The sun is a mass of incandescent gas
A gigantic nuclear furnace
Where hydrogen is built into helium
At a temperature of millions of degrees. . . .
--They Might Be Giants
The Super-Kamiokande detector, courtesy Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo
The Neutrino is a subatomic particle famous for its ability to slip through matter without interacting. Neutrinos have none of the "handles" by which most other particles affect one another: no electric charge, almost zero mass. They are so elusive that a light-year of lead, nine and one-half trillion kilometres (six trillion miles) would only stop half of the neutrinos flying through it. The only hope for detecting them is to put a large quantity of matter in one place and hope the occasional neutrino will, by dumb luck, strike an atom somewhere and interact with it. Because so many other radiation sources are releasing energy throughout the Universe, any detector trying to spot neutrinos has to deal with backgroud noise. Picking the signal out of this noise can be a challenge. To make the problem easier, neutrino detectors are built underground, often within deep mineshafts. The rock around the detector blocks any radiation not powerful enough to penetrate beneath the Earth; because neutrinos are so "slippery", they can pass through the rock and reach the detector device.
Neutrinos are valuable to astronomers precisely because they are so evasive. Since even large thicknesses of matter don't have much effect, neutrinos can flow right through things which distort or block other types of radiation. For example, our Sun is a ball of hot gases, 1,392,000 kilometres (870,000 miles) in diameter. Nuclear fusion reactions at the Sun's core heat these gases, producing vast quantities of energy. We would like to know the details of what's going on inside the Sun's core, but the gaseous layers in the way block our view. The gas atoms scatter light so well that a single photon, the basic particle of light, takes roughly fifty thousand years to reach the Sun's surface. Photons leave the core, hit nearby atoms, bounce off them, hit other atoms, and spend centuries doing more and more of the same, until they manage to leak out in the thinner regions near the surface. All that scattering and jostling obscures the details of the interior, just like a bright city skyline looks vague and indistinct when observed through a thick fog. Neutrinos avoid this problem, because they don't like to interact with the Sun's atoms. Once nuclear reactions in the core produce neutrinos, they can radiate away and rapidly escape the Sun. Neutrino detectors, then, can tell us what happens deep within the solar core, because they bring us information directly from the source. In the city analogy's terms, they zip through the fog and reveal the metropolis behind it.
The neutrino entered physics as the brainchild of Wolfgang Pauli (1900-1958). Pauli was trying to explain a puzzling feature of beta decay, a type of nuclear reaction that frequently occurs in unstable heavy elements. In beta decay, a neutron within the atomic nucleus breaks down and turns into a proton, releasing an electron which flies away from the atom. Measurements showed that the electron's energy varied: sometimes it barely crept out of the nucleus's pull, and sometimes it shot away at high speed. Physicists could explain the high-energy case fairly easily: the electron simply carried the maximum energy the reaction could produce. What about the lower-energy cases, Pauli wondered.
A basic principle, the Conservation of Energy, says that energy cannot vanish from existence. It cases where that appears to happen, it is in fact being transformed into a less obvious form. (To a physicist, watching energy vanish from a situation is like how many people feel watching money disappear from their bank account.) For example, when you throw a broken computer out the window, the Earth's gravity gives it a certain amount of energy, which shows up as the computer's speed. The higher it begins, the more energy and hence speed it gains by the time it hits the sidewalk below. When it impacts the ground, though, where does all that energy go? Answer: the kinetic energy the falling computer had from its motion went into thermal energy (both computer and sidewalk are a little warmer than before) and acoustic energy (the crash makes a sound). Also, some goes into distorting the shape of the computer, which (among other things) puts potential energy into bending pieces of metal and plastic.
What kind of phenomenon could carry away the energy the electron didn't use? Pauli dreamt up a new particle, an entity which would pick up the slack, so to speak. It would have to be hard to detect, elusive enough to explain why no one had seen it before. Pauli decided the particle would have no electric charge, and that it would be very light, either totally massless or almost so. Enrico Fermi (1901-1954) named this particle the neutrino, from an Italian word meaning "little neutral one".
A later discovery, flavor change, complicates this issue somewhat. From the mid-1960s, when the solar neutrino flux was first measured, up until about 2002, the "solar neutrino problem" caused much debate. All detectors confirmed a puzzling result: the Sun was only emitting one-third to one-half of the neutrinos we expected. Large amounts of cleverness have gone into solving this problem, and developments in the past few years have been very exciting.
References and Further Reading
On neutrinos in general:
The following articles, part of assigned class readings at MIT, may be useful to those with some experience in quantum mechanics.
Researched, written and maintained by Blake Stacey.