Photon

Photon (pronounced foh-ton)

The subatomic particle that carries the electromagnetic force and is the quantum of electromagnetic radiation; has a rest mass of zero, but has measurable momentum, exhibits deflection by a gravitational field, and can exert a force.  It has no electric charge, has an indefinitely long lifetime, and is its own antiparticle.  It is the quantum (a bundle of energy) in which light and other forms of electromagnetic radiation are emitted.

1916: A compound word phot(o)- +‎ -on.  Photo is from the Ancient Greek combining form φωτω- (phōtō-) from φῶς (phôs) (light).  The –on suffix is used often in science, in physics (eg electron) and in chemistry (eg carbon) and is from the Ancient Greek -ον (-on), used when ending neuter nouns and adjectives.  In physics, mathematics and biology, it forms nouns denoting subatomic particles (proton), quanta (photon), molecular units (codon), or substances (interferon).  In biology and genetics, it’s applied to form names of stuff considered basic or fundamental units such as codon.  In chemistry, it’s used to form names of noble gases and certain nonmetal elements such as boron or silicon.  Photon was coined in 1916 by US physicist Leonard Troland (1889-1932) as a unit of light hitting the retina but the word was little used until the 1920s when a number of scientific papers were published.  Although the findings of some of the early experiments were later disproved, the word was soon adopted by most physicists.  Photon is a noun and photonic an adjective; the noun plural is photons.  

The light quantum

The photon is also called the light quantum.  The Latin word quanta (how much; the singular form is quantum) was used in the nineteenth century to mean particles or as a measure of quantity, the latter meaning persisting in general use.  What light was actually made from was, until the early twentieth century, one of the fundamental arguments in physics; the dispute essentially whether light was a wave or a particle.  Albert Einstein (1879-1955) published his theory in a 1905 paper which described electromagnetic waves as “…spatially localized, discrete wave-packets” which he labeled das lichtquant (the light quantum).  Light has special characteristics: apart from being creation’s universal speed limit, photons have a unique property in that they are both a particle and a wave, almost certainly have no mass and carry no charge.

JPL (NASA's Jet Propulsion Laboratory) used the properties of the photon to illustrate the weird world of quantum theory.

Photons underlie Einstein’s theory of relativity.  Travelling at the speed of light (and light can be slowed-down or stopped), if a photon could visualize, the entire universe would appear as a two-dimensional timeless plane which is completely still.  However, a photon can’t visualize or even experience anything because, for a proton, time doesn't pass, physicists labeling such things null geodesics.  A photon moves from its start to its end: An interaction creates (or emits) it until another interaction destroys (or absorbs) it.  These two things, creation and destruction, are all that can happen to a photon and neither can be experienced because there’s no time; what photons do can be measured but the frame of reference is wholly external.  In an inertial reference frame, there are physical laws which don't depend on the motion of anything external to the system yet for a photon, the physical rules it obeys depend exclusively on everything which happens external to it.

Weirder still, because protons lack mass, a photon cannot visualize the rest of the universe because seeing requires interacting with other particles, antiparticles, or photons, and, once such an interaction occurs, that photon's journey is over, its destruction as instantaneous as its creation.  Thus the fundamental importance of time to the existence of three-dimensional space; were there no time, everything would happen at once.