jasoncran
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String theory is a developing theory in particle physics that attempts to reconcile quantum mechanics and general relativity.[1] It is a contender for the theory of everything (TOE), a manner of describing the known fundamental forces and matter in a mathematically complete system. The theory has yet to make testable experimental predictions, which a theory must do in order to be considered a part of science.
String theory mainly posits that the electrons and quarks within an atom are not 0-dimensional objects, but rather 1-dimensional oscillating lines ("strings"). The earliest string model, the bosonic string, incorporated only bosons, although this view developed to the superstring theory, which posits that a connection (a "supersymmetry") exists between bosons and fermions. String theories also require the existence of several extra, unobservable, dimensions to the universe, in addition to the usual four spacetime dimensions.
The theory has its origins in the dual resonance model (1969). Since that time, the term string theory has developed to incorporate any of a group of related superstring theories. Five major string theories were formulated. The main differences among them were the number of dimensions in which the strings developed and their characteristics; all of them appeared to be correct, however. In the mid 1990s a unification of all previous superstring theories, called M-theory, was proposed, which 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, some mathematical physicists (e.g. Witten, Maldacena and Susskind) believe that string theory is a step towards the correct fundamental description of nature.[2][3][4][5][unreliable source?] Nevertheless, other prominent physicists (e.g. Feynman and Glashow) have criticized string theory for not providing any quantitative experimental predictions.[6][7]
Contents [hide]
1 Overview
2 Basic properties
2.1 World-sheet
2.2 Dualities
2.3 Extra dimensions
2.3.1 Number of dimensions
2.3.2 Compact dimensions
2.3.3 Brane-world scenario
2.3.4 Effect of the hidden dimensions
2.4 D-branes
3 Gauge-gravity duality
3.1 Description of the duality
3.2 Examples and intuition
3.3 Contact with experiment
4 Problems and controversy
4.1 Is string theory predictive?
4.2 Swampland
4.3 Background independence
4.4 Supersymmetry breaking
4.5 String theory landscape
4.6 Other testability criteria
5 History
6 See also
7 References
8 Further reading
8.1 Popular books and articles
8.2 Textbooks
8.3 Online material
9 External links
[edit] OverviewString 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.[8][9][10][11] 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.
Levels of magnification:
1. Macroscopic level - Matter
2. Molecular level
3. Atomic level -- Protons, neutrons, and electrons
4. Subatomic level -- Electron
5. Subatomic level - Quarks
6. String levelSince the string theory is widely believed to be a consistent theory of quantum gravity, many hope that it correctly describes our universe, making it a theory of everything. There are known configurations which describe all the observed fundamental forces and matter but with a zero cosmological constant and some new fields.[12] There are other configurations with different values of the cosmological constant, which are metastable but long-lived. This leads many to believe that there is at least one metastable solution which is quantitatively identical with the standard model, with a small cosmological constant, which contains dark matter and a plausible mechanism for cosmic inflation. It is not yet known whether string theory has such a solution, nor how much freedom the theory allows to choose the details.
The full theory does not yet have a satisfactory definition in all circumstances, since the scattering of strings is most straightforwardly defined by a perturbation theory. The complete quantum mechanics of high dimensional branes is not easily defined, and the behavior of string theory in cosmological settings (time-dependent backgrounds) is not fully worked out. It is also not clear if there is any principle by which string theory selects its vacuum state, the spacetime configuration which determines the properties of our universe (see string theory landscape).
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.
to be continued
String theory mainly posits that the electrons and quarks within an atom are not 0-dimensional objects, but rather 1-dimensional oscillating lines ("strings"). The earliest string model, the bosonic string, incorporated only bosons, although this view developed to the superstring theory, which posits that a connection (a "supersymmetry") exists between bosons and fermions. String theories also require the existence of several extra, unobservable, dimensions to the universe, in addition to the usual four spacetime dimensions.
The theory has its origins in the dual resonance model (1969). Since that time, the term string theory has developed to incorporate any of a group of related superstring theories. Five major string theories were formulated. The main differences among them were the number of dimensions in which the strings developed and their characteristics; all of them appeared to be correct, however. In the mid 1990s a unification of all previous superstring theories, called M-theory, was proposed, which 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, some mathematical physicists (e.g. Witten, Maldacena and Susskind) believe that string theory is a step towards the correct fundamental description of nature.[2][3][4][5][unreliable source?] Nevertheless, other prominent physicists (e.g. Feynman and Glashow) have criticized string theory for not providing any quantitative experimental predictions.[6][7]
Contents [hide]
1 Overview
2 Basic properties
2.1 World-sheet
2.2 Dualities
2.3 Extra dimensions
2.3.1 Number of dimensions
2.3.2 Compact dimensions
2.3.3 Brane-world scenario
2.3.4 Effect of the hidden dimensions
2.4 D-branes
3 Gauge-gravity duality
3.1 Description of the duality
3.2 Examples and intuition
3.3 Contact with experiment
4 Problems and controversy
4.1 Is string theory predictive?
4.2 Swampland
4.3 Background independence
4.4 Supersymmetry breaking
4.5 String theory landscape
4.6 Other testability criteria
5 History
6 See also
7 References
8 Further reading
8.1 Popular books and articles
8.2 Textbooks
8.3 Online material
9 External links
[edit] OverviewString 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.[8][9][10][11] 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.
Levels of magnification:
1. Macroscopic level - Matter
2. Molecular level
3. Atomic level -- Protons, neutrons, and electrons
4. Subatomic level -- Electron
5. Subatomic level - Quarks
6. String levelSince the string theory is widely believed to be a consistent theory of quantum gravity, many hope that it correctly describes our universe, making it a theory of everything. There are known configurations which describe all the observed fundamental forces and matter but with a zero cosmological constant and some new fields.[12] There are other configurations with different values of the cosmological constant, which are metastable but long-lived. This leads many to believe that there is at least one metastable solution which is quantitatively identical with the standard model, with a small cosmological constant, which contains dark matter and a plausible mechanism for cosmic inflation. It is not yet known whether string theory has such a solution, nor how much freedom the theory allows to choose the details.
The full theory does not yet have a satisfactory definition in all circumstances, since the scattering of strings is most straightforwardly defined by a perturbation theory. The complete quantum mechanics of high dimensional branes is not easily defined, and the behavior of string theory in cosmological settings (time-dependent backgrounds) is not fully worked out. It is also not clear if there is any principle by which string theory selects its vacuum state, the spacetime configuration which determines the properties of our universe (see string theory landscape).
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.
to be continued