(Every day until December 25, I’m posting a science-related image and description.)
Day 6Astronomy is mostly about photons — particles of light — and lots of them. Sure, there are astronomical phenomena for which we only capture a small amount of light, but whenever possible researchers like to have large enough numbers of photons that they can be sure they all definitely came from the source in question.
The story is different with neutrinos, the very low-mass particles produced in quantity by stars, supernovas, neutron stars, and other cosmic sources. Doing astronomy with neutrinos is very difficult, not because the particles are rare (far from it!) but because they are hard to detect. Most neutrinos that have ever been detected came from nuclear reactions on Earth, or were produced by cosmic rays hitting atoms in Earth’s atmosphere, or originated in the Sun. However, we know many neutrinos are made in more distant events; we just have to detect and characterize them.
That’s why the image above is so exciting: it represents one of 28 neutrinos from deep space detected by the IceCube laboratory at the South Pole. IceCube consists of long strings of sensitive photon detectors extending two kilometers down into pure Antarctic ice. When a neutrino comes along and researchers are lucky, it hits a water molecule in the ice, which triggers a cascade of light and other particles. Each blob in the image above represents the amount of light in a single detector.
By studying the pattern and strength of these triggering events, the IceCube scientists can determine whether this was a neutrino or something else, and measure how much energy the particle had when it entered the ice. In that way, they determined these 28 neutrinos weren’t created in Earth’s atmosphere or anywhere in the Solar System: they were too energetic, and didn’t match known local sources. Beyond that we can’t go yet: IceCube isn’t able to trace the neutrinos back to their sources, so we don’t know right now what propelled them. However, this is a major step toward real neutrino astronomy: studying far-off events by the neutrinos they emit. Supernova 1987a confirmed that exploding stars produce neutrinos; we’d love to see more. After all, every time we’ve learned a new way of seeing — radio astronomy, X-ray astronomy, particle physics — we’ve found something new to be seen.