Almost all we know about the universe derives from the observation of photons. Radio waves (and radar), infrared (used by night-vision goggles and heat-seeking missles), visible light, ultra violet waves like those that give you a suntan, X-rays and the powerful and deadly gamma rays such as those now seen coming from neutron stars, are all electromagnetic waves composed of photons. We are learning some further things about the cosmos beyond the solar system by observing cosmic rays, which are mostly made up of either atomic nuclei minus their orbiting electrons, or one of their basic components, protons. But these positively charged particles do not point to their place of origin due to the magnetic fields of our galaxy which affect their flight paths like a magnet affects iron filings.
What is needed for deep, sharply focused examination of the universe is a telescope that can see a particle that is not much affected by the gas, dust, and swirling magnetic fields it passes on its journey. Neutrinos are a candidate. They constitute much of the total number of elementary particles in the universe, and these neutral weakly interacting particles (see section NEUTRINO) come to us almost without any disruption straight from their sources, traveling at very close to the speed of light. A (low energy) neutrino in flight would not notice a barrier of lead fifty light years thick. When we are able to see outwards in neutrino light we will doubtless get a wondrous new view of the universe.
The fundamental building blocks of the universe of which all matter is composed, consist of Fermions: the quarks (up, down, charmed, strange, top and bottom) and leptons (electron, muon and tau-on, plus a neutral particle partner for each, electron-neutrino, muon-neutrino and tau-neutrino). As a result of data from the Super-Kamiokande experiment presented in 1998, we now know with high probability that some neutrinos have mass, and thus so do all the Fermions. One of the greatest challenges to elementary particle physics is now to explain the great gap between neutrino masses and those of the electrically charged fundamental Fermions (a factor of more than a hundred billion).
Neutrinos were made in staggering numbers at the time of the Big Bang. Like the cosmic background radiation (see section on this) the neutrinos now posess little kinetic energy (the energy of motion that is to say, like the energy of an incoming meteor) due to expansion of the universe. There are expected to be at least 114 neutrinos per cubic centimeter, averaged over all space. There could be many more at earth because of condensation of neutrinos, now moving slowly under the gravitational pull of our galaxy. As of now, we only have a lower limit on the total mass in this free floating ghostly gas of neutrinos, but even so it is roughly equivalent to the total mass of all the visible stars in the universe.
These relic neutrinos would be wonderful to observe, and much thought has gone into seeking ways to do so. The problem is that the probability of neutrinos interacting within a detector decreases with the square of the neutrino's energy, for low energies. And even in the rare case when the neutrino does react in the detector the resulting signal is frustratingly miniscule. Nobody has been able to detect these lowest energy neutrinos as yet. Prospects are not good for a low-energy neutrino telescope, at least in the near future.
Next best are neutrinos from the nuclear burning of stars. Here we are more fortunate, as we have the sun close by producing a huge flux (number per unit area per unit time) of neutrinos, which have been detected now in five experiments (see SOLAR NEUTRINOS). A thirty year mystery persists in the deficit of about a half in the numbers of neutrinos detected compared to expectations, the so-called "Solar Neutrino Problem". This deficit is now thought probably to be due to neutrino oscillations. Really, the fact of calculating the expected neutrino flux from our sun and getting the answer to be close to observations represents a great triumph for our understanding of stellar burning and evolution. So, in this sense we are already doing neutrino astronomy. However, we are limited to the sun. Just as the sky is dark at night despite all the stars, the sun outshines all the rest of the cosmos in numbers of neutrinos we detect.
A marvelous event occurred at 07:35:41 GMT on 23 February 1987, when two detectors in deep mines in the US (the IMB experiment) and Japan (the Kamiokande experiment) recorded a total of 19 neutrino interactions over a span of 13 seconds. Two and a half hours later (but reported sooner) astronomers in the Southern Hemisphere saw the first Supernova to be visible with the unaided eye since the time of Kepler, 250 years ago, and this occurred in the Large Magellanic Clouds at a distance of some 50 kiloparsecs (roughly 150,000 light years). From this spectacular beginning to neutrino astronomy followed many deductions about the nature of neutrinos, such as limits on mass, charge, gravitational attraction, magnetic moment, and so on, and several hundred publications. Never had so much science and astronomy been extracted from so few events. Supernovae of the gravitational collapse type, occur when elderly stars run out of nuclear fusion energy and can no longer resist the force of gravity. The neutrinos wind up carrying off most of the in-fall energy, some 10% of the total mass-energy of the inner part of star of about 1.4 solar masses. Approximately 3 x 10^53 ergs get released with about 10^58 neutrinos over a few seconds. This is a staggering thousand times the solar energy release over its whole lifetime! The awesome visible fireworks consist of a mere one thousandth of the energy release in neutrinos.
Much can yet be learned from the death throes of stars, not only about the process of stellar collapse to a neutron star or black hole (the latter if the progenitor is very massive), but also about properties of neutrinos. For example, heavier neutrinos travel more slowly and by studying the structure of the neutrino wave passing by earth, we can perhaps extract the relative masses of the three types of neutrinos in a direct way, aside from that of the phenomenon of neutrino oscillations. As of this time (1999), four underground detectors (Super-Kamiokande in Japan, SNO in Canada, LVD, and MACRO in Italy) have significant capability for supernova detection from our galaxy. The rate of visible supernovae in our galaxy is only about one per 200 years from historical information, but many cannot be seen optically due to the obscuration of the galactic plane. From historical records and from observations of distant spiral galaxies we expect the rate of supernovae in our galaxy to be between one per twenty and one hundred years. Thus experimentalists may have to wait a long time before the next observation, and we have no way of predicting when it will occur.
Moving up in energy, physicists have realized for many years that higher energy neutrinos would be made inevitably in many of the most luminous and energetic objects in the universe. The most powerful objects seen are active galactic nuclei, which are known to produce particles with energies much higher than the most powerful human-constructed particle accelerators. There also exist enigmatic objects such as Gamma Ray Bursters, which may be the most energetic explosions observed and which are mostly at cosmological distances. These produce gamma rays up to great energies as well, and may be bountiful neutrino sources or maybe not, depending upon the mechanism for the radiation, at present a mystery. Seemingly disallowed cosmic rays have been observed in recent years, with energies more than a 100 million times greater than terrestrial accelerators (more than 10^20 eV). These mysterious particles apparently do not come from our galaxy, and indeed remain of unknown origin. In fact, after nearly a century of study, we do not know the origin of the cosmic rays generally, particularly above about 1 PeV (10^15 eV), though many models have been proposed. Whatever the source, the machinery which accelerates particles to the higher energies will inevitably also produce neutrinos. At the highest energies many speculative models have been proposed as neutrino sources, including decays of Planck mass objects left over from the Big Bang, radiation from around super-conducting cosmic strings and the like, exciting findings if verified, and of fundamental importance to particle physics and cosmology.Thus we know that high energy neutrinos surely arrive to us from the cosmos, and may teach us much as we study their directions, energy, type and variation with time. The burning question for would-be neutrino astronomers is however, are there enough neutrinos to detect? Two things make prospects more bright in the near future for higher energy neutrino astronomy than lower energies.
First the interaction probability for neutrinos goes up with energy. For the largest present underground detector, Super-Kamiokande, only about one in a trillion neutrinos of the typical energy (about 1 GeV, or the equivalent to the proton rest mass) interact when passing through the detector and can be studied. This goes up almost in proportion to the energy of the neutrino however. In fact above about 1 PeV, the earth is opaque to neutrinos and one must look for neutrinos only coming downwards. At lower energies one does neutrino astronomy backwards from optical astronomy, looking downwards, using the earth to filter out anything but neutrinos. It is this region between 1 TeV (10^12 eV) and 1 PeV, roughly, that is the favorite hunting ground for attempts to begin regular neutrino astronomy.
The second virtue of seeking higher rather than lower energy neutrinos is that the consequences of neutrino interaction with a target (earth or detector) become more detectable as the energy release is greater. The favored method is to detect muons produced by neutrinos. These muons (unlike electrons or tau-ons) fly a long distance (in closely the same direction as the neutrino) in earth before stopping, for example about one kilometer at an energy of a few TeV. These charged particles produce Cherenkov radiation, a short flash of light detectable at tens of meters distance by photomultipliers in clear water or ice. Cherenkov radiation occurs when particles exceed the velocity of light in the medium (75% of c in this case), and which is rather like an electromagnetic version of a sonic boom, or the wake of a ship. Thus a detector can effectively collect the results of neutrino interactions from a target volume much greater than the detector volume itself.
Neutrino detectors must be generally placed deep underground or water to escape the backgrounds caused by the inescapable rain of cosmic rays upon the atmosphere. The cosmic rays produce many muons which penetrate deeply into the earth, in even the deepest mines, but of course with ever decreasing numbers with depth. Hence the first attempts at high energy neutrino astronomy have been initiated underwater and under ice. The lead project, called DUMAND was canceled in 1995 on account of slow progress and budget difficulties, but managed to make great headway in pioneering techniques, studying backgrounds, exploring detector designs, and perhaps most importantly stimulating the community to consider neutrinos in astrophysics. Another long running project exists in Lake Baikal, the largest and deepest lake in the world, in Siberia. That instrument consists of large light detectors (0.4 m diameter) lowered on cables from the winter ice and connected to shore by cable. The Baikal project has reached a level of producing some modest physics results, including atmospheric neutrino detections, but is still a few years from significant neutrino astronomy since the present area amounts to only a few hundred square meters.
Two projects similar to DUMAND are underway in the Mediterranean, the more developed NESTOR Project located off Pylos in the Southwest of Greece, and the new ANTARES Project located offshore from Marseilles, France. Another project is being talked about for southern Italy as well. These projects differ in the method of supporting photo-detectors and array geometry, but basically employ the same method of bottom anchored cables with photomultipliers protected in spherical glass pressure housings, as developed for DUMAND. Both projects aim at prototype neutrino detectors in the near future (several years, with NESTOR a bit ahead). The prototype instruments will have effective areas for muon collection in the range of 20-50 thousand m^2 area. This may be compared with the largest present underground instruments which are about 1000 m^2 area, and the desired size for real astronomy of about one million m^2 (a square kilometer).
The deep ocean water is amazingly clear with optical attenuation lengths of 40-50 meters. Instruments can be spaced a few tens of meters apart to detect most muons passing nearby. An array of vertical strings of such detectors can cover a whole cubic kilometer employing roughly the same number of detectors as in the existing Super-Kamiokande deep mine instrument. Of course, placing these photo-detectors in the deep ocean is much more tricky and costly than in a tank in a mine, but the point is that such detectors are now well within the realm of technical feasibility and costs not large compared to equivalent scientific endeavors.
A different type of neutrino telescope is under construction at the South Pole, in ice, the AMANDA Project. It turns out that ice below about 1.4 km depth is quite clear (100 m attenuation length) and bubble free, though optical scattering is still somewhat of a problem (25 m effective scattering length). The experimenters have worked out a method to use hot water to drill 2 km deep holes, down which they lower strings of photomultipliers. The instruments become permanently frozen-in after about a day, but the cables can be accessed at the surface, so no complex and expensive electronics need be placed in the inaccessible holes. This array is topologically rather like the underwater arrays, turned bottom-side up. The AMANDA group has reported the detection of a few upcoming neutrino events, a demonstration of feasibility.
There have been many discussions about the relative virtues of the deep lake, ocean and ice approaches, and each has attractions and liabilities (for example, while the underwater arrays can be retrieved for service or reconfiguration, the local background light is worse and the access not easy). At this time all four (possibly five) projects are making progress and working towards detectors of a few tens of thousands of m^2 in a few years. Hence, it seems likely that real high energy neutrino astronomy with kilometer scale projects is still about a decade away. Meanwhile, calculations go on, and underground detectors wait patiently or the next galactic supernova. In the very long run, as has been the case with every venture into new parts of the electromagnetic spectrum, one can be sure that neutrino astronomy will teach us many new and unexpected wonders as we open a new window upon the universe.
Some further references on neutrino astronomy are: