Photons are basic units (quanta) of electromagnetic radiation, including visible light. Like all elementary particles, they can be characterized by three main properties: charge, mass, and spin. Their charge is zero, so is their mass, and their spin is one. Theoretically, photons by definition travel at the speed of light. All these characteristics make photons rather different from the quanta of matter. The study of photons became one of the most important directions of the 20th century’s physics (Jauch & Rohrlich, 2012). Part of this study is the exploration of a photon’s life cycle. The knowledge of how it was discovered and how the understanding of it was developing has had various technological applications.
Discovery
When the subject of a discussion is subatomic particles, the use of the word “discovery” may be somewhat misleading to someone who is not familiar with the basic concepts of microphysics. On a microscopic level, especially the subatomic level, physical processes are different from the ones we can observe on a macroscopic scale, i.e. in the world around us as we can perceive it. Sometimes, the ways elementary particles behave and interact are unattainable to the human imagination.
The discovery of a particle does not mean that it is observed in a super-powerful microscope or photographed. Instead, some phenomena are observed, which can be explained most persuasively by the existence of this article. The history of photons studies provides an example of how discoveries of particles occur and how unimaginable their characteristics sometimes are.
The existence of photons was theoretically predicted by Albert Einstein (Stone, 2015). At the beginning of the 20th century, the dominant view on the characteristics of light was based on the electromagnetic wave equation that describes the motion of electromagnetic waves through space, i.e. the wave model prevailed where light is viewed as electromagnetic radiation consisting of waves. Einstein posited that light consisted of particles.
To be more precise, he suggested the duality model, where the characteristics of light can be described both in terms of waves and in terms of particles. Before the publications of Einstein’s theories on the quanta of light, the general agreement in physics was that materials objects could emit and absorb light discretely in particular amounts, but light itself was not divided into basic units (Stone, 2015). Einstein challenged this view.
A significant experiment that confirmed Einstein’s theory was the Compton scattering experiment, which observed a deviation of the trajectory of light particles when coming into contact with electrons. The scientific community admitted that light is quantized, and Compton won the 1927 Nobel Prize in Physics for his studies. The term “photon” (from “phos,” Greek for “light”) was coined by Frithiof Wolfers and Gilbert N. Lewis a year before that, and since then it has become the common name for a particle of light.
Development
The development of microphysics and the advancement of technology and conceptual understanding allowed studying the characteristics of photons and analyzing their life cycle. The exploration of this life cycle is complicated by two major factors. First, photons theoretically have a zero rest mass, which makes conceptualizing their finite lifespan difficult. Second, they travel at a speed, at which time goes much slower than in the macroworld (Heeck, 2013). However, some milestones in the life of a photon can be described.
The emergence of a photon is associated with the release of energy by a charged subatomic particle, e.g. an electron. A released photon immediately starts traveling through space at the speed of light. According to the special theory of relativity, something that travels at this speed has no mass and the time virtually does not exist to it. At some point, a photon can be taken up by another subatomic particle (Haroche, 2013). Therefore, it can be said that the life of a photon, i.e. something occurring after is it emitted and before it is absorbed, is merely moving at the maximum speed.
However, if it is assumed that a quantum of light does not have a zero rest mass, several theories can be proposed on what happens to a photon during its lifetime. If an elementary particle has a rest mass, it can disintegrate into lighter and smaller particles. Heeck (2013) hypothesizes that the mass of a photon can be about 10–54 kg, which means that, at some point in its lifetime, a photon can decay into two particles: a neutrino and an antineutrino. It is also suggested that possibly a photon disintegrates into particles that have not been discovered yet.
To calculate the length of a photon’s life, Heeck (2013) used the cosmic microwave background (CMB), which is a kind of radiation that remained from the recombination stage of the universe development after the Big Bang. Exploring this radiation allows making conclusions about the time when energy became separated from matter after the two had been closely intertwined in the universe. At this point, photons started traveling through space. Studying the CMB to measure the photon’s lifetime is justified by the fact that CMB has characteristics of an almost perfect black body. Therefore, measuring the CBM can define the limits of the lifetime of photons.
Based on the calculations of the photon’s mass and CMB constraints, Heeck (2013) stated that the life cycle of a photon lasts three years. An important clarification is that this estimation applies to the photon’s rest frame. In our frame of reference, the photon’s lifetime is different because the particles of light travel at almost the speed of light (not the speed of light exactly because it had been posited in the described study that photons had rest mass, albeit very small, which means that their speed is slightly less than the speed of light). Considering the time dilatation, it was defined that the photon’s lifetime in our system is 1018 years, i.e. a billion years, which is more than 70 million times longer than the universe has existed.
Applications
The concept of quantized light has had various applications in science and technology. One of the main examples is the notion of stimulated emission, i.e. generating new photons in the transition process of a quantum system from the excited state to a lower energy level. Theoretical studies in this area allowed creating light amplification by stimulated emission of radiation technology, i.e. creating lasers (Keller, 2014). Lasers became a major technological achievement of the 20th century.
Their applications can be found in the spheres of medicine (surgery, dentistry, ophthalmology, etc.), computer science (printers, CDs, etc.), industry (heating, welding, cutting), and scientific research.
The understanding of how photons interact with other particles and how they are emitted and absorbed allowed developing many advanced technologies. For example, the high-resolution microscopy is based on the principle of the two-photon excitation, where two beams produce the wavelength that allows a higher penetration depth into living tissues. Another technology based on the theoretical studies of photons is fluorescence resonance energy transfer (FRET), which is used to examine molecular interactions.
FRET technology is used in molecular biology and chemistry for the studies of proteins (Keller, 2014). It also helped create various biosensors, i.e. devices that detect the presence of certain substances using biological molecules.
The study of photons significantly affected the area of advanced scientific research. For example, some modern models of quantum computing are based on photonic characteristics. Quantum computing is different from binary digital computing because the former is analog and employs the idea of superposition derived from quantum physics to operate data. The studies in this area are promising because they can bring cybersecurity standards to a new level (Keller, 2014). Also, the knowledge of how photons emerge and affect other particles helps in optical imaging, optical communication, and measuring molecular distances.
Conclusion
A photon remains a mysterious particle. Its characteristics, such as mass and speed, are still conceptually challenging to scientists. The particle’s dual wave/quanta properties are one of the most complicated notions of quantum physics. However, the knowledge about the life cycle of a photon and its interactions with other particles has been applied to many spheres of science and technology. These applications have already promoted the progress in many spheres such as medicine, industry, and scientific research. However, the unexplored potential of photonic studies is still large.
References
Haroche, S. (2013). Nobel Lecture: Controlling photons in a box and exploring the quantum to classical boundary. Reviews of Modern Physics, 85(3), 1083-1102.
Heeck, J. (2013). How stable is the photon? Physical review letters, 111(2), 218-222.
Jauch, J. M., & Rohrlich, F. (2012). The theory of photons and electrons: The relativistic quantum field theory of charged particles with spin one-half. New York City, NY: Springer Science & Business Media.
Keller, O. (2014). Light: The physics of the photon. Boca Raton, FL: CRC Press.
Stone, A. D. (2015). Einstein and the quantum: The quest of the valiant Swabian. Princeton, NJ: Princeton University Press.