It was one of the great missed connections of physics. In 1965 a particle theorist derived a formula for the collision of elementary particles. Twenty years later two gravitation theorists, using completely different techniques, derived a formula for the collision of stars or black holes. And they were the same formula. The only difference was that the first used "p" to denote momentum and the second used "P". Harvard physicist Andy Strominger jokes that "a 6-year-old could look at those two papers" and spot the similarity. But evidently no 6-year-old did, so their resemblance went unnoticed until Strominger realized it in 2014.