Photon

A photon is a massless, chargeless, stable elementary boson particle. The name is derived from the Greek word phôs, meaning “light.” The photon is the quantum, or smallest possible unit, of light. Its existence was proposed by Max Planck (1858–1947) and Albert Einstein (1879–1955) in the early twentieth century and later gained strong experimental support through studies of the photoelectric effect and Compton scattering. Photons exhibit wave-particle duality, meaning that they have characteristics of both waves and particles and are typically described as particles with wavelike properties.

Photons can be created by various processes, including the acceleration of a charged particle and the transition of an electron to a lower energy state. Another such process is the interaction of matter with antimatter, a pure conversion of matter into energy that releases at least two photons. The opposite is also true: theoretically, the head-on collision of two photons would create an electron and a positron (a positively charged electron). The energy of a single photon is given by the equation E = hf, where E is the energy of the photon, h is Planck’s constant, and f is the frequency of the photon. This equation shows that as frequency increases, so too does the energy of the particle.

Background

The debate over whether light is a particle or a wave dates back to the seventeenth century. René Descartes (1596–1650), Robert Hooke (1635–1703), and Christiaan Huygens (1629–95) each developed models that explained light as a wave. According to these scientists, particle models could not account for the refraction or diffraction properties of light. Conversely, Isaac Newton (1642–1727) championed the particle, or “corpuscular,” theory of light, arguing that waves could not travel in such straight lines.

In the early nineteenth century, it was still unclear whether light was a particle or a wave, as it exhibited properties of both. English physicist and physician Thomas Young (1773–1829) conducted the double-slit experiment, in which a coherent beam of light was shone through a single pinhole in one screen and then passed through two parallel pinholes in a second screen. (Later versions of this experiment used slits instead of pinholes, hence the name.) Beyond these two screens was a third screen that showed the pattern of the light emerging from the two pinholes. The results displayed an interference pattern that was indicative of waves interacting. Because of this experiment, the wave model of light gained widespread acceptance. It was further entrenched in the 1860s, when Scottish physicist James Clerk Maxwell (1831–79) introduced the concept of the electromagnetic field.

At the beginning of the twentieth century, however, the essential nature of light once again came into question. In 1900, German physicist Max Planck, while attempting to determine the relationship between the frequency and intensity of black-body radiation, calculated that light was in fact emitted in the form of tiny, discrete packets of energy. This discovery was later elaborated on by famous German-born physicist Albert Einstein in his 1905 explanation of the photoelectric effect, which is the tendency of certain metals to emit electrons when exposed to light at or above a particular frequency. Einstein’s calculations, which were based on Planck’s theory of black-body radiation, necessitated the existence of physical quanta, or particles, of light.

Strong experimental evidence for the particle nature of light was reported in 1923, based on experiments conducted by American physicist Arthur Holly Compton (1892–1962), when he directed a beam of electromagnetic radiation, in the form of X-rays, at a crystal. This caused the X-rays to scatter in a manner reminiscent of particles rather than waves, an effect later known as Compton scattering or the Compton effect. Compton won the Nobel Prize in Physics for that experiment in 1927. The same year, he began referring to these particles as “photons,” a term introduced in 1926 by American chemist Gilbert N. Lewis (1875–1946), and the name soon became accepted within the scientific community.

Overview

The photon is a gauge boson particle, meaning that, according to the Standard Model of particle physics, it is one of the elementary bosons that carry the fundamental forces of nature—in this case, the electromagnetic force. Other known gauge bosons include gluons, which carry the strong nuclear force, and W and Z bosons, which carry the weak nuclear force.

The discovery of photons significantly advanced understanding of quantum physics. The first step in the development of the Standard Model was the unification of the electromagnetic and weak forces by Sheldon Glashow, Abdus Salam, and Steven Weinberg in the 1960s, giving rise to what is known as the electroweak interaction. Understanding photons as the carriers of the electromagnetic interaction contributed to the development of quantum field theory, including electroweak theory. In addition, the development of photon theory contributed to the broader development of quantum mechanics, including concepts such as the uncertainty principle, which states that it is impossible to know both the momentum and the position of a subatomic particle at the same time.

The Standard Model describes photons as massless particles, although experiments continue to test this prediction by placing increasingly strict limits on any possible photon mass. If photons possessed a nonzero mass, their speed in a vacuum would depend on their frequency; however, experiments have found no evidence that photons have a nonzero mass. In addition, many calculations—including those regarding the composition of deep-space entities such as quasars, black holes, and supernovae, and those governing electric fields, relativity, and the passage of time—are based on the assumption that the mass of a photon is zero; if this is not the case, many such calculations will have to be revisited. Applying knowledge of the photon enabled the development of such tools as the laser. Photons are also used in quantum communication and photonic quantum computing, fields that rely on controlling individual particles of light.

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