The Unknown Story Behind One of the Most Important Discoveries in Physics – with Jim Al-Khalili（原声文稿）

It's estimated that tens of billions of dollars have been spent on quantum chromodynamics.

It's one of the largest scientific endeavours the human race has ever conducted, and it all started with an unpopular experiment and a stubborn physicist.

Quantum Chromodynamics, or QCD for short, is our theory of the strong force, the force ultimately responsible for holding atomic nuclei together.

The discovery of QCD was one of the biggest breakthroughs in all of science.

This is its story, told for the first time by some of the people who were actually there.

The basic idea is to go where no one has been before.

In retrospect of course it's crazy we were all This is the remarkable story of how we actually discovered quarks.

Our story starts in the mid 1960s.

Quantum mechanics, born some 40 years before, was the leading theory to describe atoms.

It described them as a cloud of electrons around the nucleus, made up of protons and neutrons.

The theory had been successful in describing the electromagnetic force, what electrons do, the interactions of light with matter, the atomic nucleus had given up its secrets.

It was made up of protons and neutrons and that these were structuralist elementary particles that stuck together by the strong nuclear force.

It was thought that the proton had already given up most of its secrets. It was merely a tiny, positively charged blob.

One physicist, Howard Giorgi, said it was really an extraordinarily boring time in particle physics.

So, perhaps not the ideal time to build the most expensive and powerful electron gun the world had ever seen. But it happened.

In 1962, against much advice, Stanford University started to build the largest electron accelerator in the world.

The Stanford Linear Accelerator, or Slack.

This is a project of massive proportions and unprecedented accuracies.

Two -and -a -half million cubic yards of earth have been moved.

One -hundred cubic yards of concrete have have been poured.

Ten thousand tons of steel bars are reinforcing the two -mile -long structure, which will contain the basic component of the accelerator, a tube of pure copper, four inches in diameter, ten thousand feet long, and straight to within the thickness of a dime.

It will permit man's most refined exploration of the atomic nucleus.

It cost well over a hundred million dollars, about a billion dollars in today's money.

And it was so crazily ambitious that it was nicknamed Project M. M for monster.

Electrons were to be accelerated to near light speed and a hundred thousandth of a second later crashed into a proton target in an immense room called End Station A.

The aim was to map the proton in ever more detail but quite honestly few were expecting it to find anything too earth shattering.

Back in 2006 I made a film about Slack for the BBC.

I remember going there and and basically standing on a hill looking down at the linear accelerator it's it really is quite an incredible sight.

This is the Stanford linear accelerator a few miles south of San Francisco.

What you can see below me is one end of what is basically a giant electron gun.

A beam of high -energy electrons is fired through a tunnel that starts off over two miles away in the hills, travels underneath the freeway and comes out at this end where it enters the experimental area.

The gray building over there is called End Station A and it's where one of the most important discoveries in physics was made.

Marty Bridenbach was part of the experimental team during the 1960s.

The End Station was, on first impression, immense.

It was a shielded building with, oh, I think it was not quite two -meter thick walls and meter thick roof, iron blocks.

The building was, I don't remember, maybe 20 by 40 meters, something like that.

And roughly 10 meters in was a pivot.

And on this pivot, three spectrometers rotated.

They were driven by electric motors and you could rotate them from control room so -called counting house.

But what was it actually for?

One prominent physicist, Maurice Goldhaber, suggested that electrons were thoroughly understood and could never produce new physics.

And everybody I think expected nothing.

Years of experiments in the 1950s and 60s had suggested that when electrons interact with a proton they simply ricochet off.

That's what's called elastic scattering. The proton remains internally unchanged.

The reason physicists thought that protons were dull was because experiments like this had been done to death in the decades before slack was built and the results were well predictable.

Then in the early 60s as accelerators gave the electrons more energy, they started to see a second kind of response.

At very specific electron energies, the proton would suddenly absorb some of this energy and briefly jump up to a higher internal energy state itself.

It's a bit like hitting a bell.

This is resonance. And because the energy of the proton has been altered, we call it inelastic scattering.

Before Slack was even built, this was already well understood.

Enter Wolfgang Ponowski, the director of Slack.

He was a short, energetic man and impervious to criticism.

He was, from the moment I met him, kind of amazing.

Despite the prevailing mood, he felt he had a duty to continue to explore the proton with ever more powerful electrons.

The basic idea is to go where no one has been before. And that's what he could do.

He could explore the proton, start simple, explore the proton by hitting it with electrons, and doing it with energies and energy losses that were immense compared to anything that had been done before.

I would say it's the totally obvious thing to do.

I like to think that Pernoski had this weird intuition that he felt strongly that something strange was going on.

We're probably never going to know the truth, but the fact is by 1968 the first results were coming back from the slack experiments.

Originally what it showed at the beginning was that lower energies the electrons were bouncing elastically off the protons.

They were conserving energy this is what people had seen before it wasn't surprising.

Well the first the first experiment I think that really ran was the elastic scattering and that was pretty much expected.

The collaboration at that point was Slack, MIT and Caltech.

Caltech decided that the deep in elastic was gonna be boring.

And so they withdrew and went off to work on the Trinos at Fermilab.

But as they ramped up the energy of the electrons, at some point the elastic scattering became inelastic scattering because the electrons were losing energy to the protons, the protons were being excited to these resonance states but at even higher energies, energies that no one had ever achieved before, where the electron beam was getting closer and closer to the speed of light, they started to see something very different.

It was still inelastic scattering but now there were no longer seeing resonances.

The scientists at Slack presented these early findings as a graph.

On the y -axis is the number of electrons detected at any given angle and the x -axis is the energy transferred from the electron to the proton.

The first peak shows low energy elastic scattering, electrons just bouncing off the proton.

Then as energy increases more peaks.

We're entering the inelastic phase and these are resonances, excited states of the proton, each revealing a very specific amount of energy transferred from the electron.

And at the high energies there was something new.

The electron was hitting the proton with so much energy that it was clearly being destroyed.

And yet the graph shows a long, shallow tail.

That implied that the electrons were rebounding, bouncing back. But off what? It just didn't make any sense.

Working at Slack in the mid -60s was physicist James Bjorken, also known as B .J. way.

He had a reputation of seeing what no one else saw, a way to cut through the complication and find the simple answer.

When he heard about the new, weird kind of inelastic scattering results the experimentalist had begun to see, he became obsessed.

I got hooked on the problem, he said, claiming he didn't look up for two years.

Bjorkens genius was that he came up with a very clever idea that gave insights into what was going on inside protons.

He'd been using an obscure and highly mathematical framework called current algebra.

And it gave him a way of thinking about the high energy electron proton collisions.

But his mathematics was, to almost everyone else, impossible to understand.

Look, I mean, I was a young experimentalist, definitely not a theorist, and I never understood current algebra.

And to this day, I don't.

Current algebra was very, was extremely mathematical.

If there was any kind of mechanical way of understanding what he was talking about it, he never explained it to us.

Current algebra led Bjorkin to a powerful new idea – scaling.

To have a sense of what he was getting at, look at these three oranges.

If we increase the magnification, you can now see a single orange. Increase it again and we see the rough texture.

And again and we see the details of pits on the skin.

So, normally greater magnification reveals new layers of detail, but the behaviour of the electrons at slack was very different.

When Bjorkin expressed the experimental results through his current algebra, he saw that as the electrons probed the proton at ever higher resolution, the overall pattern of scattering barely changed.

This is scaling, and it's what you'd expect if the electron was scattering from a point -like constituent.

Because a point is always a point no matter how much you magnify. And this was Bjorken's realisation.

The electrons bouncing back, plus the scaling, all inexorably led him to one remarkable conclusion.

Inside the proton were seemingly free -moving point -like constituents.

And in a moment of true genius, he realised that his mind -boggling maths might also show him a way to actually test this.

What Bjorken asked the experimentalists at Slap2Do was to run their deep inelastic scattering experiments again and again for a range of different electron energies.

He would calculate what was called the structure function. That would produce some curve.

Then they'd compare the different structure function curves for the different electron energies.

If the curves now aligned, then there was scaling.

And scaling meant point -like particles.

By 67, the experimentalists were able to increase the energy of the electrons up to 20 GeV, over twice the energy of anything else around at the time.

The electrons were now travelling at near light speed before they smashed into the liquid hydrogen at the target.

Detectors were set up to measure the electrons at various angles. Then Bjorken's structure functions could be calculated. The scene was set.

This is the famous plot from the Deep and Elastic experiments at Slack.

Now it's very busy, there's a lot going on, so I just want to focus on the essentials.

Ignore all this stuff here underneath the curves.

It's a graph of the structure function.

The important thing is that all these blobs here, all these dots are data points, measurements from the experiment.

What they found was that regardless of what the energy of the electron beam was, all these dots, all these experimental data points tended to fall together along this straight line at the top.

What it meant was that Bjorken's assumption was correct, that the proton really did have internal structure.

So what could all this mean?

Are there really point particles inside the proton?

Point around freely within the proton and yet they seem to be stuck together so strongly that for all intents and purposes from the outside the proton appears like a structuralist blob.

It all seemed wildly improbable and many of the slack scientists were not sure whether to just bury their findings.

But then they got some encouragement from an unexpected quarter.

A few hundred miles down the road in LA, at Kel Tech, a couple of physics geniuses were thinking in the same way, but for a totally different reason.

They were Murray Galman and Richard Feynman, two of the most interesting and innovative theoretical physicists of the 20th century.

In 2005, I went to see where they'd worked.

This office, number 456, used to belong to Feynman, but what's great is that just two doors along the corridor was the office of Murray Galman.

There was an intense academic rivalry between these two intellectual giants, but they fed off each other's creativity.

There were very different personalities, Feynman who played the buffoon Galman, the cultured elitist.

Galman used to get upset by Feynman's loud voice and Feynman certainly enjoyed winding him up.

But it's fair to say that during the 1960s and 70s these two geniuses here at Caltech dominated the world of particle physics.

In the mid -60s Galman had started to develop a new idea and later and independently Feynman came up with something similar.

They were beginning to think that there was actually a deep simplicity at work in particle physics, that the fundamental particles, protons and neutrons, might not be so fundamental after all.

Right, I don't want to go too deeply into the physics behind these ideas because it's so thrilling that I want to keep it for another film.

But the takeaway message is that both Feynman and Gelman had imagined that there were these point particles inside protons, Feynman called them partons, but Gell -Mann called them quarks.

At this stage, both Gell -Mann and Feynman thought their quarks and partons were probably just a clever mathematical explanation of how protons and neutrons might relate to each other.

But suddenly the new results from slack seem to be suggesting something much more interesting.

Perhaps quarks or partons might be more than that.

The story goes that Feynman caught wind of what was being found up in Slack and he came to visit in autumn 1968.

And he came to visit us at Slack at the county room and he had partons all worked out.

There was what we called the Feynman effect.

You would go to a lecture by Feynman about whatever Feynman wanted to talk about and it seemed totally clear.

It was wonderful. It was so good at explaining it. You understood it perfectly.

And then after it was over, you were talking to a colleague about what a wonderful talk this was from Feynman and you would realize you didn't understand.

Apparently Feynman arrived at Slack and they showed him these deep inelastic scattering experiments. He also found out about Bjorken scaling.

So he met with Bjorken to discuss them.

Bjorken was rather nervous, he was a bit ambiguous.

He didn't want to commit to what his results were actually suggesting.

Feynman, of course, this larger -than -life character was much more bullish about it.

He said, look, what you found are my partons. You found structure inside the proton.

It was because of Feynman's personality that other physicists decided, well, okay, it must be true. If Feynman says it's true, it must be.

That was the turning point.

That was when I think people were convinced that protons were made up of these smaller elementary particles inside.

In September 1968, the director of Slack, Wolfgang Panofsky, made the first tentative announcement at a conference at Vienna about the results.

Even now, he was super cautious and slightly buried the announcement in classic word salad.

He acknowledged the possibility that these data might give evidence on the behaviour of point -like charge structure within the nucleon.

But even then, most scientists were still unconvinced.

Eventually it came to mean, particularly after understanding partons, it came to mean point like structure.

But we were very, very careful about that.

The early papers, my thesis, don't mention point -like anything.

It was Panofsky, I think, in that Vienna meeting, who simply said consistent with a point -like structure.

And that was the most daring anyone had been.

We were, Or maybe it was like a different universe of scientists then, I mean today if someone had data like that they would be screaming point -like structure to the times on day one.

And it was, no, what I learned is you were very, very conservative and you were super careful not to say anything wrong, that might be wrong.

It's worth pausing for a moment to flesh out Feynman and Gell -Mann's basic idea.

It's that combinations of partons, or quarks as they became more universally called, would make up all nuclear particles.

But to do that, they imagined that they came in types, or flavours, originally up, down and strange.

The theory explained the electrical properties of protons and neutrons too.

It meant quarks had to have a fractional charge.

charge. Up was plus two -thirds, down and strange were minus one -third.

The proton and neutron were both combinations of three quarks.

The proton is up, up, down, total charge plus one.

The neutron is up, down, down, total charge zero.

Other particles had other combinations.

A pion plus is up and anti -down.

The lambda -zero particle is up, down and strange.

But although quarks seem to now make some sense of the proton, there was a problem.

You see, up to now, physicists had assumed that the nucleus was held together by what they called the strong nuclear force.

It gripped the positively charged protons and the neutral neutrons into a tight ball.

But now this picture was looking like a massive oversimplification.

If protons and neutrons were now made of point -like quarks, it meant that this strong interaction would have to operate in a very peculiar way.

The quarks inside the proton appeared like point particles to the electrons that were scattering off of them. They were able to move around freely inside the proton.

But what was different here is that for the other forces like gravity or electromagnetism, the closer the two interacting particles got, the stronger the force became.

But inside the nucleus, the strong interaction became much weaker.

And thinking in terms of quarks actually suggested this force was unique.

incredibly powerful at a distance and incredibly weak close -up many physicists including gal man were suggesting that the quarks had another property too a property they called color we can think of color for quarks a bit like electric charge is for electrons so some very often it's called color charge it shouldn't be taken literally it's not like they really have different colors as we would understand them.

And the color charge is what holds binds quarks together.

To understand the strong interaction and color charge, we have to first get to grips with another idea from quantum mechanics.

That the force between electrically charged particles, the electromagnetic force, can be thought of as an exchange of a force carrying particle, the photon.

So just as two electrons interacting will exchange photons, the carrier of the electromagnetic force, so two quarks will exchange the force carrier of the strong force.

And the carrier of the new strong interaction was dubbed the gluon.

So when one quark interacted with another, it be described as exchanging gluons.

And here's where this idea really landed.

Because it suggested that the strong interaction held quarks of different colours together to make protons and neutrons always colour neutral, or white.

It was a nifty idea, but it still wasn't adding up.

How could the strong force both grip the proton tightly together and allow the quarks to appear as free single particles inside it.

At first, most physicists would not even consider accepting quark theory until this problem was resolved.

Then, in 1973, physicists David Gross, Frank Vilcek and David Pollitzer published solutions.

They all saw something very surprising in how the strong interaction and its force -carrying particle, the gluon, actually ended up working for a single bare quark.

To explain I want to go back to the electromagnetic force and electrons and photons.

When a negatively charged electron interacts with a positively charged proton is through the electromagnetic force.

In the simplest quantum description we can represent this as the electron and the proton exchanging a virtual photon, carrying energy and momentum between them.

Quantum mechanics also suggests that this virtual photon can fluctuate to produce a virtual electron -positron pair which then flip back into the virtual photon.

Now imagine a bare electron.

Obviously because it's charged it generates an electromagnetic field around it.

The theory now suggests that field is actually a restless cloud of fluctuations, virtual electrons and positrons appearing and disappearing.

But then the bare electron will interact with these virtual particles around it.

It slightly attracts the positively charged virtual positrons and repels the negatively charged virtual electrons.

And the result is that it fundamentally changes the way the electric field is shaped.

light. The virtual particles shield the charge on the bare electron.

Close to the electron the force appears strong, but further away the virtual positrons dilute its strength.

But what Gross, Vilcek and Politz have realized was that quarks and gluons were not the same thing as electrons and photons.

Like photons, we can represent the interaction between quarks as an exchange of virtual gluons.

And this field can fluctuate, producing a virtual quark -anti -quark pair.

Again, imagine a bear quark.

We can now think of it surrounded by this cloud of fluctuations, virtual quarks flipping in and out of existence.

And this will produce some of screening. But there's a crucial difference.

You see, photons are electrically neutral, but gluons themselves have colour charge.

So they interact with themselves as well as the quarks, producing further virtual gluons too.

So the cloud around the bear quark is actually a complex mass of virtual quarks and virtual gluons.

So although the virtual quarks help screen the color charge of the bear quark, the virtual gluons do the exact opposite.

They make the effective color charge appear much stronger from further away, and much weaker close up.

So instead of screening the color charge around the quarks, it amplifies it.

So at the distances probed by Slak's electrons, the quarks interacted with each other so weakly they seemed like free point -like particles.

Gross Vilcek and Politzer called this asymptotic freedom and it's one of the great insights of quantum chromodynamics and it finally makes some sense of what Bjorken had discovered back in 69.

Slack's super -high -energy electrons probed the proton at a scale so small that the strong interaction had virtually no influence on the quarks.

And that's why they could appear to be free, point -like particles.

But the reality was that even in the mid -1970s, most in the scientific community still thought quarks were just a mathematical device.

But more clues were stacking up.

In 1974, teams at both Slack and Brookhaven National Laboratory published new evidence for a fourth quark called Charm.

And so, yeah, we started this going and then I guess it was, I don't know, a Thursday night or something. I went home to get some sleep at two in the morning.

My colleague and friend, And Jack Morehouse calls me and says, we got it.

What was needed now, though, was a genuine smoking gun.

Remember, the theory insisted that because of colour charge, quarks would never be seen individually.

So, experimentalists started to focus their attention on the carrier of the strong interaction, the gluon.

Paul Soading was a senior scientist at Germany's largest accelerator in Hamburg.

But I remember that the word Gruon did not appear at all, and the word quark did only very cautiously mentioned, because one feared that many people would say, oh, they base their arguments on quarks, they don't exist, we can't give money for that. So we had to be careful.

So quarks and gluons, even in 1976, were treated.

Gluons not at all, and quarks with care. And yeah, it was funny.

In retrospect, of course, it's crazy we were all blind, but so it was at the time.

This is the accelerator Soding used in the search for gluons called Petra. It was completed in 1978.

It was a very different beast to slack.

Instead of firing electrons at a stationary target, it accelerated them and their antiparticles positrons in opposite directions down this tube around a 2 .3km ring.

Then they would smash together and annihilate, creating spectacularly enormous energies.

This would produce quark -anti -quark pairs flying apart in opposite directions.

Now, quarks can't emerge alone, so as they fly apart, the energy in the colour field between them grows.

Enough to form a new quark -anti -quark pair from the vacuum.

These now combine to create colourless particles.

particles, red and anti -red makes a colourless meson, or with other quarks to make a barion.

Either way, a spray or jet of particles emerges.

By 1979, this huge building housed a detector called TASO.

Unlike Slack, TASO's advanced detectors could identify the patterns of particles produced.

Usually they'd see two opposing jets, one from the quark and one from the anti -quark.

But QCD predicted that occasionally a quark or anti -quark would radiate a gluon before it transformed into a colourless particle, and that gluon would produce a jet of its own.

So instead of two jets, they might occasionally C3. And that could be the smoking gun.

So it was clear there had to be something like gluons. Call them gluons or call them others.

There were other theories that called them by different names, but something binding the constituents of the nucleons should exist. That was, I think, common sense.

And Petra had another weapon too, a gifted scientist called Cao Lan Wu.

She thought about how to make it visible if there are three jets.

For example, one property of the three jets would be that they lie in one plane.

And she cleverly made an algorithm to find this plane.

It took them a few weeks, but then all the other groups had similar displays, but we were simply the first, and I think that was mainly her foresight that we could be the first with this.

With Petra's unrivaled power and Wu's new detection algorithms, the scene was finally set for the most important discovery of the decade.

Finally, in June 1979, the Petroscientists saw this.

They were expecting to see two jets coming out of the electron -positron annihilation, one from the quark and one from the anti -quark.

Instead, they saw three jets. They also saw this one.

So along with the quark and anti -quark what was also being produced was a gluon, proof that gluons were real, that quantum chromodynamics reigned supreme.

So then in I think it was August 79 there was a big conference in Chicago and all the groups by that time had evidence and it It was a big show and the gluon was declared to have been discovered.

Nobody I think reasonably could doubt anymore at that time.

So I think it was an exciting time.

Since then QCD has become more complete.

In the years following we discovered there are in fact six flavours of quarks.

quarks, up, down, strange, charm, and top and bottom.

Combinations of quarks make protons and neutrons, as well as a huge family of other particles.

One of the most impressive successes of QCD was the calculation of the mass of the proton.

Because the bare mass of the three so -called valence quarks, up, up, down, only adds up to 1 % of the measured amount.

So where does the rest of the mass come from?

It seems the strong interaction contains them in such a small space that another feature of quantum mechanics plays a role, namely the uncertainty principle.

It says that if particles are restricted to well -defined positions, then the uncertainty in their momentum, skyrockets.

And we can think of this high momentum as energy.

Add to that, the energy stored in the gluon fields that bind the quarks together, as well as the sea of virtual quark -anti -quark pairs.

And as energy and mass are equivalent, so all this energy contributes to the mass of the proton.

In 2008, a detailed supercomputer calculation of QCD suggested that the mass of the proton was extraordinarily close to its experimentally measured value within just a few percent.

A remarkable confirmation of quantum chromodynamics. And it leaves us with a rather profound thought.

When I weigh myself, what QCD tells us is that only a tiny fraction of the mass of the protons and neutrons in my body is actually quarks.

quarks. The rest of me is energy, the trapped restless energy of quarks and gluons.

The Large Hadron Collider at CERN has been essentially filling in the experimental blanks of quantum chromodynamics since its discovery in the mid 1970s and the field is as busy now as it's ever been.

CERN, Fermilab, DESI, Slack, Brookhaven and others are all using their largest accelerators, all building newer, more powerful ones, to continue to explore the physics of quarks and gluons.

Quantum chromodynamics has set the agenda for almost all particle physics research since 1969, when Bjorken's clever mathematics and Slack's Project M, their monster accelerator, both showed that protons are made of something more fundamental inside.