# String Theory

Fall, 2010

In this set of lectures Professor Susskind gives an introduction to String Theory, which he describes as a mathematical framework for theories that unify all the forces of nature, including gravity.    In string theory, fundamental objects are no longer point particles; instead they are strings or higher dimensional objects called D-branes.   These objects also require additional ingredients such as extra spatial dimensions.  (Image source: Wikipedia)

## Lectures in this Course

1. 1

### The historical origins of string theory

In the first lecture of the series Professor Susskind explains the historical origins of string theory.   Hadrons are observed to come in angular momentum sequences where a plot of angular momentum against mass squared is a straight line.  These... [more]
2. 2

### Mathematics of string motion

Professor Susskind establishes the mathematical foundation for solving the equations of motion for strings.     Breaking a string into discrete mass points, he substitutes a Fourier series for the position, generating a set of independent solutions... [more]
3. 3

### The energy spectrum of strings

This lecture develops an algebraic approach to the energy spectrum of strings.  Raising and lowering operators are associated with the modes of the strings.  The lecture finishes with the basics of string interactions.
4. 4

### Closed strings and the level matching rule

This lecture starts with a review of Noether’s theorem, which links continuous symmetries with conserved quantities (charges).   The model of a closed string is developed following the earlier procedure for open strings.   The energy spectrum is... [more]
5. 5

### Bosonic strings

Professor Susskind examines the ground state of bosonic strings more closely, returning to the tachyon problem from lecture 3.  The resolution of this problem ends up requiring the introduction of additional dimensions in which the string can... [more]
6. 6

### Strings with spin

Spin is introduced in the mass points that strings are built from, creating strings that behave like bosons and fermions.   Then Professor Susskind examines the scattering characteristics of strings and shows that the Veneziano amplitude from... [more]
7. 7

### Fermionic strings and path integrals

Fermionic strings with Fermionic mass points are introduced.   Now there are both spacelike oscillations as well as spin oscillations.    The notion of a world sheet, traced out by a string over time, is introduced.   Path integrals are defined with... [more]
8. 8

### Conformal mapping and string scattering

Professor Susskind develops the concept of conformal mapping, which can be used to dramatically simplify the solution of the 2 dimensional Laplace equation encountered in Lecture 7.   In this case the 2 dimensions correspond to the world sheet... [more]
9. 9

### Strings in compact dimensions

This lecture explores the consequences of string excitations in compact dimensions.  Compact dimensions lead to quantized momentum and also to quantized energy levels due to stretching around the compact dimensions.  The duality of the momentum and... [more]
10. 10

### T-duality, D-branes and modeling field theories

Professor Susskind explains the behavior of open and closed strings in periodic dimensions.   The endpoints of open strings are seen to be stuck on surfaces known as D-branes.   The behavior of these open strings can model more traditional field... [more]
11. 11

### String theory wrapup

Professor Susskind starts with the philosophy of reductionism, where complex objects are broken down in to larger numbers of smaller objects with more fundamental rules of behavior.   While this philosophy has served physics well,  Professor... [more]