Introduction

This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book discusses many of the methods used on the simple example of spin diffusion in liquid He and the normal classical one-component fluid as a specific application. It provides general properties of time-dependent correlation functions and contains a brief application to the Brownian motion of a heavy particle which is immersed in a fluid of light particles. Application of these methods to the normal fluid is easily distilled from our later treatment of a superfluid. The book explores the general Bogoliubov inequalities and Goldstone theorems. It describes the long-ranged magnetic order and ferromagnetic and antiferromagnetic spin waves and deals with liquid crystals, particularly with the orientational light scattering modes in nematics. The book presents certain aspects of superconductors and of charged systems in general.