1.(particle physics) a theory that postulates that subatomic particles are onedimensional strings
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String theory 

Superstring theory 
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String theory is an active research framework in particle physics that attempts to reconcile quantum mechanics and general relativity. It is a contender for a theory of everything (TOE), a selfcontained mathematical model that describes all fundamental forces and forms of matter.
String theory posits that the electrons and quarks within an atom are not 0dimensional objects, but rather 1dimensional 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 dimensions to the universe that have been compactified into extremely small scales, 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 mid1990s, in particular due to insights from dualities shown to relate the five theories, an elevendimensional theory called Mtheory is believed to encompass all of the previously distinct superstring theories.^{[citation needed]}
Many theoretical physicists (e.g., Stephen Hawking, Edward Witten, Juan Maldacena, and Leonard Susskind) believe that string theory is a step towards 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 nontrivial checks of its internal consistency.^{[1]}^{[2]}^{[3]}^{[4]} According to Hawking in particular, "Mtheory is the only candidate for a complete theory of the universe."^{[5]} Nevertheless, other physicists, such as Feynman and Glashow, have criticized string theory for not providing novel experimental predictions at accessible energy scales.^{[6]}
Contents 
String theory posits that the electrons and quarks within an atom are not 0dimensional objects, but made up of 1dimensional strings. These strings can oscillate, giving the observed particles their flavor, charge, mass and spin. Among the modes of oscillation of the string is a massless, spintwo state—a graviton. The existence of this graviton state and the fact that the equations describing string theory include Einstein's equations for general relativity mean that string theory is a quantum theory of gravity. Since string theory is widely believed^{[7]} to be mathematically consistent, many hope that it fully describes our universe, making it a theory of everything. String theory is known to contain configurations that describe all the observed fundamental forces and matter but with a zero cosmological constant and some new fields.^{[8]} Other configurations have different values of the cosmological constant, and are metastable but longlived. 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, containing 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.
String theories also include objects other than strings, called branes. The word brane, derived from "membrane", refers to a variety of interrelated objects, such as Dbranes, black pbranes and Neveu–Schwarz 5branes. 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 holelike black pbranes are identified with Dbranes, which are endpoints for strings, and this identification is called Gaugegravity duality. Research on this equivalence has led to new insights on quantum chromodynamics, the fundamental theory of the strong nuclear force.^{[9]}^{[10]}^{[11]}^{[12]} The strings make closed loops unless they encounter Dbranes, where they can open up into 1dimensional lines. The endpoints of the string cannot break off the Dbrane, but they can slide around on it.
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 (timedependent 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).
String theory can be formulated in terms of an action principle, either the NambuGoto action or the Polyakov action, which describe how strings propagate through space and time. In the absence of external interactions, string dynamics are governed by tension and kinetic energy, which combine to produce oscillations. The quantum mechanics of strings implies these oscillations exist in discrete vibrational modes, the spectrum of the theory.
On distance scales larger than the string radius, each oscillation mode behaves as a different species of particle, with its mass, spin and charge determined by the string's dynamics. Splitting and recombination of strings correspond to particle emission and absorption, giving rise to the interactions between particles. An analogy for strings' modes of vibration is a guitar string's production of multiple but distinct musical notes. In the analogy, different notes correspond to different particles. One difference is the guitar string exists in 3 dimensions, so that there are only two dimensions transverse to the string. Fundamental strings exist in 9 dimensions and the strings can vibrate in any direction, meaning that the spectrum of vibrational modes is much richer.
String theory includes both open strings, which have two distinct endpoints, and closed strings making 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, incorporated only bosonic degrees of freedom. This model 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, in more general terms, any gauge theory). However, this model has problems. What is most significant is that the theory has a fundamental instability, believed to result in the decay (at least partially) of spacetime itself. In addition, as the name implies, the spectrum of particles contains only bosons, particles which, like the photon, obey particular rules of behavior. In broad terms, bosons are the constituents of radiation, but not of matter, which is made of fermions. Investigating how a string theory may include fermions in its spectrum led to the invention of supersymmetry, a mathematical relation between bosons and fermions. String theories that include fermionic vibrations are now known as superstring theories; several different kinds have been described, but all are now thought to be different limits of Mtheory.
Some qualitative properties of quantum strings can be understood in a fairly simple fashion. For example, quantum strings have tension, much like regular strings made of twine; this tension is considered a fundamental parameter of the theory. The tension of a quantum string is closely related to its size. Consider a closed loop of string, left to move through space without external forces. Its tension will tend to contract it into a smaller and smaller loop. Classical intuition suggests that it might shrink to a single point, but this would violate Heisenberg's uncertainty principle. The characteristic size of the string loop will be a balance between the tension force, acting to make it small, and the uncertainty effect, which keeps it "stretched". As a consequence, the minimum size of a string is related to the string tension.
A pointlike particle's motion may be described by drawing a graph of its position (in one or two dimensions of space) against time. The resulting picture depicts the worldline of the particle (its 'history') in spacetime. By analogy, a similar graph depicting the progress of a string as time passes by can be obtained; the string (a onedimensional object — a small line — by itself) will trace out a surface (a twodimensional manifold), known as the worldsheet. The different string modes (representing different particles, such as photon or graviton) are surface waves on this manifold.
A closed string looks like a small loop, so its worldsheet will look like a pipe or, in more general terms, a Riemann surface (a twodimensional oriented manifold) with no boundaries (i.e., no edge). An open string looks like a short line, so its worldsheet will look like a strip or, in more general terms, a Riemann surface with a boundary.
Strings can split and connect. This is reflected by the form of their worldsheet (in more accurate terms, by its topology). For example, if a closed string splits, its worldsheet will look like a single pipe splitting (or connected) to two pipes (often referred to as a pair of pants — see drawing at right). If a closed string splits and its two parts later reconnect, its worldsheet will look like a single pipe splitting to two and then reconnecting, which also looks like a torus connected to two pipes (one representing the ingoing string, and the other — the outgoing one). An open string doing the same thing will have its worldsheet looking like a ring connected to two strips.
Note that the process of a string splitting (or strings connecting) is a global process of the worldsheet, not a local one: Locally, the worldsheet looks the same everywhere, and it is not possible to determine a single point on the worldsheet where the splitting occurs. Therefore, these processes are an integral part of the theory, and are described by the same dynamics that controls the string modes.
In some string theories (namely, closed strings in Type I and some versions of the bosonic string), strings can split and reconnect in an opposite orientation (as in a Möbius strip or a Klein bottle). These theories are called unoriented. In formal terms, the worldsheet in these theories is a nonorientable surface.
Before the 1990s, string theorists believed there were five distinct superstring theories: open type I, closed type I, closed type IIA, closed type IIB, and the two flavors of heterotic string theory (SO(32) and E_{8}×E_{8}).^{[13]} The thinking was that out of these five candidate theories, only one was the actual correct theory of everything, and that theory was the one whose low energy limit, with ten spacetime dimensions compactified down to four, matched the physics observed in our world today. It is now believed that this picture was incorrect and that the five superstring theories are connected to one another as if they are each a special case of some more fundamental theory (thought to be Mtheory). These theories are related by transformations that are called dualities. If two theories are related by a duality transformation, it means that the first theory can be transformed in some way so that it ends up looking just like the second theory. The two theories are then said to be dual to one another under that kind of transformation. Put differently, the two theories are mathematically different descriptions of the same phenomena.
These dualities link quantities that were also thought to be separate. Large and small distance scales, as well as strong and weak coupling strengths, are quantities that have always marked very distinct limits of behavior of a physical system in both classical field theory and quantum particle physics. But strings can obscure the difference between large and small, strong and weak, and this is how these five very different theories end up being related. Tduality relates the large and small distance scales between string theories, whereas Sduality relates strong and weak coupling strengths between string theories. Uduality links Tduality and Sduality.
String theories  

Type  Spacetime dimensions 
Details 
Bosonic  26  Only bosons, no fermions, meaning only forces, no matter, with both open and closed strings; major flaw: a particle with imaginary mass, called the tachyon, representing an instability in the theory. 
I  10  Supersymmetry between forces and matter, with both open and closed strings; no tachyon; group symmetry is SO(32) 
IIA  10  Supersymmetry between forces and matter, with only closed strings bound to Dbranes; no tachyon; massless fermions are nonchiral 
IIB  10  Supersymmetry between forces and matter, with only closed strings bound to Dbranes; no tachyon; massless fermions are chiral 
HO  10  Supersymmetry between forces and matter, with closed strings only; no tachyon; heterotic, meaning right moving and left moving strings differ; group symmetry is SO(32) 
HE  10  Supersymmetry between forces and matter, with closed strings only; no tachyon; heterotic, meaning right moving and left moving strings differ; group symmetry is E_{8}×E_{8} 
Note that in the type IIA and type IIB string theories closed strings are allowed to move everywhere throughout the tendimensional spacetime (called the bulk), while open strings have their ends attached to Dbranes, which are membranes of lower dimensionality (their dimension is odd — 1, 3, 5, 7 or 9 — in type IIA and even — 0, 2, 4, 6 or 8 — in type IIB, including the time direction).
An intriguing feature of string theory is that it predicts extra dimensions. In classical string theory the number of dimensions is not fixed by any consistency criterion. However, in order to make a consistent quantum theory, string theory is required to live in a spacetime of the socalled "critical dimension": we must have 26 spacetime dimensions for the bosonic string and 10 for the superstring. This is necessary to ensure the vanishing of the conformal anomaly of the worldsheet conformal field theory. Modern understanding indicates that there exist lesstrivial ways of satisfying this criterion. Cosmological solutions exist in a wider variety of dimensionalities, and these different dimensions are related by dynamical transitions. The dimensions are more precisely different values of the "effective central charge", a count of degrees of freedom that reduces to dimensionality in weakly curved regimes.^{[14]}
One such theory is the 11dimensional Mtheory, which requires spacetime to have eleven dimensions,^{[15]} as opposed to the usual three spatial dimensions and the fourth dimension of time. The original string theories from the 1980s describe special cases of Mtheory where the eleventh dimension is a very small circle or a line, and if these formulations are considered as fundamental, then string theory requires ten dimensions. But the theory also describes universes like ours, with four observable spacetime dimensions, as well as universes with up to 10 flat space dimensions, and also cases where the position in some of the dimensions is not described by a real number, but by a completely different type of mathematical quantity.^{[which?]} So the notion of spacetime dimension is not fixed in string theory: it is best thought of as different in different circumstances.^{[16]}
Nothing in Maxwell's theory of electromagnetism or Einstein's theory of relativity makes this kind of prediction; these theories require physicists to insert the number of dimensions "by both hands"^{[clarification needed]}, and this number is fixed and independent of potential energy. String theory allows one to relate the number of dimensions to scalar potential energy. In technical terms, this happens because a gauge anomaly exists for every separate number of predicted dimensions, and the gauge anomaly can be counteracted by including nontrivial potential energy into equations to solve motion. Furthermore, the absence of potential energy in the "critical dimension" explains why flat spacetime solutions are possible.
This can be better understood by noting that a photon included in a consistent theory (technically, a particle carrying a force related to an unbroken gauge symmetry) must be massless. The mass of the photon that is predicted by string theory depends on the energy of the string mode that represents the photon. This energy includes a contribution from the Casimir effect, namely from quantum fluctuations in the string. The size of this contribution depends on the number of dimensions, since for a larger number of dimensions there are more possible fluctuations in the string position. Therefore, the photon in flat spacetime will be massless—and the theory consistent—only for a particular number of dimensions.^{[17]} When the calculation is done, the critical dimensionality is not four as one may expect (three axes of space and one of time). The subset of X is equal to the relation of photon fluctuations in a linear dimension. Flat space string theories are 26dimensional in the bosonic case, while superstring and Mtheories turn out to involve 10 or 11 dimensions for flat solutions. In bosonic string theories, the 26 dimensions come from the Polyakov equation.^{[18]} Starting from any dimension greater than four, it is necessary to consider how these are reduced to four dimensional spacetime.
Two different ways have been proposed to resolve this apparent contradiction. The first is to compactify the extra dimensions; i.e., the 6 or 7 extra dimensions are so small as to be undetectable by present day experiments.
To retain a high degree of supersymmetry, these compactification spaces must be very special, as reflected in their holonomy. A 6dimensional manifold must have SU(3) structure, a particular case (torsionless) of this being SU(3) holonomy, making it a Calabi–Yau space, and a 7dimensional manifold must have G_{2} structure, with G_{2} holonomy again being a specific, simple, case. Such spaces have been studied in attempts to relate string theory to the 4dimensional Standard Model, in part due to the computational simplicity afforded by the assumption of supersymmetry. More recently, progress has been made constructing more realistic compactifications without the degree of symmetry of Calabi–Yau or G2 manifolds.
A standard analogy for this is to consider multidimensional space as a garden hose. If the hose is viewed from a sufficient distance, it appears to have only one dimension, its length. Indeed, think of a ball just small enough to enter the hose. Throwing such a ball inside the hose, the ball would move more or less in one dimension; in any experiment we make by throwing such balls in the hose, the only important movement will be onedimensional, that is, along the hose. However, as one approaches the hose, one discovers that it contains a second dimension, its circumference. Thus, an ant crawling inside it would move in two dimensions (and a fly flying in it would move in three dimensions). This "extra dimension" is only visible within a relatively close range to the hose, or if one "throws in" small enough objects. Similarly, the extra compact dimensions are only "visible" at extremely small distances, or by experimenting with particles with extremely small wavelengths (of the order of the compact dimension's radius), which in quantum mechanics means very high energies (see waveparticle duality).
Another possibility is that we are "stuck" in a 3+1 dimensional (three spatial dimensions plus one time dimension) subspace of the full universe. Properly localized matter and YangMills gauge fields will typically exist if the subspacetime is an exceptional set of the larger universe.^{[19]} These "exceptional sets" are ubiquitous in Calabi–Yau nfolds and may be described as subspaces without local deformations, akin to a crease in a sheet of paper or a crack in a crystal, the neighborhood of which is markedly different from the exceptional subspace itself. However, until the work of Randall and Sundrum,^{[20]} it was not known that gravity can be properly localized to a subspacetime. In addition, spacetime may be stratified, containing strata of various dimensions, allowing us to inhabit a 3+1dimensional stratum—such geometries occur naturally in Calabi–Yau compactifications.^{[21]} Such subspacetimes are Dbranes, hence such models are known as braneworld scenarios.
In either case, gravity acting in the hidden dimensions affects other nongravitational forces such as electromagnetism. In fact, Kaluza's early work demonstrated that general relativity in five dimensions actually predicts the existence of electromagnetism. However, because of the nature of Calabi–Yau manifolds, no new forces appear from the small dimensions, but their shape has a profound effect on how the forces between the strings appear in our fourdimensional universe. In principle, therefore, it is possible to deduce the nature of those extra dimensions by requiring consistency with the standard model, but this is not yet a practical possibility. It is also possible to extract information regarding the hidden dimensions by precision tests of gravity, but so far these have only put upper limitations on the size of such hidden dimensions.
Another key feature of string theory is the existence of Dbranes. These are membranes of different dimensionality (anywhere from a zero dimensional membrane—which is in fact a point—and up, including 2dimensional membranes, 3dimensional volumes, and so on).
Dbranes are defined by the fact that worldsheet boundaries are attached to them. Dbranes have mass, since they emit and absorb closed strings that describe gravitons, and — in superstring theories — charge as well, since they couple to open strings that describe gauge interactions.
From the point of view of open strings, Dbranes are objects to which the ends of open strings are attached. The open strings attached to a Dbrane are said to "live" on it, and they give rise to gauge theories "living" on it (since one of the open string modes is a gauge boson such as the photon). In the case of one Dbrane there will be one type of a gauge boson and we will have an Abelian gauge theory (with the gauge boson being the photon). If there are multiple parallel Dbranes there will be multiple types of gauge bosons, giving rise to a nonAbelian gauge theory.
Dbranes are thus gravitational sources, on which a gauge theory "lives". This gauge theory is coupled to gravity (which is said to exist in the bulk), so that normally each of these two different viewpoints is incomplete.
Is there a string theory vacuum that exactly describes everything in our universe? Is it uniquely determined by low energy data? 
Several major difficulties complicate efforts to test string theory. The most significant is the extremely small size of the Planck length, which is expected to be close to the string length (the characteristic size of a string, where strings become easily distinguishable from particles). Another issue is the huge number of metastable vacua of string theory, which might be sufficiently diverse to accommodate almost any phenomena we might observe at lower energies.
On the other hand, all string theory models are quantum mechanical, Lorentz invariant,^{[22]} unitary, and contain Einstein's General Relativity as a low energy limit.^{[23]} Therefore, to falsify^{[24]} string theory, it would suffice to falsify quantum mechanics, fundamental Lorentz invariance,^{[22]} or general relativity.^{[25]} Other potential falsifications of string theory would include the confirmation of a model from the swampland^{[26]}^{[27]} or observations of positive curvature in cosmology.^{[25]}^{[28]}^{[29]} However, these falsifications do not necessarily correspond to predictions which are unique to string theory, and finding a way to experimentally verify string theory via unique predictions remains a major challenge.^{[30]}
One unique prediction of string theory is the existence of string harmonics: at sufficiently high energies, the stringlike nature of particles would become obvious. There should be heavier copies of all particles, corresponding to higher vibrational harmonics of the string. It is not clear how high these energies are. In most conventional string models they would be not far below the Planck energy, around 10^{14} times higher than the energies accessible in the newest particle accelerator, the LHC, making this prediction impossible to test with any particle accelerator in the foreseeable future. However, in models with large extra dimensions they could potentially be produced at the LHC or at energies not far above its reach.
String theory as currently understood makes a series of predictions for the structure of the universe at the largest scales. Many phases in string theory have very large, positive vacuum energy.^{[31]} Regions of the universe that are in such a phase will inflate exponentially rapidly in a process known as eternal inflation. As such, the theory predicts that most of the universe is very rapidly expanding. However, these expanding phases are not stable, and can decay via the nucleation of bubbles of lower vacuum energy. Since our local region of the universe is not very rapidly expanding, string theory predicts we are inside such a bubble. The spatial curvature of the "universe" inside the bubbles that form by this process is negative, a testable prediction.^{[32]} Moreover, other bubbles will eventually form in the parent vacuum outside the bubble and collide with it. These collisions lead to potentially observable imprints on cosmology.^{[29]}^{[33]} However, it is possible that neither of these will be observed if the spatial curvature is too small and the collisions are too rare.
Under certain circumstances, fundamental strings produced at or near the end of inflation can be "stretched" to astronomical proportions. These cosmic strings could be observed in various ways, for instance by their gravitational lensing effects. However, certain field theories also predict cosmic strings arising from topological defects in the field configuration.
Theories with extra dimensions predict that the strength of gravity increases much more rapidly at small distances than is the case in 3 dimensions (where it increase as r^{−2}). Depending on the size of the dimensions, this could lead to phenomena such as the production of microblack holes at the LHC, or be detected in microgravity experiments.
String theory was originally proposed as a theory of hadrons, and its study has led to new insights on quantum chromodynamics, a gauge theory, which is the fundamental theory of the strong nuclear force. To this end, it is hoped that a gravitational theory dual to quantum chromodynamics will be found.^{[34]}
A mathematical technique from string theory (the AdS/CFT correspondence) has been used to describe qualitative features of quark–gluon plasma behavior in relativistic heavyion collisions;^{[9]}^{[10]}^{[11]}^{[12]} the physics, however, is strictly that of standard quantum chromodynamics, which has been quantitatively modeled by lattice QCD methods with good results.^{[35]}
If confirmed experimentally, supersymmetry could also be considered evidence, because it was discovered in the context of string theory, and all consistent string theories are supersymmetric. However, the absence of supersymmetric particles at energies accessible to the LHC would not necessarily disprove string theory, since the energy scale at which supersymmetry is broken could be well above the accelerator's range.
A central problem for applications is that the bestunderstood backgrounds of string theory preserve much of the supersymmetry of the underlying theory, which results in timeinvariant spacetimes: At present, string theory cannot deal well with timedependent, cosmological backgrounds. However, several models have been proposed to predict supersymmetry breaking, the most notable one being the KKLT model,^{[31]} which incorporates branes and fluxes to make a metastable compactification.
AdS/CFT relates string theory to gauge theory, and allows contact with low energy experiments in quantum chromodynamics. This type of string theory, which describes only the strong interactions, is much less controversial today than string theories of everything (although two decades ago, it was the other way around).^{[36]}
Grand unification natural in string theories of everything requires that the coupling constants of the four forces meet at one point under renormalization group rescaling. This is also a falsifiable statement, but it is not restricted to string theory, but is shared by grand unified theories.^{[37]} The LHC will be used both for testing AdS/CFT, and to check if the electroweakstrong unification does happen as predicted.^{[38]}
Gaugegravity duality is a conjectured duality between a quantum theory of gravity in certain cases and gauge theory in a lower number of dimensions. This means that each predicted phenomenon and quantity in one theory has an analogue in the other theory, with a "dictionary" translating from one theory to the other.
In certain cases the gauge theory on the Dbranes is decoupled from the gravity living in the bulk; thus open strings attached to the Dbranes are not interacting with closed strings. Such a situation is termed a decoupling limit.
In those cases, the Dbranes have two independent alternative descriptions. As discussed above, from the point of view of closed strings, the Dbranes are gravitational sources, and thus we have a gravitational theory on spacetime with some background fields. From the point of view of open strings, the physics of the Dbranes is described by the appropriate gauge theory. Therefore in such cases it is often conjectured that the gravitational theory on spacetime with the appropriate background fields is dual (i.e. physically equivalent) to the gauge theory on the boundary of this spacetime (since the subspace filled by the Dbranes is the boundary of this spacetime). So far, this duality has not been proven in any cases, so there is also disagreement among string theorists regarding how strong the duality applies to various models.
The best known example and the first one to be studied is the duality between Type IIB superstring on AdS^{5} × S^{5} (a product space of a fivedimensional Anti de Sitter space and a fivesphere) on one hand, and N = 4 supersymmetric Yang–Mills theory on the fourdimensional boundary of the Anti de Sitter space (either a flat fourdimensional spacetime R^{3,1} or a threesphere with time S^{3} × R). This is known as the AdS/CFT correspondence,^{[39]}^{[40]}^{[41]}^{[42]} a name often used for Gauge / gravity duality in general.
This duality can be thought of as follows: suppose there is a spacetime with a gravitational source, for example an extremal black hole.^{[43]} When particles are far away from this source, they are described by closed strings (i.e., a gravitational theory, or usually supergravity). As the particles approach the gravitational source, they can still be described by closed strings; also, they can be described by objects similar to QCD strings,^{[44]}^{[45]}^{[46]} which are made of gauge bosons (gluons) and other gauge theory degrees of freedom.^{[47]} So if one is able (in a decoupling limit) to describe the gravitational system as two separate regions — one (the bulk) far away from the source, and the other close to the source — then the latter region can also be described by a gauge theory on Dbranes. This latter region (close to the source) is termed the nearhorizon limit, since usually there is an event horizon around (or at) the gravitational source.
In the gravitational theory, one of the directions in spacetime is the radial direction, going from the gravitational source and away (toward the bulk). The gauge theory lives only on the Dbrane itself, so it does not include the radial direction: it lives in a spacetime with one less dimension compared to the gravitational theory (in fact, it lives on a spacetime identical to the boundary of the nearhorizon gravitational theory). Let us understand how the two theories are still equivalent:
The physics of the nearhorizon gravitational theory involves only onshell states (as usual in string theory), while the field theory includes also offshell correlation function. The onshell states in the nearhorizon gravitational theory can be thought of as describing only particles arriving from the bulk to the nearhorizon region and interacting there between themselves. In the gauge theory, these are "projected" onto the boundary, so that particles that arrive at the source from different directions will be seen in the gauge theory as (offshell) quantum fluctuations far apart from each other, while particles arriving at the source from almost the same direction in space will be seen in the gauge theory as (offshell) quantum fluctuations close to each other. Thus the angle between the arriving particles in the gravitational theory translates to the distance scale between quantum fluctuations in the gauge theory. The angle between arriving particles in the gravitational theory is related to the radial distance from the gravitational source at which the particles interact: The larger the angle the closer the particles have to get to the source in order to interact with each other. On the other hand, the scale of the distance between quantum fluctuations in a quantum field theory is related (inversely) to the energy scale in this theory, so small radius in the gravitational theory translates to low energy scale in the gauge theory (i.e., the IR regime of the field theory), while large radius in the gravitational theory translates to high energy scale in the gauge theory (i.e., the UV regime of the field theory).
A simple example to this principle is that if in the gravitational theory there is a setup in which the dilaton field (which determines the strength of the coupling) is decreasing with the radius, then its dual field theory will be asymptotically free, i.e. its coupling will grow weaker in high energies.
Some of the structures reintroduced by string theory arose for the first time much earlier as part of the program of classical unification started by Albert Einstein. The first person to add a fifth dimension to general relativity was German mathematician Theodor Kaluza in 1919, who noted that gravity in five dimensions describes both gravity and electromagnetism in four. In 1926, the Swedish physicist Oskar Klein gave a physical interpretation of the unobservable extra dimension it is wrapped into a small circle. Einstein introduced a nonsymmetric metric tensor, while much later Brans and Dicke added a scalar component to gravity. These ideas would be revived within string theory, where they are demanded by consistency conditions.
String theory was originally developed during the late 1960s and early 1970s as a never completely successful theory of hadrons, the subatomic particles like the proton and neutron that feel the strong interaction. In the 1960s, Geoffrey Chew and Steven Frautschi discovered that the mesons make families called Regge trajectories with masses related to spins in a way that was later understood by Yoichiro Nambu, Holger Bech Nielsen and Leonard Susskind to be the relationship expected from rotating strings. Chew advocated making a theory for the interactions of these trajectories that did not presume that they were composed of any fundamental particles, but would construct their interactions from selfconsistency conditions on the Smatrix. The Smatrix approach was started by Werner Heisenberg in the 1940s as a way of constructing a theory that did not rely on the local notions of space and time, which Heisenberg believed break down at the nuclear scale. While the scale was off by many orders of magnitude, the approach he advocated was ideally suited for a theory of quantum gravity.
Working with experimental data, R. Dolen, D. Horn and C. Schmid^{[48]} developed some sum rules for hadron exchange. When a particle and antiparticle scatter, virtual particles can be exchanged in two qualitatively different ways. In the schannel, the two particles annihilate to make temporary intermediate states that fall apart into the final state particles. In the tchannel, the particles exchange intermediate states by emission and absorption. In field theory, the two contributions add together, one giving a continuous background contribution, the other giving peaks at certain energies. In the data, it was clear that the peaks were stealing from the background the authors interpreted this as saying that the tchannel contribution was dual to the schannel one, meaning both described the whole amplitude and included the other.
The result was widely advertised by Murray GellMann, leading Gabriele Veneziano to construct a scattering amplitude that had the property of DolenHornSchmid duality, later renamed worldsheet duality. The amplitude needed poles where the particles appear, on straight line trajectories, and there is a special mathematical function whose poles are evenly spaced on half the real line— the Gamma function— which was widely used in Regge theory. By manipulating combinations of Gamma functions, Veneziano was able to find a consistent scattering amplitude with poles on straight lines, with mostly positive residues, which obeyed duality and had the appropriate Regge scaling at high energy. The amplitude could fit nearbeam scattering data as well as other Regge type fits, and had a suggestive integral representation that could be used for generalization.
Over the next years, hundreds of physicists worked to complete the bootstrap program for this model, with many surprises. Veneziano himself discovered that for the scattering amplitude to describe the scattering of a particle that appears in the theory, an obvious selfconsistency condition, the lightest particle must be a tachyon. Miguel Virasoro and Joel Shapiro found a different amplitude now understood to be that of closed strings, while Ziro Koba and Holger Nielsen generalized Veneziano's integral representation to multiparticle scattering. Veneziano and Sergio Fubini introduced an operator formalism for computing the scattering amplitudes that was a forerunner of worldsheet conformal theory, while Virasoro understood how to remove the poles with wrongsign residues using a constraint on the states. Claud Lovelace calculated a loop amplitude, and noted that there is an inconsistency unless the dimension of the theory is 26. Charles Thorn, Peter Goddard and Richard Brower went on to prove that there are no wrongsign propagating states in dimensions less than or equal to 26.
In 1969, Yoichiro Nambu, Holger Bech Nielsen, and Leonard Susskind recognized that the theory could be given a description in space and time in terms of strings. The scattering amplitudes were derived systematically from the action principle by Peter Goddard, Jeffrey Goldstone, Claudio Rebbi, and Charles Thorn, giving a spacetime picture to the vertex operators introduced by Veneziano and Fubini and a geometrical interpretation to the Virasoro conditions.
In 1970, Pierre Ramond added fermions to the model, which led him to formulate a twodimensional supersymmetry to cancel the wrongsign states. John Schwarz and André Neveu added another sector to the fermi theory a short time later. In the fermion theories, the critical dimension was 10. Stanley Mandelstam formulated a world sheet conformal theory for both the bose and fermi case, giving a twodimensional field theoretic pathintegral to generate the operator formalism. Michio Kaku and Keiji Kikkawa gave a different formulation of the bosonic string, as a string field theory, with infinitely many particle types and with fields taking values not on points, but on loops and curves.
In 1974, Tamiaki Yoneya discovered that all the known string theories included a massless spintwo particle that obeyed the correct Ward identities to be a graviton. John Schwarz and Joel Scherk came to the same conclusion and made the bold leap to suggest that string theory was a theory of gravity, not a theory of hadrons. They reintroduced Kaluza–Klein theory as a way of making sense of the extra dimensions. At the same time, quantum chromodynamics was recognized as the correct theory of hadrons, shifting the attention of physicists and apparently leaving the bootstrap program in the dustbin of history.
String theory eventually made it out of the dustbin, but for the following decade all work on the theory was completely ignored. Still, the theory continued to develop at a steady pace thanks to the work of a handful of devotees. Ferdinando Gliozzi, Joel Scherk, and David Olive realized in 1976 that the original Ramond and Neveu Schwarzstrings were separately inconsistent and needed to be combined. The resulting theory did not have a tachyon, and was proven to have spacetime supersymmetry by John Schwarz and Michael Green in 1981. The same year, Alexander Polyakov gave the theory a modern path integral formulation, and went on to develop conformal field theory extensively. In 1979, Daniel Friedan showed that the equations of motions of string theory, which are generalizations of the Einstein equations of General Relativity, emerge from the Renormalization group equations for the twodimensional field theory. Schwarz and Green discovered Tduality, and constructed two different superstring theories IIA and IIB related by Tduality, and type I theories with open strings. The consistency conditions had been so strong, that the entire theory was nearly uniquely determined, with only a few discrete choices.
In the early 1980s, Edward Witten discovered that most theories of quantum gravity could not accommodate chiral fermions like the neutrino. This led him, in collaboration with Luis AlvarezGaumé to study violations of the conservation laws in gravity theories with anomalies, concluding that type I string theories were inconsistent. Green and Schwarz discovered a contribution to the anomaly that Witten and AlvarezGaumé had missed, which restricted the gauge group of the type I string theory to be SO(32). In coming to understand this calculation, Edward Witten became convinced that string theory was truly a consistent theory of gravity, and he became a highprofile advocate. Following Witten's lead, between 1984 and 1986, hundreds of physicists started to work in this field, and this is sometimes called the first superstring revolution.
During this period, David Gross, Jeffrey Harvey, Emil Martinec, and Ryan Rohm discovered heterotic strings. The gauge group of these closed strings was two copies of E8, and either copy could easily and naturally include the standard model. Philip Candelas, Gary Horowitz, Andrew Strominger and Edward Witten found that the CalabiYau manifolds are the compactifications that preserve a realistic amount of supersymmetry, while Lance Dixon and others worked out the physical properties of orbifolds, distinctive geometrical singularities allowed in string theory. Cumrun Vafa generalized Tduality from circles to arbitrary manifolds, creating the mathematical field of mirror symmetry. Daniel Friedan, Emil Martinec and Stephen Shenker further developed the covariant quantization of the superstring using conformal field theory techniques. David Gross and Vipul Periwal discovered that string perturbation theory was divergent. Stephen Shenker showed it diverged much faster than in field theory suggesting that new nonperturbative objects were missing.
In the 1990s, Joseph Polchinski discovered that the theory requires higherdimensional objects, called Dbranes and identified these with the blackhole solutions of supergravity. These were understood to be the new objects suggested by the perturbative divergences, and they opened up a new field with rich mathematical structure. It quickly became clear that Dbranes and other pbranes, not just strings, formed the matter content of the string theories, and the physical interpretation of the strings and branes was revealed they are a type of black hole. Leonard Susskind had incorporated the holographic principle of Gerardus 't Hooft into string theory, identifying the long highly excited string states with ordinary thermal black hole states. As suggested by 't Hooft, the fluctuations of the black hole horizon, the worldsheet or worldvolume theory, describes not only the degrees of freedom of the black hole, but all nearby objects too.
In 1995, at the annual conference of string theorists at the University of Southern California (USC), Edward Witten gave a speech on string theory that in essence united the five string theories that existed at the time, and giving birth to a new 11dimensional theory called Mtheory. Mtheory was also foreshadowed in the work of Paul Townsend at approximately the same time. The flurry of activity that began at this time is sometimes called the second superstring revolution.
During this period, Tom Banks, Willy Fischler, Stephen Shenker and Leonard Susskind formulated matrix theory, a full holographic description of Mtheory using IIA D0 branes.^{[49]} This was the first definition of string theory that was fully nonperturbative and a concrete mathematical realization of the holographic principle. It is an example of a gaugegravity duality and is now understood to be a special case of the AdS/CFT correspondence. Andrew Strominger and Cumrun Vafa calculated the entropy of certain configurations of Dbranes and found agreement with the semiclassical answer for extreme charged black holes. Petr Hořava and Edward Witten found the elevendimensional formulation of the heterotic string theories, showing that orbifolds solve the chirality problem. Witten noted that the effective description of the physics of Dbranes at low energies is by a supersymmetric gauge theory, and found geometrical interpretations of mathematical structures in gauge theory that he and Nathan Seiberg had earlier discovered in terms of the location of the branes.
In 1997, Juan Maldacena noted that the low energy excitations of a theory near a black hole consist of objects close to the horizon, which for extreme charged black holes looks like an anti de Sitter space. He noted that in this limit the gauge theory describes the string excitations near the branes. So he hypothesized that string theory on a nearhorizon extremecharged blackhole geometry, an antideSitter space times a sphere with flux, is equally well described by the lowenergy limiting gauge theory, the N=4 supersymmetric YangMills theory. This hypothesis, which is called the AdS/CFT correspondence, was further developed by Steven Gubser, Igor Klebanov and Alexander Polyakov, and by Edward Witten, and it is now wellaccepted. It is a concrete realization of the holographic principle, which has farreaching implications for black holes, locality and information in physics, as well as the nature of the gravitational interaction. Through this relationship, string theory has been shown to be related to gauge theories like quantum chromodynamics and this has led to more quantitative understanding of the behavior of hadrons, bringing string theory back to its roots.
Some critics of string theory say that it is a failure as a theory of everything.^{[50]}^{[51]}^{[52]}^{[53]}^{[54]}^{[55]} Notable critics include Peter Woit, Lee Smolin, Philip Warren Anderson,^{[56]} Sheldon Glashow,^{[57]} Lawrence Krauss,^{[58]} and Carlo Rovelli.^{[59]} Some common criticisms include:
It is widely believed that any theory of quantum gravity would require extremely high energies to probe directly, higher by orders of magnitude than those that current experiments such as the Large Hadron Collider^{[60]} can attain. This is because strings themselves are expected to be only slightly larger than the Planck length, which is twenty orders of magnitude smaller than the radius of a proton, and high energies are required to probe small length scales. Generally speaking, quantum gravity is difficult to test because the gravity is much weaker than the other forces, and because quantum effects are controlled by Planck's constant h, a very small quantity. As a result, the effects of quantum gravity are extremely weak.
String theory as it is currently understood has a huge number of solutions, called string vacua,^{[31]} and these vacua might be sufficiently diverse to accommodate almost any phenomena we might observe at lower energies.
The vacuum structure of the theory, called the string theory landscape (or the anthropic portion of string theory vacua), is not well understood. String theory contains an infinite number of distinct metastable vacua, and perhaps 10^{520} of these or more correspond to a universe roughly similar to ours — with four dimensions, a high planck scale, gauge groups, and chiral fermions. Each of these corresponds to a different possible universe, with a different collection of particles and forces.^{[31]} What principle, if any, can be used to select among these vacua is an open issue. While there are no continuous parameters in the theory, there is a very large set of possible universes, which may be radically different from each other. It is also suggested that the landscape is surrounded by an even more vast swampland of consistentlooking semiclassical effective field theories, which are actually inconsistent.^{[citation needed]}
Some physicists believe this is a good thing, because it may allow a natural anthropic explanation of the observed values of physical constants, in particular the small value of the cosmological constant.^{[61]}^{[62]} The argument is that most universes contain values for physical constants that do not lead to habitable universes (at least for humans), and so we happen to live in the most "friendly" universe. This principle is already employed to explain the existence of life on earth as the result of a lifefriendly orbit around the mediumsized sun among an infinite number of possible orbits (as well as a relatively stable location in the galaxy).
A separate and older criticism of string theory is that it is backgrounddependent — string theory describes perturbative expansions about fixed spacetime backgrounds. Although the theory has some backgroundindependence — topology change is an established process in string theory, and the exchange of gravitons is equivalent to a change in the background — mathematical calculations in the theory rely on preselecting a background as a starting point. This is because, like many quantum field theories, much of string theory is still only formulated perturbatively, as a divergent series of approximations.
This criticism has been addressed to some extent by the AdS/CFT duality, which is believed to provide a full, nonperturbative definition of string theory in spacetimes with antide Sitter space asymptotics. Nevertheless, a nonperturbative definition of the theory in arbitrary spacetime backgrounds is still lacking. Some hope that Mtheory, or a nonperturbative treatment of string theory (such as "background independent open string field theory") will have a backgroundindependent formulation.
Two nontechnical books that are critical of string theory:
Technical and critical:
Look up string theory in Wiktionary, the free dictionary. 
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