Neutrinos are the most mysterious particles in the universe, and they power some of the most explosive events in the cosmos; new discoveries reveal how these ghosts pass through objects undetected and why they could be the reason humans exist.
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LearningTranscript
00:00our world our solar system our universe none of it would exist without a ghostly particle
00:10called the neutrino they can pass right through a wall right through a planet right through a
00:17star without even noticing they are our early warning system whenever there's trouble in the
00:24universe you can expect a flood of neutrinos neutrinos trigger star-killing explosions
00:31supernovas neutrinos can answer so many questions from why do we exist to how was the universe created
00:44these tiny particles save the infant cosmos from annihilation they cause destruction they
00:52you know sometimes they blow up a star but at the end of the day they could be the very reason
00:57that we exist at all neutrinos are the key to how the universe works
01:03in the 1960s our Sun appeared to be dying there was tantalizing evidence that our Sun might be
01:28shutting down this question was a biggie for astronomers if the Sun isn't undergoing nuclear
01:34fusion at the rate we thought it was then that's a big deal
01:38was the Sun's nuclear core shutting down stars including our own Sun are giant nuclear fusion
01:49reactors inside these fusion reactors hydrogen atoms smashed together producing heat and light in the
02:00form of photons all the light and all the heat that we receive on Earth comes from the Sun if the Sun were
02:11starting to suddenly start cooling off that would be seriously bad news for us
02:18how do we check if the Sun is shutting down
02:23we have spacecraft monitoring the solar surface but they can't see into the heart of the reactor the Sun's core
02:33you can see the surface and the Sun is very bright that makes it very easy to study sadly the core of
02:41the Sun is under 400,000 miles of Sun and that makes it pretty hard to look at studying the light made in
02:51the core doesn't help by the time it gets to us it's old news imagine a photon or this particle of light
03:01that's born in the center of a star and now imagine that it wants to reach the surface of the star it turns
03:07out that the star is so dense in the center and the star itself is so physically large that it will
03:13take it 30,000 years to escape the core
03:18it's like being at a cocktail party where you're trying to leave and every time that you make another
03:24step towards the door another group of people want to talk to you and you also want to talk to them
03:29and then it just takes 30,000 years to leave your cocktail party any information we get from sunlight
03:37about what's going on in the core is tens of thousands of years old if you want the current
03:44events the news headlines of what's going on in the Sun's core right now photons are not the way to do
03:51it you want neutrinos so what are these mysterious particles neutrino literally means tiny neutral
04:01one right we think they carry no net electrical charge and they're really really small so we call
04:06them neutrinos neutrinos don't like to interact with matter they fly through almost everything the Sun
04:15the Sun itself is generating enough neutrinos to send 60 billion of them through your thumbnail
04:23every single second and you will spend this is the craziest thing you will spend your entire life
04:31without feeling a single one neutrinos form during nuclear fusion reactions inside the core of stars
04:42hydrogen atoms collide fuse into helium and release photons of light and neutrinos in the core of the Sun
04:53nuclear bombs are going off and all of these nuclear reactions release neutrinos that's about 10 trillion
05:02trillion trillion neutrinos being created every second the trillions of neutrinos shoot out of the core
05:11and up through 323 thousand miles of the Sun to the surface a neutrino basically doesn't even notice
05:21the Sun is there it sails out at very close to the speed of light if you imagine a gridlocked highway
05:29the neutrinos would be the motorbikes that are just zooming through the traffic the solar neutrinos race
05:37towards earth most pass straight through all the neutrinos the trillions upon trillions of neutrinos passing
05:47through the earth every single second the entire earth will only interact with one neutrino out of 10
05:57billion because they pass through anything they're hard to detect I consider neutrino physicists to be
06:08the ghost hunters of the particle physics realm because we study something so elusive and they're really
06:15really hard to nail down and study hard but not impossible while most neutrinos pass through earth a few collide
06:28with atoms in the planet and we can detect those collisions to spot these tiny impacts we built
06:36underground neutrino detectors with giant sensors full of chlorine the neutrino strikes this chlorine atom
06:46it transforms into argon and then we can pick out the argon atoms from the detector and count them up to see
06:54how many neutrinos actually struck our atoms the sensors detected neutrinos from the sun but the numbers were
07:03lower than expected detectors were only detecting about a third of the number of the neutrinos that their
07:11models predicted this is called the solar neutrino problem that is a big deal that either means we're
07:19doing something wrong or our physics is wrong where were the missing two-thirds of the solar neutrinos
07:24they weren't AWOL the detector had missed them because neutrinos can change identities it turns out
07:34neutrinos can change what kind of neutrino they are as they're flying through space and we call this flavor
07:40changing neutrinos come in three different flavors think of them as different types
07:48of playing cards the king is the electron neutrino the muon neutrino is the queen and the jack is the
08:00tau neutrino the Sun produces electron neutrinos but by the time they reach earth they could be a
08:08different flavor as they travel to the earth they constantly wave back and forth trading their identities
08:15so you never know exactly what you're gonna get until it arrives at the earth and we observe it it could be
08:23anything the detectors weren't seeing the different flavors but when we fine-tuned the sensors we saw all the
08:33solar neutrinos so there were actually enough neutrinos coming from the Sun but we were only detecting a third of them
08:39flavor changing neutrinos showed the Sun was healthy the changing identities also answered an important
08:48question about neutrinos do they have mass Einstein showed that only particles without mass can travel at
08:58the speed of light and these particles don't experience time but neutrinos can change their flavor so that must
09:06happen over time and that means neutrinos can't travel at the speed of light and so they must have
09:14mass when scientists first started thinking about neutrinos they thought that they were massless and if a
09:20neutrino has no mass then it's bound to be one flavor or one type of neutrino forever
09:27experiments proved that neutrinos have mass and if they have mass they must produce gravity which means
09:39they can influence other things around them neutrinos are also involved in moments of huge cosmic violence
09:53whenever there's trouble in the universe you can expect a flood of neutrinos
10:00these floods of neutrinos are the key to some of the biggest bangs in the cosmos
10:07and new research suggests that without them there would be no solar system no planets and no us
10:17neutrinos are one of the smallest particles in the cosmos however new research suggests they play a role
10:35in some of the universe's biggest events exploding stars called supernovas
10:44the deaths of giant stars
10:49but there is a mystery surrounding their explosive ends
10:55why do these giant stars end their lives so violently this is a major puzzle in astrophysics
11:04we got a lead when we detected a huge flash of light in the large magellanic cloud
11:12a satellite galaxy of the milky way
11:14the light was a supernova explosion
11:18but three hours before the flash astronomers spotted something else
11:24a burst of neutrinos coming from the same region of the sky
11:29this was the first time we have seen neutrinos coming from a source other than the sun
11:35so there must be some sort of connection between neutrinos and supernovae but but what is that connection
11:41when a star runs out of fuel its core crushes down to a neutron star
11:48then the rest of the star collapses inwards hits the neutron star and bounces out triggering a supernova
11:57but computer models of supernovas reveal a problem the star doesn't explode
12:05when we run computer simulations of how supernova might work
12:11after this bounce the explosion stalls it peters out the supernova isn't so super
12:20it needs another source of energy to propel it to become an actual explosion
12:26could the neutrinos that appeared before the explosion be that energy source
12:33first we need to understand what created the burst of neutrinos
12:39the core of the star collapses inward
12:45and eventually the outer layers of the star fall in toward that star at an appreciable fraction of the speed of light
12:53as the core rapidly collapses the intense pressure squeezes atoms together
13:00that core of iron gets squeezed down to become a neutron star
13:05the electrons and the protons that are part of this core are under so much pressure
13:11that they fuse together to form neutrons and neutrinos in the process
13:14the neutrinos shoot out from the newly formed neutron star core
13:21carrying an enormous amount of energy
13:2499% of the energy is carried by the neutrinos
13:28neutrinos are the main event
13:30trillions of neutrinos smash into the remains of the dying star
13:35and when those neutrinos are flying out of that core region
13:40a very tiny fraction of them interact with the gas
13:44and that fraction heats the gas
13:46everything that's hanging around this newborn neutron star
13:52get heated to an unimaginable degree
13:56the heat creates pressures in the surrounding gas
13:59it builds and builds until it triggers an enormous shockwave
14:04and then the actual explosion the actual firework show begins
14:12the star explodes
14:15in one of the brightest events in the universe
14:19powered by neutrinos
14:22we think that if it weren't for neutrinos
14:25supernovas might not even exist
14:28and we might not exist either
14:31our bodies contain heavy elements
14:34like calcium in our bones
14:36and iron in our blood
14:38these elements form in supernovas
14:42and are scattered across the cosmos by the blast
14:45neutrinos are what kindle the fire
14:51in the forges of these elements
14:54and without the neutrinos
14:56you don't have the elements
14:57and without the elements
14:58you don't have planets like the earth
15:00and without planets like the earth
15:02you don't have life
15:03there's this common phrase
15:06you know we are stardust
15:07which is true
15:08but I like to think we're more like neutrino dust
15:12neutrinos reveal how supernovas explode
15:17and they also warn us
15:19when one is about to detonate
15:21so neutrinos can even be these ghostly signposts
15:25for something very violent
15:26that's happened in the universe
15:27if we detect a sudden burst of neutrinos
15:30it could be that a star has gone supernova somewhere
15:33neutrino bursts are cosmic watchdogs
15:38alerting us to danger
15:40neutrinos are definitely a sign
15:44that something troubling is happening
15:47and in 2017 a single neutrino told us about something very troubling
15:54one of the most intense sources of radiation in the universe
15:58and it was pointing
15:59right at us
16:01spring 2017
16:12scientists at the south pole are on the lookout for neutrinos
16:16these ghostly particles
16:19are extremely hard to detect
16:22neutrinos are the biggest introverts in the universe
16:26they just don't like interacting with anything
16:28so if you want to detect one of these things
16:30you need a lot of stuff
16:31you need a lot of atoms in one spot
16:33so scientists built a facility
16:36with lots of available atoms
16:38it's called ice cube
16:40with neutrino detectors
16:42buried deep beneath sheets of ice
16:45it turns out that water
16:47is a very very good detector of neutrinos
16:51to catch neutrinos
16:54you need to build a very large target
16:56for a reasonable cost
16:58large areas of ice
17:00checks both boxes
17:02so you need a lot of water
17:04that's very very clean
17:06what's the cleanest source of water on the planet
17:08the Antarctic ice sheet
17:11the Antarctic detector ice cube
17:14measures 3,280 feet across
17:18that's about the length of nine football fields
17:21it contains 5,000 sensors
17:25surrounded by more water atoms
17:27than there are stars in the universe
17:30September 22, 2017
17:35ice cube detects a neutrino
17:39colliding with a water atom
17:41when a neutrino hits an ice atom
17:44inside of ice cube
17:45a charged particle flies out
17:47and it's this charged particle
17:49that makes a signal we can detect
17:51the ejected particle
17:52appears to fly out
17:54faster than the speed of light
17:56at first glance
17:57this looks like it violates something
17:59very very important about physics
18:01that nothing can travel faster than light
18:03but light slows down
18:05when traveling through a medium
18:07like air or water
18:09and it is possible
18:11for other things
18:13other particles
18:13to outrun light
18:15in a medium
18:16as it hurtles through the ice
18:20the particle generates
18:22a burst of blue light
18:23called Cherenkov radiation
18:25it's almost like a sonic boom
18:28if you travel faster than the speed of sound
18:30there's a boom, right?
18:31when you hear that boom
18:32you also see this cone of wind
18:35it's the same thing
18:37with Cherenkov radiation
18:39you get this cone of light
18:40neutrinos carry different amounts of energy
18:45some, like the 2017 neutrino
18:49carry quite a punch
18:51and the energy of the neutrino
18:53depends on its source
18:55high energy neutrinos
18:58come from high energy events
19:00so we're looking for stuff blowing up
19:02we're looking for stuff colliding
19:04we're looking for stuff colliding
19:06and blowing up
19:07we're looking for awesome things
19:09the blue burst of Cherenkov radiation
19:13gives us a clue
19:15about the fearsome origin of the neutrino
19:17we can follow the path of that blue light
19:22and we can look backwards
19:24to see where the neutrino came from
19:26we track the neutrino
19:30to a galaxy
19:31nearly 6 billion light years away
19:33at its heart
19:36sits one of the most powerful objects
19:39in the universe
19:40a blazer
19:45a blazer
19:47is the biggest
19:49fattest form
19:51of feeding active
19:53supermassive black hole
19:55out there
19:55where material isn't just
19:57falling into the black hole
19:59it's swirling around
20:00creating a high energy accretion disk
20:02the blazer's accretion disk
20:05spins at millions of miles an hour
20:07charging particles of gas and dust
20:11the disk also generates magnetic fields
20:15that twist and tangle
20:16as they swirl around the black hole
20:18because you have magnetic fields
20:23that are twisted around
20:24they also generate electric fields
20:27the electric fields
20:28can then accelerate
20:29the charged particles
20:30along the magnetic fields
20:32and thus produce a lot of
20:34both particles and radiation
20:36coming out along jets
20:38the jets blast out of the poles
20:41of the black hole
20:42these are the most
20:46intense
20:47sources of radiation
20:49that the cosmos
20:51can ever produce
20:52and they are pointed
20:54right at us
20:55from billions of light years away
20:57do the jets create
20:59the powerful neutrinos?
21:02it's a bit of a mystery
21:03for a while
21:04it was thought
21:05that neutrinos
21:06neutrinos are produced directly by the jet
21:08but now we think that matter
21:10like protons
21:11come in from the accretion disk
21:12and they slam into each other
21:14and that's what produces the neutrinos
21:16particles racing around the accretion disk
21:20crash into the base of the jet
21:22the enormous energy there
21:23smashes the particles together
21:25producing neutrinos
21:27the jets
21:29focus the stream of neutrinos
21:31and fire them
21:32straight towards earth
21:34by just detecting one neutrino
21:36we get to see a lot
21:38of information
21:39from the inner workings
21:40of an object
21:41outside of our galaxy
21:42and that's what's really exciting
21:44about neutrinos
21:45is that it could peer
21:46into the unknown
21:47now we use neutrinos
21:51to probe even further
21:52into the universe
21:54back towards the first second
21:59of the big bang
22:00to answer
22:02the biggest question
22:03of them all
22:04how
22:05and why
22:06do we exist
22:08neutrinos are key
22:22to our understanding
22:23of how the universe works
22:25they show us
22:27that the sun
22:28is healthy
22:29they are the trigger
22:32that makes supernovas
22:34explode
22:34and they reveal
22:36the location
22:37of lethal blazers
22:39and now
22:40they may solve something
22:42that still puzzles
22:44physicists
22:44how we exist
22:47the fact that our universe
22:50appears to be
22:51filled with matter
22:52is puzzling
22:53there should have been
22:55equal amounts
22:55of matter
22:56and antimatter
22:57in the beginning
22:58and they should have
22:59annihilated one another
23:00producing just pure energy
23:01so why do we exist
23:03this is a fundamental question
23:06because this is a question
23:07about
23:07why is there something
23:09rather than nothing
23:11to answer that question
23:14we have to rewind the clock
23:16back nearly 14 billion years
23:18to the birth of the universe
23:20a speck of energy
23:23sparks into existence
23:25this energy cools
23:27and forms tiny
23:29primitive particles
23:30of matter
23:31including neutrinos
23:32the building blocks
23:34of everything
23:35we see today
23:36the early universe
23:39appears chaotic
23:40but it quickly establishes
23:42some ground rules
23:43including
23:44symmetry
23:45our universe
23:48is full of symmetries
23:50there are
23:50positive electric charges
23:52and negative electric charges
23:54there's the yin and the yang
23:55well
23:56there's also
23:57matter and antimatter
23:58the big bang
24:01stuck to the rule of symmetry
24:03and made
24:04the same amount
24:05of both forms of matter
24:06the mechanisms
24:08that we have
24:09for creating matter
24:10in the early universe
24:12create an equal amount
24:13of antimatter
24:14that symmetry
24:15is baked into
24:17the laws of physics
24:18the laws of physics
24:20also say
24:21that when matter
24:23and antimatter meet
24:24sparks fly
24:26so matter and antimatter
24:29when they touch
24:31they annihilate
24:32they just disappear
24:33in a flash of energy
24:34and as far as we understand
24:36the earliest moments
24:37of the universe
24:38matter and antimatter
24:39were created
24:40in equal amounts
24:41so they should have
24:42annihilated
24:43leaving nothing but energy
24:46which means
24:48no matter
24:48no antimatter
24:50no gas
24:51no dust
24:52no stars
24:52no galaxies
24:53no life
24:53nothing
24:54somehow
24:55matter
24:56won the battle
24:58over antimatter
24:59in the early universe
25:00in some ways
25:04the universe
25:05ignored
25:05the rule of symmetry
25:06something has to drive
25:10the universe
25:11off balance
25:12there has to be
25:13a violation
25:15of this fundamental
25:16balance in our universe
25:18that way
25:19when the matter
25:20and antimatter
25:21met
25:22and annihilated
25:23because there was more matter
25:25there would be
25:25a residual
25:26of leftover matter
25:28and there would be
25:28no antimatter
25:30how did the big bang
25:33break the symmetry
25:34between matter
25:35and antimatter
25:36so we're looking
25:38for any interaction
25:40any process whatsoever
25:41where matter behaves
25:43slightly differently
25:45than antimatter
25:46we're trying to find
25:48a flaw
25:49in physics
25:51we can't look
25:53for that flaw
25:54directly
25:54because
25:55we can't see
25:56the big bang
25:56but
25:57we can recreate it
25:59and we think
26:00neutrinos
26:01are involved
26:02this is incredibly
26:05complicated
26:05and we are diving
26:06deep
26:07into the
26:08bowels
26:09of fundamental
26:10physics
26:10and it is not
26:11a pretty sight
26:12Japanese scientists
26:16conducted an experiment
26:17called TK2
26:19they recreated
26:21part of the big bang
26:22by studying neutrinos
26:24and their symmetrical twin
26:26anti-neutrinos
26:28the goal
26:29to see if
26:30anti-neutrinos
26:31change their identity
26:33or flavor
26:34at the same rate
26:35as regular neutrinos
26:36matter and antimatter
26:39should behave
26:40exactly the same
26:42but we found
26:43something
26:44very interesting
26:45with this experiment
26:46the particles
26:48broke symmetry
26:49neutrinos
26:50and anti-neutrinos
26:52changed flavor
26:53at different rates
26:54this was a
26:57clear-cut example
26:58of matter
26:58behaving differently
27:00than antimatter
27:01and that
27:03has revolutionized
27:04our understanding
27:05of the formation
27:06of particles
27:06during the big bang
27:07what could have
27:09happened in the
27:10early universe
27:11is that more
27:12of the neutrinos
27:13converted into matter
27:14than there were
27:15anti-neutrinos
27:17decaying into antimatter
27:18and in this way
27:19you end up
27:20with a surplus
27:21of matter
27:22over antimatter
27:23even though that surplus
27:29was just one particle
27:31in a billion
27:32it was enough
27:33to build the cosmos
27:34so neutrinos
27:37in the early universe
27:38could possibly solve
27:40the matter
27:41antimatter
27:41asymmetry problem
27:42we have
27:43yes
27:46they cause destruction
27:47they
27:48you know
27:48sometimes
27:48they blow up
27:49a star
27:50but at the end
27:51of the day
27:52they did save
27:53the entire universe
27:54now
27:57scientists hope
27:58that neutrinos
27:59may solve
28:01one of the biggest
28:01mysteries
28:02in the cosmos
28:03the identity
28:04of dark matter
28:06neutrinos
28:19neutrinos have been around
28:21since the birth
28:21of the universe
28:22they may even be responsible
28:25for the formation
28:26of matter
28:26now
28:29we investigate
28:30if they play
28:31an even larger role
28:32in the development
28:33of the universe
28:34the formation
28:36of the cosmic web
28:37at the very largest
28:42scales
28:42in our universe
28:43galaxies are arranged
28:46in a very peculiar pattern
28:48we see long
28:50thin threads
28:51of galaxies
28:51and at the intersections
28:53we see dense
28:54clumps of galaxies
28:55called clusters
28:56in between them
28:57we have these vast
28:58empty regions
28:59called the cosmic voids
29:00for a long time
29:02how the cosmic web
29:04formed
29:05and held together
29:05was a mystery
29:06one of the real
29:08mysteries about
29:09our existence
29:09is why the universe
29:11was able to
29:12hold together it all
29:13all the matter
29:15was simply spread apart
29:16too sparsely
29:17to ever form
29:17galaxies or stars
29:18instead something
29:20helped to
29:20hold it together
29:21we now think
29:24the glue
29:25binding
29:25the cosmic web
29:26is a mysterious
29:28substance
29:28known as dark matter
29:30if it wasn't
29:32for dark matter
29:33in the very early universe
29:34there might be
29:35no structure at all
29:37but what is this
29:41architect
29:41of the universe
29:42this dark matter
29:43dark matter
29:45is invisible matter
29:47that we can't see
29:48so you
29:49me
29:49all of the particles
29:51everything that we see
29:52is actually only 5%
29:54of actual matter
29:56in the universe
29:56the rest
29:57is dark matter
29:58dark matter
30:01is
30:02a fancy name
30:04for something
30:04we don't understand
30:05what we do know
30:07is that there is
30:08much more stuff
30:09than we can see
30:10but we have no idea
30:12what it is
30:13it's
30:14one of the greatest
30:15open mysteries
30:16in science
30:17dark matter
30:20hardly interacts
30:21with anything
30:22a bit like
30:23neutrinos
30:23also
30:25like neutrinos
30:26dark matter
30:27was abundant
30:28and active
30:28in the infant universe
30:30so
30:31could neutrinos
30:32and dark matter
30:33be the same thing
30:35we don't know
30:37what dark matter
30:38is
30:38but we kind of
30:39know how it behaves
30:41and neutrinos
30:42sound like
30:42a pretty good
30:43candidate for it
30:44because hey
30:45they are dark
30:46they are everywhere
30:47in the universe
30:47and they do have
30:48a little bit of mass
30:49and by little
30:52we do mean
30:53little
30:54neutrinos
30:55weigh around
30:5610 billion
30:57billion
30:58billion times
30:59less
30:59than a grain of sand
31:01but neutrinos
31:03are also
31:04exquisitely abundant
31:05and so because
31:07they're so abundant
31:08their individual
31:09tiny mass
31:10can actually
31:11add up
31:12to a large
31:13diffuse mass
31:15on very large scales
31:16to investigate
31:24if neutrinos
31:25and dark matter
31:25are the same thing
31:26we must return
31:27to the big bang
31:28as the universe
31:32expands and cools
31:33primitive matter
31:34forms
31:35including
31:36dark matter
31:37and trillions
31:38of neutrinos
31:39the dark matter
31:41clumps together
31:42forming regions
31:43of higher gravity
31:44which pulls in
31:45regular matter
31:46it formed a structure
31:50a scaffolding
31:51that allowed
31:52regular matter
31:53to gravitationally
31:54begin to come together
31:55and collapse
31:56into galaxy stars
31:57and planets
31:58could the combined
32:00mass of neutrinos
32:02in the early cosmos
32:03have produced
32:04the extra gravity
32:05to help structures
32:06form
32:07could it be possible
32:10that this really
32:11is dark matter
32:12these tiny little particles
32:13but in abundance
32:14across the universe
32:15and we know
32:16more
32:18not all
32:19we know more
32:20about neutrinos
32:21than we do
32:21about dark matter
32:22but there's still
32:24a question
32:25around
32:26whether or not
32:27neutrinos
32:28can be
32:29a specific type
32:30of dark matter
32:31to answer this question
32:34we have to work out
32:36what specific type
32:37of dark matter
32:38was around
32:39in the big bang
32:40hot
32:42or cold
32:44people talk about
32:46hot dark matter
32:47and cold dark matter
32:48and really what you're saying
32:50is the speed
32:50of the particles
32:51themselves
32:52the cold dark matter
32:53is moving slowly
32:54and the hot dark matter
32:55is moving fast
32:56this speed difference
32:59is an important clue
33:00to whether neutrinos
33:02make up dark matter
33:03with hot and cold dark matter
33:06the way they interact
33:08with regular matter
33:09has a lot to do
33:09with how fast they're going
33:10so it's a good analogy
33:12to think about a river
33:13with hot dark matter
33:14you'd have a torrent
33:16basically it's going so fast
33:17it doesn't actually
33:18connect with anything
33:19it just goes right on past
33:20so there's no chance
33:21to form that larger structure
33:23if you have relatively
33:26slow moving dark matter
33:27cold dark matter
33:28think about a slow moving river
33:30a slow moving river
33:31begins to deposit silt
33:32think of that silt
33:35as the billions of galaxies
33:37that make up
33:38the cosmic web
33:38we observe that galaxies
33:41form very early
33:42in the universe
33:43and this is good
33:44for cold dark matter
33:45but it doesn't work
33:46for hot dark matter
33:47so we think
33:47cold dark matter
33:48is really dominating
33:49structure formation
33:50in the early universe
33:51but cold and slow
33:54does not describe neutrinos
33:56they move very fast
33:58close to the speed of light
33:59this is a problem
34:02with neutrinos
34:03because neutrinos
34:04would be hot dark matter
34:05that rules out neutrinos
34:08as cold dark matter
34:09the idea that neutrinos
34:13or dark matter
34:14hit another setback
34:15when we weighed the universe
34:17if you add up the total mass
34:21of all the neutrinos
34:22in the universe
34:23it would wind up being
34:24about a half a percent
34:25to 1.5 percent
34:27of the total mass
34:29of dark matter
34:30neutrinos
34:32were a good candidate
34:33for dark matter
34:34because
34:34they exist
34:35and they're very shy
34:38just like the dark matter
34:39particles are
34:40but then
34:42we were able to measure
34:43more accurately
34:44how much dark matter
34:45there is
34:45and how much neutrinos
34:46there are
34:46and there's just
34:49way less neutrinos
34:50than there's dark matter
34:51it sounds like game over
34:54but
34:55the neutrino hunters
34:57aren't giving up
34:58the search is on
34:59for a mysterious
35:00new kind of neutrino
35:02one
35:03that could solve
35:04of the riddle
35:06of dark matter
35:18neutrinos played a huge role
35:20in shaping the early universe
35:24they helped matter defeat anti-matter
35:29and the cosmos
35:30developed structure
35:33this led us to wonder
35:34if neutrinos
35:35might be
35:36dark matter
35:37but
35:39when we weighed
35:40the universe
35:41the numbers didn't add up
35:43neutrinos do have mass
35:46and there are a lot
35:47of them out there
35:48so it might be
35:49some tiny tiny fraction
35:50of dark matter
35:51is made up of neutrinos
35:52but we know
35:53that these things
35:54do not make up
35:55the bulk of dark matter
35:56it must be something else
35:58so neutrino scientists
36:00hunt for a different
36:02contender for dark matter
36:03a completely
36:04new kind of neutrino
36:06we know about
36:08three flavors
36:10of neutrinos
36:11the electron neutrino
36:13the muon neutrino
36:14and the tau neutrino
36:16but there could be
36:18a hidden
36:19fourth flavor
36:20of neutrino
36:21that could solve
36:22the riddle
36:23of dark matter
36:24we call this
36:28a sterile neutrino
36:30so called
36:32because they interact
36:33even less
36:34than regular neutrinos
36:35a particle
36:37so tiny
36:39so hard
36:40to detect
36:40could actually turn out
36:41to have lots of the secrets
36:42wrapped up inside it
36:44as to how the universe works
36:45the first step
36:49to find out
36:50if sterile neutrinos
36:51are dark matter
36:52is to prove they exist
36:54and that's tough
36:56even though sterile neutrinos
36:59are almost impossible
37:00to detect
37:01we can still hunt for them
37:04back in the day
37:05neutrinos were also said
37:06to be difficult to detect
37:09trying to find
37:11dark matter
37:12trying to find
37:12these sterile neutrinos
37:13it's almost like
37:14using one invisible
37:16undetectable thing
37:17to find another
37:17using a ghost
37:19to find a goblin
37:19we are definitely
37:22pushing the limits
37:23of science
37:24a team at Fermilab
37:27has an ingenious idea
37:28they can't spot
37:31sterile neutrinos
37:32directly
37:33because they don't
37:34interact with atoms
37:35in the detectors
37:36so they're looking
37:37for neutrinos
37:38as they change flavor
37:40into sterile neutrinos
37:42we know that normally
37:44neutrinos change type
37:46as they move through space
37:47but they have to move
37:48far enough
37:49before that change happens
37:50so tracking neutrinos
37:54over a short distance
37:55shouldn't show
37:56any flavor changing
37:57in this experiment
38:00they've constructed
38:01only a half mile long path
38:03it's not enough time
38:04for the neutrinos
38:05to change flavor
38:06in the normal way
38:07if they do see something
38:08if they see something change
38:10this could be
38:11some interesting aspect
38:12perhaps evidence
38:14for sterile neutrinos
38:15so is it possible
38:16that over short distances
38:18regular neutrinos
38:19can oscillate
38:19into this sterile neutrino
38:21the team shoots beams
38:28of muon flavor neutrinos
38:30along the detector
38:31in theory
38:34they won't have time
38:36to change flavor
38:36we can see whether or not
38:43these muon neutrinos
38:44morphed into
38:46a different type
38:47of neutrino
38:48they shouldn't change
38:50but if they do
38:51that points us
38:53towards sterile neutrinos
38:54the team compare
38:58the number of muon neutrinos
39:00reaching the detectors
39:01to those fired
39:03along the beam
39:04fewer muon neutrinos
39:08hit the detectors
39:09some neutrinos
39:11had changed flavor
39:13so we are seeing
39:16that oscillation
39:18of neutrinos
39:19changing from one type
39:20to another
39:21we had an idea
39:24of how many
39:25we should have seen
39:27but we're seeing more
39:28and that could be
39:30sterile neutrinos
39:31if sterile neutrinos
39:35do exist
39:35would they be dark matter?
39:39right now
39:40we don't know the mass
39:41of the sterile neutrino
39:43but
39:45if it's heavy enough
39:46it could be a contender
39:48if it exists
39:51it's prevalent enough
39:53to account for
39:54all the dark matter
39:54in the universe
39:55Fermilab's results
39:58haven't been verified
39:59by other scientists
40:00so
40:02it's too soon
40:03to say definitively
40:05that sterile neutrinos
40:06are real
40:06or that they make up
40:09dark matter
40:10dark matter
40:12is probably
40:13one of the biggest
40:14questions of our time
40:15and the fact that
40:17Fermilab
40:17may be
40:18one of the
40:20places
40:20to answer that question
40:22and the fact that
40:22I am working here
40:23is really fantastic
40:25because we're attempting
40:27the impossible
40:28we have to wait to see
40:32if the impossible
40:33is possible
40:34we know neutrinos
40:39have played a vital role
40:40in the history
40:41of our universe
40:42and even now
40:44they refresh it
40:46by powering supernovas
40:48without them
40:52our sun
40:53our world
40:56and even our bodies
40:58would not have formed
41:00neutrinos are pesky
41:03little particles
41:04super elusive
41:05difficult to study
41:07but when you can
41:08catch them
41:09they offer
41:10secrets
41:11to the universe
41:12the story of neutrinos
41:15has been really interesting
41:16it's like reading a book
41:17and you think you're
41:18on the last page
41:18and then you turn it
41:19and then suddenly
41:20there's a hundred new pages
41:21neutrinos are teaching us
41:24that the universe
41:24is in many ways
41:26subtle
41:26and hard to figure out
41:28and the more we learn
41:29about these things
41:30the more we learn
41:31about the universe
41:32neutrinos are the universe's
41:34great escape artists
41:36the houdini of particles
41:37in fact
41:38they may have helped us
41:39to escape the big bang
41:41and end up existing
41:42at the end of the day
41:44they're what saves us
41:45the more we understand
41:48these elusive particles
41:50the more we can
41:51gain insight
41:53into how the universe works
41:54so
41:56it's really cool
41:57and
42:00it's really cool
42:03and
42:04it's not
42:05the stars
42:06the
42:07in the day
42:08there's how we learn
42:08how they hold our
42:09therapy
42:09and
42:09helping through that
42:10I'll see you
42:11in the next one
42:12and
42:13another
42:13I zag
42:14I'll see you
42:14in the next one
42:15and
42:15then
42:17I'll see you
42:17together
42:17sadly
42:19before
42:19and
42:20I'll see you
42:20in the next one
42:21and
42:21you
42:24shouted
42:24when
42:25I
42:25I
42:25I
42:26already
42:27I