Category:String Theory

From Eurêka

Contents 1 String Theory 2 Banes 3 Superstring 4 Supergravity

String Theory

String Either (1) Fundamental one-dimensional object that is the essential ingredient in string theory, or (2) Nambu's original idea that the elementary particles could be described as extended, one-dimensional objects was called string theory. Since the ends of Nambu's strings whipped around at the speed of light they were also called light strings. Later attempts to include the spin half fermions within a string theory led to the term spinning strings. Strings that possess supersymmetry are called superstrings. Heterotic strings combine spaces of two different dimensionalities. The term string is used in a generic way to describe all these different variations, including superstrings, or (3) Hypothesized, basic constituents of matter, according to new theories of physics. In earlier theories of physics, the basic constituents of matter were point-like particles, such as electrons, which interacted with other particles at a point. According to the string theory, the basic constituents are 1-dimensional structures called strings. There are completely different strings, called cosmic strings, which can form according to some theories and which may extend for great distances in space. Postulated to have formed as a result of processes in the early Universe, cosmic strings are 1-dimensional structures of enormous energy, extending for perhaps thousands or millions of light years in space. There is no good observational evidence that either kind of strings exist.

1. Quantum string Ultimate, supreme string.

String theory Theory in fundamental physics that asserts that all matter and forces are composed of incredibly tiny loops that look like strings. It attempts to construct a model of elementary particles from one-dimensional entities rather than the zero-dimensional points of conventional particle physics. It postulates that subatomic particles actually have extension along one axis, and that their properties are determined by the arrangement and vibration of the strings. The undulations of such strings were posited to yield all the particles and forces in the universe. String theory soon ran into mathematical barriers, and decayed into five competing theories, each with thousands of solutions, most of which looked nothing like our universe. Concepts of duality, however, are allowing string theory to overcome its limitations and rise to the status of a TOE. Theory crafted in the mid-1980s, and remains very much research in progress.

1. Unified theory of the universe Postulates that fundamental ingredients of nature are not zero-dimensional point particles but tiny one-dimensional filaments called strings. String theory harmoniously unites quantum mechanics and general relativity the previously known laws of the small and the large, that are otherwise incompatible. Often short for superstring theory.

Bosonic string theory First known string theory; contains vibrational patterns that are all bosons.

Dual resonance theory Theoretical model that was developed to explain some of the experimental properties of the scattering matrix, such as the duality between s- and t-channel scattering. It was Veneziano’s version of the dual resonance model that paved the way for Nambu’s string theory.

1. Scattering matrix Matrix representing the transitions from some initial to some final state in a given interaction. The transitions may involve changes in the number of particles in the system.
2. Veneziano theory Formula that accounted for the experimental results of the dual resonance model. Nambu discovered that a string theory would reproduce the results of the Veneziano approach.

Heterotic string Gross’s version of string theory in which space-times of different dimensions are associated with the same closed loop.

1. Heterotic e-string theory (Heterotic E 8x E 8string theory) One of the five superstring theories; involves closed strings whose right-moving vibrations resemble those of the Type II string and whose left-moving vibrations involve those of the bosonic string. Differs in important but subtle ways from the Heterotic-O string theory.
2. Heterotic o-string theory (Heterotic O(32) string theory) One of the five superstring theories; involves closed strings whose right-moving vibrations resemble those of the Type II string and whose left-moving vibrations involve those of the bosonic string. Differs in important but subtle ways from the heterotic e-string theory.

Massless black hole In string theory, a particular kind of black hole that may have large mass initially, but that becomes ever lighter as a piece of the Calabi-Yau portion of space shrinks. When the portion of space has shrunk down to a point, the initially massive black hole has no remaining mass: it is massless. In this state, it no longer manifests such usual black hole properties as an event horizon.

Calabi-Yau shape Space shape into which the extra spatial dimensions required by string theory can be curled up, consistent with the equations of the theory.

1. Mirror symmetry Symmetry showing that two different Calabi-Yau shapes, known as a mirror pair, give rise to identical physics when chosen for the curled-up dimensions of string theory.
2. Calabi-Yau space Six-dimensional spaces hypothesized as arising when the ten dimensions of superstring theory are compactified down to four dimensions. They are also related to orbifold spaces.

Flop transition Evolution of the Calabi-Yau portion of space in which its fabric rips and repairs itself, yet with mild and acceptable physical consequences in the context of string theory.

Conifold transition Evolution of the Calabi-Yau portion of space in which its fabric rips and repairs itself, yet with mild and acceptable physical consequences in the context of string theory. The tears involved are more severe than those in a flop transition.

Banes

Brane Any of the extended objects that arise in string theory.

1. One-brane String.
2. Two-brane Membrane
3. Three-brane Has three extended dimensions, etc. More generally, a p-brane has p spatial dimensions.
4. Five-brane Five-dimensional membrane that moved through a 10-dimensional space serves as an alternative description of string theory.

Planck tension About 10 39 tons, the tension on a typical string in string theory.

String coupling Constant A (positive) number that governs how likely it is for a given string to split apart into two strings or for two strings to join together into one-the basic processes in string theory. Each string theory has its own string coupling constant, the value of which should be determined by an equation; currently such equations are not understood well enough to yield any useful information. Coupling constants less than 1 imply that perturbative methods are valid.

String mode Possible configuration (vibrational pattern, winding configuration) that a string can assume.

Space-time tears Fabric of space and tear and repair itself posited by Brian Greene, Paul S. Aspinwall and David R. Morisson and independently by Witten using a different method, 1992.

Grand unification Attempt to produce a unification of all the forces of nature. While some success was made in unifying the gluon force between quarks with the electroweak force, problems always arose when gravity was included. Grand unification eventually gave way to superstring theory.

Superstring

Superstring All the matter and energy in the universe, and even in space and time, stem from infinitesimal loops or “urstuff” writhing in hyperspace of 10 (or more) dimensions.

Superstring theory Explanation that tries to unify gravity with GUTs to produce a theory of everything. It is a version of string theory that incorporates ideas of supersymmetry. It is possible that these theories explain the behavior of matter in the very early universe. In superstring theories, the basic constituent of matter is a 1-dimensional structure, called a string, rather than a point-particle structure. According to superstring theory, space has more than 3 dimensions. To test this theory is beyond practicability, for a superstring is to a proton in size as a proton is to the solar system. To probe this realm directly would require a particle accelerator 1,000 light-years around.

Note : The entire solar-system is only one light-day around.

(1) Version of string theory which incorporates the ideas of supersymmetry.

(2) String theory that incorporates supersymmetry.

(3) New type of theory in physics that unifies all the forces of nature, including the gravitational force, and that may be capable of explaining all of the fundamental laws and particles of nature. (4) Proposal for the ultimate laws of nature, a theory of everything, stemming primarily from discoveries in the mid-1980's. The fundamental entity in this theory is an ultramicroscopic string-like object, with a length of typically 10-33 centimeters and effectively zero thickness. At present our understanding of string theory is very limited. The simplest predictions of superstring theory concern processes at the Planck energy, and so far very little is known about the consequences of string theory at lower energies.

1. Type I string theory One of the five superstring theories that involves both open and closed strings.
2. Type IIA string theory One of the five superstring theories that involves closed strings with left-right symmetric vibrational patterns.
3. Type IIB string theory One of the five superstring theories that involves closed strings with left-right asymmetric vibrational patterns.

Duality Broadly, two theories are said to be dual if they are apparently dissimilar but make the same physical predictions. Specifically, duality makes elementary and composite objects interchangeable: whether a particle or other entity is irreducibly fundamental or is itself made up of even more fundamental entities depends on one’s point of view. Either perspective ultimately yields the same physical results.

Note : Term “dual” replaced “super” as it had become the most overused term in particle theory with too many connotations.

1. Mirror symmetry Type of duality discovered in the late 1980s that revealed that strings in two different curled spaces sometimes yield the same particles.
2. Duality of dualities Duality between spaces, and that between elementary and composite objects, that might turn out to be connected.
3. T duality Mathematical relationship that suggests that one can shrink a circle only so far, and as the radius contracts, the circle gets smaller and smaller and then bottoms out, suddenly acting as though it is getting bigger and bigger.

Supergravity

Supergravity Unified theory that attempted to stretch Einstein’s gravity to include supersymmetry.

1. Bubble Fundamental entity that floated in 11 dimensions, postulated by Michael J. Duff. It has been conjectured that these are solitons of a particular string in 10 dimensions.
2. Eleven-dimensional supergravity Promising higher-dimensional supergravity theory developed in the 1970s, subsequently ignored, and more recently shown to be an important part of string theory.

Compactification Either, (1) The process in which a space of many dimensions effectively reduces its dimensions. Some new theories of particle physics the superstring theories claim that the Universe actually has 10 spatial dimensions but that 7 of these dimensions have become “compactified” down to subatomic size and thus are unobservable, or (2) Process of “curling up” six of the ten dimensions of superstring theory.

Orbifold Particular space used as a candidate for the compactified space of superstring theory. These six-dimensional orbifolds could be thought of as generalizations of a six-dimensional torus, but containing twenty-seven singular points.

Dimension Either, (1) Geometrical axis, or (2) Independent axis or direction in space or spacetime. The familiar space around us has three dimensions (left-right, back-forth, up-down) and the familiar spacetime has four (the previous three axes plus the past-future axis). Superstring theory requires the universe to have additional spatial dimensions.