In Our Time - Pauli's Exclusion Principle

Episode Date: April 6, 2017

After 27 years, Melvyn Bragg has decided to step down from the In Our Time presenter’s chair. With over a thousand episodes to choose from, he has selected just six that capture the huge range and depth of the subjects he and his experts have tackled. In this fifth of his choices, we hear Melvyn Bragg and his guests discuss a key figure from quantum mechanics.Their topic is the life and ideas of Wolfgang Pauli (1900-1958), whose Exclusion Principle is one of the key ideas in quantum mechanics. A brilliant physicist, at 21 Pauli wrote a review of Einstein's theory of general relativity and that review is still a standard work of reference today. The Pauli Exclusion Principle proposes that no two electrons in an atom can be at the same time in the same state or configuration, and it helps explain a wide range of phenomena such as the electron shell structure of atoms. Pauli went on to postulate the existence of the neutrino, which was confirmed in his lifetime. Following further development of his exclusion principle, Pauli was awarded the Nobel Prize in Physics in 1945 for his 'decisive contribution through his discovery of a new law of Nature'. He also had a long correspondence with Jung, and a reputation for accidentally breaking experimental equipment which was dubbed The Pauli Effect.With Frank Close Fellow Emeritus at Exeter College, University of OxfordMichela Massimi Professor of Philosophy of Science at the University of EdinburghandGraham Farmelo Bye-Fellow of Churchill College, University of CambridgeProducer: Simon TillotsonSpanning history, religion, culture, science and philosophy, In Our Time from BBC Radio 4 is essential listening for the intellectually curious. In each episode, host Melvyn Bragg and expert guests explore the characters, events and discoveries that have shaped our world

This is the BBC. Thanks for downloading this episode of In Our Time. There's a reading list to go with it on our website, and you can get news about our programs if you follow us on Twitter at BBC In Our Time. I hope you enjoy the programs. Hello, in 1925, Wolfgang Paoli 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 Powley Principle.

It asserts that no two electrons in an analogue, 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. Pauly 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 Powley and his exclusion principle are

Frank Close, fellow emeritus at Exeter College University of Oxford, Michaela Massimi, Professor of Philosophy of Science at the University of Edinburgh, and Graham Farmelow, by fellow of Churchill College, University of Cambridge. Frank Close, what were the big questions about Neatim 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 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 neon, 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 show that something weird was going on, was that if you heated elements, made them hot, they would admit light. But it wasn't just the light right across the rainbow. If you pass 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 were doing this, nobody knew. The big news, which led to the breakthroughs, was that just before Pellie'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 now talking about that we're quite near the birth of theoretical physics, which is I understand it, happened in Germany in about the 1860s and then spread over Europe. America from then on. Can we talk about one or two contributions? First of all,

Neil's bore. Was he vital to the development of this? Yes, I think Ball was probably the first step in beginning to understand what was really going on inside atoms, that he had the insights. The electrons, 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, 0, 1, 2, 3, 4 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 had lost was emitted as light of a characteristic colour. And so this 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 Pauly'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.

Mac was famous for writing a book, The Science of 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 Mac played an important role in Powery is upbringing. Mark moved to Vienna to become professor of philosophy, and three years later, Wolfgang Paoli's father moved to Vienna. He converted to Catholicism. He had Jewish origin. Marriette Berta Camilla Schutz was a prominent Austrian woman. She wrote a book on the French

Revolution, several historical essays. And when Paoli was born, Mark was invaded to become the godfather of Pauli. So the story goes that many years later, Pauli said jokingly, because Marker 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 Paoli absolutely excelled in mathematics and physics, not so much in other subjects. And at the age of a teen, he went to Munich to study with the leading spectroscopist of the time, Arnold Somerfeld. And Arnold Somerfeld was so impressed by the mathematical ability of the young Paoli that when Albert Einstein

declined the invitation to write an encyclopedia article on relativity theory, he asked his student, he's 18 year old Paoli, 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 relatively recent discovery showing incredible skills in delving mathematical details with the theory. And the result was published in 1921, was welcomed as an outstanding achievement by some of the great mathematicians of the time,

like Vile. And Paoli went so beyond just writing a simple survey of the theory, he pointed out open problems in relativity theory, such as the problem of the structure of mother, to which himself turned to 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. He's an example of a prodigy who realizes 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 it go from there? So then it starts a very hectic period of the effectively early 1920s where Paoli really began to work on a series of problems about 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 father of the Copenhagen interpretation of quantum mechanics. And from there, he moved on. I mean, later on in 1928, he got his first full professorship at the ETH in Zurich, which is one of the most... And again, he was only the youngest, if not the youngest, ever, professor there, was he in jury?

It was very young, so it 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. And yeah. But he's very much up and running, well known already as a young man, 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 one stage. they're beginning to know about the whole quantum field and quantum matter. Graham, Grammala, before we get to the exclusion principle, can you tell us about Pauley's idea of too-valuedness? I was reading that carefully, too-valuedness in electronics. Yes, well, this was perhaps his greatest contribution. We wind the clock back to about 1924. He's in 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... I love the connection you've just made. Well, it is. It is. It is. It is. No, go on, let me go. Anyway. All human life is here.

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-cop theory, which is a theory that was a part classical Newtonian, part quantum, and they were trying to understand the observations of the light given out by atoms. Now, the thing that Powley 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,

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 alter the frequency of the spectral lines, and it became a puzzle to understand those observations.

Now, what... Can I just interrupt one one second? We're talking about theoretical physics here, just for the clarity of the listeners. Does this mean he's doing experiments with stuff in a laboratory, does this mean he's sitting down and thinking things through? He very much, he wouldn't be allowed near a laboratory as we'll hear later. Well, can hear it now.

I hate saying, yeah. Might have time. Who knows what else will have later. Okay, well, let me just suppose. He was one of the classic theoretical physicist in the sense that he was very happy to talk to experiments, 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 did, and he did it brilliantly.

Okay. Now, 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 the 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 electron terms of three quantum numbers. 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, right? 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. Right. Now, from modern perspective, as we're going to hear, that was a puzzle. He didn't take that extra step. But he was the person who noticed that too-valuedness. Right, Frank, let's go to the exclusion principle. What was it? Well, the electrons are like cuckoos. 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... It's still talking about the atom. We're still talking about somebody who can't... Yeah, I just want to get back where we are. The fundamental thing, that's what we're talking 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 you start with hydrogen, which 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. 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 ring'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 ring was

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 are 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 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, I'd come back in one second. Was it recognised at the time? Did people say, woof, we've got something? Yes, the news spread very quickly. the probably announced the exclusion rule and I underline

you call it the rule he didn't call it the principle so in Germany is Auschwitzungs Regal because at the time it was just a humble empirical rule that could account

for a series of a spectroscopic anomaly 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 Principal was DRAC in 1926. And we have to bear in mind the context. Paoli announced it in a letter to Alfred Landé was a prominent experimental physicist in Chubingen at the end of 1924. The news spread. A month later, Neil Sporor

from Copenhagen sent a letter to Paoli saying, we're all very excited for the very many beautiful things you have discovered. And I don't have to hide any criticism because you yourself, Paoli, 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 Paoli in 1924, before Heisenberg matrix mechanics, before Schroederger wave mechanics, before really the foundations of quantum mechanics were laid,

was to introduce a rule that finally gave a solution to prox. that had beset physicists for decades. The problem of atomic spectra really goes back to the 19th century. So there were these an 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 states? 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 those spectroscopic anomaly. He didn't call it spin, and I call it classical and non-describable two-valuineness and his exclusion rule to make sense of the periodicity in the Mendeleu's table. The people that came after him introduced the term of electron spin. So the immediate consequence was that Paoli was visiting London in Tübingen.

There was a young PhD student from Colombia called Ralph Kronig. and he heard the news and he approached Paoli and in the kind of classical language of vector model he said maybe we can interpret this two-valuineness 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 manos one-half. And Paoli dismissed the idea as a witty nonsense so the poor chronic went away, he never published and then two Dutch-American physicists A few months later in 1925, Hulenbeck and Kow Smith 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, Famola, you want to come here. Just a brief comment that this illustrates, I think, a very important part of Powell'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. The poor Krunig had what could have been a Nobel Prize winning discovery, basically crushed by Powley. 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, is 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. He did. And dreams he was interested in. He was fascinated by the number 137. And what's all that about? Well, it's, 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 Year after his father married a woman around his Pauli's own age And Powley the next year married a cabaret dancer Not the wiser thing to do, he wasn't married a year And they were together a very little part of that time I'm sure there's some very nice camera dancers

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, he, Powley needed help, and his father steered him towards Carl Jung, which is, you asked about his relationship with the great psychoanalyst.

And then we have this improbable friendship and very respectful relationship between Young and Powley. They first met in January 1932, so Powley's just in this great crisis of his life. And as I say, it sounds very implausible. Young 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, 1909, 1912. Paoli had got interested in psychology, partly from his closeness to Neil's Bohr, who was also someone of very wide interests. Now, Pauley, as we would now say, a bit of a basket case, 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 that they never did the Young with Pauley on the couch bit, he referred him to one of his students. But Pauley did keep Young briefed on the details of his dreams. Frank, are these two things irreconcilable, or is it just the way a man lives his life?

And what do you think about this? Well, to me, I think you're seeing here Pauley 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 Powley who is here at the birth of quantum mechanics. You know, and 50, 60 years later, we still use quantum mechanics without being quite comfortable as to what's going on. Oh, your dice-Igonauts are going to love you. You'll be the patron saint when you walk out of this studio. You heard it here first. No, no, go, I interrupt you.

Can you say more? It's great. So, I've never forgotten the track that I was on. The track you were on, the track you were on was that he, it is possible that he found a similarity between exploring the unknown that Young was exploring and exploring the unknown that he as a physicist were exploring. He found

an analogy, though, 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 Young was interested in. So the fact that Young and Powley 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 principal with DRAC in 1926. So, Paul Dirac, yes, in 1926.

Can you just tell people who DRAC is? So DRAC was one of the great physicists, the fathers of the quantum mechanics together with Niels Bohr, Werner Heisenberg. What DRAC did, it 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 you 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 Ferm in Italy, another great Italian physicist, discovered this independently of Dirac,

is that anti-symmetric function 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 the Fermi Dirac statistics. So it's a statistics in quantum mechanics that tells you what's the behavior of many indistinguishable particles that follow the Pauly principle. It took 14 years for Pauly 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 is and the kind of statistical as and the kind of statistics it follows. So the theorem says that any particle that has aft integral spin, spin one aft, for example, electrons, protons, but also quarks, follow the Pauly principle, or the Fermil direct statistics, and any integral spin particles, like spin one, photons, W&Zed bosons, follow different kind of statistics, what has become known as the Boz Einstein statistics.

From that point onwards, the exclusion rule has become, become a principle, has become a cornerstone of quantum mechanics, because it governs the behaviour, not just of electrons, as was originally introduced in 1924, but the behaviour of any half-intercal 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 Piest 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 the...

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, for measurements, that the total energy before the process was not the same as it was afterwards. Those energy appeared to go missing. And Niels Bohr, about whom we spoke, thought that this may mean that energy conservation, which was a really sacred principle, might even be wrong.

Right? It was something seriously wrong with what was going on in the heart of the nucleus. It 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 each time? Now, powerly, this was an absolutely brilliant insight. Nobody had this insight at the same time. I don't know of anybody else that came up with this idea.

And so he's 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 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. Right. 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 particle was later called the 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 Powell had suggested a particle

that no experiment would ever be able to see. And later they did, and Frank you. Yes, I mean, just to make it, a remark that, as Graham says, Neal's bore 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 years, 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 powerly inventing a 50% increase. in particles to explain one phenomenon. Yeah. Avigna said it was crazy to do that.

There was something, Frank, there's something that I read in probably your notes, called the Pauli Effect. What's that? Right, well, this... I won't stop him now, like that. Yes, that... I've got my eye on the clock, Michael.

We're all right. That it was advisable to exclude Pauley from your laboratory, I think it was the Pauley effect. I mean, this is... Pauley was a theoretical physicist, There's a joke which my family can attest actually is probably true, that theoretical physicists have a habit of breaking things or things don't work. And Pauley seems to have been an extreme example of this.

That if Pauley came to your lab, things would break, even though he didn't touch them and so forth. Is this true? I mean, you were like to mythologise our heroes. I mean, how could that happen? Well, of course, that I'm sure is part of the question why Pauley and Young had so many conversations, you know, Is something going on? There's enough registered, to be serious for a moment, there's enough registered examples of him being a real old jinx.

Yes? Maybe. Right, yeah, you see. 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 Gertingen,

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 Pauley was changing trains in the station at that time. I mean, you know, these myths, I'm sure... I think we're going to move on, all right. Michaela, how has 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 that was a blank spot on the map of experimental physics. The first idea of testing the principle came in 1948 with two physicists, gold dabbard and sharp gold dabbard. And the idea was to look for... Why do it take so long? Well, think of it. What do we call it principles?

Principles 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 cornucstones, pillars of the theory so it proves sometimes very difficult to actually test them. The specific test that people were looking for were anomalous, pauli, violating a transition. So

the idea is, imagine you have a copper strip and you put electricity through it. Some atoms gets in an excited states and in that state some electrons may cascade down to the lowest energy states, although that

lowest energy states is already occupied but to 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 Fermidierax, but the first precision test came in 1990, so very, very late with Bramberg and Snow. And they found no evidence for powerly 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 a power 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 could have got the prize soon after 25 because people did see it was a very clever idea. I would say, 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 decided they had to award these prizes. And Pauley 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 a decision, said that the opinion has been that Pauley'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.

Was any little individual envis at play there? Possibly. You know what I mean? Scientists are human. 1935 must have hurt even more because they didn't award a prize and said there was no one good enough. I mean, Powell was pretty acerbic in his comments about people. I mean, how many people he told him mildly pissed off? Well, I wouldn't be my choice of worst fact. But yes.

But, Osein, he died in 1944, and he barely been in the ground five minutes, Pauley got the Nobel Prize in 1945. So it looked like Oseen had got his card marked. Talk 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 at Graham. Raym 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 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 Pauley then did mathematically, and as Graham said earlier, 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 generalised 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 analogs 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 analogs 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 Powell's theory back in 1947 or so, would have said that they had to be massless. So Pauley 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 Pallu's in the audience on the very same idea. And Pauley says, where are these massless things? And Yang said, oh, well, we're still thinking about it. And Pauley, 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 their 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. Miguel, he had a lot of great contemporaries. Some people, he isn't a 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 here's the funny thing about Paoli, that he made extraordinary contribution to physics, but somehow hasn't entered public discourse in a way the other physicists have. I mean, Boer and Heisenberg have featured in a famous theatre play,

in a way that probably the average person doesn't know about Paoli. And I mean, his mathematical talent was one of a kind, but so was also is a really sharp and compromising approach as we have already heard from Magrame and Frank, the mother played a role in his unpopularity, if you like, compared to some of his contemporary. To me, the great legacy of Paoli is this visionary ability of realizing the limits of classical physics

in dealing with quantum entities. It 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,

Adia, as unphilosophical, and the dreadful he called them. He had a massive polemic with Dirac in the 1930 against the whole theory, because again, he thought he was a completely, mathematical, very elegant, but physically dreadful theory. So to me, he remains one of the unfairly overlooked figure of the quantum mechanics. Very briefly, very briefly, Graham, do you agree with that?

Yes. I think Powley, 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 realised that he was a classicist rather than a revolutionary. Well, thank you all very much. Thanks, Michaela Massimi, Frank Kliss and Graham Farmerlo.

Next week we'll be talking about the life and times of Rosa Luxembourg, the revolutionary who argued with Lenin, helped found the German Communist Party, 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. Well, what would you like to have said that you didn't say?

I've got... Oh, he's aphorisms? That he's remarks about not even wrong. I mean, he's full of these put-downs. His first wife, always a good place to get insults. He did he? Oh, yeah, well, he... The cabaret dancer, do you remember?

I remember the camera, but she said he used to walk around the apartment polishing his barbs to make them maximally funny and poisonous. Actually, when Melvin, you made the remark about cabaret answers, I should have said it's the theoretical physicists that are the problem. Not the camera answer as well. Yeah, he was full of the...

There's another physicist called Paul Ehrenfest who walked up to Pauley, their first words were he said to Pauley, I like your physics better than I like you. he said, well, for me, it's the other way around. And they became great friends. And they did, yes. 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 thought. Two years into that.

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 that were known. Even the neutron had not yet been discovered. So the idea of inventing a third particle... Brotherford put forward the neutron. He was the bold...

an experimentalist. That's true. That's true. That's true. But I think Pali's idea of the neutrino, it was much more radical than probably we recognised today. Yeah, he is 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, just. He got a telegram at Cern and he read it out in the seminar. I mean, that must be in a wonderful moment. And he handed over the case of champagne that he had promised to give years before, when it was. was discovered. 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 deal of synchronicity. Yes. Meaningful coincidences. So how to 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 Pauli wrote this article, I remember reading it when I was. an undergrad student, it was just sheer madness. It was, honestly, it was an article on Kepler and Flood, Robert Flood, who was an alchemist at the time. So there was some sort of speculations about magical number and the magical polygons that Kepler

used for planetary orbits. 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. 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 didn't get any of that. If it was different and 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 incoheral. 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?

Well, the fine structure drawn to 1 over 100. But there's a story about Pilely 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, oh, Pauley, you were a great scientist, you can ask me one question and I will give you the answer to it.

And so Pauley 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 Pauley then says, no, no, you've made a mistake. I just have one last thing that I wish I was, be more eloquent in putting in, but as I said, I've never fully understood Powell's fascination with the psychic phenomena.

I mean, I've tried, and now other people have too, but he's made a prediction, which I, perhaps we ought to enshrine 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... And he's covered all his corners, though. He has. But you go to see...

You go to see most theoretical physicists talking 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 ingrained himself in his mind. And then all our listeners

have got a real chance to copy it down quietly. The future will be now... 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, actually, no, second time.

It makes a lot more sense, doesn't it? Yeah. 1950. I think we're going to be made an offer by the BBC that we cannot refuse. Right. Okay. Do you want tea, coffee or both or neither? There are many more science and discussion programmes from Radio 4 to download for free.

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