Horizon (1964) s40e15 Episode Script

Something very strange is happening to you right now.
A swarm of ghosts is flowing straight through your body.
There are something like a hundred trillion of them, streaming through each of us, every second, every second a hundred trillion.
Most of them pass through without doing anything, so this was ghostly or poltergeist-like.
Catching these ghosts was one of the greatest challenges scientists have ever faced.
When you think about it, it's almost unbelievable what we were doing and I'm glad we, we didn't think about it too carefully when we were doing it.
Now experiments have revealed that they're even stranger than anyone thought.
Not only have they solved several mysteries, but they're our parents.
Could the tiny particles that are passing through you right now be the very reason we all exist? They're here.
John Bahcall and Ray Davis have been friends for nearly half a century.
For most of that time these two men have been at the heart of the biggest puzzle in particle physics.
Turn some more.
It all began forty years ago with a daring underground experiment.
Ray Davis had tunnelled deep into the earth to build a trap for the most elusive particle in nature.
It was an experiment which few thought could ever succeed.
He set out to do something which sounds totally impossible.
And it produced a result which no one could believe.
Well, you know, there's got to be something wrong with that experiment; no, that can't be right.
Everyone was convinced the two of them had made an embarrassing mistake.
It was a personal shock, a very painful one.
We learned from that, but it was a painful shock.
But for decades Davis and Bahcall refused to give up, convinced that they were on to something important.
This is the story of how an experiment which no one believed has led to an astonishing discovery.
A discovery which is making scientists re-think their most fundamental theories of what the universe is made of, and where it all came from.
And it's all to do with a tiny, invisible, utterly mysterious particle called the neutrino.
But the story of the neutrino began long before Ray Davis and John Bahcall's experiment.
It all started on the 4th December 1930 with a letter, one of the most famous in the history of science.
It was written by the great Austrian physicist Wolfgang Pauli to colleagues attending a conference on a subject that was causing great puzzlement among physicists: the phenomenon of radioactive decay.
Dear Radioactive Ladies and Gentlemen, unfortunately I am unable to come to Tübingen personally since I am indispensable here, because of a ball to be held in Zurich.
In the first few decades of the 20th century atomic physics had made giant strides in understanding what the universe was made of.
Scientists believed that all atoms consisted of just two kinds of electrically charged particles.
Protons, in the atomic nucleus, surrounded by a cloud of electrons.
But strangely, some atomic nuclei were unstable.
Pauli had to deal with a very, very puzzling situation.
On the level of atomic nuclei and particles smaller than that, many things don't live forever; they disintegrate, or they decay as we say.
At the time radioactive decay was the greatest riddle in physics.
When nuclei decayed they released energy, often by ejecting an electron.
It was the energy of this electron that was the problem: there didn't seem to be enough of it.
There is a very, very well established principle in physics called the principle of conservation of energy.
It says that you don't get more energy than you had before and you don't have less energy, you don't lose energy than you had before.
Energy does not disappear.
But that's just what seemed to be happening.
The energy the nucleus lost when it decayed should all have been taken up by the electron there was nowhere else for it to go.
But it seemed the electron did not carry away as much energy as it should.
In fact what they saw was that in different decays, always with the same original nucleus, always with the same final one, the electron had differing amounts of energy, typically not all the energy that was released.
Energy was somehow disappearing.
But disappearing energy was simply not acceptable to Pauli.
The energy had to be going somewhere.
It was time to be bold.
I have had an idea for a desperate remedy, in order to save the validity of the energy law.
Pauli's idea was that there had to be a third particle involved in radioactive decay, a new kind of particle which no one had ever seen, but which was carrying away the missing energy.
He proposed in addition to the little particles that were known at that time, there was another one that would be emitted in radioactive decay along with the electron.
Pauli suggested that this new particle was very elusive, hard to detect, and this is why people have never seen it, but the particle would take up whatever energy the electron didn't, thus resurrecting and saving the principle of conservation of energy.
He did it I'm sure with great hesitation, but he did it.
It was a very bold move.
But if Pauli had solved one problem he had created another.
There was no evidence that his particle, dubbed the neutrino, really existed.
And Pauli feared there never would be.
He knew that unlike all the other atomic particles neutrinos would have no electric charge.
That would make them invisible to all instruments, able to travel right through solid matter without causing a ripple.
The neutrino was truly a ghost among particles.
They concluded that it was a practical impossibility, that no one would ever see these neutrinos, and I think that's what put people off for many years from even trying.
But then something happened that transformed physics, and the world.
The power of a nuclear bomb comes from a chain reaction of radioactive decay.
If Pauli was right then along with the blast there should be an intense pulse of neutrinos.
In the 1950s Fred Reines was a young researcher working on America's nuclear deterrent, but he really wanted to do some fundamental physics, and then he realised that the atomic weapons program was the perfect place to hunt the elusive neutrino.
He sat in an office for a long time staring at a blank pad, trying to think of an idea, and he hit on the idea of looking for the neutrino.
For him it was intolerable that the neutrino could exist and not be seen, and he had to resolve that problem.
Reines realised that if the chain reaction in a bomb produced neutrinos so should the chain reaction in a nuclear reactor.
In a reactor you get elements being produced and when they decay they give off neutrinos.
So you get lots and lots of neutrinos; it's an enormous number, ten with thirteen zeros after it, per second, going through every little square centimetre of your detector, nearby the reactor.
With such an intense source of neutrinos perhaps Reines and his colleagues could finally capture the ghost particle.
They named their enterprise 'Project Poltergeist'.
But they still faced the fundamental problem.
The neutrino had no electric charge, making it invisible.
But Fred Reines thought there was a way neutrinos could be detected by proxy.
Although neutrinos usually flowed straight through matter without any effects, just very occasionally a neutrino might collide with a nucleus and cause it to eject a charged particle like an electron.
Neutrinos were invisible, these neutrino interactions weren't.
In Reines's experiment the sign would be a distinctive double pulse of energy.
One from the ejected particle, the other from the transformed nucleus.
There was a particular signature of this detection: you saw a pulse, then you saw another pulse afterwards within a certain specified period of time.
And that very characteristic signature enabled you to pull it out from the background.
It was a question of watching an oscilloscope, waiting for that double pulse.
On June 14th 1956, Reines and his colleagues announced the detection of the neutrino.
They sent Pauli a telegram informing him of this discovery and Pauli was very, very happy, saying something like, 'all things come to him who knows how to wait'.
Pauli was right, nature needed the neutrino.
In fact, scientists soon realised we all did.
Every element vital to life, elements like carbon and oxygen, were made by a chain of nuclear reactions that would be impossible without neutrinos.
They were an essential ingredient of the universe.
Without them not even the stars would shine.
And it was this idea that brought Ray Davis and John Bahcall centre stage.
They're here.
Forty years ago Ray Davis was already a renowned designer of experiments, a man who specialised in getting the facts scientists needed to test their theories.
John Bahcall was just beginning his career.
He was drawn to astrophysics, the science of what makes stars tick.
What brought them together was a shared desire to understand how stars shine.
They believed that neutrinos would allow them to do this.
For me and for Ray it was a great challenge to see if we could look inside of a star.
In the same way that your doctor can look inside your body with ultrasound or with X-rays, we wanted to do the same thing with neutrinos: use neutrinos to look right inside the Sun, see really what the nuclear reactions are doing in the very interior.
Inside the core of every star a process called nuclear fusion was producing prodigious quantities of energy; or at least that was the theory.
No one had ever seen this happening.
Now nuclear fusion would produce not only energy, making the Sun shine, but also neutrinos, lots of them By looking at the surface of the Sun you don't learn the details of what's going on deep inside, but by looking at the neutrinos from the Sun you can.
Neutrinos were cosmic messengers, they travelled unhindered right through the Sun to the Earth.
Find the neutrinos and you would have proof that nuclear fusion really was the source of the Sun's energy.
So Davis asked Bahcall to work out exactly how many neutrinos the Sun made.
This meant creating the first detailed mathematical model of the fusion reactions inside the core.
It produced an astonishing result.
We believed that the Sun should be omitting a huge number of neutrinos all the time.
Every second through my thumbnail, and your thumbnail, about a hundred billion of these solar neutrinos would be passing through every second.
A hundred billion solar neutrinos through your thumbnail every second of every day of every year of your life and you never notice it! What can you do? For Ray the challenge was clear: confirm that John's fusion model of the Sun correctly predicted the number of solar neutrinos.
In 1965 Ray Davis embarked on one of the most difficult experiments in the history of science, to count the neutrinos coming from the Sun.
It meant building a laboratory deep underground, in a goldmine in South Dakota, to shelter from confusing background radiation from space.
The heart of the experiment was Ray's neutrino trap: six hundred tonnes of cleaning fluid, a liquid full of chlorine atoms.
When a neutrino strikes a chlorine atom it will convert the chlorine into argon.
And argon is, in particular this form of argon, will be radioactive.
Ray Davis thought you could use the radioactivity of the argon atoms to give themselves away.
The idea was the more neutrinos flowed through the tank, the more argon atoms they would make.
So by counting the argon atoms Ray would be indirectly counting the neutrinos.
But it was here that the immense difficulty of the experiment became apparent.
Trillions of neutrinos went through the tank every second.
But they interacted so rarely that John calculated just ten argon atoms would be made each week.
Finding them seemed a ludicrously impossible task.
Davis was claiming that he could take a tank consisting of three hundred and fifty zillion atoms of chlorine and other stuff, and extract from it only ten argon atoms.
It's worse than a needle in a haystack! Nevertheless every few weeks Ray would bubble helium through the cleaning fluid to sweep out the argon atoms that had accumulated.
He then brought them back to his New York laboratory to be counted.
I used to joke that he travelled all the way across the country with a little tube full of nothing.
It's not strictly true of course; it turned out to be a very important piece of nothing! But as the first results began to come through it was immediately clear that something was wrong.
John Bahcall had expected ten argon atoms per week, but Ray counted only three.
Most of the neutrinos were missing.
Right from the beginning it was apparent that Ray was measuring fewer neutrino events than I had predicted; only about a third, and that was a very serious problem.
My father and I would always talk whenever I'd come home, and I mean it was certainly very perplexing that the number was low.
It looked like Ray's daring experiment simply wasn't working.
There were even people coming and saying, "Well, you know, there's got to be something wrong with that experiment; that can't be right.
" The scepticism was understandable.
He set out to do something which sounds totally impossible.
If you have a shot of the size of the tank of cleaning fluid that he used, and then you mention how many atoms are in one of those tanks and the fact that he extracts ten or three or four, and he counts them? Correctly? Oh yeah? Gimme a break! All in all there didn't yet seem much reason to worry about missing neutrinos.
Most physicists were sure that eventually they would just turn up.
We think if Ray improves the sensitivity of his equipment he'll find the neutrinos all right.
But in any case particle physicists had plenty of other things to celebrate.
By the mid-1970s it looked like they finally had the complete recipe for the universe: the Standard Model of particle physics, a single theory that brought together all their discoveries.
It said that everything that exists was made from just twelve basic ingredients, among them neutrinos.
But there was more than one kind.
Neutrinos came in different flavours.
I actually have a neutrino, here we go! The problem is that when you look at one very carefully sometimes it appears that there's two! The Sun produced just one kind of neutrino, electron neutrinos, the only flavour Ray could detect.
But there were also muon neutrinos and tau neutrinos.
There are three neutrinos, can you give me your hand again? Neutrinos had bizarre properties.
Not only did they have no electric charge, according to the Standard Model they had no mass either, which meant they could flit invisibly through the universe at the speed of light.
The Standard Model was a tremendous advance, no matter what experiments scientists performed the Standard Model correctly predicted the answer.
Except that is for the missing neutrinos.
Except they're very slippery and very difficult to….
All through the 80s Ray continued to improve his detector, and year after year the results were the same.
He could only find one third of the neutrinos John Bahcall had predicted.
He became sure there was nothing wrong with the experiment.
We've lived with it a long time and thought of all possible tests, and we feel that our result is valid, and we realise it's, as John Bahcall calls it, a socially unacceptable result.
But if Ray was right about how many neutrinos were coming from the Sun then it seemed there must be something wrong with John's prediction.
The focus of scientific scepticism shifted.
Almost every theoretical physicist believes that we astrophysicists have just messed it up and it's our fault and we never understood what was happening in the centre of the Sun no matter how much we pretended to do so.
Perhaps the Sun's core was cooler than John thought.
Or maybe it was simply that the nuclear reactions there were starting to shut down.
That would explain why there were so few neutrinos.
It would also mean the earth was facing an icy doom.
I think it caught people's imagination.
If there's something wrong with what's going on in the Sun certainly a lot of people care about that.
But John Bahcall was confident there was nothing wrong with the Sun.
Despite a barrage of criticism he continued to insist that it must be producing far more neutrinos than Ray was detecting.
And, it didn't matter how convinced I was that they were wrong, every year for thirty years I had to demonstrate scientifically that yes the expectation for the Sun was robust, and therefore you should take the discrepancy seriously.
It became more and more puzzling.
Nobody could see what was wrong with John's theory or find fault with Ray's experiment.
This is the one I love, this is you swimming.
But at least they finally had everyone's attention.
Their neutrino anomaly had become the biggest mystery in particle physics.
Everything indicates that this apparatus is accurate and can tell us how many neutrinos are coming from the Sun.
But observation and theory don't agree.
There simply aren't enough neutrinos, and that's causing a great many raised eyebrows.
I think we need a new experiment to decide who's right and who's wrong.
In Kamioka, Japan, they had another experiment.
But it wasn't to study the Sun.
It wasn't even designed to look at neutrinos; and at first it only seemed to deepen the mystery.
In 1983 the Japanese started looking for a rare kind of nuclear decay.
They had built an experiment called Kamiokande, deep inside a mountain to shield it from radiation from space.
But there was one thing the mountain couldn't shield them from: neutrinos.
The problem was not neutrinos from the Sun, but electron and muon neutrinos produced in the upper atmosphere when cosmic rays from space collided with air molecules.
We have a lot of particles coming from the universe.
They are called cosmic rays; those cosmic rays hit the earth then produce many particles, including neutrinos.
These atmospheric neutrinos were a nuisance, easily confused with the thing they were really looking for.
But then they noticed something strange about the atmospheric neutrinos.
They found that the atmospheric neutrino is not coming as we expected.
Surprisingly we found that neutrinos coming from atmosphere is smaller than the expectation.
And we called it atmospheric neutrino anomaly.
In other words, just like Davis and Bahcall, the Kamiokande scientists found that neutrinos which should be there were going missing.
It had always seemed that the solar neutrino problem was something to do with the Sun, but now for the first time some began to wonder whether the real problem might be the neutrinos themselves.
Physicists went back to basics.
They knew there were three different types of neutrinos but that none of the experiments could detect all three.
Could this be the key to the problem? It's very suggestive of course that Ray's experiment sees a third of what John thought it should, and there are three flavours of neutrinos; and so it's not a great leap of the imagination that those two numbers might be connected.
But the connection was not obvious.
True, Davis could only detect one of the three neutrino flavours, but that was the only flavour the Sun could create.
However, that was not all there was to it.
There was a theoretical proposal that neutrinos might change from one type of flavour into another type of flavour.
This is called neutrino oscillations.
You emit the neutrino as one particular flavour but later on when you detect it, it might be another.
In this theory neutrinos would be continuously changing from type to type as they travelled through space.
What started as an electron neutrino would later look like a muon neutrino, still later a tau neutrino and then an electron neutrino again.
And in fact it can change back and forth, and back and forth, and back and forth, and that's why it's called a neutrino oscillation; and this is sort of like a pendulum.
Was this why Ray saw only a third of the neutrinos John said the Sun was making? In the time it took them to travel from the Sun's core to the Earth had electron neutrinos oscillated into muon and tau flavours his experiment couldn't detect? That would explain everything.
There was just one problem.
The problem with that is that in the Standard Model neutrinos are massless, and massless neutrinos can't do this, they can't change from one type of neutrino to the other.
It was all to do with time.
For anything to change time must pass.
But the Standard Model said the neutrino was a massless particle travelling at the speed of light.
And according to Einstein if you're travelling at the speed of light there is no time, and therefore no change.
When a particle moves fast its clocks, its internal timing mechanism, slow down.
And as it approaches the speed of light the clock slows down until it's not moving at all.
A particle which is massless is moving at the speed of light so it has no sense of what time it is.
Without mass a neutrino would be frozen in time, travelling at the speed of light but unable to change.
Neutrino oscillation is a time - dependent phenomena; it requires a neutrino clock.
That requires the neutrino to travel slower than light; that requires that the neutrino have a mass.
And so as an explanation for the Davis experiment it's not very attractive.
Because if you don't believe neutrinos have mass then they can't oscillate.
And you know whether there's a factor of three here or three there it doesn't matter, it can't be the explanation.
But then scientists made a discovery that completely transformed all their ideas about the neutrino.
Back in Japan they had completed a vastly scaled-up version of the Kamiokande experiment: Super Kamiokande.
Well this is really a marvellous opportunity.
So I decided that we go ahead, we change our detector, improve it to make our detector really capable of new type of neutrino observation.
Super Kamiokande was truly colossal: a forty-meter-high tank holding fifty thousand tonnes of ultra - pure water, surrounded by eleven thousand photomultiplier tubes.
It could still only detect electron and muon neutrinos, but because Super Kamiokande was so big it could tell what direction the neutrinos were coming from.
A neutrino comes in to your detector and produces a charged particle and the direction that the charged particle goes pretty much matches the initial direction of the neutrino.
So by reconstructing the track of the charged particle you could tell where the neutrino came from.
So you can make a plot and you could say how many neutrinos do I have coming from there, from there, from there, from there.
You make a plot on the sky.
But when they plotted where the neutrinos were coming from the Kamioka team made an astonishing discovery.
Neutrinos are produced in the atmosphere all round the Earth, not just above our heads but also 13000 kilometres beneath our feet, on the other side of the world.
Because the Earth is essentially transparent to neutrinos the Kamioka detector should have seen equal numbers of neutrinos coming from all directions.
But that's not what they found.
Neutrino flux coming from above and coming from below should be the same, but what we have observed was that neutrinos coming from below is about half of that coming from above.
The number of neutrinos that are coming down and going through a small distance in getting to us is about what you'd expect.
But the number of neutrinos that are coming up through the Earth, which are going through tens of thousands of kilometres, there are fewer of them than you would expect.
The difference could only be the time it took the atmospheric neutrinos to reach Kamiokande.
Contrary to all theory neutrinos did have a sense of time.
Just that fact, just that fact that the neutrinos coming down from above still get here but the neutrinos coming up from below don't, tells you that neutrinos have mass.
Because they tell you a neutrino knows how far it's gone.
And the only way it can know how far it's gone is if its clock isn't stopped, which means it can't be travelling at the speed of light, which means it must have a mass.
It was a bombshell.
Scientists suddenly realised that the Standard Model had got neutrinos completely wrong.
They did have mass.
They could change flavour.
So had the missing solar neutrinos been there all along, just changed in to flavours Ray's experiment could not see? There was only one way to know for sure.
All eyes turned to a nickel mine in Sudbury, Ontario, Canada.
Here two kilometres below ground a team of British, Canadian and American scientists were building a new kind of neutrino detector, one sensitive to all three flavours.
It was the deepest such experiment ever built, and it also had to be the cleanest, because the confusing background radiation came not just from space but from the very rocks themselves.
If we got an amount of dust like that into our detector it would just ruin it, it would destroy its sensitivity to the neutrinos by blocking them out with other signals.
And so we have to build the detector with fantastic levels of cleanliness, we have to just get rid of all of this stuff.
All these precautions made the Sudbury Neutrino Observatory, SNO for short, probably the least radioactive place in the universe.
When the SNO detector was finished, the exact centre of the SNO detector had the lowest level of radiation of any point in the Solar System.
After nine years' construction SNO started taking data in November 1999, looking for proof that neutrinos could change flavour.
When I joined the experiment I was betting there was no neutrino oscillations, it just seemed too bizarre.
At the heart of the detector was an acrylic sphere containing one thousand tonnes of heavy water, a substance which neutrinos could interact with in two distinct ways.
One reaction was sensitive only to electron neutrinos.
But there's a different reaction which doesn't care what kind of neutrino it is, so it allows you to see all the neutrinos.
Now measuring that reaction allows you to check John directly.
If you see the number of neutrinos that John predicts then he really does know how the Sun works.
For forty years John Bahcall's predictions of the number of neutrinos coming from the Sun had flown in the face of his friend's experiment.
Was that now, at last, about to change? Over nineteen months ten billion trillion neutrinos passed silently through the SNO detector.
Just two thousand of them reacted with the heavy water.
Almost every theoretical physicist believes that we astrophysicists have just messed it up.
There's no other really likely explanation than, than one of those two guys was wrong.
There simply aren't enough neutrinos, and that's causing a great many raised eyebrows.
socially unacceptable result.
In June 2001 the SNO team announced their estimate of the total neutrino flux from the Sun, taking for the first time all three flavours of neutrino into account.
It was almost too good to be true! The Sun works as we expect which is good, and that there is this funny business that neutrinos coming from the Sun that arrive at the Earth are not all electron neutrinos, that they have somehow changed in their nature.
For decades the question had been who was right, Ray or John? The answer was they were both right.
It was the Standard Model that was wrong.
I was called right after the announcement was made by someone from the New York Times and asked how I felt.
And without thinking I said, "I feel like dancing, I'm so happy!" And the one thing that my kids kept sending each other e-mails about all week was, "Did you see where it said in the New York Times, that dad felt like dancing!" They, they kept making fun of me about that, but I was deliriously happy.
It was, you know, it was like for three decades people had been pointing at this guy and saying this is the guy that wrongly calculated the flux of neutrinos from the Sun.
And suddenly that wasn't so, and it was like a person who had been sentenced for some heinous crime and then a DNA test is made and it is found that he isn't guilty.
And that's exactly the way I felt.
By revealing the flaws in the Standard Model, neutrino oscillation has opened up a new world of physics.
It turns out that this discovery that neutrinos have mass could have amazing consequences for the universe.
There is roughly a billion of them for every proton.
So even if you give them a very tiny mass their mass may dominate the mass of everything that we see.
All the stars and the planets and the dust and everything may have less mass than the neutrinos.
But in fact it could be even more fundamental than that.
Today the neutrino is carrying scientists towards new theories that may answer profound questions the Standard Model could never address.
In the Big Bang we would have made huge numbers of neutrinos.
And if neutrinos have mass it is possible that the matter in the universe today arose because of the decay of massive neutrinos created in the early universe.
So we may be the grandchildren of neutrinos: all the matter that makes us up may have arisen purely through the decay of neutrinos.
So in a bizarre way it may be that neutrinos tell us why we exist.
Their hunt for the most elusive thing in the universe may have brought scientists to the verge of uncovering the origin of everything around us.
And it all began forty years ago with Ray Davis's pioneering underground experiment.
Ray Davis is a hero to everybody in this field.
This was really the first time somebody really seriously tried to measure such an impossible thing coming from the Sun.
Ray Davis's persistence in the face of seemingly wrong experimental results, contradictions between not only his experiment and theory but also his experiment and other experiments, and he stuck to his guns, and he was right! Ray continued to work on his experiment well into his eighties, until he was forced to stop by the onset of Alzheimer's disease.
Then one day in October 2002 Anna Davis got an early morning phone call.
My nephew who works for Minnesota Public Radio called us at six o'clock in the morning and said, "Congratulations!" And I said, "For what?" He said, "Don't you know? Ray got a Nobel Prize in physics!" We gathered all of our five children, their spouses and our eleven grandchildren, flew them all over to Stockholm and everybody had a wonderful time for eight days.
The entourage of Davises was twenty - three people including the Nobel Prize winner himself.
So it was, it was really special.
Ray Davis shared the Nobel Prize with the scientist behind the Kamioka experiments, Masatoshi Koshiba.
I was happy.
I was happy.
That's all! The awards were a tribute to all the scientists whose work over forty years had gradually uncovered the true nature of the neutrino.
In the end to have put that much of your life into something and have it work, and not just work but to work so beautifully, that is just the most tremendous feeling a scientist can have.
And then it just hits you what you've done, that you've actually learned something about the universe that nobody ever knew before, and now you get to tell them! We are descended from neutrinos? Yeah? We are descended from neutrinos, what a kick! It's true.
We think.