photon

photon
Name	photon
Caption	Relationship between Planck's constant, frequency, and energy for a photon.
Statistics	Boson
Group	Gauge boson
Interaction	Electromagnetism
Theorized	Albert Einstein (1905)
Discovered	Arthur Holly Compton (1923)
Mean lifetime	Stable
Parity	-1
C parity	-1
Composition	Elementary particle

photon. The photon is the fundamental quantum of light and all other forms of electromagnetic radiation. It is a massless elementary particle that mediates the electromagnetic force and travels at the speed of light in a vacuum. As a gauge boson, it is the carrier of electromagnetic interaction and exhibits both wave–particle duality and quantum entanglement.

Properties

The photon has zero rest mass and zero electric charge, which allows it to travel indefinitely through empty space at the constant speed of light denoted by c. Its energy is directly proportional to its frequency, as defined by the relation involving Planck constant, while its momentum is inversely proportional to its wavelength. The intrinsic angular momentum of a photon, known as its spin, has a magnitude of one in units of ħ, and it displays a property called helicity. Photons obey Bose–Einstein statistics, classifying them as bosons, and they do not decay spontaneously. Their behavior is governed by the principles of Maxwell's equations in the classical limit and by quantum electrodynamics in the quantum realm.

History

The concept of light as composed of discrete particles was debated for centuries, with Isaac Newton advocating a corpuscular theory of light. The modern photon concept emerged from Max Planck's 1900 work on black-body radiation, where he introduced the idea of quantized energy to solve the ultraviolet catastrophe. In 1905, Albert Einstein expanded this idea in his explanation of the photoelectric effect, for which he later received the Nobel Prize in Physics. Experimental confirmation came through the work of Arthur Holly Compton with Compton scattering in 1923, and the term "photon" was later coined by Gilbert N. Lewis in 1926. The development of quantum mechanics by figures like Niels Bohr, Werner Heisenberg, and Paul Dirac fully incorporated the photon into theoretical physics.

Quantum theory

Within the framework of quantum field theory, the photon is described as the gauge boson of quantum electrodynamics, the relativistic quantum theory of electrodynamics developed by Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga. Its interactions are represented by Feynman diagrams, and it arises from the U(1) gauge symmetry of the electromagnetic field. The photon's wave function satisfies the Klein–Gordon equation, and its creation and annihilation operators are central to the formalism of second quantization. Phenomena such as spontaneous emission and the Lamb shift are key predictions of this theory, which has been validated with extreme precision by experiments like those at CERN and the National Institute of Standards and Technology.

Interactions

Photons interact primarily via the electromagnetic interaction with particles that carry electric charge, such as electrons and protons. Key processes include Rayleigh scattering, Compton scattering, and pair production in the vicinity of atomic nuclei. In materials, photons can be absorbed through the photoelectric effect, stimulating transitions in atoms and molecules as described by Einstein coefficients. They also mediate forces between charged particles through virtual exchange, a concept central to quantum electrodynamics. At high energies, photons can participate in electroweak interaction processes described by the Glashow–Weinberg–Salam model, and they can convert into other particles in powerful fields, as observed in experiments at the Large Hadron Collider.

Applications

Photons are harnessed across a vast array of technologies and scientific instruments. In telecommunication, they form the basis of fiber-optic communication systems and laser technology. Medical applications include X-ray imaging, positron emission tomography, and various forms of laser surgery. Photons are essential in consumer electronics, such as in charge-coupled device sensors in digital cameras and photovoltaic cells in solar panels. Scientific research utilizes them in instruments like the Hubble Space Telescope, laser interferometer gravitational-wave observatory, and in quantum cryptography protocols. Emerging fields like quantum computing and plasmonics also rely fundamentally on the manipulation of photons.

See also

* Electromagnetism * Quantum electrodynamics * Wave–particle duality * Laser * Photoelectric effect * Compton scattering * Standard Model * Quantum optics

Category:Elementary particles Category:Electromagnetism Category:Quantum mechanics