String Theory

Created by: Leigh Aube

Phys 221

"String theory is an effort to unify the four fundamental forces by modeling all particles as various vibrational modes of a single entity – an incredibly small string"

-- Serway & Jewett, p.1536

String theory has its origins in a model of the heavier elements proposed by Yoichiro Nambu in the late 1950s. It has since, however, become one of the most widely accepted quantum gravitational theories.

        Einstein spent 30 years searching for a unified field theory. String theory may very well be the answer to Einstein's riddle. It attempts to become a grand unified theory, to explain all forces and all matter.

Click the picture for nobelprize.org's biography of Einstein

          In sting theory, the excitation modes that are the theoretical basis for matter and the four forces are analogous to different notes on a violin string. The way identical, one-dimensional strings vibrate is responsible for the different particle properties and forces. For example, an electron contains strings vibrating with a different pattern of vibration then a proton.

With E=mc² Einstein proved that matter and energy are two faces of the same coin. Therefore, a particle increasing in mass will also increase in energy. This is included in string theory in that the least energetic strings are also the least massive. This accounts for mass properties in particles.

There are a few basic divisions in string theories. Those in which the strings can be open or form loops and those in which the stings remain fused as loops. They are also divided based upon whether or not the particle spectrum includes fermions.

The model in string theory that includes fermions is called supersymmetry. This model indicates that for every particle of matter (fermion) or that transmits a force (boson) there is an anti-particle or sparticle. It also demands that there be a balance between fermions and bosons. String theories that incorporate this model are called superstring theories. Those theories that do not incorporate supersymmetry apply only to bosons and ignore the existence of matter.

The behavior of particle interaction gives support to the veracity of string theory. According to quantum field theory, if an electron and a positron collide they do not simply change trajectory, much like balls on a pool table, as simple point particles would. Instead they annihilate each other creating a photon for an instant before reforming a new positron-electron pair which then appears to be deflected as expected by classic theory.  This can be explained by the addition of the strings from the positron joining with those from the electron and then separating as energy is released.

Finally, particle interactions occur at zero distance even though Einstein's theory of gravity makes no sense without a finite distance between the particles. String theory however spreads the interaction out over the distance of the string. Despite its tiny size, the finite distance of the strings explains how point particle interactions can theoretically occur at zero distance.