Photons Under the photon theory of light, a photon is a discrete bundle (or quantum) of electromagnetic (or light) energy. It the basic "unit" of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force. Like all elementary particles, photons are governed by quantum mechanics and will exhibit wave-particle duality – it exhibits properties from both waves and particles. For example, a single photon may be refracted by a lens or exhibit wave interference, but also act as a particle giving a definite result when its location is measured. The photon is an elementary particle, despite the fact that it has no mass. It cannot decay on its own, although the energy of the photon can transfer (or be created) upon interaction with other particles. Photons are electrically neutral and are one of the rare particles that are identical to their antiparticle, the antiphoton. Photons are spin-1 particles (making them bosons), with a spin axis that is parallel to the direction of travel (either forward or backward, depending on whether it's a "left-hand" or "right-hand" photon). This feature is what allows for polarization of light. Photons are always in motion and, in a vacuum, have a constant speed to all observers. The vacuum speed of light (more commonly just called the speed of light) is denoted by the letter "c" and is 299,792,458 m/s. According to the photon theory of light, photons have the following properties: They move at a constant velocity, c = 299,792,458 m/s (i.e. "the speed of light"), in free space. They have zero mass and rest energy. They can be destroyed/created when radiation is absorbed/emitted. They can have particle-like interactions (i.e. collisions) with electrons and other particles, such as in the Compton effect. Any single photon has a fixed, discrete energy level. Each color of light has its own unique energy level. It is not possible to increase or decrease the energy of that single photon without changing its wavelength, or else absorbing it completely and thereby ending its existence. The intensity of visible light can be increased or decreased only by changing the number of photons present. The same rules hold true for electromagnetic phenomena outside the visible range. They carry energy and momentum, which are also related to the frequency, , and wavelength, , of the electromagnetic wave by E = h = hc/, and p = h/. "h" is the symbol for Planck's Constant, which has a value of 6.6260689633 E-34 Joule*sec Why is the speed of light what it is? The speed of electromagnetic radiation through any medium is a measure of its permittivity (capacitance per unit volume) and permeability to a magnetic field. The vacuum has values of both permeability and permittivity and this effectively defines the velocity of light. Permittivity is the reaction to electrical field stress which is the potential energy part of the equation. Permeability is the kinetic energy in the magnetic field. Said in another way: if you could modify the electric and magnetic properties of the void, you would modify light's speed in it. The void's energy is still a very controversial issue (virtual particles, dark energy, etc); it has been suggested that between two very near metal plates, the Casimir Effect is due to a decrease in the energy of the void (less virtual photons), so light's speed should increase there. History of Photons The term photon was coined by Gilbert Lewis in 1926, though the concept of light in the form of discrete particles had been around for centuries and had been formalized in Newton's construction of the science of optics. In the 1800s, however, the wave properties of light (by which I mean electromagnetic radiation in general) became glaringly obvious and scientists had essentially thrown the particle theory of light out the window. It wasn't until Albert Einstein explained the photoelectric effect and realized that light energy had to be quantized that the particle theory returned. A cosmological view: "If the symmetry transformations of the bifurcated Vacuum are balanced, the charge flow among them can be visualised in the form of the still surface of a three dimensional toroid, formed by a balanced toroidal plus central rotation - it is the pure symmetry recipe of photon creation." (Click over the toroid to tilt for different views.) Courtesy of Frank van den Bovenkamp at www.heartcoherence.com