Photon is the quantum of the electromagnetic field and is the particle description of electromagnetic radiation. It is an elementary boson with zero rest mass, zero electric charge and spin one. Photons are therefore bosons rather than fermions and do not obey the Pauli exclusion principle in the way electrons and other matter fermions do. A photon in vacuum travels at the speed of light. Its energy is proportional to its frequency and its momentum is proportional to its frequency divided by the speed of light.
Photons are the force carriers of quantum electrodynamics in the sense that electromagnetic interactions are described through photon exchange in the quantum field theory framework. Real photons are observable electromagnetic radiation while virtual photons appear as internal mathematical components of perturbative calculations of electromagnetic interactions. Virtual photons are not directly detectable particles travelling between objects in the same sense as real photons. This distinction is important because simplified descriptions that say photons are always exchanged between charged particles can obscure the different roles of real and virtual field quanta.
Photon energy determines the region of the electromagnetic spectrum associated with the radiation. Radio waves have relatively low photon energies, while microwaves, infrared, visible light, ultraviolet radiation, X rays and gamma rays occupy progressively higher energy ranges. The boundaries between these regions are conventional rather than representing abrupt changes in the underlying particle. All are electromagnetic radiation and differ primarily through frequency, wavelength and photon energy. The interaction of photons with matter depends strongly on photon energy and the properties of the material.
Photons exhibit wave particle duality. Experiments can reveal interference, diffraction, reflection, refraction and polarisation, while photon detection occurs in discrete quanta. A single photon can contribute to an interference pattern over many repeated detections, demonstrating that quantum amplitudes rather than classical continuous energy flow provide the appropriate microscopic description. The electromagnetic field can be quantised so that energy exchange occurs in discrete amounts determined by photon frequency. This quantisation is fundamental to phenomena such as atomic emission and absorption.
Photons are emitted and absorbed in many physical processes. Atoms emit photons when electrons transition between allowed energy states, while atoms can absorb photons when the photon energy matches an allowed transition under the relevant selection rules. Accelerated charged particles can emit electromagnetic radiation, including synchrotron radiation and bremsstrahlung. Nuclear transitions can produce high energy photons. Particle annihilation processes can also produce photons when conservation laws permit.
Photons have no rest frame because they always propagate at the invariant speed of light in vacuum. The statement that a photon has zero rest mass does not mean that it has zero energy or momentum. Its energy and momentum are related through the relativistic energy momentum relationship. The momentum of electromagnetic radiation is experimentally observable through radiation pressure and photon momentum transfer. This is important in optical manipulation, solar radiation and interactions between light and matter.
Photons have extensive technological applications. Lasers produce highly ordered electromagnetic radiation used in communication, manufacturing, medicine, measurement and information processing. Fibre optic communication transmits information through guided electromagnetic modes with photons providing the quantum description of the energy carriers. Photodetectors convert photon interactions into electrical signals, while solar cells convert absorbed photon energy into electrical energy through semiconductor processes. Spectroscopy uses photon emission or absorption to determine physical and chemical properties.
Photon should not be understood as a tiny classical pellet of light travelling along a simple trajectory in every circumstance. Quantum theory describes probability amplitudes and field states rather than assigning a classical path to every photon. Photon polarisation is a quantum property associated with electromagnetic field degrees of freedom and is important in quantum optics. Photons can also exhibit entanglement and other nonclassical correlations. Their behaviour therefore requires quantum theory when individual quanta, coherence and measurement are important.
