Pauli's Exclusion Principle (Archive Episode)

Summary

In Our Time: Pauli’s Exclusion Principle

BBC Radio 4
Host: Melvyn Bragg
Guests: Frank Close, Michaela Massimi, Graham Farmelo
Date: December 4, 2025

Overview

This episode delves into Wolfgang Pauli’s Exclusion Principle—the quantum rule stating that no two electrons in an atom can occupy the same quantum state simultaneously. Melvyn Bragg and a panel of eminent physicists and philosophers examine the discovery’s scientific, philosophical, and personal context: how Pauli’s insight shaped our understanding of matter, why his personality loomed large in 20th-century physics, and what makes the “conscience of physics” such a unique, sometimes overlooked, figure.

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Key Topics and Discussion Points

1. Setting the Historical Stage (01:51–06:00)

• Atomic Puzzles at the Turn of the Century:

  • Scientists knew of elements, atoms, and the empirical structure of the periodic table, but not why inert and active atomic behaviors repeated in periodic fashion.
  • Discovery of the electron revealed atoms' internal structure, but classical mechanics predicted matter should collapse—posing a major paradox.

• Periodic Table as an Empirical Law:

  • The recurring chemical properties in Mendeleev’s table hinted at a deeper, unexplained order in atomic structure.

Quote:
"The periodic recurrence of common properties became known as the periodic table. It was an empirical rule. It worked, but nobody knew why."
—Frank Close (03:43)

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2. Early Quantum Models and Bohr’s Contribution (06:00–07:30)

• Niels Bohr’s Innovations:
  • Electrons confined to discrete “orbits” with quantized angular momentum.
  • Explained atomic spectra as electrons leaping between quantized “rungs,” emitting photons at characteristic energies.

Quote:
"It's like having a ladder with rungs on ... 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 color."
—Frank Close (06:16)

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3. Wolfgang Pauli: Background and Early Achievements (07:30–11:04)

• Affluent Austrian-Czech Family, Influenced by Ernst Mach:
  • Pauli’s godfather, the philosopher/scientist Ernst Mach, set an anti-metaphysical but deeply critical tone for Pauli’s thinking.
• A Teenage Prodigy:
  • Authored a seminal encyclopedia article on relativity at 18, impressing the likes of Einstein and Weyl.
  • Hectic years studying atomic models under Sommerfeld and collaborating with Niels Bohr.

Quote:
"We know that the young Pauli absolutely excelled in mathematics and physics … producing an incredible encyclopedia article on relativity theory … welcomed as an outstanding achievement by some of the great mathematicians of the time."
—Michaela Massimi (08:46)

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4. The Road to the Exclusion Principle: “Two-Valuedness” (11:21–15:49)

• Atomic Spectra Mysteries:

  • Pauli focused on alkali elements and the inexplicable splitting (“doublets”) in their spectra.
  • Proposed electrons had a hidden, non-classical “two-valuedness” (precursor to the concept of spin).

• Pauli as Theoretical Physicist:

  • Did not experiment, but reasoned deeply from data.

Quote:
"He [Pauli] wouldn't be allowed near a laboratory … he wanted to think his way into the heart of the atom. That's what he did. And he did it brilliantly."
—Graham Farmelo (13:55)

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5. The Exclusion Principle Explained and Its Immediate Impact (15:49–22:24)

• What Is the Exclusion Principle?
  • No two electrons in an atom can occupy the same quantum state.
  • Explains why matter has structure, why elements are chemically distinct, and why solids, chemistry, and the very stability of matter exist.

Quote:
"If I've got an electron already occupying one of these quantum states ... you can't put another electron in there. It's excluded."
—Frank Close (15:54)

Quote:
"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."
—Frank Close (18:06)

• Reception:
  • Initially introduced as a “rule” (“Ausschließungsregel”); Dirac rebranded it as a “principle.”
  • Niels Bohr called Pauli’s insight “sheer madness,” but recognized it solved key mysteries (19:13).

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6. Two-Valuedness Becomes Spin: The Legacy Evolves (21:09–22:24)

• From Two-Valuedness to Electron Spin:
  • Cronig suggested “spin” as the origin of two-valuedness, dismissed by Pauli but soon formalized by others.
• Foundations for Modern Quantum Mechanics:
  • Exclusion principle became linked to electron spin, periodicity, and the triumphs of quantum theory.

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7. Pauli as Person and Critic: A Complex Character (22:24–29:48)

• Acerbic, Hypercritical, Yet Brilliantly Insightful:

  • Could crush or inspire colleagues; sometimes “backed the wrong horse.”
  • Famed for harsh put-downs (later source of “not even wrong” remark).

• Fascination with the Mystical:

  • He struggled with personal crises; sought psychoanalytic help from Carl Jung.
  • Explored connections between the collective unconscious and quantum mysteries.

Quote:
"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."
—Graham Farmelo (23:41)

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8. Expansion and Formalization: From Rule to Cornerstone (29:48–32:34)

• Fermi, Dirac, and the Principle’s Generalization:

  • Dirac and Fermi formalized the mathematical behavior (Fermi-Dirac statistics) for all particles with half-integer spin.
  • Pauli’s “rule” thus became a general principle underlying the structure of all matter.

• Spin-Statistics Theorem:

  • Pauli himself eventually proved this crucial link between particle spin and statistical behavior.

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9. Further Scientific Contributions: The Neutrino and Beyond (32:34–36:19)

• Predicting the Neutrino:
  • Pauli postulated the neutrino to preserve conservation of energy in radioactive beta decay—a move bold for the time and initially unpublished.
  • The particle was real, though undetectable for decades.

Quote:
"He wrote to a conference of physicists suggesting very tentatively that ... in addition to the electron charging out of the nucleus, there was a particle that we don't see ... no electrical charge ... same spin as the electron and very, very little mass. ... Many people thought that Pauli had suggested a particle that no experimenter would ever be able to see."
—Graham Farmelo (34:30)

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10. The Pauli Effect and Experimental Testing (36:19–39:38)

• The Pauli Effect:
  • Pauli’s presence in laboratories was said to cause experimental apparatus to fail—a running joke in the physics community.
• Testing the Exclusion Principle:
  • Tests for forbidden X-ray transitions finally performed decades later; no violations ever found.

Quote:
"... theoretical physicists have a habit of breaking things ... 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..."
—Frank Close (36:33)

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11. Nobel Prize and Pauli’s Recognition (39:38–41:19)

• Delayed Nobel:
  • Pauli’s Exclusion Principle wasn’t formally recognized until 1945, partly due to conservative attitudes within physics and perhaps his own acerbic temperament.
  • Reportedly, the Nobel committee chair judged his “receptivity exceed[ed] his originality”—a controversial opinion.

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12. Later Career, Near-Misses, and Legacy (41:19–44:02)

• Missed the Higgs, but Not for Lack of Insight:

  • Pauli’s work on quantum gauge theory presaged today’s field theory, but he missed the mass-generating Higgs mechanism.
  • Dismissed promising ideas that didn’t immediately fit evidence—sometimes to his own professional detriment.

• Why Isn’t Pauli as Famous as Others?

  • Uncompromising style, lack of theatrical biography, and focus on critique might have played a role in his lesser public visibility compared to Bohr, Heisenberg, or Einstein.

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13. Enduring Influence, Notable Aphorisms, and Mystical Speculations (46:07–51:42)

• “Not Even Wrong” and the Value of Testability:

  • Pauli’s insistence on experimental testability has become legendary.
  • Famous put-downs, including “not even wrong,” enshrined his standards.

• The Enigma of 137:

  • Pauli’s fascination with the number 137 (the fine structure constant) symbolized deeper mysteries of the universe, echoed in his final days.

Quote:
"If it was different, everything would be different. And even today, you know, people say, if you're trying to guess a theoretical physicist's pin number, try 137."
—Frank Close (49:45)

• Physical and Psychic Realities—Prophetic Vision:
  • Pauli speculated to Jung that the science of the future would be “neither psychic nor physical, but somehow both, and somehow neither.” (51:20)

Quote:
"It is my personal opinion that the science of the future reality will be neither psychic nor physical, but somehow both, and somehow neither."
—Wolfgang Pauli, as paraphrased by Graham Farmelo (51:42)

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Notable Quotes & Memorable Moments

• On the importance of exclusion:
  "The consequence of that is that we're having this conversation, that the universe isn't made of goo."
  —Frank Close (18:06)

• On his acerbic wit:
  "His first wife ... said he used to walk around the apartment polishing his barbs to make them maximally funny and poisonous."
  —Graham Farmelo (46:18)

• On testability:
  "For an idea to be useful, it had to be testable ... if they didn't fit either of those categories, it was worthless in a sense, not even wrong."
  —Frank Close (46:57)

• On the exclusion principle’s reach:
  "From that point onwards, the exclusion rule has become a principle, has become a cornerstone of quantum mechanics..."
  —Michaela Massimi (32:34)

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Key Timestamps

• 01:51 – Introduction to Pauli and the Exclusion Principle
• 03:43 – Problems in atomic physics at the turn of the century
• 06:16 – Bohr’s quantum orbits and atomic structure
• 07:30 – Pauli’s early life and genius
• 10:52 – Studying with Sommerfeld, Bohr, and achieving professorship
• 11:48 – Discovery of electron “two-valuedness”
• 15:49 – Articulation and meaning of the exclusion principle
• 19:13 – Contemporaneous reactions in the scientific world
• 22:24 – Pauli’s critical demeanor and human side
• 23:41 – Fascination with Jung, alchemy, and psychic phenomena
• 29:58 – Formalization as “principle” via Dirac and Fermi
• 32:46 – Pauli’s neutrino hypothesis
• 36:19 – The “Pauli Effect”
• 37:53 – Experimental tests of the principle
• 39:48 – Nobel Prize delay and rationale
• 41:28 – Missing the Higgs mechanism
• 44:02 – Why Pauli isn’t a household name
• 46:07 – “Not even wrong” and his legacy of critical scrutiny
• 49:28 – Mystique of 137 and personal quirks
• 51:20 – Prophetic vision—psychic & physical science fused

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Tone & Language

The episode flowed from lively storytelling and scientifically dense explanation to philosophical musing and sharp-edged humor, mirroring Pauli’s own intellectual range. The panel mixed personal anecdotes with rigorous explanation, always staying true to Pauli’s reputation as both a stringent critic and a deeply imaginative thinker.

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Summary

This episode of In Our Time masterfully recounts how Wolfgang Pauli’s sharp intellect and uncompromising standards led to one of the most fundamental discoveries in physics: the exclusion principle. The discussion weaves together atomic puzzles, disciplinary rivalries, personal eccentricities, and the emergence of quantum mechanics, all while keeping an eye on scientific testability and the mysteries that still tantalize physicists. Pauli emerges as a brilliant, sometimes difficult, figure whose exclusion principle still underpins our understanding of everything from chemistry to the cosmos and whose mix of logic and fascination with the unknown continues to challenge and inspire.

Transcript

[Frank Close]

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[Michaela Massimi]

Hey, this is Sarah. Look, I'm standing out front of a.m. p.m. Right now and well, you're sweet and all, but I found something more fulfilling, even kind of cheesy. But I like it. Sure, you met some of my dietary needs, but they've just got it all. So farewell Oatmeal. So long. Use strange soggy Break up with bland.

[Frank Close]

Breakfast and taste AM PM's bacon, egg and cheese biscuit made with cage free eggs, smoked bacon and melty cheese on a buttery biscuit. Am P M. Too much good stuff.

[Michaela Massimi]

What do you think makes the perfect snack?

[Frank Close]

Hmm, it's gotta be when I'm really craving it and it's convenient. Could you be more specific when it's cravinient? Okay, like a freshly baked cookie made with real butter available right down the street at a.m. p.m. Or a savory breakfast sandwich I can grab in just a second at a.m. pM.

[Michaela Massimi]

I'm seeing a pattern here.

[Frank Close]

Well yeah, we're talking about what I.

[Michaela Massimi]

Crave, which is anything from AM pm.

[Frank Close]

What more could you want? Stop by AM PM where the snacks and drinks are perfectly crav and convenient. That's cravenience ampm. Too much good stuff.

[Graham Farmelo]

And now, to mark the end of.

[Frank Close]

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

[Melvyn Bragg]

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 20, 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 behavior of the elements to why matter is stable. And for this, he won the Nobel Prize in Physics, 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, Michaela Massimi, professor of Philosophy of Science at the University of Edinburgh, and Graham Farmeloe, BI Fellow of Churchill College, University of Cambridge. Frank Close what were the big questions about neatum in the early part of the 20th century? Paolo was born in 1900. What were the big questions then?

[Frank Close]

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 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 pass the light through a sort of spectrograph, it would make a sort of barcode of individual colors, 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 which 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. There was self destruct. So that was the great paradox that had to be sorted.

[Melvyn Bragg]

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?

[Frank Close]

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 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, 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 color. And so these spectral lines of light coming from atoms was because the electrons are jumping from one rung to another.

[Melvyn Bragg]

So we're beginning with an explanation. What date are we at now?

[Frank Close]

About 1913.

[Melvyn Bragg]

1913. Okay, let's go back to our man Michaela. What was Wolfgang Pauli's background?

[Michaela Massimi]

Pauli came from an affluent family of Czech Austrian origin, and his father went to 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, 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, married Berta Camilla Schutz, who was a prominent Austrian woman. She wrote a book on the French Revolution, several historical essays. And when Pauli was born, Mack was invited to become the godfather of Pauli. So the story goes that many years later Paoli said jokingly that because Mac 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 generality of relativity. So relatively recent discovery showing incredible skills in delving in mathematical details with the theory. And the result was published in 1921. It was welcomed as an outstanding achievement by some of the great mathematicians of the time, like Weyl. And Pauli 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 he himself turned to very easily.

[Melvyn Bragg]

And it's still a classic, that book, isn't it?

[Michaela Massimi]

It is, yeah. It remains one of the classic articles introductory to relativity theory.

[Melvyn Bragg]

He's an example of a prodigy who realizes potential.

[Michaela Massimi]

Yeah, well, certainly so, I mean it certainly made a big impression on everyone at the time and that put him firmly on the international scene.

[Melvyn Bragg]

And so then did he move on to another teacher from there? Where did he go from there?

[Michaela Massimi]

So then it starts a very hectic period of the effectively early 1920s where Pauli really began to work on a series of problems about the spectroscopic anomalies, of which Franck was already mentioning mentioning and models of the atom. So he spent a period in Copenhagen with Niels Bohr, one of the fathers of the Copenhagen interpretation of quantum mechanics.

And from there he moved on. Later on, in 1928. He got his first full professorship at the ETH in Zurich, which is one of the most.

[Melvyn Bragg]

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

[Michaela Massimi]

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. And.

[Melvyn Bragg]

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 on stage, and now they're beginning to know about the whole quantum, quantum field and quantum. Graham, Graham, Pamela, before we get to the exclusion principle, can you tell us about Paul's idea of two valuedness? I was reading that carefully. Two valuedness in electronics.

[Graham Farmelo]

Yes. Well, this, this was perhaps his, 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 sex, lovely sex in the evening, showing up very late in the, in the, in the mornings, thinking very deeply about these spectra that Frank was talking about. These are the jumps that the, the electronics.

[Melvyn Bragg]

I love the connection you've just made.

[Frank Close]

Well, it is, it is, it's just pal.

[Melvyn Bragg]

There we go. Anyway, human life is here.

[Graham Farmelo]

It is, it is. Anyway, these, at these atoms were making these jumps or transitions, right? And the experimenters were looking at the, at these discrete frequencies of light and trying to make some sense of them. This was a big problem they had. 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 imagine it just very crudely is 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.

[Melvyn Bragg]

Now, can I just interrupt one more. 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?

[Graham Farmelo]

He's very much. He wouldn't be allowed near a laboratory, as we'll hear later.

[Melvyn Bragg]

Well clear it now.

Who knows what's going later.

[Graham Farmelo]

Okay, well, let me just, let me just. He, he was one of the, a classic theoretical physicists 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, 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, 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 electrons of 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.

[Melvyn Bragg]

So what is this two valuedness?

[Graham Farmelo]

Well, he didn't know right now that.

[Melvyn Bragg]

When you say things like that.

[Graham Farmelo]

No, it's important because he was being very cautious because people were saying what does this, what does this mean? But he was quite cautious about it. He, he wrote it in his very, very clear way that it was due to a particular, non classically des 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 that from modern perspective, as we're going to hear, that was a, that was a puzzle. He didn't take that extra step, but he was the person who noticed that two valued valuedness.

[Melvyn Bragg]

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

[Frank Close]

Well, the electrons are like cuckoos. You 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, you turn around the atom.

[Melvyn Bragg]

We're talking about something that can't. Yeah, I just want to get back where we are. The fundamental thing, that's what we talk about.

[Frank Close]

The electron, which is one of the fundamental constituents of all atoms. That's 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, 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.

[Melvyn Bragg]

So what's the consequence of that?

[Frank Close]

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 squeeze in ever smaller until they can't go because they're excluded.

[Melvyn Bragg]

So, Michaela, the significance of this is vast.

[Michaela Massimi]

Absolutely.

[Melvyn Bragg]

First of all, yes, I'll come back in one second. Was it recognised at the time? Did people say, woof, we've got something?

[Michaela Massimi]

Yes, the news spread very quickly.

Paoli announced the exclusion rule and I underlined. He called it a rule, he didn't call it a principle. So in Germany it's Auschlichungsregl, because at the time it was just a humble empirical rule that could account for a series of 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 principle was Dirac in 1926. And we have to bear in mind the context. Pauli announced it in a letter to Alfred Lande, who was a prominent experimental physicist in Tubingen. 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 Schrodinger wave mechanics, before really the foundations of quantum mechanics were laid, was to introduce a rule that finally gave a solution to problem 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 that had doublets, why there are doublets? So presumably there's a double energy states, but what's the origin of that double energy states? So by introducing what Graham was referring to as this classically non describable to 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 calling classically non describable to valueness and his exclusion rule to make sense of the periodicity in the Mendeleev stable, the people that came after him introduced the term of electron spin. So the immediate consequence was that Pauli was visiting London, Tubingen. There was a young PhD student from Columbia called Ralph Kroenig, and he heard the news and 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. You can think of the electron as a spinning top that can spin clockwise or anti clockwise 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 Kroning 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 split 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.

[Melvyn Bragg]

Graham. Graham. Pamela, you want to come in?

[Graham Farmelo]

Just a brief comment that this illustrates I think a very important part of Pauli'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 criticism and he was extremely careful all the time. And this is why this. 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 surely poor Kroenig 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, right, 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.

[Melvyn Bragg]

Now, earlier in this program 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 and dreams he was interested in. He was fascinated by the number 137. And what's all that about? Well, it's.

[Graham Farmelo]

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 my. The great crisis of my life. This was a time from 1927 when his mother killed herself. Year after his, his father married a woman of around his Paoli's.

Paoli. The next year married a cabaret dancer. Not the wisest thing to do he wasn't married a year and they were together a very little part of that.

[Melvyn Bragg]

Very nice cabaret dancers maybe she was.

[Graham Farmelo]

But the relationship didn't work out very well and he was quickly on the bottle. He and the poor guy went on a great tour of America having to explain why he his arm was in a sling because he fell down the stairs while violently drunk. Anyway, he Pauli needed help and his father steered him towards Carl Jung. This is you asked about about his relationship with the great psych psychoanalyst. And then we have this improbable friendship and very respectful relationship between Young and Pauli. They first met in January 1932. So Paul is just in this great crisis of his life and you. As I say it sounds very implausible. Young was interested in physics. He was interested in UFOs as well. He was interested in the art. He had very wide interests. He had dinner with Einstein a few times. 1909, 1912. Pauli had got interested in psychology partly from his closeness to Niels Bohr, who was also very someone of very wide interests. Now Pauli went to Young and they agreed. Obviously we don't know what's going on in these sessions to have his dreams analyzed. Now to the best of my knowledge, to the rest of my knowledge that they never did the the young with with Paoli on the couch bit. He, he referred him to one of his students. But Pauli did keep Young briefed on the details of his dreams.

[Frank Close]

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[Michaela Massimi]

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[Frank Close]

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[Frank Close]

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[Melvyn Bragg]

Frank, are these two things irreconcilable or is it just the way a man lives his life? What do you think about this?

[Frank Close]

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 you know, 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 Paoli 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.

[Melvyn Bragg]

Oh, your dice like going to love you. You'll be the patron saint when you walk out of the studio.

[Frank Close]

You heard it here first.

[Melvyn Bragg]

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

[Frank Close]

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

[Melvyn Bragg]

The track you were on that he found. It is possible that he found a similarity between the exploring the unknown that Young was exploring and exploring the unknown that he is a physicist exploring. He found an analogy there. Similarity or something.

[Frank Close]

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 can 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.

[Melvyn Bragg]

Michaela, let's get back on track with this, with this physicist. His exclusion principle gained importance in the 20s and 30s. Who took it up and what importance did it gain?

[Michaela Massimi]

Right, so as I mentioned, really it was introduced as a rule. It became a principle with Dirac in 1926. So Dirac, Paul Dirac, yes, in 1926.

[Melvyn Bragg]

Can you just tell people who Dirac is?

[Michaela Massimi]

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 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 the 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 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 partic as half integral spin, spin 1/2, 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, WNZ bosons follow different kind of statistics, what has become known as the Bose Einstein statistics. From that point onwards, the exclusion rule has become a principle, has become a cornerstone of quantum mechanics because it, 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.

[Melvyn Bragg]

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?

[Graham Farmelo]

Well, this was his second piece de resistance, so to speak. We heard earlier on about how he sorted out this, 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, these nuclei can in some cases decay randomly and this is called 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, just unpredictably comes out of the, 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 this 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 each time? Now Pauli, this was absolutely brilliant insight. Nobody had this insight at the same time. I don't know of anybody else that came up with this idea.

[Melvyn Bragg]

And so he's just thinking this through.

[Graham Farmelo]

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 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. 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 undet. Right. Many people thought that at the time, they thought that Pauli had suggested a particle that no experimenter would ever be able to see.

[Melvyn Bragg]

And later they did. And, Frank, you.

[Frank Close]

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, 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.

[Graham Farmelo]

Yeah. Wigner said it was crazy to do that.

[Melvyn Bragg]

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

[Frank Close]

Right. Well, this.

[Melvyn Bragg]

I won't stop him now.

[Frank Close]

Yes.

[Melvyn Bragg]

That I've got my eye on the clock. We're all right.

[Frank Close]

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 or 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.

[Melvyn Bragg]

Is this true? I mean, you would like to mythologize our heroes? I mean, how could that happen?

[Frank Close]

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?

[Melvyn Bragg]

There's enough registered to be serious for a moment. There's enough registered examples of him being a real old jinx.

[Frank Close]

Yes, maybe rather.

[Melvyn Bragg]

Yeah.

[Frank Close]

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 Girtingen 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.

[Melvyn Bragg]

Michaela, how is his exclusion principle being tested?

[Michaela Massimi]

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 there was a blank spot on the map of experimental physics. The first idea of testing the principle came in 1948 with two physicists, Goldaber and Scharf Goldaber. And the idea was to look for.

[Melvyn Bragg]

Why did it take so long?

[Michaela Massimi]

Well, think of it. Why 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 cornerstones, pillars of the theory. So it proves sometimes very difficult to actually test them. The specific tests that people were looking for were anomalous Pauli violating 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 Fermi Diracs. But the first precision test came in 1990, so very, very late, with Rumberg 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 violation.

[Melvyn Bragg]

Graham, why did it take about a quarter of a century for Paolo to be awarded the Nobel Prize? If, as we've heard in this program, it was so very important.

[Graham Farmelo]

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 have 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 Paoli was left off. And it's worth saying here that the Carlo scene, 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.

[Melvyn Bragg]

Was any little individual envies at play there?

[Graham Farmelo]

Possibly. You know what I mean? Pitt Scientists Are Human, 1934. It must have hurt even more because they didn't award a prize and said there was no one good enough.

[Frank Close]

I mean, Paoli was pretty acerbic in his comments about people. I mean, how many people he Marley pissed off?

[Graham Farmelo]

Well, I wouldn't be my choice of worst, Frank.

But. Yes, but Osin, he died in 1944 and.

He barely been in the ground five minutes. Paoli got the Nobel Prize in 1945, so it looked like o' scene had.

[Melvyn Bragg]

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?

[Frank Close]

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 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 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 Paoli 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 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.

[Melvyn Bragg]

Michaela. 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?

[Graham Farmelo]

Yeah.

[Michaela Massimi]

So here's the funny thing about Pauli, that he made extraordinary contribution to physics, but somehow hasn't entered public discovery course in a way that other physicists have even. 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 is a really sharp, uncompromising approach, as we have already heard from Graham and Frank, that might have played a role in his unpopularity, if you like, compared to some of his contemporary. To me, the great legacy of Paolo 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 croning. It's evident from his dismissal of Heisenberg ideas and philosophical and the dreadful he called them. He had a massive polemic with Dirac in the 1930 against the whole theory, because again, he thought it was a completely mathematically very elegant, but physically dreadful theory. So to me remains one of the unfairly overlooked figure of the quantum mechanics.

[Melvyn Bragg]

Very briefly, very briefly, Graham, do you agree with that?

[Graham Farmelo]

Yes, I think Pauli unquestionably a great physicist, he did say later on, at the end, towards the end of his life, that he thought of himself as a young man, as a revolutionary. But later on he realized himself. He realized that he was a Classicist rather than a revolutionary.

[Melvyn Bragg]

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

[Michaela Massimi]

And the In Our Time podcast gets.

[Frank Close]

Some extra time now with a few.

[Michaela Massimi]

Minutes of bonus material from Melvin and his guests.

[Melvyn Bragg]

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

[Graham Farmelo]

I got. I've got.

[Frank Close]

Oh, his aphorisms, you know that his remarks about not even wrong.

[Graham Farmelo]

I mean, full of these put downs. His first wife, always a good place to get insults, right.

[Melvyn Bragg]

Doing more than one wife.

[Graham Farmelo]

Did he? Oh yeah. Oh yeah. Well, he hit the cabaret dancer. Do you remember he. I remember the cabaret D. But she said he used to walk around the apartment polishing his barbs to make them maximally funny and poisonous.

[Frank Close]

Actually, when Melvin, you made the remark about.

Cabaret arms is. I should have said it's the theoretical physicists that are the problem, not the cabaret.

[Graham Farmelo]

Yeah, he was full of the. There's another physicist called Paul Ehrenfest who walked up to Pauli. Allegedly their first words were, he said to Pauli, I like your physics better than I like you. And Paoli said, well, for me it's the other way around.

[Frank Close]

And they became great friends.

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

[Melvyn Bragg]

And it's interesting that he wasn't bold enough about the neutrino, is it? Because it's an extraordinary thought experiment two.

[Graham Farmelo]

Years before he wrote that up.

[Frank Close]

Yeah, well, as I said, I think, you know, at the time the electron and proton were all that were known. Even the new, even the neutron had not yet been discovered. So the idea of inventing a third.

[Graham Farmelo]

Particle, Rutherford put forward the neutron. He was the idea. He was an experimentalist. That's true, that's true.

[Frank Close]

But I think Pauli's idea of the neutrino. It was much more radical than probably we recognize today.

[Graham Farmelo]

Yeah. Yes, true.

[Frank Close]

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.

[Graham Farmelo]

Before he died. That's.

[Michaela Massimi]

Yes.

[Graham Farmelo]

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

[Frank Close]

And he handed over the case of champagne that he had promised to give years before when it was discovered.

[Michaela Massimi]

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 Pauli wrote this article. I remember reading it when I was an undergraduate student. It was just sheer madness. It was. No, 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.

[Melvyn Bragg]

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.

[Frank Close]

137 is a number which measures the. If you like, the. The strength of the electromagnetic forces. It's a pure number. It 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.

[Melvyn Bragg]

If it was different, everything would be different. Why is 137 so important?

[Frank Close]

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?

[Graham Farmelo]

With a fine structure, one over 100.

[Frank Close]

But there's a story about Pali 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, Paoli, you were a great scientist. You can ask me one question and I will give you the answer to it. And so Paoli 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.

[Graham Farmelo]

I just have one last thing that I wish I was more eloquent in putting in, but I, 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 I. 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.

[Melvyn Bragg]

Now. And he's covered all his corners. He has but, you know, cover a corner.

[Graham Farmelo]

But you go to see, you go 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, right, so that's how much it ingrained.

[Melvyn Bragg]

And then all our listeners have got a real chance to copy it down quietly. The future will be neither.

[Graham Farmelo]

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.

[Melvyn Bragg]

Actually, the second time, it makes a lot more sense, doesn't it? Yeah.

[Graham Farmelo]

1950, I think we're going to be.

[Melvyn Bragg]

Made an offer by the BBC that we cannot refuse.

[Frank Close]

Right. Okay.

In Our Time with Melvyn Bragg is.

[Graham Farmelo]

Produced by me, Simon Tillotson, and it's a BBC Studios production.

[Michaela Massimi]

Our culture can cancel someone in the.

[Frank Close]

Blink of an eye.

[Michaela Massimi]

Celebrities, sports stars, politicians, influencers and royalty.

[Frank Close]

Can all find themselves in the firing line. In the age of AI generated evidence.

[Michaela Massimi]

Lawsuits written in legalese you need to pass the bar to decipher. How are you supposed to separate the.

[Frank Close]

Fact from the fiction? That's where we come in. I'm Anushka Mutanda Doughty and this is Is Fame Under Fire from BBC Sounds. We'll mythbust, debunk, Pre bunk fact check and get to the truth behind the timeline.

[Michaela Massimi]

There are new episodes every week, so.

[Frank Close]

Make sure you listen to Fame Under Fire and subscribe on BBC sounds.

Hello, it's Ray Winstone. I'm here to tell you about my podcast on BBC Radio 4 histories, Toughest Heroes. I've got stories about the pioneers, the rebels, the outcasts who define tough. And that was the first time that anybody ever ran a car up that fast with no tires on. It almost feels like your eyeballs are going to come out of your head. Tough enough for you? Subscribe to History's Toughest Heroes wherever you get your podcast.