How the Universe Works

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