Photons

Photons are the quantum mechanical description of light and other forms of electromagnetic radiation. They are quanta of the electromagnetic field and the mediators of the electromagnetic interaction. Photons are bosons with zero rest mass and a spin of 1, possessing two possible helicity states. They travel at the speed of light in vacuum and carry energy proportional to their frequency, E = hf, and momentum p = E/c = h/λ.

In vacuum, photons propagate at the constant speed c. In materials, their effective speed is reduced and their wavelength shortens according to the refractive index n (λ in the material equals λ0/n). Their interactions with matter include absorption, scattering, emission, and polarization, with processes such as the photoelectric effect, Compton scattering, and pair production in the presence of nuclei.

Photons can be created in many processes: thermal and blackbody radiation, atomic and molecular transitions, fluorescence, and stimulated emission in lasers. They can be annihilated in interactions with matter or other particles, transferring energy and momentum.

From a theoretical standpoint, photons are the gauge bosons of the electromagnetic field in quantum electrodynamics (QED). They have no electric charge and no rest mass, so their energy satisfies E^2 = (pc)^2, with E = pc for massless particles. Their interactions with matter are precisely described by QED, yielding highly accurate predictions for scattering and emission.

Historically, the concept arose from Planck’s explanation of blackbody radiation and Einstein’s explanation of the photoelectric effect, with further validation through Compton scattering. The modern formalism of QED was developed in the mid-20th century by Feynman, Schwinger, and Tomonaga.

Practical uses of photons include communication, optical and laser technologies, spectroscopy, imaging, and solar energy. Photon number can change in interactions with matter, but total energy and momentum are conserved.