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00:30If they had bolted the detector in place, the nuclear bomb would have just smashed the smithereens.
00:35With links to a dramatic Cold War defection, he disappeared through the Iron Curtain and for five years disappeared off the face of the planet.
00:44And astonishing experiments that keep defying the laws of physics.
00:49Even as someone who builds these experiments for a living, it just seems mind-blowing that they ever work.
00:54Today, scientists are using neutrinos to probe the edges of our detectable universe.
01:01They're on a mission to reveal a hidden world of particles unknown.
01:08Right now, on NOVA.
01:24We live in a world of matter.
01:36A realm of tiny particles far smaller than atoms that build the universe that we know.
01:43But there is a mystery.
01:48Scientists theorize there exists a hidden parallel world of particles.
01:53So-called dark matter.
01:55So far, no one has managed to detect a single one.
02:04But now, there might be a way.
02:08Of all the particles scientists have discovered, the most elusive on the very edge of detectability are neutrinos.
02:17Neutrinos.
02:23Neutrinos are really remarkable particles.
02:26There are trillions and trillions of them streaming through our bodies, and we don't even notice.
02:31They are kind of ghost-like, and yet they're everywhere.
02:34Everywhere and nowhere.
02:37Neutrinos are so ghostly, they can pass through solid matter as if it didn't exist.
02:43And yet, they hold the secrets to why the stars shine, and what our universe is made of.
02:51The reason we care about these elusive particles is because they do play a fundamentally important role in the universe,
02:59in the nature of matter, in some of the most violent cosmic phenomena.
03:03First theorized in the 1930s, they would soon become linked to nuclear secrets and a dramatic Cold War defection behind the Iron Curtain.
03:17He goes off to Europe and never returns.
03:20Now, the quest to detect neutrinos has triggered vast experiments all over the globe.
03:28Even as someone who builds these experiments for a living, it just seems mind-blowing that they ever work.
03:34Today, scientists are on the cusp of an astonishing discovery.
03:38Tantalizing evidence suggests neutrinos could be a doorway between our world of matter and the hidden world of dark matter, waiting to be discovered.
03:51It would be a game-changer.
03:53What exactly are these particles?
03:55What is its role in the evolution of our universe?
04:00The quest for answers has driven scientists to the edge of what is experimentally possible.
04:06To reveal a universe we've never seen before.
04:24Fermilab in Batavia, Illinois.
04:28World-renowned physics laboratory.
04:30Thousands of scientists build enormous experiments to probe the very smallest particles that make up our universe.
04:42Leading one of the teams is Sam Zeller.
04:45Hey, team.
04:47My interest in physics started when I signed up for a field trip to come to Fermilab in high school.
04:52It just blew my mind.
04:54From that point on, I was a particle physicist.
04:56It turns out that the universe can be described by a small number of subatomic particles.
05:08Today, scientists have discovered 17 basic particles that make up our universe.
05:16Some are the building blocks of atoms.
05:20Others are the things that hold matter together.
05:23It's an understanding of our world that physicists call the standard model.
05:31The standard model of particle physics describes the most fundamental constituents of matter and how they interact with each other.
05:39It is, in fact, the most mathematically well-defined physical theory we as humans have ever written down.
05:46For 50 years, the standard model has withstood test after test, confirming the hierarchy of all the fundamental particles.
05:56But one type remains far more mysterious than others.
06:05They're called neutrinos.
06:07A neutrinos is a type of elementary particle, a basic fundamental building block of the universe.
06:16And they come in three different flavors.
06:20Neutrinos are everywhere.
06:22They are produced in the sun.
06:24There are neutrinos that were left over after the Big Bang.
06:29Humans emit neutrinos.
06:31Neutrinos have got no electric charge.
06:36They've almost got no mass at all.
06:38They're as near to nothing as you can imagine.
06:41They're so reluctant to interact with stuff, they pass through the Earth as if it wasn't there.
06:46And yet, at Fermilab, scientists are constructing a complex two-stage experiment with the means to create them and study them.
06:57In its first stage, a powerful ring of magnets accelerates positively charged particles called protons to colossal speeds, sending them smashing into a target.
07:13The collision creates a shower of new particles, including a powerful beam of neutrinos.
07:19One hundred and fifty trillion per second pass through the Earth at nearly the speed of light, racing towards the second stage, three giant neutrino detectors.
07:34The largest is called Icarus.
07:38Once complete, this immense tank filled with a web of electronics and cryogenic liquid will be bombarded by hundreds of trillions of neutrinos.
07:47All in the hope of catching just one each minute.
07:55That alone will be a remarkable achievement.
08:00But the scientists have even bigger ambitions.
08:04One of the big goals here at Fermilab is to try to search for possibly a new type of neutrino that no one has yet observed.
08:11Experiments have hinted there could be an even more elusive neutrino beyond the three types already known to exist.
08:21Some have suggested that it could be a link to a hidden realm of particles that could finally lead to new discoveries beyond the standard model.
08:30If we found evidence of a new type of neutrino, that would be really astounding.
08:36That's what gets me excited in the morning.
08:37That's what gets me coming into work.
08:39It would be a major and massive discovery.
08:41Making that discovery would be groundbreaking.
08:47Because while ordinary neutrinos are extremely hard to detect, this fourth type of neutrino could break the standard model.
08:57What brought them to this moment, and possibly to the brink of upending one of the bedrocks of modern physics?
09:08That story begins almost 100 years ago, half a world away, in Rome.
09:17Physicist and historian, Professor David Kaiser, has traveled here to the place where in the 1930s, scientists were investigating the inner workings of the atom.
09:34For millennia, for thousands of years, people had come to believe that the world is made of atoms, and those atoms were the smallest thing there was.
09:44In fact, the word atom even means unbreakable or indivisible, the smallest piece.
09:50But by the early 1900s, scientists had revealed a deeper hidden structure.
09:56If you think about an atom, it's about a nanometer, about a billion times smaller than a meter, roughly.
10:04The inside, the deep core of an atom, the nucleus, is about 100,000 times smaller than that.
10:10So we're really zooming in, powers of 10, powers of 10, getting to unimaginably tiny scales.
10:18During the early 20th century, scientists discovered the atom's tiny nucleus contained protons, particles with a positive electric charge.
10:28These protons held in place a cloud of negatively charged electrons that form the atom's outer limit.
10:40It seemed that protons and electrons were the only two components of all atoms, permanent and fixed.
10:49But scientists had also found something shocking.
10:54Some types of atoms seemed to break apart.
10:59That was just jaw-dropping.
11:01Literally, it contradicts the name of the thing itself.
11:04Atoms are supposed to not break down.
11:05It was as though certain atoms had too much energy.
11:11The nucleus would spontaneously transform and spit out an electron.
11:19This phenomenon was a type of radioactivity known as beta decay.
11:26It appeared to be this sort of mysterious energy leaking from or emanating from certain atoms.
11:33This process was remarkable in itself.
11:38But when scientists measured the energy of the electrons from beta decay, something was wrong.
11:45One of the basic principles in all sciences is that energy can change from one form to the other,
11:53but the total sum must be conserved.
11:58This is the principle of conservation of energy.
12:01From collisions in the macro world, to the behavior of tiny particles,
12:08the principle states that energy should never disappear.
12:12But when scientists measured the energy of the electrons from beta decay,
12:17that's exactly what seemed to happen.
12:20So every time, rather than having energy conserved,
12:25what they were seeing is that some amount of energy would be missing.
12:29Where was the energy going?
12:32It seemed that the particles themselves were breaking the fundamental rules of physics.
12:38In 1926, a young Italian physicist called Enrico Fermi was working at the University of Rome's Physics Institute.
12:53It was here that Fermi probed into the developing field of nuclear physics.
13:03Enrico Fermi was really a towering figure of 20th century physics,
13:06by any measure one of the greatest physicists of the 20th century.
13:10This is the site where Fermi built what became an absolutely world-class group of researchers.
13:15They were known as the Via Panisperna boys.
13:20This is really an iconic photograph.
13:22It captures them in the middle of what would become world-changing research.
13:26Fermi himself was remarkably young.
13:28He was just 26 years old and already had been made the big senior professor
13:32around which this young group would come together.
13:34They referred to Fermi as the Pope.
13:37He was the great leader.
13:39Rossetti was next in line.
13:40He was a cardinal.
13:42The person taking the photograph, the very young Bruno Pontecorvo,
13:46the youngest member of the group, they called him the Puppy.
13:51The group's ideas would have a profound impact on the world.
13:55In October 1931, they invited a group of the world's leading physicists
14:04to a conference held at the Physics Institute.
14:08High on the agenda was the problem of the missing radioactive energy.
14:16One scientist at the conference, the famous Wolfgang Pauli, proposed a radical idea.
14:22Wolfgang Pauli had written a letter to colleagues
14:26and he put forward what he called a desperate remedy of Fertzweifelten Ausweck.
14:31It was just ridiculous, and he says so in his letter.
14:34It's a really quite strange-sounding idea.
14:38What if there was a new type of particle in the world
14:41that no one had ever seen or detected before?
14:47Pauli suggested that instead of just an electron,
14:50perhaps there was an unknown particle
14:54that was carrying away the missing energy.
14:58Very few people seem to have been convinced
15:00that this is the right way to go.
15:03At that time, physicists were quite confident
15:05there existed two basic kinds of particles,
15:07electrons and protons.
15:09But Pauli was suggesting let's make this enormous leap.
15:12A new particle of matter seemed a step too far.
15:19But for Enrico Fermi, the Pope of Via Panasperna,
15:23he took the wacky idea and ran with it.
15:28Fermi dedicated the next two years of his life
15:31to describe the obscure ghost particle.
15:35It would be neutral and carry no electric charge.
15:37It would be tiny, far smaller than an electron.
15:42And it would pass through atoms
15:44as if they weren't there at all.
15:47He named the particle the neutrino,
15:50Italian for little neutral one.
15:53This was a really quite remarkable step.
15:59But many physicists, Fermi included,
16:02thought that it should be nearly impossible,
16:03perhaps impossible forever,
16:04to detect such a particle,
16:06even if it really exists.
16:12Outside the intellectual fervor of the lab,
16:15fascism was about to cast a shadow
16:17over the neutrino mystery.
16:19In 1939, Fermi immigrated to the USA
16:24and was quickly put to work.
16:27He helped to develop
16:29the first operational nuclear reactor
16:31that led eventually to the atomic bomb.
16:38But not everybody had forgotten
16:41about the elusive neutrino.
16:43Bruno Pontecorvo,
16:47the puppy of the Via Panisperna boys.
16:52Upon moving to England
16:53after the Second World War,
16:55he continued to think about neutrinos
16:57until his life took a shocking turn.
17:03Pontecorvo was a man
17:04who created big ideas.
17:06The work that he did on neutrinos alone
17:09could have won him certainly one Nobel Prize
17:14and been a candidate maybe for two.
17:17But it wasn't to be.
17:20In 1950, in the midst of the Cold War,
17:24Pontecorvo and his family
17:25mysteriously went missing.
17:29Bruno Pontecorvo disappeared
17:31through the Iron Curtain in 1950
17:33and for five years
17:36disappeared off the face of the planet.
17:40Only after five years of silence
17:42did he reappear in the Soviet Union.
17:50So what happened?
17:53Was he kidnapped?
17:54Was he a spy?
17:58Professor Frank Close
17:59has spent years researching Pontecorvo
18:01and his mysterious disappearance.
18:05He has come to the British National Archives
18:08in London.
18:10Earlier in his life,
18:12Pontecorvo had been a member
18:14of a Communist Party.
18:16And there are now
18:17British intelligence files
18:19under his name.
18:21You're looking at these old folders.
18:24They're worn down the sides.
18:25They have red stamps,
18:27top secret.
18:27But in the case of Pontecorvo,
18:31it is dripping with intrigue.
18:35After the war,
18:36while working for the UK's
18:38Atomic Energy Program,
18:40Pontecorvo devised a method
18:42to try and detect neutrinos.
18:45He reasoned that nuclear reactors,
18:48which derive energy
18:49from splitting atoms,
18:51should produce neutrinos
18:52in vast quantities.
18:55But the government
18:55classified his paper.
18:59Now, I conjecture
19:01that this paper
19:02was classified secret
19:04because
19:04if you could indeed
19:07detect neutrinos
19:09coming from a nuclear reactor,
19:11you would be able to work out
19:12how powerful
19:13the nuclear reactor was.
19:15So they classified it.
19:16As the Cold War escalated,
19:21the USA became paranoid
19:23of atomic espionage.
19:26In 1950,
19:28the Rosenberg spy ring
19:29was uncovered,
19:30and it triggered
19:32a communist witch hunt.
19:36A secret letter reveals
19:38the FBI wrote
19:39to a British intelligence service
19:41about Pontecorvo.
19:42The FBI now ask
19:45if we can send them
19:46any information
19:47which would indicate
19:48that Pontecorvo
19:49may be engaged
19:50in communist activities.
19:53The letter was received
19:54in London
19:55on the 19th of July.
19:57Five days later,
19:58Pontecorvo goes off
19:59to Europe
20:00and never returns.
20:03Flight manifests
20:04reveal Pontecorvo
20:06and his family
20:06flew from Rome
20:08across Europe
20:09to Helsinki,
20:11alongside two suspected
20:12KGB agents.
20:15Pontecorvo's son,
20:16just 12 years old
20:17at the time,
20:18revealed they were
20:19then driven
20:20across the border
20:21to Moscow
20:22with Bruno
20:23in the trunk.
20:25He said to me,
20:26I knew something was up.
20:30Frank believes
20:31a Soviet mole
20:32passed the FBI letter
20:33to Moscow,
20:35who then pressured
20:36Pontecorvo
20:37to defect.
20:38There's no clear evidence
20:41that he had been a spy.
20:43But whatever his reason
20:45for leaving,
20:46Bruno's time
20:47in the West
20:47was over.
20:49Was he a spy or not?
20:51We don't yet know.
20:53In any event,
20:54it was clear
20:54that Pontecorvo
20:55was a top quality scientist
20:57who had taken his brain
20:58to the Soviet Union.
20:59By 1950,
21:08the USA
21:08and the Soviet Union
21:09were engaged
21:10in a nuclear arms race.
21:13With it
21:14came a new opportunity
21:15to hunt for neutrinos.
21:17When a nuclear bomb
21:22goes off,
21:24there is this huge cascade
21:27of particles
21:29that spews out
21:31protons,
21:32electrons,
21:34a lot of light particles
21:35carrying off energy.
21:37And along with these
21:38particles spewing out,
21:40lots and lots of neutrinos
21:43come out for free.
21:45If neutrinos were real,
21:47could a nuclear weapon
21:49finally be the key
21:50to detect them?
21:53In 1951,
21:55a young American
21:56called Fred Rines
21:57was working
21:58on the U.S. nuclear program
21:59at Los Alamos
22:01National Laboratory.
22:03It was here
22:04that Rines,
22:05along with his colleague
22:06Clyde Cowan,
22:08decided to take advantage
22:09of destructive bomb tests
22:11to investigate the mystery
22:13of the missing neutrino.
22:15Rines went back
22:16to a question
22:17that had been
22:17kind of abandoned
22:18in the decades
22:19before the Second World War.
22:21The question of
22:22could physicists
22:22ever actually detect
22:24these very strange,
22:25elusive,
22:26ghost-like particles?
22:28They called their mission
22:30Project Poltergeist.
22:34For detecting the neutrino,
22:35the good news was
22:36you could calculate
22:37the chance of doing it,
22:38and the bad news was
22:39it was almost zero.
22:42Rines and Cowan
22:44needed to tip the odds
22:45in their favor
22:46and knew a nuclear bomb test
22:48could be the key.
22:51An atom bomb
22:52should produce
22:53thousands of times
22:54more neutrinos
22:55than even the biggest
22:56nuclear reactor.
22:58But it also created
23:00a problem.
23:01If they had bolted
23:03the detector in place,
23:04the nuclear bomb
23:05would have just smashed
23:06to smithereens.
23:07So instead,
23:08the proposal
23:08was to dig a shaft
23:10about 150 feet deep,
23:12right near where the bomb
23:13would eventually be
23:14detonated above ground.
23:16The team planned
23:18to drop a detector
23:19down the shaft
23:20to avoid the shockwave
23:22of the bomb.
23:22Inside that shaft,
23:25they would pad the bottom
23:26with foam and feathers
23:27and kind of like
23:28mattress cushions.
23:32It was, I mean,
23:33a creative, ambitious,
23:36and maybe slightly crazy
23:38kind of idea
23:39to try to catch
23:40these neutrinos
23:40in the midst
23:41of this very dramatic,
23:42very worldly set of events
23:43in the early years
23:44of the Cold War.
23:47Work digging the shaft
23:48had begun,
23:50but the head of physics
23:51at Los Alamos
23:52was concerned
23:53that the experiment
23:54couldn't be repeated.
23:56He urged the team
23:58to find another way.
24:00Couldn't they use
24:01a nuclear reactor instead?
24:04Late one evening,
24:06Rines and Cowan
24:07had a realization.
24:11In the same way
24:13that the nucleus
24:13of an atom
24:14could decay
24:15and release a neutrino,
24:17they knew in theory
24:19the process
24:20should be reversible.
24:22On the rare occasion
24:24a neutrino
24:25could interact
24:26with a nucleus,
24:27it should produce
24:28two new particles
24:30called a neutron
24:31and a positron.
24:34And if they travel
24:35through the right medium,
24:37those two telltale particles
24:39should produce
24:40two distinctive flashes
24:42of light.
24:42So Rines and Cowan
24:46built a detector,
24:48essentially a big tank
24:50filled with a solvent
24:52that could pick up
24:55this two coincident signal
24:59blip deep under
25:01a nuclear reactor.
25:02After five years
25:10of experiments
25:11in 1956,
25:13finally,
25:14they got their answer.
25:15They recorded the two
25:22telltale flashes
25:23of light.
25:26For the first time,
25:28they saw evidence
25:29of the elusive neutrino.
25:32What they had done
25:33was a remarkable achievement,
25:35one that seemed impossible.
25:39Neutrinos exist.
25:41They're real,
25:42they're part of the world,
25:43and not only a clever idea.
25:50Knowing neutrinos exist
25:52put a whole extra set
25:54of investigations
25:55on a kind of firmer path.
25:59If neutrinos were pouring
26:01from nuclear reactors
26:02on Earth,
26:04then surely they would
26:06be generated in abundance
26:07in the largest nuclear
26:09furnaces of all.
26:12Stars.
26:13For a long, long time,
26:17scientists had been wondering
26:18what makes the stars shine.
26:20What drives that enormous
26:21output of energy?
26:23People theorize
26:26that our sun is like
26:28a giant nuclear reactor,
26:30except rather than
26:32heavier elements
26:33breaking down
26:34into smaller ones
26:36and releasing energy.
26:37And releasing energy,
26:37you have lighter elements
26:40that fuse together
26:41through nuclear fusion.
26:45In the heart of the sun,
26:47tremendous heat and pressure
26:49force hydrogen nuclei
26:51to fuse together
26:52to make helium.
26:54And in theory,
26:56vast quantities of neutrinos
26:59that pass freely
27:00that pass freely
27:01through the sun
27:02and out into space.
27:04So, if we could detect neutrinos
27:11from the sun,
27:12we could learn
27:13about the processes
27:14that fuel it.
27:16We could peak
27:18inside the core
27:20of our sun.
27:21In the historic
27:25gold mining town
27:27of Leed,
27:28people descend
27:29into the depths
27:30of the earth.
27:33But no longer
27:34to mine precious metal.
27:37They're hunting
27:38for neutrinos.
27:39It was here
27:42in 1965
27:44that an experimentalist
27:46called Ray Davis
27:47came to try
27:48and prove
27:49what makes
27:50the sun shine.
27:52Ray Davis
27:53got very excited
27:54that there is
27:55this new thing
27:55in the world
27:56called a neutrino.
27:57He began realizing
27:58that other kinds
27:59of nuclear reactors
28:00that occur
28:00throughout the universe,
28:02like stars,
28:03they should be spewing
28:04out these neutrinos
28:05all the time.
28:07But catching them
28:08wouldn't be easy.
28:09Calculations showed
28:13that neutrinos
28:13from the sun
28:14would be so faint
28:15a detector near
28:17the earth's surface
28:18would be overwhelmed
28:19by background radiation.
28:21His only option
28:22was to go
28:23to the bottom
28:24of a mine.
28:26Beneath almost
28:27a mile of solid rock,
28:29Davis' team
28:30built a steel tank
28:31the size of a house
28:32and filled it
28:33with 100,000 gallons
28:35of dry cleaning fluid.
28:39In theory,
28:42if a neutrino
28:42from the sun
28:43collided with a chlorine atom
28:46inside the tank,
28:47it would cause a reaction
28:48that Ray Davis
28:49could detect.
28:50Here was something
28:54that was completely fresh,
28:56nobody knew anything
28:56about it,
28:58but the key thing
28:58was that if neutrinos
29:00hit chlorine,
29:01which you could get
29:02in cleaning fluid,
29:04it would turn
29:05the atoms of chlorine
29:06into a radioactive form
29:07of argon.
29:08And that's when
29:09Davis got excited
29:10because he was
29:11a radiochemist,
29:12and for him,
29:13detecting radioactive
29:15forms of argon
29:17was easy street.
29:21Scientists had calculated
29:23that around
29:23a million trillion
29:25neutrinos from the sun
29:26should pass through
29:27Davis' tank
29:28each minute.
29:30But the probability
29:31of them hitting
29:32the fluid
29:33and making an argon atom
29:35was so small,
29:37Ray Davis could only
29:38expect to find
29:3910 individual atoms
29:41of argon
29:42from 10 neutrino
29:43collisions
29:44per week.
29:46His task
29:47was almost impossible.
29:49Many of his own
29:50physicist colleagues
29:52doubted
29:53this experiment
29:54would ever work.
29:56And he was having
29:57to convince people
29:58that out of these
29:59millions and millions
29:59and millions
30:00and millions
30:00of atoms
30:00inside this tank,
30:02he could identify
30:03the collisions
30:04of one or two
30:05and convince you
30:06that these were neutrinos
30:08coming from the sun.
30:09Around each month,
30:11Davis flushed out
30:12the giant tank
30:13to extract
30:14the argon atoms.
30:16To everybody's
30:18amazement,
30:19he found them.
30:26But there was
30:27a problem.
30:30Instead of detecting
30:31the number of atoms
30:32that theory predicted,
30:34his measurements
30:35fell short.
30:36They knew
30:37the target number
30:38based on the nuclear
30:39physics theoretical
30:41explanation of how
30:42the stars shine.
30:43And that led to
30:44a very particular
30:45target number.
30:46And Davis's
30:47remarkable experiment
30:48kept coming in
30:49not close to it,
30:50not 80%,
30:51but only at one-third
30:52of that target number.
30:54What happened?
30:56Had the experiment
30:57gone wrong?
30:59Another scientist
31:00carried out a blind
31:01trial to test
31:02the accuracy
31:03of Ray's atom detection.
31:05A colleague put in
31:07500 kind of rogue atoms
31:09without telling
31:11Davis the number
31:11and Davis was able
31:12to go through
31:13the whole process,
31:15sift it through,
31:16and he counted
31:16exactly the number
31:17that had been put in.
31:19If the experimental
31:20results were accurate,
31:21then perhaps scientists
31:23had gotten their theory
31:24about neutrinos
31:25from the sun wrong.
31:26Everybody was blaming
31:28everybody else.
31:30There were even
31:30suggestions,
31:31has the sun already
31:32burnt out in the core?
31:34It was just an enormous
31:35puzzle.
31:36All these advances
31:37in understanding
31:38how stars shine
31:39and then hitting
31:40this kind of brick wall
31:41where theory and
31:42experiment just would
31:43not agree with each
31:44other.
31:46The puzzle became
31:48known as the solar
31:50neutrino problem.
31:511970, 20 years
31:59since Bruno Pontecorvo
32:00defected to the
32:01Soviet Union.
32:04Even after all
32:06that time,
32:07his life behind
32:08the Iron Curtain
32:08remained shrouded
32:09in secrecy.
32:12But in a government
32:13lab outside Moscow,
32:16Pontecorvo worked
32:17tirelessly to explain
32:18the puzzling behavior
32:19of neutrinos.
32:21He suggested that
32:26instead of just one,
32:28there may be two
32:29or even three
32:30different kinds
32:31of neutrino,
32:33known as
32:34different flavors.
32:39If this wasn't
32:40strange enough,
32:42he calculated
32:42that something
32:43peculiar might happen
32:45as they traveled
32:46through space.
32:46A neutrino would
32:52always be born
32:53as one definite
32:54flavor.
32:55But over time,
32:57it would change
32:57its identity.
33:00It would transform,
33:02mixing back and forth
33:04between the three
33:05different types.
33:05of neutrinos.
33:06This was called
33:09neutrino oscillation.
33:17Pontecorvo's idea
33:17really is,
33:18it's sort of delicious.
33:21These neutrinos
33:22could be not
33:23taking one identity,
33:25dropping that,
33:26adopting another one,
33:27dropping that,
33:28but going into this
33:29even stranger mixture
33:30where they're in
33:31neither and both
33:32states at once.
33:33It was a bold idea.
33:36No other fundamental
33:37particle seemed
33:38to spontaneously
33:39change its identity.
33:41But if neutrinos
33:42were transforming
33:43into flavors
33:43that Ray Davis'
33:45detector couldn't see,
33:47it might explain
33:48why two-thirds
33:49of the neutrinos
33:50from the sun
33:51appeared to be missing.
33:52But there was a catch.
33:57The standard model,
33:58the most precise
33:59scientific theory
34:00in human history,
34:02made one important
34:03prediction that
34:04stood in the way.
34:07The standard model
34:09of anticipated neutrinos
34:10would be completely
34:11massless.
34:12They would have
34:13no mass at all,
34:13much like the photon
34:14of light.
34:16And if they had
34:17no mass,
34:18that meant
34:18that they could
34:19not oscillate.
34:20If neutrinos
34:23had no mass,
34:24one of Albert Einstein's
34:26most important theories
34:27predicted that neutrinos
34:29could not possibly
34:30oscillate.
34:33There's this
34:34mind-boggling phenomenon
34:36from Einstein's
34:37relativity
34:37that says that
34:38a clock that is
34:39moving closer and closer
34:41to the speed of light
34:42will tick at a slower
34:43and slower rate.
34:45If that clock
34:46were moving literally
34:47at the speed of light,
34:48it would never
34:48tick at all.
34:49No time would pass
34:51for that object
34:51that moves at exactly
34:52the speed of light.
34:54According to
34:55Einstein's theories,
34:57the faster a particle
34:58travels,
34:59the more its internal
35:00clock slows down.
35:02A particle with no mass
35:04can only travel
35:06at the speed of light,
35:07which is where
35:08time stops.
35:12So if a neutrino
35:14had zero mass,
35:15it would not experience
35:17the passage of time
35:18and would never
35:20be able to change.
35:23If a particle
35:25has zero mass,
35:26what that means is that
35:27its internal clock
35:29is not ticking.
35:30There's no way
35:31for that particle
35:32to experience time.
35:34If there's no passage
35:35of time,
35:36then how could they
35:37change over time
35:39from one identity
35:40to another?
35:40If neutrino oscillation
35:44was real,
35:45neutrinos must
35:47have some mass.
35:49But could the standard
35:50model really be wrong?
35:57Throughout the 1950s
35:59and 60s,
36:00clues from experiments
36:01performed at CERN,
36:03alongside Fermilab,
36:05helped to lay the foundation
36:07of the standard model.
36:08What they found
36:10revolutionized
36:11our understanding
36:12of the particles
36:13that make up
36:14our universe.
36:16By means of this machine,
36:18it is possible
36:18to see the tracks
36:19of subnuclear particles,
36:21the smallest particles
36:22known to man,
36:24the electron,
36:25the positron,
36:27the photon,
36:28and the neutrino.
36:31Over the years,
36:33work at CERN
36:34led to groundbreaking
36:35new technologies.
36:37medical advances
36:38like PET scans,
36:40even the birth
36:41of the World Wide Web.
36:46Perhaps CERN's
36:47biggest success
36:48came in 2012.
36:51Nearly 50 years
36:53after the standard model
36:54was proposed,
36:55physicists detected
36:56the final particle
36:57it predicted,
36:59the Higgs boson.
37:02I think we have it.
37:04Thank you.
37:04Finally,
37:18all the pieces
37:19needed to describe
37:20the detectable physical universe
37:22seemed to be in place.
37:25Along with the Higgs boson,
37:27there are force carriers,
37:29like the photon of light,
37:31quarks,
37:33which form the nuclei
37:34of atoms,
37:36leptons,
37:37including the electron,
37:39muon,
37:40and tau,
37:41and three corresponding
37:43flavors of neutrinos.
37:45It is a map
37:48of what's out there,
37:49what we're made of,
37:49and how we fit.
37:51All of us,
37:52we are made of these things.
37:54And that is a kind of
37:56basic understanding
37:57of nature,
37:58of our own world,
37:59that I think is just
38:01a remarkable human achievement.
38:05And yet,
38:06for all its success,
38:07the standard model
38:09had no equations
38:10to explain how
38:11or why
38:12the neutrinos
38:13would have mass.
38:19For Ray Davis
38:20and his missing
38:21solar neutrinos,
38:22it seemed
38:24an unsolvable paradox.
38:28For decades,
38:30Davis persists,
38:32but he still only finds
38:33one-third of the neutrinos
38:35that were supposed
38:36to be coming
38:36from the sun.
38:39Well,
38:40we've been
38:41carrying on
38:42this experiment
38:43for about 20 years
38:44right here,
38:45but we're still
38:47observing a low
38:48flux of neutrinos.
38:52Eventually,
38:53the problem
38:54is too big
38:55to ignore.
38:56In the 1990s,
38:58scientists
38:58in Canada
38:59and Japan
39:00construct a new
39:02generation
39:02of supersized
39:03neutrino detectors
39:04to finally
39:06settle the mystery.
39:10One of them
39:11lies deep beneath
39:13Japan's
39:14Ikino Mountain.
39:16Scientists
39:16fit 11,000
39:18light detectors
39:19to the inside
39:20of a gigantic
39:20container
39:21and fill it
39:23with 50,000 tons
39:24of ultra-pure water.
39:28This $100 million
39:29detector
39:30is named
39:32Super K.
39:34The Super K
39:35experiment
39:35ended up
39:36being a game
39:37changer.
39:39In the rare
39:40event that a
39:41neutrino collides
39:42with the liquid
39:42in Super K,
39:44the reaction
39:44produces a trail
39:46of light
39:46which the sensors
39:47can pick up.
39:49Unlike Davis'
39:50detector,
39:51this signal
39:52allows scientists
39:53to calculate
39:54to calculate
39:54which type
39:55of neutrino
39:56has hit
39:56and the direction
39:58it came from.
40:00Super K
40:01allows scientists
40:02to test the theory
40:03of neutrino oscillation
40:05by catching them
40:06from a new source,
40:08the Earth's atmosphere.
40:12Theory suggests
40:13that when radiation
40:14from space
40:15hits the atmosphere,
40:16it creates neutrinos
40:19that travel directly
40:20through the Earth.
40:23Some travel
40:24a short distance,
40:26but others
40:27will come from
40:28the other side
40:29of the planet
40:30to reach the detector.
40:33If the neutrinos
40:34are not changing,
40:36the combination
40:37of flavors
40:37they record
40:38coming from
40:39a short distance
40:40will be the same
40:41as those coming
40:42from afar.
40:44If they are changing
40:46over a long distance,
40:48the combination
40:48of flavors
40:49will be different.
40:55After two years
40:56of recording data,
40:58the team
40:58finally has an answer.
41:01What they were seeing
41:03was that
41:04one type of neutrinos
41:06was depleting
41:08when traveling
41:09through the Earth.
41:12The Super K results
41:13combined with
41:15results from
41:16another experiment,
41:18we're able
41:18to definitively
41:20show that
41:21neutrinos
41:22can change
41:23from one type
41:24to the other.
41:26For that to happen,
41:27you must have
41:29non-zero neutrino mass.
41:31The results
41:33are groundbreaking.
41:35Neutrinos
41:36change their identity.
41:38Neutrinos
41:39have mass
41:40after all.
41:42And the standard
41:42model's prediction
41:43of the nature
41:44of neutrinos
41:45must be wrong.
41:47With the new input,
41:49the evidence
41:50that neutrinos
41:50really oscillate,
41:52they really change
41:53their identities,
41:53therefore they really,
41:54really have a mass,
41:56this long-standing,
41:57decades-long challenge
41:59to understand
41:59the solar neutrino problem
42:00finally fell into place.
42:03Nuclear fusion
42:05in the sun
42:06produces one type
42:08of neutrino.
42:09But on the long journey
42:11through space,
42:12the neutrinos
42:13oscillate
42:14and turn
42:15into a mixture
42:16of all three.
42:19On Earth,
42:22Ray Davis' detector
42:23only picked out
42:24one flavor.
42:26His results
42:28had been accurate
42:29all along.
42:3037 years after
42:35the experiment
42:36began,
42:37Ray Davis
42:38was awarded
42:39the Nobel Prize.
42:46For Bruno Pontecorvo
42:47and his theory
42:48of oscillations,
42:50sadly,
42:51the discovery
42:51came too late.
42:54Nobel Prizes
42:55aren't everything,
42:56but by the time
42:57the oscillations
42:58had been sorted out
42:59and the whole thing
43:00finally understood,
43:02Pontecorvo was dead.
43:04So that's the final
43:05tragedy of his life.
43:13After almost
43:14100 years
43:15of research
43:16and discovery,
43:17today neutrino
43:19physicists
43:19face perhaps
43:20their biggest puzzle yet.
43:23The standard
43:25model's equations,
43:26which are so precise
43:27for other particles,
43:29cannot explain
43:30why neutrinos
43:31have mass
43:32or why they oscillate.
43:34It's a sign
43:35that our understanding
43:36of matter
43:37is still incomplete.
43:41Today,
43:42neutrino experiments
43:43are in overdrive,
43:44hunting for clues.
43:47We're in the midst
43:48of really a neutrino bonanza.
43:50I mean,
43:50they're just,
43:50they're popping up
43:51all over the field
43:52of physics.
43:53At the South Pole,
43:58scientists have built
43:59the largest neutrino
44:00detector on the planet.
44:03It's made of more
44:04than 5,000 sensors
44:05drilled into a cubic
44:07kilometer
44:07of Antarctic ice.
44:11It's known
44:12as Ice Cube.
44:16Ice Cube is in this
44:17huge field around me.
44:18I'm sitting,
44:19kind of standing
44:19in the middle
44:20of Ice Cube.
44:22It's kind of amazing
44:23to think that
44:24we were able
44:26to haul something
44:26like 5 million pounds
44:27of cargo
44:28down to the South Pole.
44:29This is instrumentation,
44:31cables,
44:32drill equipment,
44:33fuel.
44:33As well as probing
44:37neutrino oscillations,
44:39Ice Cube acts
44:40like a neutrino telescope,
44:42catching cosmic neutrinos
44:44from billions
44:44of light years away.
44:46This is the universe
44:47that has really only
44:48been open to our eyes
44:50for the last 50 years.
44:52There's all kinds
44:53of discoveries
44:53that are waiting
44:54out there.
44:57With new experiments
44:58like Ice Cube,
45:00scientists believe
45:01that neutrinos
45:02may reveal discoveries
45:03beyond the standard model.
45:08Neutrinos could even
45:09help unlock
45:10one of the biggest
45:10mysteries in physics today.
45:14It seems that
45:16most of what
45:16our universe is made of
45:18is missing.
45:22The whole quest
45:24of particle physics
45:25is to explain
45:26the matter content
45:28of the universe.
45:28And we seem
45:31to be doing
45:31this phenomenally
45:32good job.
45:34You crank through
45:34the math
45:35of the standard model
45:36and everything
45:37makes sense.
45:39And yet,
45:40it only describes
45:41some very small fraction
45:43of what the universe
45:44is made out of.
45:48Looking into space,
45:50cosmologists can see
45:51the gravitational influence
45:53of a material
45:54that binds
45:55entire galaxies together,
45:57but that is
45:58completely invisible
46:00to their detectors.
46:03Scientists call
46:04this material
46:05dark matter,
46:06because nothing
46:07in the standard model
46:09can describe
46:09what it is.
46:12And yet,
46:13it seems to be
46:14what most of the matter
46:16in the universe
46:16is made of.
46:18The standard model
46:20is very good
46:21at describing
46:22about 5%
46:25of the universe.
46:2695% of the stuff
46:28is an utter,
46:28complete mystery
46:29made of dark stuff.
46:30Whether it's dark matter
46:31or dark energy
46:32and what either
46:33of those are,
46:34we don't know.
46:36All we really know
46:37about dark matter
46:38is that it creates gravity,
46:39but it's not interacting
46:41with the instruments
46:43that we have used
46:44to observe the universe.
46:45Whatever's filling space,
46:49much more of it
46:50than the ordinary matter
46:51that makes up us
46:52and our planet
46:52and our stars,
46:53it's some other,
46:55other kind of particle.
46:58Whatever dark matter
46:59particles are,
47:01scientists must look
47:02beyond the standard model
47:03to find them.
47:07Neutrinos
47:07might be the key.
47:15At Fermilab,
47:17for over 20 years,
47:19scientists have been
47:20investigating neutrino oscillations.
47:23What they've found
47:25doesn't add up.
47:28The first observation
47:30that something was amiss
47:31was in the late 1990s.
47:34Something that we don't
47:36quite understand
47:36is going on.
47:37At Fermilab,
47:42scientists fired
47:43a beam of neutrinos
47:44just 500 yards
47:46to their detector.
47:48Neutrinos oscillate
47:49too slowly
47:50for the detector
47:51to see them change
47:52over such a short distance,
47:54at least
47:55according to theory.
47:58But the detectors
47:59saw an increase
48:00in one type
48:01of neutrinos.
48:05Neutrinos seem
48:05to oscillate faster
48:07than is theoretically possible.
48:11The strange thing
48:13that we're seeing
48:14is that
48:15neutrinos
48:17seem to be
48:18changing
48:19from one type
48:20to the other
48:21much faster
48:23than expected.
48:25In order for that
48:26to happen,
48:27we think
48:28it's possible
48:29that there are
48:30extra neutrinos
48:31out there.
48:33In addition
48:34to the three flavors
48:35of neutrino
48:36that the standard
48:37model describes,
48:40there could be
48:41a fourth neutrino
48:42that affects them,
48:44making them
48:44oscillate faster.
48:48Scientists call it
48:49a sterile neutrino,
48:51and it's never
48:53been directly detected.
48:54So we call it
48:58a sterile neutrino
49:00in essence
49:01just because
49:01it interacts
49:03even less
49:04with other particles
49:05than the regular
49:06neutrinos do.
49:08A sterile neutrino
49:10would be
49:10the ultimate
49:11ghost particle.
49:13It would never
49:14collide with atoms
49:15in our world.
49:17No detector
49:17could ever see it.
49:19But it may reveal
49:21itself through
49:21its effects
49:22on the neutrinos
49:23we can see.
49:26The only way
49:27that we can tell
49:28they exist
49:29is through
49:31their effects
49:32on neutrino oscillation.
49:35If sterile neutrinos
49:37exist,
49:38it would break
49:39the neat symmetry
49:40of the standard model
49:41that organizes particles
49:43in groups of three.
49:45What if there's
49:46a fourth kind
49:47of neutrino,
49:48a so-called
49:48sterile neutrino?
49:49Well,
49:50where would you
49:51put that
49:51on our map?
49:53There's no room
49:53to kind of
49:54shoehorn in
49:55to squeeze in
49:56a fourth neutrino.
49:58So I think
49:59there really is
49:59a lot riding
50:00on this.
50:03If they're real,
50:05sterile neutrinos
50:06would have mass
50:07but not interact
50:08with our detectors
50:09just like dark matter.
50:13They could be
50:14the first particle
50:15of dark matter
50:16ever discovered
50:17and through
50:18their effects
50:19on the neutrinos
50:20we can see,
50:21they could give
50:22scientists a window
50:23into another world.
50:26The neutrino
50:27might be
50:28a kind of link,
50:29almost a kind
50:30of messenger
50:30or portal
50:31to this whole
50:32other possible
50:34kind of stuff
50:34out there.
50:38At Fermilab,
50:40scientists are
50:41edging towards
50:42the truth.
50:43I think we're
50:46getting a lot
50:46closer.
50:48Neutrino physicists
50:48are incredibly
50:49patient.
50:50It takes a long
50:50time for us
50:51to collect our
50:52data and we
50:53really want to
50:53be sure in what
50:54we're seeing
50:54before we
50:55potentially make
50:56a very important
50:57discovery.
50:59We're trying to
51:00answer some of
51:01the biggest
51:02questions in
51:02physics.
51:03I think it's
51:04really unique
51:04that neutrinos
51:05may hold all
51:06the answers.
51:07What began
51:08as a hypothetical
51:09particle that no
51:10one thought
51:11possible to
51:12detect could
51:13now be a key
51:14that unlocks
51:15what most of
51:16our universe is
51:17made of and
51:18how it works.
51:22Every time we
51:23look up, there
51:24seem to be these
51:25very curious
51:26neutrinos.
51:26They are
51:27constantly
51:27bedeviling our
51:28mental maps of
51:30how we carve up
51:31nature and try to
51:31dig in and study it
51:32and that's just
51:33amazingly exciting.
51:35So they've gone
51:36from maybe they
51:37exist, maybe they
51:38don't, we might
51:38never know, to
51:40being our sure
51:41first ticket to
51:42the next step.
51:44History has
51:44shown that with
51:46every little bit
51:47of progress, we've
51:49learned huge
51:51surprising things
51:52about our cosmos.
51:54To me, that's
51:55really exciting and
51:56I'm curious to
51:57know where else
51:59could we go.
52:00Wherever we go,
52:02neutrinos could
52:04be our guide.
52:11discover the science
52:34behind the news with
52:36the Nova Now podcast.
52:37Listen at
52:38pbs.org
52:39slash
52:39Nova Now podcast
52:41or wherever you
52:42find your favorite
52:43podcasts.
52:44To order this
52:45program on DVD,
52:46visit Shop PBS
52:47or call
52:491-800-PLAY-PBS.
52:51Episodes of Nova
52:52are available with
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52:55available on
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52:57To be continued...
52:59To be continued...
53:00To be continued...
53:01Gracias por ver el video.
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