Six years ago, atmospheric physicist, Joseph Dwyer, accidentally flew his plane into a thundercloud, and right through an unexpected pocket of antimatter. And while we know high-energy positrons - the antimatter counterpart of the electron - can be caused by cosmic rays making contact with the atmosphere, or by particularly active lightning storms, what Dwyer detected could not be explained by either of these scenarios.
"This was so strange that we sat on this observation for several years," Dwyer, from the University of New Hampshire in the US, told Davide Castelvecchi at Nature.
The flight in question took place on 21 August 2009, when Dwyer, then at the Florida Institute of Technology in Melbourne, Florida, set out to detect gamma rays (γ-rays) - extremely high-frequency electromagnetic radiation made up of high-energy photons. He installed a particle detector on a Gulfstream V aircraft, and flew it along the coast of Georgia. Unfortunately for Dwyer's nerves, he ran into a line of thunderstorms, and had no choice but to fly right through them.
"During those frightening minutes, the detector picked up three spikes in γ-rays at an energy of 511 kiloelectronvolts, the signature of a positron annihilating with an electron," says Castelvecchi.
When we talk about a positron annihilating with an electron, we're talking about when an electron and a positron collide and annihilate each other to produce gamma ray photons in a low-energy scenario, or particles such as W and Z bosons in a high-energy scenario. As a form of antimatter, a positron will have properties that are the exact opposite of what its 'normal matter' counterpart - the electron - has, including a reversed electrical charge.
While scientists think there was an abundance of both matter and antimatter just after the Big Bang, antimatter particles have become incredibly rare, because if they weren't, we would be detecting way more of the telltale bursts of energy that come with the process of annihilation. And this extreme rarity only makes them more difficult to detect, because once we know they're there, there's so much normal matter around that they'll be instantly transformed into something else entirely.
The three 511-kiloelectronvolt gamma-rays spikes that Dwyer recorded during his flight through the thunderstorm were part of a temporary cloud - measuring about 1 to 2 kilometres across - that stuck around long enough for Dwyer to fly right through. Now, six years later, he's still trying to figure out why. Castelvecchi explains at Nature:
"Electrons discharging from charged clouds accelerate to close to the speed of light, and can produce highly energetic γ-rays, which in turn can generate an electron–positron pair when they hit an atomic nucleus. But the team did not detect enough γ-rays with sufficient energy to do this.
Another possible explanation is that the positrons originated from cosmic rays, particles from outer space that collide with atoms in the upper atmosphere to produce short-lived showers of highly energetic particles, including γ-rays. … But the motion of positrons would have created other types of radiation, which the team did not see."
Dwyer plans to continue his investigation into what he detected during that fateful flight in 2009, but he's not putting himself at risk this time. He's sending balloons up into thunderstorms instead to see what they can detect. Luckily, balloons don't feel fear.