Nova (1974–…): Season 48, Episode 14 - Particles Unknown - full transcript
An international team studies the neutrino, the most common yet least understood particle in the universe.
They're the most mysterious
particles ever discovered,
tiny ghosts hidden in our world.
Now scientists are on a mission
to unlock their secrets.
They're called neutrinos.
The story of their discovery
is almost impossible to believe.
If they had bolted the detector
in place, the nuclear bomb
would've
just smashed it to smithereens.
With links to a dramatic
Cold War defection.
He disappeared through
the Iron Curtain,
and for five years,
disappeared off the face of the
planet.
And astonishing experiments
that keep defying
the laws of physics.
Even as someone who builds these
experiments for a living,
it just seems mind-blowing
that they ever work.
Today, scientists are using
neutrinos
to probe the edges
of our detectable universe.
They're on a mission to reveal
a hidden world
of "Particles Unknown."
Right now, on "NOVA."
♪♪
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.
Scientists theorize there
exists a hidden, parallel world
of particles...
So-called dark matter.
So far, no one has managed
to detect a single one.
But now there might be a way.
Of all the particles scientists
have discovered,
the most elusive, on the very
edge of detectability,
are 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 ghost-like,
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.
First theorized in the 1930s,
they would soon become linked
to nuclear secrets
and a dramatic Cold War
defection
behind the Iron Curtain.
He goes off to Europe
and never returns.
Now the quest
to detect neutrinos
has triggered vast experiments
all over the globe.
Even as someone who builds these
experiments for a living,
it just seems mind-blowing
that they ever work.
Today, scientists are on the
cusp
of an astonishing discovery.
Tantalizing evidence
suggests neutrinos
could be a doorway
between our world of matter
and the hidden world of
dark matter,
waiting to be discovered.
It would be a game-changer.
What exactly are these
particles?
What is its role in the
evolution of our universe?
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, in Batavia, Illinois.
World-renowned physics
laboratory.
Thousands of scientists
build enormous experiments
to probe the very smallest
particles
that make up our universe.
Leading one of the teams
is Sam Zeller.
Hi, team.
My interest in physics started
when I signed up
for a field trip to come
to Fermilab in high school.
It just blew my mind.
From that point on, I was
a particle physicist.
♪♪
It 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 one type remains
far more mysterious than others.
They're called neutrinos.
A neutrino is a
type of elementary particle,
a basic fundamental building
block of the universe,
and they come in
three different flavors.
Neutrinos are everywhere.
They are produced in the sun.
There are neutrinos that were
left over after the Big Bang.
Humans emit 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.
And yet, at Fermilab,
scientists are constructing
a complex two-stage experiment
with the means to create them
and study them.
♪♪
In its first stage,
a powerful ring of magnets
accelerates positively charged
particles called protons
to colossal speeds, sending
them smashing into a target.
The collision creates a shower
of new particles,
including a powerful beam
of neutrinos.
150 trillion per second pass
through the Earth
at nearly the speed of light,
racing towards the second
stage...
Three giant neutrino detectors.
The largest is called ICARUS.
Once complete,
this immense tank filled with
a web of electronics
and cryogenic liquid
will be bombarded by hundreds
of trillions of neutrinos,
all in the hope of catching
just one each minute.
♪♪
That alone will be a remarkable
achievement.
But the scientists
have even bigger ambitions.
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.
Experiments have hinted
there could be
an even more elusive neutrino
beyond the three types already
known to exist.
Some 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.
If we found evidence
for 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
in to work.
It would be a major
and massive discovery.
Making that discovery would be
groundbreaking.
Because while ordinary neutrinos
are extremely hard to detect,
this fourth type of neutrino
could break the Standard Model.
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.
In Rome.
Physicist and historian
Professor David Kaiser
has traveled here,
to the place 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 was 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 nanometer,
about a billion times smaller
than a meter, 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.
During the early 20th century,
scientists discovered the atom's
tiny nucleus contained protons,
particles with
a positive electric charge.
These protons held in place
a cloud of negatively charged
electrons
that formed the atom's
outer limit.
It seemed that protons
and electrons
were the only two components
of all atoms...
Permanent and fixed.
But scientists had 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 phenomenon 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 behavior
of tiny particles,
the principle states that
energy should never disappear.
But when scientists measured
the energy of the electrons
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 probed
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 he'd 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 a 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 have
a profound impact on the world.
♪♪
In October 1931,
they invited a group of the
world's leading physicists
to a conference held at the
Physics Institute.
High on the agenda was
the problem
of the missing
radioactive energy.
One scientist at the conference,
the famous Wolfgang Pauli,
proposed a radical idea.
Wolfgang Pauli had written
a letter to colleagues.
And he put forward what
he called a desperate remedy,
a "versweifelten 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 was 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 for Enrico Fermi,
the Pope of Via Panisperna,
he 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 fervor
of the lab,
fascism was about to cast
a shadow
over the neutrino mystery.
In 1939, Fermi immigrated
to the U.S.A.
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 British
National Archives in London.
Earlier in his life,
Pontecorvo had been a member
of a communist party.
And there are now
British intelligence files
under his name.
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
U.K.'s atomic energy program,
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 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 U.S.A. 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 a British
intelligence service
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 the 19th of July.
Five days later,
Pontecorvo goes off to Europe
and never returns.
♪♪
Flight manifests reveal
Pontecorvo and his family
flew from Rome, across Europe,
to Helsinki,
alongside two suspected
KGB agents.
Pontecorvo's son, just
12 years old at the time,
revealed they were then driven
across the border to Moscow...
With Bruno in the trunk.
He said to me,
"I knew something was up."
Frank believes a Soviet mole
passed the FBI letter to Moscow,
who then 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
that Pontecorvo
was a top-quality scientist
who had taken his
brain to the Soviet Union.
- By 1950, the U.S.A.
- 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 is 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 detect them?
In 1951, a young American
called Fred Reines
was working on the
U.S. nuclear program
at Los Alamos National
Laboratory.
It was here that Reines, along
with his colleague Clyde Cowan,
decided to take advantage
of destructive bomb tests
to investigate the mystery
of 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 favor,
and knew 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 had bolted
the detector in place,
the nuclear bomb would've just
smashed it to smithereens.
So instead, the proposal
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 shockwave 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 realization.
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 traveled
through the right medium,
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
this two coincident signal blip
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 they're 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.
Stars.
For a long, long time,
scientists have been wondering,
what makes the stars shine?
What drives that enormous
output of energy?
♪♪
People theorized 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.
In the historic gold mining
town of Lead,
people descend into the depths
of the Earth.
But no longer to mine
precious metal.
They're 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 realizing 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 showed
that neutrinos from the sun
would be so faint, a detector
near the Earth's surface
would be overwhelmed
by background radiation.
His only option was to go
to the bottom of a mine.
Beneath almost a mile of solid
rock, Davis's team built
a steel tank the size of a house
and filled it 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 Davis 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.
Scientists had calculated
that around a million trillion
neutrinos from the sun
should pass through Davis's tank
each minute.
But the probability
of them 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 percent,
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,
sift it 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 neutrinos from the
sun wrong.
Everybody was blaming
everybody else.
There were even suggestions,
has the sun already burnt out
in the core?
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 behavior
of neutrinos.
He suggested that instead
of just one,
there may be two or even three
different kinds of neutrino...
Known as different flavors.
♪♪
If this wasn't strange enough,
he calculated that something
peculiar might happen as they
traveled through space.
A neutrino would always be born
as one definite flavor,
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, 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 flavors
that Ray Davis's detector
couldn't see,
it might explain why
two-thirds of the neutrinos
from the sun appeared
to be missing.
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 is 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's
theories,
the faster a particle travels,
the more its internal clock
slows down.
A particle with no mass can only
travel at the speed of light,
which is where time stops.
So if a neutrino had zero 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?
♪♪
Throughout the 1950s and '60s,
clues from experiments
performed at CERN,
alongside Fermilab,
helped to lay the foundation
of the Standard Model.
What they found
revolutionized our understanding
of the particles that make up
our universe.
By means of this machine,
it is possible to see
the tracks
of sub-nuclear particles,
the smallest particles
known to man:
the electron, the positron,
the photon, and the neutrino...
Over the years, work at CERN
led to groundbreaking
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.
Finally, all the pieces needed
to describe the detectable
physical universe
seemed to be in place.
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 flavors
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, I think is, is just a
remarkable
human achievement.
And yet, for all its success,
the Standard Model had
no equations to explain
how or why the neutrinos
would have mass.
For Ray Davis
and his missing solar neutrinos,
it seemed an unsolvable paradox.
For decades, Davis persists,
but he still only finds
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
is too big to ignore.
In the 1990s,
scientists in Canada and Japan
construct 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 detector
is named Super-K.
The Super-K experiment ended up
being a game-changer.
In the rare event that
a neutrino collides
with the liquid in Super-K,
the reaction produces
a trail of light
which the sensors can pick up.
Unlike Davis's detector,
this signal allows scientists
to calculate
which type of neutrino has hit
and the direction it came from.
Super-K allows scientists
to test the theory
of neutrino oscillation
by 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,
the combination of flavors they
record
coming from a short distance
will be the same
as those coming from afar.
If they are changing over
a long distance,
the combination of flavors will
be different.
After two years
of recording data,
the team finally has an answer.
What they were seeing was that
one type of neutrinos was
depleting
when traveling through
the Earth.
The Super-K results combined
with results
from another experiment
were able to definitively show
that neutrinos can change
from one type to the other.
For that to happen,
you must have non-zero
neutrino mass.
The results are groundbreaking.
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 long journey through
space,
the neutrinos oscillate,
and turn into a mixture of
all three.
On Earth,
Ray Davis's detector only
picked out one flavor.
His results had been accurate
all along.
37 years after
the experiment began,
Ray Davis was awarded the
Nobel Prize.
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,
which are so precise for
other particles,
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 are
in overdrive,
hunting for clues.
We're in the midst of, really,
a neutrino bonanza... I mean,
they're just, they're popping up
all over the field of physics.
♪♪
At the South Pole,
scientists have built
the largest neutrino detector
on the planet.
It's made of more
than 5,000 sensors drilled into
a cubic kilometer
of Antarctic ice.
It's known as IceCube.
♪♪
IceCube is in this,
this huge field around me...
I'm sitting,
kind of standing in the middle
of IceCube.
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...
As well as probing neutrino
oscillations,
IceCube acts like
a neutrino telescope,
catching cosmic neutrinos
from billions
of light years away.
This is the universe that
has really
only been opened to our eyes
for the last 50 years.
♪♪
There's all kinds of discoveries
that are waiting out there.
With new experiments like
IceCube,
scientists believe that
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
contents 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 five percent
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 fired
a beam of neutrinos
just 500 yards 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 detectors saw
an increase in one type
of neutrinos.
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 flavors
of neutrino
that the Standard Model
describes,
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
on the neutrinos we can see.
The only way that we can tell
they exist
is through their effects
on neutrino oscillation.
If sterile neutrinos exist,
it would break the neat symmetry
of the Standard Model
that organizes particles
in groups of three.
What if there's 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 another world.
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.
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.
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 that unlocks
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 bedeviling
our mental maps of how we carve
up nature
and try to dig in and study it.
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?
Wherever we go,
neutrinos could be our guide.
♪♪
particles ever discovered,
tiny ghosts hidden in our world.
Now scientists are on a mission
to unlock their secrets.
They're called neutrinos.
The story of their discovery
is almost impossible to believe.
If they had bolted the detector
in place, the nuclear bomb
would've
just smashed it to smithereens.
With links to a dramatic
Cold War defection.
He disappeared through
the Iron Curtain,
and for five years,
disappeared off the face of the
planet.
And astonishing experiments
that keep defying
the laws of physics.
Even as someone who builds these
experiments for a living,
it just seems mind-blowing
that they ever work.
Today, scientists are using
neutrinos
to probe the edges
of our detectable universe.
They're on a mission to reveal
a hidden world
of "Particles Unknown."
Right now, on "NOVA."
♪♪
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.
Scientists theorize there
exists a hidden, parallel world
of particles...
So-called dark matter.
So far, no one has managed
to detect a single one.
But now there might be a way.
Of all the particles scientists
have discovered,
the most elusive, on the very
edge of detectability,
are 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 ghost-like,
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.
First theorized in the 1930s,
they would soon become linked
to nuclear secrets
and a dramatic Cold War
defection
behind the Iron Curtain.
He goes off to Europe
and never returns.
Now the quest
to detect neutrinos
has triggered vast experiments
all over the globe.
Even as someone who builds these
experiments for a living,
it just seems mind-blowing
that they ever work.
Today, scientists are on the
cusp
of an astonishing discovery.
Tantalizing evidence
suggests neutrinos
could be a doorway
between our world of matter
and the hidden world of
dark matter,
waiting to be discovered.
It would be a game-changer.
What exactly are these
particles?
What is its role in the
evolution of our universe?
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, in Batavia, Illinois.
World-renowned physics
laboratory.
Thousands of scientists
build enormous experiments
to probe the very smallest
particles
that make up our universe.
Leading one of the teams
is Sam Zeller.
Hi, team.
My interest in physics started
when I signed up
for a field trip to come
to Fermilab in high school.
It just blew my mind.
From that point on, I was
a particle physicist.
♪♪
It 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 one type remains
far more mysterious than others.
They're called neutrinos.
A neutrino is a
type of elementary particle,
a basic fundamental building
block of the universe,
and they come in
three different flavors.
Neutrinos are everywhere.
They are produced in the sun.
There are neutrinos that were
left over after the Big Bang.
Humans emit 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.
And yet, at Fermilab,
scientists are constructing
a complex two-stage experiment
with the means to create them
and study them.
♪♪
In its first stage,
a powerful ring of magnets
accelerates positively charged
particles called protons
to colossal speeds, sending
them smashing into a target.
The collision creates a shower
of new particles,
including a powerful beam
of neutrinos.
150 trillion per second pass
through the Earth
at nearly the speed of light,
racing towards the second
stage...
Three giant neutrino detectors.
The largest is called ICARUS.
Once complete,
this immense tank filled with
a web of electronics
and cryogenic liquid
will be bombarded by hundreds
of trillions of neutrinos,
all in the hope of catching
just one each minute.
♪♪
That alone will be a remarkable
achievement.
But the scientists
have even bigger ambitions.
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.
Experiments have hinted
there could be
an even more elusive neutrino
beyond the three types already
known to exist.
Some 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.
If we found evidence
for 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
in to work.
It would be a major
and massive discovery.
Making that discovery would be
groundbreaking.
Because while ordinary neutrinos
are extremely hard to detect,
this fourth type of neutrino
could break the Standard Model.
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.
In Rome.
Physicist and historian
Professor David Kaiser
has traveled here,
to the place 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 was 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 nanometer,
about a billion times smaller
than a meter, 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.
During the early 20th century,
scientists discovered the atom's
tiny nucleus contained protons,
particles with
a positive electric charge.
These protons held in place
a cloud of negatively charged
electrons
that formed the atom's
outer limit.
It seemed that protons
and electrons
were the only two components
of all atoms...
Permanent and fixed.
But scientists had 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 phenomenon 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 behavior
of tiny particles,
the principle states that
energy should never disappear.
But when scientists measured
the energy of the electrons
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 probed
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 he'd 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 a 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 have
a profound impact on the world.
♪♪
In October 1931,
they invited a group of the
world's leading physicists
to a conference held at the
Physics Institute.
High on the agenda was
the problem
of the missing
radioactive energy.
One scientist at the conference,
the famous Wolfgang Pauli,
proposed a radical idea.
Wolfgang Pauli had written
a letter to colleagues.
And he put forward what
he called a desperate remedy,
a "versweifelten 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 was 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 for Enrico Fermi,
the Pope of Via Panisperna,
he 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 fervor
of the lab,
fascism was about to cast
a shadow
over the neutrino mystery.
In 1939, Fermi immigrated
to the U.S.A.
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 British
National Archives in London.
Earlier in his life,
Pontecorvo had been a member
of a communist party.
And there are now
British intelligence files
under his name.
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
U.K.'s atomic energy program,
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 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 U.S.A. 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 a British
intelligence service
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 the 19th of July.
Five days later,
Pontecorvo goes off to Europe
and never returns.
♪♪
Flight manifests reveal
Pontecorvo and his family
flew from Rome, across Europe,
to Helsinki,
alongside two suspected
KGB agents.
Pontecorvo's son, just
12 years old at the time,
revealed they were then driven
across the border to Moscow...
With Bruno in the trunk.
He said to me,
"I knew something was up."
Frank believes a Soviet mole
passed the FBI letter to Moscow,
who then 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
that Pontecorvo
was a top-quality scientist
who had taken his
brain to the Soviet Union.
- By 1950, the U.S.A.
- 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 is 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 detect them?
In 1951, a young American
called Fred Reines
was working on the
U.S. nuclear program
at Los Alamos National
Laboratory.
It was here that Reines, along
with his colleague Clyde Cowan,
decided to take advantage
of destructive bomb tests
to investigate the mystery
of 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 favor,
and knew 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 had bolted
the detector in place,
the nuclear bomb would've just
smashed it to smithereens.
So instead, the proposal
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 shockwave 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 realization.
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 traveled
through the right medium,
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
this two coincident signal blip
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 they're 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.
Stars.
For a long, long time,
scientists have been wondering,
what makes the stars shine?
What drives that enormous
output of energy?
♪♪
People theorized 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.
In the historic gold mining
town of Lead,
people descend into the depths
of the Earth.
But no longer to mine
precious metal.
They're 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 realizing 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 showed
that neutrinos from the sun
would be so faint, a detector
near the Earth's surface
would be overwhelmed
by background radiation.
His only option was to go
to the bottom of a mine.
Beneath almost a mile of solid
rock, Davis's team built
a steel tank the size of a house
and filled it 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 Davis 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.
Scientists had calculated
that around a million trillion
neutrinos from the sun
should pass through Davis's tank
each minute.
But the probability
of them 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 percent,
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,
sift it 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 neutrinos from the
sun wrong.
Everybody was blaming
everybody else.
There were even suggestions,
has the sun already burnt out
in the core?
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 behavior
of neutrinos.
He suggested that instead
of just one,
there may be two or even three
different kinds of neutrino...
Known as different flavors.
♪♪
If this wasn't strange enough,
he calculated that something
peculiar might happen as they
traveled through space.
A neutrino would always be born
as one definite flavor,
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, 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 flavors
that Ray Davis's detector
couldn't see,
it might explain why
two-thirds of the neutrinos
from the sun appeared
to be missing.
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 is 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's
theories,
the faster a particle travels,
the more its internal clock
slows down.
A particle with no mass can only
travel at the speed of light,
which is where time stops.
So if a neutrino had zero 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?
♪♪
Throughout the 1950s and '60s,
clues from experiments
performed at CERN,
alongside Fermilab,
helped to lay the foundation
of the Standard Model.
What they found
revolutionized our understanding
of the particles that make up
our universe.
By means of this machine,
it is possible to see
the tracks
of sub-nuclear particles,
the smallest particles
known to man:
the electron, the positron,
the photon, and the neutrino...
Over the years, work at CERN
led to groundbreaking
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.
Finally, all the pieces needed
to describe the detectable
physical universe
seemed to be in place.
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 flavors
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, I think is, is just a
remarkable
human achievement.
And yet, for all its success,
the Standard Model had
no equations to explain
how or why the neutrinos
would have mass.
For Ray Davis
and his missing solar neutrinos,
it seemed an unsolvable paradox.
For decades, Davis persists,
but he still only finds
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
is too big to ignore.
In the 1990s,
scientists in Canada and Japan
construct 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 detector
is named Super-K.
The Super-K experiment ended up
being a game-changer.
In the rare event that
a neutrino collides
with the liquid in Super-K,
the reaction produces
a trail of light
which the sensors can pick up.
Unlike Davis's detector,
this signal allows scientists
to calculate
which type of neutrino has hit
and the direction it came from.
Super-K allows scientists
to test the theory
of neutrino oscillation
by 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,
the combination of flavors they
record
coming from a short distance
will be the same
as those coming from afar.
If they are changing over
a long distance,
the combination of flavors will
be different.
After two years
of recording data,
the team finally has an answer.
What they were seeing was that
one type of neutrinos was
depleting
when traveling through
the Earth.
The Super-K results combined
with results
from another experiment
were able to definitively show
that neutrinos can change
from one type to the other.
For that to happen,
you must have non-zero
neutrino mass.
The results are groundbreaking.
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 long journey through
space,
the neutrinos oscillate,
and turn into a mixture of
all three.
On Earth,
Ray Davis's detector only
picked out one flavor.
His results had been accurate
all along.
37 years after
the experiment began,
Ray Davis was awarded the
Nobel Prize.
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,
which are so precise for
other particles,
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 are
in overdrive,
hunting for clues.
We're in the midst of, really,
a neutrino bonanza... I mean,
they're just, they're popping up
all over the field of physics.
♪♪
At the South Pole,
scientists have built
the largest neutrino detector
on the planet.
It's made of more
than 5,000 sensors drilled into
a cubic kilometer
of Antarctic ice.
It's known as IceCube.
♪♪
IceCube is in this,
this huge field around me...
I'm sitting,
kind of standing in the middle
of IceCube.
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...
As well as probing neutrino
oscillations,
IceCube acts like
a neutrino telescope,
catching cosmic neutrinos
from billions
of light years away.
This is the universe that
has really
only been opened to our eyes
for the last 50 years.
♪♪
There's all kinds of discoveries
that are waiting out there.
With new experiments like
IceCube,
scientists believe that
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
contents 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 five percent
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 fired
a beam of neutrinos
just 500 yards 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 detectors saw
an increase in one type
of neutrinos.
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 flavors
of neutrino
that the Standard Model
describes,
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
on the neutrinos we can see.
The only way that we can tell
they exist
is through their effects
on neutrino oscillation.
If sterile neutrinos exist,
it would break the neat symmetry
of the Standard Model
that organizes particles
in groups of three.
What if there's 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 another world.
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.
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.
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 that unlocks
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 bedeviling
our mental maps of how we carve
up nature
and try to dig in and study it.
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?
Wherever we go,
neutrinos could be our guide.
♪♪