Photon

A photon is an elementary particle which acts as the force carrier for electromagnetism; as such, it is the quantum of all electromagnetic radiation. Photons exhibit wave-particle duality, as do all elementary particles. Since they are massless, they travel at the speed of light (c), which is related to wavelength and frequency by
 c = \lambda f 
==Notes==
In the absence of charge disturbance, it is necessary to invent an additional sort of photon which differs from those that cause the sensation of Light(au17b)
The starlight photons are far fewer than the primal heat photons(au99)
There are many, many more photons in the Universe than there are Atoms, and almost all of the entropy of the Universe is in those cold photons of the background radiation(cmn170)
Chalk is white because the molecules of which it is composed do not happen to have any state that is particularly easy to jump to by absorbing photons of any color of visible light(dft23) 
Photons are created and destroyed whenever Light is emitted or absorbed, without any significant change in the Mass of either the emitter or the absorber(fp4)
A single photon has the capacity to provide an endless stream of information(fp35)
Electrons cannot be ascribed well-defined paths in space, and even the time order in which the photon is emitted and absorbed is imprecise. The messenger photons thus have a sort of ghostly fleeting existence. To distinguish them from the permanent sort that we experience directly through the sensation of sight, the messenger photons are referred to as "virtual."(mm238)
A metal plate, which reflects light, will also reflect the evanescent virtual photons in the Vacuum. By trapping them between two parallel plates their energy can be altered slightly, leading to a measurable force on the plates(ow88)
Seven photons are the minimum required for a dark-adapted human subject to be aware of their impingement(pi196)
Because their phases are entangled with each other, each photon's phase depends on what the other photon is doing. Consequently, neither photon by itself is represented by a definite waveform; hence (according to quantum theory) neither photon possesses a definite polarization(qr202)
What we see on the screen is an interference pattern- indicating that the photon has interfered with its own possibility of arriving sooner or later. The mere possibility of the photon’s arriving sooner or later has a kind of temporal reality that smears itself out across time (QS62)
Photons have no Mass(sk6)
Photons are not conserved(sk88)
If a photon has enough energy, it can actually turn itself into a pair of Electron-like particles(sk100)
In the 1990s experimenters have come up with even more subtle experiments that show individual photons behaving as both Particle and Wave at the same time(sk114)
To make a genuine single photon, a single Atom has to be triggered into emitting just one pulse of Energy in a single transition between well-defined energy levels(sk116)
Single photons cannot be split in half(sk120)
Physicists have dreamed up an experiment (not carried out by1994) which will, if quantum theory is correct, show a single photon in two different places at the same time. The fact that an individual photon arriving at one detector simultaneously changes the probability of a virtual photon appearing out of nothing at all at a second detector on the other side of the experiment gives the illusion, when both detectors click together, that the single photon has arrived in two places at once. The original photon is detected in only one place, but its influence affects what is happening somewhere else at the same time(sk122)
Thanks to quantum uncertainty, even the inside of a photon must now, it seems, be thought of as a seething mass of Particles(sk130)
Experiments have shown that the photons indeed behave as if they were complex entities, made up of an interior jumble of quarks, electrons and other particles(sk131)
The energy of a photon is determined by its frequency. But the frequency seen by the atom depends on how fast and in which direction it is moving. This is the optical analog of the familiar Doppler effect(sm50)
Once absorbed, the photon vanishes and the electron in the atom is promoted to a state of higher energy. The quantum theory of light predicts that the higher energy state is unstable and that the electron must decay back to the ground state, a process known as spontaneous emission. As it decays the atom emits a photon with an energy corresponding to the change in electronic energy, but in an arbitrary direction, not necessarily in the same direction as the original absorbed photon. Whatever the direction the atom chooses to emit the photons it must recoil in the opposite direction(sm52)
It is impossible to increase the overall number of electrons in the universe- whereas bosons, as anyone who has ever switched a light on knows, can be manufactured in vast quantities(ssc99)
Photons are crucially important in this description of particle behavior, because they act as the carriers of the electric force(ssc186)
The photon's track on a Feynman diagram has no arrow on it not only because the photon is its own antiparticle, but because motion through time has no meaning for the photon- and that is why it is its own antiparticle(ssc190)
Although a photon has no mass, it does carry Momentum - if it didn't, it would not be able to knock electrons out of a metal surface(sss24)
De Broglie's version of the equation said that, for a photon, momentum is equal to Planck's Constant divided by wavelength. This directly relates the particle nature of the photon (momentum) to its wave character (wavelength), using P1anck's constant. But why, said de Broglie, stop there? Electrons have momentum, and we know the value of planck's constant. Rearrange the equation again and we get a relation telling us that wavelength is equal to h divided by momentum. In other words, any particle, such as an electron, is also a wave, and its wavelength depends only on its momentum and on P1anck's constant. For everyday particles, the mass involved, and therefore the momentum, is so big, compared with h, that the wave nature of matter can be ignored. For everyday objects, h divided by momentum is effectively zero. But for electrons, each with a mass of only 9 x 10 to the -28 grams, the numbers are more nearly in balance, and the wave aspect becomes significant(sss25)
You cannot chop a photon in half. It is the fundamental, basic unit of Energy(sss39)
The photon, which has spin one, and all particles with integer spin are known as bosons; so there is a fundamental difference between photons and electrons(sss48)
We can make photons just by turning on a light; they vanish again when they are absorbed by atoms and give up their energy(sss49)
Category:Elementary Particles