ADVANCES
20 Scientific American, April 2020
K M3NE TASTROPHYSICS
Underwater
Eyes
Submarine neutrino telescopes
will scan for dark matter, distant
star explosions, and more
Suspended near the bottom of the Medi-
terranean Sea off France and Italy, 126 foot-
ball-sized glass spheres are already using
the ocean itself as an instrument to search
for signals from dark matter, supernovae
and neutron star collisions. These are the
first of many such globes deployed for a
project called the Cubic Kilometer Neu-
trino Telescope, or KM3NeT.
Its target, neutrinos, are fundamental
particles that have no electrical charge and
almost no mass. “Unlike cosmic rays, neu-
trinos are not deflected by magnetic fields
in intergalactic space, making them unique
messengers,” says Walter Winter, a neu-
trino astrophysicist at the German Electron
Synchrotron (DESY) research center, who
is not involved with KM3NeT. “They are
complementary to other sources of infor-
mation like electromagnetic radiation and
gravitational waves.”
Neutrinos can pass through most other
matter with only a tiny fraction interacting;
this ghostly behavior makes them ideal
candidates for astronomy. KM3NeT is set
to be installed throughout one cubic kilo-
meter of water—enough for 400,000
Olympic swimming pools—split over two
locations, turning the surrounding water
into a giant lens. More than 6,000 spheres,
each containing 31 highly sensitive detec-
tors called photomultiplier tubes, will cling
to strings anchored to the seafloor and kept
taut by floats.
“Perhaps one or two neutrinos in a mil-
lion will interact with quarks inside the
nucleus of either hydrogen or oxygen” in the
water, says the project’s physics and sof t-
ware manager, Paschal Coyle of the Mar-
seille Par ticle Physics Center. “Because the
cosmic neutrinos possess ver y high energy,
the result of such interactions is the releaseof a charged par ticle that travels ver y fast.”
In fact, it travels through the water
faster than light can, producing an effect
Coyle likens to an optical equivalent of the
Concorde jet’s sonic boom. Researchers
can determine the original neutrinos’
energy and direction using the faint light
released—so-called Cherenkov radia-
tion—picked up by the undersea sensors.
Among the handful of astronomy-
focused neutrino telescopes in existence,
“KM3NeT is unique, especially in observing
the Southern [Hemisphere] sky with unprec-
edented directional and energy resolutions,
paired with its enormous size,” Winter says.
The French site, scheduled for comple-
tion in 2024, will detect low-energy neutri-
nos generated when cosmic rays interact
with Earth’s atmosphere. As they pass
through the planet, these particles provide
an x-ray-like view of what is inside. The
Italian site, set for 2026, will focus on cos-
mic neutrinos produced in the cataclysmic
deaths of distant stars—or in dense
regions of colliding dark matter.
Intriguingly, the telescopes’ clearest
view is looking downward; Ear th works as
a filter to block background par ticles from
the cosmic rays that continuously bombard
our world. Neutrinos are the only known
particles from those rays that make it
through the planet. — Dhananjay KhadilkarResearchers are in the process of deploying modules for
a new pair of underwater telescopes ( visualized here ).© 2020 Scientific American