Urs Schreiber
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 Aug 10 comment Obtaining supergravity from gauging global supersymmetry ... next up the ladder is the circle 3-group $\mathbf{B}^2 U(1)$. A gauge field for this is a $C$-field ncatlab.org/nlab/show/supergravity+C-field and this couples analogously to the membrane = 2-brane. In fact, in both these cases really the circle $n$-group is just one component of a more complicated nonabelian higher group. For instance the circle 2-group is part of what is called the "String 2-group" ncatlab.org/nlab/show/string+2-group and its variant the "String^c 2-group" ncatlab.org/nlab/show/string%5Ec+2-group. A gauge field for this is twisted heterotic B-field. Aug 10 comment Obtaining supergravity from gauging global supersymmetry Yes, Lie $n$-algebras and their Lie $n$-groups correspond to Lie algebras and Lie groups as $(n-1)$-branes correspond to 0-branes = point particles. The easiest example is the circle 2-group ncatlab.org/nlab/show/circle+n-group $\mathbf{B}U(1)$. A gauge field for this 2-group is a B-field ncatlab.org/nlab/show/Kalb-Ramond+field and this couples to a string = 1-brane in direct anlogy of how a $U(1)$-gauge field couples to a point particle = 0-brane (by what is called the "WZW term" ncatlab.org/nlab/show/Wess-Zumino-Witten+model). Next up the ladder is... Aug 10 answered Obtaining supergravity from gauging global supersymmetry Aug 10 comment Gauge invariant Chern-Simons Lagrangian This nice argument of course requires that the 3-manifold we are interested in is "co-bounding" (the boundary of a 4-manifold) in the first place. Depending on which extra structure one has (e.g. spin structure etc.) this is not always the case. But the good message is that essentially this argument goes through fully generally if one refines it "locally" as expressed in the language of stacks. An exposition of this is at the beginning of this article: ncatlab.org/schreiber/show/… Aug 9 comment The Role of Rigor You haven't seen clarity yet. Aug 9 revised What is kappa symmetry? edited body Aug 9 answered What is kappa symmetry? Aug 5 comment Is string theory a quantum theory of gravity? The distinction between "postdiction" and "prediction" is not interesting. For instance, suppose tomorrow somebody solves the mass gap problem for Yang-Mills theory. Then we won't say "QFT postdicts the mass gap of Yang-Mills", just because we knew it already from experiment and numerics. Instead we will say "QFT explains the mass gap of Yang-Mills in that it derives it from simpler axioms". And this is true also about how string theory yields gravity. Aug 5 comment Is string theory a quantum theory of gravity? (Not to mention that via AdS/CFT supposedly properties of quantum black holes are encoded in the dynamics of a sYM CFT on an asymptotic boundary of spacetime.) Aug 5 comment Is string theory a quantum theory of gravity? It is not true that perturbative string theory says nothing about black hole singularities, because of "the existence of a classical spacetime". Instead, it knows about all properties of such black holes that are "protected" by supersymmetry (and these are more than for plain supersymmetric back holes). This works by following a weakly coupled configuration of branes (gravity back reaction approximately turned off) as the coupling constant is turned on. All the detailed microscopic computations of BH entropy work this way ncatlab.org/nlab/show/black+holes+in+string+theory . Aug 5 comment Is string theory a quantum theory of gravity? The starting point of perturbative string theory is not an "unmotivated and unexplainable assumption about the nature of particles". Instead, it's a natural variation of the perturbative description in QFT. It may be physically wrong, but it is by no means unmotivated and unexplainable. Here it helps to remember the "worldline formalism" for QFT ncatlab.org/nlab/show/worldline+formalism . If you can see this and not naturally wonder if there could be a higher dimensional worldvolume generalization of this formalism, you have to return your theoretical physics license ;-) Jul 30 awarded Nice Answer Jul 15 comment About the stability of the ground state of the bosonic string I wouldn't say so. Also the closed string tachyon condensation has been studied with avail, and better understood, if admittedly less so than the open string tachyon. A list of references is here ncatlab.org/nlab/show/tachyon#ReferencesInStringTheory . Jul 14 revised About the stability of the ground state of the bosonic string added 4 characters in body Jul 14 answered About the stability of the ground state of the bosonic string Jul 14 answered How exactly are Calabi-Yau compactifications done? Jul 13 revised String Theory and Standard Model in CERN added 2 characters in body Jul 13 answered String Theory and Standard Model in CERN Jul 13 revised What is the relationship between string theory and quantum field theory? added 153 characters in body Jul 13 answered What is the relationship between string theory and quantum field theory?