Millions of light-years away, a star explodes as a supernova and sends a host of subatomic particles called neutrinos in all directions. One of these heads towards our Solar System and, after millions of years, this tiny neutrino enters Earth’s atmosphere and collides with an atom inside a detector below the ice of Antarctica. The detectable signal produced not only confirms the neutrino’s existence, but also indicates where it has come from.

This is the amazing process that has now been confirmed to be taking place by the IceCube Collaboration in Antarctica. Neutrinos, nearly massless high-energy particles with no charge, are known to have sources here on Earth and in the Solar System, such as the Sun. But astronomers wanted to prove that they were also created elsewhere in the universe, covering vast distances of the cosmos. Now, they have that proof – and these neutrinos could act as subatomic signposts to exotic phenomena. The results are published in the journal Physical Review Letters.

The existence of cosmic neutrinos was hinted at in 2013 when two – dubbed Bert and Ernie – were found by the IceCube Observatory. However, astronomers needed to confirm that these were definitely not coming from a source in the Solar System. So they fired up the detector again and recorded 35,000 more neutrinos. Twenty-one of these were confirmed to have an energy high enough to indicate they came from beyond the Solar System – and possibly beyond the Milky Way.

“It is sound confirmation that the discovery of cosmic neutrinos from beyond our galaxy is real,” said Albrecht Karle, a professor from the University of Wisconsin-Madison and a senior author on the study, in a statement.

^{Shown in red, the neutrinos were found across the sky. IceCube Collaboration.}

The neutrinos were found by detecting 21 ultra high-energy muons. These are secondary particles created when neutrinos bump into other atoms. As neutrinos are almost massless, they are incredibly hard to detect aside from spotting these muons.

To detect them, the IceCube Observatory uses thousands of optical sensors beneath the ice at the South Pole. It can spot the muons because they move faster than the speed of light in a solid. Note that the speed of light isn’t being broken here – rather, light changes speed depending on what medium it is traveling through. In a vacuum it travels at its limit, but in things like glass and ice it travels slower. But muons are not limited in this way; they travel faster through matter, producing noticeable Cherenkov radiation – a light wave produced in their wake, similar to a boat moving through water. 

The importance of finding the cosmic neutrinos is that they might point towards exotic phenomena in the universe. While we mentioned that they can form in supernovae, they are also thought to orginate in black holes, during star formation and elsewhere. But no single source has been found as the main culprit of neutrinos, something that might be discovered with future research.

“Cosmic neutrinos are the key to yet unexplored parts of our universe and might be able to finally reveal the origins of the highest energy cosmic rays,” said collaboration spokesperson Olga Botner of Uppsala University in Sweden in a statement. “The discovery of astrophysical neutrinos hints at the dawn of a new era in astronomy.”