When the Nobel Foundation awarded Ray Davis and Masatoshi Koshiba the 2002 Nobel Prize in Physics, it could have chosen to emphasize any of their many accomplishments. Davis made his name detecting neutrinos from the sun—the first of these notoriously elusive particles ever seen from beyond our planet—and Koshiba discovered them coming from the great supernova explosion of 1987. Their work was an experimental tour de force and helped to establish that neutrinos, which theorists had assumed were massless, in fact have a small mass. Yet the Nobel Foundation recognized Davis and Koshiba, above all, for establishing a new branch of science: neutrino astronomy.

With their work, neutrinos graduated from a theoretical novelty to a practical way to probe the universe. In addition to studying neutrinos to glean the particles’ properties, scientists can now use them to lift the veil on some of the hidden mysteries of the universe. In an undertaking akin to the construction of giant optical telescopes a century ago, astronomers have been designing and building vast neutrino telescopes in anticipation of seeing new wonders. These observatories have already caught tens of thousands of neutrinos and made pictures of the sun in neutrinos. Neutrinos from other cosmic sources are hard to tell apart from those produced in Earth’s upper atmosphere, but instruments should be able to do so by this time next year.

At that point, the floodgates of discovery will open, and a particle once derided as unobservable may become indispensable. Neutrinos can reveal things that light is blind to. When we study the sun with light, we are seeing only the surface—just the uppermost few hundred kilometers of gas. Although the energy powering sunlight originates in nuclear reactions at the core, sunlight itself is absorbed and reemitted trillions on trillions of times by the intervening layers of gas, and only very near the surface does it stream freely into space. In contrast, with neutrino eyes we directly see the central fusion engine—the hottest, innermost 1 percent of the sun’s volume. The neutrinos created there pass through the sun’s outer layers almost as if they were empty space.

Neutrinos also will allow us to peer deep into supernovae, other stellar explosions such as gamma-ray bursts, and disks swirling around supermassive black holes. The observatories now under construction should catch sight of about one supernova a year within the nearest 50 or so galaxies. They may also see some of the hundreds of gamma-ray bursts that go off every year, not to mention even more exotic celestial objects that may be going entirely unnoticed. But like every powerful tool, the neutrino takes some getting used to. It requires astronomers to approach their subject in a new way.

The Benefits of Being Antisocial
To a particle physicist, a neutrino is similar to an electron, except for its lack of electric charge, which makes it immune to the electric and magnetic forces that dominate the everyday world. When you sit on a chair, electric repulsion prevents you from falling through. When chemicals react, atoms swap or share electrons. When a material absorbs or reflects light, charged particles react to an oscillating electromagnetic field. Neutrinos, being electrically neutral, pass right through solid matter, play no role in atomic or molecular physics, and are almost completely invisible.

The known types of neutrinos do participate in the weak nuclear force that is responsible for radioactive beta decay and fusion of the heavier elements, but this force, as its name suggests, is feeble except over extremely short distances. Thus, neutrinos barely interact with other particles. To detect them, physicists and astronomers must monitor large volumes of matter, looking for the rare occasion when a neutrino leaves a mark. If cosmic neutrinos collectively have as much energy as cosmic rays (the protons and ions that bombard our planet), as astronomers expect, it will take a cubic kilometer of material to capture a decent sample of them. The biggest observatories are approaching this size.

Physicists have also postulated other neutrinos, so-called sterile neutrinos, that are so standoffish they barely respond even to the weak force; the force of gravity may be their dominant connection to the rest of the universe. These neutrinos are even more challenging to detect.

Aloof though they may be, neutrinos are active participants in the drama of the cosmos. They are a necessary by-product of beta decay, which warms the debris of exploded stars and the interiors of planets and is a crucial intermediate step in stellar nuclear fusion. They are also decisive in one of the two major types of supernovae, those that result from the implosion of a massive star at the end of its life. The implosion compresses the core of the star to nuclear densities and releases 1058 neutrinos in a span of 10 to 15 seconds. In such numbers, even the most antisocial particle cannot help but become the life of the party. Neutrinos account for 99 percent of the total energy released by the cataclysm. Observing them thus lets us see the 99 percent of the picture that ordinary telescopes miss, including the decisive early stages. The detection of neutrinos from the 1987 event confirmed the basic theory of stellar collapse [see “The Great Supernova of 1987,” by Stan Woosley and Tom Weaver; Scientific American, August 1989]. The detectors now available will be able to provide a real-time movie of stellar collapse, rebound and explosion.

Whatever their origins, neutrinos have no difficulty reaching Earth. Not only do they pass through gas and dust, they can cross the entire universe no matter how high their energy is. That is not true of light. The most energetic form of light, gamma rays, is attenuated by the cosmic background radiation—the haze of microwaves left over from the big bang and the accumulated starlight and radio waves of past eons. Gamma-ray photons with energies of 100 tera-electron-volts (TeV) go barely a few tens of millions of light-years. Energetic cosmic rays are blocked, too.

Neutrinos are thus one of the few ways astronomers have to study the most powerful phenomena in nature. They may be hard to catch but are worth the effort.

Flavorful Science
Besides being aloof, neutrinos have another distinctive feature: their strange ability to metamorphose. Like all elementary matter particles, they come in three versions, called flavors. The electron (e) has two heavier replicas, the muon (µ) and the tau (τ), and each has a neutrino partner: the electron-neutrino (νe), the muon-neutrino (νµ) and the tau-neutrino (ντ).

But whereas the electron, the muon and the tau have specific masses, the three neutrino flavors do not. If you measure the mass of a neutrino with a given flavor, you get one of three answers at random, with a certain probability for each. Conversely, if you measure the flavor of a neutrino with a given mass, you get one of three answers. A neutrino can have either a specific flavor or a specific mass but not both at the same time. Neutrino mass states are labeled by number, ν1, ν2 and ν3 , which are distinct states from νe, νµ and ντ.

Neutrinos thus violate a basic intuition we have about objects. A basketball weighs 22 ounces, a baseball five ounces. But if balls behaved like neutrinos, a basketball would sometimes be 22 ounces, sometimes five ounces. In this respect, neutrinos are rather like people, with our multiple group identities. For instance, scientists may have both an institutional affiliation and a political party affiliation at the same time. Surveys show that 6 percent of scientists are Republicans, but that does not mean 6 percent of scientific laboratories are associated with the Republican party. Rather, in a typical laboratory, six out of 100 randomly chosen scientists happen to be Republican. Likewise, a ν1 neutrino interacting in a detector may manifest itself as a νe, νµ or ντ with a calculable probability.

Flavor determines how neutrinos partake in the weak nuclear force, and mass determines how they propagate through space. For instance, beta decay produces neutrinos of a single flavor, νe. As these particles fly through space, their flavor is unimportant; it is their mass state that dictates their behavior. The νe is a mixture of a ν1, ν2 and ν3 in proportions that, for technical reasons, physicists call mixing angles. Instead of a single type of particle, physicists must now keep track of three. Eventually the neutrinos react with material in a detector, and here again it is the flavor that matters. If the relative proportions of mass states have remained unchanged, they will add up to the original flavor again (which, for beta decay, is νe).

But that need not be the case. When the particles are propagating as mass states, they become vulnerable to new effects that can alter the mixture, thereby changing their flavor. This process is what causes neutrinos to metamorphose.

By the principles of quantum mechanics, each mass state corresponds to a wave with a certain wavelength. The waves overlap and interfere with one another. To use an acoustic metaphor, a neutrino is like a sound wave consisting of three pure tones. As anyone who has ever tuned a musical instrument knows, superposed sound waves having slightly different pitch (or wavelength) exhibit “beats,” an oscillation of their sound intensity. In the case of neutrinos, a difference in mass acts like a difference in pitch, and the beats cause an oscillation of flavor with distance.

The sun, for example, produces electron-neutrinos. Before they reach Earth, they become a mix of all three flavors. The pioneering experiments of Davis and Koshiba were sensitive only to electron-neutrinos, so they missed the muon- and tau-neutrinos into which many of the electron-neutrinos had metamorphosed during their journey. It took a detector sensitive to all three neutrino flavors, the Sudbury Neutrino Observatory detector in Canada in 2001 and 2002, to detect a representative sample of particles [see “Solving the Solar Neutrino Problem,” by Arthur B. McDonald, Joshua R. Klein and David L. Wark; Scientific American, April 2003].

Another well-established example of neutrino metamorphosis occurs when neutrinos are created in Earth’s upper atmosphere. Cosmic rays collide with nuclei in the air, creating unstable particles called pions that subsequently decay into electron- and muon-neutrinos. These neutrinos then propagate through the air and solid planet as mass states. The farther they travel by the time they are detected, the more of the muon-neutrinos turn into tau-neutrinos. Consequently, neutrino observatories see half as many muon-neutrinos coming up from below (having traveled from the opposite side of the planet) as from overhead (having gone from the upper atmosphere straight to the ground).

Fun with Ratios
For astronomers, flavor is to neutrinos what polarization is to light: a property that can encode information. Just as a celestial source can emit light with a given polarization, it produces neutrinos with certain flavors, and by measuring the flavor, astronomers can figure out what processes must have operated within the source. The trick is to mentally undo the metamorphosis that the neutrinos underwent on their journey.

If we could measure precisely the energy of a neutrino and how far it traveled, we would know where in the oscillation cycle it ended up and could calculate the relative proportions of the three flavors. We lack this precision. Over large distances and long times, neutrinos oscillate so many times that we cannot keep track of the flavor mix—it looks like a blur to us. Instead we take a statistical average, described by a so-called flavor propagation matrix. From this matrix, astronomers can deduce what an observed ratio must originally have been.

For instance, it is thought that many neutrinos come from extreme-energy collisions of photons with protons. This process occurs in cosmic-size particle accelerators—found at the shock fronts of supernova remnants and in the jets squirted out by black holes of all sizes—as well as in deep space where cosmic rays slam into the cosmic background radiation. The collisions produce charged pion particles, which decay to muons and muon-neutrinos. The muons in turn decay to electrons and electron-neutrinos, among other things. The resulting stream of neutrinos is one part νe, two parts νµ and no ντ—a flavor ratio of 1:2:0. Looking up the pertinent values in the propagation matrix, we find that this ratio evolves into 1:1:1. If an Earthly experiment sees other than 1:1:1, then the pion-decay chain cannot be the source of the neutrinos.

In some cases, the pion might lose energy by colliding with other particles or emitting radiation while traveling on a curved trajectory in a magnetic field. If so, the muon into which it decays becomes irrelevant as a high-energy neutrino source, and the initial flavor ratio is instead 0:1:0. According to the propagation matrix, the ratio at Earth will be 4:7:7 rather than 1:1:1. If an experiment finds that the flavor is 1:1:1 for lower-energy neutrinos but 4:7:7 for higher-energy ones, astronomers can infer the particle density and magnetic field strength of the source.

Neutrinos can also come from so-called beta-beam sources. In cosmic particle accelerators, high-speed atomic nuclei can exchange pions or simply fall apart, leading to a fast beam of neutrons. The neutrons undergo radioactive beta decay, emitting a pure stream of electrons and electron-neutrinos, for a flavor ratio of 1:0:0. After processing with the propagation matrix, the flavor ratio that emerges at Earth is 5:2:2.

Whatever the initial mix of flavors is, the two flavors νµ and ντ arrive at Earth in equal numbers. This equality, which reflects a deeper symmetry that physicists have yet to explain, is noteworthy because tau-neutrinos will always turn up in telescopes even though no known astrophysical source produces them.

The flavor ratio can discriminate the workings of celestial objects as no other source of information can. Together with gamma rays and cosmic rays, neutrinos will spell out the dynamical mechanism and energy budget of nature’s mightiest dynamos. They can determine whether cosmic particle accelerators are purely electromagnetic (in which no neutrinos are produced) or involve heavy particles (in which neutrinos do emerge). They might even help solve a mystery that is on every astronomer’s top-10 list: How are the highest-energy cosmic rays produced? Some cosmic rays are so potent that they seem to defy known physics. Neutrinos can probe the interior of whatever is spitting them out.

They can reveal other natural processes as well. The decay of dark matter particles might yield neutrinos in a ratio of 1:1:2, which evolves to roughly 7:8:8. In certain quantum theories of gravity, the very fabric of spacetime undulates on microscopic scales. Very high energy neutrinos have very short wavelengths that might be sensitive to these fluctuations. The fluctuations might act to scramble the flavor, leading to an observed ratio of 1:1:1. In the future, physicists may be able to use the measurement of a ratio other than 1:1:1 to rule out certain classes of theories and determine the energy levels at which quantum-gravitational effects come into play.

Another exotic process is the decay of a heavy neutrino into a lighter variety, which would alter the flavor ratio. From studying solar neutrinos, physicists have found that ν1 is lighter than ν2, but they do not know which of ν1 and ν3 is the lightest. If astronomers found a flavor ratio of 4:1:1, it would mean that neutrinos are indeed unstable and that ν1 is the lightest. A ratio of 0:1:1 would favor ν3.

Historically, astronomy began with observations of the universe in visible light and gradually expanded to infrared, microwave, radio, x-rays and gamma rays. Neutrinos continue the trend. The coming decade will be the golden age of neutrino astronomy.