Not so long ago we saw the universe as particles moving in space, then 'String theory' was introduced to better explain the organization of the universe, but more recently this has evolved into 'M theory'.
In standard
particle physics, particles are thought to be points moving through space, tracing out a line known as the '
World Line'. For the different interactions that we see in nature to occur there must be more possible degrees of freedom (dimensions) for a particle to move in than just their position and velocity. Such dimensions could be mass, electric charge or
spin. The standard model of the universe was designed around
Quantum Field Theory (QFT), which allows theories to be developed and be consistent with
quantum mechanics and
special relativity.These theories successfully described three of the four known interactions in nature (Electromagnetism, Strong and Weak nuclear force). They also described the interaction between electromagnetism and weak nuclear force (
Electroweak Theory). The fourth interaction (Gravity), which is described in Einsteins theory of
General Relitivity does not seem to fit. When the rules of QTF are applied to general relativity the results seen are impossible. For example; the force between two
gravitons would be infinite, which can't happen.
String Theory replaces the range of different particles with a single funamental building block. This is known as a 'string'. These strings can be open ended or 'tied at the ends' forming a loop or ring. As a string moves through time it traces out a sheet or a tube, depending on which type of string it is. Strings can vibrate freely. Different vibrations create the different particle types. For example; one vibrational mode makes a string appear as an electron, another could make a photon, even a graviton. String theory recieved a lot of attention because it could describe the interaction of two gravitons, whereas the previous theorys could not. String theory also has the necessary dimensions to describe other interactions. This gave hope for a new theory which would unify all particles and forces, 'The Theory of Everything'
The known subatomic particles are classifed depending upon the way in which they
spin. The two types are called
Bosons and Fermions. Bosons carry forces, and fermions make up the matter of which we are made. Originaly String Theory did not describe Fermions, only Bosons. This meant matter particles were not included. To obtain a theory which includes both Fermions and Bosons,
Supersymmetry was used and this gave us
Superstring theory. The problem with this is that five different theorys developed and only three of them had no mathmatical inconsistencies. For two of the theorys the funamental object is a closed string, while in another, open strings are used. Two more theorys were developed by mixing the best parts of Bosonic strings and Superstrings, these are Heterotic String Theorys. Studying these theorys was problematic, as we did not have all the neccesary tools to test all possible values of the paramaters involved.
We are all used to the idea of three dimensions (3D), but time is also a dimension. This makes four dimensions that we can percive. String Theory predicts that space-time has ten dimensions, not four. It is thought that the 'extra' dimensions are tightly packed on complex shapes known as
Orbifolds and Calabi-Yau Manifolds. These dimensions, or degrees of freedom are such things as charge on an electron. It is belived that the charge arises because the elctron is moving in one of these other dimensions.
Recently techniques have been developed to thoroughly explore the theorys. This led to the developmet of
M-Theory. This theory describes the previous five theorys of superstrings as just different aspects of the same theory. M-Theory seems to tell us that there is another dimesion, bringing the total to eleven. It is said that M-Theory should be seen as an eleven dimenisonal theory that looks ten dimensional at some of the points in its parameters. Now this gave us a new funamental object,
Branes (membranes). These Branes look like strings if the eleventh dimension is curls into a small circle (like looking at a tube from the end).
Next Page: Other Theory Previous Page: Zero Point Energy & Overunity
Linear Mode
