Every second, day and night, hundreds of trillions of invisible particles are zipping through the Earth and everything on it—including you. These tiny bits of matter, called neutrinos, constantly bombard the Earth at nearly the speed of light (299,792 kilometers [186,282 miles] per second). During your entire lifetime, however, only a few of them will collide with an atom in your body.

Because they are almost unimaginably small and light, neutrinos have posed some of the biggest mysteries in modern physics, and they have been extremely hard to study. More than 70 years passed from the first hints that neutrinos exist to the discovery of a type of neutrino known as a tau-neutrino. A group of researchers at the Fermi National Accelerator Laboratory (Fermilab) near Batavia, Illinois, discovered evidence for the elusive tau-neutrino in July 2001, completing the picture of the neutrino family. At about the same time, a team of scientists in Canada reported that they had solved a long-standing mystery of physics: why scientists were detecting fewer neutrinos than expected from the sun.

Neutrinos are important because they play a key role in the reactions that power the sun and other stars. They also play an important role in several critical reactions that take place in the nucleus (central part) of an atom. Studies of neutrinos may also provide astronomers with information about the centers of galaxies, which are often obscured by gigantic clouds of dust and gas. Physicists who are studying these ghostly particles of matter believe that the efforts required to learn about them will one day pay off in a deeper understanding of the structure and age of the universe.

Most neutrinos that bombard the Earth come from the sun, though some are created inside atoms in Earth's atmosphere and others arrive from outside the solar system. Neutrinos are so elusive that nearly all of them go straight through the entire Earth without hitting a single atom. Because neutrinos interact with matter so rarely, scientists must build huge detectors in order to catch just a few of the extremely large numbers of neutrinos that pass through the Earth each day. These detectors contain many tons of matter in order to increase the chances of snagging their prey. The scientists who use these instruments must also be patient, because it can take years to accumulate enough data to learn anything useful about neutrinos.

Neutrinos are only one of many subatomic (smaller than an atom) particles. Atoms are the basic units of all matter, but they are made of even smaller particles. The two main components of the atom's nucleus are positively charged protons and electrically neutral neutrons. Protons and neutrons, in turn, are composed of even smaller particles called quarks. Atoms also contain electrons, negatively charged particles that orbit the nucleus at high speed. Physicists call electrons, quarks, and neutrinos elementary particles because so far there is no evidence that these particles are made up of any smaller units.