The second most abundant particle in the universe after the photon is the neutrino. It is a shifty little thing. It has no electric charge, very little mass, travels very near the speed of light, interacts only with the weak nuclear force and passes through Earth as though it were not there.

Wolfgang Pauli, a key physicist in the development of quantum theory, invented the neutrino in 1930 as a solution to a problem. The problem was that when a type of nuclear radiation called beta decay occurs, the ejected electrons do not have enough energy and momentum.

In beta decay a neutron in the nucleus of an atom becomes unstable and emits a negatively charged electron. This leaves behind an excess positive charge, which appears in the nucleus as a proton. Because energy and momentum before and after the beta decay are not equal, the disintegration of the neutron at first appeared to violate the conservation of energy and momentum, two of the stalwart laws of physics.

To “save” the conservation laws, Pauli proposed that a new particle yet unseen carried away the excess energy and momentum.

Separately over the next three decades, physicists discovered that there are three members of the family of leptons: electrons, mu mesons and tau mesons. For each of the three leptons, the Standard Model assigns a corresponding neutrino.

In 1956, physicists first detected the electron neutrino from nuclear power plants, where nuclear fission produces them in large quantities. Detection of the muon neutrino followed soon thereafter in 1962, and the tau neutrino not until 2010.

For years physicists debated whether the neutrino had mass or was massless like the photon. They knew that if it had mass it had to be very small.

In the sun, beta decay during nuclear fusion produces neutrinos in astoundingly large numbers, providing an excellent source of neutrinos to study. A theoretical problem developed when measurements of the flux of neutrinos coming from the sun was only one-third to one-half the number that calculations had predicted.

From 1998 to 2002, Takaaki Kajita and Arthur McDonald, winners of the 2015 Nobel Prize in physics, determined that the neutrinos were changing identity, spontaneously oscillating between the three flavors as they traveled through space.

Due to a quantum oddity, neutrinos must have mass because it takes time for the neutrino to evolve from one state to another during the oscillations. A massless photon that travels at the speed of light does not experience time.

The neutrino’s mass is estimated to be unimaginably small, in the range of one-millionth of an electron’s mass.

Although the mass of neutrinos is small, the universe is full of them. Each star in every galaxy produces them in abundance. Billions pass through every square inch of our bodies undetected every second.

There are so many of them that their overall mass is similar to the visible mass of the universe, which brings to mind the mysterious “dark matter,” which is another major puzzle in physics.