# string theory

Written by khantaal4 on Fri Dec 30, 2011 7:46 am in blog health for teenagers under Motivational -
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I'm not sure what you are trying to do here. Clearly this is material from another site, so simply copying it into your blog is both against the TOS and also rather a waste of time isn't it?

I've not removed the posting yet - obviously if there is some valid purpose behind this then I'm willing to listen, but otherwise I think it will have to go.......

Bikerman

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String theory is an active research framework in particle physics that attempts to reconcile quantum mechanics and general relativity.[1] It is a contender for a 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 novel experimental predictions at accessible energy scales, leading some scientists to claim that it cannot be considered a part of science.[2]

String theory posits mainly 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 four known spacetime dimensions.

The theory has its origins in an effort to understand the strong force, the dual resonance model (1969). Subsequent to this, five different superstring theories were developed that incorporated fermions and possessed other properties necessary for a theory of everything. Since the mid-1990s, in particular due to insights from dualities shown to relate the five theories, an eleven-dimensional theory called M-theory is believed to encompass all of the previously-distinct superstring theories.

Many theoretical physicists (e.g., Hawking, Witten, Maldacena and Susskind) believe that string theory is a step toward the correct fundamental description of nature. This is because string theory allows for the consistent combination of quantum field theory and general relativity, agrees with general insights in quantum gravity (such as the holographic principle and Black hole thermodynamics), and because it has passed many non-trivial checks of its internal consistency.[3][4][5][6][unreliable source?] According to Stephen Hawking in particular, "M-theory is the only candidate for a complete theory of the universe."[7] Nevertheless, other physicists (e.g. Feynman and Glashow) have criticized string theory for not providing any quantitative experimental predictions.[8][9]

Contents

1 Overview

2 Basic properties

2.1 Worldsheet

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 Predictability and testability

4.1 Predictions

4.1.1 String harmonics

4.1.2 Cosmology

4.1.3 Supersymmetry breaking

4.1.4 AdS/CFT correspondence

4.1.5 Coupling constants

4.2 Criticism

4.2.1 High energies

4.2.2 Number of solutions

4.2.3 Background independence

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

Overview

String theory posits that the electrons and quarks within an atom are not 0-dimensional objects, but made up of 1-dimensional strings. These strings can oscillate, 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.[10][11][12][13] 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 level

Since string theory is widely believed[who?] 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 that describe all the observed fundamental forces and matter but with a zero cosmological constant and some new fields.[14] 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 that 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 as to whether there is any principle by which string theory selects its vacuum state, the spacetime configuration that determines the properties of our universe (see string theory landscape).

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