Abstract: This introductory chapter outlines the differences between classical physics and quantum physics by positing three imaginary worlds. World 1 is governed by the laws of classical Newtonian mechanics, World 2 by classical field physics, and World 3 by quantum physics. The point is that classical physics deals with well-defined, continuously-existing physical quantities, including trajectories, while quantum physics is fundamentally about physical quantities defined in terms of probability functions.

Keywords: particles, trajectories, classical physics, quantum physics, probability function,