# Essentials : The Universal Symphony

In string theory, the nature of reality changes. We are used to thinking that the world is composed of particles, an assumption formalised by the Standard Model. In string theory, however, particles are no longer fundamental: at very short distances they appear to be tiny vibrating strings. These strings play nicely with both quantum mechanics and relativity. They contain a remarkable [...]

# String Dynamics : A Tangled Reality

Welcome to the theory where everything is made of strings. They’re much as you might imagine from everyday life; strings can wiggle and contort in myriad ways. In doing so they give rise to particles and forces. Remarkably, the motion of a string can encode both particles and forces. To understand string theory we must study the physics of strings. It’s good to start with a musical [...]

# Quantum Strings : What's in a Particle?

We’ve learnt that all strings vibrate as a superposition of modes. Each mode is a particular type of vibration and has an associated energy. In quantum string theory every mode is identified with a fundamental particle. The equations describing the mode correspond exactly with those defining the particle. For example, the mathematical laws governing photons naturally emerge as [...]

# Superstrings : We're Missing Half the Jigsaw!

Supersymmetry (SUSY) was proposed in the early 1970s as a further symmetry in nature. The Standard Model divides particles into two camps called fermions and bosons. All the usual matter particles we observe – like electrons and quarks – are fermions. Every normal force carrying particle – like a photon or graviton – is a boson. Roughly speaking, SUSY claims that [...]

# Extra Dimensions : Any More for Any More?

Extra dimensions are string theory’s most outlandish prediction. String theory demands that our cosy 4D view of the world is wrong. In fact the universe of strings must have ten dimensions! This is immediately at odds with our perception of reality, but we can resolve the paradox by requiring the six unseen dimensions to be incredibly small. So what makes a dimension? Intuitively each [...]

# Dualities : Two for the Price of One

Sometimes different mathematical theories describe the same physics. We call this situation a duality. In many cases, calculations which are very complicated in one theory become much easier in the other. Usefully, string theory is awash with dualities. These variously offer us new perspectives on reality, improve our ability to compute hard sums and unite disparate areas of physics. [...]

# M-theory : It's an Elephant!

There are five different superstring theories, each ten-dimensional, all seemingly incompatible. But in 1995, Edward Witten proposed that the five theories were actually all part of a large, mysterious and uncharted framework that he dubbed M-theory. We don’t have the full equations for M-theory, but there are many hints as to how it works. Witten showed that the five theories are [...]

# D-branes : Life on a Magic Carpet

M-theory is not just populated by strings, but also by membranes called D-branes. These are multi-dimensional surfaces that move through the eleven dimensions of M-theory. We can have D-branes of up to nine spatial dimensions (though that’s a little hard to visualise)! A point is a D0-brane, a string a D1-brane, a sheet a D2-brane and so on. Eleven-dimensional M-theory can look [...]

# AdS-CFT : Through the Looking Glass

The AdS/CFT correspondence is one of the largest areas of research in string theory. AdS/CFT stands for Anti-de Sitter/Conformal Field Theory, an expression that’s not particularly elucidating. AdS/CFT is a particular, and deeply surprising, example of a duality. It relates two very different theories and at first sight seems obviously wrong. It states that there is [...]