Bosonic string theory
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Bosonic string theory is the original version of string theory, developed in the late 1960s. Although it has many attractive features it has a pair of features that render it unattractive as a physical model. 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 energy is negative implying that it travels faster than light. The existence of such a particle, commonly known as a tachyon, 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. In a spacetime of 26 dimensions, however, 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. This would leave only the familiar four dimensions of spacetime visible.
In the early 1970s, supersymmetry was discovered in the context of string theory, and a new version of string theory called superstring theory (supersymmetric string theory) became the real focus. Nevertheless, bosonic string theory remains a very useful "toy model" to understand many general features of perturbative string theory, and string theory textbooks usually start with the bosonic string. The first volume of Polchinski's String Theory and Zwiebach's A First Course in String Theory are good examples.
See also Polyakov action, Nambu-Goto action
