Original subtitles

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.

DvX3M www.opensubtitles.org

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, 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 "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 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.