## String Theory

String Theory asserts that elementary particles have an internal structure: each particle is a string curled into a
ring of a very small size. Different particles are different vibrational modes of the string. These simple
assumptions have far-reaching consequences. Perhaps the most remarkable feature of string theory is its relationship
to quantum gravity: in contrast to the Standard Model, string theory is capable of explaining all known particles
and all types of interactions, gravity included. String Theory has also proved to be mathematically rich, and has
given rise to a whole new area of research, known as **Holographic Duality**.

Two robust predictions of String Theory are extra dimensions and supersymmetry. If we want to connect string theory with experiment we first need to explain why observed spacetime has only four dimensions rather than ten. This is usually achieved by compactifying six of the ten dimensions on a compact six-dimensional manifold that is sufficiently small to have avoided detection so far. Supersymmetry not being observed, has to be broken on the way from the string scale to the length scale probed by current experiments.

Mathematical consistency of String Theory is very restrictive and there are but a few well- defined string theories all living in ten dimensions, such as the celebrated E8×E8 heterotic string. It has been realized that all these models are different facets of a unique but still enigmatic eleven-dimensional structure called M-theory. On the contrary, the String Theory is believed to possess a myriad of consistent vacua forming an intricate landscape. Vacuum selection leads to an enormous number of different predictions that are all equally valid a priori. This spells trouble for any theory which is supposed to predict from first principles the behavior of elementary particles that we observe. Many ideas have been proposed to address these issues. One is referred to as brane-world compactifications, where the gauge theories of the Standard Model live inside the world-volume of a stack of D(irichlet)-branes. Gauge fields then correspond to open strings with both ends attached to the branes in a stack, while quarks and leptons are open strings connecting different types of D-branes.

Being a consistent theory of **Quantum Gravity**, String Theory is capable to address deep questions
about the nature black holes in a well-defined setting. Quite remarkably, the synthesis of ideas from
**Quantum Gravity**, String Theory and black hole physics resulted in the formulation of the
**Holographic Duality** relating gravity, quantum or classical, to non- gravitational physics at the
boundary of space-time. The duality can be equally well used to gain deeper understanding of **Quantum
Gravity**, and to explore strongly-coupled quantum system in the regime inaccessible to conventional methods.
The **Holographic Duality** thus shed an entirely new light on the quark confinement problem and has
deepened our understanding of the quark-gluon plasma. Most recently, its applications have extended to condensed
matter physics with applications to quantum criticality and superconductivity.

Physicists at Nordita are working on many aspects of String Theory, including supersymmetry, holographic duality, black holes, and the non-relativistic limit of strings.