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00:00An invisible danger hurtles towards Earth at close to the speed of light.
00:08These are intergalactic alien interlopers on our Milky Way.
00:13Cosmic 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, vastly
00:24more energetic than anything we can even create in a laboratory in a nuclear fusion reactor
00:29anywhere.
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?
00:39Are they coming from things in between the galaxies?
00:41Where do cosmic rays come from?
00:42Truth 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.
00:59If 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
01:15of things.
01:16But probably at the very top of that list, cosmic rays.
01:21These space invaders are not what they seem.
01:24When you hear the name cosmic rays, you might think it's like a beam, like a laser beam
01:28of light.
01:29No, no, no, no, no, no, no.
01:30It's a tiny little death particle.
01:34To fight them, we must first understand them.
01:42April 2019, NASA'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, but we don't
01:57know how.
02:00Our sun looks like a beautiful glowing orb, bringing energy and light to Earth and allowing
02:08life to thrive.
02:10But if you look at it up close, you will see a tumultuous storm of events.
02:16The amount of energy the sun is emitting every second is the equivalent of 100 billion one
02:22megaton bombs.
02:24It's a dangerous neighborhood.
02:27Suddenly, the probe is caught head on in a powerful blast.
02:35It's perfectly positioned to monitor the outburst from the inside.
02:41The entire outer third of the sun is a boiling cauldron, and tied up in that plasma are magnetic
02:48fields.
02:49They get tied and twisted, and energy is stored in them, so they rise toward the surface.
02:53And there they rearrange, they reconnect, they twist, they spin.
02:58When the magnetic field lines snap, energy bursts out.
03:04And sometimes that energy release is explosive, and that's what results in flares, which are
03:10these huge bursts of light.
03:13The probe discovers that after a solar flare, the sun's surface stores electrically charged
03:20particles.
03:22Sometimes there's a second explosion, called a coronal mass ejection, releasing superheated
03:29electrically charged gas called plasma.
03:34These giant balls of plasma go flying off the surface of the sun, and in those balls
03:40of plasma are contained these charged particles.
03:46The charged particles move fast, but they hit a roadblock, a cloud of slower-moving
03:52particles that always surrounds the sun, the solar wind.
03:57Well, the coronal mass ejection is moving into the solar wind much faster than the wind
04:03is moving.
04:04So it sort of runs into it and creates this shockwave, and ends up piling up particles
04:09at the edge.
04:10The shockwave and particles slam together.
04:14In the collision, the particles steal energy and speed, like a baseball accelerating off
04:20a bat.
04:22The 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, and these guys reach us in about
04:41an 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
05:03else, they 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, and when a BB hits you, it might sting for a little bit, but
05:31you're not going to get too worried about it.
05:33A bigger concern, galactic cosmic rays.
05:38They travel faster and have more energy.
05:41If solar rays are like BBs, then galactic cosmic rays are like rifle bullets.
05:48They're far more dangerous.
05:50They're moving a lot faster, but they're also more rare.
05:54Faster still are the universe's most wanted ultra high energy cosmic rays.
06:00If you thought galactic cosmic rays were bad, it's because you haven't met an ultra high
06:04energy cosmic ray.
06:06These are the biggest, baddest, meanest cosmic rays in the universe.
06:14These ultra high energy cosmic rays are like hypersonic missiles.
06:18They are screaming, and they come from the most energetic events in the universe.
06:27The ultra high energy cosmic missiles are the rarest, but also the swiftest.
06:34These cosmic ray particles are moving fast.
06:38These 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 99.999999999999999999999999, 21 nines.
06:56That's fast!
06:57That's wild.
06:59That's scary.
07:01All 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
07:14that golf ball every single second.
07:19It's a deadly hail of particle bullets.
07:22And out in space, our astronauts are caught in the crossfire.
07:27Cosmic rays represent one of the greatest dangers for human spaceflight.
07:33NASA plans to send astronauts back to the moon, where radiation levels from cosmic rays
07:39are 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, and that is a long way away.
07:49At least a six-month journey, and more often about a nine-month journey.
07:54That's a big problem.
07:56I am hoping that one day I can go to Mars as an astronaut, but I'm definitely afraid
08:02of 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
08:14their astronauts in these really dangerous situations.
08:19Only one group of people have been exposed to these high levels of cosmic rays.
08:25The crew members of the Apollo missions.
08:29July 1969.
08:30That's one small step for man, one giant leap for mankind.
08:39One of the astronauts, Buzz Aldrin, sees something strange.
08:45During Apollo 11, Buzz Aldrin reported seeing tiny little flashes sometimes when he was
08:50looking 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.
09:08The astronauts describe 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
09:19blue diamond.
09:21What's happening is that a cosmic ray is entering the eyeball and then striking molecules
09:29and giving off a flash of light.
09:33An 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.
09:49Outside of the eye's lens, there are these fiber cells that are transparent.
09:52Well, when a cosmic ray travels through them, it can damage those cells and make them cloudy,
09:58causing cataracts.
09:59When NASA examines the astronauts' helmets, they find tiny tracks etched through them,
10:05evidence of cosmic ray impacts.
10:08When 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
10:19astronaut's brain, which just makes me feel weird to think about.
10:25What might that long term radiation do to your brain, to your ability to reason and
10:31problem solve in one of the most dangerous environments that humanity has ever placed
10:35itself?
10:36The farther we venture from our home planet, the more danger we face.
10:53Cosmic rays, highly energetic space particles, may be the most serious threat to human space
11:00exploration.
11:02The Hollywood conception of outer space is it's full of dangers like aliens wielding
11:08ray guns or black holes or asteroid showers.
11:11But in reality, the biggest danger facing astronauts is invisible.
11:16It's the cosmic radiation.
11:19Cosmic rays damaged Apollo astronauts' eyes after just a few days' exposure.
11:25A one-way trip to Mars takes nine months.
11:29Future missions are going to be spending much longer times in space, which means we
11:33really need to consider how cosmic rays will impact us.
11:37We don't understand all the long term effects from a steady rain of cosmic rays, but the
11:45astronauts are going to have to deal with it.
11:48To find out more, scientists bombarded human cells with man-made cosmic ray particles.
11:55They discovered cosmic rays physically cut through DNA, chopping it apart.
12:04Damage to DNA in your cell is by far the worst kind, because your DNA is the cell's operating
12:12manual.
12:13It's the blueprint, so the cell knows how it should be functioning normally.
12:18You can trigger that cell to turn tumorous, to start producing cancer.
12:25In 2019, scientists took the experiment further and simulated a trip to Mars for mice.
12:33For six months, they blasted the rodents with a steady stream of lab-made cosmic ray particles.
12:41The experiment found profound alterations to the mice's normal behavior.
12:47They learnt new tasks much more slowly.
12:51Their memory was affected and they forgot things they had already learned.
12:56They were more anxious and prone to giving up on tasks they'd normally complete.
13:01If you put some of these irradiated mice into a swimming test, rather than trying to swim
13:06to safety, many of them just simply gave up.
13:11This is important because we need our astronauts to be fully functioning.
13:15The reason why you do crewed missions is because the human brain is much better than any
13:21computer.
13:22If even one of them has a problem, it can even put the mission and their lives in jeopardy.
13:30Other studies discover cosmic rays can accelerate aging, alter genes, and cause cardiovascular
13:36disease.
13:39That sounds bad enough, but there's a more immediate danger.
13:44When cosmic rays penetrate spaceships, they can fry electronic systems, and that's enough
13:50to jeopardize a mission.
13:52Our operations in space depend on electronics, on computers.
13:57And the worst case scenario is that the wrong cosmic ray comes at the wrong time and hits
14:04the wrong circuit, and it leads to a cascading series of failures that can totally jeopardize
14:11a mission.
14:13We see evidence of this onslaught in mission cameras.
14:17Even when we have a detector in space, like on the Hubble Space Telescope, if you saw
14:21a raw image, it doesn't look like the beautiful images that are shown to the public.
14:26They're just crossed with cosmic rays, and those cosmic rays are destroying that detector
14:31slowly over time.
14:35So how can we protect astronauts and their equipment?
14:39The obvious answer is to add shielding.
14:44That's one thing to say, like, oh, just add more stuff.
14:47But have you seen rocket launches and how hard they are, how expensive it is to get
14:52stuff up into space?
14:55NASA does have a plan.
14:58The spacecraft for the Artemis moon landing mission will be packed for optimum cosmic
15:03ray protection.
15:04So one of the ways you can get around the mass limit is to basically get dual use out
15:10of everything, your supplies, your fuel, your water, and you can use those as shielding.
15:16But it's not that simple.
15:18Just as more powerful bullets penetrate armor, more energetic cosmic rays pierce the shielding
15:24on spaceships.
15:26The solar ones, yeah, you can just put up some material, some shielding, and it'll generally
15:30block them.
15:31But the higher energy ones, they can just burrow on through.
15:35If they hit one of the atoms in the shielding that is protecting our astronaut, it can create
15:41a shower of particles.
15:44That radiation particle might have missed any of the cells in your body, but you've
15:49now turned it into a blast shredding through everything in the spacecraft.
15:54And so it turns out your shielding becomes the weapon that the cosmic rays use against
16:00you.
16:02But NASA is recruiting an unexpected ally.
16:05The sun.
16:06Can we protect our astronauts by fighting fire with fire?
16:23Powerful cosmic rays smash through spaceships.
16:28But how can objects smaller than an atom carry enough energy to be dangerous to astronauts?
16:35Moving objects carry energy.
16:37We call this kinetic energy.
16:39And when they strike something, they transform that energy.
16:45When I hit my hand, the kinetic energy of my fist transforms into sound and heat and
16:52vibration.
16:53My hand hurts a little from that impact, from the transformation of energy.
16:59It's the same thing with cosmic rays.
17:02When they slam into a human brain cell or a computer chip, they dump some energy, causing
17:08damage.
17:10How much damage depends on their kinetic energy.
17:14And that comes down to two things, mass and speed.
17:19Intuitively, things that are moving at the same speed, if they're more massive, they
17:24carry more energy.
17:25A bigger asteroid slamming into Earth will do more damage than a smaller asteroid.
17:31If you double the mass of an object, its kinetic energy also doubles.
17:37Although mass is important, it's not as important as speed.
17:44Speed matters even more than mass.
17:47The kinetic energy depends directly on the mass, but it depends on the square of the
17:52speed.
17:53Here's what that means.
17:54You double the mass, you have double the kinetic energy.
17:57You double the speed, you have four times the kinetic energy.
18:05When it comes to speed, cosmic rays are the elite.
18:09An ultra-high energy cosmic ray, detected in 1991, hit the atmosphere so fast, scientists
18:17called it the Oh My God Particle.
18:23This particle was higher energy than they thought they would ever, ever see.
18:28Until this fluorescent streak in the Utah sky, no one believed the particle could reach
18:34the Earth, traveling so close to the speed of light, making cosmic ray particles far
18:39more dangerous than expected.
18:41As you approach the speed of light, energy, momentum, mass, they start to act a little
18:47bit differently.
18:49Einstein's equations of relativity become important.
18:51The physics changes, and the energy it has becomes much, much, much stronger.
18:57If a particle is moving at close to the speed of light, that means that its energy is almost
19:02at the maximum allowed by the laws of physics.
19:06It's amazing to think that something as tiny as a proton could actually be dangerous
19:12to a human being.
19:14But amazingly, that proton is moving so fast, it carries as much energy as a baseball thrown
19:19at 100 miles an hour.
19:21A baseball contains over a trillion, trillion protons.
19:25Imagine all that energy carried by just one particle.
19:29So now you get a sense of just how risky these can be.
19:33Ultra-high energy cosmic rays, like the Oh My God Particle, are like supersonic missiles.
19:40They are the fastest, but they're so rare, astronauts are unlikely to be hit by one.
19:46Solar cosmic rays are like BB pellets, abundant, but our spacecraft can block them.
19:53The biggest threat to our astronauts, however, are galactic cosmic rays.
19:57They come from elsewhere in the Milky Way.
20:00The combination of their speed and frequency makes them the most dangerous.
20:06These galactic cosmic rays are much more powerful than the solar cosmic rays, and they've traveled
20:12enormous distances to mess you up.
20:17Luckily, our astronauts have a surprising protector, a guardian of the solar system,
20:25the sun.
20:27As well as spitting out these high energy solar cosmic ray particles, the sun is also
20:34streaming out lots of much lower energy particles of the solar wind.
20:40The outward moving solar wind acts as a force field, and the cosmic rays have to work their
20:46way upstream to get to Earth, far inside this bubble.
20:52The solar wind extends 11 billion miles around the solar system, generating a magnetic field
20:59that repels incoming galactic cosmic rays.
21:03It's almost like the deflector shield of the Starship Enterprise.
21:08So the sun's magnetic field partially helps protect the Earth and any astronauts from
21:15the incoming radiation.
21:18Not long ago, our Voyager spacecraft made it to that boundary between the sun's bubble
21:23and the galaxy, and was able to study that region.
21:27And we see the difference between inside the sun's bubble and what's going on outside the
21:32sun's bubble.
21:33The sun has our back billions of miles away, and that's pretty cool.
21:39The Voyager space probes discovered a moving battlefield.
21:45The solar wind behaves a bit like a storm front on Earth.
21:49Sometimes it advances, sometimes it retreats.
21:54When the sun's activity is the highest, it's spitting out more solar energetic protons.
21:59But those solar cosmic rays are much less damaging than the galactic ones.
22:04So the net is a benefit.
22:06So actually, ironically, you might find that for astronauts, it is safer to launch missions
22:13to Mars during a period of higher solar activity, because although you have more of the solar
22:20particle radiation, you also get a better shielding effect from the solar wind.
22:28The sun's activity goes through an 11-year cycle of highs and lows.
22:33The protective bubble follows the same cycle, allowing NASA to predict the safest times
22:39to launch.
22:41This is a thorny problem, and you know, we have very smart people working on it.
22:44But we want to explore space as much as we can, but we have to lower the risk to the
22:50astronauts as much as possible.
22:54NASA's fight against the cosmic invaders continues, but the biggest mystery remains.
23:01What exactly is launching the deadliest galactic cosmic rays?
23:16Every second, quadrillions of bits of space shrapnel race towards Earth at close to the
23:21speed of light.
23:24Galactic cosmic rays.
23:27The galactic cosmic rays are like a rifle bullet.
23:30You do not want to get hit by one of these.
23:34They are invaders from outside the solar system.
23:37We know they're made by something powerful within our galaxy, so the source should be
23:43easy to detect.
23:46You'd think if one of them hits a detector on Earth, that we'd just be able to point
23:50back in a straight line and say, it came from over there, and then look, is there something
23:54else over there, like a supernova explosion that could explain the source of this?
23:59The problem is that cosmic rays get bent as they move by magnetic fields.
24:06The electric charge on a cosmic ray makes it act like a little magnet, and the Milky
24:11Way is full of other magnets.
24:15If I'm a cosmic ray just barreling through the galaxy, and I encounter a magnetic field,
24:20I'm going to slightly change directions.
24:23Maybe here, maybe there, maybe up there.
24:26My trajectory is going to become scrambled.
24:29And after a few million years or so, basically all the information about where it started
24:34has been lost.
24:35It's going in a completely random direction for all practical purposes.
24:40But galactic cosmic rays also have a sidekick.
24:44One that is far less elusive.
24:47Gamma rays.
24:49When a galactic cosmic ray hits a regular atom out in space, it causes this big reaction.
24:57It emits all sorts of other particles, including gamma rays, which are basically extremely
25:02energetic photons of light.
25:05Critically, gamma rays don't get bent by magnetic fields because they don't have an electric
25:09charge.
25:10So they just beeline off in a straight line along whatever direction the cosmic ray was
25:14moving in in the first place.
25:17So we can look back at where gamma rays are coming from in the sky, and that tells us
25:22where there are a lot of cosmic rays having collisions.
25:27And they've led us to a prime suspect.
25:32Supernovas.
25:37Supernova are some of the most powerful explosions in the universe, and so they're ripe grounds
25:42for these highly energetic, extremely fast particles to be created.
25:48When a giant star runs out of fuel, it can no longer support its own weight.
25:53It collapses inward, triggering a huge explosion, powerful enough to smash atoms into tiny pieces.
26:04The explosion pushes out an expanding cloud of gas and dust, the supernova remnant.
26:11And that material, as it's moving out at a thousand miles a second, generates an incredibly
26:15powerful shockwave.
26:17And that shockwave could be where a particle swept up in the shock gets accelerated.
26:25The magnetic fields inside the cloud trap the subatomic particles.
26:31Cosmic rays inside of a supernova remnant are a lot like being in a pinball machine.
26:38So you have the shockwave as the flipper, and then your magnetic fields are these bumpers
26:43prohibiting it from actually leaving.
26:47They're bouncing back and forth across this incredibly energetic shock.
26:51And each time they bounce back and forth, the key is they pick up a little more energy.
26:56When a galactic cosmic ray gains enough energy, the magnetic fields can no longer hold on
27:02to it.
27:03It escapes.
27:07The supernova theory explains the birth of many of these cosmic bullets.
27:12But then we discovered a super gamma ray so powerful, it must have a completely different
27:19origin story.
27:20So this gamma ray was incredibly high energy, which means that the cosmic ray responsible
27:26for it was probably also extremely high in energy.
27:29If you fire a bullet into a pinball machine, it's not going to bounce back and forth.
27:34It's just going to break through the machinery.
27:37And the problem is that these are vastly more energetic than that.
27:41So there's no way they could have been bouncing around all the way up to their current energies
27:46inside of that particular pinball machine.
27:51There must be something else in the Milky Way creating galactic cosmic rays.
27:57Something more powerful than a supernova.
28:02The question is, what?
28:06January 2021, at an observatory high up on the side of a Mexican volcano, blue light
28:15zaps through water tanks, signs of incoming gamma rays.
28:22Their trail stretches back across the Milky Way, crossing billions of miles, but suddenly
28:29goes cold.
28:31Instead of originating in a huge explosion, the trail ends in a cold, sparse cloud of
28:37dust.
28:40Molecular clouds at first glance seem like one of the most boring, innocuous places in
28:46the universe.
28:47You can barely even see them without an infrared telescope.
28:51They're not events like supernova that have enormously high energies.
28:56So you wouldn't expect to create super energetic particles.
29:00Something must be hidden in the cloud.
29:03Something powerful enough to accelerate the cosmic rays.
29:07We just don't know what.
29:09We can't see inside the molecular clouds, so it could be that deep inside them there
29:15are clusters of newborn stars that are cranking out these cosmic rays.
29:20But we don't know if even the crankiest of stars are capable of producing cosmic rays
29:26at these energies.
29:30Just two months later, in March of 2021, we get another clue.
29:36Scientists detect gamma rays coming from the Cygnus Cocoon Nebula.
29:41It's a dense molecular cloud with a difference.
29:45At the center is a cavity.
29:48Hundreds of closely packed stars push against the dust and gas, including huge bright stars
29:56called spectral type O and B.
30:00Spectral type O and B stars are some of the hottest stars in our universe.
30:06The massive stars blast out solar winds far stronger than the wind produced by our sun.
30:12When you think about all these stars forming together, they are all putting off a wind
30:17of high energy particles from their surfaces.
30:20These winds collide and form big shock structures between all of these young stars.
30:24They're getting so much energy from so many different winds coming from so many different
30:29directions that it forms a boiling mass of shock waves and magnetic fields.
30:35It's a pinball machine on a far bigger scale.
30:39The magnetic fields are stronger than a supernovas, trapping and accelerating the more energetic
30:45cosmic rays for longer.
30:48One important thing about star clusters is that they're around for millions and millions
30:52of years.
30:53It's not just a one-off event like a supernova.
30:56And so you've got this magnetic field and these shocks happening over a long period
31:01of time, and that may be what you need to accelerate cosmic rays.
31:06Molecular clouds may shoot out galactic cosmic rays, but what fires the hypersonic space missiles,
31:13the ultra high energy cosmic rays?
31:17That culprit may be hiding out in distant galaxies, supermassive black holes.
31:33Ultra high energy cosmic rays are the hypersonic missiles of the particle world.
31:39If a photon of light, the fastest thing in the universe, had a race with an ultra high
31:43energy cosmic ray, it would be so close that after 200,000 years, that photon would be
31:50half an inch ahead of the ultra high energy cosmic ray.
31:55They appear to come from beyond our Milky Way galaxy.
32:00Our galaxy is 100,000 light years across.
32:03The next nearest galaxy to us is 2 million light years away.
32:07So these are traveling to us across millions and billions of light years.
32:14How do you accelerate this tiny little particle to such insane velocities?
32:22What is the power source?
32:24What in the universe has that kind of capability?
32:28Where's the Death Star here?
32:30Their speed makes them dangerous, but it also makes it easier to find their source.
32:37Ultra high energy cosmic rays are moving so rapidly that they're really not affected
32:41that much by magnetic fields.
32:43It's like a bullet going through a fisherman's net.
32:46And so they're coming mostly in a straight line.
32:50When they're coming in a straight line, we can point back to their origin, and that's
32:54something we can use to figure out where and how they're getting accelerated.
33:01In the Argentinian desert, the Pierre Auger Cosmic Ray Detector completes a 12-year study
33:07of the sky.
33:10It confirms that most galaxies have a supermassive black hole at their center, but only a few
33:16are active, shooting out energy.
33:19These active supermassive black holes also blast out ultra high energy cosmic rays.
33:28Supermassive black holes are already extremely powerful, so it makes a lot of sense to me
33:32that the ultra high energy cosmic rays could originate at supermassive black holes.
33:38The M87 galaxy is 54 million light years away.
33:44It's famous because we took a photo of the supermassive black hole at its core.
33:49So the Event Horizon Telescope image of the swirling vortex of gas around that central
33:55black hole, that shadow that you can't actually see, that could be a site for the unbelievably
34:02energetic acceleration of cosmic rays.
34:06In March 2021, scientists analyzed the data further.
34:11This new image of M87 shows very clear magnetic field lines, which is really stunning and
34:17reminds us of how much energy could be contained close to the supermassive black hole.
34:23Supermassive black holes have enormous power, but how do they transfer some of that energy
34:28to a tiny particle?
34:30One possibility for how supermassive black holes could accelerate such enormously energetic
34:35cosmic rays is that they actually drag or capture via their gravity pre-existing normal
34:42cosmic rays, which are already extremely energetic, and then give them an extra boost to even
34:48higher energies.
34:49So supermassive black holes bend the fabric of space-time around them, and even light
34:55particles can get stuck.
34:58Cosmic rays are no different.
34:59They can also be attracted by the supermassive black holes and get drawn into their orbit.
35:06It makes sense that the black hole captures passing cosmic rays.
35:10But how do the particles escape its clutches and hurtle towards us?
35:16M87 has a fearsome weapon in its arsenal.
35:20Enormous jets of energy blast out of its poles.
35:25So M87's jets are spectacularly large, larger than the entire galaxy that houses this black
35:31hole that's launching those jets.
35:33The powerful jets may give the cosmic rays a speed injection, transforming them from
35:39galactic rifle bullets into ultra-high-energy hypersonic missiles.
35:45So imagine if you had a regular bullet that you fired out of a gun at high speed, and
35:52as it's flying, a little rocket motor in the bullet kicks in and takes it up to even
35:58higher speed, that's sort of what's happening to the cosmic rays in these jets.
36:06Black holes may be the supervillains we've been looking for, firing out the fastest cosmic
36:11bullets.
36:12But cosmic rays have a superpower of their own.
36:17They're time travelers.
36:27Cosmic rays race through the universe at close to the speed of light.
36:33Like subatomic bullets, they can pierce spaceships and harm astronauts.
36:41But down on Earth, we're protected.
36:46Out of all of the rocky inner planets in the solar system, the Earth is the only one to
36:53generate its own deflector shield against this cosmic radiation.
36:58That's amazing, and that's where life is.
37:00I don't think that's actually all that much of a coincidence.
37:04The Earth creates its own magnetic field.
37:07The Earth has this wonderful, active, molten core of metal.
37:11All of that metal is moving around inside the Earth, and that moving metal generates
37:15a strong magnetic field.
37:17These cosmic rays are electrically charged.
37:19They follow a magnetic field.
37:21So our magnetic field deflects most of the cosmic rays around it.
37:25The shield is not perfect.
37:28Some cosmic rays do get through, but then they hit our second line of defense, the atmosphere.
37:36One of the things we have to be thankful for is our atmosphere.
37:38Not only does it give us air to breathe, but it protects us from these space bullets.
37:43The atmosphere is like a missile defense system.
37:48Cosmic rays collide with air molecules, shattering into safer, smaller particles.
37:55The most common ones are called muons.
37:59The muons are the children of the cosmic rays.
38:04They're produced by these high-energy collisions in our upper atmosphere that create these
38:09showers of muons that then come down to the surface.
38:13There's as many as four of these cosmic rays passing through my hand every second.
38:17They're passing through your body right now.
38:20Muons are so abundant, we don't need a high-tech observatory to detect them.
38:25Just a few things you'd find in a high school science lab.
38:28A small aquarium that I've attached a small piece of felt to the bottom.
38:34Some frozen carbon dioxide, some dry ice, hence the safety gloves.
38:39A flat piece of metal like this.
38:43Some isopropyl alcohol.
38:47Then I flip the whole thing over onto the bottom, and I wait.
38:55So what's happening is that the alcohol in the felt is evaporating and sinking down.
39:02And because that bottom layer is so cold from the dry ice, it forms a super-saturated cloud
39:08of alcohol vapor.
39:12When the charged particles pass through the cold vapor, they create tiny, ghostly trails.
39:19What we're looking for are the muons, the subatomic particles generated when a cosmic
39:25ray strikes the upper atmosphere.
39:28Each silvery thread in the cloud chamber is the sign of a cosmic ray.
39:35These muons should never make it down to Earth at all.
39:38They only live for 2.2 microseconds before breaking up.
39:44Not enough time to travel through six miles of Earth's atmosphere.
39:48Naively we would think there's no way that a muon could make it from the upper atmosphere
39:52to where we are now without decaying.
39:55It turns out they do, and the only way they do this is they effectively time travel.
40:00The muons move at 98% the speed of light.
40:05They move so fast they experience what Einstein called time dilation.
40:12Albert Einstein taught us that we live in a space-time, and so that means that all measurements
40:17of lengths and durations of time are relative.
40:22From a muon's perspective, we humans move incredibly slowly.
40:28They're moving so fast that for them, time is stretched out.
40:33What we found by measuring the energy and the lifetime of muons is that as muons got
40:41closer to the speed of light, their lifetime increased because to them time is slowing
40:48down exactly the way Einstein predicted.
40:51Their lifespan is extended by more than 20 times from our perspective.
40:56So they make it to the ground.
41:02Dark rays are the ultimate space travelers.
41:06Their awe-inspiring speed allows us to unlock hidden processes and test our theories of
41:12physics.
41:14They're way more energetic than anything we can do in a laboratory on Earth.
41:18That means we can unlock all kinds of new domains about physics at the highest, most
41:23extreme energies.
41:25They're our best link to the farthest reaches of the cosmos.
41:29To me, it's really exciting that we're actually sampling pieces of matter from distant stars,
41:37from distant galaxies, and we're getting them here at Earth and studying them.
41:43There are so many amazingly violent events in the universe.
41:47The birth of black holes, exploding stars.
41:50These cosmic rays that are going through your body right now are messengers from those events.
41:55In some way, you're still connected to those events millions of light years away.
42:01These are messengers from the universe telling us about how it works.