All language subtitles for How.The.Universe.Works.S09E09.Secret.Lives.Of.Neutrinos.1080p.AMZN.WEB-DL.DDP2.0.H.264-alfaHD_track4_[eng]

NARRATOR: Our world,
our solar system,

our universe.

None of it would exist

without a ghostly particle
called the neutrino.

They can pass right through a wall,
right through a planet, through
a star without even noticing.

They are our early warning system.

Whenever there's trouble
in the universe

you can expect a flood of neutrinos.

Neutrinos trigger star
killing explosions.

Supernovas.

Neutrinos can answer
so many questions,

from why do we exist
to how was the universe created.

These tiny particles save
the infant cosmos from annihilation.

They cause destruction, you know,
sometimes they blow up a star,

but at the end of the day,

they can be the very reason
that we exist at all.

Neutrinos are the key
to how the universe works.

(EXPLOSION)

In the 1960s,
our sun appeared to be dying.

There was tantalising evidence

that our sun might be shutting down,

this question was a biggy
for astronomers,

if the sun isn't undergoing
nuclear fusion

at the rate we thought it was,
then that's a big deal.

Was the sun's nuclear core
shutting down?

Stars, including our own sun,
are giant nuclear fusion reactors.

Inside these fusion reactors,
hydrogen atoms smash together...

producing heat and light
in the form of photons.

All the light and all the heat
that we receive on Earth

comes from the sun,

if the sun were to suddenly
start cooling off,

that would be seriously bad news
for us.

How do we check
if the sun is shutting down?

We have spacecraft
monitoring the solar surface.

But they can't see into the heart
of the reactor, the sun's core.

You can see the surface,
and the sun is very bright,

that makes it very easy to study,

sadly, the core of the sun
is under 400,000 miles of sun,

and that makes it pretty hard
to look at.

Studying the light
made in the core doesn't help.

By the time it gets to us,
it's old news.

Imagine a photon,
or this particle of light

that's born in the centre of a star,

and now imagine that it wants
to reach the surface of the star,

it turns out that the star
is so dense in the centre

and the star itself
is so physically large,

that it will take it 30,000 years
to escape the core.

It's like being at a cocktail party,

where you're trying to leave

and every time that you make
another step towards the door

another group of people
wanna talk to you.

And you also wanna talk to them.

And then it just takes 30,000 years
to leave your cocktail party.

Any information we get from sunlight
about what's going on in the core

is tens of thousands of years old.

If you want the current events,

the news headlines of what's
going on in the sun's core,

right now, photons are not the way
to do it, you want neutrinos.

So, what
are these mysterious particles?

Neutrino literally means
tiny neutral one, right?

We think they carry
no net electrical charge,

and they're really, really small,
so we call them neutrinos.

Neutrinos don't like to interact
with matter.

They fly through almost everything.

The sun itself is generating
enough neutrinos

to send 60 billion of them
through your thumbnail

every single second,

and you will spend,
this is the craziest thing,

you will spend your entire life
without feeling a single one.

Neutrinos form
during nuclear fusion reactions

inside the core of stars.

Hydrogen atoms collide,
fuse into helium

and release photos of light
and neutrinos.

In the core of the sun,
nuclear bombs are going off,

and all of these nuclear reactions
release neutrinos.

That's about ten trillion,
trillion, trillion

neutrinos being created
every second.

The trillions of neutrinos
shoot out of the core

and up through 323,000 miles
of the sun to the surface.

A neutrino basically doesn't
even notice the sun is there,

it sails out at very close
to the speed of light.

If you imagine a gridlocked highway,

the neutrinos would be
the motorbikes

that are just zooming
through the traffic.

The solar neutrinos
race towards Earth.

Most pass straight through.

All the neutrinos, the trillions
upon trillions of neutrinos

passing through the Earth
every single second,

the entire Earth will only interact

with one neutrino
out of ten billion.

Because they pass through anything,
they're hard to detect.

I consider neutrino physicists

to be the ghost hunters
of the particle physics realm,

because we study something
so elusive,

and they're really, really hard
to nail down and study.

Hard but not impossible.

While most neutrinos
pass through Earth,

a few collide with atoms
in the planet,

and we can detect those collisions.

To spot these tiny impacts, we built
underground neutrino detectors

with giant sensors full of chlorine.

When a neutrino strikes
this chlorine atom,

it transforms into argon,

and then we can pick out
the argon atoms from the detector

and count them up to see

how many neutrinos actually
struck our atoms.

The sensors detected neutrinos
from the sun,

but the numbers
were lower than expected.

Detectors were only detecting

about a third of the number
of the neutrinos

that their models predicted.

This is called
the Solar Neutrino Problem.

That is a big deal,

that either means
we're doing something wrong

or our physics is wrong.

Where were the missing two thirds
of the solar neutrinos?

They weren't AWOL,
the detector had missed them

because neutrinos
can change identities.

And it turns out
neutrinos can change

what kind of neutrino they are
as they're flying through space.

And we call this flavour changing.

Neutrinos come
in three different flavours.

Think of them as different types
of playing cards.

The king is the electron neutrino,

the muon neutrino is the queen,
and the jack is the tau neutrino.

The sun produces electron neutrinos,

but by the time they reach Earth
they could be a different flavour.

As they travel to the Earth

they constantly wave back and forth,

trading their identities,

so you never know
exactly what you're gonna get

until it arrives at the Earth
and we observe it.

It could be anything.

The detectors weren't seeing
the different flavours.

But when we fine-tuned the sensors
we saw all the solar neutrinos.

So, there were enough
neutrinos coming from the sun,

but we were detecting
a third of them.

Flavour changing neutrinos
showed the sun was healthy.

The changing identities also
answered an important question

about neutrinos, do they have mass?

Einstein showed
that only particles without mass

can travel at the speed of light,

and these particles
don't experience time.

But neutrinos can change
their flavour,

so that must happen over time,

and that means neutrinos
can't travel at the speed of light,

and so, they must have mass.

When scientists first started
thinking about neutrinos

they thought
that they were massless,

and if a neutrino has no mass

then it's bound to be one flavour
or one type of neutrino forever.

Experiments proved
the neutrinos have mass.

And if they have mass
they must produce gravity,

which means, they can influence
other things around them.

Neutrinos are also involved

in moments of huge, cosmic violence.

Whenever there's trouble
in the universe

you can expect a flood of neutrinos.

These floods of neutrinos
are the key

to some of the biggest bangs
in the cosmos.

And new research suggests
that without them,

there would be no solar system,
no planets and no us.

Neutrinos are one of the smallest
particles in the cosmos.

However, new research suggest
they play a role

in some of the universe's
biggest events.

Exploding stars, called supernovas.

The deaths of giant stars.

But there is a mystery surrounding
their explosive ends.

Why do these giant stars
end their lives so violently?

This is a major puzzle
in astrophysics.

We got a lead when we detected
a huge flash of light

in the Large Magellanic Cloud,
a satellite galaxy of the Milky Way.

The light was a supernova explosion.

But three hours before the flash,
astronomers spotted something else.

A burst of neutrinos coming
from the same region of the sky.

This was the first time
we have seen neutrinos coming
from a source

other than the sun,

so, there must be some
sort of connection

between neutrinos and supernova,
but...but what is that connection?

To find a link we need to travel
deep inside a giant, dying star,

where a battle is raging.

A star owes its existence
to the careful balance

between the force of energy flowing
out of the core of the star,

and the force of gravity pushing in
toward the core of the star.

So, if these forces go out of
balance something's gonna change.

That balance changes
as the giant star starts to age.

When nuclear fuel runs out
in the centre of a star,

fusion shuts down,
gravity starts to take over.

Gravity makes the star collapse.

The extra pressure triggers
a new burst of nuclear fusion,

making heavier elements and more
energy to support the star, for now.

That process works fine

as you build up heavier
and heavier nuclei

all the way to iron.

And then it tries to fuse
that iron into heavier elements

to get some more energy out of it,
but it can't.

Creating iron doesn't release
energy, it uses it up.

That means that when a star
gets iron in its core

it's like you've poisoned
the nuclear process.

You have this massive,

crushing weight of the star
with a core of iron,

and nothing left to support it.

So all that material crushes
and squeezes that iron core down,

squeezes it so tightly

that all the electrons
get shoved inside of the protons,

turning them into neutrons.

And very, very quickly you convert
this massive ball of iron

into a very small,
very compact neutron star.

When a star runs out of fuel, its
core crushes down to a neutron star,

then the rest of the star
collapses inwards,

hits the neutron star
and bounces out,

triggering a supernova.

The computer models of supernovas
reveal a problem,

the star doesn't explode.

When we run computer simulations
of how supernova might work,

after this bounce the explosion
stalls, it peters out,

the supernova isn't so super,

it needs another source of energy
to propel it

to become an actual explosion.

Could the neutrinos
that appeared before the explosion

be that energy source?

First, we need to understand
what created the burst of neutrinos.

The core of the star
collapses inward,

and eventually, the outer layers of
the star fall in toward that star

at an appreciable fraction
of the speed of light.

As the core rapidly collapses,

the intense pressure
squeezes atoms together.

That core of iron gets squeezed down

to become a neutron star.

The electrons and protons
that are part of its core

are under so much pressure
that they fuse together

to form neutrons and neutrinos
in the process.

The neutrinos shoot out from the
newly formed neutron star core,

carrying an enormous
amount of energy.

99% of the energy is carried
by the neutrinos.

Neutrinos are the main event.

Trillions of neutrinos smash
into the remains of the dying star.

And when those neutrinos
are flying out of that core region,

a very tiny fraction of them
interact with the gas.

And that fraction heats the gas.

Everything that's hanging around
these new born neutron star

get heated
to an unimaginable degree.

The heat creates pressures
in the surrounding gas.

It builds and builds until
it triggers an enormous shockwave.

And then the actual explosion,
the actual firework show begins.

(EXPLOSION)

The star explodes

in one of the brightest events
in the universe.

Powered by neutrinos.

We think that if it weren't
for neutrinos,

supernovas might not even exist.

And we might not exist either.

Our bodies contain heavy elements

like calcium in our bones,
and iron in our blood.

These elements form in supernovas

and are scattered across the cosmos
by the blast.

Neutrinos are what kindle the fire

in the forges of these elements

and without the neutrinos
you don't have the elements

and without the elements you don't
have planets like the Earth

and without planets like the Earth,
you don't have life.

There's this common phrase,
you know,

we are stardust which is true,

but I like to think
we're more like neutrino dust.

Neutrinos reveal
how supernovas explode,

and they also warn us
when one is about to detonate.

The neutrinos can even
be these ghostly signposts

for something very violent that's
happened in the universe, right?

We detect a sudden
burst of neutrinos,

it could be that a star
has gone supernova somewhere.

Neutrinos are
an early warning system

because they barely interact
with anything,

they race out of a dying star
ahead of the light.

The neutrinos just slip
right on through,

cos remember, they're ghosts,
they're ninjas,

they get right through
and they can arrive at the Earth

before the light does.

Supernovas happen suddenly,
without warning,

so we rely on neutrinos
to give us a heads up.

Neutrinos from supernova actually
get to us faster than the light,

within tens of seconds we're seeing
neutrinos hit our detectors.

If you have multiple detectors
on the Earth,

they will all receive
the same neutrino blast

as it washes over the Earth,

but different detectors

will receive that blast
at different times.

And you can use that
to triangulate on the sky

where that blast came from.

And we can use that
as a warning signal

to alert our telescopes
around the country

to look in this direction,
in this portion of the sky,

because a firework display,
supernova, is about to happen.

This gives us the time to focus
our telescopes on the supernova

and watch the light show as it
follows the initial neutrino burst.

Neutrino bursts are cosmic watchdogs
alerting us to danger.

Neutrinos are definitely a sign

that something troubling
is happening.

And in 2017, a single neutrino told
us about something very troubling.

One of the most intense sources
of radiation in the universe,

and it was pointing right at us.

Spring, 2017,

scientists at the South Pole
are on the lookout for neutrinos.

These ghostly particles
are extremely hard to detect.

Neutrinos are the biggest
introverts in the universe,

they just don't like
interacting with anything.

So if you wanna detect these things,

you need a lot of stuff and you need
a lot of atoms in one spot.

So scientists built a facility
with lots of available atoms.

It's called IceCube.

With neutrino detectors
buried deep beneath sheets of ice.

It turns out that water is a very,
very good detector of neutrinos.

To catch neutrinos, you need
to build a very large target

for a reasonable cost.

Large areas of ice checks
both boxes.

So you need a lot of water
that's very, very clean.

What's the cleanest source of water
on the planet?

The Antarctic ice sheet.

The Antarctic detector IceCube
measures 3,280 feet across.

That's about the length
of nine football fields.

It contains 5,000 sensors
surrounded by more water atoms

than there are stars
in the universe.

So if you wanna catch neutrinos
colliding with other particles,

the thing you need to do
is put a lot of targets

in front of that gun, right?

You need to pack a lot of particles
in the path of the neutrino

to give it a higher probability
of interaction.

The actual detectors are holes,
drilled in the ice.

And these holes contain
chains of detectors

that are three times longer than
the tallest building in the world,

and these are sunk down into the ice

and use all that incredible volume
of ice to detect neutrinos.

September 22nd, 2017.

IceCube detects a neutrino
colliding with a water atom.

When a neutrino hits an ice atom
inside of IceCube,

a charged particle flies out,

and it's this charged particle
that makes a signal we can detect.

The ejected particle appears to fly
out faster than the speed of light.

At first glance, this looks like

it violates something
very important about physics,

that nothing can travel
faster than light,

but light slows down

when travelling through a medium,
like air or water,

and it is possible for other things,

other particles to outrun light
in a medium.

As it hurdles through the ice,

the particle generates
a burst of blue light

called Cherenkov radiation.

It's almost like a sonic boom,

if you travel faster than the speed
of sound there's a boom.

When you hear that boom,
you also see this cone of wind.

It's the same thing
with Cherenkov radiation,

you get this cone of light.

Neutrinos carry
different amounts of energy.

Some, like the 2017 neutrino,
carried quite a punch.

And the energy of the neutrino
depends on its source.

High energy neutrinos
come from high energy events.

So we're looking for stuff
blowing up,

we're looking for stuff colliding,

we're looking for stuff
colliding and blowing up,

we're looking for awesome things.

The blue burst of Cherenkov
radiation gives us a clue

about the fearsome origin
of the neutrino.

We can follow the path
of that blue light

and we can look backwards to see
where the neutrino came from.

We tracked the neutrino to a galaxy
nearly six billion light years away.

At its heart sits one of the most
powerful objects in the universe.

A blazar.

A blazar is the biggest, baddest
form of feeding, active,

super massive black hole out there,

where material isn't
just falling into the black hole,

it's swirling around creating
a high energy accretion disk.

The blazar's accretion disk spins
at millions of miles an hour.

Charging particles of gas and dust.

The disk also generates magnetic
fields that twist and tangle

as they swirl around the black hole.

Because you have magnetic fields
that are twisted around,

they also generate electric fields.

The electric fields can then
accelerate the charged particles

along the magnetic fields,

and thus produce a lot of
both particles and radiation

coming out along jets.

The jets blast out of the poles
of the black hole.

These are the most intense
sources of radiation

that the cosmos can ever produce,

and they are pointed right at us
from billions of light years away.

Do the jets create
the powerful neutrinos?

It's a big of a mystery.

For a while it was thought
that neutrinos are produced
directly by the jet,

but now we think that matter,
like protons,

come in from the accretion disk
and they slam into each other,

and that's what produces
the neutrinos.

Particles racing around
the accretion disk

crash into the base of the jet.

The enormous energy there
smashes the particles together,

producing neutrinos.

The jets focus
the stream of neutrinos

and fire them
straight towards Earth.

By just detecting one neutrino,
we get to see a lot of information

from the inner workings of
an object outside of our galaxy

and that's what's
exciting about neutrinos,

is that it could peer
into the unknown.

Now, we use neutrinos to probe
even further into the universe.

Back towards the first second
of the Big Bang.

To answer the biggest question
of them all,

how, and why, do we exist?

The fact that our universe

appears to be filled with matter
is puzzling.

There should have been
equal amounts of matter
and anti-matter in the beginning

and they should have annihilated
one another,
producing just pure energy.

So why do we exist?

This is a fundamental question,
because this is a question

about why is there something
rather than nothing?

To answer that question,
we have to rewind the clock back

nearly 14 billion years
to the birth of the universe.

A speck of energy
sparks into existence.

This energy cools and forms tiny,
primitive particles of matter,

including neutrinos.

The building blocks
of everything we see today.

The early universe appears chaotic,

but it quickly establishes some
ground rules, including symmetry.

Our universe is full of symmetries.

There are positive electric charges
and negative electric charges.

There's the yin and the yang.

Well, there's also matter
and anti-matter.

The Big Bang stuck
to the rule of symmetry

and made the same amount
of both forms of matter.

The mechanisms that we have

for creating matter
in the early universe

create an equal amount
of anti-matter,

that symmetry is baked
into the laws of physics.

The laws of physics also say that
when matter and anti-matter meet,

sparks fly.

So matter and anti-matter,
when they touch they annihilate,

they just disappear
in a flash of energy.

And as far as we understand, the
earliest moments of the universe,

matter and anti-matter
were created in equal amounts,

so they should have annihilated

leaving nothing but energy.

Which means no matter,
no anti-matter, no gas, no dust,

no stars, no galaxies,
no life, nothing.

Somehow, matter won the battle over
anti-matter in the early universe.

In some ways, the universe
ignored the rule of symmetry.

Something has to drive
the universe off balance.

There has to be a violation

of this fundamental balance
in our universe.

That way, when the matter and
anti-matter met and annihilated,

because there was more matter,

there would be a residual
of left over matter

and there would be no anti-matter.

How did the Big Bang
break the symmetry

between matter and anti-matter?

So we're looking for any
interaction, any process whatsoever

where matter behaves slightly
differently than anti-matter.

We're trying to find
a flaw in physics.

We can't look for that flaw directly
because we can't see the Big Bang,

but we can recreate it, and we
think neutrinos are involved.

This is incredibly complicated,

we are diving deep into the bowels
of fundamental physics

and it is not a pretty sight.

Japanese scientists conducted
an experiment called TK2.

They recreated part of the Big Bang
by studying neutrinos

and their symmetrical twin,
anti-neutrinos.

The goal? To see if anti-neutrinos
change their identity, or flavour,

at the same rate
as regular neutrinos.

Matter and anti-matter
should behave exactly the same.

But we found something very
interesting with this experiment.

The particles broke symmetry.

Neutrinos and anti-neutrinos
changed flavour at different rates.

This was a clear cut example

of matter behaving differently
than anti-matter.

And that has revolutionised
our understanding

of the formation of particles
during the Big Bang.

What could have happened
in the universe

is that more of the neutrinos
converted into matter

than there were anti-neutrinos
became into anti-matter,

and in this way you end up with a
surplus of matter over anti-matter.

Even though that surplus
was just one particle in a billion,

it was enough to build the cosmos.

So neutrinos, in the early universe
could possibly solve

the matter, anti-matter asymmetry
problem we have.

Yes, they caused destruction,

you know,
sometimes they blow up a star,

but at the end of the day they...
they did save the entire universe.

Now, scientists hope that neutrinos

may solve one of the biggest
mysteries in the cosmos,

the identity of dark matter.

Neutrinos have been around
since the birth of the universe.

They may even be responsible
for the formation of matter.

Now, we investigate if they play
an even larger role

in the development of the universe.

The formation of the cosmic web.

At the very largest scales
in our universe,

our galaxies are arranged
in a very peculiar pattern.

We see long, thin threads
of galaxies

and at the intersections
we see dense clumps of galaxies
called clusters.

In between them
we have these vast empty regions

called the cosmic voids.

For a long time,
how the cosmic web formed

and held together was a mystery.

One of the real mysteries
about our existence

is why the universe was able
to hold together at all.

All the matter was simply spread
apart too sparsely

to ever form galaxies or stars.

Instead, something helped
to hold it together.

We now think the glue
binding the cosmic web

is a mysterious substance
known as dark matter.

If it wasn't for dark matter
in the very early universe,

there might be no structure at all.

But what is this architect
of the universe?

This dark matter?

Dark matter is invisible matter
that we can't see,

so you, me, all of the particles,
everything that we see,

is actually only five percent
of actual matter in the universe.

The rest is dark matter.

Dark matter is a fancy name
for something we don't understand.

What we do know is that there is
much more stuff than we can see,

but have no idea what it is.

It's one of the greatest
open mysteries in science.

Dark matter hardly interacts
with anything.

A bit like neutrinos.

Also, like neutrinos,

dark matter was abundant
and active in the infant universe.

So, could neutrinos and dark matter
be the same thing?

PHIL: We don't know what dark matter
is but we kinda know how it behaves

and neutrinos sound like
a pretty good candidate for it,

because, hey, they are dark, they
are everywhere in the universe

and they have
a little bit of mass.

And by little, we do mean little.

Neutrinos weigh around 10 billion,
billion, billion times

less than a grain of sand.

But neutrinos are also
exquisitely abundant

and so because they're so abundant,

their individual tiny mass
can actually add up

to a large diffuse mass
on very large scales.

To investigate if neutrinos
and dark matter are the same thing,

we must return to the Big Bang.

As the universe expands and cools,
primitive matter forms,

including dark matter
and trillions of neutrinos.

The dark matter clumps together
forming regions of higher gravity,

which pulls in regular matter.

It formed a structure of scaffolding
that allowed regular matter

to gravitationally begin
to come together

and collapse into galaxies,
stars and planets.

Could the combined mass of neutrinos
in the early cosmos

have produced the extra gravity
to help structures form?

Could it be possible that
this really is dark matter?

These tiny little particles, but
in abundance across the universe.

And we know more, not all,

we know more about neutrinos
than we do about dark matter,

but there's still a question
around whether or not

neutrinos can be a specific type
of dark matter.

To answer this question,

we have to work out what specific
type of dark matter was around

in the Big Bang.

Hot, or cold.

People talk about hot dark matter
and cold dark matter,

and really what you're saying
is the speed
of the particles themselves.

The cold dark matter
is moving slowly,

and the hot dark matter
is moving fast.

This speed difference
is an important clue

to whether neutrinos
make up dark matter.

With hot and cold dark matter,

the way they interact with regular
matter has a lot to do
with how fast they're going,

so it's a good analogy
to think about a river.

With hot dark matter,
you'd have a torrent,

basically it's going so fast
it doesn't actually connect
with anything,

it just goes right on past,

so there's no chance to form
that larger structure.

If you have relatively slow moving
dark matter, cold dark matter,

think about a slow moving river.

A slow moving river
begins to deposit silt.

Think of that silt
as the billions of galaxies

that make up the cosmic web.

We observed that galaxies formed
very early in the universe

and this is good
for cold dark matter,

but it doesn't work
for hot dark matter,

so we think cold dark matter
is really dominating structure
formation in the early universe.

But cold and slow
does not describe neutrinos.

They move very fast,
close to the speed of light.

This is a problem with neutrinos,

because neutrinos would be
hot dark matter.

That rules out neutrinos
as cold dark matter.

The idea that neutrinos are dark
matter, hit another set-back

when we weighed the universe.

If you add up the total mass of
all the neutrinos in the universe,

it would wind up being about
a half a percent to 1.5 percent

of the total mass of dark matter.

Neutrinos were a good candidate
for dark matter

because they exist

and they're very shy just like
the dark matter particles are.

But then, we were able
to measure more accurately

how much dark matter there is
and how much neutrinos there are

and there are just way less
neutrinos than there's dark matter.

Neutrinos do have mass and
there are a lot of them out there,

so it might be
some tiny, tiny fraction

of dark matter is made up
of neutrinos,

but we know that these things do not
make up the bulk of dark matter,

it must be something else.

So neutrino scientists

hunt for a different contender
for dark matter,

a completely new kind of neutrino.

We know about three flavours
of neutrinos.

The electron neutrino, the muon
neutrino and the tau neutrino.

But there could be a hidden
fourth flavour of neutrino

that could solve the riddle
of dark matter.

We call this a sterile neutrino.

So called because they interact
even less than regular neutrinos.

A particle so tiny,
so hard to detect,

could turn out
to have lots of the secrets

wrapped up inside it
as to how the universe works.

The first step to find out if
sterile neutrinos are dark matter,

is to prove they exist
and that's tough.

Even though sterile neutrinos
are almost impossible to detect,

we can still hunt for them.

Back in the day,
neutrinos were also said

to be difficult to detect.

Trying to find dark matter, trying
to find these sterile neutrinos,

it's almost like
using one invisible,

undetectable thing to find another,
using a ghost to find a goblin.

We are definitely pushing
the limits of science.

A team at Fermilab
has an ingenious idea.

They can't spot sterile
neutrinos directly

because they don't interact
with atoms in the detectors.

So they're looking for neutrinos

as they change flavour
into sterile neutrinos.

We know that normally neutrinos
change type

as they move through space,

but they have to move far enough
before that change happens.

So tracking neutrinos
over a short distance

shouldn't show any flavour changing.

In this experiment
they've constructed
only a half mile long path,

it's not enough time
for the neutrinos to change flavour
in the normal way.

If they do see something,
if they see something change,

this could be some
interesting aspect,

perhaps evidence
for sterile neutrinos.

So is it possible
that over short distances,

regular neutrinos can oscillate
into this sterile neutrino?

The team shoots beams
of muon flavour neutrinos

along the detector.

In theory, they won't have time
to change flavour.

We can see whether or not
these muon neutrinos

morphed into a different type
of neutrino.

They shouldn't change,

but if they do that points us
towards sterile neutrinos.

The team compare the number of muon
neutrinos reaching the detectors

to those fired along the beam.

Fewer muon neutrinos
hit the detectors,

some neutrinos had changed flavour.

So we are seeing
that oscillation of neutrinos

changing from one type to another.

We had an idea of how many
we should have seen,

but we're seeing more,
and that could be sterile neutrinos.

If sterile neutrinos do exist,
would they be dark matter?

Right now, we don't know the mass
of the sterile neutrino.

But if it's heavy enough,
it could be a contender.

If it exists, it's prevalent enough

to account for all the dark matter
in the universe.

Fermilab's results haven't been
verified by other scientists.

So, it's too soon
to say definitively

that sterile neutrinos are real.

Or that they make up dark matter.

Dark matter is probably one of
the biggest questions of our time

and the fact that Fermilab
may be one of the places
to answer that question,

and the fact that I am
working here, is really fantastic,

because we're attempting
the impossible.

We have to wait to see
if the impossible is possible.

We know neutrinos
have played a vital role

in the history of our universe.

And even now, they refresh it
by powering supernovas.

Without them, our sun...

our world and even our bodies,
would not have formed.

Neutrinos are pesky
little particles,

super elusive, difficult to study,

but when you can catch them
they offer secrets to the universe.

The story of neutrinos has been
interesting, it's like reading and
you're on the last page,

and then you turn it and suddenly
there's a hundred new pages.

Neutrinos are teaching us

that the universe is, in many ways,
subtle and hard to figure out

and the more we learn
about these things

they more we learn
about the universe.

Neutrinos are the universe's
great escape artists,

the Houdini of particles.

In fact, they may have helped us
to escape the Big Bang

and end up existing.

At the end of the day,
they're what saves us.

The more we understand
these elusive particles,

the more we can gain insight
into how the universe works.

So, it's really cool.

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