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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, right 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 saved the infant cosmos

from annihilation.

They cause destruction.

They, 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.

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

There was tantalizing evidence that

our sun might be shutting down.

This question was a biggie 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 a 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 center 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 center,

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 want to talk to you, and you also

want to 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 photons 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 10 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 motor bikes 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 10 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.

It turns out neutrinos can change what kind

of neutrino they are as they're flying through space,

and we call this flavor changing.

Neutrinos come in three different flavors.

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 flavor.

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 flavors.

But when we fine-tuned the sensors,

we saw all the solar neutrinos.

So there were actually enough neutrinos coming from

the sun, but we were only detecting a third of them.

Flavor-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 flavor,

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 flavor

or one type of neutrino forever.

Experiments proved that 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 suggests 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 supernovae,

but... but what is that connection?

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.

But 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 the protons that are part of this 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

this newborn 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 shock wave.

And then the actual explosion,

the actual fireworks show, begins.

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 forages 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.

So 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.

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 want to detect one of these things,

you need a lot of stuff.

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.

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, very important

about physics, that nothing can travel faster than light.

But light slows down when traveling through a medium like

air or water, and it is possible

for other things, other particles, to outrun light

in a medium.

As it hurtles 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, right? - 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,

carry 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 track 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, supermassive 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 bit 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 really 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?

Neutrinos are key to our understanding

of how the universe works.

They show us that the sun is healthy.

They are the trigger that makes supernovas explode,

and they reveal the location of lethal blazars.

And now they may solve something that still

puzzles physicists... How we exist.

The fact that our universe appears to be filled

with matter is puzzling.

There should have been equal amounts of matter

and antimatter 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 antimatter.

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 antimatter.

That symmetry is baked into the laws of physics.

The laws of physics also say

that when matter and antimatter meet...

sparks fly.

So matter and antimatter,

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 antimatter were created in

equal amounts.

So they should have annihilated,

leaving nothing but energy.

Which means, no matter, no antimatter, no gas,

no dust, no stars, no galaxies, no life, nothing.

Somehow matter won the battle

over antimatter 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 antimatter met

and annihilated, because there was more matter,

there would be a residual of leftover matter,

and there would be no antimatter.

How did the Big Bang break

the symmetry between matter and antimatter?

So we're looking for any interaction,

any process whatsoever where matter behaves slightly

differently than antimatter.

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.

I'm... 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 re-created part of the Big Bang by

studying neutrinos

and their symmetrical twin, antineutrinos.

The goal... to see if antineutrinos change their

identity or flavor at the same rate as regular neutrinos.

Matter and antimatter should behave exactly the same,

but we found something very interesting with

this experiment.

The particles broke symmetry.

Neutrinos and antineutrinos changed flavor at

different rates.

This was a clear-cut example

of matter behaving differently than antimatter.

And that has revolutionized our understanding

of the formation of particles during the Big Bang.

What could have happened in the early universe

is that more of the neutrinos converted into matter

than there were antineutrinos became into antimatter,

and in this way, you end up with a surplus of matter

over antimatter.

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,

antimatter asymmetry problem we have.

Yes, they cause destruction.

They... you know, sometimes they blow up a star,

but, at the end of the day, 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,

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

to 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 5% 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 we 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?

We don't know what dark matter is,

but we kind of 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 do have a little bit of mess.

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, a 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 setback 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% of

the total mass of dark matter.

Neutrinos were a good candidates 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's just way less neutrinos

than there is dark matter.

It sounds like game over,

but the neutrino hunters aren't giving up.

The search is on for a mysterious new kind

of neutrino,

one that could solve the riddle

of dark matter.

Neutrinos played a huge role in shaping

the early universe.

They helped matter defeat antimatter,

and the cosmos develop structure.

This led us to wonder if neutrinos might be dark matter.

But when we weighed the universe,

the numbers didn't add up.

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 flavors of neutrinos...

The electron neutrino,

the muon neutrino, and the tau neutrino.

But there could be a hidden fourth flavor 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 actually 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 flavor 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 flavor changing.

In this experiment, they've constructed

only a half-mile-long path.

It's not enough time from the neutrinos

to change flavor 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 flavor

neutrinos along the detector.

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

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 flavor.

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.

A story of neutrinos has been really interesting.

It's like reading a book,

and you think you're on the last page, and then

you turn it, and then suddenly there's 100 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,

the 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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