Neutrinos!
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neu·tri·no /nuːˈtriːnoʊ/ n. (pl. neu·tri·nos) 1 A particle with no electric charge and almost no mass. It hardly ever touches anything, so it passes through people, planets and stars as if they were empty space. 2 After light itself, the most common particle in the universe. The Sun makes neutrinos in vast numbers in the nuclear fusion at its core. Exploding stars, cosmic rays striking the atmosphere, nuclear reactors and radioactive rock inside the Earth make them too, and a sea of them left over from the Big Bang still fills space. [It., 'little neutral one', from neutro 'neutral'; named by Enrico Fermi in the early 1930s]
- About 65 billion neutrinos from the Sun pass through every square centimeter of you, every second.
- Most neutrinos would pass through a light-year of solid lead without hitting anything.
- At night, neutrinos from the Sun come up through the floor. They have just crossed the whole Earth, and it barely stopped any of them.
- The Sun makes neutrinos in its core, where hydrogen fuses into helium. Every helium nucleus made there sends out two of them.
- A neutrino is out of the Sun two seconds after it is made and reaches the Earth about eight minutes later. Light made in the core at the same moment needs tens of thousands of years to get out.
- Your body sends out neutrinos, a few thousand a second, from the radioactive potassium in it.
- Wolfgang Pauli proposed the neutrino in 1930 to account for energy that went missing in radioactive decay. Nobody detected one until 1956.
- The first detectors caught only about a third of the neutrinos expected from the Sun. The rest had changed type on the way. Neutrinos come in three types and switch between them as they travel, a discovery that won the 2015 Nobel Prize in Physics.
- Super-Kamiokande in Japan watches for neutrinos in 50,000 tons of ultra-pure water, a kilometer underground, with about 11,000 light sensors.
- IceCube, at the South Pole, uses a cubic kilometer of Antarctic ice as a neutrino detector.
- Super-Kamiokande has made a picture of the Sun out of neutrinos. It works at night too, straight through the Earth.
- In 1987, detectors in Japan, the United States and the Soviet Union caught about two dozen neutrinos from an exploding star in a neighboring galaxy, a few hours before its light arrived.
- A neutrino has less than a millionth of the mass of an electron.