From: Zardoz ® 03/11/2001 8:27:47
Subject: QUANTUM MECHANICS FAQ post id: 482674

LET THERE BE LIGHT


For Kids…

Could someone give a description of a 'photon'.

From: Chris (Avatar)
Newton was the first to come up with the "corpuscular theory of light" - he thought light travelled as a stream of tiny particles. In the last century diffraction experiments sought to show that light was definitely a wave. Maxwell's equations describe light classically as a wave.

Then Einstein published a paper on the photoelectric effect showing that the effect could only be explained by considering light as a particle. Now we know that light can exhibit the characteristics of either a particle or a wave, and when it is acting like a particle, it's called a photon.

Brief review of quantum field theory (QFT): there are four fundamental interactions: electromagnetism (e/m), weak and strong nuclear, and gravity. Light is a type of e/m field, as is gamma radiation, X-Rays, UV, infrared, microwave, radio waves. Each field is described in quantum theory as having a smallest indivisible "chunk", called the field's "quantum". The quantum of the e/m field is the photon. The photon is real when carrying radiation in one of the field guises - ie a light photon, a gamma ray photon, etc. There is also a virtual photon which is exchanged by particles interacting via e/m. Eg the electromagnetic repulsion of two electrons is occasioned by the exchange of virtual photons between the two electrons.

The photon has zero rest mass and zero electric charge. It has no colour charge (QCD) and no weak charge. It has a quantum spin of 1 (it's a boson). It has a characteristic frequency according to which type of photon it is (highest frequency for gamma rays, lowest for long radiowaves). It has a discrete energy proportional to its frequency, and momentum propotional to its frequency as well. It has a characteristic wavelength inversely proportional to its frequency.