Closing in on the cosmic origins of the “OMG Particle”
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 kilometers 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 hot air balloon, taking with him instruments to measure levels of radiation as he ascended. He found the readings increased as he went up – contrary to the prevailing belief that they would decline with altitude – and concluded that this radiation must be caused by something coming from space, not Earth. That something became known as cosmic rays, though we now know them to be particles, often protons or clusters of protons and neutrons.
When cosmic rays hit our atmosphere, they usually collide with molecules in the atmosphere, producing a shower of energetic particles that rain down. (These descendants of the original particle still contain a lot of energy and have been suspected of interfering with the electronics of aircraft.) It is this shower of secondary particles that we have learned to detect, allowing us to infer the energy of the cosmic ray that produced it. We now know that cosmic rays come in a range of energies. The least energetic are the most common, with each square centimeter of the outer atmosphere being hit once a minute by one of them. The most energetic are much rarer – they strike only once a century per square kilometer.
The rays that Hess detected were relatively modest in energy, it turns out, measuring less than 1 gigaelectronvolt (GeV). It wasn’t until the 1960s that more extreme versions were found, when physicist John Linsley used an array of ground detectors in New Mexico to spot the shower created by a cosmic ray with the vastly greater energy of 100 exaelectronvolts (EeV).
That was a staggering find. But the best was yet to come. In the 1980s, a larger project called the Fly’s Eye telescope array was built in Utah [at Dugway Proving Ground, see photo above]. It had more than 100 detectors, each equipped with a 1.5-meter-wide mirror to look for the flash of particles colliding in the atmosphere. Each of the telescope’s detectors were designed to point at a different part of the field of view, in a similar way to insects’ compound eyes. It was this that earned the telescope its name. “We were hoping we might pick up something really unusual,” says David Kieda at the University of Utah, who worked on the telescope at the time.
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