Pauli's Exclusion Principle (Archive Episode)

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And now, to mark the end of his 27 memorable years presenting in our time, we have Melvin Bragg to introduce the next in our series of his most cherished episodes.

I've always been fascinated by the vastness of space, the unimaginable distances between stars and galaxies, and how long light takes to travel across the universe.

But sometimes it's the minuteness of something that makes my jaw drop in the studio.

How did brilliant men and women imagine the arrangements of electrical charges around an atomic nucleus, in this case almost a century ago, before computers, prompting ideas that underpin quantum mechanics?

Unless this is your field, the chances are that if you ever heard of Pauli's exclusion principle, you heard it here first, one morning in April 2017.

Hello, in 1925 Wolfgang Pauli made a decisive contribution to atomic theory through his discovery of a new and fundamental law of nature, the exclusion principle, or as it became known, the Pauli principle.

It asserts that no two electrons in an atom can be at the same time in the same state or configuration.

It was groundbreaking as it explained a huge range of phenomena, from the chemical behaviour of the elements to why matter is stable and for this he won the Nobel Prize in Physics in 1945.

Pauli astonished and intrigued his peers.

He also correctly predicted the existence of the neutrino and was called the conscience of physics.

Yet he was fascinated by mysticism, alchemy and dreams, which he explored with the psychoanalyst Carl Jung.

With me to discuss Pauli and his exclusion principle are Frank Close, Fellow Emeritus at Exeter College University of Oxford, Michael Lamassimi, Professor of Philosophy of Science at the University of Edinburgh, and Graham Farmelow, Bi-Fellow of Churchill College, University of Cambridge.

Frank Close, what were the big questions about Niatom in the early part of the 20th century? Paolo was born in 1900. What were the big questions then?

Well, at the time of his birth, the end of the 19th century, they knew that matter was made of elements and that the elements were all made of atoms

and that you could order the atoms of of the different elements by mass hydrogen the lightest then helium all the way up to uranium the heaviest naturally occurring and each element had sort of unique properties and yet there were also some common features that kept appearing for example some elements are inert like neon and argon and helium other ones are very active and they noticed that when you looked at this ordering that the inert elements appeared sort of regularly and either side of them would be an element that was very active, like sodium or chlorine, for example.

And this periodic recurrence of common properties became known as the periodic table. It was an empirical rule.
It worked, but nobody knew why. So clearly something was going on.

The second thing that showed that something weird was going on was that if you heated elements, made them hot, they would emit light. But it wasn't just the light right across the rainbow.

If you passed the light through a sort of spectrograph, it would make a sort of barcode of individual colours. And these became known as spectra.
And again, why atoms are doing this, nobody knew.

The big news, which led to the breakthroughs, was that just before Pauli's birth, they discovered that atoms had got some inner structure. The electron was discovered.

The electron is negatively charged, and the atom was soon shown to consist of negatively charged electrons whirling around a central nucleus with positive charge.

And the metaphor that this gave rise to was the idea that they were like miniature planetary systems.

The problem with that is that it's impossible, or at least it was impossible according to the laws of physics that Isaac Newton had set up over 200 years before, that electrons whirling around a nucleus held together by the electrical force, not the gravitational force, would spire into the nucleus in a fraction of a second.

Basically, atoms, us, nothing would exist.

So this was clearly an impossible situation. They were self-destruct.
So that was the great paradox that had to be sorted.

We're quite near the birth of theoretical physics, which, as I understand it, happened in Germany in about the 1860s and then spread over

Europe and America from then on. Can we talk about one or two contributions? First of all, Niels Bohr, was he vital to the development of this?

Yes, I think Bohr was probably the first step in beginning to understand what was really going on inside atoms, that he had the insights that the electrons, they're they're not free to travel anywhere.

They are restricted to what he called orbits.

And he quantified this using maths. He said that

the rotary motions they whirl around, the angular momentum, can't be any old value.

It has to be an integer multiple, naught, one, two, three, four, times some fundamental quantity, which became the quantum. So electrons can't go anywhere, they have to have one of these magic values.

And this gives rise to an analogy that it was like having a ladder with rungs on.

If you hold the ladder vertically, you can be on a high rung with high energy or a low rung with low energy, but you can't be between rungs. So the electrons had to be on a rung somewhere.

And they could jump from a high rung to a low rung. And when they did, the energy that they lost was emitted as light of a characteristic colour.

And so these spectral lines of light coming from atoms was because the electrons are jumping from one rung to another. So we're beginning with an explanation.
What date are we at now?

About 1913. 1913.
Okay, let's go back to

our man, Michaela. What was Wolfgang Pauli's background?

Pauli came from an affluent family of Czech-Austrian origin and his father went to a school in Prague with the oldest son of the great physicist and philosopher Ernst Mach.

Mach was famous for writing a book, The Science of Mechanics, Mechanics, where he famously criticized Newton's absolute space and was hugely influential.

Even Einstein regarded Mach as a precursor of relativity theory. So the figure of Mach played an important role in Pauli's upbringing.

Mach moved to Vienna to become professor of philosophy and three years later Wolfgang Pauli's father moved to Vienna. He converted to Catholicism.
He had Jewish origin.

He married Berta Camila Schutz, who was a prominent Austrian woman. She wrote a book on the French Revolution, several historical essays.

And when Pauli was born, Mach was invited to become the godfather of Pauli.

So the story goes that many years later Paoli said, jokingly, that because Mach was such a great influence on him, he was baptized not so much Catholic, but anti-metaphysical,

a line of reasoning that remained for the rest of his career.

We know that the young Pauli absolutely excelled in mathematics and physics, not so much in other subjects, and at the age of 18 he went to Munich to study with the leading spectroscopist of the time, Arnold Sommerfeld.

And Arnold Sommerfeld was so impressed by the mathematical ability of the young Pauli that when Albert Einstein declined the invitation to write an encyclopedia article on relativity theory, he asked his student, his 18-year-old Pauli, whether he wanted to write the article.

And so here we have a young university student producing an incredible encyclopedia article on relativity theory.

We have to remember that the special relativity was introduced in 1905 and 1916 is general theory of relativity, so a relatively recent discovery, showing incredible skills in delving mathematical details with the theory.

And the result was published in 1921 and was welcomed as an outstanding achievement by some of the great mathematicians of the time, like Weil.

And Paoli went so beyond just writing a simple survey of the theory, he pointed out out open problems in relativity theory, such as the problem of the structure of matter, to which he himself turned very easily.

And it's still a classic, that book, isn't it? It is, yeah, it remains one of the classic articles introductory to relativity theory. It's an example of a prodigy who realises potential.
Yeah.

Well, certainly so. I mean it certainly made a big impression on everyone at the time and put him firmly on the international scene.
And so then did he move on to another teacher from there?

Where did he go from there?

So then it starts a very ectic period of the effectively early nineteen twenties where Pauli really began to work on a series of problems about the spectroscopic anomalies, of which Frank was already mentioning, and models of the atom.

So he spent a period in Copenhagen with Nils Bohr, one of the fathers of the Copenhagen interpretation of quantum mechanics. And

from there he moved on. I mean later on in nineteen twenty eight he got his first full professorship at the ETH in Zurich, which is one of the most

professional. And again, he was one of the youngest, if not the youngest, ever professor there, was he?

He was very young, so he was only 28 years old. And mind you, the story goes that the professorship was originally offered to his rival Werner Heisenberg, and Heisenberg declined.

So there was a bit of a story of rivalry between him and his contemporary Werner Heisenberg at the time.

But he's very much up and running, well known already as a young man, very, very very highly respected. And on the case of this

very exciting development of what Frank said at the beginning, people knew very little, if anything, about Monster Age, and now they're beginning to know about the whole quantum quantum field and quantum memory.

Graham Pamela, before we get to the exclusion principle, can you tell us about Pauli's idea of two-valuedness?

I was reading that carefully. Two-valuedness in electronics.
Yes, well, this was perhaps

his greatest contribution. We wind the clock back to about 1924.
He's Hamburg.

He's a night owl visiting the red light district, having lovely sex in the evening, showing up very late

in the mornings, thinking very deeply about

these spectra that Frank was talking about. These are the jumps that the electricity is.

I love the connection you've just made.

Well, it is. It is.
It is powerless.

Let me go. Anyway, all human life is here.
It It is, it is. Anyway,

these atoms were making these jumps or transitions, right? And the experimenters were looking at

these discrete frequencies of light and trying to make some sense of them.

This was a big problem.

They had what you might call a half-cock theory, which is a theory that was a part classical Newtonian, part quantum, and they were trying to understand the observations for light given out by atoms.

Now, the thing that Pauli did so brilliantly was concentrate on one particular set of problems, and that was what's called the alkali elements, lithium, sodium, potassium, and so on.

Now, the reason why these were special was that those particular elements

people had worked out consist of

shells, which you can imagine

just very crudely as a kind of sphere, like a soccer ball of electrons, with one electron on the outside, which you call a valence electron. So each of those has basically that structure.

Now if you subject those atoms to a magnetic field you can you can alter the frequency of the spectral lines and it became a puzzle to understand those observations.

Now what I just interrupt one more second. We're talking about a theoretical physicist here.

Just for the clarity of the listeners, does this mean he's doing experiments with stuff in a laboratory or does this mean he's sitting down and thinking things through?

He'd he'd very much he wouldn't be allowed near a laboratory as we'll hear later uh we'll hear it now yeah

might be time who knows what's going to happen later

okay well let me just let me just just play so he he was one of the a classic theoretical physicist in the sense that uh he was very happy to talk to experimenters but he didn't get his hands dirty in the laboratory he wanted to think his way into the heart of the atom that's what he that's what he did and he did it brilliantly okay now um

he

said that he could account for

those spectral lines that were a puzzle if,

this is the key thing, the electron didn't just have

what we call three quantum numbers that specified the state of the electron.

That was what was widely understood at the time, that you could specify the state of an electron under three quantum numbers.

But if the electron had that outer electron, the valence electron had what he called a two-valuedness, right?

Now that accounted for the spectral lines and also for the number of electrons that were in that shell. So what is this two-valuedness? Well, he didn't know.

I love it when you say things like that.

No, it's important because he was being very cautious because people were saying,

what does this mean? But he was quite cautious about it.

He wrote it in his very, very clear way that it was due to a particular non-classically describable two-valuedness of the valence electron.

In other words, he was saying that there was something doubled about that, but he wasn't prepared to say what it was.

Now, from a modern perspective, as we're going to hear,

that was a puzzle that he didn't take that extra step. But he was the person who noticed that two-valued valuedness.

Right, Frank, let's go to the exclusion principle. What was it?

Well, the

electrons are like cuckoos.

You know, put two in the same nest, and that's one too many. If you've got an electron already occupying one of these quantum states, we're talking about the atom,

we're talking about someone who can't see. Yeah,

so I just want to get back where we are, the fundamental thing. That's what we talked about.

So the electron, which is one of the fundamental constituents of all atoms, that if there's an electron already in an atom at some place, you can't put another electron in there. It's excluded.

I mean, an example is if I wrap the table.

You know, my hand doesn't pass through the table because the electrons in the outer rim of my knuckle are trying to occupy a state that's already being occupied by an electron in the wood of the table.

So it's excluded. So

that fact that electrons can't just go any place, that you have to put them in special places because occupied states are already excluded, gives rise to structure.

It gives rise to the different chemical natures of the atoms. That if you start with hydrogen, we've got a single electron on the bottom rung.

I mean the different rungs in the ladder have got different shapes, if you like. They can accommodate different amounts.
The bottom rung, the simplest one, can only occupy with two.

That was the two-valuedness that Graham was mentioning. One electron, that's hydrogen.
Two electrons, that's helium, and you fill that rung. And helium is chemically inert because the rung is full.

Now if you want to go to the next element lithium you have to go to the next rung. Lithium is very active.
The next rung's got a different shape.

It turns out you can fill that and they're eventually filled when you've got up to about 10 altogether. And there I think you're now at neon if I'm keeping track of things which again is inert.

Every time

a rung was filled you got chemical inertness. Add one or remove one, you get chemical activity.
And the filling of the rungs was because of his exclusion principle.

You can't put an electron on a rung that is already full. You can't put one in a state that's already occupied.
So what's the consequence of that?

The consequence of that is that we're having this conversation.

That

the universe isn't made of goo. I mean, electrons exist and

the forces of nature exist. And if that was the whole story,

they could just be floating around like goo, like photons, for example, of light. That doesn't have an exclusion principle.

You can add more and more photons and make laser beams as intense as you like. If electrons were like that, electrons could be flying around at random.

It's the exclusion principle which forces them to go into different places in the jigsaw and build up structures. So you get atoms and chemistry, you get solids, you get crystals.

You even, in the cosmos, the death throes of stars are involved with the exclusion principle.

As the star collapses, the constituents are trying to to squeeze in ever smaller until they can't go because they're excluded. So, Michaela, the significance of this is vast.

Absolutely.

First of all, yes, but I come back in one second. Was this recognised at the time? Did people say, woof, we've got something? Yes, the news spread very quickly.

Pauli announced the exclusion rule, and I underline, he called it a rule, he didn't call it a principle.

So in German it's a Auschlissungsregel, because at the time it was just a humble empirical rule that could account for a series of spectroscopic anomalies, as Graham said, and exactly for

some outstanding problems about the periodic table that Frank was referring to. So as far as I know, the first person that called it principle was Dirac in 1926.

And we have to bear in mind the context. Pauli announced it in a letter to Alfred Landé, who was a prominent experimental physicist in Tübingen, at the end of 1924.
The news spread.

A month later, Niels Bohr from Copenhagen sent a letter to Pauli saying, We are all very excited for the very many beautiful things you have discovered.

And I don't have to hide any criticism because you yourself, Pauli, have described the whole thing as sheer madness.

And the reality is that people really were scratching their heads about the exclusion rule and what it meant. But

the visionary insight of Pauli in 1924,

before Heisenberg matrix mechanics, before Schrödinger wave mechanics, before really the foundations of quantum mechanics were laid, was to introduce a rule that finally gave a solution to problems that had beset physicists for decades.

The problem of atomic spectra really goes back to the 19th century. So there were this anomaly, like the alkali metals, they had doublets.

Why there are doublets? So presumably there's a double energy state, but what's the origin of that double energy state?

So, by introducing what Graham was referring to as this classically non-describable two-valuedness and the exclusion rule, he could solve at once both the problem of spectroscopic anomaly, because we now need the electron spin to make sense of the spectroscopic anomaly.

He didn't call it spin, and by call it classically non-describable two-valuedness, and his exclusion rule to make sense of the periodicity in the Mendeleev's table.

The people that came after him introduced the term of electron spin. So the immediate consequence was that Pauli was visiting London, Tübingen.

There was a young PhD student from Colombia called Ralph Kronig.

And he heard the news and he approached Pauli and in the kind of classical language of vector model he said maybe we can interpret this two-valuedness in terms of a spinning top.

You can think of the electron as a spinning top that can spin clockwise or anticlockwise and that gives you the two values plus one-half and minus one-half.

And Pauli dismissed the idea as a witty nonsense. So the poor Kronig went away, never published.
And then two Dutch-American physicists, a few months later in 1925, Hullenbeck and Cowsmith,

published a paper where finally the idea of the electron spin was introduced.

So with that idea in place, the electron spin, that Pauli anticipated with the idea of the two-valuedness and the exclusion rule, all of a sudden some anomalies could be explained and the foundations of quantum mechanics could begin.

Graham, Graham Pamela, you want to come in? Just a brief comment: that this illustrates, I think, a very important part of

Paoli's character.

A brilliant deducer, very, very creative in doing this. But what he was almost as famous for as his physics was being a great critic.
And he was extremely careful all the time.

And this is why you said, what's his classical two-valuedness? Everyone now calls it spin. But he didn't take that step because he couldn't be absolutely sure.

Paul Krunig had what could have been a Nobel Prize-winning discovery basically crushed by Paoli. And he did this a lot.

He often backed the wrong horse, although he also backed right horses, but he could be wrong. And his personality sometimes upset people because he could take ideas and crush them in people's arms.

Now, earlier in this programme, you've proved to be the expert on his personality. And one

factor that might astonish most people, we've talked about theoretical physics, it's as hard as it gets and logic and so on. But he was interested in alchemy, he was interested in psychoanalysis,

and he struck up a friendship with Carl Jung.

What's in dreams he was interested in? He was fascinated by the number 137. Now, what's all that about? Well,

this is really difficult to understand because we said he's a rectilinear, brutally logical, honest thinker, very, very tough critic, and he goes into a field that some people might say was a bit flaky, right?

But he goes into it, he jumps into it with both feet. Now, this happened at a time that he called the great crisis of my life.
This was a time from 1927 when his mother killed herself.

A year after his father married a woman of around Paoli's own age. And Pauli, the next year, married a cabaret dancer.
I mean, not the wisest thing to do. He wasn't married a year.

And they were together a very little part of that. She was a very nice cabaret dancer.
Maybe she was, but

the relationship didn't work out very well and he was quickly on the bottle. And the poor guy went on a great tour of America, having to explain why

his arm was in a sling because he fell down the stairs

while violently drunk. Anyway,

Pauli needed help and his father steered him towards Carl Jung. This is what you asked about

his relationship with the great psychoanalyst. And then we had this improbable friendship and

very respectful relationship between Jung and Pauli. They first met in January 1932

so Pauli's just in this great crisis of his life and you as I said it sounds very implausible.

Jung was interested in physics, he was interested in UFOs as well, he was interested in the arts, he had very wide interests, he had dinner with Einstein a few times in 1909, 1912.

Pauli had got interested in psychology partly from his closeness to Niels Bohr, who was also

someone of very wide interests.

Now, Powley went to Young and they agreed, obviously we don't know what's going on in these sessions,

to have his dreams analysed. Now, to the best of my knowledge,

to the best of my knowledge,

they never did the Young

with Powley on the couch bit.

He referred him to one of his students. But Pauli did keep

Young briefed on the details of his dreams.

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Frank,

are these two things irreconcilable or is it just the way a man lives his life? What do you think about this? Well, to me, I think you're seeing here Pauli is being a genuine theoretical physicist.

He's asking questions and he's prepared to consider things.

And to me, I'm just making this up as I go along, but Young with his idea of the collective unconscious, the feeling that there is something going on beyond that that we are immediately aware of, is not radically different from Pauli, who is here at the birth of quantum mechanics.

And 50, 60 years later, we still use quantum mechanics without being quite comfortable as to what's going on.

Oh, your guy psychoanalysis is going to love you. You'll be the patron saint when you walk out of this studio.

You heard it here first.

I interrupted you. Can you say more? It's great.

So I've never forgotten the track that I was on.

The track you were on was it that he found he it is possible that he found a similarity between exploring the unknown that Jung was exploring and exploring the unknown that he is a physicist exploring.

He found an analogy there, a similarity or something. Yes, I mean for 50 years the mysteries of quantum mechanics have been around.

They've created all manner of humbug, but there have been very serious theorists who have investigated the question as to whether there are what's called hidden variables, that

the quantum mechanics as we currently understand it is actually a manifestation of something deeper. There are these hidden variables behind the scene.

I mean, experiment now suggests that isn't the case, but it's a very serious theoretical idea.

And qualitatively, I can see a parallel between that and the idea of the collective subconscious that Jung was interested in.

So the fact that Jung and Pauli had a lot of interesting intellectual discussion to me makes quite a lot of sense.

Michaela, let's get back on track with

this physicist. His exclusion principle gained importance in the 20s and 30s.
Who took it up and what importance did it gain? Right, so as I mentioned, really, it was introduced as a rule.

It became a principle with Dirac in 1926.

Dirac? Paul Dirac, yes, in 1926. Can you just tell people who Dirac is? So Dirac was one of the great physicists, the fathers of the quantum mechanics, together with Niels Bohr, Werner Heisenberg.

What Dirac did that was really important in 1926, He was working on a system of what we call indistinguishable particles.

Those are particles that have exactly the same properties, mass, charge, and spin. And so he was working on the mathematics, how do you describe the function for a system of many particles?

And there are two kinds of functions.

There are symmetric functions, where the state of the system remains the same if we permute, if we swap, if you like, the two particles, or anti-symmetric functions, where by permuting the two particles, the final state is different.

So what Dirac discovered, and Enrico Fermi in Italy, another great Italian physicist, discovered this independently of Dirac, is that anti-symmetric functions vanish when two electrons are in the same orbit, which is exactly Pauli's principle.

So from that point onwards, 1926,

the Pauli's rule became a Pauli principle and was reformulated in terms of what has become known as as the Fermidirac statistics. So it's a statistics in quantum mechanics that tells you

what's the behavior of many indistinguishable particles that follow the Pauli principle. It took 14 years for Pauli to prove an important theorem called the spin statistics theorem.

And what the theorem does is to show the link between the kind of spin a particle has and the kind of statistics it follows.

So the theorem says that any particle that has half integral spin, spin one-half, for example electrons, protons, neutrons, but also quarks, follow the Pauli principle or the Fermi-Dirac statistics and any integral spin particles like spin one, photons, W and Z bosons follow different kind of statistics, what has become known as the Bose-Instant statistics.

From that point onwards, the exclusion rule has become a principle, has become a cornerstone of quantum mechanics, because it governs the behavior not just of electrons, as was originally introduced in 1924, but the behavior of any half-integral spin particles that has been discovered.

So, an incredible achievement and an incredible far-reaching validity of the principle. Thank you very much.
Graham,

how did he arrive at his prediction of the existence of the neutrino, and why didn't he, as it were, claim it?

Well, this was his second piece de resistance, so to speak. We heard earlier on about how he sorted out this muddle of trying to understand the light coming out of atoms.
This was a different problem.

This was to do with the atomic nuclei, the little tiny positively charged cores of atoms identified early in the 20th century. Now

these nuclei can, in some cases,

decay randomly, and this is what we call radioactive decay. There are different types of radioactive decay.
Now there's one particular type of decay where out of this nucleus

charges a very high energy electron, just

unpredictably comes out of

the atomic nucleus. Now the problem was that First of all,

if you look at the energy of those processes, it seemed from measurements that

the total energy

before

the process was not the same as it was afterwards. Those energy appeared to go missing.

And Niels Borb, about whom we've spoken, thought that this may mean that energy conservation, which was a really sacred principle, might even be wrong.

There was something seriously wrong with what was going on in the heart of the nucleus.

There was another puzzle, too, that the electron didn't come out with one particular energy, it came out with a range of energies, right? Now, this was odd.

If there were just two particles produced, why didn't the electron come out with the same energy energy each time? Now,

Powell, this was another absolutely brilliant insight. Nobody had this insight at the same time.
I don't know of anybody else that came up with this idea. He's always just thinking this through.

This was an example, exactly, of him just thinking his way into what might happen.

And it was so bold, rather like the uncertainty, the excuse me, the exclusion principle, as we sometimes call it, which he considered not publishing. He also, with this one, did not publish it.

He wrote to a conference of physicists suggesting very tentatively that what was going on was that in addition to the electron charging out of the nucleus, there was a particle that we don't see.

Now this particle he deduced very cleverly from looking at the data would have no electrical charge, it would have the same spin as the electron and very, very little mass.

Now, this

so he suggested this particular was later called the neutrino. neutrino right

and what he in fact was suggesting that instead of there being two particles coming out there would be three one of which was a mystery so to speak and he even thought this particle would be undetectable right

many people thought that at the time they thought that the Pauli had suggested a particle that no experimenter would ever be able to see and later they did and Frank you yes I mean just to make a remark that as Graham says Niels Bohr was prepared to consider that energy wasn't being conserved in nuclear processes.

That shows how radical a problem this was. But also, you know, to modern ears, people might be thinking, well, what's the big deal about inventing a particle? Don't you invent them all the time?

Yes, today we do. But back in 1930, everything at that stage, only two particles were known, the electron and the proton.

So here was Pauli inventing a 50% increase in particles to explain one phenomenon. Yeah.
But Wigner said it was crazy to do that.

There There was something, Frank, there's something that I read in probably your nerves called the Pauli effect. What's that?

Right, well, this.

I will stop him now.

Yes, that. I've got my eye on the clock, McKenna.
We're all right. That it was advisable to exclude Pauli from your laboratory, I think, was the Pauli effect.

I mean, this is Pauli was a theoretical physicist, and there's a joke which my family can attest, actually, is probably true, that theoretical physicists have a habit of breaking things while things don't work.

And Pauli seems to have been an extreme example of this: that

if Pauli came to your lab,

things would break, even though he didn't touch them and so forth.

Is this true? I mean, you would like to mythologise our heroes. I mean, how could that happen? Well, of course, that, I'm sure, is part of the question why Pauli and Young had so many conversations.

You know, is something going on?

There's enough registered.

To be serious, Ramon, there's enough registered examples of him being a real real old jinx.

Yes?

Maybe. Right,

because of course the question is: the moment you get a small reputation, you start getting things attracted to you that may or may not be true. For example, there's the story that

at Goettingen there was somebody doing an experiment in the lab, and the experiment went wrong, and they said, Oh, it's good job that Powell isn't here.

And then apparently, it was discovered that Paoli was changing trains in the station at that time. I mean, you know, these myths, I'm sure,

developed.

Michaela, how is his exclusion principle being tested?

So, for a long time, there wasn't really a test of the exclusion principle, and some physicists complained that the lack of a test was a blank spot on the map of experimental physics.

The first idea of testing the principle came in 1948 with two physicists, Gould Aber and Scharf Goldaber. And the idea was to look for.
How did it take so long?

Well, think of it. Why do we call it principles? Principles are are foundational laws of nature and in a way they lend themselves to be tested a lot less than other kinds of laws of nature.

They play the role of cornerstones, pillars of the theory, so that it proves sometimes very difficult to actually test them.

The specific tests that people were looking for were anomalous, probably violating a transition.

So the idea is: imagine you have a copper strip and you put electricity through it. Some atoms get in an excited state

and in that state some electrons may cascade down to the lowest energy states, although that lowest energy state is already occupied by two electrons according to the Pauli's principle.

If that happens, X-rays might be emitted. So the search for what is called the K-shell X-rays became

what scientists were looking for. really from 1948 onwards.

And there have been a series of tests throughout the 1980s because people were looking for statistics different from the Fermidirax.

But the first precision test came in 1990, so very, very late, with Ramberg and Snow. And they found no evidence for Pauli violating K-shell X-rays.

So they fixed a limit of 10 to the minus 26 for possible violations.

Graham, why did it take about a quarter of a century for Pauli to be awarded the Nobel Prize, if, as we've heard in this programme, it was so very important, significant?

It is a bit of a mystery. he

he could have got uh the the prize soon after 25 because people did see it was a very clever idea

I would have said incidentally that the structure of atoms was pretty good circumstantial evidence for the principle but By 1933, the Nobel Committee was still arguing about quantum mechanics because we now think of it as the most revolutionary successful theory of the 20th century.

But in terms of direct, unequivocal confirmation, it had been

pretty thin pickings if you were setting the highest standards. But in 1933 they came they decided they had to award these prizes and

Pauli was left off and it's worth saying here that the Carlo Seen who was chairing the group that was advising the people that made the decision said that the opinion has been that Pauli's receptivity exceeds his originality which is a bit harsh incidentally it is a bit harsh and I suspect he was very hurt when he didn't get the prize and there's any little individual envies at play there possibly you know what I mean Pit Scientists are human.

1934 must have hurt even more because they didn't award a prize and said there was no one good enough. I mean, Pauli was pretty acerbic in his comments about people.

I mean, how many people he put it mildly pissed off? Well, I wouldn't have been my choice of words, Frank, but yeah, but

yes. But O'Cine,

he died in 1944, and

he barely been in the ground five minutes. Pauli got the Nobel Prize in 1945.
So it looked like O'Sine had got his card marked.

Talking about something else he didn't quite get to or he may have got to Frank how close did he get to the Higgs boson?

Well

with hindsight this is an example of one of the things that he completely missed that Graham referred to but it was hardly his fault.

After the war

quantum mechanics

was combined with relativity and applied not just to particles but to fields like the electromagnetic field and this gave rise to the the theory called quantum electrodynamics one part of which is that light consists of little bundles called photons which have no mass at all and this theory is wonderful what Pauli then did mathematically and as Graham said earlier you know he he liked to just play with the maths and see where it led him he took this theory and replaced the numbers by what we call matrices and generalized the idea to what's now called non-abelian gauge theories.

But he discovered that this wonderful mathematical idea would not work because it implied that there were analogues of the photons that carried electric charge.

Now if there were massless electrically charged analogues of photons basically we wouldn't be here. You just couldn't create stuff.
And so he dropped the idea. Now today

we know that there are analogues of these things. The W bosons, which are the transmitters of the weak force of radioactivity, are like electrically charged photons, but they're very, very massive.

Whereas Pauli's theory back in 1947 or so would have said that they had to be massless. So Pauli dropped it because he said these things don't exist.

Then Yang, a future Nobel laureate, but at that stage still, I think, a young postdoc or even student, was giving a talk and Pauli was in the audience on the very same idea.

And Pauli says, where are these massless things?

And Yang said, oh, well, we're still thinking about it. And Pauli, being very critical, was quite annoyed about this.
And basically, Yang almost had to quit on the seminar there and then.

Of course, what neither of them knew at the time was that Higgs and others years later would discover a loophole in the argument that enabled mass to work its way in behind the scenes and give mass to these particles.

So, in a sense, the basic ideas of what led to the modern theories were already there with that one missing ingredient.

Michaela,

he had a lot of great contemporaries.

Some people,

he isn't the name that pops up, is it? With Bohr and Heisenberg, of course, not Einstein and so on. How do you rate him? How is he rated at the moment?

Yeah, so this is the funny thing about Pauli, that he made an extraordinary contribution to physics, but somehow hasn't entered public discourse in a way that other physicists have.

I mean, Bohr and Heisenberg have featured in a famous theater play in a way that probably the average person doesn't know about Pauli.

And I mean, his mathematical talent was one of a kind, but so was also his really sharp, uncompromising approach, as we have already heard from Graham and Frank.

They might have played a role in his unpopularity, if you like, compared to some of his contemporary.

To me, the great legacy of Pauli is his visionary ability of realizing the limits of classical physics in dealing with quantum entities.

He was one of the few people at the time really working still within the old quantum theory, they realized the limits of applying classical models to describing quantum entities, and that's evident from his dismissal of the spinning model with Kroning.

It's evident from his dismissal of Heisenberg idea as unphilosophical and the dreadful, he called them.

He had a massive polemic with Dirac in the 1930s against the whole theory because, again, he thought it was a completely mathematically, very elegant but physically dreadful theory.

So, to me, it remains one of the

unfairly overlooked figures of the quantum mechanics. Very briefly, very briefly, Graham, do you agree with that?

Yes, I think Paoli unquestionably a great physicist.

He did say later on,

towards the end of his life, that he thought of himself as a young man, as a revolutionary, but later on he realised himself, he realized that he was a classicist rather than a revolutionary.

Well, thank you all very much. Thanks, Michaela Massimi, Frank Close, and Graham Farmelow.

Next week, we'll be talking about the life and times of Rosa Luxemburg, the revolutionary who argued with Lenin, helped found the German Communist Party, and was arrested and murdered in 1919.

Thanks for listening.

And the In Our Time podcast gets some extra time now with a few minutes of bonus material from Melvin and his guests.

But what would you like to have said that you didn't say?

I've got his aphorisms, you know, that

his remarks about

not even wrong. I mean, he's full of these put-downs.
His first wife, always a good place to get insults, right?

He did more than one one wife, did he? Oh yeah, oh yeah well he he the cabaret dancer, do you remember? He uh remember the cabaret dancer.

But she said he used to walk around the apartment polishing his barbs to make them maximally funny and poisonous.

Actually w when Melvin, you made the remark about uh um cabaret dancers, I I should have said it's the theoretical physicists that are the problem.

Not the cabaret dancers

Yeah, he was full of the uh uh th there's another physicist called Paul Ehrenfest uh uh who wa walked up to Powley, allegedly their first words were he said to Pauli, I like your physics better than I like you and Pauli said well for me it's the other way around

and they became great friends. They did.

But I mean he's not even wrong. It's sort of I think that's a very good example that we should take

to heart because any number of people will be now sending us their latest theories of the universe.

And they're usually not even wrong in the sense that you cannot do a test experimentally to assess whether this idea is or is not the way to go.

And his criticism was that for an idea to be useful, it had to be testable so that you could show that either it was right or that it wasn't right.

And if they didn't fit either of those categories, it was worthless, in a sense, not even wrong. And it's interesting that he wasn't bold enough about the neutrino, is it?

Because it's an extraordinary

experiment. Two years before he wrote that up.
Yeah. Well, as I said, I think, you know, at the time, the electron and proton were all known.

Even the neutron had not yet been discovered. So the idea of inventing a third particle.

Rutherford put forward the neutron. He was a very good idea.
He was an experimentalist. That's true.
That's true.

But I think Pali's idea of the neutrino was much more radical than probably we recognise today. Yeah, yeah, that's true.

And the fact that it almost violated his not even wrong principle in the sense that he thought it would not be possible to detect it, and it was 25 years before it was detected.

Before he died, though. Yes,

he got a telegram at CERN and he read it out in a seminar. I mean, that must be a wonderful moment.

And he handed over the case of champagne that he had promised to give years before when it was discovered? I didn't care. And they, I mean, he wrote a book with Jung.
So there is this book that...

Do you know about this book? I mean, I remember having a copy of it at home. So there's a book that they published together with an article by Jung on the idea of synchronicity.

Meaningful coincidences. Meaningful coincidences.

So how two events may happen at the same time, even if there is no causal connection between them, some sort of telepathy, whatever you want to call it.

And Paoli wrote this article, I remember reading it when I was an undergraduate student,

it was just sheer madness.

Honestly, it was an article on Kepler and Flood, Robert Flood, who was an alchemist at the time.

So there were some sort of speculations about magical number and the magical polygons that Kepler used for planetary orbits. And so

why was he fixated on the number 137?

And when he was dying, he was taken ill one day and he died the next, but he died in a room, he pointed excitedly, I'm using the word excited because one of you did, to the door, and the room number was 137.

And he went on about 137, an awful lot of money. 137

is a number which measures the, if you like,

the strength of the electromagnetic forces. It's a pure number that appears in...
If it was different, everything would be different. And it's sort of a thing that fixated him.
And many people.

Arthur Eddington. I was telling you that.
If it was different, everything would be different. Why is 137 so important? In quantum electrodynamics,

there is a scale that has to be set somewhere. And this scale is encoded in a number which happens to have the value empirically almost 137.

So near to it, people thought it was precisely 137, and that this somehow was significant.

And even today, you know, people say, if you're trying to guess a theoretical physicist's pin number, try 137, okay? With a fine structure, it's 1 over 100.

But there's a story about Piley which incorporates the 137 and his great sense of he knew it all and his critical faculties in one thing, which is apparently after he died, he goes up to heaven and God says, Ah, Pauli, you are a great scientist.

You can ask me one question and I will give you the answer to it. And so Pauli said, well, the question I want to know is, why 137?

And so God then starts describing how he created the universe and how the theories all work that will lead to this. And Pauli then suddenly says, no, no, you've made a mistake.

I just have one last thing that I wish I was more eloquent in putting in, but as I said, I've never fully understood

Pauli's fascination with the psychic phenomena. I mean, I've tried, and other people have too.
But he's made a prediction, which

perhaps we ought to enshrine in the In Our Time archive. He said that in his view, the science of future reality will neither be psychic nor physical, but somehow both and somehow neither.

Now, he's covered all his corners,

he has, but you know,

you go to see, you go to see most theoretical physicists, they talk about the future being a kind of superposition of psychic reality.

But that's what he said in a letter, which you can read. So, that's how much it ingrains.

Then, all our listeners have got a real chance to copy it down quietly.

The future will be neither

he said, It is my personal opinion that the science of the future reality will be neither psychic nor physical, but somehow both and somehow neither.

Well, I see that a second time, it makes a lot more sense, doesn't it? Yeah. 1950.

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