Why neutrinos matter - Sílvia Bravo Gallart

They're everywhere,
but you will never see one.

Trillions of them are flying
through you right this second,

but you can't feel them.

These ghost particles are called neutrinos
and if we can catch them,

they can tell us about
the furthest reaches

and most extreme environments
of the universe.

Neutrinos are elementary particles,

meaning that they can't be subdivided
into other particles the way atoms can.

Elementary particles are the smallest
known building blocks

of everything in the universe,

and the neutrino is one
of the smallest of the small.

A million times less massive
than an electron,

neutrinos fly easily through matter,
unaffected by magnetic fields.

In fact, they hardly ever
interact with anything.

That means that they can travel
through the universe in a straight line

for millions, or even billions, of years,

safely carrying information
about where they came from.

So where do they come from?

Pretty much everywhere.

They're produced in your body
from the radioactive decay of potassium.

Cosmic rays hitting atoms
in the Earth's atmosphere

create showers of them.

They're produced by nuclear
reactions inside the sun

and by radioactive
decay inside the Earth.

And we can generate them
in nuclear reactors

and particle accelerators.

But the highest energy neutrinos
are born far out in space

in environments that
we know very little about.

Something out there,
maybe supermassive black holes,

or maybe some cosmic dynamo
we've yet to discover,

accelerates cosmic rays to energies
over a million times greater

than anything human-built
accelerators have achieved.

These cosmic rays,
most of which are protons,

interact violently with the matter
and radiation around them,

producing high-energy neutrinos,

which propagate out
like cosmic breadcrumbs

that can tell us about the locations

and interiors of the universe's most
powerful cosmic engines.

That is, if we can catch them.

Neutrinos' limited interactions
with other matter

might make them great messengers,

but it also makes them
extremely hard to detect.

One way to do so is to put a huge volume
of pure transparent material in their path

and wait for a neutrino to reveal itself

by colliding with the nucleus of an atom.

That's what's happening
in Antarctica at IceCube,

the world's largest neutrino telescope.

It's set up within
a cubic kilometer of ice

that has been purified by the pressure

of thousands of years
of accumulated ice and snow,

to the point where it's one
of the clearest solids on Earth.

And even though it's shot through with
boreholes holding over 5,000 detectors,

most of the cosmic neutrinos racing
through IceCube will never leave a trace.

But about ten times a year,

a single high-energy neutrino
collides with a molecule of ice,

shooting off sparks of charged
subatomic particles

that travel faster through the ice
than light does.

In a similar way to how a jet
that exceeds the speed of sound

produces a sonic boom,

these superluminal charged particles
leave behind a cone of blue light,

kind of a photonic boom.

This light spreads through IceCube,

hitting some of its detectors
located over a mile beneath the surface.

Photomultiplier tubes amplify the signal,

which contains information about
the charged particles' paths and energies.

The data are beamed
to astrophysicists around the world

who look at the patterns of light

for clues about the neutrinos
that produced them.

These super energetic collisions
are so rare

that IceCube's scientists give each
neutrino nicknames,

like Big Bird and Dr. Strangepork.

IceCube has already observed

the highest energy
cosmic neutrinos ever seen.

The neutrinos it detects should finally
tell us where cosmic rays come from

and how they reached
such extreme energies.

Light, from infrared,
to x-rays, to gamma rays,

has given us increasingly energetic

and continuously surprising
views of the universe.

We are now at the dawn
of the age of neutrino astronomy,

and we have no idea
what revelations IceCube

and other neutrino telescopes may bring us

about the universe's most violent,
most energetic phenomena.