Scientists using a telescope buried under the Antarctic ice sheet have found the first sign of high-energy neutrinos that come from outside the solar system. The discovery is a huge step to finding the source of cosmic rays — high-energy particles that speed through space and appear to come at Earth from all directions.
The details of the study will be published in the journal Science on Friday, Nov. 22.
The origin of cosmic rays, and what could be accelerating the particles in them, has been a mystery since their discovery more than 100 years ago. That's partly because cosmic rays are electrically charged, and as a result, the particles get deflected from their original track as they interact with the magnetic fields in space. The particles travel across the galaxy in looping paths that make it nearly impossible for detectors on Earth to trace where cosmic rays come from.
But neutrinos are different. A neutrino is one of the fundamental particles that make up the universe. The nearly massless particle has no electric charge, and is therefore not affected by magnetic fields.
Neutrinos are everywhere — zipping through our bodies and entire planets right now — maintaining their speed and direction as they strike Earth at the speed of light. Once neutrinos get here, they are still tough to detect because they interact weakly with other particles. But if observed and confirmed, the particles can act as pointers to the place where they originated since they move in a straight line.
Previously, scientists have observed low-energy neutrinos that originate in Earth's own atmosphere. The IceCube Neutrino Observatory was designed to look for high-energy neutrinos — those that come from the outer reaches of our galaxy and beyond. Detecting these neutrinos would be the first solid evidence of neutrinos coming from cosmic sources, like exploding stars, gamma rays bursts, or black holes, that are millions, or even billions of light years from Earth.
IceCube has been scanning for high-energy neutrinos since 2010, using thousands of sensors placed inside a cubic kilometer block of ice located a mile below the surface. As neutrinos pass through and collide with atoms inside the ice, they sometimes give off other charged particles. The charged particles will give off light as they travel through the ice. The telescope detects this light.
Most neutrinos that IceCube detects come from the Earth's atmosphere and are not of interest to scientists studying cosmic rays. But in the summer of 2012, team members reported two neutrinos with energies above what would be expected if they came from the atmosphere.
Looking back through their records, scientists found 26 more super-energetic neutrinos, including the most energetic neutrinos ever observed.
These particles have the characteristics that scientists predict of neutrinos that come from extraterrestrial sources.
The new research still doesn't solve the riddle of where cosmic rays come from — more observations are needed to trace the high-energy events back to any one location, Gregory Sullivan, a physics professor who led a team of researchers from the University of Maryland, said in a statement.
But the finding proves that IceCube has the ability to see these high-energy events, and this sensitivity will improve as new and better neutrino detectors are put to work. "The era of neutrino astronomy has begun," Sullivan said.
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