String theory
As string theory refers to a collection of closely related hypothetical physical models, instead of the elementary particles - use with one-dimensional spatial extent so-called strings (English for threads or strings ) as fundamental objects - these are objects of dimension zero. This is in contrast to the usual models of quantum field theory, emanating from zero-dimensional particles.
String theories ( quantum chromodynamics ) were used in the 1960s to describe the strong interaction and the gluons were perceived as spatially extended strings between the quarks. Since the 1980s, string theory experienced new interest, this time as a candidate of a unified theory that will connect the standard model of elementary particle physics and gravitation together. Their main application she finds it in the supersymmetric version of string theory ( superstring theory ), which includes a symmetry between bosons and fermions. In the 1990s, it turned out that the previously known superstring theories and 11- dimensional supergravity coupled to each other and ( called M-theory ) part of a broader theory, which includes even higher-dimensional objects ( so-called " brane "). Whether it ever is in string theory is a scientific theory that can make falsifiable experimental predictions, is not clear.
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Overview
In contrast to the standard model of particle physics are the fundamental building blocks that make up our world, no particles within the meaning of points ( ie zero-dimensional objects), but one-dimensional vibrating objects in string theory. This one-dimensional objects are called strings. Can be thought of as elementary excitation of vibrations of the strings, with the frequency according to quantum mechanics, an energy equivalent.
In further developments of string theory, the so-called brane theories are considered essential objects not only one-dimensional (or at the time of inclusion (1 1 )-dimensional ) strings considered, but also higher-dimensional objects (" brane " called ) used.
String theory avoids the problems encountered in the classical quantum field theory of singularities and developed to its Domestication renormalization. They arise especially for point particles from their self-interaction, which is the case of extended one-dimensional objects, for example " smeared " and thus mitigated.
By accepting this one-dimensional structure of the strings automatically emerge many desirable properties of a more fundamental theory of physics. Most stands out that any string theory that is consistent with quantum mechanics, must include a quantum gravity that you previously can not describe consistently without strings.
The characteristic length scale of the string should be of the order of the Planck length, size, are important from the effects of quantum gravity:
On much larger length scales, as they are accessible in laboratories today, these objects would be indistinguishable from zero-dimensional point-like particles. Nevertheless, the vibrational states and the structure of these tiny strings would make them appear as different elementary particles of the Standard Model of elementary particle physics. For example, a vibration condition of the string would be associated with a photon, another state with a curd. This unifying effect of string theory is one of its greatest strengths, but still no known solution to this theory reproduces exactly the number of particles that knows the standard model.
In the space-time a particle passes over a line, called world line: the particle has no spatial extension, but it moves along the "time". In contrast, a string has a two-dimensional world sheet ( "World Sheet " ), since he also has a spatially one-dimensional expansion. The interactions of elementary particles, described in the usual quantum field theory of point particles with Feynman diagrams in the space- time can be thought of by " thickening " of the Feynman diagrams in one spatial direction (see the picture on the left ).
Kinds of strings
Closed and open strings
Strings can be either open or closed. A " closed string" has no endpoints in its topology and is therefore a circle equivalent. An "open string" has two ends and is topologically equivalent to a distance. Not all string theories contain open strings, but every theory must contain closed strings, as interactions of open strings can always create closed.
The oldest string theory containing open strings, was the type -1 string theory.
With open as closed strings are always characteristic vibration modes (modes) connected. A certain vibration of a string enclosed can be identified as the graviton. In certain string theories, the oscillation with the lowest energy of an open string tachyon a dar. Other vibrational modes of open strings show the properties of photons or gluons.
Orientation
Strings can also "guidance" to possess, which can be thought of as string internal arrow which distinguishes them from strings with the opposite orientation. In contrast, there is also a " non-oriented string", the arrow key can be assigned to any of these.
Historical development
Originally, the discovery of the strings (as "dual models " ) is a formula of Gabriele Veneziano in 1968 as part of the scattering matrix theory of strongly interacting particles. 1970 gave Yoichiro Nambu, Holger Bech Nielsen and Leonard Susskind an interpretation in the form of one-dimensional strings. Initially, only for particles with integer spin ( bosons ) formulated the description of particles with half-integer 1971 was soon followed by spin ( fermions ) in the string model by Andre Neveu, John Schwarz and Pierre Ramond. This resulted in the course of the 1970s the realization that in the string models supersymmetry between bosons and fermions must exist. Initially it was hoped to describe strings the strong interaction, but the discovery that the quantum theory of strings only in 26 dimensions ( boson string ) or ten dimensions ( superstring ) is possible, the theory shifted around 1974 initially a damper. Through the work of Joel Scherk and Others, however, soon became clear that superstring theory came as a candidate for a unified theory of natural forces including gravity in question. The gravity results automatically with closed strings as massless spin -2 excitation, the other known forces of nature (all gauge theories ) corresponding to the massless spin-1 Bosonenanregungen. The extra dimensions would then " rolled up " in some way ( compacted ), as already in the prior art since the 1930s, Kaluza-Klein theories (see Kaluza-Klein compactification ).
1984 discovered Michael Green and John Schwarz, that cancel out in superstring theories the one- loop divergences in the perturbation theory only at very specific symmetry groups ( the rotation group in 32 dimensions SO (32) and the special Lie group E8). In addition the occurrence of " abnormality " is avoided in these symmetries, i.e., a symmetry-breaking due to quantum mechanical effects in certain interaction diagrams. This led to a revival of the theory and a number of other discoveries (so-called " first superstring revolution "). They had demonstrated that the theory for the gauge theories describing the low-energy limit of string theory case, the particle spectrum, results in significant limitations. In addition, Green and Black constructed explicitly the first superstring theories, whose existence was previously only suspected.
To get to the " compactification " ( the " rolling ") of the extra dimensions a realistic model of elementary particles in the observable four dimensions, Edward Witten concluded, inter alia, also a number of limitations to the Kompaktifizierungs -manifold ( were preferred so-called Calabi -Yau manifolds ).
First, it was hoped, hard to find limiting principles here, but it was discovered during the 1980s that this was not the case and the theory was room for a very large number of possible " vacuums ".
As candidates for the superstring theories arose in the 1980s following five theories:
- The type I string theory, with open ends of the strings (but coupling to closed strings by contact of the ends, corresponding to gravitational interaction ) and the symmetry SO (32) charge at the ends
- The type IIA and type IIB string theory, with closed strings; in type IIA the massless fermions have both handedness ( left / right), in II B only one handedness ( chirality )
- Two variants of the heterotic string theory, closed strings, which are sometimes referred to with reference to their symmetry groups E8xE8 or SO (32) heterotic E- and O- heterotic string theory. They were found by the " Princeton String Quartet " by David Gross. In them right - and left-handed modes ( RH, LH) are described by different theories: RH by a 10- dimensional superstring theory ( description of bosons and fermions ), LH, by a 26- dimensional bosonic string theory, but compacted to 10 dimensions with the gauge field loads arise E8 × E8 or SO (32).
Edward Witten conjectured in 1995 that the various string theory types different approximations of a broader theory of M- theory. There is no complete and consistent formulation of this theory yet succeeded, but it is the subject of intensive research. Arguments to show that they are aspects of a single theory in these theories were provided by showing dualities between the different string theories, that is, it has been shown that they use the same system, only in areas such as various degrees of coupling constants describing. Similar dualities were also used for different solutions ( " vacuums ", ie ground states ) found in string theory. This was the so-called "second superstring revolution " in the mid- 1990s led to a revival of the then something stagnant theory.
An interesting result of this union of the sub- theories was that the elfdimensionale supergravity, which had previously fallen somewhat into isolation, has been recognized as another limiting case of M- theory. However, this does not contain strings, but is a particle approximation of two - and five-dimensional branes. This illustrates that a general string theory describes more than just one-dimensional strings, and in fact, in the late 1990s has shown that higher-dimensional branes (D - branes ) play an important role in string theory play (Joseph Polchinski ).
String theory has evolved over the years into a very active research area with a large number of publications per year, which is reflected, among other things, that some participating researchers (especially Edward Witten ) are among the most cited scientists in the entire physics.
Experimental verification
According to string theory, there is a vibration spectrum of an infinite number of modes of vibration, which, however, much too high masses ( energies) have to be directly observed can. If one also considers the limited size of the strings in the order of the Planck length, it means that the vibration modes have masses that are a multiple of about 1019 GeV. This is many orders of magnitude beyond what can be seen today; direct detection of the vibration modes is not possible. Instead you tried just to find the " massless " Suggestions for string theory -specific properties for the low-energy, compared to the Planck mass. For this purpose, but you'd have to Kompaktifizierungsmechanismus of 10 or 11 to 4 dimensions - or the Planck mass of 1019 to the W - Bosonenmasse of about 80 GeV or the proton mass of about 1 GeV - a better understanding of string theory, which has not been the case is.
Nevertheless, there are already a plethora discussed solutions for the observable low-energy sector in four space-time dimensions.
In general, however, a possible discovery of supersymmetry in the currently running experiments (eg the Large Hadron Collider ( LHC) ) is considered to be support of string theory. However, there are also the mechanism of supersymmetry breaking at the string theorists not have a match. As a further means of checking the string theory possible clues to the compactification of the extra dimensions in the cosmic background radiation were discussed by Gary Shiu, including the previously most accurate data should be available from the Planck space telescope in 2012.
Reception
String theory has in the course of their development a considerable echo in various media, often caused in the form of popular scientific literature. In the recent past, books and newspaper articles have been published in which string theory is considered negative. The authors argue that string theory makes no falsifiable statements and is thus under the current philosophy of science considers that no scientific theory. The string theorist Edward Witten believes that supersymmetry a falsifiable prediction of string theory is, which would invalidate the criticism.