String Theory  

In General > s.a. bosonic strings; history of particle physics; string phenomenology; String Field Theory.
* Idea: The theory of a 2D surface in a d-dimensional flat M, which can be thought of as a field theory on 2D Minkowski, the field being the metric induced on the world-sheet by d-dimensional Minkowski; For a realistic theory one needs to use superstrings, because bosonic strings have tachyons and no fermions.
* Motivation: It automatically includes "gravitons", and, in its supersymmetric version, gauge interactions, quark and lepton fields (more naturally than GUTS); It has no ultraviolet divergences and is thought to be finite (this is almost known to all orders), anomaly-free, and is the only known consistent theory of interacting particles of spin > 2; It has almost no free parameters (only the string tension), and only one fundamental object.
* Problems: Vacuum non-uniqueness; Dependence of low-energy phenomenology on the compactification scheme (no completely satisfactory model is known yet); Vanishing of the cosmological constant; Divergent perturbation series (no renormalization is needed, but the series is still asymptotic); Lack of underlying general principle; 1994, No known experimental signature.
* Goals: Search for non-metric string theory (in the spirit of topological field theories); Define the second-quantized theory (non-perturbative).
* 4D strings: They are not really (geometrically) strings, but just theories with similar actions in 4D.

* Action: The area of the string in superspace, plus a Wess-Zumino term.
* Consistency: Consistent and well-behaved only in 10D, with gauge group SO(32) or E8 × E8 (496 generators).
* Types: There are five consistent types of string theories, Type I (unoriented, open or closed, admit only N = 1 supersymmetry); Type II (oriented, closed, with the N = 2 supersymmetry required for divergence cancellations–but in type II theories, gauge groups cannot be introduced by attaching quantum numbers to the ends of the strings); Heterotic (oriented closed, N = 1, made chiral by requiring left and right-moving excitations to be different; clockwise modes live in 10D with extra fermionic dimensions, while counterclockwise ones in a 26D bosonic space–the compactification of the extra 16 generates E8 × E8); and SO(32) (these groups are associated with the Yang-Mills charges).
* And phenomenology: The particles governed by the two E8s can have only gravitational interactions; Maybe we see only one group, the other one accounting for the missing mass.
@ References: Siegel NPB(86) [classical mechanics, Lagrangian and Hamiltonian]; Guttenberg PhD(07)-a0807 [general backgrounds].

Other Types of Strings
* Ambitwistor string theories: A family of chiral (holomorphic) string theories whose target space is the space of complexified null geodesics in a general spacetime; Like conventional strings, they are critical in 10 dimensions and describe supergravity, but unlike conventional strings, they do not admit a tower of higher massive modes (and are correspondingly not thought to be ultraviolet finite).
@ References: Álvarez NPB(83) [with boundaries]; Shapiro & Taylor PRP(90) [spacetime supersymmetric]; Hadasz & Rog PLB(96)ht [with particles at the ends]; Geyer et al CQG(15) & CQG+(15) [ambitwistor strings].

References > s.a. non-local theories; quantum spacetime; symplectic structures in general and in physics.
@ Articles: Green & Schwarz PLB(82), NPB(83), PLB(84), PLB(84) [anomaly-free], PLB(85) [likely finite]; Candelas, Horowitz, Strominger & Witten NPB(85); Gross, Harvey, Martinec & Rohm PRL(85), NPB(85), NPB(86) [E8 × E8 closed N = 1 heterotic]; Narain PLB(86); Susskind FP(13); Duff FP(13) [response to critics]; Polchinski a1512-conf [unification of quantum mechanics and general relativity].
@ I / II: Schwarz PT(87)nov; Davies & Brown 88; Linden PW(90)aug; Kaku 94; Bernstein 96; Duff SA(98)feb; Musser SA(98)oct; Greene pw(00)mar; Susskind pw(03)nov; Greene 03 [interview SA(03)nov]; Chalmers pw(07)sep; Cartwright & Frigg pw(07)sep; Gubser 10; Conlon 15.
@ Books: Schwarz ed-85; Green, Schwarz & Witten 87; Polyakov 87; Brink & Henneaux 88; Freund & Mahanthappa ed-88; Hatfield 92; Bailin & Love 94; Polchinski 98 [solutions manual Headrick a0812]; Kaku 99; Becker et al 07; Kiritsis 07; Zwiebach 09; Szabo 11; West 12; Dine 16; in Manoukian 16; Schomerus 17.
@ Intros, reviews: Schwarz PRP(82), in(85); Sen lecture notes; Horowitz in(86); Bailin CP(89); issue PTRS(89)#1605; Schwarz ht/96-conf, ht/97; Dienes PRP(97); Kiritsis ht/97, ht/97-ln; Schwarz & Seiberg RMP(99)ht/98; Sen ht/98-proc; Rudolph ht/98-ln; Álvarez & Meessen JHEP(99); Yoneya ht/00-conf; Schwarz ht/00-MG9, hx/00-ln; Sen NPPS(01)ht/00; Mohaupt LNP(03)ht/02; de Boer NPPS(03)ht/02; Schwarz ap/03-conf [update]; Marolf AJP(04)jun-ht/03; Barbón EPJC(04)ht [ideas]; Johnson IJMPA(05); Krishnan ht/06-ln; Zapata ht/06; Schwarz PTPS(07)ht, IJMPA(10)-a0812-ln; Tseytlin a0808-ln; Belhaj a0808-ln; Sahoo EJP(09); Tong a0908-ln; Bedford a1107-ln; Mukhi CQG(11)-a1110 [25 years]; Stelle LNP(13)-a1203; issue FP(13)#1; Witten PT(15)nov [what every physicist should know].
@ And mathematics: Deligne et al 99 [string theory for mathematicians]; > s.a. finite groups [monshine]; mathematical physics [relationship].
@ Quantization: Ohta PRD(86) [BRST]; Carlip & DeWitt-Morette PRL(88) [sign of the metric]; Grassi et al CQG(03) [covariant]; Tseytlin IJMPA(03) [semiclassical]; Guttenberg et al JHEP(04)ht [type II]; Grigore RVMP(07) [bosonic and supersymmetric strings]; > s.a. renormalization.
@ Conceptual: Witten PTRS(89) [and higher symmetry]; Polyakov PAN(01)ht/00; Woit AS(02)phy/01; Marshakov PU(02)ht [motivation]; Witten ht/02-in [status]; Schnitzer phy/03 [history/philosophy of science]; Kim phy/04 [historical perspective]; Johansson & Matsubara SHPMP(11)-a0912 [philosophical perspective]; Rickles FP(13)-a1004 ['no miracles' argument]; blog sa(15)dec [is string theory science?]; Dawid a1812 [implications of the absence of free parameters]; Gilbert & Loveridge a2004 [interviews and analysis]; Huggett & Wüthrich a2005-ch [emergence of spacetime].
@ Critical assessments: Larsson mp/01; Faraggi ht/03-proc; Schroer IJMPD(08)phy/06, ht/06; Hedrich P&P-phy/06, JGPS(07)phy/06; Smolin 06; Woit 06; Emam AJP(08)jul-a0805; Schroer IJMPD(08)-a0805; Zapata a0905 [results, acceptance, and AdS/cft]; Schroer a0906; Hedrich a1101 [epicycles!]; Giddings FP(13)-a1105; Rovelli FP(13)-a1108; Duff FP(11)-a1112-in, response Schroer a1201 [answering the critics]; Popławski a1210 [and matter with intrinsic spin]; 't Hooft FP(13); Dawid FP(13), 13 [r CP(14), FP(14)]; Copsey a1303 [orientifold instabilities]; Martins a1701 [Solomoff induction].
@ Nambu-Goto action: Grigore JPA(92); Ramos NPB(98) [reduced covariant phase space quantization]; > s.a. bosonic strings.
@ In curved space: Jain IJMPA(88); Viswanathan & Parthasarathy PRD(97); Sánchez IJMPA(03)ht-in; Orlando & Petropoulos JPCS(06)ht.
@ Background independence: Rahman ht/97, ht/97; Hohm IJMPD(18)-a1806-GRF.
@ Uniqueness (including p-branes): Bars & Pope GRG(89).
@ And fundamental physics: Gibbs ht/96; Schlesinger FPL(02)ht/00, Brustein & de Alwis PRD(01), news Quanta(15) [universality].
@ In 4D: Dine ed-88; Meusburger & Rehren CMP(03)mp/02 [algebraic quantization of bosonic string]; Alexandre & Mavromatos ht/07; Swain a1110 [and simplicial gravity].
@ Networks: Krogh & Lee NPB(98)ht/97; Sen JHEP(98)ht/97; Bhattacharyya et al PRL(98)ht [U-duality]; Verlinde & Vonk ht/03.
@ Substructure: Bergman ht/96, ht/96; Thorn ht/96-conf.
@ Compactification: Reffert a0706-ln [geometric tools]; Alexandrov PRP(13) [twistor approach, rev]; Shahbazi note1(15), note2(15).
@ And loop space / spin networks: Ansoldi et al PRD(96), CQG(99); Starodubtsev gq/02; Sathiapalan IJMPA(03)ht/02; Miković ht/03; Miković ht/05-ln [spin-foam formulation]; Silva PRD(20)-a2008, a2009 [holographic duality]; > s.a. bosonic strings.
@ Related topics: Karliner et al IJMPA(88) [size, shape]; 't Hooft NPB(90) [black hole interpretation]; Álvarez-Gaumé HPA(91) [random surfaces]; Witten ht/95; Gibbs ht/95 [and knots]; Abel et al ht/99-fs [thermodynamics]; Polchinski & Susskind ht/01 [hadron size]; Schreiber JHEP(04)ht, JHEP(04)ht [(super)-Pohlmeyer invariants]; Adam GRG(09) [perturbation theory]; Martinec FP(13) [and geometry]; Chishtie & McKeon CJP(15)-a1401 [canonical structure].
> Related topics: see anomalies; cosmic strings; instantons; Moduli Space; non-commutative field theory; non-commutative geometry.

Online Resources > s.a. The Official String Theory Website; Wikipedia article.
> Video clips: Discover clip of Brian Greene on String Theory in Two Minutes of Less.

"The unification of quantum mechanics and general relativity remains the primary goal of theoretical physics,
with string theory appearing as the only plausible unifying scheme." A Nicolaidis, arXiv:0812.1946

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