Introduction

How do we describe the state of a quantum mechanical physical system in a

mathematically precise way? How do we ascribe physical properties to this

system? How does the system evolve in time? How do its properties change

when it interacts with another system or a force? How do we measure and

determine its properties? These are the questions whose answers are encoded

in the laws of quantum mechanics to be set down in this chapter. They have

been inscribed in this form after nearly half a century of experimental obser-

vations, theoretical modeling, and intellectual gymnastics to tie up the two in

a satisfactory and robust structure.