  The wave-particle duality is the main point of demarcation between quantum
and classical physics, and is the quintessential mystery of quantum mechanics.
Young's two-slit interference experiment is the arch prototype of actual and
gedanken experiments used as a testing ground of this duality. Quantum
mechanics predicts that any detector capable of determining the path taken by a
particle through one or the other of a two-slit plate will destroy the
interference pattern. We will examine both the experimental and theoretical
attempts to test this assertion, including a new kind of experiment, and to
grasp the underlying truth behind this mystery from the earliest days to the
present. Where positions differ, the views of both sides are presented in a
balanced approach.
