Lecture 3 | String Theory and M-Theory
Why Physics Stack Exchange recommends it
Physics Stack Exchange users link it when answering questions about string interactions, tachyons, and how massless particles arise in string theory, citing Susskind's accessible treatment.
Recommended by 1 people across 1 different months, 2012 to 2022.
What it is
Lecture 3 of Leonard Susskind's Stanford Continuing Studies course on string theory and M-theory. It reviews harmonic oscillators, the spin of massless particles like photons and gravitons, the low-lying string spectrum, the tachyon problem, and the basics of string interactions.
What people said
Recommends Dilaton · 2012-02 · Physics Stack Exchange · on “What happens when two strings collide?”
Prof Susskind talks a bit about string interactions at the end of this lecture: [this video] The whole course is quite accessible, easy to understand, and gives a (only slightly technical) good introduction.
Cites Leandro M. · 2015-02 · Physics Stack Exchange · on “Do tachyons move faster than light?”
A tachyon is a particle with an imaginary rest mass. This however does not mean it "travels" faster than light, nor that there's any conflict between their existence and the special theory of relativity. The main idea here is that the typical intuition we have about particles -- them being billiard ball-like objects -- utterly fails in the quantum world. It turns out that the correct classical limit for quantum fields in many situations is classical fields rather than point particles, and so you must solve the field equations for a field with imaginary mass and see what happens rather than just naively assume the velocity will turn out to be faster than light. The mathematical details are a
Cites Mitchell Porter · 2022-05 · Physics Stack Exchange · on “How does massless particles arise in string theory when it starts with the assumption that all strings are composed of massive little pieces?”
Susskind gets to masslessness at 54:10 in third lecture [this video] Leading up to that he focuses on motion of the string's center of mass, that is the only degree of freedom which matters regarding mass of the ground state, everything else is an excitation