations in the graviton field take the spacetime away from the classical solution. But this is not true then.

    Another question came to my mind. Why is it not possible to define string theory nonperturbatively with a discretised spacetime geometry? Lattice gauge theory constructions can not be generalised to curved spacetimes?

  44. Dear Rezso, yours seemed to be a good intuition and it was widespread.

    But with the discovery of dualities, the belief was shown to be wrong. Even at strong coupling, the spacetime at infinity may be as frozen and fixed as it is at a weak coupling. The value of moduli at infinity are simply sharp and well-defined and they determine superselection sectors that are always sharply separated. Some effects connected with the strong fluctuation of the metric tensor exist but at least for moduli spaces preserving SUSY, this doesn't interfere with the possibility to pick the superselection sectors and separate them from others.

    There's no known "lattice string theory" in the spacetime and I think it's extremely unlikely that such a thing exists because lattice definitions - e.g. for lattice QCD - implicitly assume that the continuum limit, the full theory, is exactly local. But string theory is a theory of gravity which is not perfectly local, for example, because it is capable of sending the information from the black hole interior outside.

    If you meant AdS/CFT-based definitions of string theory in terms of a gauge theory, then indeed, you may also imagine that this gauge theory is defined as a limit of a lattice. But this gauge theory definition only applies to spacetimes that are asymptotically AdS_d times something specific: in this sense, the gauge-theoretical definition is background-dependent. It still allows the spacetime in the bulk, interior, to do anything, but it's very hard to change AdS5 x S5 near the boundaries of the spacetime to an M^4 times Calabi-Yau space somewhere inside, for example. ;-)

  45. Off Topic:

    The Physics SE election results are out now it is:

    Qmechanic: good


    Manishearth: not good :-(

    Manishearth has no clue about fundamental or theoretical physics (not even at the level of a good popular book) and often speaks rather dismissive about it. And addheres 200% to the party line of the SE Overlords and Dictators that slightest part of freedom and self governement the Physics SE site ever had is gone. And with this Ron will definitively never be able to come back, even if he wanted to.
    In the last about three days, almost no new questions about fundamental or theoretical physics have come in, neither were there many answers to old questions about such topics.

    I just hope Manishearth will not start to close such questions considering them as "off topic and too esoteric" because he has neither a clue nor any interest in such topics :-(((
    Some guys even complained in chat about to many QM, ST etc questions being asked (which is certainly no longer the case, so they have obtained what they want)

    Now I should probably really better stay away from Physics SE.

  46. Manish GoregaokarDec 10, 2012, 12:24:00 PM

    Like I've told you countless times, I won't close a question as OT just because I don't know what it's talking about--quite the opposite, I would stay away from it (or ask someone to take a look at it). You already know that I'm using the harmless meaning of the word "esoteric".

    I _never_ speak dismissively about it. I plan to be a theoretical physicist myself, I am well in awe of the topic. And I have an large interest in it, just that I cannot yet fulfill that interest, as my knowledge is insufficient to understand stuff about it.

    What I _have_ said is that it is not a moderator's business to distinguish between right and wrong--we only clean up. And in cases where my knowledge is insufficient to make a decision, like I said, I shall let others handle it.

    Regarding the "threat":I can't find that comment either, but I'm pretty sure I clearly mentioned that it wasn't a threat. I was just letting you know that your semi-offensive comments might lead up to a ban. I like you, even though we've had opposing opinions, and I didn't want you banned.

    Regarding the "party line": I still don't feel that it's a dictatorship, but you're right, I do agree with the SE rules. Most of them. (I try to adhere to all, though). But I'm not really willing to argue on this point anymore, we've been talking past each other on meta for a while now.

  47. Hi Manishearth,

    I have just lost all my trust in people powerful at SE, and I am still upset about how Ron has been chased away.

    So many people have spoken out for him, and tried to explain how importent he and his contributions are for our community, at least for people interested in rather theoretical topics. But it was all in vain.

    As he is so imortant for the site, why cant the moderators not show some mercy and shorten the punishment, such that he could come back before Christmas if he wants to? But I have learned that rather hell will freeze before something like this would happen.

    My above comment is not fair (I apologize for it) and having "slept about it" another night (a German saying) I see that it is really inappropriate. I rather thought you are a funny, cheerful helpful guy, and this is still the case ;-). So I wish you all the best with your diamond-job :-)

    But after all that has happened, I better dont come back, for a longer time at least, I feel still too troubled.

  48. Dear Lubos,

    I was thinking about your answer, and I realised that I don't understand something fundamental. You wrote that string theory is not perfectly local. But I think that the divergences in QFT follow exactly from locality. So, if I am allowed to build nonlocal models at the Planck-scale, than why can't I simply build a nonlocal QFT to describe gravity?

  49. Dear Rezso, to some extent, you may indeed say that string theory's violation of locality - well, the extended nature of its fundamental objects - is what cures the UV divergences. But various other properties of the theory are interlocked.

    Now, "nonlocal QFT" is a bit of an oxymoron. In particle physics, QFT is used as a synonym with relativistic QFT which is, because of the Lorentz invariance, exactly local. QFTs of the normal sort are local - or at least small deformations of local QFTs where the small deformation isn't enough to cure the divergences because their primary source, one lying in the local QFT starting point, is still there.

    It's very hard to construct consistent nonlocal theories because in relativity, locality is pretty much necessary for causality. If you don't guarantee that an object can't influence other objects that are further than c.dt, so that superluminal signals or actions would be needed, then it means that the same objects may also influence their own past. But if you can influence the past and the past still influences the future, you get closed time-like curves or causal chains and all the paradoxes that would spoil various sci-fi movies with time machines.

    So only a certain very special, constrained type of nonlocality is allowed. In fact, string theory may still be considered to be completely local in many respects. For example, the analyticity conditions in open string field theory still obey the inequalities that may be derived mostly from the locality condition in QFTs.

    Also, if you view a string as a bound state of string bits, the interactions and motion of the string bits is still local in spacetime - because the dynamics is local on the world sheet. The nonlocality occurs if you represent an extended string by "something located at one point, the center of mass". But even if you do so, as shown by Erler and Gross, there's still some version of locality that exactly holds, at least in string field theory for open strings.

    In the quantum gravity regime, some nonlocality is there in string theory and gets the information away from the black hole. But it's extremely subtle - all effective field theories for some regions look like perfectly local quantum field theories, at least to all orders in the perturbative expansion in hbar. The nonlocality is a tiny, exponentially suppressed nonperturbative effect, some kind of a tunneling. The relationship between the nonlocality needed to solve the information loss paradox and the nonlocality needed to regulate the UV divergences is indirect although they probably ultimately boil down to similar properties of the theory.

    At any rate, finding a "nonlocal QFT" that still avoids basic inconsistencies is hard and you may say that string/M-theory *is* this nonlocal QFT, the closest thing to nonlocal QFT you may get.

  50. Bloody hell, only just saw this discussion...that's Ron, Arnold and Dilaton (Anna is staying?) This is not going well. Great shame, since Physics SE offers a good format - good physics discussion without the wandering around that happens in physicsforums. It is becoming a bit homeworkey at the moment though there seems to be a market for a bit more in-depth type forum. Let me know if you find one.

    All the best,
    Phil (twistor59).

    That's "59" Dilaton, not "69". The summer of 69 was when I bought my first real 6-string.

  51. Hmmm, this sounds interesting. :)

    So you say that while a QFT cannot be Lorentz-invariant and nonlocal at the same time, string theory can do this in a subtle way. But if I look at string theory in a crude way, with a low energy effective QFT, then the subtle nonlocalness will disappear and I will get the divergences.

    By the way, my intuition about the connection between locality and the

    divergences came from a nonrelativistic QFT: the BCS-BEC transition in ultracold atoms. Interaction = collisions between the atoms. If I use a nonlocal, soft potential, then the theory is finite, but a local, Dirac-delta potential generates a divergence and I have to renormalise the theory.

  52. Dilaton may have remembered 69 because it's one of the world's most famous positions x^mu. ;-)

  53. Wow, this is really heavy impressive nice comment :-)

  54. Hey Phil :-D, I like your alias, twistor59 ;-)

    Unfortunately it has gone wrong, our own moderators were not able and they did not even try to protect Ron and the integrity of the community.

    People actively doing things at Physics SE seem to have completely changed. Many new guys who are not two fond of theoretical questions and non physics people from Stack Overflow still permanently hanging around in our chatroom and everywhere else :-/...

    Seems they intend to permanently have an eye on what is going.

    In addition, since some days (it almost coincides with the time Ron has been killed) there are almost no more new hign level theoretical or fundamental physics questions comming in.

    @Lumo: This string theory question needs a better answer than the dismissive one Dmckee has given ...

    But I better dont try ... ;-)

  55. Hah! anyway, if you come across a good theo physics discussion format, give me a shout at I'm not planning to leave physics SE will just see how it goes, but if there' s many more balls rolling down inclined planes and light inextensible strings on frictionless pulleys, I might just lose my marbles. (the perfectly elastic ones that is!)

  56. Cough, splutter, blush.....I was always a fan of number 97.6 myself :-)

  57. Manish GoregaokarDec 10, 2012, 10:27:00 PM

    I am sorry to hear that you've lost trust in SE, but happy that we can part on good terms :)

    I'd rather not discuss the ban here, I've already exhausted myself talking about it on meta :S

    Hope you decide to come back soon!

  58. @Lumo: This string theory question needs a better answer than the dismissive one Dmckee has given ...

    But I better dont try it ... :-P ;-)

  59. How can I get in touch regarding details? Like how do you tex? Link pictures? What's the kB limit on a post? I didn't imagine you were serious, but I'll do it. My email is likebox at gmail.

  60. Do it Ron!! It does not mean you will languish in an Argentine jail afterward, that's just a myth.

  61. I have contacted you by e-mail, Ron, with TeX/formatting instructions. No strict limit but I suppose you have some common sense to realize that some excessively long and boring text won't be read by too many people.

  62. Well, hence I got no answer I will just assume a NO. Actually, despite being a very basic question, it seems that no one really can deny this kind of simple logic argumentation (background independence is a Must in ST, but we just don 't know how to implement it) =P

  63. Dear Numcracker, I have answered your Yes/No question but maybe you didn't ever want to listen to the answer because it disagrees with your preconceptions, right?

    Background independence in the strongest sense in the modern sense isn't a "must" however intriguing it could be to have such a description.

  64. Well, so you have failed to tell us how background independence is a fact in a subset of M-Theory (called Supergravity) while the full theory is not BG independent. This is the paradox I mention.

  65. Hi, there's no paradox. All the (geometric) backgrounds of string theory are approximated by SUGRA coupled to matter at low energies and string theory shares all of the SUGRA's background independence in the operational sense, especially if one means the background independence in the Einsteinian sense only. String theory really confirms the Einsteinian background independence at all energy scales when confined to a sector with a known low-energy field approximation.

    The configuration space of string theory is more complicated than anything we could just deduce from a particular SUGRA theory because SUGRA always breaks down in extreme limits while string theory always allows one to continue the well-defined analysis through - and once we get through, we make a transition to another branch of the landscape or whatever which is described by a generally qualitatively different SUGRA theory.

  66. Many thanks, I got it now ;-)

  67. Hi Twistor :-)

    Now I have setup a Meta question to gather ideas about how we could setup a new physics Q&A site outside the SE network:

    I'm curious about how long it will stay open ...

  68. Ron Maimon,

    Could you let me know what your opinion is of the Widom-Larsen theory that NASA is fond of to explain LENR?

    I've done a number of stories about LENR over the past several years but I'm always trying to get several opinions from different physicists.

    LENR to me is still a big puzzle I struggle to assemble, as a journalist. Best,


  69. Due to the wikipedia reference, I wonder if you're the same as User:Likebox:


  70. I wonder if you' are still interested in a new q & a site like a physics.overflow: