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00:00An invisible danger hurdles towards Earth at close to the speed of light.
00:07These are intergalactic alien interlopers on our Milky Way.
00:12Cosmic rays.
00:14Getting hit by a cosmic ray is like getting hit by a cosmic bullet.
00:19Cosmic rays are billions of times more energetic than any other types of particles.
00:24Vastly more energetic than anything we can even create in a laboratory,
00:28in a nuclear fusion reactor, anywhere.
00:30They pierce spaceships, putting our astronauts in danger.
00:34But the source of their power is a mystery.
00:37Are they coming from other galaxies? Are they coming from things in between the galaxies?
00:41Where do cosmic rays come from? Truth is, the most powerful ones, we haven't got a clue.
00:45The race is on to solve the mystery of the fastest particles in the universe.
00:58If I were to make a list of the dangers of space, it would be a long list.
01:10You know, there's hard vacuum, huge swings in temperatures, micrometeorites, all kinds of things.
01:16But probably at the very top of that list, cosmic rays.
01:20These space invaders are not what they seem.
01:23When you hear the name cosmic rays, you might think it's like a beam, like a laser beam of light.
01:29No, no, no, no, no, no, no.
01:30It's a tiny little death particle.
01:32To fight them, we must first understand them.
01:37April, 2019.
01:46NASA's Parker Probe flies closer to the sun than ever before.
01:52We know the sun produces some of the cosmic rays that fill the solar system.
01:57But we don't know how.
02:00Our sun looks like a beautiful glowing orb bringing energy and light to Earth and allowing life to thrive.
02:10But if you look at it up close, you'll see a tumultuous storm of events.
02:16The amount of energy the sun is emitting every second is the equivalent of 100 billion one megaton bombs.
02:24It's a dangerous neighborhood.
02:27Suddenly, the probe is caught head on in a powerful blast.
02:34It's perfectly positioned to monitor the outburst from the inside.
02:41The entire outer third of the sun is a boiling cauldron.
02:46And tied up in that plasma are magnetic fields that get tied and twisted.
02:50And energy is stored in them.
02:52So they rise toward the surface.
02:54They flare.
02:55They rearrange.
02:56They reconnect.
02:57They twist.
02:58They spin.
02:59When the magnetic field lines snap, energy bursts out.
03:04And sometimes, that energy release is explosive.
03:08And that's what results in flares, which are these huge bursts of light.
03:13The probe discovers that after a solar flare, the sun's surface stores electrically charged particles.
03:20But sometimes, there's a second explosion, called a coronal mass ejection, releasing superheated, electrically charged gas called plasma.
03:32These giant balls of plasma go flying off the surface of the sun.
03:39And in those balls of plasma contain these charged particles.
03:45The charged particles move fast.
03:49But they hit a roadblock.
03:51A cloud of slower moving particles that always surrounds the sun.
03:56The solar wind.
03:58Well, the coronal mass ejection is moving into the solar wind much faster than the wind is moving.
04:04So it sort of runs into it and creates this shock wave and ends up piling up particles at the edge.
04:10The shock wave and particles slam together.
04:14In the collision, the particles steal energy and speed, like a baseball accelerating off a bat.
04:21The particles transform into something far more powerful, a solar cosmic ray.
04:29They're light, but they're moving incredibly fast.
04:34The Earth is on average 93 million miles away from the sun.
04:39And these guys reach us in about an hour.
04:42That's 93 million miles an hour.
04:44That's pretty fast.
04:46The cosmic rays speed towards Earth.
04:49We're under attack.
04:51Cosmic rays are by far the most energetic particles that we know to exist in the universe.
04:59And when things with very high energy, no matter how small they are, impact something else,
05:04they deposit that energy, right?
05:06And so cosmic rays can be very dangerous.
05:09Solar cosmic rays aren't the only threat we face.
05:12Other space bullets arrive from beyond our solar system.
05:17There are different kinds of cosmic rays, just like there are different kinds of bullets.
05:22At the lowest end of the spectrum are these solar cosmic rays.
05:27These are like the BBs.
05:29And when a BB hits you, it might sting for a little bit, but you're not going to get too worried about it.
05:34A bigger concern?
05:35Galactic cosmic rays.
05:38They travel faster and have more energy.
05:41If solar rays are like BBs, the galactic cosmic rays are like rifle bullets.
05:48They're far more dangerous.
05:50They're moving a lot faster.
05:52But they're also more rare.
05:54Faster still are the universe's most wanted.
05:57Ultra high energy cosmic rays.
06:00If you thought galactic cosmic rays were bad, it's because you haven't met an ultra high energy cosmic ray.
06:06These are the biggest, baddest, meanest cosmic rays in the universe.
06:13These ultra high energy cosmic rays are like hypersonic missiles.
06:18They are screaming and they come from the most energetic events in the universe.
06:23The ultra high energy cosmic missiles are the rarest, but also the swiftest.
06:32These cosmic ray particles are moving fast.
06:37These mysterious particles are moving incredibly close to the speed of light.
06:42I'm not talking about 99% the speed of light.
06:45They're moving through space at like 99.99999999999999999999999999999999999999999999999999999999.
06:5521 nines.
06:56That's fast.
06:57That's wild.
06:58That's scary.
06:59All three types of cosmic rays are racing through the solar system.
07:06If 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.
07:18It's a deadly hail of particle bullets.
07:22And out in space, our astronauts are caught in the crossfire.
07:26Cosmic rays represent one of the greatest dangers for human space flight.
07:33NASA plans to send astronauts back to the moon, where radiation levels from cosmic rays are 200 times greater than on Earth.
07:42And that is just the start.
07:44One of NASA's big goals is to send humans to Mars.
07:47And that is a long way away.
07:49At least a six month journey and more often about a nine month journey.
07:53That's a big problem.
07:55I am hoping that one day I can go to Mars as an astronaut, but I'm definitely afraid of cosmic rays.
08:03And the more that I read about it, the bigger of a threat it seems.
08:06So 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.
08:17Only one group of people have been exposed to these high levels of cosmic rays.
08:26The crew members of the Apollo missions.
08:29July 1969.
08:32That's one small step for man, one giant leap for mankind.
08:39One of the astronauts, Buzz Aldrin, sees something strange.
08:44During Apollo 11, Buzz Aldrin reported seeing tiny little flashes sometimes when he was looking around.
08:51That's pretty weird, but what's weirder is that he saw them when his eyes were closed.
08:57Later missions also report seeing odd flashes of light.
09:02A streak in the lower left side of the left eye moving down.
09:08The astronauts described the flashes as spots, streaks, and clouds.
09:14Apollo 15 commander David Scott reported seeing one that was blue with a white cast like a blue diamond.
09:21What's happening is that a cosmic ray is entering the eyeball and then striking molecules and giving off a flash of light.
09:32An alternative theory is that it triggers the layer of sensitive cells in your retina.
09:39So you perceive a streak of light even though no light ever actually existed.
09:45The cosmic rays cause long-term damage.
09:48Inside of the eye's lens there are these fiber cells that are transparent.
09:53Well, when a cosmic ray travels through them, it can damage those cells and make them cloudy causing cataracts.
09:59When NASA examines the astronauts' helmets, they find tiny tracks etched through them.
10:05Evidence of cosmic ray impacts.
10:07When we say that cosmic rays are like tiny little bullets, we're not joking around.
10:13And some of these burrowed all the way through the helmet, which means it ended up in the astronaut's brain,
10:21which just makes me feel weird to think about.
10:24What might that long-term radiation do to your brain, to your ability to reason and problem solve
10:31in one of the most dangerous environments that humanity has ever placed itself?
10:36The farther we venture from our home planet, the more danger we face.
10:41Cosmic rays, highly energetic space particles, may be the most serious threat to human space exploration.
11:00The Hollywood conception of outer space is it's full of dangers like aliens wielding ray guns
11:07or black holes or asteroid showers. But in reality, the biggest danger facing astronauts is invisible.
11:14It's the cosmic radiation.
11:16Cosmic rays damaged Apollo astronauts' eyes after just a few days' exposure.
11:22A one-way trip to Mars takes nine months.
11:26Future missions are going to be spending much longer times in space,
11:30which means we really need to consider how cosmic rays will impact us.
11:35We don't understand all the long-term effects from a steady rain of cosmic rays,
11:42but the astronauts are going to have to deal with it.
11:45To find out more, scientists bombarded human cells with man-made cosmic ray particles.
11:52They discovered cosmic rays physically cut through DNA, chopping it apart.
12:00Damage to DNA in your cell is by far the worst kind.
12:07Because your DNA is the cell's operating matter, it's the blueprints so the cell knows how it should be functioning normally.
12:15You can trigger that cell to turn tumerous, to start producing cancer.
12:21In 2019, scientists took the experiment further and simulated a trip to Mars for mice.
12:31For six months, they blasted the rodents with a steady stream of lab-made cosmic ray particles.
12:38The experiment found profound alterations to the mice's normal behaviour.
12:45They learnt new tasks much more slowly.
12:49Their memory was affected and they forgot things they had already learnt.
12:53They were more anxious and prone to giving up on tasks they'd normally complete.
12:58If 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.
13:09This is important because we need our astronauts to be fully functioning.
13:13The reason why you do crewed missions is because the human brain is much better than any computer.
13:20If even one of them has a problem, they can even put the mission and their lives in jeopardy.
13:27Other studies discover cosmic rays can accelerate aging, alter genes, and cause cardiovascular disease.
13:37That sounds bad enough, but there's a more immediate danger.
13:42When cosmic rays penetrate spaceships, they can fry electronic systems, and that's enough to jeopardize emission.
13:50Our operations in space depend on electronics on computers.
13:55And the worst case scenario is that the wrong cosmic ray comes at the wrong time and hits the wrong circuit.
14:04And it leads to a cascading series of failures that can totally jeopardize a mission.
14:10We see evidence of this onslaught in mission cameras.
14:14Even 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.
14:24They're just crossed with cosmic rays, and those cosmic rays are destroying that detector slowly over time.
14:33So how can we protect astronauts and their equipment?
14:37The obvious answer is to add shielding.
14:41That's one thing to say, like, oh, just add more stuff.
14:45But have you seen rocket launches?
14:48And how hard they are?
14:49How expensive it is to get stuff up into space?
14:52NASA does have a plan.
14:55The spacecraft for the Artemis Moon landing mission will be packed for optimum cosmic ray protection.
15:02So one of the ways you can get around the mass limit is to basically get dual use out of everything.
15:09Your supplies, your fuel, your water.
15:12And you can use those as shielding.
15:14But it's not that simple.
15:16Just as more powerful bullets penetrate armor, more energetic cosmic rays pierce the shielding on spaceships.
15:24The solar ones, yeah, you can just put up some material, some shielding, and it'll generally block them.
15:29But the higher energy ones, they can just burrow on through.
15:33If they hit one of the atoms in the shielding that is protecting our astronauts, it can create a shower of particles.
15:42That 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.
15:52And so it turns out your shielding becomes the weapon that the cosmic rays use against you.
15:59But NASA is recruiting an unexpected ally, the Sun.
16:05Can we protect our astronauts by fighting fire with fire?
16:19Powerful cosmic rays smash through spaceships.
16:24But how can objects smaller than an atom carry enough energy to be dangerous to astronauts?
16:31Moving objects carry energy.
16:34We call this kinetic energy.
16:36And when they strike something, they transform that energy.
16:41When I hit my hand, the kinetic energy of my fist transforms into sound and heat and vibration.
16:50My hand hurts a little from that impact, from the transformation of energy.
16:55It's the same thing with cosmic rays.
16:58When they slam into a human brain cell or a computer chip, they dump some energy, causing damage.
17:05How much damage depends on their kinetic energy.
17:09And that comes down to two things, mass and speed.
17:14Intuitively, things that are moving at the same speed, if they're more massive, they carry more energy.
17:21A bigger asteroid slamming into Earth will do more damage than a smaller asteroid.
17:26If you double the mass of an object, its kinetic energy also doubles.
17:32Although mass is important, it's not as important as speed.
17:37Speed matters even more than mass.
17:43The kinetic energy depends directly on the mass, but it depends on the square of the speed.
17:49Here's what that means.
17:50You double the mass, you have double the kinetic energy.
17:53You double the speed, you have four times the kinetic energy.
17:57When it comes to speed, cosmic rays are the elite.
18:06An ultra-high energy cosmic ray, detected in 1991, hit the atmosphere so fast, scientists called it the Oh My God particle.
18:16This particle was higher energy than they thought they would ever, ever see.
18:24Until this fluorescent streak in the Utah sky, no one believed a particle could reach the Earth traveling so close to the speed of light,
18:33making cosmic ray particles far more dangerous than expected.
18:38As you approach the speed of light, energy, momentum, mass, they start to act a little bit differently.
18:45Einstein's equations of relativity become important.
18:48The physics changes and the energy it has becomes much, much, much stronger.
18:53If 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.
19:02It's amazing to think that something as tiny as a proton could actually be dangerous to a human being.
19:10But amazingly, that proton is moving so fast, it carries as much energy as a baseball thrown at 100 miles an hour.
19:17A baseball contains over a trillion, trillion protons. Imagine all that energy carried by just one particle.
19:25So now you get a sense of just how risky these can be.
19:29Ultra high energy cosmic rays, like the Oh My God particle, are like supersonic missiles.
19:35They are the fastest, but they're so rare, astronauts are unlikely to be hit by one.
19:41Solar cosmic rays are like BB pellets. Abundant, but our spacecraft can block them.
19:48The biggest threat to our astronauts, however, are galactic cosmic rays.
19:53They come from elsewhere in the Milky Way.
19:56The combination of their speed and frequency makes them the most dangerous.
20:01These galactic cosmic rays are much more powerful than the solar cosmic rays.
20:07And they've traveled enormous distances to mess you up.
20:12Yes you are.
20:14Luckily, our astronauts have a surprising protector.
20:18A guardian of the solar system.
20:21The sun.
20:23As well as spitting out these high energy solar cosmic ray particles,
20:29the sun is also streaming out lots of much lower energy particles of the solar wind.
20:36That outward moving solar wind acts as a force field,
20:40and the cosmic rays have to work their way upstream to get to Earth, far inside this bubble.
20:48The solar wind extends 11 billion miles around the solar system,
20:53generating a magnetic field that repels incoming galactic cosmic rays.
20:59It's almost like the deflector shield of the starship Enterprise.
21:04So the sun's magnetic field partially helps protect the Earth and any astronauts from incoming radiation.
21:14Not long ago, our Voyager spacecraft made it to that boundary between the sun's bubble and the galaxy,
21:21and was able to study that region.
21:23And we see the difference between inside the sun's bubble and what's going on outside the sun's bubble.
21:29The sun has our back billions of miles away, and that's pretty cool.
21:35The Voyager space probes discovered a moving battlefield.
21:39The solar wind behaves a bit like a storm front on Earth.
21:44Sometimes it advances, sometimes it retreats.
21:49When the sun's activity is the highest, it's spitting out more solar energetic protons.
21:55But those solar cosmic rays are much less damaging than the galactic ones.
22:00So the net is a benefit.
22:02So actually, ironically, you might find that for astronauts it is safer to launch missions to Mars
22:10during a period of higher solar activity.
22:14Because although you have more of the solar particle radiation,
22:19you also get a better shielding effect from the solar wind.
22:24The sun's activity goes through an 11-year cycle of highs and lows.
22:29The protective bubble follows the same cycle, allowing NASA to predict the safest times to launch.
22:36This is a thorny problem, and, you know, we have very smart people working on it.
22:41But we want to explore space as much as we can.
22:44But we have to lower the risk to the astronauts as much as possible.
22:48NASA's fight against the cosmic invaders continues.
22:53But the biggest mystery remains.
22:56What exactly is launching the deadliest galactic cosmic rays?
23:11Every second, quadrillions of bits of space shrapnel race towards Earth at close to the speed of light.
23:17Galactic cosmic rays.
23:21The galactic cosmic rays are like a rifle bullet.
23:25You do not want to get hit by one of these.
23:28They are invaders from outside the solar system.
23:31We know they're made by something powerful within our galaxy, so the source should be easy to detect.
23:39You'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,
23:46it came from over there, and then look, is there something else over there?
23:49Like a supernova explosion that could explain the source of this.
23:53The problem is that cosmic rays get bent as they move by magnetic fields.
24:00The electric charge on a cosmic ray makes it act like a little magnet.
24:05And the Milky Way is full of other magnets.
24:09If I'm a cosmic ray just barreling through the galaxy and I encounter a magnetic field,
24:15I'm going to slightly change directions.
24:17Maybe here.
24:18Maybe there.
24:19Maybe up there.
24:20My trajectory is going to become scrambled.
24:23And after a few million years or so, basically all the information about where it started has been lost.
24:29It's going in a completely random direction for all practical purposes.
24:34But galactic cosmic rays also have a sidekick.
24:38One that is far less elusive.
24:41Gamma rays.
24:43When a galactic cosmic ray hits a regular atom out in space, it causes this big reaction.
24:50It emits all sorts of other particles, including gamma rays, which are basically extremely energetic photons of light.
24:59Critically, gamma rays don't get bent by magnetic fields because they don't have an electric charge.
25:05So they just beeline off in a straight line along whatever direction the cosmic ray was moving in in the first place.
25:11So we can look back at where gamma rays are coming from in the sky,
25:16and that tells us where there are a lot of cosmic rays having collisions.
25:20And they've led us to a prime suspect.
25:23Supernovas.
25:27Supernova are some of the most powerful explosions in the universe.
25:34And so they're ripe grounds for these highly energetic, extremely fast particles to be created.
25:40When a giant star runs out of fuel, it can no longer support its own weight.
25:47It collapses inward, triggering a huge explosion, powerful enough to smash atoms into tiny pieces.
25:57The explosion pushes out an expanding cloud of gas and dust, the supernova remnant.
26:04And that material, as it's moving out at a thousand miles a second, generates an incredibly powerful shockwave.
26:11And that shockwave could be where a particle swept up in the shock gets accelerated.
26:19The magnetic fields inside the cloud trap the subatomic particles.
26:24Cosmic rays inside of the supernova remnant are a lot like being in a pinball machine.
26:29So you have the shockwave as the flipper, and then your magnetic fields are these bumpers prohibiting it from actually leaving.
26:41They're bouncing back and forth across this incredibly energetic shock.
26:45And each time they bounce back and forth, the key is they pick up a little more energy.
26:50When a galactic cosmic ray gains enough energy, the magnetic fields can no longer hold on to it.
26:57It escapes.
27:02The supernova theory explains the birth of many of these cosmic bullets.
27:07But then we discovered a super gamma ray so powerful, it must have a completely different origin story.
27:15So this gamma ray was incredibly high energy, which means that the cosmic ray responsible for it was probably also extremely high in energy.
27:24If you fire a bullet into a pinball machine, it's not going to bounce back and forth.
27:28It's just going to break through the machinery.
27:31And the problem is that these are vastly more energetic than that.
27:35So there's no way they could have been bouncing around all the way up to their current energies inside of that particular pinball machine.
27:43There must be something else in the Milky Way creating galactic cosmic rays.
27:52Something more powerful than a supernova.
27:56The question is, what?
28:01January 2021.
28:04At an observatory high up on the side of a Mexican volcano, blue light zaps through water tanks.
28:12Signs of incoming gamma rays.
28:17Their trail stretches back across the Milky Way, crossing billions of miles, but suddenly goes cold.
28:25Instead of originating in a huge explosion, the trail ends in a cold, sparse cloud of dust.
28:33Molecular clouds at first glance seem like one of the most boring, innocuous places in the universe.
28:42You can barely even see them without an infrared telescope.
28:46They're not events like supernova that have enormously high energies, so you wouldn't expect to create super energetic particles.
28:54Something must be hidden in the cloud.
28:57Something powerful enough to accelerate the cosmic rays.
29:01We just don't know what.
29:03We can see inside the molecular clouds.
29:06So it could be that deep inside them there are clusters of newborn stars that are cranking out these cosmic rays.
29:15But we don't know if even the crankiest of stars are capable of producing cosmic rays at these energies.
29:24Just two months later, in March of 2021, we get another clue.
29:29Scientists detect gamma rays coming from the Cygnus Cocoon Nebula.
29:34It's a dense molecular cloud with a difference.
29:40At the center is a cavity.
29:42Hundreds of closely packed stars push against the dust and gas, including huge bright stars called spectral type O and B.
29:54Spectral type O and B stars are some of the hottest stars in our universe.
29:59The massive stars blast out solar winds far stronger than the wind produced by our sun.
30:06When you think about all these stars forming together, they are all putting off a wind of high energy particles from their surfaces.
30:14These winds collide and form big shock structures between all of these young stars.
30:19You're getting so much energy from so many different winds coming from so many different directions
30:24that it forms a boiling mass of shock waves and magnetic fields.
30:30It's a pinball machine on a far bigger scale.
30:33The magnetic fields are stronger than a supernova's, trapping and accelerating the more energetic cosmic rays for longer.
30:42One important thing about star clusters is that they're around for millions and millions of years.
30:47It's not just a one-off event like a supernova.
30:50And so you've got this magnetic field and these shocks happening over a long period of time.
30:56And that may be what you need to accelerate cosmic rays.
30:59Molecular clouds may shoot out galactic cosmic rays.
31:04But what fires the hypersonic space missiles, the ultra-high energy cosmic rays?
31:11The culprit may be hiding out in distant galaxies, supermassive black holes.
31:18Ultra-high energy cosmic rays are the hypersonic missiles of the particle world.
31:31If a photon of light, the fastest thing in the universe, had a race with an ultra-high energy cosmic ray,
31:37it would be so close that after 200,000 years that photon would be half an inch ahead of the ultra-high energy cosmic ray.
31:46They appear to come from beyond our Milky Way galaxy.
31:51Our galaxy is 100,000 light years across.
31:56The next nearest galaxy to us is 2 million light years away.
32:00So these are traveling to us across millions and billions of light years.
32:05How do you accelerate this tiny little particle to such insane velocities?
32:14What is the power source?
32:16What in the universe has that kind of capability?
32:20Where's the Death Star here?
32:22Their speed makes them dangerous, but it also makes it easier to find their source.
32:28Ultra-high energy cosmic rays are moving so rapidly that they're really not affected that much by magnetic fields.
32:36It's like a bullet going through a fisherman's net.
32:38And so they're coming mostly in a straight line.
32:41When they're coming in a straight line, we can point back to their origin.
32:46And that's something we can use to figure out where and how they're getting accelerated.
32:53In the Argentinian desert, the Pierre Auger cosmic ray detector completes a 12-year study of the sky.
33:02It confirms that most galaxies have a supermassive black hole at their center.
33:08But only a few are active, shooting out energy.
33:12These active supermassive black holes also blast out ultra-high energy cosmic rays.
33:19Supermassive black holes are already extremely powerful.
33:24So it makes a lot of sense to me that the ultra-high energy cosmic rays could originate at supermassive black holes.
33:31The M87 galaxy is 54 million light years away.
33:36It's famous because we took a photo of the supermassive black hole at its core.
33:42So the Event Horizon Telescope image of the swirling vortex of gas around that central black hole,
33:48that shadow that you can't actually see, that could be a site for the unbelievably energetic acceleration of cosmic rays.
33:59In March 2021, scientists analyzed the data further.
34:04This new image of M87 shows very clear magnetic field lines,
34:09which is really stunning and reminds us of how much energy could be contained close to the supermassive black hole.
34:16Black holes have enormous power, but how do they transfer some of that energy to a tiny particle?
34:22One possibility for how supermassive black holes could accelerate such enormously energetic cosmic rays is that they actually drag or capture via their gravity pre-existing normal cosmic rays,
34:36which are already extremely energetic, and then give them an extra boost to even higher energies.
34:41So supermassive black holes bend the fabric of space-time around them, and even light particles can get stuck.
34:50Cosmic rays are no different. They can also be attracted by the supermassive black holes and get drawn into their orbit.
34:56It makes sense that the black hole captures passing cosmic rays.
35:03But how do the particles escape its clutches and hurtle towards us?
35:08M87 has a fearsome weapon in its arsenal. Enormous jets of energy blast out of its poles.
35:17So M87's jets are spectacularly large, larger than the entire galaxy that houses this black hole that's launching those jets.
35:26The powerful jets may give the cosmic rays a speed injection, transforming them from galactic rifle bullets into ultra-high energy hypersonic missiles.
35:37So imagine if you had a regular bullet that you fired out of a gun at high speed,
35:44and as it's flying, a little rocket motor in the bullet kicks in and takes it up to even higher speed.
35:53That's sort of what's happening to the cosmic rays in these jets.
35:59Black holes may be the supervillains we've been looking for, firing out the fastest cosmic bullets.
36:05But cosmic rays have a superpower of their own. They're time travelers.
36:14Cosmic rays race through the universe at close to the speed of light.
36:23Like subatomic bullets, they can pierce spaceships and harm astronauts.
36:29But down on Earth, we're protected.
36:35Out 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.
36:48That's amazing. And that's where life is. I don't think that's actually all that much of a coincidence.
36:54The Earth creates its own magnetic field.
36:58The Earth has this wonderful active molten core of metal.
37:01All of that metal is moving around inside the Earth, and that moving metal generates a strong magnetic field.
37:07These cosmic rays are electrically charged. They follow a magnetic field.
37:11So our magnetic field deflects most of the cosmic rays around it.
37:15The shield is not perfect. Some cosmic rays do get through. But then they hit our second line of defense, the atmosphere.
37:27One 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.
37:34The atmosphere is like a missile defense system. Cosmic rays collide with air molecules shattering into safer, smaller particles.
37:46The most common ones are called muons.
37:50The muons are the children of the cosmic rays.
37:54They're produced by these high energy collisions in our upper atmosphere that create these showers of muons that then come down to the surface.
38:03There's as many as four of these cosmic rays passing through my hand every second.
38:08They're passing through your body right now.
38:10Muons are so abundant, we don't need a high-tech observatory to detect them.
38:15Just a few things you'd find in a high school science lab.
38:19A small aquarium that I've attached a small piece of felt to the bottom.
38:25Some frozen carbon dioxide, some dry ice, hence the safety gloves.
38:30A flat piece of metal like this.
38:34Some isopropyl alcohol.
38:37Then I flip the whole thing over onto the bottom.
38:42And I wait.
38:45So what's happening is that the alcohol in the felt is evaporating and sinking down.
38:52And because that bottom layer is so cold from the dry ice, it forms a super saturated cloud of alcohol vapor.
39:00When the charged particles pass through the cold vapor, they create tiny ghostly trails.
39:08What we're looking for are the muons, the subatomic particles generated when a cosmic ray strikes the upper atmosphere.
39:18Each silvery thread in the cloud chamber is the sign of a cosmic ray.
39:24These muons should never make it down to Earth at all.
39:29They only live for 2.2 microseconds before breaking up.
39:34Not enough time to travel through six miles of Earth's atmosphere.
39:39Naively, we would think there's no way that a muon could make it from the upper atmosphere to where we are now without decaying.
39:45It turns out they do.
39:47And the only way they do this is they effectively time travel.
39:51The muons move at 98% the speed of light.
39:56They move so fast, they experience what Einstein called time dilation.
40:02Albert Einstein taught us that we live in a space-time.
40:06And so that means that all measurements of lengths and durations of time are relative.
40:12From a muon's perspective, we humans move incredibly slowly.
40:18They're moving so fast that for them, time is stretched out.
40:24What we found by measuring the energy and the lifetime of muons is that as muons got closer to the speed of light,
40:33their lifetime increased because to them time is slowing down exactly the way Einstein predicted.
40:41Their lifespan is extended by more than 20 times from our perspective.
40:47So they make it to the ground.
40:52Cosmic rays are the ultimate space travelers.
40:56Their awe-inspiring speed allows us to unlock hidden processes and test our theories of physics.
41:04They're way more energetic than anything we can do in a laboratory on Earth.
41:08So that means we can unlock all kinds of new domains about physics at the highest, most extreme energies.
41:14They're our best link to the farthest reaches of the cosmos.
41:19To me, it's really exciting that we're actually sampling pieces of matter from distant stars, from distant galaxies,
41:29and we're getting them here at Earth and studying them.
41:32There are so many amazingly violent events in the universe, the birth of black holes, exploding stars.
41:40These cosmic rays that are going through your body right now are messengers from those events.
41:45In some way, you're still connected to those events millions of light years away.
41:50These are messengers from the universe telling us about how it works.
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