“Einstein comes along and says, space and time can warp and curve, that’s what gravity is. Now string theory comes along and says, yes, gravity, quantum mechanics, electromagnetism — all together in one package, but only if the universe has more dimensions than the ones that we see.”

— Brian Greene – Professor of physics and mathematics at Columbia University

Welcome to a world with eleven dimensions, where all matter is actually composed one-dimensional loops of string oscillating at different frequencies and where the physicists’ Holy Grail of a unified theory is realised. The Standard Model of physics isn’t without its limitations, and string theory claims to solve one of the major problems it faces, the quantum (subatomic world) application of our understanding of gravity. The bizarre claims the theory proposes have made it one of the most controversial schools of thought in physics, but it is arguably the present leading candidate for the much-coveted “theory of everything.”

## The Problem with Physics

Einstein’s theory of gravity – the Theory of General Relativity – solved the inherent problems with Newton’s explanation of the force, and remains science’s closest model of the universe’s celestial architect. It describes things on the large scale, like you, the earth and the galaxies in the universe. The rules of the very small things (like atoms) are described by quantum mechanics, where things generally get a lot weirder and are largely governed by the force of electromagnetism. The problem is that these two models have never been effectively combined.

The Standard Model describes the subatomic particles and their interactions; explaining the matter you can see around you, its composition and the majority of the forces which act upon it. One of the key principles of quantum mechanics is wave-particle duality, which states that particles are also waves, and that each force has an associated particle. The photon – light – is the force-particle of electromagnetism, for example. This gets a little overwhelming, as theoretical physicist Sunil Mukhi comments, “Electrons, muons, neutrinos, quarks, W bosons, gluons, gravitons... the list is rather large and the whole collection of particles begins to look like a zoo.”

Yet even with this exhaustive list of the fundamental components of matter and the forces we experience, gravity cannot be applied in a quantum context. The graviton, the theoretical particle which is thought to carry the force of gravity, is not included in the Standard Model and has never been observed. The equations which physicists use to describe everything else cannot be successfully applied to gravity. This doesn’t make much of a difference in practice (gravity depends on mass, so particles wouldn’t exert much of it anyway), but theoretically it presents a serious problem.

## String Theory’s Solution

One of the best things in physics is unification. Einstein unified energy and mass with his famous equation, simplifying our understanding of the universe with a few symbols that could fit on the back of a postage stamp. The theory is considered “elegant” because it expresses something accurately and concisely, and that is exactly what string theory aims to do. Instead of the aforementioned “zoo” of particles, everything could be composed of the same basic constituents, loops of quantum “string.”

Strings are required because they aren’t “point-like” particles such as the ones proposed by the Standard Model. Point-like particles are the reason the equations for quantum gravity never quite add up, so loops of strings don’t encounter the same issues when it comes to the maths. The quantum string’s inherent tension gives the theory a scale on which to calculate mass, and the stable oscillations of the string create the particles we observe in nature.

A guitar string is a perfect analogy for the production of fundamental particles in string theory. The frequency at which the string vibrates determines the note that is produced, and in music there are only thirteen stable notes. In string theory, the “notes” made by the oscillations of the quantum string manifest as things like quarks and electrons, their mass coming directly from the intensity of the vibrations. The equations all worked, but there was one important caveat. The maths for string theory only works if there are a total of eleven dimensions, instead of four.

It may seem unbelievable, but if the theory is correct it explains all of the forces of nature, as well as providing valuable insight into mysteries such as dark matter and dark energy. Theoretical physicist and leading string theorist Michio Kaku explains that “the laws of chemistry that we struggled with in high school would be the melodies that you can play on these vibrating strings. The Universe would be a symphony of these vibrating strings and the mind of God that Einstein wrote about at length would be cosmic music resonating through this nirvana… through this eleven dimensional hyperspace.”