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[♩INTRO]

Scientists have been making particle accelerators for more than a century.

These machines use electromagnetic fields to boost particles to unimaginably

high speeds, and over the years, that's been useful for all sorts of science.

Like, today, in the largest accelerators, like the Large Hadron Collider,

scientists can smash these particles together to simulate the early universe

and test their most fundamental theories about physics.

But even though we've accelerated particles to more than

99.9999% the speed of light,

our accelerators are nothing compared to some of the ones in nature.

So by learning how natural accelerators work,

we can better understand the most energetic events in the universe.

And one day, maybe we can even use these accelerators for research,

in addition to building our own.

Some of the most familiar natural particle accelerators are thunderclouds,

which can get particles going to 99.97% the speed of light!

It all starts when pressure differences set up strong winds

moving both up and down the cloud.

The upward winds carry water droplets from the warmer air

close to Earth's surface,

while the downdrafts carry ice from the chilly upper atmosphere.

As the updrafts and downdrafts cross, water and ice collide,

and the water freezes into a type of hail called graupel.

Then, when graupel hits ice, the two materials often trade electrons.

The electrons can go either way, depending on different conditions,

but for the sake of example, let's say they favor the ice.

This leaves positively-charged graupel moving towards the top of the clouds,

while ice particles and their extra electrons move down.

In the end, you get separate groups of oppositely-charged particles

which is similar to a giant floating battery.

Then, sooner or later, a charged particle from the cloud will connect with

oppositely-charged particles on the ground,

and the battery will discharge with a giant spark: lightning.

But that's not the only kind of flash that appears to come from these clouds.

Scientists had noticed for a few decades that thunderstorms also seemed to be

accompanied by intense bursts of high-energy gamma rays.

That's normally the kind of radiation you see from powerful cosmic events,

like exploding stars, so it was pretty surprising

to see that kind of energy coming from Earth.

And researchers weren't sure what was causing it,

but they thought it was possible those gamma rays

were being emitted by electrons accelerating through the clouds.

After all, a giant battery is great at accelerating charged particles

from one end to the other.

The problem was, electrons in a thundercloud have to confront air,

water, and ice particles, which can slow them way down.

And in order to create the high-energy gamma rays they were observing,

the thunderclouds would need to be accelerating

about a hundred million billion high-energy electrons.

But as unlikely as that might sound,

scientists now believe that's exactly what happens.

See, the faster electrons go, the less drag they experience in air.

As a result, if one electron manages to pick up speed,

the thundercloud's electric field can keep it going—and even accelerate it faster.

Then, when this fast electron collides with an air particle, it can knock out

electrons with enough energy that they too can be accelerated.

This creates an avalanche effect, as each high-speed electron knocks out even

more electrons and they all shoot through the cloud.

Those electrons will radiate a small number of gamma rays.

But when those gamma rays interact with air, they'll often produce more

high-energy particles that cause even more avalanches.

All together, these acceleration events produce the high-energy bursts

that accompany these storms.

Now, these events are just temporary, and eventually the field will collapse

as the cloud battery discharges.

But while they last, they're some of the most energetic events on Earth.

Just outside the Earth, our planet's magnetic field forms another powerful particle

accelerator, in the donut-shaped rings of radiation known as the Van Allen belts.

The Van Allen belts are made up of charged particles from the Sun

that get trapped in Earth's magnetic field lines.

And some of these particles travel as fast as 99.7% of the speed of light.

For a long time, though, no one really knew how they got moving that fast.

At first, scientists thought the particles were mainly being accelerated

by the Earth's magnetic field.

See, as electrons get drawn toward Earth from farther away,

they can pick up speed, sort of like a ball rolling down a hill.

That's probably part of the story, but a 2013 analysis of satellite data from a

geomagnetic storm showed that something else had to be happening too.

That's because, if particles were just accelerating towards Earth,

you'd expect the most energetic particles to be near the inner edge of the belt,

where Earth's magnetic field is strongest.

But the experiment found that particles actually picked up most of their energy

when they were near the center of the belt.

And scientists think they understand why.

See, on the side of the Earth that faces away from the Sun, a bunch of charged

particles stream out behind the planet in what's called a plasma sheet.

And that sheet isn't stagnant.

If outbursts from the Sun disturb Earth's magnetic

field, electrons can pop out of that sheet and head toward Earth.

As they move towards the Earth, they release some energy in strong

electromagnetic waves called chorus waves.

When these chorus waves reach the Van Allen belts,

they interact with the electrons there.

Most of those electrons lose energy to the chorus waves,

making the waves even more powerful.

But every now and then, chorus waves hit an electron at just the right angle

to give it a big boost of energy, accelerating it to high speeds.

These natural particle accelerators can boost electrons

up to almost the speed of light, producing the super-high-speed particles

that surround Earth in the Van Allen belts.

Aside from thunderclouds and the Van Allen belts,

most high-energy particles come from deep in the universe.

High-energy particles hitting Earth from outer space are known as cosmic rays,

and scientists detected the first ones in 1912.

But at the time, it was pretty much a mystery where they were coming from.

Today we know that at least some come from the explosions of massive stars,

known as supernovas—but it's not as simple as it sounds.

See, the explosion itself produces a lot of energy, but that alone isn't enough to

accelerate particles to the speeds we see

ones just a hair below the speed of light.

Instead, the key to this natural accelerator is shock waves.

When a star explodes, it puts a bunch of pressure on the surrounding plasma,

and that wave of pressure moves through the plasma

the same way a sound wave moves through air.

But the pressure wave doesn't just move smoothly through,

because the plasma is made of charged particles.

And when charged particles move, they produce an electromagnetic field.

That's where things get interesting.

Because electromagnetic fields also exert a force on the charged particles,

the particles in this plasma end up both creating a field and getting pushed by it.

And this weird fact of physics is what gives the shock wave

its ability to accelerate particles.

See, at the front of these waves, there's a shock front where plasma

is extremely compressed, making the electromagnetic field especially strong.

And that creates a strong push on the charged particles.

But these particles aren't perfect surfers that just ride the crest of the wave.

Since the electromagnetic field extends both ahead of the front and behind it,

sometimes a charged particle gets pushed back and forth across this front.

If it moves backward it loses energy, while if it moves forward it gains energy.

But it's not as simple as one step forward, one step back.

Thanks to an odd property of plasma, a particle moving backward over the shock

front loses less energy than a particle going forward gains.

So each time a particle moves backward and forward,

it gains a little bit of energy overall.

And thanks to that periodic boost, the particle gradually picks up speed.

It's kind of like if you had a ping pong ball

bouncing between a paddle and the floor.

If you move the paddle closer on each bounce,

the bouncing will keep getting faster and faster.

In the supernova scenario, most particles just cross the shock front a few times

and gain a little bit of energy before escaping.

But a few particles stick around long enough to make lots of crossings,

and these can eventually gain a lot of energy.

Like, after 1000 crossings,

an electron can pick up 20,000 times its original energy.

This mechanism for accelerating electrons actually shows up in shock waves

all over the universe.

But since supernovas are extraordinarily powerful,

they also produce unusually strong shock waves.

And that makes them a major source of the high-energy cosmic rays we've

observed on Earth, accelerating particles up to nearly the speed of light.

Finally, even supernovas can't account for the most

energetic particles detected on Earth.

They're known as ultrahigh-energy cosmic rays.

And they go ludicrously close to the speed of light.

Like, this close.

Scientists still aren't sure where these particles come from, but they think they

might come from the high-energy environment around supermassive black holes.

These black holes are at the center of most galaxies,

and many of them are surrounded by a disk of matter spiraling inward.

In some cases, black holes also have powerful jets spewing materials from their poles.

Now, since black holes are famously good at pulling things toward them,

scientists aren't entirely sure how they form these jets

that blast material away in the first place.

One hypothesis is that they might be fueled by magnetic fields

that form in the region surrounding the black hole.

But however they form, what we do know is that the ends of these jets form

shocks as the jet material interacts with surrounding plasma.

And some ideas have suggested that these shocks accelerate particles

a lot like supernovas do—only more.

Now, they're not sure if ultrahigh-energy particles get all their speed from

black holes, or if black holes just boost particles that are already moving fast.

But since black holes are such extreme sources of energy,

it's likely that somehow they're involved in producing

the highest-energy particles we can detect.

And that's given some scientists hope that maybe one day we could use

black holes as laboratories to study fundamental physics the same way

we use human-made accelerators like the Large Hadron Collider.

It's definitely not straightforward to use a natural accelerator for science the way

we use our existing machines, because after all,

they only exist in extreme environments.

And that makes it pretty inconvenient to try to

design a controlled, repeatable experiment.

But in situations where natural accelerators outdo our homemade ones,

some scientists are hopeful that we can make it work.

Whether or not we pull that off, though, these natural accelerators

still let us witness some of the most energetic events in the universe

including some that we could never reproduce here on Earth.

And studying these extremes can help us understand the fundamental processes

that underpin the most ordinary and exotic phenomena in the world.

Thanks for watching this episode of SciShow!

If you like our show,

there's a good chance you'll also like a new channel called What's Watt.

It's all about electricity and is powered by Nexans,

a leading cable company committed to promoting sustainable energy.

They have long episodes that go deep into electrical science,

and also shorter episodes with fun facts — like one about electric cars.

The show is hosted by Frederic Lesur, one of Nexans' top engineers

and science communicators, and features some science YouTubers

you might already know.

Like, in the first season, there's Vanessa Hill from Braincraft

and Athena Brensberger from Astroathens.

If you want to check them out, we recommend starting with their first episode.

[♩OUTRO]