The case of the purloined identity
It has been said that a neutrino can pass through a wall of lead
stretching from the earth to the moon and not interact with a single
atom. The electrically neutral and almost incomprehensibly small
neutrino – it is dwarfed in size by an electron – was
theorized in 1930 by Wolfgang Pauli but not detected until 1956.
Neutrinos were created in the Big Bang and are now created in the
nuclear fusion process that powers the sun and other stars. They
come in three different types, or what physicists call "flavors"
– the electron neutrino, the muon neutrino, and the tau neutrino.
Originally the flavors were thought to be distinct, but scientists
now believe the picture is more complicated and that neutrinos mix
and change between flavors.
That neutrinos are more like wraiths than physical particles makes
them extremely valuable in efforts to map the universe. Neutrinos
can come to us from regions in space too densely packed with matter
for light to penetrate. They also come unaltered by any of the myriad
magnetic fields that permeate the cosmos, which means their journey
can be traced as a straight line back to their point of origin.
Neutrinos have always been an enigma. For decades, scientists
believed neutrinos were massless. When it was discovered that neutrinos
do have mass – albeit very tiny – scientists were left
scratching their heads. Said Leon Lederman, who shared the 1988
Nobel Prize in physics for showing there was more than one type
of neutrino, said, “It shows us that we really don't know
Neutrinos have also inspired commentary from nonscientists. In
Cosmic Gall, an ode to neutrinos, Pulitzer-Prize-winning
author John Updike said:
“Neutrinos they are very small.
They have no charge and have no mass
And do not interact at all.
The earth is just a silly ball
To them, through which they simply pass,
Like dustmaids down a drafty hall
Or photons through a sheet of glass.
They snub the most exquisite gas,
Ignore the most substantial wall,
Cold-shoulder steel and sounding brass,
Insult the stallion in his stall,
And, scorning barriers of class,
Infiltrate you and me! Like tall
And painless guillotines, they fall
Down through our heads into the grass..."
The case of the sun’s missing neutrinos has been solved. Like many
good mysteries, there was a surprising twist involving switched identities
and the possibility of one or more sequels.
According to the predictions of the Standard Model, the scientific theory
that for the past three decades has served as our basis for understanding
the fundamental particles and forces of nature, neutrinos have no mass.
However, based on the Standard Model and our understanding of thermonuclear
reactions, scientists were only able to detect about a third of the number
of electron neutrinos (the type the sun produces) that they were expecting.
Where were the missing solar neutrinos?
The arrival in the year 2000 of the Sudbury Neutrino Observatory (SNO)
gave scientists their first chance ever to directly measure not only electron
neutrinos, but muon and tau neutrinos as well, even though they weren’t
expected based on the Standard Model. A little more than a year later,
they had an answer to the mystery:
there were no "missing" solar neutrinos;
the electron neutrinos had been changing their type or “flavor”
during their 93 million mile journey from the sun to the earth.
Two years of SNO observations confirmed that contrary to the Standard
Model’s predictions, neutrinos do have a mass – by some estimates
about 1/60,000th that of an electron. Having a mass enables neutrinos
to oscillate in transit, meaning they can change flavor from electron
to muon or tau neutrinos.
The SNO observations have since been independently confirmed by another
major neutrino observatory, KamLAND, which is located in central Japan.
Observations at KamLAND showed that antineutrinos (the mirror image counterpart
to neutrinos) emanating from nearby nuclear reactors are also “disappearing,"
which indicates they, too, have mass and can change from one flavor to
The results from SNO and KamLAND are enough to convict neutrinos of
having mass, which means changes are in store for the Standard Model.