Photon
Quantum of light and force carrier for electromagnetism.
A photon is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can only move at one speed, the speed of light measured in a vacuum. The photon belongs to the class of boson particles. As with other elementary particles, photons are best explained by quantum mechanics and exhibit wave–particle duality, their behavior featuring properties of both waves and particles.
- type
- Elementary particle
- classification
- Boson
- mass
- Zero rest mass (experimental upper limit ~10⁻⁵³ g)
- charge
- No electric charge
- spin
- ±ħ
- speed
- Speed of light in vacuum
- lifetime
- Stable (greater than 10¹⁸ years)
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.
Reader's Guide
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 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. Photons obey Bose–Einstein statistics, not Fermi–Dirac statistics, meaning they do not obey the Pauli exclusion principle, and more than one photon can occupy the same bound quantum state. The photon has no electric charge, is generally considered to have zero rest mass, and is a stable particle.
Did You Know?
- Photons are emitted when a charge is accelerated and emits synchrotron radiation.
- The photon can be considered its own antiparticle.
- In free space, at least two photons must be created during particle–antiparticle annihilation to conserve momentum.
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