Neutrino

The size of a subatomic particle is expressed in terms of a cross-sectional area; the larger the cross section, the more likely the particle is to interact with ordinary, solid matter, which consists largely of empty space occupied by matrices of widely separated atoms. The cross section of a neutrino is very small. For a beam of neutrinos passing through the center of the Earth, only one in a trillion neutrinos would be blocked by the Earth. A slab of lead 100 light years thick will only absorb roughly one third of the neutrinos striking it. This property makes neutrinos difficult to find; they zip right through almost any detector. About 60 billion solar neutrinos pass through every square centimeter of the Earth's surface every second.

The neutrino's small cross section makes it a useful probe of the Sun's interior, for the energy-producing nuclear reactions in the Sun's core yield very great quantities of neutrinos. A unit of energy produced at the center of the Sun takes several million years to migrate to the Sun's surface; the neutrinos, however, zipping through the Sun's substance at nearly the speed of light, make it to Earth in about eight minutes. Neutrinos, therefore, can reveal what is going on in the Sun's core now rather than a few million years ago.