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. It was first experimentally observed in the 1920s, though its existence had been theorized by Albert Einstein (1879–1955) and Max Planck (1858–1947) at the turn of the century. 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 radial 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.

The first experimental demonstration of the particle nature of photons came in 1922, when American physicist Arthur Holly Compton (1892–1962) conducted an experiment in which 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 that had been coined the previous year 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. This unification would not have been possible without an understanding of the particle nature of photons. In addition, the existence of photons as quantized units is necessary to explain 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.

One ongoing question regarding photons is whether they have mass, as either a massless or a massive photon would have significant implications for relativity theory and our understanding of the speed of light. For all practical purposes, quantum physics treats the photon as massless, but the ultimate truth of this is uncertain. If photons are not massless, then their speed in a vacuum would be slower than the speed of light and would depend on the frequency of the particle. In addition, many calculations—including those regarding the composition of deep-space entities such as quasars, black holes, and supernovas, 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.

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