Photon

A photon is an elementary particle, the quantum of the electromagnetic field including electromagnetic radiation such as light, and the force carrier for the electromagnetic force (even when static via virtual photons). The photon has zero rest mass and is always moving at the speed of light.

Quotes

 * The theory of the light quantum, which Einstein initiated in 1905 and which is with us still, is certainly the most revolutionary development in the history of physics and arguably in the history of science. The creators of the theory, men such as Einstein, Niels Bohr, Werner Heisenberg, Erwin Schrödinger, Wolfgang Pauli, and Paul Dirac, were often struck by the apparent "absurdity"—the utterly noncommonsensical aspects—of the world depicted by the quantum theory.
 * Jeremy Bernstein, Quantum Profiles (1991), "John Stewart Bell: Quantum Engineer"


 * Light is something like raindrops—each little lump of light is called a photon—and if the light is all one color, all the "raindrops" are the same.
 * Richard Feynman, QED: The Strange Theory of Light and Matter (1985), p. 14.


 * My favorite deep, elegant and beautiful explanation is Albert Einstein's 1905 proposal that light consists of energy quanta, today called photons. Actually, it is little known, even among physicists, but extremely interesting how Einstein came to this position. It is often said that Einstein invented the concept to explain the photoelectric effect. Certainly, that is part of Einstein's 1905 publication, but only towards its end. The idea itself is much deeper, more elegant and, yes, more beautiful.
 * Anton Zeilinger, Einstein's Photons (2012)


 * At the first of the 1960's Rochester Coherence Conferences, I suggested that a license be required for use of the word photon, and offered to give such license to properly qualified people.
 * W. E. Lamb, Anti-photon, Appl. Phys. B 60, 77-84 (1995).


 * All the indistinguishable photons illuminate the array of N slits, or grating,  simultaneously. If only one photon propagates, at any given time, then that individual photon illuminates the whole array of N slits simultaneously.