Theory of Everything

String Theory: the multi-dimensional universe

In the standard model of particle physics, particles are considered as points moving through space, tracing out a line called the World Line. Unfortunately, there are a great number of different fundamental particles, which makes the construction of a Unified Theory difficult.

In String Theory, the myriad of particle types is replaced by a single fundamental loop, a `string'. As the string moves through time it traces out a tube, the 'World Sheet'. The string is free to vibrate, and different vibrational modes on the string represent the different particle types. One mode of vibration, makes the string appear as an electron, another as a photon. There is even a mode describing the graviton, the particle carrying the force of gravity, which is an important reason why String Theory has received the attention it has. The original hope was that String Theory will be able to unify all of the known forces and particles together into a single

 



Imagine a single string traveling in a flat spacetime in dimensions, that it is traveling across space while time is ticking. A string is a one-dimensional object, that is, if you want to travel along a string, you can only go forwards or backwards in the direction of the string, there is no sideways or up and down on a string. The string can move sideways or up and down in spacetime, though, and as the string moves around in spacetime, it sweeps out a surface in spacetime called the string worldsheet, a two-dimensional surface with one dimension of space and one dimension of time.
    
The string worldsheet is the key to all the physics of the string. A string oscillates as it travels through the dimensional spacetime. Those oscillations can be viewed from the two-dimensional string worldsheet point of view as oscillations in a two-dimensional quantum gravity theory. In order to make those quantized oscillations consistent with quantum mechanics and special relativity, the number of spacetime dimensions has to be restricted to 26 in the case of a theory with only forces (bosons), and 10 dimensions if there are both forces and matter (bosons and fermions) in the particle spectrum of the theory.
   
Where then does gravity come in?

 

If the string traveling through spacetime is a closed string, then the spectrum of oscillations includes a particle with 2 units of spin and zero mass, with the right type of interactions to be the graviton, the particle that is the carrier of the gravitational force. Where there are gravitons, then there must be gravity. Where is the gravity in string theory?


Strings and Spacetime Geometry


 

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