本书是一本真正意义上的现代弦论的入门书籍，内容现代全面，简明，将20世纪80年代以来弦论中最易于理解，最重要的方面全面囊括其中。物理的一个核心理论是用一维扩充弦代替零维类点粒子，弦论已经成为成功地各种基本自然力统一起来的最重要理论。本书从弦论的基本定义开始，通述了经典观点和现代观点。特别地，书中详述了扰动弦论及其共形场论，同时探讨了非扰动的各个方面和弦相互作用的整体性。陈列了一些现代话题如黑洞、他们的显微熵和Ads/CFT对应，并且包括了将近500研究生水平的习题，这些都使本书的独立性增强，内容更加详实。读者对象：物理专业的研究生和更高层次的科研人员。
1 Introduction
1.i Prehistory
1.2 The Case for String Theory
1.3 A Stringy Historical Perspective
1.4 Conventions
Bibliography
2 Classical String Theory
2.1 The Point Particle
2.2 Relativistic Strings
2.3 0scillator Expansions
2.3.1 00sed strings
2.3.2 0pen strings
2.3.3 The Virasoro constraints
Bibliography
Exercises
3 Quantization of Bosonic Strings
3.1 Covariant Canonical Quantization
3.2 Light-cone Quantization
3.3 Spectrum of the Bosonic String
3.4 Unoriented Strings
3.4. 1 0pen. strings and Chan-Patonfactors
3.5 Path Integral Quantization
3.6 Topologically Nontrivial World-sheets
3.7 BRST Primer
3.8 BRST in String Theory and the Physical Spectrum
Bibliography
Exercises
4 Conformal Field Theory
4.1 Conformal Transformations
4.1.1 The case oftwo dimensions
4.2 Conformally Invariant Field Theory
4.3 Radial Quantization
4.4 Mode Expansions
4.5 The Virasoro Algebra and the Central Charge
4.6 The Hilbert Space
4.7 The Free Boson
4.8 The Free Fermion
4.9 The Conformal Anomaly
4.10 Representations of the Conformal Algebra
4.11 Affine Current Algebras
4.12 Free Fermions and O(N) Affine Symmetry
4.13 Superconformal Symmetry
4.13.1 N =(1.0)2 SIAperconformal symmetry
4.13.2 N =(2.0)2 superconformal symmetry
4.13.3 N =(4.0)2 superconformal symmetry
4.14 Scalars with Background Charge
4.15 The CFT of Ghosts
4.16 CFT on the Disk
4.16.1 Free massless bosons on the dislc
4.16.2 Free masslessfern竹ions on the dislc
4.16.3 The projective plane
4.17 CFT on the Torus
4.18 Compact Scalars
4.18.1 Modular invariance
4.18.2 Decompaaification
4.18.3 The torus propagator
4.18.4 Marginal deformations
4.18.5 Multiple compaa scalars
4.18.6 Enhanced symmetry and the string Brout-Englert-lliggs effea
4.18.7 T-duality
4.19 Free Fermions on the Torus
4.20 Bosonization
4.20. 1 "Bosonization " of bosonic ghost system
4.21 0rbifolds
……
5 Scattering Amplitudes and 'Vertex Operators
6 Strings in Background Fields
7 Superstrings and Supersymmetry
8 D-branes
9 Compactifications and Supersymmetry Breaking
10 Loop Corrections to String Effective Couplings
11 Duality Connections and Nonperturbative Effects
12 Black Holes and Entropy in String Theory
13 The Bulk／Boundary Correspondence
14 String theroy and Matrix Models
Although I wrote a short textbook in 1997， I have not yet learned my lesson. Therefore， I embarked on a direct confrontation with deadlines that， although not as spectacular as the ones in Polchinski's book， managed to wreak havoc on my academic and personal schedule. i have not even put sufhcient credence in a colleague's statement "it is never too late to stop writing a book." I can only hope now that the result was worth the effort.
This is a textbook on the collection of ideas categorized under the name of "string theory." This is a large domain that has as its central goal the unification of all interactions induding gravity. There have been several surges ofprogress in different directions in the past twentyyears， and this book tries to give the student ofthe field some ofthe salientideas.
It is not the purpose of this book to provide deep insights into the theory. I can only leave this to more competent colleagues. Its purpose is to provide the fastest possible introduction to the basic formalism and structure of string theory and its main properties and ramifications that are on a reasonably solid footing today.
This effort is unlike writing a textbook about the Standard Model of the fundamental interactions. It is less clear here what will turn out to be just mathematics， what will transform into real physics， and what will be neither ofthe above. However， the scope and deep interest of the endeavor， namely， to understand the basic mysteries of the universe at the most fundamental level， has driven more than a generation ofbright physicists and has provided breakthroughs in our theoretical understanding of both gravitational and gauge theories. It is this interest that drives researchers today， together with the hope that the theory will eventually be seriously confronted with experimental data.
There are several current and past areas of research in string theory .that have not been treated in this book. The reasons were varied. They include the subjects of the Feynman diagrams， it is obvious that there is essentially a universal three-point interaction in the theory. This interaction is dictated by the two-dimensional geometry of closed Riemann surfaces as is obvious from figure 1.2. The other related reason is the presence of an infinite tower of excitations with masses in multiples of the string scale. Their interactions are carefully tuned to become soft at distances larger than the string length ~s but still longer than the Planck length ~P.
For open strings the situation is subtler. There， UV divergences are present， but are interpreted as IR closed-string divergences in the dual dosed-string channel. This UV-IR open-dosed string duality is at the heart of many of the recent developments in the field.
Another key ingredient ofstring theoryis that it unifies gravity with gauge interactions. It does this in several different ways. The simplest is via the traditional KK approach. Super-string theory typically is defined in ten dimensions. Standard four-dimensional vacua can be obtained via compactification on a six-dimensional compact manifold. However， gauge symmetry can also arise from D-branes that sometimes are part of the vacuum （as in orientifolds）. There is even gauge symmetry coming from a nongeometrical part of the theory as happens in the heterotic string. The unified origin of gravity and gauge symmetry extends even further to other interactions. For example， the Yukawa interac-tions， crucial for giving mass to the SM particles， are also intimately related to the gauge interactions.
Unlike earlier Kaluza-Klein approaches to unification， string theory is capable of pro-viding， upon appropriate compactifications， chiral matter in four dimensions. This hap-pens via a subtle interplay between anomaly-related interactions and the process of compactification.
Another characteristic ingredient of string theory is that the presence of space-time fermions in the theory implies the appearance o
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