Neutrinos: the ghost particles

Neutrinos are the most common elementary particles in the universe after photons. They are produced in the Sun's core, in supernova explosions, in nuclear reactors, in particle accelerators and in the natural decay of radioactive elements. They travel near the speed of light, have nearly zero mass and carry no electrical charge. That combination makes them practically impossible to detect: they can pass through a light-year of lead without interacting with anything.

The IceCube detector, buried under 2.5 kilometers of Antarctic ice, is one of the most extraordinary instruments ever built by human science. A cubic kilometer of polar ice, instrumented with more than 5,000 optical sensors, waiting to capture the rare events where a neutrino interacts with a water molecule and produces a flash of light. Across the entire detector volume, a few hundred events of interest occur per year.

Why so much effort to detect something that interacts with nothing. Precisely because of that: neutrinos arrive from the universe without being deflected or absorbed, carrying direct information about the most energetic processes in the cosmos. The first extragalactic neutrinos detected by IceCube in 2013 opened a new astronomical window. There are things about the universe that can only be seen with neutrinos.