Why String Theory?

By Sean Carroll | September 21, 2012 8:41 am

Breathless press reports notwithstanding, string theory is very far from being dead. If you’re interested in what it is and what’s going on within the field, I can recommend a new website called Why String Theory? (And of course, accompanying twitter feed @WhyStringTheory.) It was set up by Oxford undergraduates Charlotte Mason and Edward Hughes, working under Joseph Conlon. It’s a very engaging and professional-looking site, featuring a great deal of explanatory material.

Developing pedagogical sites like this is a great project for undergrads; the only looming issue is keeping the site going once the students move on to bigger and better things. Hopefully this one is kept up — I think an initial surge of interest has already been taxing the poor web server.

CATEGORIZED UNDER: Internet, Science
  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean Carroll

    After the post on gender bias, I wanted to get something up that wouldn’t cause any controversy.

  • http://www.filip.milstan.net Filip

    This is nice. It will be very interesting to read it (especially after some critics about the String theory I read in [1]). Thanks

    [1] http://www.math.columbia.edu/~woit/wordpress/

  • martenvandijk

    Gender plays a role in string theory: by definition there is not much in string theory as it hardly covers anything.

  • Sili

    String Theory is a conspiracy to get more women into physics by making them think it’s all about knitting.

  • meh

    Why don’t we have a discussion about when it’s appropriate for a woman to breastfeed her baby in public. I’m sure that’ll be a nice respectful & and rational discussion. πŸ˜€

  • Dilaton

    “After the post on gender bias, I wanted to get something up that wouldn’t cause any controversy”

    Then you should rather not have written this post either :-(…

    To be sure, I like the site announced in this CV article a lot of course πŸ˜› !

    But writing the keyword “string theory” or anything related to it on a publicly accessible site just turns the corresponding comment section into a horrible hell of non constructive flame wars, etc :-(

    … trying to run away and hide now, it will certainly happpen very soon here too …

  • AI

    Why are string theorists so keen on selling empty hype to laymen?

    If those laymen are interested in physics there is plenty of other interesting physics for them to learn, physics that could actually be useful to them cause it deals with real world.

  • Mark Weitzman

    So far a site where I can’t find a single equation – not very promising

  • http://www-thphys.physics.ox.ac.uk/people/JosephConlon/ Joseph Conlon

    @Filip, Dilaton: thanks!

    @AI: of course there is much other interesting physics beside string theory. The site is for those members of the public who would like to find out about string theory – a research area after all funded by public money.

    @Mark: by design. If you want string theory with equations, the most accessible text is Zwiebach’s book.

  • A Boy Scout

    Why String Theory?
    Why Knot?

  • Dilaton


    @AI just delivered the proof that my prediction of flame wars and very noncostructive comments wherever somebody dares to mention a certain s-word is right :-(.

    A just say to people who hate, dislike ST, etc just DONT CLICK such sites and move on to something else you enjoy better, but leave people who are intererested in it and want to learn about it in peace ok…? Why the hell cant you never just mind your own business and let other people do what they like ?!

    Duh sorry but I had to say this …

    Hi @Joseph Conlon πŸ˜€
    Yep, this site is a lot of fun to read. Even though I already know a tiny little bit from Lenny Susskind’s lectures, Lumo, and started the corresponding demystified book, I often just browse through it because this new site is so cute and reading it makes me happy :-).
    The only thing I’m not sure if I like it is the link to a not particularly ST friendly blog; its owner will certainly not appreciate your new nice site :-/. But then again I think you are well aware of this since you have written that the links are choosen to have some rants too … LOL πŸ˜€

  • Mitchell Porter

    Joseph Conlon, I’m sorry to say that I like string theory but I don’t like the site. I have trouble identifying the problem: too bland? too humble? too dumbed-down? It’s like a science project made by a talented (but not ultra-talented) high-schooler. The content is at the level of what you can already find in Wikipedia.

  • Dilaton

    Nah Mitchell c’mon … πŸ˜‰

    You are not a layman so it is not for you anyway … πŸ˜›

  • James Gallagher

    Why String theory?

    In the end, because unfortunately Einstein over-impressed on everyone the greatness of Riemmanian Geometry by his tortuous development of the beautiful mathematical theory of GR.

    Yes the Great man developed a beautiful approximation to Nature using it, but for fuck’s sake, wake up people, these continuous geometry methods are surely the wrong way to go.

    Go back to what we are sure of, the discovery of probability amplitudes and their evolution equations (Heisenberg and Schrodinger co-discovered this, Heisenberg in the form of a Matrix of atomic transition probabilities/intensities, Schrodinger in the form of an elegant wave equation (although neither realized at the time that they were describing probability amplitudes))

    In fact, Heisenberg’s original idea that everything must be described by what we can observe, can now be seen more accurately to require that everything is described by a probability amplitude – for it is the probabilities that we observe (eg intensities of lines in atomic transitions).

  • Brian Too

    So disappointed. I expected to see material concerning G-strings. Instead I find only coverage of the A through F-strings!

    1 x 10^500 possible solutions and you can’t give me G-strings? C’mon!

  • http://darkbuzz.blogspot.com/ Roger

    Lots of hype. Slick web site. No substance. Just what we would expect from proponents of string theory.

  • Dilaton

    @Sean Carroll Why in the world have you posted this?

    Now I already have a second data point confirming my prediction of flame wars and nonconstructive comments.
    The excess will soon grow strong enough (5 sigma and more) such that I can claim discovery of a complete contamination of this comment section by trolls and sourballs, which makes it unreadable for people who come here just to learn interesting things :-(

    Talking about or mentioning ST on publicly available sites, such as CV for example, is no good and should really be avoided !

  • James Gallagher


    you need to chillax

  • Mark Weitzman

    @Conlon – I own many string theory books including those by Green,Schwartz, Witten, Polchinski, Ziewbach, Becker,Becker, Schwartz – my point was so many people think string theory is all hype or imaginary – at least show some of the founding equations to give it some credibility. Compare with the http://www.superstringtheory.com web site which has some pages with equations and advanced analysis.

  • http://vtullow.blogspot.com vince

    >> show some of the founding equations to give it some credibility

    equations = credibility

    pages with equations = good pages

  • Ashlie

    I can’t believe the amount of stupid comments on this site. God forbid any intelligent conversation happens. I’m very disappointed.

  • martenvandijk

    @ Ashlie

    Do well and don’t look upwards.

  • http://www.math.columbia.edu/~woit/blog Peter Woit

    I agree with Sean that this is a great project for undergrads, at least for those planning a career in advertising…

  • Dilaton

    @Peter Woit,

    It is you and your fans who prohibit any serious and intelligent discussion on physics sites intended to maintain constructive conversations from which everybody can learn something.

    So may I kindly ask you to stop popping up everyplace where somebody dares to mention ST, BSM physics, or fundamental physics in a broader sense and spoiling the corresponding comment sections ? You are nether in the slightest interested in nor competent to talk about such topics.

    On your blog and in your books you can rant about whatever you want and insult whoever want, but the world absolutely does not need you constantly poisoning the whole physics blogosphere outside your site (!) with your scornfulness and destructive attitudes.

    I know making research you dont like completely disappear by trying to be omnipresent in popular media of any kind (and probably at influential real world positions too …?) to negatively influence science and spread dishonest lies is your “raison d’etre”.

    Just get yourself a live, put your long nose into your business, and do something constructive !

    (From Prof. Strassler’s “crocodile’s tears” comment remember that any response to me containing the term “character assassination” would impress nobody, just in case …)

  • http://darkbuzz.blogspot.com/ Roger

    @Dilaton: You are the one who is posting insults here.

  • AI

    Man up dude, jesus, get yourself together (to quote Brett from previous thread).

  • Brett

    Lord have muh-cy, my ea-uhs, they are a-burnin’. I like what #14 said, but I also disagree with it. This is an argument that Einstein would have had with many early quantum physicists, so I’ll turn away now, or will I? no, I won’t. If we only stuck to what we were sure of, we would have missed out on a lot in the past 3 centuries.

    #10 is making a good point as well; the two are awfully similar in some aspects. It definitely seems like the founders of string theory gathered inspiration from it.

    I think string field theory looks pretty good. But back to #14, I do think it takes time for people to realize when there is a mistake in an existent theory. We are currently questioning G.R. because of the necessity to explain Dark Matter. We are bound by the simplistic certainty that everything must be described by what we can observe because sooner or later, we won’t be able to deny that what we expect is grossly different from what we are observing. I believe a fundamental concept in the creation of string theory was this: break nature down into the most fundamental form possible and then we’ll be able to explain everything because we will have revealed where everything “comes from”. Geometry is the most fundamental form possible. Of course, that’s coming from someone who believes that Time is the most F.U.B.A.R. scientific concept there is right now.

  • James Gallagher

    I must apologise for using double mm instead of double nn in my post #14 above, but otherwise I stick to the message, Riemannian geometry is a beautiful (and sometimes effective) mathematical model, but it doesn’t describe Nature at its most basic level.

  • Brett

    I guess the real question left to be answered is: what is the most basic level of nature? Maybe Craig Hogan can figure it out.

  • Neil

    Of course we would not be having these insult exchanges if ST could connect to the experimental world, which it does not.

  • andy

    The website doesn’t work without JavaScript enabled. I suppose a website about lots of inaccessible mathematical theory requires navigational inaccessibility as well?

  • http://www.eskesthai.com/ Platohagel

    Good for these young people to further the initiative for understanding through web development. As well, good to see future researchers being developed in theoretical approaches.


  • jeigh

    Who plays Sheldon in this charade?

  • Mitchell Porter

    @Neil #30: “Of course we would not be having these insult exchanges if ST could connect to the experimental world, which it does not.”

    String theory is criticised, sometimes by the same person, for predicting everything and predicting nothing.

    The reality is just that it has developed very very slowly. Modern string phenomenology starts in the late 1980s with grand unified theories derived from the heterotic string. It was understood qualitatively that phenomenologically plausible features (N=1 supersymmetry, nice gauge groups) could be obtained from specific compactifications.

    Since then, the range of models has broadened, there has been a lot of technical progress in understanding the mathematics, but calculation is still very difficult for any specific vacuum. Here’s a string theorist reporting from a conference two months ago:

    “in many string models, we don’t actually have enough control to calculate physical coupling constants; often an overall proportionality factor is missing, and simply assumed to be ‘of order one’ (i.e. of magnitude between about .1 and 10). I think this is a very important point; ‘string phenomenology’ as it stands is a bit of a misnomer, because as far as I know, nobody has yet been able to do an honest calculation of all quantities like masses and coupling constants in a realistic string model.

    “Eran [Palti] offered these ultra-local models as a case where proper calculations can be done, and gave examples where parameters which might have been assumed to be order one actually come out to be much smaller (e.g. ∼10^βˆ’8). ”

    –from http://x-sections.blogspot.com/2012/07/string-phenomenology-days-4-5.html , which also has a link to Palti’s talk

    At the other extreme, we have the “G2-MSSM”, which is an example of trying to do “string-inspired phenomenology”. The G2-MSSM is a particular region of the parameter space of the Minimal Supersymmetric Standard Model, into which (it is argued by Kane, Acharya, and others) certain compactifications of M-theory (on “manifolds with G2 holonomy”) should arrive in the low-energy limit. They are trying to work around the difficulties of performing a definitive top-down calculation, by seeing how far they can get with approximations and plausible assumptions about how such compactifications behave generically. So they are trying to fit all particle and cosmological data, including recent observations like the 125 GeV Higgs and the 130 GeV gamma-ray line that may be a signal of dark matter.

    So it’s not the case that we can write down a unique master equation for string theory and get back a unique falsifiable prediction. It is also not really the case that string theorists can produce a model for any scenario at all. The attempts to fit the data are based on a lot of assumptions about how the models work, and they are still struggling and learning about how the models *really* work.

  • Gavin Flower

    @Peter Woit: Is Not Even Wrong! Nor is he right…

    Simply because something is incredibly difficult and doesn’t give answers you would like, doesn’t mean it is of no positive value.

    Peter should propose a simpler theory that explains things better than String Theory. There is no law of the Universe that says theories must conform to our prejudices, nor that they should be tractable!

    There are aspects of Evolution that I hate, that does not mean Evolution is wrong.

  • chris

    “String theory is criticised, sometimes by the same person, for predicting everything and predicting nothing.”

    predicting everything is predicting nothing.

  • Jimbo

    Hoorahing ST theory by Sean is understandable: It’s his latest paper w/Lisa Randall !
    Methinks the world of theoretical physics will soon relegate ST to history, thanx to LHC, which has gutted All aspects of BSM physics vis a vis ST, e.g., SUSY, LXDs, & mini-BHs. Yet like a golem, it stalks the landscape, & refuses to die in the fantasy minds of aspiring students, theorists, & experimentalists who’ve devoted careers to it. They are in denial, & will remain so.
    The arxiv posts new ST-related papers everyday, many trying to reverse-engineer the Higgs mass out of some contrived variant of SUSY which would make Occam go insane. The UK mtg on phenom this week has a neat talk focussed on “Saving SUSY”, an excerpt from which says it all:
    “It’s ironic that the solution to the absence of SUSY is to add even more stuff: composite 3rd generation or Higgs, R-parity violating couplings, scalar gluons, or new singlets”. Adding more layers to an already contrived house of cards will only ensure that it crashes. If theorists would put as much effort into a fresh look at the phenom flaws of the SM, such as the 3-sigma muon anomaly, rather than continuing SUSY on life-support, theoretical physics would move forward, intead of treading water while the sharks circle.

  • Dilaton

    @Sean Carroll

    I just wanted to post a positive addition to what Mitchell Porter said, but I sadly see that this would be a completely pointless thing to do here :-(.

    Please please please , have at least some mercy with the poor students who did the work and stop the spitting and spatting on the site they have designed and put up here by removing this blog post !

    It must be horrible for these young people if they see this terrible comment section so brutally attacking them.

    Please Sean Carroll take the post down, you cant be that cruel … ?

  • James Gallagher


    haha, “poor students”, you mean Oxford University Physics Undergraduates, many of whom will/can move into the finance sector and destroy the world like their predecessors if their silly attempts at understanding nature don’t work out.

    You’re well OTT, it’s fine to attack a few over-privileged(lucky to get oxbridge) students for a pretty shite attempt at an advert for String Theory.

    String Theory has devoured some of the greatest minds of our time in a tortuous/brave/brilliant attempt to describe reality well beyond anything humanity has ever witnessed in the past – there, that’s hyperbole for you.

  • Meh


  • Bright Matter

    For sure. String Theory will last as long as Buddhism. Never-ending cycle of life and death, reincarnation into a new form. Billions of people believe this is real. Similarly, with 10**(500) forms and convolutions, String Theory will last forever. In theory.

  • Mitchell Porter

    @Jimbo #37: There are lots of papers trying to fit the Higgs mass into supersymmetric *field* theory, not string theory. There are also many field theory papers trying to explain recent experimental anomalies like the “forward-backward asymmetry” of the top quark, some of which use supersymmetry, some of which do not. The muon anomaly has been around much longer so the interest is less focused, but many papers have been written there too. There are papers asking whether CERN’s new boson is a dilaton rather than a Higgs. So the field is vigorously engaged with the data. There may still be huge blindspots, in the form of facts or ideas that are being neglected, but theoretical effort is already about much more than just a struggle to save SUSY.

    At the current level of knowledge, it’s still only field theory that produces immediately predictive models of particle physics. String phenomenologists are still refining their understanding of how to produce something that looks qualitatively like the standard model, and they have tended to focus on a standard view of BSM physics too – weak-scale N=1 supersymmetry, grand unification. But it’s unclear to me how much of this stringy research program becomes valueless, if we suppose that supersymmetry just doesn’t show up at the LHC. It would depend very much on what the new directions in field theory are. The same goes for any new explanation of the muon anomaly. Some approaches aren’t that different from current theory, e.g. see arxiv:1208.2630

  • Michael

    String theory is as much science as creationism is.

  • Shantanu

    Mitchell or other string theorists, can you help me answer these questions (or resolve my doubts) about string theory below

    o does string theory predict violation of equivalence principle?
    Thibault Damour and Cliff Will claim it does. Most others claim it does not.

    o does string theory predict violation of lorentz invariance?
    Nanopoulus claims it does. Most others claim it does not.

    o Does string predict Horava gravity?
    That’s what Peter Horava claimed in his colloquium at Berkeley(video available)

    o Does string theory predict MOND?
    Verlinde claimed so in his colloquium at Caltech(reported by a user on a woit’s blog)

    Anyhow to a non-expert like me, it seems string theory is consistent with any observation(even
    contradictory ones), which means it can never be falsified or proven.

    Also do you agree with the grades given by Strominger in his colloquium at Harvard (Fall 2010)?
    video again available online

  • Mitchell Porter

    I’m not a string theorist, I just read the papers. But regarding that list of claims…

    Conceptually, you can analyze what goes on in string theory as follows. There is a theoretical core, consisting of the “theories” linked by the dualities that were worked out in the mid-1990s. Within that theoretical core, there is a phenomenological core consisting of the vacua which people study as possible descriptions of the real world. Then outside the theoretical core, there are various heterodox versions of string theory.

    So very briefly, I *think* that what Damour and Horava talk about is inside the theoretical core but way outside the phenomenological core; Nanopoulos is outside the theoretical core, in “noncritical string theory”; and Erik Verlinde is off on his own. His idea was *inspired* by work in AdS/CFT, but entropic gravity has severe problems. (There seems to be a revised version coming, not yet written up.)

    As things stand, string theory is like field theory, it’s a framework which yields many models. To get unique predictions, you have to pick a specific vacuum. String cosmology is still a wildcard, we don’t really know how it works. It could turn out to be more predictive than just anthropic selection. But if it isn’t, the challenge will still remain of finding a string vacuum that matches experiment. Any individual vacuum is extremely predictive; if you can find one, and show that it predicts the measured particle masses and mixings, in principle you could use it to predict the next n decimal places, and then go do the measurements. But as I said in comment #34, string phenomenology is not at that level yet.

  • P


    I agree with Mitchell’s response. I’d be skeptical about some of the bullet points you’ve raised. They are definitely not the core of things discussed in the string community.

    I will follow up on Mitchell’s nice comments too, though. String theory generically predicts that the world is described by a spontaneously broken non-abelian gauge theory. This is correct, of course, but still isn’t our world in a precise way. “The challenge will still remain of finding a string vacuum that matches experiment” is precisely the right idea, though. The first thing that must be done in being more precise is matching the “discrete” data we see in the world, such as the standard model gauge group and matter representations. This can and has been done in concrete, globally consistent string compactifications (I believe O(100) or O(1000) exist at this point). The next difficulty is one of moduli stabilization: within a compactification that realizes the discrete data appropriately, we must study the effective potential on the moduli space – i.e. what string vacuum we’re in – to determine the masses, mixings angles, and other similar data that we know exist in particle physics. This is a formidable task and has not been accomplished to date.

    So, in short – Mitchell is absolutely right that any string vacuum is extremely predictive, and nearly all vacua have been ruled out since their low energy gauge group and matter content are not the standard model. However, there are O(1000) that are pretty close . . .


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  • John Merryman

    “Of course, that’s coming from someone who believes that Time is the most F.U.B.A.R. scientific concept there is right now.”

    Shouldn’t it be a clue that exponential complexity is a possible signal there is a fundamental mis-conception? A la epicycles.

    My candidate is that since physics treats time as a measure from one event to the next, it only re-enforces the impression of sequence as fundamental, rather than a measure of change, due to action. As in the earth isn’t traveling the fourth dimension from yesterday to tomorrow, but tomorrow becomes yesterday because the earth rotates.

    Duration doesn’t transcend the present, but is simply the state of the present between events, so there is no physical dimensionality.

    One would think that if time is a dimension from past to future, then a frame with a faster clock rate would travel into the future faster, but the opposite is true. It ages/burns quicker, so it moves into the past faster. Witness the twin in the faster frame has died when her slower twin returns.

    The cat is not both dead and alive, because it is the collapse of future probability which yields current actuality. Time emerges from the effect of change, rather than is the cause of it.

    Knowledge is created inductively, as future becomes past, but is used deductively, as the past is used to predict the future.

    We reach for the future, but can only grasp the past.

  • Tom

    Here’s a paper that show that superstring theory is incorrect:


  • Shantanu

    P and Mitchell,
    I am not sure I understand your replies. You can find papers by Damour and Will on arxiv
    which point out that dilaton fields (from string theory) violate equivalence principle.
    So is this not part of “theoretical core”? Anyhow given that needs to find the right “string theory vaccum” we are still far off.

    Anyhow to give another example of contradictory claims in string theory,
    last year there was a panel debate involving Freese, Smolin, Levin, Gates, Greene, Gleiser (video
    available on the web). Jim Gates claimed that string theory is consistent with no extra dimensions. Brian Greene claimed it does. so who is right and who is part of the “core string theory community”? Also in the same debate it was also pointed out that some models also predict extra time dimensions.
    So what is the correct claim by ST regarding extra space and extra time dimensions?

  • Mitchell Porter

    #50 The equivalence principle holds in 11 dimensions, and if the shape and size of the extra dimensions is stable, then it will also hold (to great precision) in 4 dimensions, but if the extra dimensions are changing, then you will get EP violations in 4 dimensions. The dilaton is the radius of the 11th dimension, so it is one of the “moduli” that parametrize the extra dimensions. But phenomenologists normally assume that the moduli are stable (because changing moduli could e.g. make the fine structure constant vary), which is why this scenario is in the theoretical core but not in the phenomenological core – it is dynamically possible but normally you wouldn’t think that it was happening in the real world.

    I have not seen Gates vs Greene, but apparently Gates was talking about non-critical string theory, like Nanopoulos. String theory normally starts in 10 dimensions (the “critical dimension”) in order to avoid a quantum inconsistency (the “conformal anomaly”), and then 6 of the dimensions are assumed to be small. Non-critical string theory can start in any number of dimensions, and deals with the anomaly by adding an extra “Liouville” field along the string to compensate. It’s not a subject I know about and I don’t understand what its foundations are supposed to be. There are a few notable string theorists who take an interest in it, but most stick to the normal theory. Extra time dimensions are a similar story, it’s a minority interest and I can’t make any sense of it as physics. I can assure you that most string models only assume one dimension of time. :-)

  • Dilaton

    @Mitchell Porter you seem to forget what nice phenomenology Cumrun Vafa gets from F-theory … πŸ˜‰

    I like your comments

  • Jill Harry

    Hi All,
    I loved this article. I would like to refer a link of a you tube video for viewers that will give you simple concept: http://www.youtube.com/watch?v=y6JrsZoohOU


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Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .


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