The 20 words defined in this lexicon reflect the ways in which light irradiates the atmosphere, the universe, and our perception of the world. Because no single system--scientific, religious, philosophical, or cultural--can possibly encompass every meaning of light, this lexicon is systematically unsystematic, exploring each of these realms through words that serve as synecdoches for ways in which we understand light and its myriad effects. Each of Earth's poles has an aurora, which can occasionally grow large enough to be seen near the equator, inspiring visions of apocalypse. For the Inuit, who are more accustomed to it, the aurora is perceived as a football game played by spirits in the heavens. The scientific explanation is no less astounding.

Thanks to advances in machine learning over the last two decades, it's no longer in question whether humans can beat computers at games like chess; we'd have about as much chance winning a bench-press contest against a forklift. But ask the current computer champion, Google's AlphaZero, for advice on chess theory, like whether a bishop or a knight is more valuable in the Ruy Lopez opening, and all you'll get is a blank stare from a blinking cursor. Theory is a human construct the algorithm has no need for. The computer knows only how to find the best move in any given position because it's trained extensively--very extensively--by practicing against itself and learning what works. Even with a lead time of 18 months, the neural network was able to see El Niño events coming.

Can AI teach itself the laws of physics? Will classical computers soon be replaced by deep neural networks? Sure looks like it, if you've been following the news, which lately has been filled with headlines like, "A neural net solves the three-body problem 100 million times faster: Machine learning provides an entirely new way to tackle one of the classic problems of applied mathematics," and "Who needs Copernicus if you have machine learning?". The latter was described by another journalist, in an article called "AI Teaches Itself Laws of Physics," as a "monumental moment in both AI and physics," which "could be critical in solving quantum mechanics problems." The trouble is, the authors have given no compelling reason to think that they could actually do this.