When
the sun was young and faint and the Earth was barely formed, a gigantic
black hole in a distant, brilliant galaxy spat out a powerful jet of
radiation. That jet contained neutrinos — subatomic particles so tiny
and difficult to detect they are nicknamed “ghost particles.”
Four
billion years later, at Earth’s South Pole, 5,200 sensors buried more
than a mile beneath the ice detected a single ghostly neutrino as it
interacted with an atom. Scientists then traced the particle back to the
galaxy that created it.
The cosmic achievement, reported Thursday by a team of more than 1,000 researchers
in the journal Science, is
the first time scientists have detected a high-energy neutrino and been
able to pinpoint where it came from. It heralds the arrival of a new
era of astronomy in which researchers can learn about the universe using
neutrinos as well as ordinary light.
Neutrinos
are so small that they seldom bump into atoms so humans can't feel
them. They don't shed light, so our eyes can't see them. Yet these very
qualities make them invaluable for conveying information across time and
space, scientists say. Light can be blocked and gravitational waves can
be bent, but neutrinos are unscathed as they travel from the most
violent events in the universe into a detector at the bottom of the
Earth.
Scientists call the kinds of signals they
can detect through space, like radio waves or gravitational waves or
now neutrinos, “messengers.” If you're trying to understand complex and
chaotic phenomena happening billions of light-years away, it's helpful
to have a messenger like a neutrino: one that doesn't get lost.
“They're very clean, they have simple interactions, and that means every single neutrino interaction tells you something,” said
Heidi Schellman, a particle physicist at Oregon State University and computing coordinator for a different neutrino detection project, the
Deep Underground Neutrino Experiment, who was not involved with the new research.
Neutrinos
arrive on Earth at varying energy levels, which are signatures of the
processes that created them. By pairing neutrino detections with light
observations, Schellman said, scientists will be able to answer
questions about distant cataclysms, test theories about the composition
of the universe, and refine their understanding of the fundamental rules
of physics.
The IceCube lab lit by stars and the southern lights. (Martin Wolf/IceCube/NSF)
The
high-energy neutrino reported Thursday was created in the fast-moving
swirl of matter around a supermassive black hole at the center of the
galaxy. When this black hole generates a brilliant jet of radiation, and
that jet is aimed directly at Earth, scientists call the galaxy a
“blazar.”
Subsequent analysis revealed this blazar had also produced a flare of more than a dozen neutrino events several years earlier.
The new discovery, from the South Pole neutrino detector called
IceCube,
has also solved a mystery that stumped scientists for generations: What
is the source of mysterious cosmic rays? These extremely energetic
particles have been detected raining down from space since 1912, but
researchers could not figure out what phenomenon could produce particles
moving at such high speeds.
Astroparticle physicist and IceCube spokesman
Darren Grant
said it’s as though scientists have spent 100 years listening to
thunder with their eyes closed and never known what caused the booming
sound. It wasn't until they looked up and saw lightning that the
spectacle finally made sense. Both sound and light — or in this case,
cosmic rays and neutrinos — are coming from the same event.
“That’s
why this is exciting,” Grant said of the neutrino detection. “It’s a
brand new vision on what's happening in the universe.”
What is a neutrino?
Our
universe is suffused with neutrinos, so named because they are
uncharged (or “neutral”) and infinitesimally puny (about a millionth of
the mass of an electron). They are created in nuclear reactions — at
power plants, in the center of the sun, and amid even more extreme
events — when protons accelerate, collide and then shatter in a shower
of energetic particles.
Neutrinos are the
second most abundant type of particle in the universe, after photons. If
you held your hand toward the sky, about a billion neutrinos from the
sun would pass through it in a single second.
But you wouldn't feel their presence, because these ethereal particles rarely interact with normal matter.
And
the reality is, most of what we call “matter” is just empty space. If a
hydrogen atom were the size of Earth, the proton at its center would
fit inside the Ohio State football stadium. The electron orbiting it
would be even smaller, and a neutrino could be compared to a lone ant.
Neutrinos
are said to come in “flavors” — called electron, muon and tau — and on
the rare occasions that they collide with other matter they generate
corresponding charged particles. Many neutrino detectors work by looking
for the flash of light emitted by these charged particles as they move
through water or ice.
Flavored specks that are
found everywhere yet felt by no one; matter that seems solid but is
actually mostly empty — this is the bizarre science of particle physics.
It's difficult to wrap your mind around, and almost hard to believe.
Yet
scientists assure us they are not just making things up. Since the
1950s, when neutrinos were detected for the first time, researchers have
observed low-energy versions of these ghostly particles coming from the
sun and a 1987 supernova in a nearby galaxy.
Maps of neutrinos emanating from the surface of the Earth have even been used to identify the sites of nuclear reactors.
But
high-energy neutrinos, generated only in extreme environments where
protons are accelerated to astonishing speeds, have been harder to pin
down. To be detected, a neutrino had to form long ago in a far away
cataclysm, travel across intergalactic space, fly through our galaxy,
enter our solar system, sail on to Earth, and then happen to interact
with a particle minding its own business in the ice below the South
Pole.
And, in a process that seems just as
improbable, in the time since the neutrino left its source 4 billion
years ago, life on Earth had to arise, expand, and evolve to the point
that a few enterprising Homo sapiens were willing to go to the extreme effort of detecting it.
“It's crazy,” said
Chad Finley,
an astroparticle physicist at Stockholm University who spent 10 years
working as point source coordinator for the IceCube team. “These are
particles that seldom interact with anything. That has to be the
unluckiest neutrino ever.”
On the other hand, he mused, he and his colleagues are some pretty lucky humans.
“Ghost” hunting on ice
This
was the detection scientists were dreaming of when the National Science
Foundation began building the $279 million IceCube Neutrino Observatory
in 2005. Working during the South Pole summer, when the sun never sets
and temperatures hover at a balmy negative 18 degrees Fahrenheit,
scientists and engineers melted dozens of mile-deep holes in the ice and
dropped strings of spherical sensors into them. (Neutrino detectors are
typically buried or submerged to filter out other cosmic signals that
would obscure the tiny particles.)
The result
was a grid array of sensors spread across a cubic kilometer of glacier
and capable of catching a ghost. The sensors record the energy level and
direction of the flash of light emitted by the charged particle created
when a neutrino crashes into other matter. From that information,
scientists can extrapolate the energy level of the neutrino and where it
came from.
Since
the observatory was completed in 2010, IceCube scientists have detected
dozens of high-energy neutrinos coming from outside the solar system.
But they were never able to connect those particles with a source that
could be observed by conventional telescopes.
Establishing
such a connection was a “holy grail of the field,” Finley said, in
large part because of the link between neutrinos and the enigma of
cosmic rays. These are extremely energetic protons and atomic nuclei
moving through space at almost the speed of light. They're considered
one of the threats to humans on a potential mission to Mars: During the
months-long journey through space, cosmic rays would damage the cells of
astronauts and could cause radiation sickness.
But
unlike neutrinos, cosmic rays have a charge, which means their path can
be deflected by magnetic fields. This allows Earth's magnetic field to
protect us from these powerful particles, but it also makes it
impossible for scientists to figure out where the particles come from.
Extensive
research suggests that whatever process accelerates protons to such
speeds also generates high-energy neutrinos. So if IceCube could figure
out where neutrinos were coming from — a task made simpler by the fact
that neutrinos are such dependable “messengers” — they'd know the source
of cosmic rays as well.
“Neutrinos are the smoking gun,” Finley said.
An IceCube sensor is dropped into a mile-deep hole in the Antarctic ice (Mark Krasberg, IceCube/NSF)
On Sept. 22,
an alert
went out to the international astronomy community: IceCube had seen the
signature of a muon neutrino coming from just above the right shoulder
of the constellation Orion in the night sky.
Swiftly, scores of scientists
began pointing their telescopes in
that direction, staring at the right region of the universe in every
wavelength of the electromagnetic spectrum. Researchers using NASA's
Fermi space telescope saw a burst of gamma rays coming from the presumed
source. Gamma rays are associated with the particle acceleration that
produces both neutrinos and cosmic rays.
Other
observatories saw flares of X-rays, radio waves and visible light. Taken
together, these observations revealed a blazar — a giant elliptical
galaxy with a spinning supermassive black hole at its core. As a blazar
spins, twin jets of light and charged particles — one of which is aimed
toward Earth — spurt from its poles.
The blazar
was given the catchy name “TXS 0506+056" — the first known source of a
neutrino, and a possible answer to the century-old cosmic ray mystery.
As
a matter of due diligence, Finley suggested that the IceCube team go
back through their old data to examine whether any other neutrinos had
come from the same direction. He didn't expect to find anything —
neutrinos react so rarely that finding more from a single source would
be like lightning striking twice in the same spot.
So
he was shocked to discover that IceCube had recorded more than a dozen
neutrino events from what they now knew was the same blazar between late
2014 and early 2015. It was so improbable that Finley found himself
repeating the words uttered by
Isidor Isaac Rabi, a Nobel prize-winning U.S. physicist, when he discovered the
muon: “Who ordered that?”
“An absolutely beautiful messenger”
Combined
with gravitational wave detection and traditional light astronomy, the
observation of a neutrino from a known source gives researchers three
ways to observe the cosmos, and they say we're now in
the era of “multi-messenger astrophysics.” (Since gravitational waves
are often described as the way we “hear” the universe and light is how
we “see” it, some scientists wondered whether neutrinos would be how we
“smell” it.)
Of all these “senses,” neutrinos
are in some ways the most reliable. High-energy light from distant
sources rarely makes it to Earth, because photons are so reactive they
get lost along the way. Neutrinos, on the other hand, will travel in a
straight line right from their origin point to a detector.
“It's an absolutely beautiful messenger,” Grant said.
An artist's conception of a blazar. (NASA/JPL-Caltech/GSFC)
Neutrinos'
ghostly quality also means they can be used to probe celestial objects
light can't penetrate. Schellman pointed out that astronomers using
regular telescopes can't see beneath the surface of the sun, but 30
years of observations of the low-energy neutrinos that emanate from our
star's center have allowed scientists to peer into its core. By looking
at their energy levels, researchers could understand the fusion process
that creates the neutrinos and generates the sun's energy. This research
also revealed that it takes 100,000 years for energy at the center of
the sun to make it to the surface, “which means the sun is going to keep
working for at least 100,000 years,” Schellman said.
So that's one disaster Earthlings don't have to worry about.
The
neutrinos detected by IceCube are millions of times more energetic than
those coming from the sun, but they offer the same kinds of insights
into the intense environments from which the particles emanate. The
telescopes looking at TXS 0506+056 could only capture what happened on
the surface of the blazar; the neutrinos carry signatures of the
processes at its very center.
It's in these
extreme settings that the laws of nature are stretched to their limits.
What neutrinos reveal about the acceleration of charged particles and
the voracious behavior of black holes could help scientists refine the
rules of physics — or rethink them.
And there
are even more energetic neutrinos out there — ones that make the
powerful IceCube particles look practically wimpy. To Schellman, this
suggests that other, even more chaotic and cataclysmic, sources of
neutrinos are still waiting to be found.
“There are things we don’t even know about yet,” she said. “This is just the start.”
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