What Is Light?

I happen to have discovered a direct relation between magnetism and light, also electricity and light, and the field it opens is so large and I think rich.

– Michael Faraday, in a Letter to Christian Schönbein (13 Nov 1845)

The story of the manipulation and utilization of electromagnetic (EM) radiation on Earth begins about 550 million years ago around the start of the Cambrian explosion when the first complex eyes developed [1], allowing living things to use light not just as a source of energy, but as a source of information; enabling fast locomotion and navigation. It is even argued that the development of the eye is what caused the Cambrian explosion since the unveiling of the size, shape, color, and behavior of other creatures created a strong evolutionary pressure to adapt [2]. Creatures continued to evolve as time progressed, leading to extremely sophisticated light detection equipment allowing for long-range vision, low-light sensing, and other even more exotic functions such as detection of circularly-polarized light in the case of the mantis shrimp [3]. Eventually, however, about 300,000 years ago [4], a species arrived that used their eyes in an entirely new way, not just to avoid predators or secure sustenance, but as a way to observe the universe around them and seek deeper truths about existence itself. This species was Homo Sapiens.

The mantis shrimp can detect circularly polarized light.

We began by simply turning our eyes to the stars and beholding the wonder of the night sky, or to the mountains and lakes to question what forces could form such structures, or to the ground beneath our feet where small creatures with an animus that we did not understand went about industriously to perform their daily tasks. The first instrument that we developed to enhance our light detection prowess in a way that expanded our understanding of the Universe was arguably the telescope, as in 1609 Galileo Galilei turned his own two-lens refractive telescope to the sky and discovered both the moons of Jupiter and the rough surface of the Moon [5]. Isaac Newton enhanced our understanding of light in 1704 by publishing Opticks [6], where he precisely described how light refracts, reflects, diffracts, and disperses, the final action revealing how white or ‘pure’ light consists of spectral components, the first hint that light as we understood it may exist in different forms than we experience with our eyes, as we can see in the Figure below where the entire electromagnetic spectrum is represented. Following discoveries by Ampere and Faraday, among many others, in 1865 James Clerk Maxwell derived an electromagnetic wave equation [7] with a propagation speed that matched that of light (accurate measurements of the speed of light happened as early as 1728 by James Bradley [8]), leading him to conclude that light itself was electromagnetic radiation and traveled as transverse coupled electric and magnetic waves through space.

The electromagnetic spectrum, with symbols representing the various technologies and sources that utilize specific frequency bands located near those frequency bands.

Maxwell’s revelation was a watershed moment in human history, as the foundational understanding needed to develop our modern communication structures was finally in place. Not long after Maxwell’s landmark insights, Heinrich Hertz conducted a series of experiments between 1886 and 1889 that proved the existence of electromagnetic waves that obeyed Maxwell’s wave equation [10]. Before Hertz’s experiments, many had proposed the idea of communication via electromagnetic waves (at the time known as ‘wireless telegraphy’), but no one had a concrete idea of how it might be accomplished. Guglielmo Marconi would lead the development and commercialization of radio communication starting in the mid-1890’s, going from 34-mile transmissions in 1897 to transatlantic transmissions in 1901. Radio would see rapid deployment in both military and commercial applications in the first half of the 20th century, largely enabled by the use of amplifying vacuum tubes and the invention of FM radio in 1933. The invention of the transistor in 1947 by Bardeen, Brattain, and Shockley at Bell Labs would first revolutionize radio before it would revolutionize computing, as the commercialization of transistor radios led to smaller, cheaper devices that were widely available.

Portrait of James Clerk Maxwell

The late 20th century and the early 21st century saw a flurry of advances in radio communication, to the point where now essentially every single person in the developed world owns or has access to several devices that use radio signals to communicate or transmit data. Every person walking on the surface of the earth is awash with electromagnetic radiation carrying information from satellites, cell towers, personal hand-held devices, WiFi routers, and much more. One simply cannot participate in the modern world without having dominion over a portion of the electromagnetic spectrum by way of one ingenious device or another.

So what is light exactly? Well, as we saw above, light is nothing but electromagnetic waves; the same signals your devices use to communicate. But what are those? In our framework for understanding Nature, we have four fundamental forces: the weak force, the strong force, gravity, and the electromagnetic force (the weak and EM interactions have been theoretically unified, but that is unimportant here). In electromagnetics, we assign a fundamental characteristic of matter called charge. Charge is a measure of how hard electromagnetic forces push or pull on a given object. Objects with charge are also sources of electromagnetic fields, which are best understood as arrows that, when multiplied by the charge of an object, tell you how hard and in which direction the object will be pushed by the electromagnetic force.

Electromagnetic fields are described by Maxwell’s equations:

    \begin{gather*}\nabla\cdot\Bf{D} = \rho_f\\\nabla\cdot\Bf{B} = 0\\\nabla\times\Bf{E} = -\pder[\Bf{B}]{t}\\\nabla\times\Bf{H} = \Bf{J}_f + \pder[\Bf{D}]{t}.\end{gather*}

If we start with Faraday and Ampere’s laws in the absence of free charges and current, as well as any media for which the relative permeability \mu is much different than 1 (an assumption that holds up in most cases);

    \begin{gather*}\nabla\times\Bf{E} = -\pder[\Bf{B}]{t}\\\frac{1}{\mu_0}\nabla\times\Bf{B} = \pder[\Bf{D}]{t}\end{gather*}

and then we take the curl of Faraday’s law and the time derivative of Ampere’s law and apply a vector identity for the double curl operator, we can obtain the EM wave equation in terms of \Bf{E}. I’m brushing past this point rather quickly here, but this is actually a momentous manipulation of Maxwell’s equations. It was the discovery of this equation with its requisite wave speed equivalent to the speed of light that led Maxwell to conclude that light was nothing but electromagnetic radiation!

    \begin{gather*}\nabla^2\Bf{E}-\nabla(\nabla\cdot\Bf{E})-\ten{\eps}\cdot\frac{1}{c^2}\pders[\Bf{E}]{t}=0.\end{gather*}

If the reader is familiar with partial differential equations, they will recognize this as nothing more than an ordinary wave equation. This tells us that we can have electromagnetic disturbances that propagate through space according to the wave equation. These are electromagnetic waves. What’s more, this equation has a wave speed equal to the speed of light c. So, put simply, matter with net charge moves around the universe and creates oscillatory disturbances that propagate through space in the form of electromagnetic waves. These waves are the answer to the question what is light?

An oscillating dipole radiates electromagnetic waves.

Bibliography

This article is an excerpt from Prof. Rodríguez’s PhD thesis. If you would like to cite it for your own work, please cite:

– J. A. Rodríguez, Electromagnetic Wave Manipulation with Plasma Metamaterials, Ph.D. thesis, Stanford University (2023).

[1]: Mark A.S. McMenamin. Dynamic Paleontology: Using Quantification and Other Tools to Decipher the History of Life. Springer Chem, Switzerland, 2016.

[2]: Andrew Parker. In the blink of an eye. Perseus, Cambridge, MA, 2003.

[3]: Yakir Luc Gagnon, Rachel Marie Templin, Martin John How, and N. Justin Marshall. Circularly polarized light as a communication signal in mantis shrimps. Current Biology, 25(23):3074–3078, 2015.

[4]: Jean-Jacques Hublin, Abdelouahed Ben-Ncer, Shara E. Bailey, Sarah E. Freidline, Simon Neubauer, Matthew M. Skinner, Inga Bergmann, Adeline Le Cabec, Stefano Benazzi, Katerina Harvati, and Philipp Gunz. New fossils from jebel irhoud, morocco and the pan-african origin of homo sapiens. Nature, 546(7657):289–292, 2017.

[5]: Galileo Galilei. Sideris Nuncius. Apud Thomam Baglionum, Venice, 1610.

[6]: Isaac Newton. Opticks: or, a treatise of the reflexions, refractions, inflexions, and colours of light. The Royal Society, London, 1704.

[7]: James Clerk Maxwell. A dynamical theory of the electromagnetic field. Philosophical Trans- actions of the Royal Society of London, 155, 1865.

[8]: James Bradley. Iv. a letter from the reverend mr. james bradley savilian professor of astronomy at oxford, and f. r. s. to dr. edmond halley astronom. reg. &c. giving an account of a new discovered motion of the fix’d stars. Philosophical Transactions of the Royal Society of London, 35(406):637–661, 1728.

[9]: Anton A. Huurdeman. The Worldwide History of Telecommunications. John Wiley & Sons, Hoboken, NJ, 2003.