String theory entered the public arena in 1988 when a BBC radio series Desperately Seeking Superstrings was broadcast.

Thanks to good marketing and its inherently curious name and features, it’s now part of popular discourse, mentioned in TV’s Big Bang Theory, Woody Allen stories, and countless science documentaries.

But what is string theory and why does it find itself shrouded in controversy?

## Life, the universe and the theory of everything

Today we think of string theory in two ways.

It’s seen as a theory of everything – that is, a theory that aims to describe all four forces of nature within a single theoretical scheme.

These forces are:

- Electromagnetic force
- Gravitational force
- Weak nuclear force
- Strong nuclear force.

Electromagnetism and gravity are familiar to most people. The nuclear forces occur at a subatomic level, and are unobservable by the naked eye.

String theory is also used to describe quantum gravity, a theory that combines Einstein’s theory of gravity and the principles of quantum theory.

## Tangled beginnings

But string theory began life more modestly, as a way to describe strongly interacting particles called hadrons.

Hadrons are now understood to be composed of quarks connected by gluons but string theory viewed them as quarks connected by strings (tubes of energy).

Understood this way, string theory buckled under both new experimental evidence (leading to the crowning of quantum chromodynamics which describes the interactions of quarks and gluons) and also internal problems.

String theory involved too many particles, including a massless particle with so-called spin 2 – spin being the name used for the angular momentum of particles.

As it happens, this is exactly the property possessed by the graviton – the carrier of gravitational force in the particle physics picture of the world.

## Beyond four dimensions

This discovery meant that with a bit of skilful rebranding (and rescaling the energy of the strings to match the strength of gravitation), string theory shed its hadronic past and was reborn as a quantum theory of gravity.

All those other particles that were also problematic for the original string theory were able to capture the remaining non-gravitational forces too. This is how string theory took on its current role as describing all four forces together: a theory of everything.

But it could not shed many of its curious features.

One such feature was the necessity of many more space-time dimensions than are actually observed.

In a “bosonic” version of string theory (i.e. *without* matter or fermions, there would have to be 21 dimensions – 20 space dimensions and one time dimension.

In a theory *with* fermions, there would have to be nine spatial dimensions and one temporal, ten dimensions all together.

The problem is that we only perceive four dimensions: height, width, depth (all spatial) and time (temporal).

## Supersizing symmetry, downsizing dimensions

The “super” in “superstring theory” refers to a symmetry, known as supersymmetry, relating bosons and fermions.

There are five possible theories that involve matter in ten dimensions. This was previously seen as a problem since it was expected that a theory of everything should be unique.

The six unseen dimensions (ten minus the four dimensions of everyday life) are made too small to be observable, using a process known as compactification.

## Beautiful maths

It is from this process that much of the extraordinarily beautiful (and fiendishly difficult) mathematics involved in string theory stems.

We have no trouble thinking of each event in the world as labeled by four numbers or coordinates (e.g., x,y,z,t).

A string-theoretic world adds another six coordinates, only they are crumpled up into a tiny space of radius comparable to the string length, so we don’t see them.

But, according to string theory, their effects can be seen indirectly by the way strings moving through spacetime will wrap around those crumpled, curled up directions.

There are very many ways of hiding those six dimensions, yielding more possible stringy worlds (perhaps as many as 10^{500}!).

## How long is a piece of string?

This is why string theory is so controversial. It seemingly loses all predictive power since we have no way of isolating our world amongst this plenitude.

And what good is a scientific theory if it cannot make predictions?

One response is to say that these various theories are not really so different. In fact there are all sorts of exact relations known as dualities connecting them.

More recent developments based on these dualities include a new type of object with higher dimensions – so called Dp-branes.

These too can wrap around the compact dimensions to generate potentially observable effects.

Most importantly, they can also provide boundaries on which endpoints of strings sit.

Just to complicate things more, a new kind of theory has been discovered, this time in 11 dimensions: 11 dimensional supergravity - it is also very beautiful mathematically.

## Dial M for Multiverse

String theorists are fond of saying that these six theories are aspects (special limits) of a deeper underlying theory, known as M-theory. In this way, uniqueness is restored.

Or is it?

We still have the spectre of the 10^{500} solutions or worlds. The great hope is that the number of solutions with features like our own world’s (with its four visible dimensions, particles of various types interacting with particular strengths, conscious observers, and so on) will be small enough to be able to extract testable predictions.

So far, though, the only real way of getting our world out of the theory involves the use of a multiverse (a realistically interpreted ensemble of string theoretic worlds with differing physical properties) combined with the anthropic principle (only some of these worlds have what it takes to support humans).

Needless to say, this does not entirely sit easy with critics of string theory!

But string theory has been making strides in other areas of physics. Notably in the physics of plasmas and of superconductors.

Whether this success can be repeated within its proper realm (fundamental physics) remains to be seen.