These notes are based on the string theory lecture course by Prof. Amihay Hanany, taught at Imperial College London in 2025. While they broadly follow the structure of the course, at times I decided to …

While this might have been a natural assumption in the early days of string theory, the perspective has changed during the last couple of decades, and D-branes (the result of considering Dirichlet …

Given an initial and final shape for the string, all possible trajectories are surfaces that describe the motion of the initial shape into the final one. Topologically these surfaces are cylinders. To each of …

String theory builds on the conceptual foundations that have been established in elementary particle physics in the 1970s. It is a quantum theory that aims at reproducing the interaction and symmetry …

Professor Zwiebach’s “A First Course in String Theory” is the main textbook for this course, which we’ll loosely follow (and have assigned readings and problems taken from).

As a basis for such “string-string scattering”, we will need to quantize a single free string, cf. Fig. 5. After quantization, one finds a discrete set of vibrational modes with different masses

Exercise: Assuming the string world sheet to be timelike, check that we can impose the boundary condition (3.4) on any closed string, while keeping the coordinate condition