Heisenberg's Uncertainty Principle (Archive Episode)

Summary

In Our Time: Heisenberg's Uncertainty Principle

Podcast Information:

• Title: In Our Time
• Host: Melvyn Bragg
• Episode: Heisenberg's Uncertainty Principle (Archive Episode)
• Release Date: August 14, 2025

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Introduction

In the August 14, 2025 episode of In Our Time, host Melvyn Bragg delves into the profound contributions of Werner Heisenberg to the field of quantum mechanics. Accompanied by expert guests—Faye Dauker, Professor of Theoretical Physics at Imperial College London; Harry Cliff, Research Fellow in Particle Physics at the University of Cambridge; and Frank Close, Professor Emeritus of Theoretical Physics at the University of Oxford—the discussion explores Heisenberg's groundbreaking work, the formulation of the Uncertainty Principle, and its enduring impact on both science and technology.

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Heisenberg's Background and Early Influences

Werner Heisenberg, at the young age of 23, revolutionized physics with his formulation of quantum mechanics. Bragg sets the stage by explaining Heisenberg's formative years:

"As we'll hear, this was momentous." ([01:28])

Harry Cliff highlights Heisenberg's early inspirations:

"He read Plato while he was hiking in the Bavarian mountains... he believed that the smallest units of matter are not particles in an ordinary sense, but forms, ideas only expressed in mathematical language." ([02:30])

This classical background, combined with his burgeoning interest in mathematics and physics, set the foundation for Heisenberg's future breakthroughs.

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Development of Quantum Mechanics

Before Heisenberg's contributions, quantum theory was already taking shape, beginning around 1900. Frank Close provides a succinct overview:

"Max Planck... assumed that electromagnetic waves are not a sort of smooth legato wave, but more like a staccato bunch of what we call photons... Einstein picked up on this idea with the photoelectric effect." ([04:31])

Heisenberg's pivotal moment came after attending a Niels Bohr lecture, which spurred him to develop a mathematical framework that could accurately describe atomic emissions—a leap that would birth quantum mechanics.

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Heisenberg vs. Schrödinger: Matrix Mechanics vs. Wave Mechanics

Heisenberg's approach to quantum mechanics was notably abstract, utilizing matrices to represent physical quantities:

"He discovered that the position of an electron in an atom is represented by a completely new, unexpected mathematical entity called a matrix." ([07:48])

In contrast, Erwin Schrödinger introduced a wave-based formulation a year later, which was more intuitive to physicists accustomed to wave mechanics:

"Schrodinger has this wave description of say, an electron around an atom. And a wave is something that's intuitively much easier to understand for physicists." ([14:02])

Despite initial preferences for Schrödinger's approach, it was later recognized that both matrix mechanics and wave mechanics are mathematically equivalent, providing different perspectives on the same quantum phenomena.

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The Uncertainty Principle: Origins and Implications

Heisenberg's most renowned contribution, the Uncertainty Principle, emerges directly from his matrix formulation:

"There is a limit to how well you can simultaneously know the position of a quantum particle and its momentum. And that is what the uncertainty principle states." ([24:38])

Frank Close elaborates on the mathematical foundation:

"The uncertainty principle comes from the way that matrices multiply with each other... there is a trade-off on what you can know on the average about both of them." ([25:54])

This principle fundamentally alters our understanding of measurement in quantum mechanics, emphasizing that certain pairs of properties—like position and momentum—cannot both be known to arbitrary precision.

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Philosophical Implications and the Heisenberg Cut

The Uncertainty Principle extends beyond mathematics, influencing philosophical interpretations of reality:

"If you're a physicist, that's very funny." ([28:51])

Frank Close discusses the observer's role:

"It makes the observer, the experimenter part of the system in a way... The choices you make in your observation determine the results you get." ([28:51])

This introduces the concept of the Heisenberg Cut, a division between the quantum system and the classical world of observers, highlighting the indeterminate nature of quantum states until measured.

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Heisenberg's Later Life and Legacy

Heisenberg's career was not without controversy, particularly regarding his role in Germany during the rise of Nazism. Melvin Bragg recounts:

"He spends a lot of effort trying to prevent the dismissal of his Jewish colleagues... but he does accommodate to a large extent with the regime." ([36:12])

His legacy in physics remains monumental, despite the complex interplay of his scientific achievements and political stances.

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Technological Impact of Quantum Mechanics

The principles established by Heisenberg have had far-reaching technological implications. Frank Close underscores:

"From the standard model of particle physics to the behavior of semiconductor materials in phone chips, quantum mechanics affects our daily lives." ([30:58])

Looking forward, the guests anticipate continued advancements:

"Quantum computing... has become an area that's potentially going to have revolutionary impacts on the way we live." ([58:07])

Heisenberg's work not only transformed theoretical physics but also paved the way for modern technological innovations.

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Conclusion

Heisenberg's Uncertainty Principle represents a cornerstone of modern physics, reshaping our comprehension of the microscopic world and influencing a plethora of scientific and technological domains. Through rigorous mathematical formulations and profound philosophical implications, Heisenberg challenged classical intuitions, fostering a new era where observation itself becomes a fundamental part of understanding reality. The collaborative insights of Faye Dauker, Harry Cliff, and Frank Close illuminate the enduring significance of Heisenberg's legacy, affirming his pivotal role in the pursuit of knowledge and the ever-evolving landscape of science.

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Notable Quotes:

• Werner Heisenberg's Reflection on Breakthrough:

  "At first I was deeply alarmed. I had the feeling that I had gone beyond the surface of things and was beginning to see a strangely beautiful interior and felt dizzy." ([12:11])

• Frank Close on the Revolutionary Nature of Heisenberg's Work:

  "The equations of motion, the dynamics that apply to the micro world have been identified and you can now apply them to the microworld, which you could never do before." ([18:06])

• Melvin Bragg's Summary on Quantum Theory's Observer Dependence:

  "The thing that's really controversial about it is it makes the observer, the experimenter part of the system in a way... The choices you make in your observation determine the results you get." ([28:51])

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For those intellectually curious about the intricate dance between mathematics, philosophy, and the boundaries of human knowledge, this episode of In Our Time offers a comprehensive and engaging exploration of one of physics' most fundamental principles.

Transcript

[Melvin Bragg]

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[Unknown]

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[Faye Dauker]

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[Frank Close]

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[Frank Close]

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[Unknown]

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[Harry Cliff]

Not even a little?

[Unknown]

Not even a crumb.

[Frank Close]

What if.

[Unknown]

No, please.

[Melvin Bragg]

Mine.

[Unknown]

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[Alex von Tunzelman]

This is History's Heroes. People with purpose, brave ideas and the courage to stand alone, including a pioneering surgeon who rebuilt the shattered faces of soldiers in the First World War.

[Unknown]

You know, he would look at these men and he would say, don't worry, sonny, you'll have as good a face as any of us when I'm done with you.

[Alex von Tunzelman]

Join me, Alex von Tunzelman for History's Heroes. Subscribe to History's Heroes wherever you get your podcasts. BBC Sounds Music Radio podcasts, Inner Times.

[Faye Dauker]

On its annual break. And we'll be back on BBC Radio 4 and BBC Sounds on the 18th of September. Until then, each week we're offering an episode chosen from our archive of over a thousand programs, which I hope you'll enjoy. Have a good summer.

[Frank Close]

Hello.

[Faye Dauker]

At the age of 23, the German physics student Werner Heisenberg effectively created quantum mechanics, for which he later won the Nobel Prize. He made this breakthrough in a paper in 1925 when he worked backwards from what he observed of atoms and their particles and did away with the idea of continuous orbit, replacing this with equations. As we'll hear, this was momentous. And from this flowed what's known as his uncertainty principle, the idea that, for example, you can accurately measure the position of an atomic particle or its momentum, but not both. With me to explain and discuss Heisenberg and his uncertainty principle are Faye Dauker, professor of Theoretical physics at Imperial College London, Harry Cliff, research fellow in particle physics at the University of Cambridge, and Frank Close, professor emeritus of theoretical physics and fellow emeritus at Exeter College at the University of Oxford. Frank, what was there in his background that suggested he was going to go in that direction?

[Harry Cliff]

Well, he was born in 1901 in Bavaria, and his father was a teacher of classics and Greek. And I think that the young Werner was very interested in the ideas of Plato. He read Plato while he was hiking in the Bavarian mountains. And the reason, I think, that that was important for him Is that later he made a remark that which was that the smallest units of matter are not particles in an ordinary sense, but forms, ideas only expressed in mathematical language. So I think it was that classical background from his father that perhaps made him look that way. But he became interested obviously in maths and physics, and that was what he went to study as an undergraduate at Munich and then Gottingen from 1920-23. I think the seminal moment for him was in 1922 he went to a lecture given by Niels Bohr. Bohr was famous for having come up with the model of the atom as a miniature solar system with like a nuclear sun in the middle and electrons like planets whirling around on the outside. And this peaked Heisenberg philosophically, because nobody had seen these electrons orbiting around. So what really made you believe that they were there? And so he spent a year then working with Bohr in Copenhagen, where Bohr was based. And I think it was during that year that the mathematical scheme that Heisenberg developed first began to mature based upon what you can see. Namely, the one thing we do know about atoms is that they emit light. They, they don't emit a whole rainbow, but they emit like a barcode of individual lines. And why was that? And how could you mathematically describe that? And in 1925, he came up with the equations that did that. And that is what we call now the birth of quantum mechanics.

[Faye Dauker]

Frank, can you tell us the essence of quantum theory before Heisenberg stepped into the picture?

[Harry Cliff]

Well, quantum theory as such really began around 1900. I mean, up to that time, nature appeared to be continuous. Light, There's a whole spectrum of light. Motion is a continuous thing. But that is how things are in the large scale world which we are aware of day to day, and which the scientists up to that time have been examining. But Max Planck, a German physicist, had the insight that if he assumed that instead of continuous, nature was actually discrete, that's what the word quantum describes. There were certain things that could be explained that otherwise made no sense. For example, he assumed that electromagnetic waves are not a sort of smooth legato wave, but more like a staccato bunch of what we call photons, particles. And by making that assumption, he was able to explain the way that hot bodies radiate light. Without that assumption, the classical theory of Maxwell just didn't work. And then Einstein picked up on this idea and supposed that indeed these little particles of light existed as they crossed space and hit things. And with that, he was able to describe what happened when light hit metals and kicked electrons out, called the photoelectric effect. So these were the first two indications that if you assumed that nature was discrete on the small scale, things worked well. Then along comes niels Bohr. In 1913, Ernest Rutherford has discovered the existence of the atomic nucleus. And the picture of the atom that emerges is pretty much one that is a good model today, as I mentioned earlier, the idea of a miniature solar system with the nuclear sun at the middle and the electrons, the planets, whirling around on the outside, that was the sort of picture that emerged. The problem is that if you use the classical theory, the electrons whirling around would just spiral into the nucleus in a fraction of a second and we wouldn't be here. So Bohr made the assumption that this discrete idea, this quantum idea, applied to electrons in atoms, that they couldn't go anywhere. They were like on rungs of a ladder that they could step down. And as you step from one rung to the next, a high energy rung to a low energy rung, the energy difference is radiated as light. And that is why you see a spectrum of lines, a barcode for the atoms. And that was Bohr's model. But again, it was all ad hoc and it worked. But it was a bit like imagining. Back in the 17th century, people were aware that apples fall to earth, or if you kick something, it moves. But the equations, the mechanics, Isaac Newton's laws of dynamics, had not yet been written down. And it was a bit like that. The idea that nature is discrete on the very small scale was clearly true, but the equations of the quantum mechanics had not yet been written down. And that is what Heisenberg made the first step in doing.

[Faye Dauker]

Thank you, Fay, can you set the scene for why Heisenberg in 1925, why his paper was so distinctive?

[Frank Close]

So, as Frank described, there was an ad hoc model of the atom of atoms, but no internal dynamics to describe why the model had the structure that it does. So Heisenberg steps in and makes a number of conceptual moves in 1925. So he takes on board the Bohrian structure of states that the electron in the atom can be in. And they are discrete, so you can number them, you can label them, 1, 2, 3, 4, and they are labeled by their energies. So there are higher energy states, lower energy states. So he takes that and doesn't change that particular idea. What he does change is that he denies the idea that the states correspond to the electron having particular orbits in space, that the electron is going around the nucleus at a Fixed radius from the nucleus and with a fixed period, periodic rotation. So he just denies that. He says, forget that idea. As Frank said, that's not. He argues, that's not an observable thing. We cannot experimentally determine where the electron is inside the atom. So let's just say that that's not even speakable. We won't even speak about that. We just think of these states as being abstract states of the electron. So that's the first thing. He then centers a concept of the transition between these atomic states that he had already worked on with my collaborator, Hendrik Kramers. So the transitions are not deterministic. You can't predict with certainty when or whether an electron in one state will transition to another particular state. There's just some probability for each possible transition. So he takes that and the intensities of the radiation that the atoms emit. That was an experimentally determinable quantity that experimentalists were measuring in the lab. And the probabilities of these transitions translate exactly into the intensities of the. Of the radiation of those particular frequencies. So if you can calculate the probabilities of the transitions between these atomic states, then you can predict the intensities of the particular frequencies of light that would be emitted. So all of that was somehow already in the literature. But he takes that and he says, okay, I need to predict the actual energies of the. Of these atomic states, and I need to predict the probabilities. How am I going to do that? And he works backwards. You said that in your introduction. That's exactly right. He works backwards from the form of the measured experimental outcomes of these intensities and these probabilities and these energies. And he asked, what would the position of the electron have to be like in order to give me these particular results, these particular experimental results? And what he discovered was that it led him to a really startling proposal that the position of an electron in an atom is not given by some. A number. It's not here or there or here. But the position is represented by a completely new, unexpected mathematical entity called a matrix. This is something very abstract. It's not something that can be conceptualized as an actual place where the electron is in space. And he also postulated that the matrix that corresponds to the position of an electron satisfies an equation of motion. So it's a dynamical thing. This is the quantum dynamics that Frank described that was needed, necessary to complete the quantum formalism to a fully, fully fledged theory. And the equation of motion for this matrix, position or position matrix, was the Analogue of Newton's second law. So in one way Heisenberg was being very revolutionary, saying that positions are not conceptualizable as being in space, three dimensional space, they're these matrices. But on the other hand, the equation of motion that these matrices obey is just the normal expected, familiar 200 year old Newtonian equation of motion for evolution of position.

[Faye Dauker]

Thank you, Harry. Harry. Cliff, can we just develop this paper how it came about? And is he talking about practicalities here or thought experiments? What's going on?

[Melvin Bragg]

Well, there's a lovely story actually around the kind of key insight that leads to this paper, which is Heisenberg is in Gottingen working with Max Born at the time, but he suffers from terrible hay fever and he's sort of, so he's driven mad by it. So he goes to one place he knows where there aren't any trees, which is this island called Helgeland off the German coast in the, in the North Sea. So he retreats out there, stays in this lodging house essentially and is alone with his thoughts and the wind and the sea and bits of rock and it releases hay fever. And there's this lovely description of him, sort of in the middle of the night he's been doing these calculations and he has the kind of key realization that Faye's been been describing. And there's this lovely quote from him where he says, I think gives a sense of what it's like to make a breakthrough like this. He said, at first I was deeply alarmed. I had the feeling that I had gone beyond the surface of things and was beginning to see a strangely beautiful interior and felt dizzy. So there's this real moment, I think it's quite romanticized in the history of science about this, this breakthrough that he has. And as Faye said, the kind of algebra discovers or that he finds applies to these quantum transitions he doesn't actually recognize as matrix algebra to begin with. It's, he has these strange rules about how you multiply these different, what they're called amplitudes together in a particular, according to particular rules. And it's when he shares his paper with his colleagues born in Jordan, that I think, I think it's born who is, you know, a senior, more senior academic in Girting and who sort of in the back of his mind thinks, I recognize this strange algebraic law. I think I learned about this years ago. And it's the way matrices, multiple matrix matrices, these grids of numbers multiply together. So it's kind of, he's almost sort of stumbled upon this algebra and actually then is discovered the maths already exists and it's. But it's very unfamiliar to physicists at the time. And I think physicists really struggle to get their head around Heisenberg's approach because it's so abstract and he uses a mathematical language that at the time is really unfamiliar to people. And it's actually just a year later, another German physicist called Erwin Schrodinger, who comes up with a different approach to the same problem, which is based rather than on these strange mathematical objects called matrices, on something much more familiar, which is a wave. So Schrodinger has this, this wave description of say, an electron around an atom. And a wave is something that's intuitively much easier to understand for physicists. They're very used to dealing with the algebra and the science of waves. So actually it's later realized that really Schrodinger's wave picture and Heisenberg's matrix mechanics are actually two different mathematical ways of ultimately describing the same thing. But there's a period where Schrodinger's approach is really adopted much more enthusiastically by the community because it's familiar. And this, this really irks Heisenberg. And it becomes actually quite a sort of a bad tempered sort of debate between Schrodinger and Heisenberg as to which picture is the correct one.

[Faye Dauker]

Who wins?

[Melvin Bragg]

Well, in the end, actually, I mean, so what Schrodinger tries to say is that you can think of the electron as a physical wave. So when it, when it sort of goes into one of these states around the atom, it's a sort of physical thing that adopts this strange wave like structure. And what Heisenberg and others eventually show is that actually this isn't right, that the wave isn't really a physical thing. And it is later reinterpreted, I think, by Born, who says that actually this is not a physical wave, it's a mathematical object. And what this wave describes is not the sort of the physical nature of the electron, but it tells you the probability of finding the electron at a particular place in space. So in some ways it's. No, it's, in a sense, conceptually it's not so different from Heisenberg. Heisenberg has this way of representing this information as a grid of numbers. Essentially. Schrodinger equally has this wave, but it's not a physical object, it's a mathematical description of the electron.

[Faye Dauker]

So nothing is observed. Is everything a thought experiment then?

[Melvin Bragg]

Well, I mean, no things are observed. So these spectral lines that Frank talks about, these are the key bits of evidence about what's going on in the quantum realm. So what you do see in experiments are these particular frequencies of light that are absorbed and emitted as electrons transition. But you never see the wave that Schrodinger describes. And also, you know, and Heisenberg's description is sort of, in a way, not visualizable at all. So in some sense, you have to kind of give up on the idea of a mental picture of what's happening. And that's, I suppose, in some ways, what's quite revolutionary about what Heisenberg is saying. He says you stick to what you can measure, and you shouldn't concern yourselves with kind of an imagine, imagined picture of what's actually going on, because it's only what you see in experiments that ultimately matters.

[Faye Dauker]

Did previous theories in this area just fall away after this, then? Did it displace, say, everything that Newton had said?

[Melvin Bragg]

No. I mean, I think that there's a. Sometimes a misunderstanding in the history of science that you have. You have one picture of the world, there's a revolution, and that overturns what was there before, but that's not really the way things happen. You kind of realize that actually there's a domain in which the old physics doesn't work, but it still works very well. So if you want to know about, you know, tennis balls going through the air, Newton's laws of motion are perfectly good for that. But in some ways, I suppose it's an extension of Newtonian mechanics, and it now applies to the behavior of things that are much smaller when you zoom out. Newton's laws still work perfectly well, but they break down when you get down to the scale of atoms and molecules.

[Harry Cliff]

To pick up on what Harry is saying there, that Newton's laws apply very, very well to things that are big and move around relatively slowly. By that, I mean slowly compared to the speed of light. And the two great revolutions of the early 20th century were that Einstein asked, what happens if you go to very fast things and made the relativistic extension of Newton's laws. And now we've got the other extreme of very small things, which is the quantum extension of Newton's laws. So Newton's laws are a limiting case of Einstein at slow speeds and Heisenberg at large scales.

[Faye Dauker]

Can you sum up for listeners he's talked of in the highest terms by you three and by other physicists at the time. And since what his breakthrough was, what he actually broke through, and what was different after he'd done it in 1925.

[Harry Cliff]

One of the most remarkable things about his breakthrough was he was only 23 when he made it. I mean, that to me is one of the astonishing things. Harry just made the remark about Schrodinger, not avoiding the question, but to put it in a bit of context, he came up with this set of matrices to describe the quantum world of the atom. It is pretty difficult to construct the matrices. And the fact that Schrodinger comes along a year later with this wave equation approach and the tools that you need mathematically to deal with that are called differential equations. And they're the things that all students have learned and are familiar with by the time you get to graduate school and meet these sort of ideas. So we get taught the Schrodinger wave approach. And actually, I think one tends to use that approach. There's only a few occasions I can think of where I actually used Heisenberg matrices. And because the Schrodinger approach became so not easy, but relatively speaking, convenient to use, that's why we tend to think of waves all the time. But that quickly gets you into these philosophical problems of waves in Watts and probabilities and does God play dice with the world? And so on and so forth, which may be an artifact of the waves, which, in Heisenberg's opinion, aren't really there. So that's avoiding your question slightly. So what is it that really has been done at this point? The equations of motion, the dynamics that apply to the micro world have been identified and you can now apply them to the microworld, which you could never do before. And one of the first things which Heisenberg's approach discovers is that hydrogen. We talk of the hydrogen atom, the simplest thing, a single proton with a single electron whirling around the outside. But hydrogen tends to exist as a molecule of two hydrogen atoms together, two protons, each at atoms length apart, but sharing their two electrons. The electron swapping backwards and forwards, being exchanged between one hydrogen atom and the other. And Heisenberg's matrix approach turned out to have a very profound implication, which is this, that the electron is a lump of charge, but it also acts like a little magnet in the jargon, we say it spins. It can spin up or spin down like a north pole up or a south pole up. And this is the one place where Heisenberg's matrices really come to play. And one can illustrate them. A simple little column of just two numbers. If you think of a ground floor and a first floor, if the first floor is occupied, you put the one upstairs and the zero downstairs. If the Ground floor is occupied, the one goes downstairs and the zero upstairs. Those are the two matrix descriptions of an electron spinning up or down. And the matrix approach of Heisenberg is perfect for this. And he turns out to predict that there are two different forms of molecular hydrogen called ortho and para. And his matrices predict that one of these should be three times more common than the other at room temperature. And in 1929, these two forms are experimentally discovered and the abundance confirmed in line with what Heisenberg has predicted, and that is mentioned in the citation in his Nobel Prize by the Stockholm Noble Academy. I think that that was the experimental proof that showed that the. This formalism that he'd created was able to predict things and explain things which had not previously been understood about something as fundamental as hydrogen. Quickly then, people started applying these new techniques to electrons in all manner of stuff. Electrons in metals, electrons in insulators. Why do metals conduct? Why do insulators not conduct? These were questions you could not approach before. And suddenly, within the space of a few years, they were all falling into place, thanks to this new quantum mechanics. And the final thing, I think in 1928, was that a Russian theorist called George Gamow applied quantum ideas to the atomic nucleus. That one of the things that had been known since 1896 or so was that nuclei have a property called radioactivity. They emit this strange radiation, and one form is called alpha radioactivity. Gamow applied quantum mechanics to the atomic nucleus and explained how alpha radioactivity happens previously, something totally unexplained.

[Faye Dauker]

Thank you, Fay. In what ways was his approach revolutionary?

[Frank Close]

It was both revolutionary, but also, I would say, completely embedded in its time. No breakthrough is made in isolation from everything else. Heisenberg was in constant communication with colleagues and embedded in his intellectual milieu. So these ideas of atomic states and transitions between states, they were already there. Another aspect of his milieu was the movement in philosophy which people call positivism or instrumentalism. And he would certainly have been aware of that, not least because that tradition is generally accepted to have had quite a lot of influence on Einstein and his development of special relativity. So he would have been very aware of those ideas that physics should deal only with observable quantities. So he was primed to accept the strangeness of this new way of representing position. It was so embedded in people's consciousness that particles, bodies, every physical entity should have a position in space. It should have a position in space and move around in time. But he was just ready to make the leap. And say, no, we don't have to have such a picture.

[Faye Dauker]

I go.

[Alex von Tunzelman]

This is history's heroes. People with purpose, brave ideas, and the courage to stand alone, including a pioneering surgeon who rebuilt the shattered faces of soldiers in the First World War.

[Unknown]

You know, he would look at these men and he would say, don't worry, sonny, you'll have as good a face as any of us when I'm done with you.

[Alex von Tunzelman]

Join me, Alex von Tanzelmann, for History's Heroes. Subscribe to History's Heroes wherever you get your podcasts.

[Faye Dauker]

Harry. Harry Cliff, how did the uncertainty principle emerge, then?

[Melvin Bragg]

So this is probably what Heisenberg is most associated with, because it has his name attached to it. And it comes really from this matrix description of quantum mechanics. And it comes ultimately, mathematically, from the way that matrices multiply with each other, which is different from ordinary numbers. If you take two ordinary numbers, say 1 and 2, actually, it's a bad example, but 1 and 2 is fine. 1 times 2 is 2, and 2 times 1 is also 2. It doesn't matter which order you multiply them. You can switch them around. And it's the same with a matrix. That's not true. So there are certain matrices that describe particular properties of an electron. For example, so you have a matrix that describes the position of the electron that Fey referred to. And also there's this corresponding quantity called the momentum, which is essentially related to how fast the electron is going multiplied by its mass. And you have these two matrices, and the order that you multiply these matrices together matters. So if you can think of them in some ways as representing, measuring either the position or the momentum of the electron. So it matters whether you measure the position first and then the momentum, or the momentum first and then the position, and they give you different answers. And from this, mathematically, what that essentially, if you work through the consequences of this, you find that there is a limit to how well you can simultaneously know the position of a quantum particle and its momentum. And that is what the uncertainty principle states.

[Faye Dauker]

What are the consequences of that? Frank?

[Harry Cliff]

Profound. In a word, that is how the universe is. I mean, as Harry said, there's a trade off. You can, if you know the position perfectly, you can't know anything about the momentum and vice versa. So there's a trade off on what you can know on the average about both of them. I mean, I can give an example because people might be thinking, well, this is a bit odd if you think of a wave on a pond, and if I want to know the position, I just look at where the high spot on the ripple is. But that tells me nothing at all about how fast the wave is moving. To get a measurement of its speed, I've got to watch some ripples pass me. And the more ripples that pass, the more precisely I will know the speed of that wave. But of course, the less I know about the position because there's been so many waves that have gone past, and vice versa, if I want to measure the position precisely, I can't know anything at all about the speed. So that is an example that is familiar. And now I'm getting into the area where I start, sort of feeling I'm going in a whirlpool around some great black hole of ignorance or enlightenment. I can't be sure which waves are a very convenient model to understand why the Uncertainty Principle can apply to things. Is it the uncertainty Principle that is fundamental and waves are a nice model that help us visualize it, or is it waves that are fundamental and the Uncertainty Principle is a consequence of that? Now, I'm in the first camp because I think that actually the moment you start inventing these waves, they're a very nice model and we think of them like that, but the moment they start becoming too much reality in quotes, you get into all of these horrible paradoxes about Schrodinger's cat and so forth, which is not for me. I mean, I'm out of the studio. Should we go there? So that is the profound nature of it, where it applies and what it matters. Position and momentum are complementary. Energy and time is the other side to this. If you know at an instant where something is, you know nothing at all about its energy. If you know its energy precisely, you know nothing at all precisely about the time of the measurement. And this surprisingly explains why CERN is so big. I mean, people often say, why do you need this huge accelerator device, 27km to measure these things that are so small? Answer blame Heisenberg. These things are so small, you need to have incredible precision in space to resolve them. And to get that precision, you have to have extremely high energy, and vice versa.

[Melvin Bragg]

There's a good joke to get some of it this across, maybe in a bit more easy, understandable way, which is. So Heisenberg is driving along in his motorcar and he's stopped by a police officer who says, do you know how fast you were going, sir? He says, no, but I knew exactly where I was. Um, so if you're a physicist, that's very funny.

[Harry Cliff]

But I mean, I think one of.

[Melvin Bragg]

The things we haven't really talked about, which I think is important about the uncertainty principle is it says. The thing that's really controversial about it is it makes the, the observer, the experimenter part of the system in a way. So it matters what you choose to measure determines what you will actually observe. And that so, so you. There's no longer this, it's sort of, there's no longer this idea of an experiment as an objective thing that just looks at nature as it is. The choices you make in your observation determine the results you get. So if you choose to measure momentum or you choose to measure position, you will get, you will change the system fundamentally and change what you see. And that is, I think, you know, one of the most difficult ideas for people to accept when this is come up with in 1927.

[Harry Cliff]

I think Fay alluded to this. When you say about that Heisenberg said nobody's seen these electrons in orbits. I mean, he did a sort of thought experiment about what would you actually have to do to detect one of these electrons in an orbit. The answer is, well, you'd have to shine light on it and then the light would scatter back to you. And of course, in the process, because an electron is such a tiny thing, the action of the light that went out hitting it has kicked it off somewhere else. So you know where it was but not where it is sort of thing. And this also perhaps gives an intuitive feeling for why it is that the uncertainty principle doesn't really concern us in our day to day affairs. It's absolutely the essence of the whole thing. When you're down at the very small atomic scale, where the little particles are so light and photons hit them and kick them all over the place. But by the time you get to macroscopic stuff like us, the effects are so tiny I can know precisely where Melvin is and I can know precisely that you're not moving at this moment because you're so wrapped by what is going on in the studio. But if I could somehow shrink you down to the size of the atom, I, I'd be able to know one of those, but not both of them.

[Faye Dauker]

I was going to ask that what kind of. I'll turn to you fair on this. What value. I mean, and this is a lump and proletarian question, I hope you don't mind me asking it. What value does this have for most people, this discovery? People think we are better informed in order to do what?

[Frank Close]

It's hard to overestimate the impact that quantum mechanics has had scientifically and Technologically, it affects our daily lives. So from the predictions of the abundances of the light elements produced in the first few minutes after the Big Bang. Through the standard model of particle physics that's tested and explored at cern. To the behavior of semiconductor materials that are in the chips of every. Of all of our phones. So it has had a huge impact on all of science. Much technology that we all use. So you simply can't overestimate just how successful it has been. And yet it's such an interesting situation whereby. I know I may drive Frank out of the studio in a moment. But I would claim that we cannot deduce, we cannot recover classical physics from quantum physics. It is not possible to take quantum physics as Heisenberg set it out. And from that deduce, recover. Have classical physics, the physics that describes the behavior of macroscopic objects. The things in this room from that quantum world. From the rules of quantum theory as Heisenberg laid them down. Because he was very clear that the theory is empty unless there's an observer. If there's an observer observing the system. Then the theory, the quantum mechanical dynamics and the rules of prediction. Give you predictions about the results of observations that the observer makes on the system. If there's no observer external to the system, then you cannot make any predictions. People call this the Heisenberg cut. It's necessary. You have to have divide the world, the whole world into two pieces. One, the quantum system to which the quantum dynamics applies. There are quantum states. There are these matrices that correspond to. I won't say describe. They correspond to positions of particles. And all of that takes place in this abstract mathematical space which is elsewhere, then the rest of the world. On the other side of the Heisenberg cut is the classical world. The world of observers, of experiments, of apparatuses. And we can make definite statements about the outcomes of the measurements that we make. And there's this mysterious and absolutely not set out in any axiomatic way, interaction between the two. Of course, one of the heuristics of quantum theory is that when you interact with. When you measure, when you observe the quantum system, then your decision about what to observe and what to measure, how you set up the apparatus determines what the possible outcomes of your. Of your experiment are going to be. But the actual interaction is not described by the theory. My students all say, oh, yeah, well, what's a measurement? And I say, right question, wrong place. Let me teach you the rules of quantum mechanics as laid down by Heisenberg.

[Harry Cliff]

So when you say wrong place, you mean they should go and ask that in the philosophy department. No, no, no, no.

[Frank Close]

I mean, wrong place and time. I said, come and ask me later, Right? So we have to get through this material. Come and talk to me later. So it's the question that springs to everyone's mind when you first learn it, because it's central. The concept of measurement is central. Observation. It's central. And without it, Heisenberg's rules simply do not work. They're just empty. They don't say anything. And so we're left with this puzzle. It's fascinating, a fascinating situation for us all to be in. As theorists, we have this phenomenally successful theory. Einstein called it our most successful physical theory. I don't deny that. Don't disagree. But on the other hand, it leaves us without any picture at all of a quantum system as it is in space. Okay, Frank.

[Harry Cliff]

I'm a Luddite physicist. I mean, to me, this is all very interesting, but I. I remember a cartoon I saw many years ago of somebody in an elevator who looked washed out. And somebody said that, that Smithers, he was doing great. And then he started worrying about quantum mechanics. Strangely, the rules, if you apply them, they make predictions. I mean, that is indeed the test for me. If your theory makes a prediction that can be tested, an experiment either confirms or denies, then you know what you're doing. I'm also amazed that. I mean, as you alluded to this last six months, I've benefited from magnetic resonance imaging, positron emission tomography. They are things that the quantum world has led to as tools that we now use in the macro world. So without quantum mechanics, a lot of the things we're taking for granted today, we would never have been invented, whatever the philosophy behind it. I don't know.

[Faye Dauker]

Unfortunately, we have a limited time. We don't have the five hours I really deeply like at this moment. I really would. I'm joking. It's absolutely fascinating for somebody who gave up physics at the age of 14 or was asked to give up physics at the age of 14 in order to take up Latin. Imagine that. Harry, I'm going to switch now. We're talking about theories, but what about the man? One of the things about the man is that the politics of the 1930s affected him and his position with regard to Germany.

[Melvin Bragg]

Yeah, so. So Heisenberg lived through a very turbulent period in German history. So when he's a young man just going into university, Germany is coming out of the humiliation of the First World War. There are armed groups on the street, and Heisenberg I think, is dismayed by what's happened to his country. He is a sort of patriotic German. But then as you get into the 30s, the rise of Nazism, antisemitism becomes increasingly prevalent in the. In the academic world. And there are certain physicists, a German physicist, Philip Leonard, is one example, Johannes Stark, who are deeply opposed to what they see as Jewish physics. So this is the sort of ideas proposed by Einstein around relativity, but also by extension, quantum theory as well. So Heisenberg's reaction, I think, to this at first is his view is that somehow physics should be separate from politics, that actually politics is beneath the dignity of a sort of an academic aristocrat, that this is a sort of essentially the ultimate ivory tower, that he shouldn't have to deal with this. And his. His reaction to what's going on around him is, I think, quite. He doesn't come out of it sort of smelling of roses. He's not an enthusiastic supporter of the Nazis, although when they do come to power, he does express some sympathy with some of the things they're trying to do, a sort of national revival, because he wants. He wants Germany to sort of be on the up. But at the same time, he does spend a lot of effort trying to prevent the dismissal of his Jewish colleagues from their university positions and also to persuade his colleagues not to leave Germany. And it's clear that in the 30s, his key objective is really to preserve German physics so that when the Nazis go, there will be still physics of high quality in Germany. And that's his primary focus. But as a result, he does accommodate to a large extent with the. With the regime that he finds himself living under.

[Faye Dauker]

How does this affect his reputation in the physics. In the world of physics?

[Melvin Bragg]

Well, it's interesting because Heisenberg actually comes under quite virulent attack by people like Stark. So I think it's in 1937, Stark is writing articles in Nazi publications attacking Heisenberg as a sort of not, you know, he's not Jewish himself, but saying he's a sort of supporter of Jewish physics. And this leads to an SS investigation into Heisenberg. So he's actually put under investigation by. By the ss. He becomes very desperate. He's interrogated in Berlin, in the SS headquarters. He ends up writing to Himmler directly, pleading his case. Eventually, he's exonerated after quite a sort of traumatic investigation, at which point he is kind of keen to prove his usefulness to the regime. And this extends into the Second World War, when he becomes involved in the German nuclear Research as well. So I think. I think it definitely harms his reputation. He's given many opportunities, actually to leave Germany. So he's offered positions in America, for example, at Columbia University. And many of his colleagues are perplexed as to why he refuses to leave Germany, given the sort of pressures he's come under. Personally, I think a lot of his colleagues view him quite critically for not having taken a more courageous stand against what was going on in his country at the time.

[Faye Dauker]

Faye, what's his continuing impact?

[Frank Close]

So at the moment, I'm working on a project on quantum field theory. So one of the things that I think we've alluded to already is that the principles of quantum mechanics, which were essentially discovered in 1925 by Heisenberg and then separately, in a sort of different form by Schrodinger, those principles were very swiftly formalized. And then it was realized that they could be applied to any physical system whatsoever, so long as the classical form of the theory obeyed Newton's laws. You could do this process which people call quantization. So you take the classical form of the theory, Newton's laws, and you turn the handle almost Heisenberg, Schrodinger handle, and you produce the quantum theory. And that this was so universal that it. That there wasn't, I think people thought at the time, there's no limit to what we can apply this to. It's so universal, we can apply it to any system at all. And one of the systems that people applied it to was field theory. So electromagnetism was a field theory, the fields of Faraday and Maxwell. And so quantum field theory is a quantum theory that is completely in accord with those axioms that were essentially laid out by Heisenberg in 1925. So I'm doing a project on quantum field theory, and we say the word Heisenberg probably about 20 times a day. So there's the Heisenberg operators, there's a Heisenberg equation, there's a Heisenberg picture. Although, on the other hand, my research area is quantum gravity. So I would like to understand how quantum matter can be compatible with our understanding of space time. As Einstein laid it out in general relativity. Gravity does not fit into this paradigm. You cannot take general relativity, turn the Heisenberg Schrodinger handle, and produce a quantum theory for gravity that works. And the reason is that the theory of gravity that we have, our best theory of gravity is a theory of space time itself. So in order for there to be a quantum theory of gravity, there has to be a Quantum theory of space time. So space time itself must be part of the quantum system. And that raises this issue which I've, which I've mentioned, that in order for the Heisenberg Schrodinger rules to apply, you need something outside the system to do the observing. But if your system is space time itself, then kind of by definition there's nothing outside it, because everything that happens, happens in space and time. So how could you make that work? So the struggles in producing a theory of quantum gravity are as much conceptual as they are technical. And this idea of the necessity of there being an observer and hanging everything, all the, all your meaning on the results of measurements and observations is actually a barrier now to making progress in quantum gravity, in my view.

[Harry Cliff]

Finally, Frank, what Faber is just saying, the universe, why are we here at all? And I don't mean here today, I mean the whole, the whole thing. She's saying, and it has been suggested, semi seriously, or maybe even seriously, that the universe's existence is itself an example of Heisenberg's uncertainty principle. The idea that you can overdraw the energy accounts by a small amount for a small amount of time, so long as the product of the two is constrained by this quantum uncertainty. And one of the surprising things is that the universe itself, because there's a lot of gravity around, when you're in a gravitational field, you have negative potential energy. There's a lot of positive energy around in all of our MC squared and so forth. It is possible that the sum total energy of the whole gravitational universe is nothing. In which case, Heisenberg says, you can borrow that nothing forever. And so the universe could be a quantum fluctuation satisfying Heisenberg's uncertainty principle. The problem, or a problem with that is. So where was that encoded? Who or what encoded that principle that enabled a universe to erupt out of nothing with a total energy balance which Heisenberg, millennia eons later, would formulate answers on a postcard.

[Faye Dauker]

Well, thank you all very much. Exhilarating. Thanks, Frank. Frank Close, Faye Dauker and Harry Cliff, and our studio engineer, nevermysterian. Next week, uprising in Algeria in 1871 against the rule of France, when that country was reeling from the Paris Commune and the loss of Alsace Lorraine to the new Germany. That's a McCranley rebuilt. Thanks for listening.

[Frank Close]

And the In Our Time podcast gets some extra time now with a few.

[Alex von Tunzelman]

Minutes of bonus material from Melvin and his guests.

[Faye Dauker]

That was terrific. I'm afraid I'm gonna ask you to do some more.

[Harry Cliff]

When you were asking you know about how the Heisenberg uncertainty principle gets used. I know that you're always loving these amazingly small things or huge numbers and the things that you keep trotting around in particle physics all the time. This complementarity between energy uncertainty and time uncertainty is key. I mean, time uncertainty, most of the particles that we find are unstable. They live for 10 to the minus 24 seconds. That's the time it takes light to cross one tenth the size of an atomic nucleus, that small amount of time. But Heisenberg tells us that if you try to measure the mass of that particle, its MC squared, its energy will be uncertain because of the limited time. And we can measure that uncertainty in energy, and that's the way that we do it. At cern, they discovered the Z boson years ago by tuning the beams to start making it and the beams showing nothing. Then gradually they built up to a peak and then down the other side. So there was a spread in the energy. And from Heisenberg, that spread in energy was interpreted as the lifetime of the particle. So we can measure times of that minute amount by using Heisenberg. And I think it's fair to say those are the smallest measures of time that we actually do measure in practice. And it's using Heisenberg to do it, Harry.

[Melvin Bragg]

And one of the things we didn't talk about, I think, is, I don't know if we emphasize enough how so revolutionary the new quantum mechanics was because, you know, there's the sort of famous debates between Bohr and Einstein and also Einstein and Heisenberg. So, I mean, Einstein really reacted very strongly against Heisenberg's sort of really pragmatic. We only worry about what you can measure. And, you know, Einstein really wanted to hang on to this idea of a mental picture of what was going on, which Heisenberg sort of approach denied, but also this fact of the universe being probabilistic and not being deterministic any. I mean, we did. We did talk about it, I think, but maybe that sort of should be emphasized a bit and just how radical that was. It's a complete break with, you know, how you think about the world in the 19th century, where if you know the positions of all the atoms in this room and how fast they're going, you can predict arbitrarily far in the future what's going to happen. And quantum mechanics says, no, you're not allowed to know that. You can only say what the probability of certain outcomes is in the future.

[Faye Dauker]

Who says that's the only thing you.

[Melvin Bragg]

Can say, well, I mean, that's what, that's sort of fundamental to quantum mechanics. So now you know when you, even if you know everything you can know about a system because of the uncertainty principle, because of quantum mechanic laws of quantum mechanics, all you can say from this position in time is that there are certain possible outcomes and we can assign probabilities to those outcomes, but we can't say which outcome we're going to arrive at. And that is, that is the sort of, I suppose the philosophical, the really big change that comes in with quantum mechanics.

[Harry Cliff]

To know what's going to happen to one particle in the future of the universe, you've got to know two things precisely both where it is now and how fast it is moving now. And Heisenberg says you can't know both of those. And so that's the uncertainty that you're bringing in. So in principle, does he say you.

[Faye Dauker]

Can'T know both of those together or you can't know both of those at all?

[Harry Cliff]

You can't know both of them. To perfect precision, there's a trade off. And the amount of trade off is controlled by the size of the discrete quantum.

[Melvin Bragg]

And in fact, if you know, say the position of an electron exactly, you know nothing about its momentum at all. So you lose all information about how fast it's going and corresponding the other way around. So if you know exactly how fast it's going, you have no idea where it is. Well, Frank sort of talks about this, this idea of a wave. So if you can a wave, imagine a wave on the surface of a pond that goes on forever, that mathematically has a well defined momentum. So you can express that as a pure state of momentum effect. So you know the momentum of that wave perfectly. But a wave that's infinite in extent has no position, it's everywhere. So you can't say where it is. Equally, if you want to localize that wave, one of the ways you can do it is by adding up lots of different waves with different frequencies in such a way that the peaks and troughs add up and cancel out that you get and you end up with a spike at one location. So you've got now location, but that's made up of a huge number of different waves with different frequencies and you don't know what momentum is anymore. So that, that's a sort of, I don't know how intuitive that is, but that's one way of thinking about what's going on.

[Harry Cliff]

The listeners invite them. Now let's draw a wave on a piece of paper as a series of dots. So please put down the first dot. And what's the wavelength of that wave? No idea at all. Put some more dots in and I can now begin to get an idea of what the wavelength is. But you spread those dots over a range. So there's a trade off between position and wavelength.

[Melvin Bragg]

So much better than what I said. Actually.

[Harry Cliff]

What you said is the mathematical realization of that. It's called Fourier analysis. It's got a long history back into the centuries.

[Frank Close]

What Harry and Frank are describing is indeed the mathematical reasons for the theorem which we call the uncertainty principle, the uncertainty relation. But what's really revolutionary about it is that it doesn't refer to the position or the momentum of the particle in itself, because that you have to deny. So what's really revolutionary is that you say it doesn't have a position, it doesn't have a momentum. And this is just an uncertainty which expresses itself experimentally. It takes the form of statistical uncertainty about sequences of measurements. You do many, many repeated measurements of the same thing over and over on an identically prepared quantum system. And the uncertainty relation relates to the statistics of those measurements. It doesn't say actually if you, in the axiomatic formulation, it doesn't say anything about what? About the position or the momentum of the particle. That's what's so fascinating about the 1927 paper about the uncertainty, relational uncertainty principle. Heisenberg is having this debate with himself in print on the page. So he's debating Schrodinger. So as Harry said, there was this, this discussion, debate, argument between Heisenberg and Schrodinger at the time. And you can, you can see that in, in the 1927 paper, he's responding to Schrodinger in print, in the, in the paper, but he's also responding and debating with himself and he's trying to hold true to this idea that you should not ever talk about a particle as having a position and a momentum. But then he uses that picture to describe this so called Heisenberg microscope, whereby you imagine that it does, the particle does have a position. You're trying to locate it using by shining light on it, blah, blah. But that whole Heisenberg microscope argument relies on you having this picture of it being, having a position and momentum which you're supposed to deny. So it's simply, he's struggling and you can see it, it's totally fascinating, struggling with himself about wanting not to talk about position and momentum and space, but then needing to talk about position and momentum and space in order to give people a heuristic way of understanding the uncertainty relation.

[Harry Cliff]

I think that's really what I was trying alluding to when we had this debate in the, during the, the program, when I said, I'm a Luddite physicist, that if I start asking myself, trying to understand what's really going on there, I end up in a fog. If I use the rules that have been developed, they work. And in that sense, I'm in the latter camp. Like an engineer, I will use the rules and let others worry about why they are. And I think it also, it was interesting what you said about Heisenberg's debate, the gap between the micro world and the Heisenberg cut. Whether you look at Heisenberg's or Schrodinger and waves and things, you end up. There's a problem. It's the Heisenberg cut or it is the Schrodinger's cat. Whichever way you look at it, there's this dichotomy. How do you get from the quantum world to the macro world?

[Frank Close]

So I take what you say perfectly frank, and I, I in your lab, I completely agree that as a practical matter, the rules of quantum theory work for making predictions about your, your, the results of your experiment. But what's practical depends on what you're trying to do. So if you're trying to find a theory of quantum gravity in which the whole universe is quite quantum, then as a practical matter, the rules of quantum theory, as laid down by Heisenberg will not work. But they cannot because there's no external observer. So you have to, you're forced. As a practical matter, if you can think of trying to find a theory of quantum gravity as a practical matter. As a practical matter, as a working scientist, you have to go beyond it.

[Faye Dauker]

Harry, do you want to come in?

[Melvin Bragg]

I'm an experimental physicist, so I'm feeling slightly out of my depth here, to be honest.

[Harry Cliff]

But, I mean, one thing, I was.

[Melvin Bragg]

Going to ask Faye about this, actually. So with quantum gravity, as I understand it, my very limited understanding, you're talking about effects that only really manifest themselves at extremely high energies, extremely short distances. In terms of. So as the question is, in terms of the practical things you would observe in an experiment, what. What is a theory of quantum gravity actually trying to solve? Because at the moment, as far as I might, as I said, you're much more expert than me, but as far as I know, there aren't really any experimental inconsistencies with the two theories we have for the universe at the moment, quantum Mechanics and general relativity, they cover everything we've ever seen perfectly well. And it's more a sort of. We're worrying about things that might happen and in very extreme conditions that we haven't yet observed. So, I mean, I suppose if we're going to take that practical approach that Frank takes to quantum gravity, what are the problems where. Where you think, why do we need such a theory? And is it just. Is it. Is this more philosophical and kind of we would like a quantum theory of gravity because we think these two things should be reconciled. But what's the actual practical problem we're trying to address?

[Harry Cliff]

Well, I'm on phase side here that this is exactly analogous to what Einstein was worrying about in 1900, about what happens if he lived on the light wave and by doing those thought experiments, found the contradictions and moved forward. And likewise the questions about the imagining what happens if you do an experiment where you're making quantum black holes fluctuate out? At least you can imagine that, although we can't do it in the laboratory, we can imagine that and we can show that quantum theory as at present formulated and general relativity cannot live together. And something has to give. Is that a fair statement or.

[Frank Close]

Yes, absolutely. But I think there's more than just an intellectual problem there of trying to bring together two theories which at the moment are in contradiction in certain extreme regimes. So, for example, the. What people call the standard model of cosmology today is a very simple model, but there's. There are parameters in the. In the model that so thus far are just phenomenological parameters. You just choose them to fit the data. And two things about those parameters. One is that there's starting to be real tension between are observations and the standard model. So there's something called the Hubble tension, which is measurements of the Hubble constant or Hubble parameter today from late time measurements of observations of, for example, supernovae and early time measurements of, for example, the cosmic microwave background radiation. So those two measurements of this Hubble parameter today are in tension. And some people, increasing number of people, believe that that tension is a real tension. It's, you know, it's something which we can now say, yes, there's definitely a discrepancy between the model and our observation. So that's the first thing. The second thing is some of those parameters are very strange. So, for example, at the Big Bang, the hot, dense state that the universe began in, we believe space is very, very, very, very, very, very, very flat. You would expect it on dimensional grounds to be Very curved. That everything is of the scale of what people call the Planck scale, that there should be. There is curvature on the Planck scale in time, but in space it's super, super, super flat. As far as we, we know, it's consistent with being perfectly flat. But there's a bound on how much curvature there can be in space. Very strange. And that is what people call a fine tuning problem. Why should it be so flat then? So those initial conditions of the universe, so there's the dynamics of the universe, we believe it's general relativity and other classical theories, but the initial conditions are unexplained. Why is the world, the universe, the way it is? Why, why did the cosmos start out the way it did? And we expect, we believe, we hope that a theory of quantum gravity would tell us why the universe started out the way it did, because in some sense it would tell us what happened before the Big Bang. So what led up to the Big Bang? That would be the deep quantum regime in which there's no classical space time at all. We would have the full quantum theory of space time, which was, probably looks like nothing that we have now. And that would then give us those initial conditions. So it is a scientific problem today, not just an intellectual exercise that may bear fruit in the future.

[Harry Cliff]

The fact that we can ask such questions actually is the result of the things that Heisenberg did and have been developed from that. I mean, the fact that such questions can be asked and tackled scientifically today are a legacy of the things that Heisenberg did in 1925 and grew from it.

[Melvin Bragg]

And also actually in the future, moving away from science into technology. Some of, you know, the big breakthroughs people are expecting technologically, a lot of them are quantum related. So quantum computing, which has become, you know, increasingly a kind of growth area, like a lot of my PhD students who work in experimental physics are now going off to. One of my old students is now writing software for quantum computers as his job. And this has become an area that's, you know, potentially going to have revolutionary impacts on the way we live, particularly coupled with AI and other developments. You have quantum sensing. So, you know, the future I think is going to be quantum as well. And with. Yeah, so the leg, the legacy of Heisenberg isn't just scientific, it's also now. Well, it has had a huge technological impact already, but it's going to continue to do so in the future as well.

[Faye Dauker]

Thank you very, very much. On time. Here comes our producer, man Simon.

[Frank Close]

Yeah.

[Harry Cliff]

Thank you.

[Frank Close]

You have to go.

[Melvin Bragg]

I've got to go.

[Harry Cliff]

Three T's.

[Melvin Bragg]

Thank you very much.

[Harry Cliff]

In Our Time with Melvin Bragg is produced by Simon Tillotson and it's a.

[Melvin Bragg]

BBC Studios audio production.

[H]

I'm David Runciman and from BBC Radio 4. This is Post War from the Cradle.

[Faye Dauker]

To the grave, they said.

[H]

Eighty years on, we're telling the story of the 1945 election and the creation of post war Britain.

[Harry Cliff]

There must be a revolution in our way of living.

[H]

This is the Britain that many of us grew up in and which still shapes an idea of who we think we are.

[Harry Cliff]

Even Winston Churchill thrown out. All right, he may have won the.

[Faye Dauker]

War, but here you're going to win.

[H]

The peace post war with me. David Runciman. Listen on BBC sounds.

[Faye Dauker]

Can we have the Britain we desire?

[Alex von Tunzelman]

This is history's heroes. People with purpose, brave ideas and the courage to stand alone, including a pioneering surgeon who rebuilt the shattered faces of soldiers in the First World War.

[Unknown]

You know, he would look at these men and he would say, don't worry, sonny, you'll have as good a face as any of us when I'm done with you.

[Alex von Tunzelman]

Join me, Alex von Tunzelman for History's Heroes. Subscribe to History's Heroes. Wherever you get your podcasts.