A photon (from the Greek phõs phōtṓs, light, on the model of electron) is the smallest quantity (quantum) of energy that can be transported and it was the realization that light traveled in discrete quanta that were the origins of Quantum Theory. A photon is massless, has no electric charge, and is a stable particle with two possible polarization states.
The term (from greek φῶς gen. φωτός “phòs, photòs” which means light) was coined in Paris in July 1926 by optical physicist Frithiof Wolfers, a few months later was reused by American chemist Gilbert Lewis and immediately adopted by many physicists, becoming definitive.
With the emergence of quantum field theory the photon has acquired to all effects the role of particle associated to electromagnetic field, classified as an elementary vector boson of zero mass mediator of electromagnetic interaction (gauge boson). It is usually indicated with the greek letter γ (gamma), symbol probably derived from gamma rays.
The concept of the photon was introduced in quantum physics to explain the contradictions that emerged between classical electromagnetism and experiments carried out at the turn of the late nineteenth century and the twentieth century. According to the classical theory developed by Maxwell, light, radio waves and UV rays are all electromagnetic radiation, that is, electric and magnetic fields that propagate in the matter and the vacuum following wave dynamics. The photon was introduced as an elementary constituent of these radiations by Max Planck and Albert Einstein between 1900 and 1905, as an entity that cannot be further divided.
A. Einstein deduced that the high frequency component of radiation in thermal equilibrium in a cavity had thermodynamic behavior in some aspects analogous to that of a perfect gas, that is a discrete set of particles. A first evidence of the physical reality of the photon was provided in 1915 by the experimental study of the surface photoelectronic effect, interpreted by Einstein already in 1905. The apparent dualism between the corpuscular properties of photons and the undulatory properties of radiation (interference, diffraction, etc..) is resolved in quantum mechanics. The photon is coupled to all charged particles in proportion to their electric charge and the exchange of photons mediates electromagnetic interactions between charged particles (electrodynamics).
In classical physics, each wave, according to the superposition principle, can always be decomposed as the sum or the contribution of two or more other waves. In contrast, the quantum mechanics postulates for electromagnetic waves, in agreement with the experiments, the existence of a “quantum” of indivisible fundamental energy, which therefore has both wavelike and particle properties (a phenomenon known as wave-particle duality).
From the standpoint of the particle, the photon has zero mass and does not carry any electric charge. Its intrinsic angular momentum, the spin, can assume only two values of ±1 which correspond to the different classical polarization states. In the void, photons always propagate at the speed of light (there is no observer against which they are stationary) and their range of action is unlimited. This means that a photon can continue to travel in space-time indefinitely without any limit until it is absorbed by another particle. For this reason, it is still possible to detect the photons emitted in the early life of the universe, which form the cosmic background radiation.
Until the eighteenth century many theories had introduced a corpuscular model for light. One of the first texts to present this hypothesis is a compendium of studies of the Iraqi scientist Alhazen, translated in 1270 by the Polish monk Vitellione, which under the overall title of De Aspectibus collects together several works, including the Book of Optics, 1021, known in the West under the title of Alhazen’s Perspective. In the book light rays are considered as streams of particles that “have no sensible characteristic except energy”. Since the particle model does not explain phenomena such as refraction, diffraction and birefringence, René Descartes proposed in 1637 a wave model, followed by Robert Hooke in 1665, and Christian Huygens in 1678. However, the corpuscular theory remains dominant, mainly due to the influence of Isaac Newton’s discoveries. In the early nineteenth century, Thomas Young and Augustin-Jean Fresnel definitively demonstrate the interference and diffraction of light, confirming the soundness of the wave model, which by 1850 was generally accepted. In 1865 Maxwell’s equations lay the foundations of electromagnetism, identifying light as electromagnetic radiation, and subsequent discoveries of Heinrich Hertz give further proof, making the particle model seem wrong.
Maxwell’s equations, however, do not take into account all the properties of light: they show the dependence of light energy from the intensity of radiation, and not from frequency, while some experiments regarding photochemistry show that in some cases the intensity does not contribute to the energy carried by the wave, that depends only on frequency. Also the black body researches, carried out by various scientists in the second half of the nineteenth century, in particular Max Planck, show that the energy that every system absorbs or emits is an integer multiple of a fundamental quantity, the quantum of electromagnetic energy.
The studies on photoelectric effect made at the beginning of the twentieth century by several scientists, mainly Albert Einstein, finally showed that the separation of electrons from their own atom depends only on the frequency of radiation from which they are hit, and therefore the hypothesis of a quantized energy became necessary to describe the energy exchange between light and matter.
The “quantum” was introduced as an elementary constituent of these radiations by Max Planck in 1900, as an entity not further divisible. In the context of his studies on the black body, the German physicist, assuming that atoms exchange energy through “finite packets”, formulated a model in agreement with experimental data. In this way he solved the problem of infinite emission in black body radiation (problem known as “ultraviolet catastrophe”), that emerged applying Maxwell equations. The true nature of light quanta remained initially a mystery: Planck himself introduced them not directly as real physical entities but rather as a mathematical device to make the accounts to balance.
The theory of light quanta (Lichtquant) was also proposed by Albert Einstein in 1905, following his studies on photoelectric effect, to explain the emission of electrons from the surface of a metal hit by electromagnetic radiation, effect that exhibited data in disagreement with Maxwell’s wave theory. Einstein introduced the idea that not only atoms emit and absorb energy in “finite packets”, the quanta proposed by Max Planck, but it is the same electromagnetic radiation to be constituted by quanta, that is discrete amounts of energy, then named photons in 1926. In other words, since electromagnetic radiation is quantized, energy is not uniformly distributed over the entire amplitude of electromagnetic wave, but concentrated in fundamental energy vibrations.
Although the German physicist accepted the validity of Maxwell’s equations, in 1909 and 1916 he shows that many experiments can be explained only assuming that energy is localized in point quanta that move independently from each other, even if the wave is distributed with continuity in space. For his studies on the photoelectric effect and the subsequent discovery of light quanta Einstein received the Nobel Prize in Physics in 1921.
Einstein’s quantum hypothesis was not accepted for several years by an important part of the scientific community, including Hendrik Lorentz, Max Planck and Robert Millikan (winners of the Nobel Prize in Physics in 1902, 1918 and 1923, respectively), according to whom the real existence of photons was an unacceptable hypothesis, considering that in interference phenomena electromagnetic radiation behaves like waves. The initial skepticism of these great scientists of that time should not be surprising, since even Max Planck, who first hypothesized the existence of quanta (even if with reference to atoms, which emit and absorb “energy packets”), thought, for some years, that quanta were only a mathematical device to make the accounts and not a real physical phenomenon. But later the same Robert Millikan demonstrated experimentally Einstein’s hypothesis on the energy of photon, and then of the emitted electron, that depends only on the frequency of radiation, and in 1916 he made a study on electrons emitted by sodium that contradicted the classic Maxwell’s wave theory.
The corpuscular aspect of light was definitively confirmed by the experimental studies of Arthur Holly Compton. In fact the American physicist in 1921 observed that, in collisions with electrons, photons behave as material particles with energy and momentum that are conserved; then in 1923 he published the results of his experiments (Compton effect) that confirmed in an indisputable way Einstein’s hypothesis: electromagnetic radiation is made of quanta (photons) that interacting with electrons behave as single particles and each photon interacts with only one electron. For the experimental observation of the linear momentum of photons and the discovery of the homonymous effect Arthur Compton received the Nobel prize in 1927.
The problem of combining the wave and particle nature of light occupied Einstein’s remaining life, and was solved by quantum electrodynamics and the standard model.
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