Photon

A photon is an elementary particle and the fundamental unit, or quantum, of the electromagnetic field. This means it constitutes all forms of electromagnetic radiation, from light to radio waves, and acts as the carrier of the electromagnetic force. Photons have no mass and are restricted to a single speed: the speed of light in a vacuum. They are classified as bosons. Like all elementary particles, photons are understood through quantum mechanics and display wave–particle duality, meaning they behave like both waves and particles.

The modern understanding of the photon emerged in the early 20th century, building on Max Planck's work. Planck, studying how matter and radiation reach thermal equilibrium, suggested that energy in a material object exists only in discrete, equal-sized packets. Albert Einstein then applied this idea to explain the photoelectric effect, proposing that light itself consists of these discrete energy units. The term "photon" was popularized by Gilbert N. Lewis in 1926 for these units, and many later experiments confirmed Einstein's theory.

In the Standard Model of particle physics, photons arise as a necessary consequence of fundamental symmetries in spacetime. Their intrinsic properties—such as zero electric charge, zero rest mass, and stability—are determined by gauge symmetry. Photons have no electric charge, and experimental limits place their mass below about 10⁻⁵³ grams, with a lifetime exceeding 10¹⁸ years (the universe is roughly 1.38×10¹⁰ years old). Single photons travel at the speed of light in a vacuum, as confirmed by measurements using heralded single-photon sources. In a vacuum, a photon has two possible polarization states. As the gauge boson for electromagnetism, all other quantum numbers (like lepton number, baryon number, and flavor) are zero. Photons obey Bose–Einstein statistics, not Fermi–Dirac statistics, so they do not follow the Pauli exclusion principle; multiple photons can occupy the same quantum state.

Photons are emitted when a charged particle accelerates, producing synchrotron radiation. They are also released during molecular, atomic, or nuclear transitions to lower energy levels, with energies spanning from radio waves to gamma rays. Additionally, photons arise from the annihilation of a particle and its antiparticle, such as an electron and positron.

The energy of a photon is proportional to its frequency (ν) via the Planck constant (h): E = hν. This can also be expressed using angular frequency (ω) or wavelength (λ): E = ħω = hc/λ, where ħ is the reduced Planck constant and c is the speed of light. A photon's momentum (p) is given by p = ħk, where k is the wave vector (with magnitude k = 2π/λ). The magnitude of momentum is p = hν/c = h/λ. Since the photon has zero mass, its energy and momentum satisfy the special relativity relation E = pc, consistent with E² = p²c² + m²c⁴ when m = 0.

The photon concept has driven major advances in physics, including lasers, Bose–Einstein condensation, quantum field theory, and the probabilistic interpretation of quantum mechanics. It has practical applications in photochemistry, high-resolution microscopy, and measuring molecular distances. Photons are also key to quantum computing, optical imaging, and quantum cryptography.