All language subtitles for How.the.Universe.Works.S10E02.Curse.of.the.Cosmic.Rays.1080p.WEB.h264-KOMPOST

An invisible danger
hurtles towards Earth

at close to the speed of light.

These are intergalactic alien
interlopers on our Milky Way.

Cosmic rays.

Getting hit by
a cosmic ray is like

getting hit by a cosmic bullet.

Cosmic rays are billions of
times more energetic than any

other types of particles,

vastly more energetic
than anything

we can even create in

a laboratory, in a nuclear
fusion reactor, anywhere.

They pierce spaceships,
putting our astronauts

in danger.

But the source of their power
is a mystery.

Are they coming from
other galaxies?

Are they coming from things in
between the galaxies?

Where do cosmic rays come from?

Truth is,
the most powerful ones,

we haven't got a clue.

The race is on
to solve the mystery

of the fastest particles
in the universe.

If I were to make a list
of the dangers of space,

it would be a long list.

You know, there's hard vacuum,
huge swings in temperatures,

micro meteorites,
all kinds of things.

But probably at the very top
of that list, cosmic rays.

These space invaders
are not what they seem.

When you hear the name
cosmic rays,

you might think
it's like a beam,

like a laser beam of light.
No, no, no, no, no, no.

It's a tiny little
death particle.

To fight them,

we must first understand them.

April 2019.

NASA's Parker probe flies
closer to the sun than

ever before.

We know the sun produces some of

the cosmic rays that fill
the solar system,

but we don't know how.

Our sun looks like
a beautiful glowing orb,

bringing energy and light to
Earth and allowing life

to thrive.

But if you look at it up close,

you'll see a tumultuous storm
of events.

The amount of energy
the sun is emitting every

second is the equivalent of
100 billion one megaton bombs.

It's a dangerous neighborhood.

Suddenly,
the probe is caught head-on

in a powerful blast.

It's perfectly
positioned to monitor

the outburst from the inside.

The entire outer
third of the sun is a boiling

cauldron, and tied up in that
plasma are magnetic fields.

They get tied and twisted,
and energy is stored in them,

so they rise towards
the surface, and there,

they rearrange, they reconnect,
they twist, they spin.

When the magnetic
field lines snap,

energy bursts out.

And sometimes, that energy
release is explosive

and that's what
results in flares,

which are these huge
bursts of light.

The probe discovers
that after a solar flare,

the sun's surface stores
electrically charged particles.

But sometimes, there's
a second explosion called

a coronal mass ejection,
releasing superheated,

electrically charged gas
called plasma.

These giant balls of plasma
go flying off the surface of

the sun,
and in those balls of plasma

are contained
these charged particles.

The charged particles move
fast, but they hit a roadblock,

a cloud of
slower-moving particles

that always surrounds
the sun, the solar wind.

Well, the coronal mass ejection
is moving into the solar wind

much faster
than the wind is moving.

So it sort of runs into it
and creates this shockwave

and ends up piling up
particles at the edge.

The shockwave and particles

slam together...
In the collision,

the particles steal
energy and speed,

like a baseball accelerating
off a bat.

The particles transform into
something far more powerful...

A solar cosmic ray.

They're light,
but they're moving

incredibly fast.

The Earth is on average
93 million miles away from

the sun, and these guys reach
us in about an hour.

That's 93 million miles an hour.

That's pretty fast.

The cosmic rays speed
towards Earth.

We're under attack.

Cosmic rays are, by far,
the most energetic

particles that we know
to exist in the universe.

And when things with very high
energy, no matter how small

they are, impact something else,

they deposit that energy, right,
and so cosmic rays

can be very dangerous.

Solar cosmic rays aren't
the only threat we face.

Other space bullets arrive
from beyond our solar system.

There are different kinds of
cosmic rays,

just like there are different
kinds of bullets.

At the lowest end
of the spectrum

are these solar cosmic rays.

These are like the BBs,
and when a BB hits you,

it might sting for a little
bit, but you're not gonna get

too worried about it.

A bigger concern...
Galactic cosmic rays.

They travel faster
and have more energy.

If solar rays are like BBs,

the galactic cosmic rays are
like rifle bullets.

They're far more dangerous.

They're moving a lot faster.

But they're also more rare.

Faster still are
the universe's most wanted...

Ultra-high-energy cosmic rays.

If you thought galactic
cosmic rays were bad, it's

because you haven't met
an ultra-high-energy cosmic ray.

These are the biggest, baddest,

meanest cosmic rays in
the universe.

These ultra-high-energy
cosmic rays

are like hypersonic missiles.

They are screaming,
and they come from

the most energetic events in
the universe.

The ultra-high-energy
cosmic missiles

are the rarest
but also the swiftest.

These cosmic ray particles
are moving fast.

These mysterious particles
are moving incredibly

close to the speed of light.

I'm not talking about
99 percent the speed of light.

They're moving through space at

like 99.999...

- 9999...
- 9999...

- 99999...
- 999999...

99999... 21 nines.

That's fast.

That's wild. That's scary.

All three types of cosmic rays

are racing
through the solar system.

If I were to hold up a golf
ball in the middle of space,

almost 100 cosmic rays pass
through that golf ball every

single second.

It's a deadly hail of
particle bullets,

and out in space,

our astronauts are caught in
the crossfire.

Cosmic rays represent one of

the greatest dangers for
human space flight.

NASA plans to send
astronauts back to the moon,

where radiation levels
-OVER RADIO: Lift-off.

From cosmic rays are
200 times greater than

on Earth,
and that is just the start.

One of NASA's big goals is to
send humans to Mars,

and that is a long way away,

at least a six-month journey,
and more often,

about a nine-month journey.
That's a big problem.

I am hoping that one day, I can
go to Mars as an astronaut,

but I'm definitely afraid of
cosmic rays,

and the more that I read
about it,

the bigger of a threat it seems.

So I think that NASA and other
space organizations

are going to need to work on

how to protect
their astronauts in these

really dangerous situations.

Only one group
of people have been

exposed to these high levels
of cosmic rays,

the crew members of
the Apollo missions

July 1969.

That's one small step for man,

one giant leap for mankind.

One of the astronauts,
Buzz Aldrin,

sees something strange.

During Apollo 11,

Buzz Aldrin reported
seeing tiny little

flashes sometimes
when he was looking around.

That's pretty weird.

But what's weirder
is that he saw them

when his eyes were closed.

Later missions also report
seeing odd flashes of light.

A streak in
the lower left side of the...

left eye, moving down.

The astronauts describe
the flashes as spots,

streaks, and clouds.

Apollo 15 Commander David
Scott reported seeing one that

was blue with a white cast,
like a blue diamond.

What's happening is that
a cosmic ray is entering

the eyeball

and then striking molecules
and giving off

a flash of light.

An alternative theory is that
it triggers the layer of

sensitive cells in your retina,

so you perceive
a streak of light even though

no light ever actually existed.

The cosmic rays
cause long-term damage.

Inside of the eye's lens,

there are these fiber cells
that are transparent.

Well, when a cosmic ray
travels through them,

it can damage those cells
and make them cloudy,

causing cataracts.

When NASA examines
ROWE: the astronauts' helmets,

they find tiny tracks etched
through them,

evidence of cosmic ray impacts.

When we say that cosmic rays
are like tiny little bullets,

we're not joking around.

And some of these burrowed all
the way through the helmet,

which means it ended up
in the astronaut's brain,

which just makes me feel weird
to think about.

What might that
long-term radiation

do to your brain,
to your ability to reason

and problem solve in one of
the most dangerous environments

that humanity has ever
placed itself?

The farther we venture
from our home planet,

the more danger we face.

Cosmic rays, highly
energetic space particles,

may be the most serious threat
to human space exploration.

The Hollywood
conception of outer space is

it's full of dangers like aliens

wielding ray guns or black holes

or asteroid showers.

But in reality, the biggest

danger facing astronauts
is invisible.

It's the cosmic radiation.

Cosmic rays damaged
ROWE: Apollo astronauts' eyes

after just a few days' exposure.

A one-way trip to Mars takes
nine months.

Future missions are going to be

spending much longer times
in space,

which means we really need to

consider how cosmic rays will
impact us.

We don't understand
all the long-term effects

from a steady rain of
cosmic rays,

but the astronauts are gonna
have to deal with it.

To find out more,
scientists bombarded

human cells with manmade
cosmic ray particles.

They discovered cosmic rays
physically cut through DNA,

chopping it apart.

Damage to DNA in your cell is

by far the worst kind,
because your DNA

is the cell's operating
manuals, the blueprints

so the cell knows how it
should be functioning normally.

You can trigger that cell to

turn tumorous,
to start producing cancer.

In 2019, scientists
ROWE: took the experiment

further and simulated
a trip to Mars...

for mice... For six months,

they blasted the rodents with
a steady

stream of lab-made
cosmic ray particles.

The experiment found
profound alterations

to the mice's normal behavior.

They learned new tasks
much more slowly.

Their memory was affected,
and they

forgot things they had
already learned.

They were more anxious
and prone to

giving up on tasks
they'd normally complete.

If you put some of these
irradiated mice into

a swimming test, rather than
trying to swim to safety,

many of them just simply
gave up.

This is important,
because we need

our astronauts to be
fully functioning.

The reason why you do
crewed missions is because

the human brain is much better
than any computer.

If even one of them
has a problem,

it can even put the mission
and their lives in jeopardy.

Other studies discover
cosmic rays can accelerate

aging, alter genes, and cause
cardiovascular disease.

That sounds bad enough,

but there's
a more immediate danger.

When cosmic rays
penetrate spaceships,

they can fry electronic
systems, and that's enough to

jeopardize a mission.

Our operations in space depend
on electronics, on computers.

And the worst case scenario is
that the wrong cosmic ray comes

at the wrong time and hits

the wrong circuit, and it leads
to a cascading series of

failures that can totally
jeopardize a mission.

We see evidence of this
onslaught in mission cameras.

Even when we have
a detector in space

like on
the Hubble Space Telescope,

if you saw a raw image,

it doesn't look like
the beautiful images

that are shown to the public.

They're just crossed with
cosmic rays, and those

cosmic rays are destroying
that detector slowly over time.

So how can we protect
astronauts and their equipment?

The obvious answer is to
add shielding.

It's one thing to say
like, oh, just add more stuff.

But have you seen rocket
launches and how hard they are,

how expensive it is to get
stuff up into space?

NASA does have a plan.

The spacecraft for the Artemis
moon landing mission

will be packed for optimum
cosmic ray protection.

So one of the ways
you can get around

the mass limit is
to basically get dual use

out of everything...
Your supplies,

your fuel, your water, and you
can use those as shielding.

But it's not that simple.

Just as more powerful bullets
penetrate armor,

more energetic cosmic rays
pierce the shielding

on spaceships.

The solar ones...
Yeah, you can just put up some

material, some shielding, and
it'll generally block them.

But the higher energy ones,
they can just burrow

on through.

If they hit one of the atoms
in the shielding that is

protecting our astronauts,

it can create
a shower of particles.

That radiation
particle might have missed

any of the cells in your body.

But you've now turned it into

a blast, shredding through
everything in the spacecraft.

And so it turns out
your shielding becomes

the weapon that the cosmic
rays use against you.

But NASA is recruiting
an unexpected ally... the sun.

Can we protect our astronauts
by fighting fire with fire?

Powerful cosmic rays
smash through spaceships.

But how can objects smaller
than an atom carry enough

energy to be
dangerous to astronauts?

Moving objects carry energy.

We call this kinetic energy,
and when

they strike something,
they transform that energy.

When I hit my hand,
the kinetic energy of

my fist transforms into sound
and heat and vibration.

My hand hurts a little from

that impact, from
the transformation of energy.

It's the same thing
with cosmic rays.

When they slam into a human
brain cell or a computer chip,

they dump some energy,
causing damage.

How much damage depends on
their kinetic energy,

and that comes down
to two things...

Mass and speed.

Intuitively, things that are
moving at the same speed,

if they're more massive,
they carry more energy.

A bigger asteroid slamming
into Earth will

do more damage than
a smaller asteroid.

If you double
the mass of an object,

its kinetic energy also doubles.

Although mass is important,

it's not as important as speed.

Speed matters
even more than mass.

The kinetic energy depends
directly on the mass,

but it depends on
the square of the speed.

Here's what that means...
You double the mass,

you have double the kinetic
energy, you double the speed,

you have four times
the kinetic energy.

When it comes to speed,
cosmic rays are the elite.

An ultra-high-energy cosmic ray

detected in 1991 hit
the atmosphere so fast,

scientists called it the
"Oh My God Particle."

This particle was higher
energy than they thought

they would ever, ever see.

Until this fluorescent
streak in the Utah sky,

no one believed a particle
could reach the Earth traveling

so close to the speed of light,

making cosmic ray particles
far more dangerous

than expected.

As you approach
the speed of light,

energy, momentum, mass,
they start

to act a little bit differently.

Einstein's equations of
relativity become important,

because the physics changes,
and the energy it has

becomes much, much,
much stronger.

If a particle is moving
at close to the speed of light,

that means that its energy is
almost at the maximum allowed

by the laws of physics.

It's amazing to think
that something as tiny as

a proton could actually be
dangerous to a human being.

But amazingly, that proton is
moving so fast,

it carries as much energy
as a baseball

thrown at 100 miles an hour.

A baseball contains over
a trillion, trillion protons.

Imagine all that energy
carried by just one particle.

So now you get a sense of just
how risky these can be.

Ultra-high-energy
cosmic rays like

the Oh My God Particle are
like supersonic missiles.

They are the fastest,
but they're so rare,

astronauts are unlikely
to be hit by one.

Solar cosmic rays are like
BB pellets... abundant,

but our spacecraft can
block them.

The biggest threat to
our astronauts,

however, are galactic
cosmic rays.

They come from elsewhere
in the Milky Way.

The combination of their speed

and frequency makes them
the most dangerous.

These galactic cosmic rays
are much more powerful than

the solar cosmic rays,

and they've traveled
enormous distances

to mess you up.

Luckily, our astronauts
have a surprising protector,

a guardian of the solar system,
the sun.

As well as spitting out these

high-energy solar cosmic
ray particles,

the sun is also streaming out
lots of much lower energy

particles of the solar wind.

That outward moving
solar wind acts

as a force field, and
the cosmic rays have to work

their way upstream to get to
Earth far inside this bubble.

The solar wind extends
11 billion miles

around the solar system,

generating a magnetic field
that repels incoming

galactic cosmic rays.

It's almost like
the deflector shield

of the Starship Enterprise.

So the sun's magnetic field
partially helps protect

the Earth and any astronauts

from the incoming radiation.

Not long ago,
our Voyager spacecraft

made it to that boundary
between the sun's bubble

and the galaxy and was able

to study that region, and we
see the difference between

inside the sun's bubble
and what's going on

outside the sun's bubble.

The sun has our back, billions

of miles away,
and that's pretty cool.

The Voyager
space probes discovered

a moving battlefield.

The solar wind behaves a bit
like a storm front on Earth.

Sometimes, it advances.

Sometimes, it retreats.

When the sun's
activity is the highest,

it's spitting out more
solar energetic protons,

but those solar cosmic rays
are much less damaging

than the galactic ones,

so the net is a benefit.

So actually,
ironically, you might find that,

for astronauts, it is safer
to launch missions to Mars

during a period of higher
solar activity, because although

you have more of
the solar particle radiation,

you also get a better
shielding effect

from the solar wind.

The sun's activity goes through

an 11-year cycle
of highs and lows.

The protective bubble follows
the same cycle,

allowing NASA to predict
the safest times to launch.

This is a thorny problem,
and, you know,

we have very smart people
working on it, but we want to

explore space as much as we can,

but we have to lower the risk

to the astronauts
as much as possible.

NASA's fight against
the cosmic invaders continues,

but the biggest mystery remains.

What exactly is launching
the deadliest

galactic cosmic rays?

Every second,
quadrillions of bits

of space shrapnel
race towards Earth

at close to
the speed of light...

Galactic cosmic rays.

The galactic cosmic rays
are like a rifle bullet.

You do not want to get hit
by one of these.

They are invaders from
outside the solar system.

We know they're made by
something powerful

within our galaxy,

so the source should be
easy to detect.

You'd think if one of them

hits a detector on Earth,
that we'd just be able

to point back
in a straight line and say,

"It came from over there."

And then look, is there
something else over there,

like a supernova explosion that

could explain the source
of this?

The problem is
that cosmic rays get

bent as they move
by magnetic fields.

The electric charge
on a cosmic ray

makes it act like
a little magnet,

and the Milky Way
is full of other magnets.

If I'm a cosmic ray just
barreling through the galaxy,

and I encounter
a magnetic field,

I'm gonna slightly
change directions.

Maybe here, maybe there,
maybe up there.

My trajectory is going to
become scrambled.

And after
a few million years or so,

basically all the information
about where it started has

been lost...
It's going in a completely

random direction for all
practical purposes.

But galactic cosmic rays
also have a sidekick,

one that is far less elusive...
Gamma rays.

When a galactic cosmic ray
hits a regular atom out

in space,
it causes this big reaction.

It emits all sorts
of other particles,

including gamma rays,

which are basically extremely
energetic photons of light.

Critically, gamma rays don't
get bent by magnetic fields,

because they don't have
an electric charge,

so they just beeline off in
a straight line along whatever

direction the cosmic ray was
moving in the first place.

So we can look back at where
gamma rays are coming from in

the sky, and that tells us
where there are a lot of

cosmic rays having collisions.

And they've led us
to a prime suspect...

supernovas.

Supernova are some of

the most powerful explosions
in the universe,

and so they're ripe grounds
for these highly energetic,

extremely fast particles
to be created.

When a giant star
runs out of fuel,

it can no longer support its
own weight.

It collapses inward,

triggering a huge explosion,
powerful enough

to smash atoms into tiny pieces.

The explosion pushes out
an expanding cloud

of gas and dust,
the supernova remnant.

And that material,
as it's moving out at

1,000 miles a second,

generates an incredibly
powerful shockwave.

And that shockwave
could be where

a particle swept up in
the shock gets accelerated.

The magnetic fields
inside the cloud

trap the subatomic particles.

Cosmic rays inside of
the supernova remnant

are a lot like being
in a pinball machine.

So you have the shockwave as
the flipper, and then your

magnetic fields
are these bumpers,

prohibiting it from
actually leaving.

They're bouncing back
and forth across

this incredibly energetic shock,
and each time

they bounce back and forth,

the key is they pick up
a little more energy.

When a galactic cosmic ray
gains enough energy,

the magnetic fields can no
longer hold on to it.

It escapes.

The supernova theory
explains the birth

of many of these cosmic bullets.

But then we discovered
a super gamma ray

so powerful, it must have
a completely different

origin story.

So this gamma ray was
incredibly high energy,

which means that the cosmic
ray responsible for it was

probably also extremely
high in energy.

If you fire a bullet into
a pinball machine,

it's not gonna bounce back
and forth.

I'm just gonna break
through the machinery.

The problem is that these

are vastly more energetic
than that.

So there's no way they could
have been bouncing around all

the way up to their current
energies inside of that

particular pinball machine.

There must be
something else in the Milky Way

creating galactic cosmic rays,

something more powerful
than a supernova.

The question is, what?

January 2021.

At an observatory

high up on the side of
a Mexican volcano,

blue light zaps through
water tanks,

signs of incoming gamma rays.

Their trail stretches back
across the Milky Way,

crossing billions of miles,
but suddenly goes cold.

Instead of originating
in a huge explosion,

the trail ends in a cold,
sparse cloud of dust.

Molecular clouds,
at first glance, seem like

one of the most boring,

innocuous places in
the universe.

You can barely even see them
without an infrared telescope.

They're not events
like supernova

that have enormously
high energies.

So you wouldn't expect it
to create super

energetic particles.

Something must be
hidden in the cloud,

something powerful enough to
accelerate the cosmic rays.

We just don't know what.

We can't see inside
the molecular clouds,

so it could be that,
deep inside them,

there are clusters of newborn
stars that are cranking out

these cosmic rays,

but we don't know if even
the crankiest of stars are

capable of producing cosmic
rays at these energies.

Just two months later,

in March of 2021,
we get another clue.

Scientists detect
gamma rays coming

from the Cygnus Cocoon Nebula.

It's a dense molecular
cloud with

a difference...
At the center is a cavity.

Hundreds of closely packed
stars push against the dust

and gas, including

huge bright stars
called spectral type O and B

Spectral type O and B stars are

some of the hottest stars in
our universe.

The massive stars
blast out solar winds far

stronger than the wind
produced by our sun.

When you think about all these
stars forming together,

they are all putting off
a wind of high-energy particles

from their surfaces.

These winds collide
and form big shock structures

between all of
these young stars.

You're getting
so much energy from

so many different winds,
coming from so many

different directions, that it
forms a boiling mass of

shockwaves and magnetic fields.

It's a pinball machine

on a far bigger scale...
The magnetic fields are stronger

than a supernova's,

trapping and accelerating
the more energetic cosmic rays

for longer.

One important thing about
star clusters is that

they are around for millions
and millions of years.

It's not just a one-off event
like a supernova.

And so you've got
this magnetic field

and these shocks happening
over a long period of time,

and that may be what you need
to accelerate cosmic rays.

Molecular clouds may
shoot out galactic cosmic rays,

but what fires the hypersonic
space missiles,

the ultra-high-energy
cosmic rays?

The culprit may be hiding out
in distant galaxies...

Supermassive black holes.

Ultra-high-energy
cosmic rays are

the hypersonic missiles of
the particle world.

If a photon of light,
the fastest thing in

the universe, had a race with
an ultra-high-energy cosmic ray,

it would be so close,
that after 200,000 years,

that photon would be
half an inch ahead of

the ultra-high-energy
cosmic ray.

They appear to come from
beyond our Milky Way galaxy.

Our galaxy is
100,000 light years across.

The next nearest galaxy to us
is two million light years away.

So these are traveling to us

across millions and billions
of light years.

How do you accelerate this tiny

little particle to such
insane velocities?

What is the power source?

What in the universe has
that kind of capability?

Where's the Death Star here?

Their speed makes
them dangerous,

but it also makes it easier to
find their source.

Ultra-high-energy cosmic rays
are moving so rapidly,

that they're really not
affected that much by

magnetic fields.

It's like a bullet going
through a fisherman's net.

And so they're coming
mostly in a straight line.

When they're coming in a
straight line, and we can point

back to their origin,

and that's something we can
use to figure out where

and how they're getting
accelerated.

In the Argentinian desert,

the Pierre Auger Cosmic Ray
Detector completes

a 12-year study of the sky.

It confirms that most
galaxies have a supermassive

black hole at their center,

but only a few are active,
shooting out energy.

These active
supermassive black holes also

blast out ultra-high-energy
cosmic rays.

Supermassive black holes are
already extremely powerful.

So it makes a lot
of sense to me that

the ultra-high-energy cosmic
rays could originate at

supermassive black holes.

The M87 galaxy is
54 million light years away.

It's famous
because we took a photo

of the supermassive black hole
at its core.

So the event horizon telescope
image of the swirling vortex of

gas around
that central black hole,

that shadow that
you can't actually see,

that could be a site for
the unbelievably energetic

acceleration of cosmic rays.

In March 2021,

scientists analyze
the data further.

This new image
of M87 shows very clear

magnetic field lines,

which is really stunning and
reminds us of how much energy

could be contained close to
the supermassive black hole.

Black holes have enormous power,

but how do they transfer some
of that energy

to a tiny particle?

One possibility for how
supermassive black holes

could accelerate such
enormously energetic

cosmic rays is that they
actually drag or capture via

their gravity

preexisting normal cosmic rays,
which are already

extremely energetic,

and then give them an extra
boost to even higher energies.

So supermassive
black holes bend the fabric

of spacetime around them,

and even light particles can
get stuck, and cosmic rays are

no different... they can also be
attracted by the supermassive

black holes and get drawn
into their orbit.

It makes sense that
the black hole captures

passing cosmic rays,

but how do the particles
escape its clutches

and hurtle towards us?

M87 has a fearsome weapon
in its arsenal.

Enormous jets of energy
blast out of its poles.

So M87's jets are
spectacularly large,

larger than the entire galaxy
that houses this black hole

that's launching those jets.

The powerful jets
may give the cosmic rays

a speed injection,

transforming them from
galactic rifle bullets into

ultra-high-energy
hypersonic missiles.

So imagine if you had
a regular bullet that

you fired out of a gun
at high speed,

and as its flying,
a little rocket motor in

the bullet kicks in and takes
it up to even higher speeds.

That's sort of what's
happening to

the cosmic rays in these jets.

Black holes may be
the supervillains

we've been looking for,

firing out the fastest
cosmic bullets,

but cosmic rays have
a superpower of their own.

They're time travelers.

Cosmic rays race through
the universe at close

to the speed of light...
Like subatomic bullets,

they can pierce spaceships
and harm astronauts.

But down on Earth,
we're protected.

Out of all of the rocky inner
planets in the solar system,

the Earth is the only one
to generate

its own deflector shield
against this cosmic radiation.

That's amazing.
And that's where life is.

I don't think that's actually
all that much of a coincidence.

The Earth creates
its own magnetic field.

The Earth has this wonderful
active molten core of metal.

All of that metal is moving
around inside the Earth,

and that moving metal generates
a strong magnetic field.

These cosmic rays are
electrically charged.

They follow a magnetic field.

So our magnetic field
deflects most

of the cosmic rays around it.

The shield is not perfect.

Some cosmic rays do get through,

but then they hit
our second line of defense...

The atmosphere.

One of the things we have to
be thankful for is

our atmosphere... not only
does it give us air to breathe,

but it protects us
from these space bullets.

The atmosphere is like
a missile defense system.

Cosmic rays collide
with air molecules,

shattering into
safer, smaller particles.

The most common ones
are called muons.

The muons are the children
of the cosmic rays.

They're produced by
these high-energy collisions

in our upper atmosphere that

create these showers of
muons that then come down to

the surface.

There's as many as four of
these cosmic rays passing

through my hand every second.

They're passing through
your body right now.

Muons are so abundant,

we don't need a high-tech
observatory to detect them,

just a few things you'd find
in a high school science lab.

A small aquarium that I've
attached a small piece of felt

to the bottom.

Some frozen carbon dioxide,
some dry ice,

hence the safety gloves,
a flat piece of metal like this,

some isopropyl alcohol.

Then I flip the whole thing over

onto the bottom, and I wait.

So what's happening is that
the alcohol in the felt is

evaporating in sinking down,
and because

that bottom layer is
so cold from the dry ice,

it forms a super saturated
cloud of alcohol vapor.

When the charged particles
pass through the cold vapor,

they create tiny ghostly trails.

What we're looking for
are the muons,

the subatomic particles
generated when a cosmic ray

strikes the upper atmosphere.

Each silvery thread
in the cloud chamber

is the sign of a cosmic ray.

These muons should never
make it down to Earth at all.

They only live for 2.2
microseconds before breaking up,

not enough time to travel

through six miles of
Earth's atmosphere.

Naively, we would think there's
no way that a muon could make it

from the upper atmosphere
to where we are now

without decaying.

It turns out they do,
and the only

way they do this is
they effectively time travel.

The muons move at 98 percent
the speed of light.

They move so fast,

they experience what Einstein
called time dilation.

Albert Einstein taught us that
we live

in a space time,
and so that means

that all measurements of
lengths and durations of time

are relative.

From a muon's perspective,

we humans move
incredibly slowly.

They're moving so fast that,
for them,

time is stretched out.

What we found by measuring
the energy and the lifetime of

muons is that as muons got
closer to the speed of light,

their lifetime increased,
because to them, time is

slowing down, exactly the way
Einstein predicted.

Their lifespan is
extended by more than 20 times

from our perspective,

so they make it to the ground.

Cosmic rays are
the ultimate space travelers.

They're awe-inspiring speed
allows us to unlock

hidden processes and test
our theories of physics.

They're way more energetic
than anything

we can do in a laboratory
on Earth.

So that means we can unlock
all kinds of

new domains about physics
at the highest,

most extreme energies.

They're our best link
to the farthest reaches of

the cosmos.

To me, it's really exciting
that we're actually sampling

pieces of matter from distant

stars, from distant galaxies,
and we're getting them here at

Earth and studying them.

There are so many amazingly
violent events in the universe,

the birth of black holes,
exploding stars.

These cosmic rays that are
going through your body

right now are messengers
from those events.

In some way,
you're still connected to

those events, millions
of light years away.

These are messengers
from the universe,

telling us about how it works.