# String Theory

July 11, 2010 by TMO

String theory is a developing theory in particle physics which attempts to reconcile quantum mechanics andgeneral relativity. String theory posits that the electrons and quarks within an atom are not 0-dimensional objects, but rather 1-dimensional oscillating lines (“strings”), possessing only the dimension of length, but not height or width. The theory poses that these strings can vibrate, thus giving the observed particles their flavor, charge, massand spin. The earliest string model, the bosonic string, incorporated only bosons, although this view evolved to thesuperstring theory, which posits that a connection (a “supersymmetry”) exists between bosons and fermions, two fundamentally different types of particles. String theories also require the existence of several extra, unobservable, dimensions to the universe, in addition to the usual three spatial dimensions (height, width, and length) and the fourth dimension of time. M theory, for example, requires that spacetime have eleven dimensions.

The theory has its origins in the dual resonance model — first proposed in 1969 by Gabriele Veneziano — which described the strongly interacting hadrons as strings. Since that time, the term string theory has evolved to incorporate any of a group of related superstring theories. Indeed, the “strings” are no longer considered fundamental to the theory, which can also be formulated in terms of points or surfaces. As such, five major string theories were developed, each with a different mathematical structure, and each best describing different physical circumstances. The main differences between each theory were principally the number of dimensions in which the strings developed, and their characteristics (some were open loops, some were closed loops, etc.), however all these theories appeared to be correct. In the mid 1990s, string theorist Edward Witten of the Institute for Advanced Study considered that the five major versions of string theory might be describing the same phenomenon from different perspectives. Witten’s resulting M-theory, a proposed unification of all previous superstring theories, asserted that strings are really 1-dimensional slices of a 2-dimensional membrane vibrating in 11-dimensional space. As a result of the many properties and principles shared by these approaches (such as the holographic principle), their mutual logical consistency, and the fact that some easily include the standard model of particle physics, many of the world’s greatest living physicists (such as Edward Witten, Juan Maldacena and Leonard Susskind) believe that string theory is a step towards the correct fundamental description of nature.

In particular, string theory is the first candidate for the theory of everything (TOE), a manner of describing the known fundamental forces (gravitational, electromagnetic, weak and strong interactions) and matter (quarks and leptons) in a mathematically complete system. However, prominent physicists such asRichard Feynman and Sheldon Lee Glashow have criticized string theory for not providing any quantitative experimental predictions.[7][8] Like any other quantum theory of gravity, it is widely believed that testing the theory directly would require prohibitively expensive feats of engineering. Although direct experimental testing of String Theory involves grand explorations and development in engineering, there are several indirect experiments that may prove partial truth to String Theory. Supersymmetry (an idea developed in the early 1970s through String Theory research) is theoretically established through String Theory and it does appear to weave into current experimentally understood High Energy Physics (Particle Physics) (Supersymmetry could possibly be discovered at CERN where energies are being probed that could motivate the emergence of Supersymmetric Particles. Also the existence of Extra Compactified Dimensions (Calabi-Yau manifold) could possibly be discovered at CERN by the permeation of a Graviton into a higher dimensional space (Membrane (M-Theory)).

String theory posits that the electrons and quarks within an atom are not 0-dimensional objects, but 1-dimensional strings. These strings can move and vibrate, giving the observed particles their flavor, charge, mass and spin. String theories also include objects more general than strings, called branes. The word brane, derived from “membrane”, refers to a variety of interrelated objects, such as D-branes, black p-branes and Neveu-Schwarz 5-branes. These are extended objects that are charged sources for differential form generalizations of the vector potential electromagnetic field. These objects are related to one another by a variety of dualities. Black hole-like black p-branes are identified with D-branes, which are endpoints for strings, and this identification is called Gauge-gravity duality. Research on this equivalence has led to new insights on quantum chromodynamics, the fundamental theory of the strong nuclear force. The strings make closed loops unless they encounter D-branes, where they can open up into 1-dimensional lines. The endpoints of the string cannot break off the D-brane, but they can slide around on it.

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