We live in a world of matter.
A realm of tiny particles far smaller than atoms,
that build the universe that we know.
But there is a mystery.
Of all the particles scientists have discovered...
..the strangest and most elusive of them all seemed to defy our
understanding of how the universe works.
They're called neutrinos.
Neutrinos are really remarkable particles.
There are trillions and trillions of them streaming through our bodies,
and we don't even notice.
They are kind of ghostlike, and yet they're everywhere.
Everywhere and nowhere.
Neutrinos are so ghostly, they can pass through solid matter
as if it didn't exist, and yet they hold the secrets
to why the stars shine and what our universe is made of.
The reason we care about these elusive particles is because they do
play a fundamentally important role in the universe, in the nature
of matter, in some of the most violent cosmic phenomena.
Today, the quest to detect neutrinos has triggered
multimillion-dollar experiments all over the globe.
Even as someone who builds these experiments for a living,
it just seems mind-blowing that they ever work.
Now, tantalising new evidence suggests neutrinos could be a link
between our familiar world of matter...
..and an unknown world of hidden particles waiting to be discovered.
It would be a game changer.
What exactly are these particles?
The quest for answers has driven scientists to the edge of what
is experimentally possible...
..to reveal a universe we've never seen before.
Fermilab - world-renowned physics laboratory.
Here, thousands of scientists build vast experiments to probe
the very smallest particles that make-up our universe.
Leading one of the teams is Sam Zeller. Hey, team.
My interest in physics started when I signed up for a field trip
to come to Fermilab in high school, that just blew my mind.
From that point on, I was a particle physicist.
Turns out that the universe can be described by a small number
of subatomic particles.
Today, scientists have discovered 17 basic particles
that make up our universe.
Some are the building blocks of atoms.
Others are the things that hold matter together.
It's an understanding of our world that physicists call
the Standard Model.
The Standard Model of particle physics describes the most
fundamental constituents of matter
and how they interact with each other.
It is, in fact, THE most mathematically well-defined physical
theory we as humans have ever written down.
For 50 years, the Standard Model has withstood test after test,
confirming the hierarchy of all the fundamental particles.
But at Fermilab, the team are hunting for a particle
that may turn the entire theory on its head.
They're searching for a new and strange type of neutrino.
A neutrino is a type of elementary particle, a basic fundamental
building block of the universe.
And they come in three different flavours.
Neutrinos are everywhere.
They are produced in the sun,
there are neutrinos that were left over after the Big Bang.
Humans make neutrinos.
Neutrinos have got no electric charge.
They've almost got no mass at all.
They're as near to nothing as you can imagine.
They're so reluctant to interact with stuff, they pass through
the Earth as if it wasn't there.
Yet despite their elusive behaviour, scientists at Fermilab
are constructing an elaborate experiment to find
an even stranger kind of neutrino...
..one that's never been detected before.
One 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.
This mysterious fourth type of neutrino would lie
outside the three already known to exist, and could be a link
to an unknown realm of new particles.
If it's real, it would rewrite the equations of the Standard Model
and transform our understanding of what the universe is made of.
If we found evidence of a new type of neutrino,
that would be really astounding.
That's what gets me excited in the morning, that's what
gets me coming into work.
It would be a major and massive discovery.
In the experiment's first stage, a powerful ring of magnets
accelerates particles called protons to colossal speeds,
sending them smashing into a graphite target.
This collision creates a shower of new particles,
including a powerful beam of neutrinos.
Trillions pass directly through the solid ground
at nearly the speed of light, racing towards the second stage...
..three giant neutrino detectors.
This is the direction the neutrinos go.
They're produced back behind us.
Fermilab produces roughly about 150 trillion neutrinos per second,
and only about one of them every minute will make its presence known
in one of our detectors.
Neutrinos interact with atoms so rarely, to increase the odds
of catching them, the team have to build big.
This is ICARUS, this is the ICARUS detector.
We're about to deploy it in the world's most-intense neutrino beam
at Fermilab, and the entire community
is super-excited about this.
Soon, this colossal tank packed with a web of electronics
will be filled with 600 tons of cryogenic liquid.
All in the hope of catching just one neutrino each minute.
Inside the control room,
one of the experiment's other detectors, called MicroBooNE,
is already online.
This is a live snapshot of what's occurring in the MicroBooNE
detector as we speak.
We expect about one neutrino interaction every few minutes.
This, for example, might actually be a neutrino interaction.
When a neutrino interacts with the liquid in the detector,
it creates a telltale scattering of particles
that the computers will record.
Hidden in this data may be the signature
of the mysterious fourth neutrino.
What brought them to this moment and possibly to the brink of upending
one of the bedrocks of modern physics?
That story begins almost 100 years ago...
..half a world away.
Physicist and historian Professor David Kaiser has travelled
to Rome, where, in the 1930s, scientists were investigating
the inner workings of the atom.
For 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.
In fact, the word atom even means unbreakable or indivisible,
the smallest piece.
But by the early 1900s, scientists had revealed
a deeper hidden structure.
If you think about an atom, it's about a nanometre,
about a billion times smaller than a metre, roughly.
The inside, the deep core of an atom, the nucleus,
is about 100,000 times smaller than that.
So we're really zooming in powers of ten, powers of ten, getting
to unimaginably tiny scales.
In the early 20th century, scientists had discovered the atom's
tiny nucleus contained protons - particles with a positive
electric charge.
These protons held in place negatively charged electrons
that formed the atom's outer limit.
It's seen that protons and electrons were the only two components
of all atoms, permanent and fixed.
But scientists have also found something shocking.
Some types of atoms seemed to break apart.
That was just jaw dropping, literally.
It contradicts the name of the thing itself.
Atoms are supposed to not break down.
It was as though certain atoms had too much energy.
The nucleus would spontaneously transform...
..and spit out an electron.
This was a type of radioactivity known as beta decay.
It appeared to be this sort of mysterious energy,
leaking from or emanating from certain atoms.
This process was remarkable in itself, but when scientists
measured the energy of the electrons from beta decay,
something was wrong.
One of the basic principles in all sciences is that energy
can change from one form to the other,
but the total sum must be conserved.
This is the principle of conservation of energy.
From collisions in the macro world to the behaviour of tiny particles,
the principle states that energy should never disappear.
But when scientists measured the electrons coming from beta decay,
that's exactly what seemed to happen.
So every time, rather than having energy conserved,
what they were seeing is that some amount of energy would be missing.
Where was the energy going?
It seemed that the particles themselves were breaking
the fundamental rules of physics.
In 1926, a young Italian physicist called Enrico Fermi was working
at the University of Rome's Physics Institute.
It was here that Fermi began to probe into the developing field
of nuclear physics.
Enrico Fermi was really a towering figure of 20th-century physics,
by any measure, one of the greatest physicists of the 20th century.
This is the site where Fermi built
what became an absolutely world-class group of researchers.
They were known as the Via Panisperna boys.
This is really an iconic photograph.
It captures them in the middle of what would become
world-changing research.
Fermi himself was remarkably young.
He was just 26 years old and already had been made the big senior
professor around which this young group would come together.
They referred to Fermi as The Pope - he was the great leader.
Rasetti was next in line, he was The Cardinal.
The person taking the photograph, the very young Bruno Pontecorvo,
the youngest member of the group, they called him The Puppy.
The group's ideas would go on to change the course of history.
In October 1931, they invited a group of the world's greatest
physicists to a conference held at the institute.
High on the agenda was the problem of the missing radioactive energy.
The conference itself was really quite remarkable.
Seven people who agreed to come had already received the Nobel Prize.
14 others would go on to win the prize in the years
after the meeting. It was really just a kind of constellation
of extraordinary talent.
It included Marie Curie, Werner Heisenberg and Niels Bohr.
Niels Bohr, already by this point, a sort of grand
figure in the field.
Bohr suggested that if these puzzles persist in trying to understand
the behaviour of nuclei, then maybe we have to just break
the laws of physics.
Maybe in nuclear physics, the rules must change.
The conservation of energy, one of the most fundamental principles
in science, was at stake.
But one scientist at the conference, the famous Wolfgang Pauli,
proposed an even more radical idea.
Wolfgang Pauli had written a letter to colleagues, and he put forward
what he called a desperate remedy, a verzweifelten Ausweg.
It was just ridiculous, and he says so in his letter.
It's a really quite strange-sounding idea.
What if there was a new type of particle in the world
that no-one had ever seen or detected before?
Pauli suggested that instead of just an electron,
perhaps there was an unknown particle that was carrying
away the missing energy.
Very few people seem to have been convinced
that this is the right way to go.
At that time, physicists were quite confident there existed two basic
kinds of particles, electrons and protons.
But Pauli was suggesting, let's make this enormous leap.
A new particle of matter seemed a step too far.
But Enrico Fermi, The Pope of Via Panisperna,
took the wacky idea and ran with it.
Fermi dedicated the next two years of his life
to describe the obscure ghost particle.
It would be neutral
and carry no electric charge.
It would be tiny - far smaller than an electron,
and it would pass through atoms as if they weren't there at all.
He named the particle the neutrino - Italian for "little neutral one".
This was a really quite remarkable step.
But many physicists, Fermi included, thought that it should be nearly
impossible, perhaps impossible forever, to detect such a particle,
even if it really exists.
Outside the intellectual fervour of the lab,
fascism was about to cast a shadow over the neutrino mystery.
In 1939, Fermi immigrated to the USA,
and was quickly put to work.
He helped to develop the first operational nuclear reactor...
..that led eventually to the atomic bomb.
But not everybody had forgotten about the elusive neutrino.
Bruno Pontecorvo, The Puppy of the Via Panisperna boys.
Upon moving to England after the Second World War,
he continued to think about neutrinos until his life took
a shocking turn.
Pontecorvo was a man who created big ideas.
The work that he did on neutrinos alone
could have won him certainly one Nobel Prize
and been a candidate maybe for two.
But it wasn't to be.
In 1950, in the midst of the Cold War, Pontecorvo and his family
mysteriously went missing.
Bruno Pontecorvo disappeared through the Iron Curtain in 1950.
And for five years disappeared off the face of the planet.
Only after five years of silence did he reappear,
in the Soviet Union.
So what happened?
Was he kidnapped?
Was he a spy?
Professor Frank Close has spent years researching Pontecorvo
and his mysterious disappearance.
He has come to the National Archives in London.
Earlier in his life, Pontecorvo had been a member
of a communist party, and there are now MI5 files under his name.
You're looking at these old folders.
They're worn down the sides, they have red stamps, top secret.
The case of Pontecorvo, it is dripping with intrigue.
After the war, while working for the UK's atomic energy programme,
Pontecorvo devised a method to try and detect neutrinos.
He reasoned that nuclear reactors, which derive energy from splitting
atoms, should produce neutrinos in vast quantities.
But the government classified his paper.
Now, I conjecture that this paper was a classified secret
because if you could indeed detect neutrinos coming from a nuclear
reactor, you would be able to work out how powerful
the nuclear reactor was.
So they classified it.
As the Cold War escalated, the USA became paranoid of atomic espionage.
In 1950, the Rosenberg spy ring was uncovered,
and it triggered a Communist witch hunt.
A secret letter reveals the FBI wrote to MI5 about Pontecorvo.
The FBI now ask if we can send them any information which would indicate
that Pontecorvo may be engaged in Communist activities.
The letter was received in London on July 19th.
Five days later, Pontecorvo goes off to Europe and never returns.
Pontecorvo had vanished.
Nobody knew where he'd gone.
"Dearest Bruno," this is from his father,
"We have had no more news and we imagined all sorts of mishaps.
"Hoping to get the best news soon. Love to you all, Papa."
Pontecorvo and his family had travelled to Rome,
when suddenly they flew across Europe to Helsinki,
alongside two suspected KGB agents.
Pontecorvo's son, just 12 years old at the time,
revealed they were then driven to Moscow,
with Bruno hidden in the boot.
So why did Pontecorvo leave?
The secret MI5 letter suggests the influence of an infamous
Soviet mole.
The SIS representative is the interesting mention in here,
because that is none other than Kim Philby,
who really was perhaps the most dangerous traitor of them all.
And Philby's task primarily was any information that came
across his desk was sent to Moscow.
Frank believes Philby tipped off the Soviets about the MI5 letter,
who then may have pressured Pontecorvo to defect.
There's no clear evidence that he had been a spy.
But whatever his reason for leaving, Bruno's time in the West was over.
Was he a spy or not?
We don't yet know.
In any event, it was clear the Pontecorvo was a top-quality
scientist who had taken his brain to the Soviet Union.
By 1950, the USA and the Soviet Union were engaged
in a nuclear arms race.
With it came a new opportunity to hunt for neutrinos.
When a nuclear bomb goes off...
..there's this huge cascade of particles
that spews out protons, electrons,
a lot of light particles carrying off energy.
And along with these particles spewing out,
lots and lots of neutrinos come out for free.
If neutrinos were real, could a nuclear weapon finally
be the key to detecting them?
In 1951, a young American called Fred Reines was working
on the US nuclear programme at Los Alamos National Laboratory.
It was here that Reines, along with his colleague Clyde Cowan,
decided to pursue some fundamental physics
and hunt for the missing neutrino.
Reines went back to a question that had been kind of abandoned
in the decades before the Second World War.
The question of could physicists ever actually detect
these very strange, elusive ghost-like particles?
They called their mission Project Poltergeist.
For detecting the neutrino, the good news was you could calculate
the chance of doing it, and the bad news was it was almost zero!
Reines and Cowan needed to tip the odds in their favour,
and thought a nuclear bomb test could be the key.
An atom bomb should produce thousands of times more neutrinos
than even the biggest nuclear reactor.
But it also created a problem.
If they'd bolted the detector in place, the nuclear bomb
would have just smashed into smithereens.
So instead, the proposal, which I assume he gave
with a straight face, he was being very serious,
was to dig a shaft about 150 feet deep, right near
where the bomb would eventually be detonated above ground.
The team planned to drop a detector down the shaft to avoid
the shock wave of the bomb.
Inside that shaft, they would pad the bottom with foam
and feathers and kind of like mattress cushions.
It was, I mean, a creative, ambitious and maybe slightly crazy
kind of idea to try to catch these neutrinos in the midst
of this very dramatic, very worldly set of events
in the early years of the Cold War.
Work digging the shaft had begun.
But the head of physics at Los Alamos was concerned
that the experiment couldn't be repeated.
He urged the team to find another way.
Couldn't they use a nuclear reactor instead?
Late one evening, Reines and Cowan had a brainwave.
In the same way that the nucleus of an atom could decay and release
a neutrino, they knew, in theory, the process should be reversible.
On the rare occasion a neutrino could interact with a nucleus,
it should produce two new particles called a neutron and a positron,
and if they travelled through the right liquid,
those two telltale particles should produce two distinctive
flashes of light.
So Reines and Cowan built a detector.
Essentially a big tank filled with a solvent
that could pick up these two...coincident signal...blips.
This characteristic signal deep under a nuclear reactor.
After five years of experiments, in 1956,
finally they got their answer.
They recorded the two telltale flashes of light.
For the first time, they saw evidence of the elusive neutrino.
What they had done was a remarkable achievement,
one that seemed impossible.
Neutrinos exist, they're real and a part of the world,
they're not only a clever idea.
Knowing neutrinos exist
put a whole extra set of investigations
on a kind of firmer path.
If neutrinos were pouring from nuclear reactors on Earth,
then surely they would be generated in abundance in the largest
nuclear furnaces of all...
The stars.
For a long, long time, scientists had been wondering
what makes the stars shine?
What drives that enormous output of energy?
People theorise that our sun is like a giant nuclear reactor,
except, rather than heavier elements breaking down into smaller ones
and releasing energy, you have lighter elements that fuse
together through nuclear fusion.
In the heart of the sun, tremendous heat and pressure force
hydrogen nuclei to fuse together to make helium.
And in theory, vast quantities of neutrinos that pass freely
through the sun and out into space.
So if we could detect neutrinos from the sun,
we could learn about the processes that fuel it.
We could peek inside the core of our sun.
ARCHIVE: There was gold in them thar hills.
Gold on the mountains, in the rivers and in the dark depths
far below the surface of the Earth.
In the historic gold-mining town of Lead, people still descend
into the depths of the Earth...
..but no longer to mine precious metal.
They are hunting for neutrinos.
It was here in 1965 that an experimentalist
called Ray Davis came to try and prove what makes the sun shine.
Ray Davis got very excited that there is this new thing
in the world called a neutrino.
He began realising that other kinds of nuclear reactors that occur
throughout the universe like stars, they should be spewing out
these neutrinos all the time.
But catching them wouldn't be easy.
Calculations show that neutrinos from the sun would be so faint,
background radiation would overwhelm any detector
near the Earth's surface.
His only option was to go to the bottom of a mine.
Shielded beneath almost a mile of solid rock, Ray Davis
built his detector - a steel tank the size of a house,
filled with 100,000 gallons of dry-cleaning fluid.
In theory, if a neutrino from the sun collided with a chlorine
atom inside the tank, it would cause a reaction
that Ray Davies could detect.
Here was something that was completely fresh,
nobody knew anything about it.
But the key thing was that if neutrinos hit chlorine,
which you could get in cleaning fluid, it would turn the atoms
of chlorine into a radioactive form of argon.
And that's when Davis got excited, because he was a radiochemist.
And for him, detecting radioactive forms of argon was easy street.
All Davis needed now was a target number to test the theory
of what powered the sun.
There were very compelling theoretical calculations
that suggested that stars should shine in a particular way.
And that meant there should be a predictable number of neutrinos
streaming toward the Earth from the sun.
Scientists had calculated that around a million trillion
neutrinos from the sun should pass through Davis's tank each minute.
But the probability of the neutrinos hitting the fluid and making
an argon atom was so small, Ray Davis could only expect to find
ten individual atoms of argon from ten neutrino
collisions per week.
His task was almost impossible.
Many of his own physicist colleagues doubted this experiment
would ever work.
He was having to convince people that out of these millions
and millions and millions and millions of atoms inside this tank,
he could identify the collisions of one or two
and convince you that these were neutrinos coming from the sun.
Around each month, Davis flushed out the giant tank
to extract the argon atoms.
To everybody's amazement...
..he found them.
But there was a problem.
Instead of detecting the number of atoms that theory
predicted, his measurements fell short.
They knew the target number based on the nuclear physics
theoretical explanation of how stars shine,
and that led to a very particular target number,
and Davis's remarkable experiment kept coming in,
not close to it, not 80%, but only at one third of that target number.
What happened?
Had the experiment gone wrong?
Another scientist carried out a blind trial to test the accuracy
of Ray's atom detection.
A colleague put in 500 kind of rogue atoms without telling Davis
the number, and Davis was able to go through the whole process,
sifted through, and he counted exactly the number
that had been put in.
If the experimental results were accurate, then perhaps
scientists had gotten their theory about the sun wrong.
Everybody was blaming everybody else.
There were even suggestions, has the sun already burnt out in the core?
And it was just an enormous puzzle.
All these advances in understanding how stars shine and then hitting
this kind of brick wall where theory and experiment
just would not agree with each other.
The puzzle became known as the solar neutrino problem.
1970. 20 years since Bruno Pontecorvo defected
to the Soviet Union.
Even after all that time, his life behind the Iron Curtain
remained shrouded in secrecy.
But in a government lab outside Moscow, Pontecorvo worked
tirelessly to explain the puzzling behaviour of neutrinos.
He suggested that instead of just one, there may be two or even
three different types of neutrinos.
These were known as different flavours,
called the electron, muon and tau neutrinos.
If this wasn't strange enough, he calculated that something
peculiar might happen as they travelled through space.
A neutrino would always be born as one definite flavour.
But over time, it would change its identity.
It would transform, mixing back and forth
between the three different types.
This was called neutrino oscillation.
Pontecorvo's idea really is...it's sort of delicious.
These neutrinos could be not taking one identity, dropping that,
adopting another one, dropping that, but going into this even stranger
mixture, where they're in neither and both states at once.
It was a bold idea. No other fundamental particle seemed
to spontaneously change its identity.
But if neutrinos were transforming into flavours
that Ray Davis' detector couldn't see,
it might explain why two-thirds of the neutrinos from the sun
appeared to be missing.
There's been the thought that maybe the neutrino
isn't behaving quite the way we thought.
Some Soviet scientists had pointed out that when neutrinos
come from the sun, they can be converted into another
kind of neutrino that we would not be able to detect.
But there was a catch.
The Standard Model, the most precise scientific theory in human history,
made one important prediction that stood in the way.
The Standard Model anticipated neutrinos
would be completely massless.
They would have no mass at all, much like the photon of light.
And if they had no mass, that meant that they could not oscillate.
If neutrinos had no mass, one of Albert Einstein's most important
theories predicted that neutrinos could not possibly oscillate.
There's this mind-boggling phenomenon from Einstein's
relativity that says that a clock that is moving closer and closer
to the speed of light will tick at a slower and slower rate.
If that clock were moving literally at the speed of light,
it would never tick at all.
No time would pass for that object that moves at exactly
the speed of light.
According to Einstein, the faster a particle travels,
the more its internal clock slows down.
The laws of physics state a particle with no mass
can only travel at the speed of light, which is where time stops.
So if a neutrino had no mass, it would not experience the passage
of time and would never be able to change.
If a particle has zero mass, what that means is that its internal
clock is not ticking.
There's no way for that particle to experience time.
If there's no passage of time,
then how could they change over time from one identity to another?
If neutrino oscillation was real, neutrinos must have some mass.
But could the Standard Model really be wrong?
At CERN in the 1950s and '60s, clues from experiments performed
here helped to devise the Standard Model.
What they found revolutionised our understanding of the particles
that make-up our universe.
ARCHIVE: By means of this machine, it is possible to see the tracks
of subnuclear particles, the smallest particles known to man.
The electron, the positron, the photon and the neutrino.
The work at CERN led to ground-breaking new technologies,
medical advances like PET scans,
even the birth of the World Wide Web.
Perhaps CERN's biggest success came in 2012.
Nearly 50 years after the Standard Model was proposed,
physicists detected the final particle it predicted.
The Higgs boson.
I think we have it.
APPLAUSE
The Higgs boson had been found, and that was just electrifying.
It was the last remaining particle that had been hypothesised
to be part of the Standard Model, a new piece that helps explain
how and why we're here.
And that news just rippled really far and wide.
Today, the Standard Model describes every fundamental particle
in the detectable universe.
Along with the Higgs boson, there are force carriers,
like the photon of light.
Quarks, which form the nuclei of atoms.
Leptons, including the electron, muon and tau.
And three corresponding flavours of neutrinos.
It is a map of what's out there, what we're made of
and how we fit, all of us,
we are made of these things.
And that is a kind of basic understanding of nature
of our own world that I think is just a remarkable human achievement.
And yet, for all its success, the Standard Model could not explain
how or why the neutrinos would have mass
and therefore might oscillate.
For Ray Davis and his missing solar neutrinos,
it seemed an unsolvable paradox.
For decades, Davis persisted.
But he still only found one third of the neutrinos
that were supposed to be coming from the sun.
Well, we've been carrying on this experiment for about 20 years
right here, but we're still observing a low flux of neutrinos.
Eventually, the problem was too big to ignore.
In the 1990s, scientists in Canada and Japan constructed
a new generation of supersized neutrino detectors to finally
settle the mystery.
One of them lies deep beneath Japan's Ikeno Mountain.
Scientists fit 11,000 light detectors to the inside
of a gigantic container and fill it with 50,000 tons
of ultra-pure water.
This $100 million experiment is called Super-K.
The Super-K experiment ended up being a game changer.
Here we had a brand-new, shiny experiment, which had the special
ability of being able to tell not only where the neutrinos
were coming from, which direction they were coming from,
but also whether they were muon type neutrinos,
or electron type neutrinos.
When neutrinos collide with the water in Super-K,
the reaction produces a trail of light which the sensors can pick up.
The signal allows scientists to calculate which type of neutrino
is hit and the direction it came from.
To test the theory of neutrino oscillation,
Super-K focused on catching them from a new source -
the Earth's atmosphere.
Theory suggests that when radiation from space hits the atmosphere,
it creates neutrinos that travel directly through the Earth.
Some travel a short distance.
But others will come from the other side of the planet
to reach the detector.
If the neutrinos are not changing as they travel, the combination
of neutrino types they record coming from a short distance
should be the same as those coming from afar.
If they ARE changing as they travel, the combination of types
will be different.
After two years of taking data, the team finally have an answer.
What they were seeing was that one type of neutrinos
was depleting when travelling through the Earth.
So that was an indication that neutrinos can change
from one type to the other.
For that to happen,
you must have non-zero neutrino mass.
The results were ground-breaking.
Neutrinos change their identity - neutrinos have mass, after all.
And the Standard Model's prediction of the nature of neutrinos
must be wrong.
With the new input, the evidence that neutrinos really oscillate,
they really change their identities, therefore, they really,
really have a mass,
this long-standing, decades-long challenge to understand
the solar neutrino problem finally fell into place.
Nuclear fusion in the sun produces one type of neutrino.
But on the journey through space, the neutrinos oscillate
and turn into a mixture of all three.
On Earth, Ray Davis's detector could only pick out
one of the three types.
His results had been accurate all along.
37 years after the experiment began,
Ray Davis was awarded the Nobel Prize.
APPLAUSE
For Bruno Pontecorvo and his theory of oscillations,
sadly, the discovery came too late.
Nobel Prizes aren't everything, but by the time the oscillations
had been sorted out and the whole thing finally understood,
Pontecorvo was dead.
So that's the final tragedy of his life.
After almost 100 years of research and discovery, today neutrino
physicists face perhaps their biggest puzzle yet.
The Standard Model's equations cannot explain why neutrinos
have mass or why they oscillate.
It's a sign that our understanding of matter is still incomplete.
Today, neutrino experiments costing over £3 billion
are in overdrive, hunting for clues.
We're in the midst of really a neutrino bonanza.
I mean, they're popping up all over
the field of physics.
In Germany, scientists are trying to pin down exactly how much
a neutrino weighs.
In the mines of South Dakota, neutrino experiments
hope to reveal how matter itself emerged from the Big Bang.
And all over the globe, scientists now use neutrinos
as a tool to probe the deepest corners of the cosmos.
Even space filled with big, thick, dense matter like planets and stars,
these neutrinos can zip through them as if there's nothing
there at all.
They become a kind of cosmic, you know, telegram for what
the universe might have been like at the time of the Big Bang,
or in the furthest reaches of space, as far from our own planet Earth
as we can imagine.
This is neutrino astronomy.
And in the quest for cosmic neutrinos, scientists adventure
to the ends of the Earth.
At the South Pole, scientists have built the largest
neutrino detector on the planet.
It's made of 5,000 sensors drilled into a cubic kilometre
of Antarctic ice.
It's known as Ice Cube.
Ice Cube is in this this huge field around me, I'm sitting, kind of
standing in the middle of Ice Cube.
It's kind of amazing to think that we were able to haul
something like five million pounds of cargo down to the South Pole.
This is instrumentation, cables, drill equipment, fuel.
Like the Super-K detector, when a neutrino strikes an atom
in the ice, it produces a cone of light which the sensors
can pick up.
This reveals the neutrino's cosmic origin, making Ice Cube
a neutrino telescope.
We're able to conceptually kind of put pushpins where these
neutrino events came from,
and that's exactly why we call it a neutrino telescope.
Today, Ice Cube has detected neutrinos from billions
of light years away.
Some launched by giant black holes in the hearts of distant galaxies.
This is the universe that has really only been open to our eyes
for the last 50 years.
There's all kinds of discoveries that are waiting out there.
With new experiments like Ice Cube, neutrinos may reveal discoveries
beyond the Standard Model.
Neutrinos could even help unlock one of the biggest mysteries
in physics today.
It seems that most of what our universe is made of...
..is missing.
The whole quest of particle physics is to explain
the matter content of the universe.
And we seem to be doing this phenomenally good job.
You crank through the math of the Standard Model
and everything makes sense.
And yet it only describes some very small fraction
of what the universe is made out of.
Looking into space, cosmologists can see the gravitational influence
of a material that binds entire galaxies together.
But that is completely invisible to their detectors.
Scientists call this material dark matter because nothing
in the Standard Model can describe what it is.
And yet it seems to be what most of the matter
in the universe is made of.
The Standard Model is very good at describing
about 5% of the universe.
95% of the stuff is an utter, complete mystery,
made of dark stuff, whether it's dark matter or dark energy.
And what either of those are, we don't know.
All we really know about dark matter is that it creates gravity,
but it's not interacting with the instruments
that we have used to observe the universe.
Whatever is filling space...
..much more of it than the ordinary matter that makes up us
and our planet and our stars, it's some other, other kind of particle.
Whatever dark matter particles are, scientists must look
beyond the Standard Model to find them.
Neutrinos might be the key.
At Fermilab, for over 20 years, scientists have been
investigating neutrino oscillations.
What they've found doesn't add up.
The first observation that something was amiss was in the late 1990s.
Something we don't quite understand is going on.
At Fermilab, scientists fire the beam of mostly muon type
neutrinos just 500 metres to their detector.
Neutrinos oscillate too slowly for the detector to see them change
over such a short distance, at least according to theory.
But the data showed electron type neutrinos mysteriously
appearing in the detector.
Neutrinos seem to oscillate faster than is theoretically possible.
The strange thing that we're seeing is that neutrinos seem
to be changing from one type to the other,
much faster than expected.
In order for that to happen, we think it's possible
that there are extra neutrinos out there.
In addition to the three flavours of neutrino
that the Standard Model predicts,
there could be a fourth neutrino that affects them,
making them oscillate faster.
Scientists call it a sterile neutrino.
And it's never been directly detected.
So we call it a sterile neutrino.
In essence, just because it interacts even less
with other particles than the regular neutrinos do.
A sterile neutrino would be the ultimate ghost particle.
It would never collide with atoms in our world.
No detector could ever see it.
But it may reveal itself through its effects
with the neutrinos we can see.
The only way that we can tell they exist
is through their effects on neutrino isolation.
If sterile neutrinos exist, it would break the neat symmetry
of the Standard Model that organises particles in groups of three.
What if there is a fourth kind of neutrino,
a so-called sterile neutrino?
Well, where would you put that on our map?
There's no room to kind of shoehorn in, to squeeze in a fourth neutrino.
So I think there really is a lot riding on this.
If they're real, sterile neutrinos would have mass,
but not interact with our detectors, just like dark matter.
They could be the first particle of dark matter ever discovered,
and through their effects on the neutrinos we can see,
they could give scientists a window into an unknown world.
Now, the neutrino might be a kind of link, almost a kind of messenger
or portal to this whole other possible kind of stuff out there.
At Fermilab, scientists are edging towards the truth.
Physicist Angela Fava is making the finishing touches
to the ICARUS detector.
Trigger in five hertz...
OK, on now...
We are starting what we call the cold commission,
so the cooling-down phase, in a couple of weeks.
Then we will need to get it filled with liquid argon.
Fingers crossed, then you start seeing
your neutrino tracks immediately.
Once complete, ICARUS will join MicroBooNE and eventually
a third detector called SBND.
A sterile neutrino would act like a catalyst,
speeding up how ordinary neutrinos can transform
from one flavour to another.
If the team see these fast oscillations, it could prove
the sterile neutrino is real.
This would be really a great discovery.
I mean, game changing, it opens up perspective for knowing
the universe in a different way.
Once we'll have all the detectors up and running, it will take still
at least three years, but definitely we are confident that we can
answer this question.
Let's wait for the data and see.
I think we're getting a lot closer.
Neutrino physicists are incredibly patient.
It takes a long time for us to collect our data
and we really want to be sure in what we're seeing
before we potentially make a very important discovery.
We're trying to answer some of THE biggest questions in physics.
And I think it's really unique that neutrinos may hold all the answers.
What began as a hypothetical particle that no-one
thought possible to detect
could now be a key to unlock what most of our universe
is made of and how it works.
Every time we look up, there seem to be these very curious neutrinos.
They are constantly bedevilling our mental maps of how we carve
up nature and try to dig in and study.
And that's just amazingly exciting.
So they've gone from "maybe they exist, maybe they don't,
"we might never know..."
to being our surest ticket to the next step.
History has shown that with every little bit of progress,
we've learned huge, surprising things about our cosmos.
To me, that's really exciting, and I'm curious to know
where else could we go?