String theory

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String theory From Wikipedia, the free encyclopedia Jump to: navigation, search Interaction in the subatomic world: world lines of pointlike particles in the Standard Model or a world sheet swept up by closed strings in string theoryString theory is a model of fundamental physics whose building blocks are one-dimensional extended objects (strings) rather than the zero-dimensional points (particles) that are the basis of the Standard Model of particle physics. String theorists are attempting to adjust the Standard Model by removing the assumption in quantum mechanics that particles are point-like. By removing this assumption and replacing the point-like particles with strings, it is hoped that string theory will develop into a sensible quantum theory of gravity. Moreover, string theory appears to be able to "unify" the known natural forces (gravitational, electromagnetic, weak and strong) by describing them with the same set of equations. No experimental verification or falsification of the theory has yet been possible, thus leading many experts to turn to one of several alternate models, such as Loop quantum gravity. However, with the construction of the Large Hadron Collider near Geneva, Switzerland scientists may produce relevant data. Studies of string theory have revealed that it predicts not just strings, but also higher-dimensional objects (branes). String theory strongly suggests the existence of ten or eleven (in M-theory) spacetime dimensions, as opposed to the relativistic four (three spatial and one time).[1]Contents[hide] 1 Overview 2 History 3 Basic properties 3.1 Worldsheet 3.2 Dualities 3.2.1 T-duality 3.2.2 S-duality 3.3 Extra dimensions 3.4 Integrated Functionality 3.5 D-branes 4 Gauge-gravity duality 5 Problems and controversy 6 Popular culture 7 See also 8 References and further reading 8.1 Footnotes 8.2 Popular books and articles 8.3 Textbooks 8.4 External links [edit] Overview The basic idea behind all string theories is that the fundamental constituents of reality are strings of extremely small scale (possibly Planck length, about 10-35 m) which vibrate at specific resonant frequencies.[2] Thus, any particle should be thought of as a tiny vibrating object, rather than as a point. This object can vibrate in different modes (just as a guitar string can produce different notes), with every mode appearing as a different particle (electron, photon etc.). Strings can split and combine, which would appear as particles emitting and absorbing other particles, presumably giving rise to the known interactions between particles. In addition to strings, string theories also include objects of higher dimensions, such as D-branes and NS-branes. Furthermore, all string theories predict the existence of degrees of freedom which are usually described as extra dimensions. String theory is thought to include some 10, 11 or 26 dimensions, depending on the specific theory and on the point of view. Interest in string theory is driven largely by the hope that it will prove to be a consistent theory of quantum gravity or even a theory of everything. It can also naturally describe interactions similar to electromagnetism and the other forces of nature. Superstring theories include fermions, the building blocks of matter, and incorporate supersymmetry, a conjectured (but unobserved) symmetry of nature. It is not yet known whether string theory will be able to describe a universe with the precise collection of forces and particles that is observed, nor how much freedom the theory allows to choose those details. String theory as a whole has not yet made falsifiable predictions that would allow it to be experimentally tested, though various planned observations and experiments could confirm some essential aspects of the theory, such as supersymmetry and extra dimensions. In addition, the full theory is not yet understood. For example, the theory does not yet have a satisfactory definition outside of perturbation theory; the quantum mechanics of branes (higher dimensional objects than strings) is not understood; the behavior of string theory in cosmological settings (time-dependent backgrounds) is still being worked out; finally, the principle by which string theory selects its vacuum state is a hotly contested topic (see string theory landscape). String theory is thought to be a certain limit of another, more profound theory - M-theory - which is only partly defined and is not well understood. A key consequence of the theory is that there is no obvious operational way to probe distances shorter than the string length.[3] [edit] History Main article: History of string theory String theory v • d • e bosonic string theory superstring theory type I string type II string heterotic string M-theory (simplified) string field theory strings branes Related topics quantum field theory gauge theory conformal field theory topological field theory supersymmetry supergravity general relativity quantum gravity See also string theory topics String theory was originally developed and explored during the late 1960s and early 1970s, to explain some peculiarities of the behavior of hadrons (subatomic particles such as the proton and neutron which experience the strong nuclear force). In particular, Yoichiro Nambu (and later Lenny Susskind and Holger Nielsen) realized in 1970 that the dual resonance model of strong interactions could be explained by a quantum-mechanical model of strings. This approach was abandoned as an alternative theory, quantum chromodynamics, gained experimental support. During the mid-1970s it was discovered that the same mathematical formalism can be used to describe a theory of quantum gravity. This led to the development of bosonic string theory, which is still the version first taught to many students. Between 1984 and 1986, physicists realized that string theory could describe all elementary particles and the interactions between them, and hundreds of them started to work on string theory as the most promising idea to unify theories of physics. This is known as the first superstring revolution. In the 1990s, Edward Witten and others found strong evidence that the different superstring theories were different limits of a new 11-dimensional theory called M-theory. These discoveries sparked the second superstring revolution. In the mid 1990s, Joseph Polchinski discovered that the theory requires the inclusion of higher-dimensional objects, called D-branes. These added an additional rich mathematical structure to the theory, and opened many possibilities for constructing realistic cosmological models in the theory. In 1997 Juan Maldacena conjectured a relationship between string theory and a gauge theory called N=4 supersymmetric Yang-Mills theory. This conjecture, called the AdS/CFT correspondence has generated a great deal of interest in the field and is now well-accepted. It is a concrete realization of the holographic principle, which has far-reaching implications for black holes, locality and information in physics, as well as the nature of the gravitational interaction. Through this relationship, string theory may be related in the future to quantum chromodynamics and lead, eventually, to a better understanding of the behavior of hadrons, thus returning to its original goal. Recently, the discovery of the string theory landscape, which suggests that string theory has an exponentially large number of different vacua, led to discussions of what string theory might eventually be expected to predict, and to the worry that the answer might continue to be nothing. [edit] Basic properties String theory is formulated in terms of an action principle, either the Nambu-Goto action or the Polyakov action, which describes how strings move through space and time. Like springs, the strings want to contract to minimize their potential energy, but conservation of energy prevents them from disappearing, and instead they oscillate. By applying the ideas of quantum mechanics to strings it is possible to deduce the different vibrational modes of strings, and that each vibrational state appears to be a different particle. The mass of each particle, and the fashion with which it can interact, are determined by the way the string vibrates — the string can vibrate in many different modes, just like a guitar string can produce different notes. The different modes, each corresponding to a different kind of particle, make up the "spectrum" of the theory. Strings can split and combine, which would appear as particles emitting and absorbing other particles, presumably giving rise to the known interactions between particles. String theory includes both open strings, which have two distinct endpoints, and closed strings, where the endpoints are joined to make a complete loop. The two types of string behave in slightly different ways, yielding two different spectra. For example, in most string theories, one of the closed string modes is the graviton, and one of the open string modes is the photon. Because the two ends of an open string can always meet and connect, forming a closed string, there are no string theories without closed strings. The earliest string model - the bosonic string, which incorporated only bosons, describes - in low enough energies - a quantum gravity theory, which also includes (if open strings are incorporated as well) gauge fields such as the photon (or, more generally, any Yang-Mills theory). However, this model has problems. Most importantly, the theory has a fundamental instability, believed to result in the decay (at least partially) of space-time itself. Additionally, as the name implies, the spectrum of particles contains only bosons, particles which, like the photon, obey particular rules of behavior. Roughly speaking, bosons are the constituents of radiation, but not of matter, which is made of fermions. I