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  • This episode is made in partnership with our friends at What's Watt,

  • a new show about electricity.

  • To learn more about this invisible topic,

  • head to their channel by clicking the link in the description.

  • [♩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 goingand 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 supernovasbut 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 doonly 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 factslike 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]

This episode is made in partnership with our friends at What's Watt,

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From Thunderstorms to Black Holes: 4 Natural Particle Accelerators

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    joey joey に公開 2021 年 07 月 02 日
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