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Bosonic string theory is the original version of string theory
String theory

String theory is a developing branch of theoretical physics that combines quantum mechanics and general relativity into a quantum gravity. The String s of string theory are one-dimensional oscillating lines, but they are no longer considered fundamental to the theory, which can be formulated in terms of points or surfaces too....
, developed in the late 1960s
1960s

The 1960s list of decades were the years from the start of 1960 to the end of 1969. The term also refers to an era more often called The Sixties, denoting the complex of inter-related cultural and political trends in the west, particularly United States, United Kingdom, France, Canada, Brazil, Australia, Spain, Italy, and Ger...
. Although it has many attractive features, it has a pair of features that render it unattractive as a physical model
Model (physical)

A physical model is a smaller or larger physical copy of an object. The object being modelled may be small or large .The geometry of the model and the object it represents are often similar in the sense that one is a rescaling of the other; in such cases the Scale is an important characteristic....
. Firstly it predicts only the existence of bosons whereas we know many physical particles are fermions. Secondly, it predicts the existence of a particle whose mass is imaginary
Imaginary number

In mathematics, an imaginary number is a complex number whose square value is a real number not greater than zero. The imaginary unit, denoted by i or j, is an example of an imaginary number....
 implying that it travels faster than light.






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Bosonic string theory is the original version of string theory
String theory

String theory is a developing branch of theoretical physics that combines quantum mechanics and general relativity into a quantum gravity. The String s of string theory are one-dimensional oscillating lines, but they are no longer considered fundamental to the theory, which can be formulated in terms of points or surfaces too....
, developed in the late 1960s
1960s

The 1960s list of decades were the years from the start of 1960 to the end of 1969. The term also refers to an era more often called The Sixties, denoting the complex of inter-related cultural and political trends in the west, particularly United States, United Kingdom, France, Canada, Brazil, Australia, Spain, Italy, and Ger...
. Although it has many attractive features, it has a pair of features that render it unattractive as a physical model
Model (physical)

A physical model is a smaller or larger physical copy of an object. The object being modelled may be small or large .The geometry of the model and the object it represents are often similar in the sense that one is a rescaling of the other; in such cases the Scale is an important characteristic....
. Firstly it predicts only the existence of bosons whereas we know many physical particles are fermions. Secondly, it predicts the existence of a particle whose mass is imaginary
Imaginary number

In mathematics, an imaginary number is a complex number whose square value is a real number not greater than zero. The imaginary unit, denoted by i or j, is an example of an imaginary number....
 implying that it travels faster than light. The existence of such a particle, commonly known as a tachyon
Tachyon

A tachyon is any hypothetical particle physics that travels faster-than-light. The first description of tachyons is attributed to German physicist Arnold Sommerfeld; however, it was George Sudarshan, Olexa-Myron Bilaniuk, Vijay Deshpande and Gerald Feinberg that advanced a theoretical framework for their study....
, would conflict with much of what we know about physics, and such particles have never been observed.

Another feature of bosonic string theory is that in general the theory displays inconsistencies due to the conformal anomaly
Conformal anomaly

Conformal anomaly is an anomaly i.e. a quantum phenomenon that breaks the conformal symmetry of the classical theory.In string theory, conformal symmetry on the worldsheet is a local Weyl symmetry and must therefore cancel if the theory is to be consistent....
. But, as was first noticed by Claud Lovelace, in a spacetime of 26 dimensions, with 25 dimensions of space and one of time, the inconsistencies cancel. Another way to look at this is that in general bosonic string theory predicts unphysical particle states called 'ghosts'. In 26 dimensions the no-ghost theorem predicts that these ghost states have no interaction whatsoever with any other states and hence that they can be ignored leaving a consistent theory. So bosonic string theory predicts a 26 dimensional spacetime. This high dimensionality isn't a problem for bosonic string theory because it can be formulated in such a way that along the 22 excess dimensions, spacetime is folded up to form a small torus
Torus

In geometry, a torus is a surface of revolution generated by revolving a circle in three dimensional space about an axis coplanar with the circle, which does not touch the circle....
. This would leave only the familiar four dimensions of spacetime visible.

In the early 1970s, supersymmetry
Supersymmetry

In particle physics, supersymmetry is a symmetry that relates elementary particles of one Spin to another particle that differs by half a unit of spin and are known as superpartners....
 was discovered in the context of string theory, and a new version of string theory called superstring theory
Superstring theory

Superstring theory is an attempt to explain all of the Elementary particle and fundamental forces of nature in one theory by modelling them as vibrations of tiny supersymmetry strings....
 (supersymmetric string theory) became the real focus. Nevertheless, bosonic string theory remains a very useful "toy model
Toy model

In physics, a toy model is a simplified set of objects and equations relating them that can nevertheless be used to understand a mechanism that is also useful in the full, non-simplified theory....
" to understand many general features of perturbative string theory, and string theory textbooks usually start with the bosonic string. The first volume of Polchinski
Joseph Polchinski

Joseph Polchinski is a physicist working on string theory. He graduated from Canyon del Oro High School in Tucson, Arizona in 1971, obtained his B.S....
's String Theory and Zwiebach's A First Course in String Theory are good examples.

See also

  • Nambu-Goto action
    Nambu-Goto action

    The Nambu-Goto action is the simplest invariant action in bosonic string theory. It is the starting point of the analysis of string behavior, using the principles of Lagrangian mechanics....
  • Polyakov action
    Polyakov action

    In physics, the Polyakov action is the two-dimensional action of a conformal field theory describing the worldsheet of a string in string theory....


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