This book describes the changing views of the physics community toward photons, and how photons are viewed today in several contexts. The first portion, a ninechapter Memoir with few equations and many definitions, explains the changing view of physicists toward radiation and its wave-particle photons, written for those with interest but possibly without technical background. It gives operational definitions that have been used for photons and their association with quantum-state manipulations that include Quantum Information, astronomical sources and crowds of photons, the boxed fields of cavity Quantum Electrodynamics It defines, qualitatively, the historical photons of Planck, Einstein, Compton, and Bohr, the later photons of Dirac, Feynman, and Glauber, and the photon constituents of the Standard Model of Particle Physics. It points to contemporary photons as causers of change to atoms, as carriers of messages, and as subject to controllable creation and alteration. A second portion, of three tutorial appendices, explains the mathematical background of quantum theory and radiation needed by those whose profession involves photonics and who therefore want more detailed understanding of the Memoir portion: quantum theory and the Schrodinger equation for quantum-state manipulation; Maxwell equations for electromagnetism with wave modes that become photons through a quantization postulate, possibly exhibiting quantum entanglement; and the coupling of atoms and fields to create quasiparticles that are seen as slow and stored light pulses. As with other Memoirs, the present book has idiosyncrasies of the author. Most notably, on the opening page of each chapter, and at the end of the book, is a cartoon drawn by the author, as a grad student, that reflects the changing views of a PhD aspirant toward the grad school experience as he progressed through the graduate school of MIT in the 1950s.
