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 also predicts a particle called the tachyon possessing some unsettling properties, and it has no fermions. All of its particles are bosons, that determines the name. The physicists have also calculated that bosonic string theory requires 26 spacetime dimensions, 25 spatial dimensions and one dimension of time.
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