If you ever wondered what is the most recent definition of light and don’t want to spend a career in physics to find out the answer, then you’ll find it here. A bit technical but also a brief introduction to the nature of light.
Light is a ray
Since antiquity, the behavior of light has been analyzed with great scrutiny. From the Greek’s concept of visual fire [1] to the first geometrical treatment of light by Euclid’s “Optics” written around 300 B.C. Geometrical [or ray] Optics was mainly pushed forward by Alhazen, Snell, Descartes and Newton, culminating with Hamilton’s modern geometrical optics approach. Although this treatment of light was important in the explanation of phenomena like reflection, refraction and the formation of images, which would allow the harnessing of optical technologies (telescopes, microscopes), it offered no insight into the true nature of light [2].
Light is a wave
At the beginning of the eighteenth century, two different explanations on the nature of light arose. The first one proposed that light was a wave that propagates through the luminiferous ether, just like sound waves were able to propagate through a dense medium. This wave theory of light was originally published by Huygens in his Traite de la lumiere in 1690. The second approach consisted of the ‘corpuscular’ or particle theory of light developed by Isaac Newton in his ‘Opticks’ in 1704 [3]. Although no direct evidence of either possibility was observed at the time, more ‘weight’ was given to Newton’s approach due to his higher authority in other fields of Physics. It was not until the experiments on Interference carried out by Thomas Young in 1801 that the ‘corpuscular’ theory of light was rejected in favor of the wave theory.
By trying to find an answer to the nature of light waves, Young speculated that the ether could be the cause for electric phenomena and light [1]. In 1845 Michael Faraday found a connection between light and magnetism, giving a hint that light could be a transverse vibration of his newly developed concept of ‘lines of force’ that he applied previously to explain electric and magnetic phenomena. Along came J.C Maxwell, in his paper “On Faradays lines of force” where he explained all electric and magnetic phenomena known at the time by using a mechanical model, in which the medium (ether it was believed) was filled with hexagonal molecular vortices, predicting the existence of electromagnetic waves. By polishing his ideas, in 1865, he got rid of the mechanical model to explain the existence of the electromagnetic waves that propagate through the ether, where the velocity of these waves, that he calculated, was that of light exactly. This was the birth of Classical Electrodynamics and the unification of the old wave theory of light with electromagnetism. Light was an electromagnetic wave.
Light is a particle
At the end of the 19th century, physicists were studying the way a blackbody emits radiation. After several attempts to explain the radiation spectral shape of a blackbody (Wien and Rayleigh), it was not until Max Planck came up with the solution, although in doing so, he had to sacrifice the idea that energy is emitted continuously. He presented his ideas on 14 December 1900 in Germany, which is often regarded as the date that Quantum Mechanics was born [4]. That same decade, Albert Einstein, by explaining the photoelectric effect, a process that consists of knocking down electrons from an electrically charged metal plate by using light, arrived at the startling conclusion that all electromagnetic radiation was made of discrete energy lumps, or energy quanta (photons), giving a full 180 degree turn to the concept of light due to Maxwell’s theory of Electromagnetism, in which radiation was considered to be continuous.
Light is a wave and a particle
Further developments in quantum mechanics by DeBroglie, Heisenberg and Schrodinger left the “nature of light” as a conundrum, since this new quantum mechanics was not consistent with special relativity, unlike Maxwell’s classical Electrodynamics that fitted accurately to the relativistic frames of reference, leading to an “acceptance” of light, both as a wave and a particle in the so called “wave-particle duality”.
Light is a vibration
It was not until Paul Dirac started to remove this apparent paradox in his seminal paper in 1927 [5] where he combined quantum mechanics with Maxwell’s theory to give a first glimpse into what eventually evolved as the “theory of light-matter interactions” or “Quantum Electrodynamics” (QED for short), a theory fully consistent with special relativity where light is defined as a localized vibration of a continuous electromagnetic field that spreads throughout all of space. These vibrations go by the name of photons [6]. This idea, officially named “second quantization”, expanded to all subatomic particles, where the building blocks of nature are not the particles themselves, but fields, which are fluidlike objects that spread throughout the whole universe [7], and the particles (e.g. electrons, photons) are just vibrations of these continuous fields (e.g. electron field, electromagnetic field).
A note on reductionism:
Even though QED allows a deeper understanding of the nature of light, this does not diminish the usefulness and importance of the different pictures of light on a non-fundamental level (i.e. wave and ray optics). All scientific knowledge consists of explanations, and the structure of these explanations does not agree with the reductionist point of view, where science [they believe] is about analyzing things into components [8].
Citation
For academic purposes, please cite this as: Felipe Ortiz-Huerta, “Lux, A brief history of light”, https://felipeortiz.home.blog/2019/12/19/lux/
References
[1] O. Darrigol, A history of optics from Greek antiquity to the nineteenth century. OUP Oxford, 2012.
[2] S. M. Dutra, Cavity quantum electrodynamics: the strange theory of light in a box. John Wiley & Sons, 2005.
[3] I. Newton, Opticks, or, a treatise of the reflections, refractions, inflections & colours of light. Courier Corporation, 1979.
[4] Al-Khalili Jim, Quantum: A guide for the perplexed. Hachette UK, 2012.
[5] P. A. M. Dirac, “The quantum theory of the emission and absorption of radiation,” in Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, 1927, vol. 114, no. 767, pp. 243–265.
[6] T. Lancaster and S. J. Blundell, Quantum field theory for the gifted amateur. OUP Oxford, 2014.
[7] David Tong, Quantum Fields, The Real Building Blocks of the Universe. The Royal Institution Public lectures, https://www.youtube.com/watch?v=zNVQfWC_evg&t=174s
[8] D. Deutsch, The fabric of reality. Penguin UK, 1998.
Magna-Machina 