The theory passed another recent test, which led to a science writer’s reflections on the strangeness of pi, the irrational number that pops up everywhere:
Pi seems to pop up all the time—not just explicitly in circles but in the hydrogen atom and the way needles fall across lines. The reason a factor of pi appears in an equation for gravitational waves is a little headier, however: the waves interact with themselves.
“When a gravitational wave is traveling out, it sees the curvature of spacetime, including the energy that was generated by the gravitational waves produced in the past,” Berti says. The first stone you drop into a calm pond sends out smooth ripples across the surface. If you drop another stone immediately after, the surface is no longer smooth—leftover ripples from the previous stone will interfere with new ripples from the second one. Gravitational waves work similarly, but the medium is spacetime itself, not water.
Daniel Garisto, “Pi in the Sky: General Relativity Passes the Ratio’s Test” at Scientific American
More unexpected places pi appears.