This work is a short introduction to string theory from very basic physical principles.

We present the foundations upon which bosonic string theory for closed strings is built. We then specialize the theory to the case of open strings, highlighting the differences between the two types of strings and introducing the concept of D-branes.

Finally, we provide a brief introduction to superstring theory, which offers a much more promising candidate for a theory of everything by resolving the problems that arise in bosonic string theory.

If you want to read it completely you can find it below:

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