1956

Neutrino Astronomy

Wolfgang Pauli (1900–1958)

Early-twentieth-century physicists, such as Wolfgang Pauli, who were trying to understand the nature of the radioactive decay of certain elements knew that they had big holes in their understanding of the atom. Energy released during radioactive decay, for example, couldn’t be explained just by the protons and electrons created in the process. The discovery of the neutron in 1933 didn’t help—it was too massive. An additional low-mass, electrically neutral elementary particle also had to be involved. Pauli had theorized the existence of a new subatomic particle that would later be called the neutrino (“little neutral one”).

Actual evidence for the existence of the neutrino came in 1956, in high-energy particle accelerator collision experiments. Subsequent experiments in the 1960s showed that neutrinos come in different varieties, or “flavors,” each associated with other kinds of elementary particles (including electrons), and that each flavor of neutrino also has an antiparticle. It was becoming evident that atoms are busy places.

Discovery of the neutrino opened up the entirely new field of neutrino astronomy. Without a charge, and having almost negligible mass, neutrinos easily pass through even enormous quantities of ordinary matter at nearly the speed of light and with little attenuation. Neutrinos generated in the nuclear fusion reactions that occur deep inside the Sun and other stars thus pass almost effortlessly through the star and can be detected at Earth; by contrast, photons—sunlight—generated in the Sun’s interior can take more than 40,000 years to bounce around and escape from that dense, opaque environment.

An image of the Sun taken by collecting 500 days of neutrino measurements using the detectors in the Japanese Super Kamiokande observatory.

Today, special neutrino detectors enable us to understand more details of nuclear reactions occurring deep within the Sun, like supernovae, black holes, and even the Big Bang, as well as other inaccessible environments.

The interior of the Super Kamiokande chamber uses 11,200 photomultiplier tubes immersed in 50,000 tons of pure water to detect and measure neutrinos from the Sun and other cosmic sources.