In the time it takes you to read this sentence, uncountable trillions of neutrinos have passed through your body. These ghostly particles rain down on us from the sun, but also from sources outside our solar system. Just a tiny fraction of neutrinos will run into anything on Earth, but scientists just detected one from outside our galaxy for the first time ever. It came from a supermassive black hole some 3.7 billion light years away, and then it collided with some ice in Antarctica.
Neutrinos are created by radioactive decay in stars, during supernovae, or as matter spirals into a black hole. They have the lowest known mass of any elementary particle, are electrically neutral, and only interact weakly with other matter. That means neutrinos fly right through planets, stars, and even you at nearly the speed of light. Scientists on Earth have managed to devise methods to detect the few neutrinos that do smack into atoms, and the National Science Foundation’s IceCube Neutrino Observatory spotted a very special Neutrino last year.
On Sept. 22, 2017, scientists using the IceCube observatory detected a high-energy neutrino striking the Antarctic ice. It had an energy of 300 trillion electron volts. That’s 45 times more energy than the Large Hadron Collider can produce in a collision. That provided good evidence that the neutrino came from outside our solar system. The team was able to calculate the probable path of the neutrino.
The IceCube observatory operates out of the Amundsen–Scott South Pole Station, so the team had to look at the possible neutrino sources in the sky over that location. Data from the Fermi Large Area Telescope Collaboration pointed to an object known as a blazar. These are active galaxies with supermassive black holes at the center. That also describes quasars, but the difference is a blazar is spewing a jet of particles and radiation in the direction of Earth. Around the time IceCube detected the impact, Fermi noted that the blazar TXS 0506+056 was brighter (in gamma rays) than it had been in more than a decade, and it was in just the right place to match the trajectory of the neutrino.
This is the first time we’ve detected a neutrino from such a distant source. The study of these particles can help unravel the mysteries lurking in the most extreme environments of the universe. Imagine what secrets are hidden in all the neutrinos that passed through your body while reading this.