Little Neutral Ones

In John Updike's memorable description, "The earth is just a silly ball/To them, through which they simply pass."

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.

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