The helicopter was flying high through the night sky with its door slightly ajar. Johannes Eser and Matthew Rodencal were in the back controlling a laser pointing out through the gap. They aimed towards a balloon 35 kilometres above them and fired.
It sounds like a scene from a spy movie, but Eser and Rodencal, then at the Colorado School of Mines, were actually testing a plan to spot ultra-high-energy cosmic rays, the most energetic particles ever discovered. They stream across the universe before slamming into our atmosphere and emitting a tiny flash of light. The laser was supposed to mimic that flash.
This twilight helicopter ride happened nearly a decade ago, but is part of a saga that goes back to at least 1991. In October that year, we detected the single most energetic particle ever seen. It had the kinetic energy of a bowling ball dropped from shoulder height, crammed into a subatomic-sized package. It quickly became known as the “Oh-My-God particle” and, naturally enough, scientists were desperate to know where it came from.
Since then, we have spotted many similar particles. Huge ground-based detectors have provided us with maps of where they might come from, together with a shortlist of the extreme cosmic objects that could produce them. But truth be told, we still don’t have all the answers. That is why scientists now want to take the cosmic ray hunt into the atmosphere – and ultimately into space – in an effort to solve the mystery once and for all.
This story really began with another balloon in 1911. At that time, physicist Victor Hess climbed into a …