You'd never know it, but 6 trillion subatomic particles pass through every square inch of your body every second at nearly the speed of light. Most are leftovers from the big bang, but others arrive fresh from their superhigh-energy origins near black holes, deep inside gamma-ray bursts and supernovas, and within the core of our Sun. They zip across space, pass through your flesh and bones as though you didn't exist, and continue heedlessly on their way.
Before these particles were actually discovered, the Austrian physicist Wolfgang Pauli hypothesized their existence. In a letter to his colleagues, written in December 1930 and addressed to "Dear Radioactive Ladies and Gentlemen" (yes, that's physics humor), Pauli proposed an electrically neutral particle that he called a neutron. It was, he admitted, "a desperate remedy to save…the law of conservation of energy"—a law that, to the surprise of his colleagues, appeared to be failing on the subatomic level.
Two years later the English physicist James Chadwick discovered a relatively massive neutral particle residing contentedly in the atomic nucleus. Soon the name "neutron" was bestowed on it. But that nuclear neutron was not Pauli's; his hypothetical savior had to be much less massive. A year later the Italian physicist Enrico Fermi named Pauli's still-undiscovered particle the neutrino, Italian for "little neutral one."
Along with the photon, the electron, and the less-familiar quark, the neutrino lays claim to being one of the fundamental, indivisible building blocks of nature. Pauli had tactfully remarked in his 1930 letter that if such a particle existed, physicists should already have seen one. Not long afterward he confessed, in a candid assessment of what he had wrought, "I have done a terrible thing. I have postulated a particle that cannot be detected."
But it could be. Indeed, it was. Just after the Second World War two American physicists, Clyde L. Cowan Jr. and Frederick Reines, realized that the place to search would be a nuclear reactor, where, as in a nuclear bomb, disruptive changes to atomic nuclei lead to the prodigious emission of neutrinos. So they looked in the Savannah River Plant, a just-finished underground fission reactor near Aiken, South Carolina, built to produce tritium and plutonium for the Cold War nuclear arsenal of the United States. The physicists' first task was to find a way to capture these most antisocial of particles. Their second task was to disentangle the properties, behavior, and effects of the neutrino from those of all other subatomic particles liberated by their experiment. In 1956, based on their detection of a unique particle "signature," they announced the discovery of the neutrino.
Pauli proposed his new particle because of his confidence in the laws of conservation, which are among the most highly tested and fertile ideas in science. "Conservation," to a physicist, does not refer to recycling or to safeguarding endangered habitats. It's the shorthand way to say that certain properties of nature remain unchanged during a controlled experiment, no matter what you do to it, no matter what anybody else does to it, and no matter what nature does to itself. Conserved properties include momentum, the total quantity of mass and energy, and the net electric charge. Run the experiment, and when you're done, the stuff you take out of the box must be the same as the stuff you put into the box--for all properties described by the laws of conservation.
Take momentum, which is motion coupled with direction. Imagine twin ice skaters standing still and facing each other, palms touching. This two-skater system has zero momentum, and since it's resting on slippery ice, it has only negligible attachment to Earth. If the twin skaters--two objects with the same mass--push away from each other, they will glide apart in opposite directions at the same speed. The momentum of one skater cancels that of the other, leaving the system as it started, with a net momentum of zero.
Arithmetically, momentum is just mass times velocity, so various kinds of pairs can still cancel. For example, if one skater has twice the mass of the other, the chubbier one will glide away at half the speed of the thinner one, again leaving the system's total momentum at zero. Rockets do much the same thing. Spent fuel spews out the back while the body recoils forward, leaving the momentum of the entire system unchanged from its prelaunch repose on the launch pad.
Even when rocket engines are anchored to the ground while fired (which is what goes on at testing facilities), something's got to give. Typically, the rockets are mounted horizontally and connected securely to Earth by cement piers. When the high-velocity exhaust blasts out the nozzles, it's planet Earth that recoils, ever so slightly, in the opposite direction. So a lazy but perverse engineer could point all the world's test rockets due east--in the direction of Earth's spin--and ignite them, just to shorten the workday.
The conservation of total mass and energy has illustrious roots. Be fore Einstein proposed his most famous equation, mass-energy conservation was instead the conservation of mass and, separately, the conservation of energy. The universe was endowed with a certain amount of each, presumed from the experiments of the day to be changeless. But at the turn of the twentieth century, the discoveries of radioactivity and other bizarre phenomena within the atom indicated that mass could become energy, and energy could become mass. The conversion recipe was none other than E = mc².
[pagebreak]Another conserved quantity is electric charge. Protons carry a unit of positive charge, electrons carry the same amount of negative charge, and neutrons carry no charge at all. Charge conservation requires that at no time during an experiment is the net charge anything other than what you started with. And that's as true for particle accelerators on Earth as for supernova explosions in distant galaxies.
Armed with the conservation laws of momentum, mass-energy, and electric charge, you're more or less where Pauli was in 1930. Back then, life was simpler. Particle physicists were not yet talking about quarks, muons, gluons, or Higgs bosons. What they did discuss was a subatomic process called beta decay, in which a proton and an electron spontaneously fly apart, accompanied by unbalanced momentum and a loss of mass-energy. Had the conservation laws lost their grip on nature? Or could the existence of an unforeseen and undiscovered particle resolve the conundrum?
Discoveries in physics often emerge from one's confidence in competing ideas. Rather than dismantle the foundations of physics, Pauli postulated that the escaping proton and electron (both later shown to have come from a decayed neutron) were not the sole products of the decay. His additional particle was to have no charge, some momentum, and vanishingly small, possibly zero, mass-energy.
Turns out, the key to beta decay was not the neutrino but its antimatter counterpart, the antineutrino. A decaying neutron yields a proton, an electron, and an antineutrino. Under the dictates of additional conservation laws, unknown to Pauli and his contemporaries, that's just what you'd expect. Two of those laws decree that no process can change the net numbers of heavy particles (baryons) and light particles (leptons). If your experiment starts with one baryon (a proton or a neutron), it must end with one baryon. That means a neutron can morph into a proton. And if it starts with zero leptons (an electron or a neutrino), it must end with zero leptons.
Wait a minute. Beta decay starts with zero leptons but ends with two leptons: an electron and an antineutrino.
Not to worry. The antineutrino is not simply a light particle; it's an anti--light particle. So in the particle count, an electron and an antineutrino cancel, resulting in zero net leptons. The laws of conservation triumph yet again.
Pauli died in 1958. Fortunately for him, he lived just long enough to see Cowan and Reines detect his "undetectable" particle. Today neutrinos remain among the most challenging subatomic particles to catch, even though everybody and everything is steeped in trillions of them. Problem is, they interact so rarely with other kinds of matter that you need enormous, clever traps to boost your chances of detecting any at all.
Nearly all the evidence for neutrinos from space comes from detectors buried deep underground, which hold enormous quantities of odd liquids surrounded by unusual hardware. These underground "telescopes" can catch intrepid neutrinos from any direction, even those that have passed all the way through Earth from below. In one detector the neutrinos enter a tank filled with 600 tons of chlorine-laden dry-cleaning fluid. Every so often, in a kind of reverse beta decay, one of the passing neutrinos changes a resident neutron within a chlorine atom into a proton, thereby changing the chlorine to radioactive argon. The presence of an argon atom serves as a tracer of the neutrino's visit. Other creative designs track the flash of blue light emitted by the particle products of neutrino interactions. Those tanks are filled with ultrapure water or a mixture of baby oil and benzene.
My favorite setup, though, is a not-yet-finished neutrino observatory called IceCube. Its "tank" is a cubic kilometer of clear, dense Antarctic ice, in which the investigators will suspend a lattice of sensors, lowered through deep holes melted by a hot-water drill.
Unfortunately, Pauli didn't live long enough to see how populous the particle zoo would become--how many categories and subcategories and families and flavors particle physicists would postulate and discover in the decades that followed his death. Nor could he have imagined that neutrinos themselves would land in the middle of one of the greatest astrophysical conundrums of the twentieth century.
In March 1964, in the journal Physical Review Letters, the late American astrophysicist John N. Bahcall published his calculations showing that vast quantities of neutrinos should continually flee the Sun as the nuclear furnace in its core transforms hydrogen into helium. In a tandem paper, Bahcall's friend and colleague Raymond Davis Jr. described an experiment he was building in the disused Homestake Gold Mine in Lead, South Dakota. In search of evidence for solar neutrinos, he would place a tank of chlorine-rich liquid deep belowground. As usual, the encounters between neutrinos and atoms would be exceedingly rare; Bahcall calculated that the experiment should record about ten neutrinos a week. But even those few neutrinos would reveal what was going on in the Sun's center, thus eliminating the need ever to visit the place.
For years, however, only about three of the ten neutrinos showed up. That gap became the tenacious "solar-neutrino problem." Some physicists copped an attitude, suggesting that astronomers didn't fully understand how the Sun manufactures energy—knowledge that underpins much of modern astrophysics. Shaken but not stirred, Bahcall was so sure the Sun was not misbehaving that he committed much of his career to demonstrating why. Meanwhile, Davis continued to refine his measurements. And the solar-neutrino problem endured.
Normally, physicists hand the laws of physics to astrophysicists, and it's those laws that guide questions and constrain answers. But every now and then, astro folks teach the physics folks a thing or two about how the universe works. Indeed, Bahcall was right all along. The missing neutrinos were there. They just weren't the kind of neutrinos that would turn chlorine into argon. Apparently, without telling anybody, they had left the Sun with one identity—the one the experiment was designed to detect—but reached Earth in a different guise, requiring a different experiment to be detected at all.
[pagebreak]Turns out, neutrinos come in three flavors, representing three regimes of energy in the universe. Not that you asked, but they're called the electron neutrino (low energy), the muon neutrino (middle energy), and the tau neutrino (high energy). So if your apparatus is designed to detect electron neutrinos, such as the ones forged in the core of the Sun, then the other neutrinos will pass undetected. Furthermore, if your experiment is designed to detect neutrinos of any regime, but antineutrinos are what come your way, you'll miss them, too. As with so much else in life, you need to know in advance what you're looking for.
But detection is only part of the challenge. Next comes the urge to compile a list of the neutrinos properties, beyond its neutral charge and its elusiveness. How about mass? All attempts to measure this basic property had failed so miserably that, until recently, physicists were uncertain whether the neutrino had any mass at all.
Here's where things get spooky.
According to Einstein's special theory of relativity, an onlooker who views a material object traveling at ever-greater speeds will see the object's mass increase, its time slow down, and its length shorten in the direction of motion. At the speed of light, its mass would become infinite, its time would stop, and its length would shorten to zero--all of which led Einstein to the sensible conclusion that physical objects can never attain light speed. Not only that, the reverse is true as well: if the thing has no mass whatsoever (if it's a photon, say), it must always travel at the speed of light.
So if the neutrino exists but has no mass, then it must travel at the speed of light. And if it travels at the speed of light, its own passage of time has stopped, leaving it with no internal "clock" to judge how old it is. To an outside observer, the neutrino's identity would forever be what it has ever been.
But if the neutrino has mass, it must travel more slowly than light, and must therefore bear an internal clock that actually ticks--one that recognizes the passage of time. And if the neutrino undergoes the passage of time, as other particles do, then it can transform itself. Unlike the neutron, however, which can decay into fundamental particles, the neutrino is already a fundamental particle. All it can do, then, is transform into another variety of neutrino. So if someone were to build an apparatus that could detect muon neutrinos or tau neutrinos, rather than only the garden-variety electron neutrinos detectable in Davis's setup, maybe all ten of Bahcall's neutrinos would show up.
And that's exactly what's happened.
John Bahcall had proceeded on the perfectly plausible assumption that the Sun's supply of electron neutrinos would simply remain electron neutrinos. But by the time they arrived on Earth, two-thirds of them had changed into muon and tau neutrinos, a process called neutrino oscillation. Imagine that somebody threw you a baseball, but it turned into a football in midflight. If you were looking only for the baseball, the football might pass unnoticed.
Once you know a neutrino can transform itself, you know it has a self-timer. You also know it cannot be traveling at the speed of light, which means it must have mass. As of March 2006, courtesy of a beam of muon neutrinos sent from Illinois to Minnesota, physicists can say with confidence that the mass of the neutrino is no more than 1/2,000,000 the mass of the already tiny electron, itself checking in at about 1/2,000 the mass of the proton.
Knowing that the neutrino can switch identities and has very small (but nonzero) mass, astrophysicists have revisited earlier calculations that assumed a massless neutrino. Their efforts have lengthened the list of cosmic dramas in which the neutrino plays more than a bit part. Astrophysicists have not seen the last of the little neutral ones. For all we know, neutrinos hold the answers to questions already posed, as well as to questions not yet imagined.