| photon | |
|---|---|
| Name | Photon |
| Composition | Elementary particle |
| Type | Gauge boson |
| Generation | Force carrier |
| Interaction | Electromagnetic interaction |
| Mass | 0 (rest mass) |
| Discovered | 1905 (conceptual), 1926 (terminology) |
| Discoverer | Albert Einstein (concept), Gilbert N. Lewis (name) |
photon
The photon is the quantum of the electromagnetic field and the fundamental force carrier for the electromagnetism in the Standard Model. It is massless, carries energy and momentum, and exhibits both particle-like and wave-like behavior; photons underpin phenomena from blackbody radiation to laser operation and are central to Quantum electrodynamics and modern quantum optics.
A photon is an elementary, massless boson with intrinsic angular momentum (spin) equal to 1, described by the gauge field of U(1) gauge theory in the Standard Model. Photons travel at the speed of light in vacuum and obey the relativistic energy–momentum relation E = pc for zero rest mass. They possess quantized energy E = hν related to frequency ν via Planck's constant h, and momentum p = h/λ where λ is the wavelength. The photon has two physical polarization states corresponding to helicity ±1; the longitudinal polarization is excluded by gauge invariance and the Lorentz group representation for massless spin-1 particles. Photons are electrically neutral and do not carry color charge, therefore they do not participate in the strong interaction. The photon mediates the electromagnetic force between charged particles such as the electron and the proton.
The concept of quantized light emerged from efforts to explain blackbody radiation; Max Planck introduced energy quanta in 1900. Albert Einstein proposed in 1905 that light itself is quantized to explain the photoelectric effect, a pivotal argument leading to the photon concept and earning Einstein the Nobel Prize in Physics. The term "photon" was popularized by Gilbert N. Lewis in 1926. The development of quantum mechanics by Niels Bohr, Werner Heisenberg, and Erwin Schrödinger provided a theoretical framework for matter, while Paul Dirac formulated a quantum field description of light that led directly to quantum electrodynamics (QED). Experimental confirmations came from studies by Robert A. Millikan on the photoelectric effect, the observation of Compton scattering by Arthur H. Compton, and later precision tests at institutions like CERN and Bell Labs.
In quantum field theory, the photon is an excitation of the quantized electromagnetic field described by QED, the relativistic quantum field theory developed by Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga. QED provides Feynman diagram rules for photon emission and absorption and predicts phenomena such as the Lamb shift and the anomalous magnetic moment of the electron. In quantum optics, the photon is treated both as a particle in number states (Fock states) and as coherent states introduced by Glauber, important for describing lasers and squeezed states. Photon statistics distinguish classical light from nonclassical sources: photon antibunching and sub-Poissonian statistics are signatures of single-photon sources used in quantum information and quantum cryptography (e.g., BB84 protocol implementations). The photon is central to tests of Bell's theorem and investigations of quantum entanglement between spatially separated systems, such as experiments by Alain Aspect and subsequent work at universities like MIT and Caltech.
Photons interact with charged particles primarily via electromagnetic coupling described by QED vertices; common processes include absorption, emission, scattering, and pair production. In atoms, photon absorption and emission produce electronic transitions described by the Bohr model extensions and selection rules derived from angular momentum conservation. Coherent interactions lead to phenomena such as stimulated emission (basis of the laser, invented by Theodore Maiman building on Einstein's theory) and Rabi oscillations observed in cavity quantum electrodynamics experiments. Nonlinear optics, studied in laboratories like Bell Labs and Fermi National Accelerator Laboratory, arises when intense photon fields induce higher-order polarization responses in media, producing effects such as second-harmonic generation and four-wave mixing. In plasmas and astrophysics, photons couple to collective modes, while in condensed matter systems photons interact with quasiparticles like polaritons and excitons.
Photons enable a broad array of technologies that sustain modern infrastructure and national cohesion: telecommunications via fiber-optic links developed by companies like Corning Incorporated and standards set by organizations such as the ITU, laser systems for manufacturing and medicine, imaging modalities (optical microscopes, photodetectors, CCDs developed by institutions like Bell Labs and NASA), and photovoltaic energy conversion in solar cells commercialized by firms like First Solar. In information technology, single-photon sources and detectors underpin protocols in quantum key distribution and proposals for quantum computing architectures (ion traps at NIST, photonic chips at IBM Research and University of Oxford). Photonics supports national security, metrology (atomic clocks relying on optical transitions), and deep-space communication in programs run by agencies like ESA and NASA.
Photon detection techniques range from classical photometry to single-photon counting. Devices include photomultiplier tubes developed in 1930s laboratories, semiconductor avalanche photodiodes, superconducting nanowire single-photon detectors pioneered in research groups at NIST and NIST, and transition-edge sensors used in astronomy (e.g., at Infrared Telescope Facility). Experiments employ interferometry (Michelson and Mach–Zehnder interferometers), spectrometers, and correlation measurements (Hanbury Brown and Twiss experiment) to probe coherence and statistics. High-energy photon detection uses calorimeters and scintillators at facilities such as CERN and SLAC National Accelerator Laboratory. Quantum optics testbeds at universities (e.g., Harvard University, University of Oxford) and national labs enable manipulation of single photons, quantum state tomography, and Bell-test experiments.
While the photon is well described by QED, open questions and theoretical extensions persist. Searches for a nonzero photon mass are constrained by precision tests (e.g., limits derived from galactic magnetic fields and laboratory experiments), and speculative extensions include tiny violations of Lorentz invariance probed in experiments by collaborations like ICECUBE and Fermi Gamma-ray Space Telescope. The role of photons in theories beyond the Standard Model, such as grand unified theories and string theory, appears via additional gauge bosons or hidden-sector photons (dark photons) investigated by experiments at CERN and dedicated fixed-target programs. Foundational issues remain in reconciling quantum measurement with relativistic quantum fields; ongoing work in quantum information theory and experimental tests by groups at Caltech and Perimeter Institute examine decoherence, entanglement, and the photon’s role in quantum foundations.
Category:Quantum physics Category:Elementary particles Category:Optics