Although billions of neutrinos traverse every square centimeter of the earth each second, observing them is difficult since they only interact with matter through the, appropriately named, weak force, which is so weak that most neutrinos pass through the entire Earth without interacting even once. However, with complex equipment and lots of time, a very few of these neutrinos can be detected.

Sudbury Neutrino Observatory —designed to catch solar neutrinos. Shown here is an external view of the detector. Photo courtesy of Lawrence Berkeley National Lab.

Stars, like our sun, produce HUGE amounts of neutrinos—over two hundred trillion trillion trillion— every second. But this pales in comparison to a supernova--like 1987A--which emits1000 times more of these elusive particles than the sun will produce in its 10-billion year lifetime.

Neutrinos that originate in the sun are called solar neutrinos. Since they escape the sun's core unimpeded, these solar neutrinos, when detected, can provide information about the reaction processes deep inside the sun. However, data from several neutrino detectors reveal only approximately half the number of neutrinos that are expected from prevailing theories. This is a puzzle commonly known as the Solar Neutrino Problem..

A new detector, the Sudbury Neutrino Observatory (SNO), located in Sudbury, Ontario, Canada, may offer a solution to this problem. Neutrinos are known to come in three types commonly referred to as "flavors." The neutrinos produced by the sun are electron "flavored," and all neutrino detectors before SNO were designed to observe only these neutrinos. Recent findings suggest the possibility that as they propagate through space neutrinos can transform themselves, or "oscillate", from one flavor to another. So, while experiments were counting only electron neutrinos, other flavored solar neutrinos could have been passing through the detectors unnoticed. SNO, which began taking data in 1999, is designed to detect all three flavors of neutrinos. If the experiment confirms neutrino flavor oscillation, SNO will earn its nickname—our window on the sun.