# String Theory

” String theory ” in this article we’ll take deep concept about string theory. And also we’ll take the deep analysis and research of Ph.D. scholars about String theory……

**What is string theory?**

**Preliminaries**

During the twentieth century, Physics has provided an extremely accurate view of the fudamental components of matter (elementary particles) and the laws that regulate their behavior (fundamental interactions). That is, it has provided an explanation to the question *` What are things made of?*.

Today we know that matter is made of atoms, which in turn are made up of a nucleus and a cloud of electrons that orbit it. The nucleus is in turn composed of protons and neutrons, which in turn are composed of quarks. Both electrons and quarks behave, with the current experimental precision, as point particles, without structure. All matter in the Universe is therefore composed of quarks and leptons (electrons are a specific type of particles called leptons). Likewise, the forces in Nature can be understood in terms of four fundamental forces: gravitational, electromagnetic (which unifies electricity and magnetism), strong interaction (which links quarks to form protons and neutrons, already the protons and neutrons to form nuclei) and the weak interaction (which is capable of transforming some particles into others, and that underlies radioactive phenomena). Within the framework of Quantum Mechanics, these interactions are interpreted in turn as exchanges of certain particles, the quanta of the interaction field. These quanta are the photon for the electromagnetic interaction, the W / Z bosons for the weak interaction and the gluons for the strong interactions. The gravitational interaction, once framed within Quantum Mechanics, would have its corresponding carrier particle, the graviton. the quanta of the interaction field. These quanta are the photon for the electromagnetic interaction, the W / Z bosons for the weak interaction and the gluons for the strong interactions. The gravitational interaction, once framed within Quantum Mechanics, would have its corresponding carrier particle, the graviton. the quanta of the interaction field. These quanta are the photon for the electromagnetic interaction, the W / Z bosons for the weak interaction and the gluons for the strong interactions. The gravitational interaction, once framed within Quantum Mechanics, would have its corresponding carrier particle, the graviton.

This description of Nature and its behavior at the most fundamental level underlies the explanation of everyday phenomena (such as the fall of bodies, planetary orbits, electric currents, etc.), but remains valid at much higher energies, such as the very high temperatures of the primitive Universe, or those reached in the current experiments of particle collisions.

However, this description is undermined from its foundations, since it is based on two pillars of Theoretical Physics that are, in their present form, mutually incompatible. The description of the electromagnetic interactions, strong and weak, called the Standard Model (of Elemental Particles) is framed within the Quantum Field Theory, an advanced form of Quantum Mechanics. However, the description of gravitational interaction is based on Einstein’s Theory of General Relativity, which is a classical theory, and therefore does not include quantum effects.

The inclusion of quantum effects in the gravitational interaction following usual procedures entails pathological responses at very high energies, of the order of the Planck scale (present in the primordial Universe at 10^{-44} seconds, or equivalently 10^{17} times higher than the energies accessible in particle accelerators). For more extensive information on the problem of Gravitation and Quantum Mechanics, see the article Gravity and quanta, by Prof. Enrique Alvarez (IFT, Madrid).

Although the problem arises in a regime currently not accessible to the experiment, it remains one of the fundamental problems of Theoretical Physics: the formulation of a theory that describes gravitational interaction consistently at a quantum level, and that by so much to reconcile General Relativity with Quantum Mechanics (and therefore the gravitational interaction with the remaining fundamental interactions). A discussion of the problems of unifying interactions, and the role of string theory in this regard, can be found in the article

String Theory , by Prof. Sunil Mukhi (Tata Institute, India).

### String theory

The natural proposal to achieve this unified description is the modification of the behavior of the particles at very high energies, so that the pathological behavior of gravity at energies of the order of the Plank scale is corrected. The modifications would be very small in the most familiar situations, but they would essentially enter into the explanation of Nature’s behavior in a very intense gravity system, where the curvature of spacetime is very high (radii of curvature of the order of length from Planck, that is 10^{-35} m), as in black holes, or at the beginning of the Universe.

String theory (or superstrings) proposes precisely such a modification. Specifically part of the hypothesis that elementary particles are not point, but large objects in one dimension (really strings). The size of these strings is very small, much smaller than the smaller length scales measured experimentally (^{10-17} m). Although it is normally assumed that this size is of the order of the Planck length (10^{-35} cm), in some models this size could be larger (of the order of 10^{-18}cm). At very low energies, there is not enough resolution to observe the size of the strings, and their behavior is reduced to that of point particles. However, at very high energies, the extensive nature of the strings begins to manifest and modifies the behavior of the particles so that their gravitational interactions, calculated in theory, do not exhibit any pathological behavior.

An introduction in Spanish to string theory and other related fields can be found in the chapter on string theory in the virtual book Straddling Time , by Patricio T. Díaz Pazos (see also Super Strings ).

**Some general introductions to string theory (in English) can be found at**

- The Elegant Universe
- Superstrings , by John M. Pierre.
- String Theory , by Robbert Dijkgraaf.
- The Official String Theory web site
- Beyond String Theory
- The Second Superstring Revolution , by John Schwarz.
- Allegro (Ma Non Troppo), Passage for Strings in Warped Passages, by Lisa Randall.

**String theory has profound implications in our vision of Nature.**

In string theory, different particles are simply different modes of vibration of a single type of string. Moreover, certain vibration modes correspond to the carrier particles of the fundamental interactions. Therefore, it implies a definitive unification, where all particles and interactions receive an explanation in terms of a single type of object.

- The search for the last constituents of the subject article by Prof. Luis Ibáñez (IFT, Madrid), in the journal of the Royal Spanish Physical Society.
- Unification and duality in string theory article by Prof. Luis Ibáñez (IFT, Madrid), in the journal Research and Science.
- What is string theory? by Prof. Alberto Guijosa (UNAM, Mexico).

The mathematical consistency of the theory implies that our Universe has additional spatial dimensions, curved on themselves and of a size that makes them unobservable to current energies, but that influence the behavior of particles at very high energies (potentially accessible in future experiments, and certainly experienced in the early Universe).

The description of gravitational systems in string theory naturally incorporates the concept of holography. This idea, proposed by ‘tHooft and Susskind in the connection of black holes, is that the degrees of freedom of a gravitational theory can be encoded in a hypersurface of one dimension less (such as a two-dimensional hologram encodes a three-dimensional image).

The AdS / CFT correspondence in string theory allows a quantitative description of gravitational phenomena, such as black hole microphysics, in terms of a dual holographic theory, described as a quantum field theory.

An introduction to holography, AdS / CFT correspondence, and its implications can be found in the talk by Juan Maldacena (IAS, Princeton) Black Holes, Strings and Quantum Gravity.

- Beyond Einstein: holography, article by Prof. César Gómez, member of the IFT, in the journal of the Royal Spanish Society of Physics.
- Holography, article on page BK2.
- The holographic principle and the M theory, translation of the DAMTP general public page.

Conversely, the AdS / CFT correspondence can be applied to understand complicated phenomena in strong coupling field theories (such as quarks and gluons plasma hydrodynamics) using the dual gravitational description, in the classical approximation.

- Black holes, the most perfect low viscosity fluid?
- The Liquid Universe hints at the strings , article in Physics in action.

From a more abstract point of view, classical space and time are concepts derived from string theory. String theory proposes in several limits, drastically modified versions of Einstein’s spacetime. For example, in certain situations the geometry in string theory is modified so that the spacetime coordinates do not commute with each other.

- Noncommutative geometry and quantum spacetime , article by Prof. José L. Fernández Barbón, member of the IFT, in the journal Research and Science.

Despite all the progress in the field, string theory is in some ways a theory still under construction, whose ultimate form is framed in the so-called (and still mysterious) theory M. This theory, whose structure is treatable in particular situations simple, it would include strong coupling effects in string theory, and would treat equally so-called fundamentals and other non-disturbing objects (p-branes) present in the theory.

For more information on string theory and M theory, you can consult

- What is the M theory?, by Prof. Carmen Núñez (IAFE, Argentina).
- Magic and mystery in the unification of Physics, by Prof. Hugo García-Compeán (CINVESTAV, Mexico).

String theory remains one of the most active fields in Theoretical Physics. The annual Strings conference gathers every year the order of 500 researchers in the field to share their ideas and discuss the advances of the theory.

Einstein’s General Theory of Relativity Stanford University

Modern Physics: Special Relativity Stanford

String Theory and M-Theory Stanford

Leonard Susskind offers a lecture on the string theory and high energy physics. he’s a world honor theoretical man of science and uses graphs to assist demonstrate the theories he’s presenting.

String theory (with its shut relative, M-theory) is that the basis for the foremost formidable theories of the physical world. it’s deeply influenced our understanding of gravity, cosmology, and high energy physics. during this course we’ll develop the essential theoretical and mathematical ideas, as well as the string-theoretic origin of gravity, the idea of additional dimensions of house, the affiliation between strings and black holes, the “landscape” of string theory, and also the holographic principle .

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### Is there a theory at all?

Physics, in reality, are two sciences. There is quantum mechanics, the strange and tumultuous world where particles appear and disappear and cats are both alive and dead. And there is general relativity, Einstein’s majestic vision of massive objects that curve space and time.

Since these two different visions of the world emerged in the early twentieth century, generations of physicists have tried to unify them into a single theory that, ideally, would describe the four basic forces of nature. Even Einstein tried, and failed. Now, after a few particularly frustrating decades with little new evidence to guide us, current physicists may be about to get tempting clues about how the forces fit together.

The tracks are expected to arrive from the Large Hadron Collider, a ring of superconducting magnets in the Alps designed to impact protons with each other at energies never before seen on Earth. The collider began operating in March 2010, and is expected to reach its maximum power in 2014, when it will attempt to collide protons at twice the current energy.

Even then, the LHC will be far from powerful enough to recreate the only unified force that physicists believe existed for a fraction of a second after the Big Bang – you would need a collider as large as the universe itself to do that. But the LHC might be able to test some of the predictions made by the main theory that links gravity and other forces.

Superstring theory – or string theory for short – unites all physics in a package, reducing the disconcerting particle taxonomy of the current bestiary of physics, the Standard Model, to identical fragments of strings, each less than one billionth of a billionth of a billionth of a centimeter long. According to string theory, the particles that carry the three forces included in the Standard Model – the photon (electromagnetism), the gluon (strong nuclear force) and the bosons W and Z (weak nuclear force) are only the same Tiny dancers each following different rhythms.

And, unlike the Standard Model, string theory has a place for gravity.

Although there are proposals along with string theory that try to explain how all the forces of nature would fit in, most of these theories have big problems. Some, for example, predict the existence of particles that cannot exist.

The main obstacle of string theory is that it requires that there be many more things in the universe than physicists can study, making the theory very difficult to test. For example, most versions of string theory require that the universe have 10 or 11 dimensions -9 or 10 of space and a temporal one, instead of thefour that folks experience: up-down, front-back, left-right and past-future.

“The forces square measure unified in eleven dimensions, however they divide after they move to four dimensions,” says Gordon Kane, a man of science at the University of Michigan in urban center.In order for string theory to say something about how forces arise, physicists have to calculate how these additional dimensions are rolled up, or “compactified,” into the four that are familiar to us.

String theory also makes a population of companion particles appear in the shadow for each of those currently known to exist – an idea called supersymmetry. In fact, supersymmetry may be necessary to unify electromagnetic forces, strong and weak, so it is important even if string theory is not correct.

**When the forces collide**

Many physicists have high hopes that the LHC will find evidence of supersymmetric particles and additional spatial dimensions.

“Even if we don’t go to the other dimensions, in a sense the other dimensions will come to us,” says Harvard physicist Lisa Randall.

In the 1990s, when working with Raman Sundrum, now at the University of Maryland at College Park, Randall showed that it might be possible to detect the disintegration of a gravity-bearing particle that came from an extra dimension. Finding such a particle in the LHC would verify the existence of the additional dimensions and suggest why gravity is much weaker than the other three forces.

“I think it would be quite surprising,” says Randall.”But this is one of the things we could find, and it is one of the things we should look for.”

Most physicists believe that the LHC is more likely to find evidence of supersymmetric partners of Standard Model particles. The appearance and properties of the partners would establish some useful restrictions on how the universe compacts the 11 dimensions predicted by string theory.

For example, if the lighter superparticle turns out to be the wino, the supercompany of the boson W, bearer of the weak force, would be consistent with a version of the string theory known with the appellative expressive of “Theory M compactified in a collector 7- D of holonomy G2?.

Such supersymmetric particles may have already been observed, in fact – not on Earth, but in space. Some of the dark matter that is believed to form more than 80 percent of the matter in the universe could be composed of supersymmetric particles, remnants of the first moments of the universe. In recent years, two space instruments, the Gamma Fermi Ray Telescope and the Italian PAMELA mission, have seen signs of dark matter in the Milky Way, in the form of gamma and antimatter rays that could have been produced by the collision of supersymmetric particles.

Since the LHC and future colliders can, for the moment, take physicists only so far together after the Big Bang, the scientific understanding of a unified theory will eventually have to lead to the exploration of the vastness of the universe.Some physicists surprise if such a technique, that depends on finding and deciphering the clues left naturally, will turn out results adore the high-precision experimental knowledge that crystal rectifier to the quality Model throughout the 20th century.But string theory is not science of the twentieth century – in fact, string theorist Edward Witten has described it as “physics of the twenty-first century that fell by accident in the twentieth century.” currently that the twenty first century has arrived, it’s time for string theory to be tested.

String theory is a fundamental model of physics that basically assumes that seemingly point material particles are actually “vibrational states” of a more basic extended object called “string” or “filament.”

According to this proposal, an electron is not a “point” without internal structure and zero dimension, but a mass of tiny strings that vibrate in a space-time of more than four dimensions. A point can do nothing but move in a three-dimensional space. According to this theory, at the “microscopic” level it would be perceived that the electron is not really a point, but a loop-shaped string. A rope can do something besides moving; It can swing in different ways. If it oscillates in a certain way, then, macroscopically we would see an electron; but if it oscillates otherwise, then we would see a photon, or a quark, or any other particle of the standard model. This theory, expanded with others such as superstrings or Theory M, aims to move away from the point-particle conception.

**How are interactions in the subatomic world ?:** space-time lines like subatomic particles. in the Standard Model (left) or Closed rope with no ends and in a circle, as the string theory states (right).

The following formulation of a string theory is due to Jöel Scherk and John Schwuarz, who in 1974 published an article in which they demonstrated that a theory based on one-dimensional objects or “strings” instead of point particles could describe gravitational force. Although these ideas did not receive much attention at that time until the first superstring revolution of 1984. According to the formulation of string theory arising from this revolution, string theories can in fact be considered a general case of Kaluza’s theory. Klein quantized. The fundamental ideas are two:

The basic objects of the theory would not be point particles but extended one-dimensional objects (in the five conventional string theories these objects were one-dimensional or “strings”; currently in the M-theory they’re conjointly admitted of upper dimension or “p-branes”). This renormalizes some eternity of perturbative calculations.The space-time in which the strings and p-branes of the theory move would not be the ordinary 4-dimensional space-time but a Kaluza-Klein type space, in which 6 compactified dimensions are added to the four conventional dimensions in the form of Calabi-Yau variety. So conventionally in string theory there is 1 temporal dimension,

The unobservability of the additional dimensions is linked to the fact that these would be compactified, and would only be relevant at scales as small as Planck’s length. Similarly, with conventional measurement accuracy, closed strings with a length similar to Planck’s length would resemble point particles.

### Subsequent Developments

After the introduction of string theories, the necessity and convenience of introducing the principle that the speculation was supersymmetric was considered; that’s, to admit Associate in Nursing abstract symmetry that relates fermions and bosons. presently most string theorists work on supersymmetric theories; thus, string theory is presently known as particle theory..This last theory is essentially a supersymmetric string theory; that’s, it’s invariant beneath scientific theory transformations.

There are currently five superstring theories related to the five known ways of implementing supersymmetry in the string model. Although this multiplicity of theories baffled specialists for more than a decade, current conventional knowledge suggests that the five theories are borderline cases of a unique theory about a space of 11 dimensions (the 3 of the space,1 temporal and 6 additional resabiadas or “compacted” and 1 that encompasses them forming “membranes” from which part of their gravity could be escaped in the form of “gravitons”). This unique theory, called M theory, of which only some aspects would be known, was conjectured in 1995.

### Variants of the theory

The superstring theory of outer space is current. In his early (mid-1980s) appeared about five string theories, which were later identified as particular limits of a single theory: Theory M . The five versions of the theory currently existing, among which several duality relationships can be established are:

**Type I**theory , where both “strings” and open and closed D-branes appear , which move over a 10-dimensional space-time. The D-branes are 1,5 and 9 spatial dimensions.**Type IIA**theory is additionally a 10-dimensional theory however it uses solely closed strings andD-branes. It incorporates two gravities (theoretical particles associated with graviton through supersymmetry relationships). Use D-branes of dimension 0,2,4,6, and 8.- Theory
**type IIB**. - The
**heterotic-O theory**, based on the symmetry group O (32). - The
**heterotic-E theory**, based on the exceptional Lie group E_{8}. It was proposed in 1987 by Gross, Harvey, Martinec and Rohm.

The term tightrope theory actually refers to the theories of 26-dimensional bosonic strings and the 10-dimensional superstring theory, the latter discovered by adding supersymmetry to the bosonic string theory. Nowadays string theory is usually referred to as the supersymmetric variant, while the old one is known by the full name of “Bosonic string theory”. In 1995, Edward Witten conjectured that the five different superstring theories are borderline cases of an unknown 11-dimensional theory called M-Theory. The conference where Witten showed some of his results initiated the so-called Second superstring revolution .

In this theory M intervene as fundamental physical animated objects not only one-dimensional strings, but a whole variety of non-disturbing objects, extended in several dimensions, which are collectively called papuas (this name is an apheresis of “membrane”).

### Controversy over the theory

Although string theory, according to its defenders, could become one of the most predictive physical theories, capable of explaining some of the most fundamental properties of nature in geometric terms, the physicists who have worked in that field to date they have not been able to make concrete predictions with the precision necessary to confront them with experimental data. These prediction problems are due, according to the author, to the fact that the model is not falsifiable, and therefore not scientific, or that «The theory of superstrings is so ambitious that it can only be entirely correct or completely wrong.The only drawback is that their arithmetic area unit thus new and then tough that for many decades we’ll not recognize what they’re.Falsificationism and string theory Main article:

Demarcation criteria.

String theory or Theory M may not be falsifiable, according to its critics. Several authors have declared their concern that String Theory is not falsifiable and as such, following the theses of the philosopher of science Karl Popper, String Theory would be equivalent to a pseudoscience.

**Science philosopher Mario Bunge has recently stated:**

Consistency, sophistication and sweetness square measure ne’er enough in research.String theory is suspicious (of pseudoscience). It seems scientific because it addresses an open problem that is both important and difficult, to build a quantum theory of gravitation. But the theory postulates that physical space has six or seven dimensions, instead of three, simply to ensure mathematical consistency. Since these extra dimensions are unobservable, and since the theory has resisted experimental confirmation for more than three decades, it seems like science fiction, or at least, failed science.

Particle physics is inflated with sophisticated mathematical theories that postulate the existence of strange entities that do not interact appreciably, or at all, with ordinary matter, and as a consequence, are safe by being undetectable. Since these theories are in discrepancy with the whole of Physics, and violate the requirement of falsificationism, they can be described as pseudoscientific, even if they have been swarming a quarter of a century and continue to be published in the most prestigious scientific journals.

Mario Bunge,2006.

However, in the current state of science, the technological step has been taken that can finally begin the search for evidence on the existence of more than three spatial dimensions, since CERN and its new particle accelerator will try, among other things, to discover if the Higgs boson exists and if that particle expands only in 3 dimensions or if it does so in more than 3 dimensions, and it is intended to be achieved by studying the discrepancies in the measurements and observations of the mass of said particle if finally it is found, so in conclusion the string theory would be, recently, trying to enter the field of falsifiability.

**Superstring theory**

Superstring theory is a theoretical scheme to explain all the fundamental particles and forces of nature in a single theory, which models the particles and physical fields as vibrations of thin supersymmetric strings, which move in a space-time of more than 4 dimensions

One of the motivations used by superstring theorists is that the scheme is one of the best candidate theories to formulate a quantum theory of gravity. The superstring theory is a shorthand of the supersymmetric string theory because, unlike bosonic string theory, this is the version of string theory that, through supersymmetry, incorporates fermions.

The superstring theory comprises five alternative theories or formulations of combined string theories, in which supersymmetry requirements have been introduced.The name string theory is presently used as a equivalent word, since all wide studied string theories ar, in fact, particle theories.The fundamental idea is that the particles are actually strings that vibrate in resonance at a frequency of the Planck length and where the graviton would be a spin spin 2 and null mass.

Recently it has been possible to prove that several of these formulations are equivalent and after all of them there could be a unified theory or theory of everything. The five existing theories would be nothing more than particular boundary cases of this unified theory, provisionally referred to as Theory M. This theory M attempts to explain all existing subatomic particles at once and unify the four fundamental forces of nature. It defines the universe formed by a multitude of vibrant strings, since it is a version of string theory that incorporates fermions and supersymmetry.

The main problem of current physics is to be able to incorporate the force of gravity as explained by the theory of general relativity to the rest of the already unified physical forces. The superstring theory would be a method of unifying these theories. The theory is far from being finished and profiled, since there are many undefined variables, so there are several versions of it.

The underlying drawback in theoretical physics is to harmonize the idea of relativity theory, wherever gravitation and large-scale structures (stars, galaxies, clusters) square measure delineate,with quantum mechanics, where the other three fundamental forces that are described are described. They act at the atomic level.

The development of quantum field theory of an invariable force results in infinite (and useful) probabilities. Physicists have developed mathematical techniques of renormalization to eliminate those infinities of 3 of the four basic forces – electromagnetism, sturdy nuclear and weak nuclear- but not of gravity. The development of the quantum theory of gravity must, therefore, come in a different way than those used for other forces.

The basic idea is that the fundamental constituents of reality are strings of a Planck length(close to ^{10-35} m) that vibrate at resonance frequencies. Each string, in theory, features a distinctive resonance or harmony. completely different|completely different} harmonies confirm different basic forces the tension in the rope is of the order of Planck’s forces (1044 N). the graviton(name proposed for the particle that carries the gravitational force), for example, is predicted by the theory that it is a string with zero amplitude. Another key idea of the theory is that measurable differences between ropes that recapitulate small dimensions in themselves and many that move in large dimensions (eg affecting a dimension of size R equal to one of size 1/ R cannot be detected)). The singularities are avoided because of the observable consequences of the “great collapse” never reach zero sizes. In fact, the universe can begin a small “big collapse” of processes, string theory says that the universe can never be smaller than the size of a string, at which point it could begin to expand.

**The problem of dimensions**

Although the noticeable physical universe has 3 spatial dimensions and a temporal dimension, nothing prohibits a theory from describing a universe with more than four dimensions, especially if there is a mechanism of “apparent unobservability” of the additional dimensions. That is the case of string theory and superstring theory that postulate compactified additional dimensions and that would only be observable in physical phenomena that involve very high energies. In the case of superstring theory, the consistency of the theory itself requires a spacetime of 10 or 26 dimensions. The conflict between observation and theory is resolved by compacting dimensions that cannot be observed in the range of usual energies. In fact, superstring theory is not the first physical theory that proposes extra spatial dimensions; At the beginning of the 20th century, a geometric theory of the electromagnetic and gravitational field known as the Kaluza-Klein theory was proposed that postulated a 5-dimensional space-time. Subsequently, the idea of Kaluza and Klein was used to postulate the 11-dimensional supergravity theory that supersymmetry also uses.

The human mind has difficulty visualizing larger dimensions because it is only possible to move in 3 spatial dimensions. One way to deal with this limitation is not trying to visualize larger dimensions at all but simply thinking, when making equations that describe a phenomenon, that more equations should be made than usual. This opens up the questions that these ‘extra numbers’ can be investigated directly in any experiment (where results in 1,2,+1 dimensions would be shown to human scientists). Thus, in turn, the question arises whether these types of models that are investigated in this abstract modeling (and potentially impossible experimental devices) can be considered ‘scientific’.

One theory that generalizes it is the brane theory, where the strings are replaced by elementary constituents of the “membrane” type, hence their name. The existence of 10 dimensions is mathematically necessary to avoid the presence of mathematical inconsistencies in its statement.

**Number of superstring theories**

Theoretical physicists were disturbed by the existence of five different string theories. This happened under the so-called second superstring

revolution in the 1990s where the 5 string theories were postulated, being different borderline cases of a single theory: the M theory.

String Theory | ||

Types | Spatial Dimensions | Details |

Bosonica | 26 | Only bosons , not fermions , means only forces, not matter, with open and closed strings; biggest flaw: a particle with imaginary mass called tachyon |

I | 10 | Supersymmetry between force and matter, with open and closed strings, free of tachyons, symmetry group SO (32) |

IIA | 10 | Supersymmetry between force and matter, only with closed strings, free of tachyons, fermions without mass that rotate in both directions |

IIB | 10 | Supersymmetry between force and matter, only with closed strings, free of tachyons. mass-free fermions that rotate in only one direction |

HO | 10 | Supersymmetry between force and matter, only with closed strings, free of tachyons, heterotic , differ between ropes of right and left movement, symmetry group is SO (32) |

HE | 10 | Supersymmetry between force and matter, only with closed strings, free of tachyons, heterotic , differ between ropes of right and left movement, symmetry group E _{8}×E _{8} |

The five consistent superstring theories are:

Type I string theory has a ten-dimensional supersymmetry (16 superloads). This theory is special in the sense that it is based on an open and closed orientation, while the rest are based on ropes with closed orientations.

Type II string theory has two supersymmetries in the sense of 10 dimensions (32 superloads). There are in fact two types of Type II ropes called type IIA and IIB. They differ mainly in the fact that theory IIA is non-chiral (conserving parity), while theory IIB is chiral (violating parity).

The heterotic string theory relies on a peculiar hybrid of a sort I particle and a bosonic string. There ar two sorts of heterotic strings that disagree in their ten-dimensional gauge group: the heterotic string E8× E8 and therefore the therefore (32).(The heterotic name therefore (32) could be a bit inaccurate within the therefore (32) of the Lie cluster, the theories ar a quantitative relation of Spin (32)/ Z2 that’s not such as therefore (32).)The chiral theories of gauge may be inconsistent in its anomalies. This occurs when a loop of the Feynman Diagram causes a break in the quantum mechanics of gauge symmetry. Nullifying anomalies is limited to possible string theories.

### Integrating general relativity with quantum mechanics

General relativity usually refers to situations involving large massive objects in distant regions of spacetime where quantum mechanics is reserved for atomic scale scenarios (small regions of spacetime). The two are very hard used together, and the most common case where their study is combined is black holes. Having “density peaks” or maximum amounts of matter possible in space, and a very small area, the two must be used in synchrony to predict conditions in certain places; Even when used together, the equations crumble and provide impossible answers, such as imaginary distances and less than one dimension.

The biggest problem with its congruence is that, at dimensions smaller than those of Planck, general relativity predicts a certainty, a fluid surface, while quantum mechanics predicts a probability, a deformed surface; They are not compatible.Superstring theory solves this demand, commutation the classical plan of point particles with loops. These loops would have an average diameter of a Planck length, with extremely small variations, which completely ignores the predictions of quantum mechanics at dimensions smaller than those of Planck, and that for their study does not take into account those lengths.

### Falsificationism and superstring theory

The main criticism of the String Theory is that it is, fundamentally, impossible to falsify, due to its intrinsic nature: it has sufficient mathematical flexibility so that its parameters can be molded to fit with any type of observed reality. To illustrate the confusing situation that dominates this field of research, it is enough to cite the recent Bogdanov scandal, two brothers who managed to publish absurd and meaningless theories in prestigious scientific journals. The German physicist Max Niedermaier concluded that it was pseudoscience, written with a dense technical jargon, to avoid the system of peer review of theoretical physics. According to the physicist-mathematician John Baez, his work ” It is a hodgepodge of seemingly plausible phrases that contain the correct technical words in the approximately correct order. But there is no logic or cohesion in what they write.”According to physicist Peter Woit in the prestigious magazine Nature:” The Bogdanoffs’ work is significantly more incoherent than anything else published. But the growing low level of coherence across the field allowed them to think they had done something sensible and publish it.”

**M theory**

In physics, M-Theory (sometimes called U-Theory) is the proposition of a “Universal Theory” that unifies the five theories of Super Strings.Based on the work of many theoretical scientists (including: Chris Hull, Paul meliorist, Ashoke Sen, Michael Duff and John H. Schwarz), Edward Witten, of the Institute for Advanced Study, suggested the existence of the Superstrings at a conference in the USC in 1995, using the M-Theory to explain a number of previously observed dualities, giving the spark for a new investigation of string theory called the second superstring revolution.

In this theory 11 dimensions are identified, where supergravity interacts between membranes of 2 to 5 dimensions. This would evidence the existence of infinite parallel Universes, some of which would be like ours with greater or lesser differences, and others that would be unthinkable with 4 or 5 dimensions. This would explain the weakness of gravity, as the graviton particle would be the only one that could pass through all the membranes, losing its strength.

In the early 1990s, it was shown that the various theories of Super Strings were related by dualities, which allowed physicists to relate the description of an object in a SuperString theory to eventually describe a unique object from another theory. These relationships imply that every of the particle theories could be a totally different facet of one theory, planned by Witten, and known as “M-Theory”The M-Theory is not complete; However, it can be applied to many situations. The theory of electromagnetism was also in the same state in the mid-nineteenth century; there were separate theories for magnetism and electricity and, although they were known to be related, the exact relationship was not clarified until James Clerk Maxwell published his equations in his 1864 work, A Dynamic Theory of the Electromagnetic Field. Witten had suggested that a general formula of M-theory would probably require the development of a new mathematical language. Some scientists have questioned the tangible successes of the M-Theory given its incomplete status and limited predictive power even after years of intense research.

It was believed before 1995 that there were five consistent superstring theories, which are called respectively: Type I string theory, Type IIA string theory, Type IIB string theory, SO Heterotic Theory (32)(HO string), and Theory Heterotic E8× E8 (HE string).

As their names suggest, some of these string theories are related to each other. In 1990, theorists discovered that some of these relationships were so strong that they could be used as their identification. Type IIA and Type IIB string theory are connected by dual-T; this means that essentially the description of the Type IIA string theory of a circle of radius R is exactly the same in the description of circle IIB of radius 1/ R, which are distances measured in units of Planck distance.

This is a very deep result. First, it is an intrinsically mechanical-quantum result: identification is not truly classical. Second, because we can build a space by joining circles in various ways, it can be noted that any space described by String Theory IIA can also be seen as a space different from that described by Theory IIB. This means that we can identify Theory IIA with Theory IIB: any object that can be described by Theory IIA has an equivalent, although apparently different, description in terms of Theory IIB. This suggests that both Theory IIA and Theory IIB are aspects of the same theory.

**Characteristics of the M theory**

The M theory contains much more than strings.It contains each objects of bigger and lesser spatiality. These objects are called P-branes * where p denotes their dimensionality (thus,1-brane could be a string and 2-brane a membrane) or D-branes (if they are open strings). Objects of larger dimensions were always present in string theory but could never be studied before the Second Revolution of the Superstrings due to their non-disturbing nature. It has even been suggested that the Big bang was produced by the collision of two of these membranes, sprouting our Universe.

**Note: The Theory-M, conceives an organization of spheres / membranes endless but with an underlying order. For this hypothesis, holographic order, will define among others, the dynamism and / or relationships within the system.-**

**D-brana**

In theoretical physics, D-branes are a special class of P-branes, named in honor of mathematician Johann Dirichlet by physicist Joseph Polchinski. Dirichlet boundary conditions have long been used in the study of liquids and the theory of potential, where they involve specifying a certain amount along a whole border. In fluid dynamics, setting a Dirichlet boundary condition could mean assigning a known fluid velocity to all points on a surface; When studying electrostatics, Dirichlet boundary conditions can be established by setting known voltage values at particular locations, such as conductor surfaces. In any case, the locations in which the values are specified are called a D-brane.

D-branes are typically classified by their dimension, which is indicated by a number written after D. A D0-brane is a single point, a D1-brane is a line, a D2-brana is a plane, and a D25-brana fills the hyper-dimensional space considered in the Bosonic string theory.

**D-branes in string theory**

Theoretical background Most versions of string theory involve two types of string: open strings with unlinked endpoints and closed strings that form closed loops. Exploring the consequences of the Nambu-Goto action, it is clear that energy can flow along a rope, sliding to the end point and disappearing. This poses a problem: energy conservation states that energy should not disappear from the system. Therefore, a consistent string theory must include places where energy can flow when it leaves a string; These objects are called D-branes. Any version of string theory that allows open strings must necessarily incorporate D-branes, and all open strings must have their endpoints attached to these branes. For a string theorist,

All elementary particles are expected to be vibratory states of the quantum strings, and it is natural to wonder if the D-branes are made in some way with the strings themselves. In one sense, this turns out to be true: among the spectrum of particles that the vibrations of the string allow, we find a type known as tachyon, which has some rare properties, such as imaginary mass. D-branes can be imagined as large collections of coherent tachyons, in a manner similar to the photons of a laser beam.

Strings that are restricted to D-branes can be studied by means of a quantum theory of renormalizable 2-dimensional fields.

World Cosmology of Branas

This has implications in cosmology, as a result of string theory implies that the universe has a lot of dimensions than expected (26 for Bosonic string theories and 10 for superstring theories) we have to find a reason why additional dimensions do not They are obvious. One possibility would be that the visible universe is a very large D-brana that extends over three spatial dimensions. Material objects, consisting of open strings, are linked to the D-brane, and cannot move “transversely” to explore the universe outside the brane. This panorama is called a brane cosmology. The force of gravity is not due to open strings; the gravitons that carry gravitational forces are vibrating states of closed strings.

**Black holes**

Another important use of D-branes has been the study of black holes. The D-branes theory allows you to assign the quantum states of black holes.

**The branes and the bulk**

The central idea is that the visible part of our four-dimensional universe is limited to a brane within a higher dimensional space called the “bulk” or “bulk” in Spanish. The additional dimensions, compact, are rolled in a space of Calabi-Yau. In the “bulk” model, other branes may be moving through the bulk. Interactions with the Bulk, and possibly with other branes, can influence our brane universe and hence it can introduce effects not seen in more standard cosmological models.

### Explanation of the weakness of gravity

This is one of the attractive features of this theory, in which it explains why the weakness of gravity is with respect to the rest of the fundamental forces of nature, solving the so-called hierarchy problem. In the branes scenario, the other three forces of nature, electromagnetism and the weak and strong nuclear forces, are confined as ropes anchored to our 3-brane universe, differing gravity, which is thought to be like a closed rope not anchored , and therefore, much of its attractive force “leaks” or escapes the “bulk.” As a consequence, the force of gravity must appear more strongly on small scales, where less gravitational force has been “filtered.”

Models based on the cosmology of branes There are two large groups of theories based on the cosmology of branes. The first group mixes aspects of M theory with inflationary cosmology. The second group, of more recent formulation, argues the existence of a branology cosmology based on the M theory without resorting to the inflationary model. The Randall-Sundrum model (RS1 and RS2) can be adjusted to the criteria of any model of both groups.

In inflationary cosmology, the universe acquires its observable characteristics (horizon, flatness and magnetic monopoly problem) after the Big Bang, while in the ecpirotic and cyclic models the observable characteristics derive from a moment before the big bang due to a clash between branes. Cosmologist Alexander Vilenkin argues that in inflationary models time is self contained within the universe marking the Big Bang its beginning (finite time). Meanwhile the cosmologist Neil Turok proposes that in the models where there are clashes between branes, time already existed before the Big Bang (infinite time).

In models based on inflationary cosmology, an infinite ocean can be imagined that due to fluctuations in quantum physics branes form like bubbles in boiling water. In this way, Big Bangs emerge from moment to moment and some bubbles disappear and others grow (inflation) due to the same fluctuations. Figuratively each universe could be considered as a bubble (brana) swimming in an infinite ocean of boiling water (false vacuum). The formulation of this theory is strongly influenced by the interpretation of quantum mechanics by Hugh Everett

**Cyclic Model **In the models based on the collision of branes, unlike the inflationary models, each brana already existed before the big bang and the characteristics they carried before the collision are printed on the characteristics of the next universe formed after the collision. The pre-big bang, ecpirotic and cyclic models belong to this group of theories.

At the heart of modern cosmology there is a mystery: Why does our universe seem so exquisitely prepared to create the necessary conditions for life? In this ‘tour de force’ by some of the greatest discoveries of science, Brian Greene shows how the answer to the enigma can be in the amazing idea of a multiverse.

Brian Henry Graham Greene is probably the known soul of particle theory, the concept that minuscule strands of energy moving in a very higher dimensional reference frame produce each particle and force within the universe.

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