I n my first months as a physics journalist nearly a decade ago, I kept running into an inscrutable string of characters: AdS/CFT. Thoroughly intimidated, I decided to just ignore it. But I couldn’t keep my head in the sand for long. I soon learned that those characters are shorthand for a surprising connection between the seemingly inharmonious worlds of gravity and quantum mechanics. And even more bizarrely, this “anti-de Sitter/conformal field theory” correspondence suggests that gravity eliminates the distinction between volume and area. This broader idea is known as the holographic principle, and it now strikes me as the most profound proposal in theoretical physics in the last 30 years. Theoretical physicists tend to vote with their feet, and AdS/CFT sparked a stampede. The three foundational papers on the topic in the late 1990s have garnered tens of thousands of citations, making them by far the most highly cited theoretical physics works of the digital era.…