# Essentials : More Than the Sum of Its Parts

String theory is not a monolithic topic and those who work on it are not clones. People differ in interests and aptitude, and their purposes vary widely. So who uses string theory, and what do they hope to get from it? Most obviously, string theory is wielded to tackle quantum gravity. This can then be employed to study black holes and the cosmology of the early universe. String [...]

# Quantum Gravity : Towards the Holy Grail

The classic reason for working on string theory is to solve the problem of quantum gravity. Physicists aim to obtain a consistent quantum theory of gravitational interactions, and then to understand in detail how it works. This motivation still drives research today. Everyday work involves examining the stringy version of general relativity. Research topics are typically theoretical: [...]

# Black Holes : Where's My Encyclopedia Gone?

Black holes are one of the most famous predictions of Einstein’s theory of general relativity. They are trapped regions of space, characterised by electric charge, mass and rate of rotation, from which nothing can escape. Supermassive black holes exist at the centre of probably every galaxy, with a mass ten to one hundred billion times greater than the sun. General relativity [...]

# Cosmology : A Guidebook to the Universe

String theory has several potential applications to cosmology, the study of the universe, covering the entire history of the cosmos. One line of enquiry focuses on the initial singularity – what happened when the universe was squeezed to a size close to the Planck scale. This happened shortly after the Big Bang. In principle, this is a period ideally suited to any quantum theory of [...]

# Beyond The Standard Model : Now What?

The Standard Model (pictured right) is one of the great successes of 20th century physics. But even without including gravity, we know that the Standard Model cannot be the whole story. There must be a deeper conceptual structure lurking behind it. The Standard Model signals its own incompleteness in several ways. Here are three. The hierarchy problem is the mystery of why gravity is [...]

# Field Theory : How Strong Is Strong

Quantum field theories (QFTs) are the language in which the Standard Model is written. They each contain a coupling constant that determines the strength of interactions between particles. When it is small (weak coupling) the theory is easy to solve, in principle. Typically physicists use perturbation theory to obtain good approximate answers. But when the coupling constant is [...]

# Condensed Matter : Extreme String Theory

A more recent direction of research uses string theory to think about systems in condensed matter physics, the physics of solids and liquids. At first sight this appears bizarre: string physics takes place on tiny scales! How could string theory possibly model systems of many atoms with energies determined by the ambient temperature? What makes this feasible – although not necessarily [...]

# Mathematics : The Emperor's New Shape

Many reasons for doing string theory involve physics. However strings have also had an important impact on mathematics. Ideas from string theory have an independent existence in the mathematical world, separate from any question about their relationship to reality. This is particularly true for a duality called mirror symmetry. Remember that string theory requires six extra [...]