A few months ago, one of the writers for Physics World asked his readers to send in their nominations for the most beautiful equation of all time. Being a journal devoted to physics, and not economics or pure mathematics or accounting, the respondants offered a number of candidates in that field: the equation defining Hubble’s constant, the Balmer formula, Maxwell’s equations. (Strangely, Newton’s Law of Universal Gravitation seems to have been relatively unpopular, perhaps because of lingering suspicions that Newton stole the idea from hunchbacked microscope enthusiast Robert Hooke. Given Newton’s smearing of Leibniz over the discovery of calculus, perhaps the Physics World readership felt that the Royal Society, alchemy, and gambling studies were enough historical credit for the nonce.) I probably would have chosen a pure math equation, possibly the prime number theorem or one of the leading votegetters, Euler’s identity, which elegantly ties together every important number in undergraduate-level math (*e*, π, *i*, 1, and 0). Of the nominees selected by the magazine’s readership, I have a sneaking fondness for the oldest of them (not counting `1 + 1 = 2`), however. Some 2500 years ago, Pythagoras of Samos was leading a cult of philosopher-mathematicians. Dedicated to studying the mathematics, purifying their souls, and avoiding beans, the Pythagoreans viewed mathematics as a window into the true nature of reality. They showed how to construct the Platonic solids, discussed the personalities of the integers, and proved an Egyptian theorem that today bears Pythagoras’ name: "for a right angled triangle the square on the hypotenuse is equal to the sum of the squares on the other two sides". The fact that `a ^{2} + b^{2} = c^{2}` is hardly novel today (although there are some notable implications), and there are a staggering number of ways to demonstrate it, including some not based on geometry at all. Chinese mathematician Liu Hui proved it in the third century B.C.(had he been born someplace a bit more central to Western civilization, it might be known the Hui theorem); President James Garfield proved it; in Gödel, Escher, Bach, Douglas Hofstadter uses the fact that a computer has proved it as the starting point for discussing artificial intelligence and the meaning of creativity and intent. A sabremetrical formula for estimating a baseball team’s expected wins shares its name. And once upon a time, a New York Times editorial urged humanity to contact Mars by means of carving a gigantic geometric proof on the Siberian steppes. Any equation can be simple or well-known or meaningful, but what could define beauty better than vast triangles carved into the permafrost in order to signal our intelligence to an alien race?