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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?
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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
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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
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Jul
13
answered String Theory and Standard Model in CERN
Jul
13
revised What is the relationship between string theory and quantum field theory?
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Jul
13
answered What is the relationship between string theory and quantum field theory?
Jul
13
comment String theory and the SM spectrum
@Michael Brown, yes, I think back in the 20th century many hopes were just as irrational as many supposed debunkings are these days. It would be good if the grander theoretical physics community had a better sense of the logical foundations of its subject, such as to then be able to make objective, substantive judgements. Much of the criticism of string theory is based on confusion already about QFT (such as the common "string theory doesn't work because the perturbation series does not converge" or "because it doesn't have a unique vacuum").
Jul
12
comment String theory and the SM spectrum
It's not about being a "fan" or a "stringer". It's about logical reasoning in science. String theory might be wrong, but not because it has more than one vacuum and not because it doesn't uniquely predict the observed elementary mass spectrum. That's not a logically sensible criticism. You might as well complain that it doesn't predict your telephone number. I would like to see people concentrate and have decent arguments about string theory. Feel free to try to trash it, that's what science is about, but know some good reasons.