The first part is an introduction to conformal field theory and string perturbation theory. The second part deals with the search for a deeper answer to the question posed in the title. Contents: 
1. Conformal Field Theory 
2. String Theory 
3. Vacua and Dualities 
4. String Field Theory or Not String Field Theory 
5. Matrix Models

https://arxiv.org/pdf/hep-th/9411028v1.pdf

What is string theory

Quasinormal modes are eigenmodes of dissipative systems. Perturbations of classical gravitational backgrounds involving black holes or branes naturally lead to quasinormal modes. The analysis and classification of the quasinormal spectra require solving non-Hermitian eigenvalue problems for the associated linear differential equations. Within the recently developed gauge-gravity duality, these modes serve as an important tool for determining the near-equilibrium properties of strongly coupled quantum field theories, in particular their transport coefficients, such as viscosity, conductivity and diffusion constants. In astrophysics, the detection of quasinormal modes in gravitational wave experiments would allow precise measurements of the mass and spin of black holes as well as new tests of general relativity. This review is meant as an introduction to the subject, with a focus on the recent developments in the field.

/pdf/quasinormal-modes-of-black-holes-and-black-branes-3f4tq2403g.pdf

Quasinormal modes of black holes and black branes

Black branes in AdS5 appear in a four parameter family labeled by their velocity and temperature. Promoting these parameters to Goldstone modes or collective coordinate fields—arbitrary functions of the coordinates on the boundary of AdS5—we use Einstein's equations together with regularity requirements and boundary conditions to determine their dynamics. The resultant equations turn out to be those of boundary fluid dynamics, with specific values for fluid parameters. Our analysis is perturbative in the boundary derivative expansion but is valid for arbitrary amplitudes. Our work may be regarded as a derivation of the nonlinear equations of boundary fluid dynamics from gravity. As a concrete application we find an explicit expression for the expansion of this fluid stress tensor including terms up to second order in the derivative expansion.

/pdf/nonlinear-fluid-dynamics-from-gravity-h5lern3bur.pdf

Nonlinear fluid dynamics from gravity

Perturbations of black holes, initially considered in the context of possible observations of astrophysical effects, have been studied for the past 10 years in string theory, brane-world models, and quantum gravity. Through the famous gauge/gravity duality, proper oscillations of perturbed black holes, called quasinormal modes, allow for the description of the hydrodynamic regime in the dual finite temperature field theory at strong coupling, which can be used to predict the behavior of quark-gluon plasmas in the nonperturbative regime. On the other hand, the brane-world scenarios assume the existence of extra dimensions in nature, so that multidimensional black holes can be formed in a laboratory experiment. All this stimulated active research in the field of perturbations of higher-dimensional black holes and branes during recent years. In this review recent achievements on various aspects of black hole perturbations are discussed such as decoupling of variables in the perturbation equations, quasinormal modes (with special emphasis on various numerical and analytical methods of calculations), late-time tails, gravitational stability, anti--de Sitter/conformal field theory interpretation of quasinormal modes, and holographic superconductors. We also touch on state-of-the-art observational possibilities for detecting quasinormal modes of black holes.

Quasinormal modes of black holes: From astrophysics to string theory

Black branes in AdS5 appear in a four parameter family labeled by their velocity and temperature. Promoting these parameters to Goldstone modes or collective coordinate fields -- arbitrary functions of the coordinates on the boundary of AdS5 -- we use Einstein's equations together with regularity requirements and boundary conditions to determine their dynamics. The resultant equations turn out to be those of boundary fluid dynamics, with specific values for fluid parameters. Our analysis is perturbative in the boundary derivative expansion but is valid for arbitrary amplitudes. Our work may be regarded as a derivation of the nonlinear equations of boundary fluid dynamics from gravity. As a concrete application we find an explicit expression for the expansion of this fluid stress tensor including terms up to second order in the derivative expansion.

/pdf/nonlinear-fluid-dynamics-from-gravity-vgh1egef29.pdf

Nonlinear Fluid Dynamics from Gravity

Department of Physics, Kwansei Gakuin University, Sanda, Hyogo, 669-1337, Japan(Dated: October 1, 2008)We study causal hydrodynamics (Israel-Stewart theory) of gauge theory plasmas from theAdS/CFT duality. Causal hydrodynamics requires new transport coeﬃcients (relaxation times)and we compute them for a number of supersymmetric gauge theories including the N = 4 SYM.However, the relaxation times obtained from the “shear mode” do not agree with the ones from the“sound mode,” which implies that the Israel-Stewart theory is not a suﬃcient framework to describethe gauge theory plasmas.

Causal hydrodynamics of gauge theory plasmas from AdS/CFT duality

We study ``holographic superconductors'' in various spacetime dimensions. We compute most of the static critical exponents in the linear perturbations and show that they take the standard mean-field values. We also consider the dynamic universality class for these models and show that they belong to model A with dynamic critical exponent $z=2$.

Universality class of holographic superconductors

We investigate the vortex lattice solution in a (2+1)-dimensional holographic model of superconductors constructed from a charged scalar condensate. The solution is obtained perturbatively near the second-order phase transition and is a holographic realization of the Abrikosov lattice. Below a critical value of the magnetic field, the solution has a lower free energy than the normal state. Both the free-energy density and the superconducting current are expressed by nonlocal functions, but they reduce to the expressions in the Ginzburg-Landau theory at long wavelengths. As a result, a triangular lattice becomes the most favorable solution thermodynamically, as in the Ginzburg-Landau theory of type II superconductors.

Vortex lattice for a holographic superconductor

We compute the strong coupling limit of the shear viscosity for the $\mathcal{N}=4$ super-Yang-Mill theory with a chemical potential. We use the five-dimensional Reissner-Nordstr\"om-anti-deSitter black hole, so the chemical potential is the one for the $R$-charges $U(1{)}_{R}^{3}$. We compute the quasinormal frequencies of the gravitational and electromagnetic vector perturbations in the background numerically. This enables one to explicitly locate the diffusion pole for the shear viscosity. The ratio of the shear viscosity $\ensuremath{\eta}$ to the entropy density $s$ is $\ensuremath{\eta}/s=1/(4\ensuremath{\pi})$ within numerical errors, which is the same result as the one without chemical potential.

Viscosity of gauge theory plasma with a chemical potential from AdS / CFT correspondence

This is the draft/updated version of a textbook on "real-world" applications of the AdS/CFT duality for beginning graduate students in particle physics and for researchers in the other fields. The aim of this book is to provide background materials such as string theory, general relativity, nuclear physics, nonequilibrium physics, and condensed-matter physics as well as some key applications of the AdS/CFT duality in a single textbook. Contents: (1) Introduction, (2) General relativity and black holes, (3) Black holes and thermodynamics, (4) Strong interaction and gauge theories, (5) The road to AdS/CFT, (6) The AdS spacetime, (7) AdS/CFT - equilibrium, (8) AdS/CFT - adding probes, (9) Basics of nonequilibrium physics, (10) AdS/CFT - nonequilibrium, (11) Other AdS spacetimes, (12) Applications to quark-gluon plasma, (13) Basics of phase transition, (14) AdS/CFT - phase transition, (15) Exercises.

/pdf/ads-cft-duality-user-guide-mvszwn8onh.pdf

Makoto Natsuume

Papers

Causal hydrodynamics of gauge theory plasmas from AdS/CFT duality

Universality class of holographic superconductors

Vortex lattice for a holographic superconductor

Viscosity of gauge theory plasma with a chemical potential from AdS / CFT correspondence

AdS/CFT Duality User Guide