Photon

Lore & Background

The modern photon concept originated during the first two decades of the 20th century with the work of Albert Einstein, who built upon the research of Max Planck. While Planck was trying to explain how matter and electromagnetic radiation could be in thermal equilibrium with one another, he proposed that the energy stored within a material object should be regarded as composed of an integer number of discrete, equal-sized parts. To explain the photoelectric effect, Einstein introduced the idea that light itself is made of discrete units of energy. Lewis popularized the term photon for these energy units. Subsequently, many other experiments validated Einstein's approach. In the Standard Model of particle physics, photons and other elementary particles are described as a necessary consequence of physical laws having a certain symmetry at every point in spacetime. The intrinsic properties of particles, such as charge, mass, and spin, are determined by gauge symmetry. The photon has no electric charge, is generally considered to have zero rest mass, and is a stable particle. The experimental upper limit on the photon mass is very small, on the order of 10⁻⁵³ g; its lifetime would be more than 10¹⁸ years. Single photons have been shown to travel at the speed of light in vacuum. Photons are emitted when a charge is accelerated and emits synchrotron radiation. During a molecular, atomic, or nuclear transition to a lower energy level, the photons emitted have characteristic energies ranging from radio waves to gamma rays. Photons can also be emitted when a particle and its corresponding antiparticle are annihilated. The photon also carries spin angular momentum, which is related to photon polarization, with two possible values, either +ħ or −ħ.

Reader's Guide

The photon concept has led to momentous advances in experimental and theoretical physics, including lasers, Bose–Einstein condensation, quantum field theory, and the probabilistic interpretation of quantum mechanics. It has been applied to photochemistry, high-resolution microscopy, and measurements of molecular distances. Moreover, photons have been studied as elements of quantum computers, and for applications in optical imaging and optical communication such as quantum cryptography. The photon is the gauge boson for electromagnetism, and therefore all other quantum numbers of the photon (such as lepton number, baryon number, and flavour quantum numbers) are zero. Photons obey Bose–Einstein statistics, and not Fermi–Dirac statistics; they do not obey the Pauli exclusion principle, and more than one photon can occupy the same bound quantum state. The classical formulae for the energy and momentum of electromagnetic radiation can be re-expressed in terms of photon events. For example, the pressure of electromagnetic radiation on an object derives from the transfer of photon momentum per unit time and unit area to that object. Current commonly accepted physical theories imply or assume the photon to be strictly massless; if photons were not purely massless, their speeds would vary with frequency, and Coulomb's law would be modified.

Did You Know?

• The photon has no electric charge and is generally considered to have zero rest mass, with an experimental upper limit on its mass on the order of 10⁻⁵³ g.
• The photon's spin angular momentum has two possible values, either +ħ or −ħ, corresponding to two possible pure states of circular polarization.
• In free space, at least two photons must be created when a particle and its antiparticle annihilate, due to conservation of momentum.