Superstring theory

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

One of the motivations put forward by superstring theorists is that schema is one of the best candidate theories for formulating a quantum theory of gravity. Superstring theory is a shorthand for supersymmetric string theory because, unlike bosonic string theory, this is the version of string theory that, by supersymmetry, incorporates fermions.

Superstring theory comprises five alternative theories or formulations of combined string theories, in which supersymmetry requirements have been introduced. The name string theory is currently used synonymously, since all widely studied string theories are, in fact, superstring theories.

The fundamental idea is that they are actually strings that vibrate in resonance at a frequency of Planck’s length and where the graviton would be a spin 2 string with zero mass.

The five existing theories would only be particular limit cases of this unified theory, provisionally named as Theory M. This theory M tries to explain all the existing subatomic particles at the same time and unify the four fundamental forces of nature. It defines the universe formed by a multitude of vibrating strings, since it is a version of string theory that incorporates fermions and supersymmetry.

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


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  • 1 Background
  • 2 The space-time dimensions
  • 3 Number of superstring theories
  • 4 Integrating general relativity with quantum mechanics
  • 5 See also
  • 6 Sources


The underlying problem in theoretical physics is to harmonize the theory of general relativity, where gravitation and large-scale structures (stars, galaxies, clusters) are described, with quantum mechanics, where the other three fundamental forces 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 renormalization techniques to remove those infinities from three of the four fundamental forces – electromagnetism, strong nuclear, and weak nuclear – but not from gravity. The development of the quantum theory of gravity must, therefore, come in a different way than that used for the 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 has a unique resonance, or harmony. Different harmonies determine different fundamental forces. The tension in the rope is of the order of Planck’s forces (1044 N). The graviton (proposed name for the particle carrying the gravitational force), for example, is predicted by theory to be a chord with zero amplitude. Another key idea of ​​the theory is that measurable differences cannot be detected between strings that recapitulate on small dimensions in themselves and many that move on large dimensions (eg affecting a dimension of size R equal to one of size 1 / R ). Singularities are avoided because the observable consequences of the “great collapse” never reach size zero. In fact the universe can start a small “big crash” of processes, string theory says that the universe can never be smaller than the size of a string, at that point it could start to expand.

The space-time dimensions

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

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 than usual should be made. This raises questions that these ‘extra numbers’ can be directly investigated in any experiment (where results in 1, 2, 2 + 1 dimensions would be shown to human scientists). Thus, in turn, the question arises whether these types of models investigated in this abstract modeling (and potentially impossible experimental apparatus) can be considered ‘scientific’.

A theory that generalizes it is the branes theory, where the strings are replaced by elemental constituents of the “membrane” type, hence its name. The existence of 10 dimensions is mathematically necessary to avoid the presence of tachyons, particles faster than light, and “ghosts”, particles with zero probability of existence.

Amount 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 limiting cases of a single theory: the M theory.

The five consistent superstring theories are:

  • Type I string theory has supersymmetry in the ten-dimensional sense (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 strings with closed orientations.
  • Type II string theory has two 10-dimensional supersymmetries (32 superloads). There are in fact two types of Type II ropes called Type IIA and IIB. They differ mainly in the fact that IIA theory is non-chiral (preserving parity), while IIB theory is chiral (violating parity).
  • Heterotic string theory is based on a peculiar hybrid of a Type I superstring and a bosonic string. There are 2 types of heterotic chords that differ in their ten-dimensional gauge group: the E8 × E8 heterotic chord and the SO (32). (The heterotic name SO (32) is somewhat inaccurate in the SO (32) of the Lie Group, the theories are a ratio of Spin (32) / Z2 that is not equivalent to SO (32).)

Chiral gauge theories may be inconsistent in their anomalies. This occurs when a loop in the Feynman Diagram causes a break in the quantum mechanics of gauge symmetry. Nulling 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 space-time where quantum mechanics is reserved for atomic-scale scenarios (small regions of space-time). The two are very rarely 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 should be used in sync 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 Planck’s, general relativity predicts a certainty, a fluid surface, while quantum mechanics predicts a probability, a deformed surface; they are not compatible. Superstring theory meets this requirement, replacing the classical idea of ​​point particles with loops. Those loops would have an average diameter of a Planck length, with extremely small variations, that completely ignores the predictions of quantum mechanics at dimensions less than Planck’s, and that for its study does not take into account those lengths.


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