What is a Brane?

Greg Moore has written an article for the latest Notices of the AMS entitled “WHAT IS… a Brane?”. He begins by noting that “The term ‘brane’ has come to mean many things to many people” and one of the difficulties of the subject is that one has to figure out from context what sort of brane someone is talking about.

For the case of branes of dimension greater than one, in general no one knows how to consistently quantize such objects. See Warren Siegel’s research summary for comments about this. He notes that now that M-theory shows that non-perturbatively strings are membranes in 11d, one doesn’t have a finite quantum theory of gravity, since membranes are infinite even in perturbation theory. All one has is an effective low-energy supergravity theory, which is what one had before one got involved with string theory. While at Siegel’s web-site, check out his latest parody paper called “The Everything of Theory”, which includes the following lines:

“The real problem with string theory is that there is no alternative. However, the reason there is no alternative is that no one ever bothers to look for one; in fact, there is a strong resistance to even considering looking for one. Consequently, practically all theoretical high energy physics (and even most of phenomenology) is now string theory. Thus, string theory is not so much the Theory of Everything (since it explains nothing), but rather the “Everything of Theory”, since it now encompasses all of theory. This era in string research is strongly reminiscent of the Dutch tulip trade just before the Tulip Crash of 1637. ”

I, for one, am missing the joke here…

For some new hyping of a different kind of brane, see the latest Nature, which has a piece on Nima Arkani-Hamed. Equal time is given to LQG, with a similar piece on Martin Bojowald.

Update: Lubos Motl has a long posting about branes and M-theory, which explains many things. As usual though, he insists that the full dynamics of the branes in M-theory is completely determined and unique even though he doesn’t know what it is in any phenomenolgically realistic background. To get a finite quantum theory of gravity that has anything to do with the real world out of M-theory, you need to show that you can get well-defined, finite results for the dynamics of these branes in the case of four large dimensions, the rest small. As far as I can see any claim to have this now is purely wishful thinking.

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23 Responses to What is a Brane?

  1. Peter says:

    I think you’ll find that if you talk privately to many string theorists, they are very unhappy with the present state of the field. A common opinion would be that the standard picture of a 10/11 d string/M-theory TOE has been incredibly overhyped and now looks unlikely. But they’re not sure what else to work on. One of the few intellectually alive sub-fields of particle theory is that of trying to really understand QCD via its supposed string dual. A lot of people are working on that, keeping quiet about their doubts about the TOE. Siegel is one of the few not keeping so quiet (he has tenure, for one thing…).

  2. Anonymous says:

    I read much of Siegel’s webpages and I’m surprised he seems to be able to hold contradictory opinions in the same webpage! In his “serious” pages, he is a proponent of string theory and he describes his work on it but in his parodies, he becomes very critical of string theory.

  3. Peter says:

    Well if some of the features of the physics look like they come from a vibrating string, it’s perfectly reasonable to try to model strong interactions with a quantum string. It just doesn’t work, or at least not in anything like the simplest such way of “quantizing a string”.

    I’d heard that lots of people had suggested a fourth quark with the right quantum numbers around 1964, but never that Zweig was one of them. The idea only became really compelling in 1970 with the GIM mechanism.

  4. D R Lunsford says:

    Peter – you mention Glashow and his 4th quark. Remember that George Zweig had his model of “aces”, which were really just 4 quarks at the outset.


  5. D R Lunsford says:

    Peter –

    It looks like Siegel is carrying on the parton and Veneziano dual-resonance ideas in his own way, with modern tools. So he’s sort of going back to before gauge theory and s.s.b.!

    One thing I never understood – it seems that string theory got started because the dynamics of these models looked like that of a vibrating string. Why did they make the leap of faith to say that it really *is* a string??


  6. Peter says:

    DMS: Sure, if when particles collide at the LHC, energy starts disappearing into extra dimensions, that would be a big deal. It also would be a big deal if these collisions caused the archangel Michael to appear at the collision point. These both seem about equally likely to me.

    Glashow and others had good arguments for a fourth quark. This was a simple extension of the three quark model that made the whole thing simpler and more elegant. The extra dimension stuff is nothing at all like that.

    ksh95: Yes p-branes for p>1 are described by a non-renormalizable sigma-model. Look at your favorite way of quantizing the string, increase the dimension of the world-sheet above 2 and see what happens.

    Danny: I don’t really understand exactly what Siegel has in mind, but he is clearly more interested in finding a 4d string that would help one understand QCD, rather than a 10d string that would explain gravity.

  7. DMS says:

    Getting to the article, I think I would agree with Nima that *IF* brane concept turns out to be correct, it will indeed be the biggest thing in science in 300 years. Don’t you think?

    Of course, I don’t think it will be found, but who knows…

    Maybe, it will be like the charm episode in particle physics, when Glashow made a bet that charm would be found with an estimate of the mass, and he won the bet( If my memory serves me right, it was based on the GIM mechanism based one-loop calculation of the K0-K0bar mixing.). But my understanding is that no one believed his prediction (unlike the case now where anyone who “matters” believes in it).

  8. ksh95 says:

    Whoa, whoa, whoa,

    You’re telling me that branes give infinite results, even in perturbation theory?

    What’s this about? Are they doing S-matrix calculations or something else?

    I’ll ask around, but in the meantime if some one could kindly explain the present state of affairs I’d be much ablidged.

  9. Thomas Larsson says:

    A Brane New World always reminds me of a Swedish dotcom company, by Huxley’s original name A Brand New World. It defaulted a couple of years ago, after burning more venture capital than Boo.com.

    As they say on the stock market: never try to catch a falling knife. No matter how much the string stock falls, there is always 100% left to the bottom.

  10. plato says:

    sorry Peter it won’t happen again

  11. D R Lunsford says:

    Peter –

    I read thru all of Siegel’s comments and it seems that he regards the useful part of ST as a phenomenological expression of some kind of theory in which X can be regarded as a bound state of Y andZ, Y and Z can be regarded as bound states of X and W, W can be regarded.. etc. etc. IIRC, ST had its origin in a naive nuclear model in which nucleons were literally banded together, one wrote a Lagrangian for the bands with their tension etc. etc. Is this a correct summary?


  12. Peter says:

    This has really gotten out of hand. I’m quite annoyed that people are ignoring my request to not post multiple off-topic comments and not to respond to those who do. This is damaging something that belongs to me and I’m about to start deleting all such comments. Please do not continually submit multiple comments attempting to turn this into a forum for discussion of your private interests. This is extremely rude and I’m not going to put up with it any more.

  13. plato says:

    Okay don’t let it happen again.:)

    Alan Sokal saids:A lot of the blame for this state of affairs rests, I think, with the scientists. The teaching of mathematics and science is often authoritarian

    Now who are you going to believe and quote, when you’ve cried wolf?

    The more one adds, the crazier it gets:)

  14. I’m sorry Plato.

    As you guys guessed, Plato’s assitant doesn’t exist.

  15. plato says:

    Plato’s Assistant,

    It is unfortunate that you have refused to keep yourself confined to the club, (url listed on my name), that we will have to suspend you until further notice.:)

    Brane and Brain, are not the same thing.:)

    As to understanding the gist of my resources, you might find such accumulation of the data very interesting, and why, any attempts to devalue, is only a rejection of the reality that has been moved into this century:)

    Cater to Peter’s request, if you dare 🙂

  16. D R Lunsford says:

    Apparently, there really was a Tulip Crash:


    Most amusing is the noted battle of “Wittstock”:


    Obviously this was the first real-world test of flower power. (King Jiminy Hendrix of the Duchy of Fender prevailed over Count Rey Josef und die Fische. After the victory, a great celebration of music, a free “concerto grosso” was held at Maximilan Yasgar’s fuedal estate. This was the first appearence of the band then known as Jefferson Bargepole with Grace Slick, a stimulating teenager from San Francisco di Zitterbewegung.)

  17. Chris W. says:

    This must be the Bogdanoffs‘ secret. Thousands of students could save themselves the expense and risk of buying term papers and phony theses with only a minimal decline in the quality of the work they submit.

  18. D R Lunsford says:

    Peter –

    A few simple mods of the Postmodernism Generator should do fine! Just throw in a few classic formulas pulled from Dirac or Messiah, and make the obligatory reference to the CMB, and a postscripted estimate of the cosmological constant, references to Witten, etc. Making up references would be fun!

    10. Stringfellow, Ariel. “Step-Ladder Potentials for Propagating Slinkies” Physics Yarns p. 278-314


  19. Peter says:

    It always seemed to that it couldn’t be that hard to modify that guy’s software to generate scientific papers about branes. Come on, doesn’t anyone want to try this?

  20. Plato's Assistant says:

    Monsieur Plato is on holiday. These comments are computer-generated. Disparaging remarks concerning their coherence (or rather, the lack thereof) will, as usual, be ignored.

  21. plato says:


    Mushrooms won’t help you in your calculations, and push you any further.:)

    Even if it’s a primordial kind of thing that you think helps you, you must recognize other information is present from way back when? Please do not poison yourself with the analogies of higher dimensions that you attribute.

    How did you get as far as you did?:)

  22. D R Lunsford says:


    Where can I get some of these mushrooms?

    Thanks in advance!


  23. Plato says:

    It’s Brane New World?

    Branes reside in the hidden dimensions, known as “the bulk.” While matter and light stick to the branes, gravity traverses both branes and bulk. The hidden dimensions cannot be seen because only gravity can go there.

    Maybe the tulip trade is reminscent and inferred by Veltman and Hooft, as a realization of a cosmological principle that can be mapped holographically, even if Hooft is overwhelmed?

    Finally, we describe the embedding of branes in the 5d bulk using the phase space geometric methods developed here. In this language the boundary conditions at the branes can be described as a 1d curve in the phase space. We discuss the naturalness of tuning the brane potential to stabilize the brane world system.

    You just have to adjust your views to the bulk is all?:)

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