What is string theory?

I am a theoretical physicists doing research in string theory. Let me try and explain in simple words, intended for a general public, what it is all about.

What is the physical theory describing the laws of our Universe? On the one hand, the world of microscopic particles and their interactions (electro-magnetic and nuclear forces) is described very precisely by quantum field theory. On the other hand, the universe at large scales - and the gravitational force that causes the motion of stars and planets - is the domain of Einstein's general relativity.

These two pillars of twentieth century physics - quantum field theory and general relativity - describe the behaviour of the Universe at their respective scales extremely well. However, they are incompatible with each other. How can the same Universe be described by two different, incompatible theories?

String theory is our best attempt to overcome this contradiction. It is a quantum theory that automatically incorporates general relativity, thus unifying the description of all the forces of Nature in a consistent framework.

The starting point of string theory is quite simple: if we could "zoom in" very close to the elementary particles, we would discover that they are not point-like objects, but rather tiny vibrating strings. Different patterns of vibration of the string constitute what we call different particles. From this simple construction, a very complex and mathematically beautiful theory emerges, whose consequences are still not fully understood.

String theory drastically modifies our notion of space-time, by for instance postulating that the dimensions of the space where we live are not the three we are used to observe, but rather nine, six of which are "curled up" in tiny geometric structures we cannot see. Unfortunately, this is also one of the causes why it has so far turned out to be impossible to devise a concrete experiment to test the theory...

In the last ten years, string theory has also brought us a conceptual revolution on gauge theories (the quantum field theories describing the interactions of elementary particles) and their relation to quantum theories of gravity. This is the gauge/string duality, according to which two seemingly very different theories, a gauge theory in a space of lower dimension and a gravitational theory in a space of higher dimension, are actually dual descriptions of the same theory: one encodes the other as a hologram, opening new perspectives on the understanding of both.

Most of my research has been done in the context of the gauge/string duality, as explained in more detail in the My research section below.

If you want to know more about strings, I suggest you start from Beyond String Theory, Jan Troost's excellent introduction to the subject.

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My research

This section briefly describes my past and current research. Notice that, although I have tried to restrain from giving too much technical detail, this description is probably only accessible to physicists.

As mentioned in the What is string theory? section above, I have mostly been working on the gauge/string theory or gauge/gravity correspondence, the deep relation that is possible to establish between quantum field theories with local gauge symmetry and gravity/string theories.

The correspondence traces its origin to the double nature of D-branes, extended objects present in string theory:

• D-branes are hyperplanes where open string endpoints lie. At low energies, the dynamics of these open strings realizes a gauge theory on the volume of the branes.
  
  
• D-branes are also boundaries of closed string world-sheets, described at low energies by classical solutions of supergravity.

This complementarity of descriptions is the basis of the holographic duality that relates a lower dimensional non-gravitational field theory and a higher dimensional gravitational theory. The main example of the correspondence, or AdS/CFT duality, states the equivalence between N=4 supersymmetric Yang-Mills theory and type IIB superstring theory compactified on five-dimensional Anti-de Sitter space times a five-sphere. The duality is particularly useful due to the fact that the strongly coupled regime of one theory, which is hard to study, is mapped onto the weakly coupled regime of the dual theory, which is much more tractable.

My research has been devoted to the study of the duality and of its extensions, having in mind the ultimate goal to use holography to describe experimentally testable physical systems.

In fact, N=4 Super Yang-Mills theory, the gauge theory involved in the prototypical AdS/CFT duality, is very far from describing the real world, due to its high degree of supersymmetry and due to the fact that it is conformal, namely invariant under changes of the physical scale.

I have therefore been interested in generalizations of the correspondence towards more "realistic" gauge theories, with reduced supersymmetry and broken conformal invariance, in order to get closer to a holographic description of quantum chromodynamics, the theory of strong interactions in Nature. To this aim, I have obtained relevant information on gauge theories with N=2 or N=1 supersymmetry (or no supersymmetry at all) and/or broken conformal invariance by constructing and studying supergravity solutions arising from configurations of D-branes in various string compactifications.

I have also been interested in generalizations of the gauge/string correspondence beyond the supergravity approximation, as well as in models of non-critical strings that are useful to understand the open/closed string duality which is at the origin of the correspondence.

More recently, I have been concentrating on the possibility of using holography to describe three-dimensional physical systems that may have experimental applications in the context of condensed matter physics. In particular, quantum critical systems, which are relevant to the description of unconventional superconductors and gases of cold atoms, are described by strongly coupled conformal field theories and thus constitute an ideal arena to apply holographic ideas.

For additional information, you may take a look at my statement of research interests.

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