## Definition A **photon** is the quantum (the elementary particle) of the electromagnetic field — the indivisible packet of energy and momentum that light, radio waves, X-rays, and all other forms of electromagnetic radiation consist of. Photons are massless, travel at the speed of light $c$, and are the carriers of the electromagnetic force between charged particles. ## Discovery The concept was introduced by Albert Einstein in 1905, in the same year as his papers on Special Relativity and Brownian motion. Einstein analysed several phenomena — fluorescence, the photoelectric effect, and thermal radiation — that could not be explained by treating light as a continuous wave. He proposed that: > "The energy of a ray of light is not distributed continuously through space but consists of a finite number of energy quanta localised at points, which move without dividing and are absorbed or produced only as complete units." This paper, for which Einstein received the Nobel Prize in Physics (1921), established the photon as real. Max Planck had already used discrete energy packets as a mathematical convenience in 1900, but Einstein was the first to assert their physical reality. ## Energy and Momentum The energy of a photon is proportional to its frequency $\nu$: $ E = h\nu = \frac{hc}{\lambda} $ where $h$ is Planck's constant and $\lambda$ is the wavelength. Its momentum is: $ p = \frac{h}{\lambda} = \frac{E}{c} $ ## The Photon as a Quantum of a Field In the language of Quantum Field Theory, the photon is not a classical particle but an *excitation* (a quantum) of the electromagnetic field — the field first conceived by Faraday and described mathematically by Maxwell. This view connects the photon to the broader framework of the [[Standard Model of Particle Physics]], in which every fundamental force is mediated by carrier particles (gauge bosons): the photon carries the electromagnetic force, gluons carry the strong force, and the W/Z bosons carry the weak force. ## Wave–Particle Duality In quantum mechanics, a photon is neither purely a wave nor purely a particle. It exhibits wave-like interference and diffraction; it also arrives and departs as an indivisible lump of energy. This duality is not paradox but the signature of the quantum regime. ## Role in Atomic Physics Bohr's model of the atom (1913) explained atomic spectra by positing that electrons occupy quantised energy levels and emit or absorb exactly one photon each time they jump between levels. The photon energy $h\nu$ equals the energy difference between levels. This was the first quantitative success of the quantum hypothesis after Einstein's 1905 paper. ## Related - [[Quantum Mechanics]] - [[Standard Model of Particle Physics]] - [[General Relativity]] ## Sources - [[Seven Brief Lessons on Physics (Rovelli 2014)]]