C (3:03)

Today to discuss your book. Great to be with you, Greg.

D (3:06)

Harry, why did you write Space Oddities and who is the target reader?

C (3:09)

Well, the book really came out of my own research. So for the last God, ten years or so, I've been working on anomalies that we've been seeing at the Large Hadron Collider particularly. So maybe we'll get into this at some point, but I work on a type of particle called a bottom quark, which is a fundamental particle. They're very interesting for various reasons, but we've been making a series of measurements that have been showing them deviating from the predictions of our current best theory of fundamental physics. So that's been kind of a very exciting journey as a, as an experiment and, and also personally in my own research. And so I've been thinking a lot about anomalies in my day job and then looking around sort of science more broadly, particularly in cosmology and in astrophysics and in particle physics, you kind of realize there are a whole host of these anomalies all the time. I mean, actually, this is generally true if you take any time in history, but particularly now when we are in this really curious position where we have two very well tested theories of the universe. One that describes particle physics, the very small one that describes the evolution of the universe as a whole, the standard cosmological model. And yet we know these models are incomplete in some way. We've been looking for a long time for something new to fill in the gaps. And these anomalies could be the clues to that, that thing that comes next, that next view of the universe. So I kind of really wanted to kind of explore the role that anomalies play in science in general. So there's a bit of history in there talking about anomalies past, but then also focusing on these kind of, these six big stories in the book that are kind of sampled from those, those subjects. So that's kind of where the motivation came from. And then in terms of the target audience, I think it's really anyone who's kind of interested in, in, in physics and the kind of physics is a spectator sport, I suppose. I mean, you don't have to be a scientist to appreciate the progress of science. I actually think kind of following science as a spectator is a really enjoyable, rewarding thing to do. Just like you might follow your favorite football team. And this is sort of giving you the latest score, if you like, from the current season of work in physics. So it's hopefully giving people a view kind of behind the curtain on what actually goes on in the nitty gritty of experiments and in research institutions.

D (5:22)

Yeah, that's a great answer. And I should point out in the book you talk about how the history of physics is chock full of weird results that presage a transformative discovery. Do you think that's the case even for the six or so anomalies you discuss in the book and where we are today?

C (5:38)

I think the thing you have to start with is that for most anomalies, the explanation is not some revolution in how you think about the universe. And there's kind of, I think in the book I list these kind of three boring explanations that you have to eliminate before you can conclude you've already seen something genuinely new and exciting. And there are kind of three obvious explanations for an anomaly before you get to that. One is that you've made a mistake with your experiment. And that happens all the time. Because like, you know, astronomical particle physics experiments or all scientific experiments are really complicated. There's loads of factors you have to control for. And even very, very smart, hard working teams of people can miss things that they just didn't anticipate. There are many famous examples of these kind of results that got people very excited and then turned out to be mirages that was due to, you know, for example, due to a cable not being plugged in properly. And one famous famous case where people thought they'd seen neutrinos going faster than the speed of light, that it was traced to a cable that hadn't been plugged in. If I had been a real result though, that would have overturned like the foundations of physics, you know, relativity and all the rest of it. So that's kind of number one boring explanation. Number two boring explanation is it's just A statistical fluke. So whenever we make a measurement in physics and in science in general, you measure something, that's always some uncertainty associated with that measurement. How precisely do we think we know this quantity? And when you're making measurements and comparing them to some theoretical prediction, if your uncertainty is relatively large, there is a good chance that it's just random statistics that have meant the results drifted away from the expectation. It's like if you pick up a coin and you're asked, is this a fair coin? There's a probability that you toss 10 heads in a row. Say, it's not very likely, but it can happen every so often. So that's the kind of statistical problem we have to guard against. And then the final one that also happens sometimes is that you're comparing to a theory. Well, theorists also make mistakes. So theorists can miscalculate something and you realize that actually your measurement's fine, but the prediction you were comparing to was wrong. And that has also happened repeatedly in the history of science. But it's only when you can eliminate these three boring explanations that you can then say, okay, we're really sure this is something new. And for the six anomalies in the book, in fact, actually, just even since I've written the book, and this is the danger of writing about current science, at least a couple of them are now looking less solid than they were. There's one that's looking perhaps more solid than it was. Maybe we'll get onto which one that is. But, I mean, I say in the book, it's always. This is always the case. So most of them go by the wayside, and you kind of. You have to pursue these things and track down the source of the anomaly and. And eliminate it. And even when it turns out that the anomaly is due to some experimental mistake or some theoretical miscalculation, you generally learn something quite important in that process, which allows you to then build the next experiment. So even when they disappear, they're very useful in terms of sharpening our scientific tools, as it were. But sometimes they do lead to something big. So I have to say, if I was a betting person, I think there's a good chance we're going to learn something profound from at least one of these anomalies. But I suspect most of them will turn out to be not the signs of something revolutionary.

D (8:57)

Okay, perfect. So I want to ask you about which one you think we are going to learn from and represents a breakthrough. Before we go there, I do want to point out what is nice about your book. You sort of hit on it. It is a bit warts and all of. Not mistakes, but reasons made me for thinking we have a new discovery relative to the standard particle model is sort of human error that cable not being plugged in or just the data. And I like the fact that you show that throughout the book. And then obviously you definitely show the, you know, follow the weirdness, follow the strange with the data, setting up the experiments and then, yeah, the attitude towards mistakes. I think you quote one of your colleagues as saying, you know, I like mistakes. I'm happy to have another brought us that much closer understanding. So it is a very human and sympathetic in terms of showing the humility of the scientists, which sometimes we might not always see. So totally great, great observations there in your book, Harry. Let's just start at the beginning here. Why is there a crisis in physics? You've already touched upon these two standard models of particle physics and cosmological model. Could you go into a little more detail there?

C (10:08)

Yeah, I mean, I don't know. I think saying there's a crisis in physics in general is too. Is too big a statement. There is certainly you could argue there are crises in certain areas of physics, depending on how you look at it. Other people might say, oh, it's not a crisis, it's really exciting, you know. So it kind of depends on your perspective. But I suppose there's this general story in particle physics of the last decade and a half, which is that we have this standard model, this theory that was put together in the 20th century, in the sort of 60s and 70s, really, that is stunningly successful. It describes more or less every experiment we've done. It describes the basic building blocks of the universe, the known fundamental forces, with the exception of gravity. And it's tremendous. But we know that this standard model is not the final word on fundamental physics. One reason is it doesn't include gravity. But there are more immediate reasons. Like, you know, when astronomers look into the universe, their measurements show that 95% of what's out there in terms of matter and energy are not substances described by the Standard Model. There are these things called dark matter, dark energy. These are just terms to cover our ignorance. We know something about their properties, but we have no evidence for what they actually are physically yet. And so we kind of know that there is new stuff to find, and we've known this for many years now. And, you know, when I started out in particle physics, which was quite a while ago now, back in sort of 2008, 2009, that was just before the Large Hadron Collider switched on. So this is this huge 27 kilometer circumference particle collider buried under the ground near Geneva in Switzerland, on the Swiss, French border. And the great optimism at that time was that when we switched on this huge machine that had taken decades to build, we would discover clues to this next view of the universe. So we'd find dark matter, or we'd find something else that showed us how to extend the Standard Model and how to include all these things that we're clearly missing. And what has happened at the lhc, broadly, there's been loads of amazing science and lots of detailed measurements and discoveries, but the major achievement I suppose you have to point to is the discovery of this thing called the Higgs boson. But that was a particle predicted by the Standard Model. It was the last missing piece that was kind of finishing off that theory. But it hasn't really provided us yet with any solid clues as to what comes next. And so in particle physics that I think for people who came in, in that period, in that period of great optimism, the last 10, 15 years has been a sort of challenging time, because you keep looking, you keep looking, you keep looking. And the data says, well, you know, there's nothing new showing up. We're measuring the Standard Model more and more precisely. But it seems this theory is really holding up, and that's kind of very contradictory. So there's a kind of. Some people call that a crisis. Other people say, well, actually, maybe this is, you know, it's a kind of cliche, but this is an opportunity. Because what this is telling us is that the way we've thinking about these problems in the past is clearly wrong, and we need a kind of wholesale rethink about our approach to finding new physics, finding new particles, what have you. So we're kind of at this interesting juncture where the kinds of things we're now looking for in experiments has shifted from where it was a decade or more ago. So that's one. And then in cosmology, it's different. I think in some sense, the Standard model of particle physics is extremely solid and kind of well understood. The standard model of cosmology is a bit different. So this is a theory that describes the beginning of the universe as evolution and through the Big Bang and the formation of large scale structure, galaxies and stars and so on. And it's pretty good at kind of, you know, mapping the broad patterns that we see in the data, in the. What is actually very good, mapping the observations. But the Two key ingredients in this model are dark matter and dark energy, these two unknown substances. So when your theory is like, you know, the key ingredients are things we don't understand, that suggests, and most cosmologists, I think, would say, even people who put together this theory, that this is, you know, there is very likely to be revisions to this theory because the two major ingredients are unknown to us at the moment. So there are all kinds of assumptions about what dark matter and dark energy are like, how they behave, which go into this model, which don't necessarily need to be true. And so the crisis in cosmology, I think that is being driven now by this particular anomaly that has been plaguing or plaguing or stimulating. Interesting. I suppose, in cosmology for the last decade or more, which is anomaly to do with the rate of the expansion of the universe. And there are people who remain skeptical about this anomaly. Other people see it as a kind of a real challenge to the cosmological theory. And some people call that a crisis. But I think a crisis sounds like the field's in trouble. I think when you have an anomaly like that, if that's actually very exciting. So it's kind of really, I think, you know, crisis maybe isn't quite the right word for that.

D (15:08)

Interesting. And I should say in the book, I think you just said this, dark matter and energy represent roughly 95% of what we think makes up the universe. Is that correct?

C (15:18)

Yeah, that's right. Yeah.

D (15:19)

Yeah. And you actually suggest that the standard monocle particle physics, which describes 17 different elementary particles, none of them has the right properties for dark matter. How you sort of answer this, but just missing that 95%, how big an issue is that? It feels like, you know, we're in the early innings, or I think you quote one of your colleagues saying, we really just don't know much about the universe at this point. Is that accurate?

C (15:45)

Yeah, I mean, I think, you know, if you don't know what 95% of the universe is, Yes. I mean, you can argue that we know the 5% pretty well. So this standard model of particle physics describes that 5%, which is basically stuff made of atoms and light and the familiar forces that we know about, we do understand that sector of the universe very well and very precisely with very, very high precision predictions and measurements. But we know at the fundamental level, we know more or less nothing about this dark sector. And so, you know, there's kind of a big question there, which is, if you think about the 5% of which makes up the visible universe, that 5% is a very, very rich place, right? There's all kinds of phenomena, planets, life, stars, radiation, all kinds of things going on in the universe. Magnetic fields, black holes, et cetera. And the question is, you know, is that 95, there's a kind of assumption, it's a kind of Occam's Razor type assumption, which is that dark matter and dark energy are something relatively simple. So dark matter is one extra particle that we've just not found. And that one extra particle explains that 27% slice of dark matter. And then dark energy is some other kind of something in empty space, which is also probably quite simple, which accounts for the remaining 68% or whatever it is. But that needn't be true. And it could be. And Lisa Randall, the Harvard theoretical physicist, has written about this, that it's quite possible that dark matter is just as interesting as the atomic matter that makes up the universe. So this 27% Dark Sector, Dark matter sector could have all kinds of things, structures, phenomena in it that mirror kind of what we see in the visible world. So if those sectors are very similar, sort of getting to your point of how much do we know? If these sectors are really simple and they're just one extra particle and one extra field, then actually you could argue, okay, well, they make up the bulk of the universe. But it's just two new things we need to find. And once we've found around, and then we'll kind of understand a pretty good view of the whole picture. But if that idea is wrong, there could be all kinds of phenomena hidden in this dark sector that we have yet to explore. So we're in a position of kind of. It's like what Donald Rumsfeld called the known unknowns and the unknown unknowns. So we know. We don't know a lot, but we don't know how much. We don't know if that makes sense.

D (18:14)

No, absolutely. It's always fascinating to hear we're still missing 95%. Because you do think, as you pointed out in the book and others have said, the Standard Model has been very precise. It seems like we've yet to break it or go to a new physics. So it seems to be holding up well, but obviously still needs to be completed. You spend some of the early part of the book talking about the discovery and our understanding of the atom and eventually leading it to the idea that the universe is made entirely of sort of ethereal quantum fields. Could you talk about how we, how we reach that conclusion?

C (18:51)

Yeah, I mean, so the story I mean, this is actually a story that I cover in a lot of detail. My first book, which is called how to make an Apple Pie From Scratch, which really traces how we figured out what the physical universe is made from. And that kind of began, you know, long, long ago. People often start with the Greeks, because it sounds clever to start with the Greeks, but, I mean, it really starts with kind of chemistry. In the 17th and 18th and 19th centuries, people start to figure out that the way chemicals react with each other suggests this atomic nature of matter. But it's not really until the very late 19th, early 20th century that we start to get direct evidence for matter and then subatomic particles. But the kind of. And that happened broadly, experimentally. It was driven by people in laboratories with glass tubes and gases and high voltages and eventually accelerators, probing deeper and deeper and deeper into the structure of matter and discovering these sub components. But the kind of. The view that we. I guess, a lot of us carry around in our heads about what the world is made from. When we think of atoms or we think of particles, if we think about them at all, like electrons, we probably think of little balls, like little kind of hard things that marbles whizzing about in space. But that isn't what modern particle physics says about the universe. What it says actually, is that particles themselves are not really fundamental in some sense, that they are actually manifestations of something deeper and more fundamental, which are these things called quantum fields. A quantum field is much less tangible than a particle particle. You can kind of visualize a little ball, a field, I suppose, if you want to. It's not a perfect analogy, but it's kind of like an invisible fluid. So, you know, we. We are kind of. There are probably some fields we're more familiar with than others. Like, we're probably familiar with the idea of an electromagnetic field or a magnetic field in particular. So, you know, if you have a. If you. I'm sure at some point, most people in their lives have had two magnets, maybe at school, and that, you know, you push the North Poles towards each other and you feel this force. And there's nothing in the gap. You can't see anything in the gap between the poles, but you can feel this physical thing, and that is. That is the effect of a magnetic field. And so these fields are things that permeate the universe. They're everywhere. They're invisible. But what particle physics says is that for every particle that we know about, like the electron or quark or a Higgs boson, there is a corresponding quantum field. And each particle is a little vibration in that corresponding field. So there is something called the electron field that fills the whole universe. It's kind of like an invisible fluid. You can think of it that way. And every electron is a little ripple in that fluid. So every electron in the universe is part of the same ocean, if you like, the same, the same body. And, and that, that is already the modern view of, of what matter ultimately is made from. We are kind of, we are made out of these disturbances in these fields that fill the universe. And that view of matter was arrived at really through experimentation and theorizing that went on in the sort of 30s and 1940s. And this kind of view solidified in the kind of latter half of the 20th century, really through various experimental breakthroughs, theoretical breakthroughs that I go into in detail in the book.

A (22:07)

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D (23:03)

Great. You quote Feynman saying, the first principle is not to fool yourself. And you are the easiest person to fool. Or scientists susceptible to this law. And could you talk about it in particular respect to the Dhamma Libra team?

C (23:17)

Yeah. So, I mean, if you're a scientist, you go into science because you want to learn something about the world, but the reality is that most of the time, your experiments don't. You always learn something from your experiments, but you don't necessarily discover something fundamentally new that's very, very rare. And so when you do see some blip in your data that doesn't really fit, there's a very natural reaction which is, oh my God, goodness, maybe I've seen something that's going to really change how we think about particle physics or astronomy. And you have to really guard against that because As I said, 99 times out of 100, you haven't made a Nobel Prize winning discovery. There is probably some bug in your experiment or some problem with your analysis that you haven't accounted for. And so you have to. Where Feynman says you can't fool, you mustn't fool yourself, and you are the easiest person to fool. That's what he's talking about. It's this urge to discover, which is why we're in the game of physics or science in general. But also acknowledging that you're very flawed and you're much more likely to make a mistake than you are to make a groundbreaking discovery. So, I mean, I don't want to be so. Dharma Libra is a particular story that I discuss in the book. This is a dark matter experiment that's been running in Italy in a place called Gran Sasso, which is this mountain to the sort of northeast of Rome mountain range. And it's this deep underground experimental facility with lots of different experiments in it. And one of them, this dark matter experiment has for many, many years now, I don't know, 20 years, maybe more, been claiming a signal consistent with dark matter. So this is a detector that sits underground and it waits for dark matter particles to bump into it and it gives off a little signal if it does. And there have been lots of other experiments looking for dark matter and none of them have seen any. Anything. They've got more and more sensitive over the years. But this one experiment has been claiming that it's been seeing a signal and it's not consistent with any of the other experiments really. Most other experiments in the world now have ruled out the signal that Dharma Libra claimed to have seen. And there's a lot of, of, you know, mystery, I think, still about what the cause of this signal is. And the Dharma Libra team, I think, remain to this day, you know, convinced, or at least outwardly out publicly. They say this is a genuine signal, there's no mistake. And because they're quite, they've been quite protective, I suppose is a polite way of saying it, of their methods and their data, not really allowing others to scrutinize what they've done. And so it's very hard for the external community to figure out what is causing this signal. And other people have built replicas of the experiment to try and see if they see a signal and they don't. So this is a kind of case of, I think, where maybe wishful thinking or not being willing to admit there could be a mistake not saying they have made a mistake. I mean, we don't really know what causes, is causing this signal, but it seems much more likely that it is a problem with the experiment than it is genuinely a dark matter signal. So that's a kind of example of where the desire to discover can, can kind of override your kind of humility and, you know, perhaps not, you know, not scrutinizing your own methods and results as, as, as strictly as you should.

D (26:38)

Interesting. I want to get into neutrinos, and you talk about two, I think, anomalies there, high energy neutrinos and the stolen neutrino problem. But could you just define a neutrino? I'm going to quote your opening line, I think, from the chapter. The strangest and yet most ubiquitous of all things that make up the cosmos are neutrinos. They are also wrapped up in some of its deepest mysteries.

C (27:02)

Yeah, I mean, so neutrinos, they're probably quite unfamiliar. If you know any particle physics or any atomic physics, you're probably familiar with the idea that there is, you know, an atom has electrons going around the outside. These are negatively charged particles. And inside the atoms, the center of the atom, there's this positively charged nucleus. Well, neutrinos are sort of partners of the electron in some sense, but they have no electric charge. And because they have no electric charge, they very, very rarely interact with ordinary matter. So the reason that ordinary matter holds together, the reason that we kind of can touch objects, is because the particles that make up that stuff have electric charge. And that kind of holds them together and allows them to interact with each other. But neutrinos have no electric charge. So they can kind of go straight through solid objects. They can straight through the Earth. So there are trillions and trillions and trillions of neutrinos streaming through your body every second. And these come from the sun and from the upper atmosphere and from distant places in space. But we are completely oblivious to this because they just don't ever. They very, very ready bump into an atom. So they're all around us. They are completely ubiquitous, but they're very, very difficult to detect. And the way that they are generally detected today is you dig an enormous hole in the ground and you fill it with lots and lots of really pure water. And then you surround that with light detectors and they're these amazing. There's an amazing instrument in Japan called Super Kameo Kande, which is this vast underground water tank. And the photographs of it worth googling. It's kind of these golden Orbs in this huge underground lake, basically. And what happens is, every so often in this huge tank of water, a neutrino will bump into an electronic and create a little flicker of light, and that will get picked up by one of the detectors. But this only happens incredibly infrequently Given the huge numbers of neutrinos going through it. So they're very, very hard to detect. They were kind of first hypothesized back in the 1930s. It took until the 50s until they were discovered. And since then, they've kind of thrown up various strange properties throughout the kind of. Throughout the last sort of 50, 60, 70 years. The most famous of them was this thing called the solar neutrino problem, which is that the majority, I think, of neutrinos that arrive at Earth come from the center of the Sun. So in the center of the sun, you have these nuclear fusion reactions going on that are responsible for generating the Sun's light and heat. And this is the fusion of hydrogen into helium. And these nuclear reactions produce neutrinos as part of the reaction chain. And one of the ideas for sort of when the understanding of stars and the sun was being put together, one of the problems is there's no way to see inside a star, because the center of the sun is like the light that comes from it is bounced off all the kind of atoms in the Sun. It takes tens of thousands of years to get to the surface. And by the time it gets to us, all the information about what was happening in the middle of the sun is lost. But the neutrinos, in principle, they are produced in the middle of the sun because they don't interact. They just come straight out of the sun and they reach you in eight minutes or so, like light does from the surface of the Sun. So the idea was, if we could detect neutrinos from the center of the sun, we could test the model of the nuclear reactions that we think is driving the sun and creating sunlight ultimately. And these experiments were the first experiments doing this was done in the 50s and 60s. And they found this problem, which is that there were about a third too few neutrinos coming from the Sun. And this caused, like, a lot of head scratching. People wondered whether there was something wrong with the astrophysical model about how stars work. Maybe we'd got that wrong. But a lot of further work showed that didn't seem to be the case. There was a really serious thought for a while that actually the sun was kind of powering down and that we just Caught it as the core was sort of switching off. And if that was quite a scary prospect because if the sun stops generating energy, then at some point in the future, the Earth's going to get very, very cold and not very hospitable. But it turned out actually it wasn't the Sun's fault at all. The sun was shining quite happily. It was neutrinos misbehaving. So it turns out the neutrinos have this very strange property which is that they can change their nature as they travel. And they're kind of unique in this sense. So in terms of the fundamental particles, there are actually three different types of neutrino. They're for various reasons. There's electron neutrinos, muon neutrinos and tau neutrinos. They have strange names, doesn't really matter why, but they're three different types basically. And it turns out the nutri, if you make an electron neutrino in the sun as it travels towards the Earth, it can oscillate, we call it into a different type of neutrino, a muon neutrino or a tau neutrino. It's kind of like, you know, it's like. So Jekyll and Hyde act sort of mid flight and as a result, when they get to the Earth, some of them are changed to a type of neutrino that we can't detect and it looks like there's too few. And when you actually correct for this, when you then look for the muon and the tau neutrinos, you see, and add them all together, you see, ah, the sun is actually behaving just as we thought. And this property of neutrino oscillations is, you know, really sort of a fundamental phenomena that can teach us a lot about the fundamental nature of the universe. In particular, people are very interested in understanding whether neutrinos had a role in allowing matter to survive the Big Bang. And so studying neutrino oscillations today is at a very active area of research. So that's a kind of big picture view of what these things are and why they've been interesting in the last 50, 60, 70 years.

D (32:39)

Fascinating. You mentioned the three types of neutrinos and you point out one of them carries quote, muon ness. Could you talk about why muons are quote, weird little things? I'm paraphrasing Will. And there's a mystery around their magnetism and how, how we attempt to solve that. I really enjoyed that chapter.

C (32:58)

Yeah. So a muon is Again, probably a particle that most people haven't heard of. It was a muon is basically like an electron, but 200 times heavier. And they were first found in the mid the, the mid to late 1930s by two American physicists, Carl Anderson and Seth Nedermeyer. And they found these particles in cosmic, basically coming from cosmic rays. So particles that collide with the upper atmosphere create a shower of particles that come down and you can detect. And amongst them were these strange particles called muons. And no one really knew what they were for. They caused a lot of confusion at the time because we kind of thought, well, we have electrons and protons and neutrons, that makes up atoms. Neuron didn't seem to play any role in any of that. No one really knew what they were for. One famous physicist said, who ordered that when the discovery was made, as if it was like a pizza that turned up and no one knew who'd ordered it. But muons are kind of, they're very interesting things. And for a long time there has been an anomaly in a property of the muon, which is its magnetism. So muons and electrons, they have electric charge, but they also behave like little bar magnets. So they have a north and a south pole. And the standard model of particle physics makes a prediction for how strong that magnet that the muon carries with it should be. And the reason this is an interesting thing is that the magnetism of the muon doesn't just depend on the muon. What you, when you actually, when you put a muon in the vacuum, in empty space. Empty space isn't really empty. It's full of these quantum fields that we've discussed. So these invisible fluid like things. And there are 17 particles that we know about. So there are 17 quantum fields. And in the vacuum, the muon interacts with all of these fields. And so when you see a muon, when you measure its properties, you're actually measuring the muon plus the 17 fields that kind of are interacting with it, around it, basically. And so in order to calculate the magnetism of the muon, you need to calculate the effect of all of these 17 fields. You kind of think of them like dressing the muon. They're almost like a kind of clothing that the muon's wearing, and that changes how it appears. And you can predict this in the standard model, and then you can make a measurement of it. And the reason this is interesting is, well, we think there are other particles out there, like dark matter. If there's dark matter, that means there's also going to be a quantum field associated with dark matter. And that field will then form part of the muon's clothing if you like. It's like an extra garment or an extra colored thread that makes up what it's wearing. And that will subtly change the muon's properties. In particular, it will change its magnetism. So if see a difference between your measurement of the magnetism of the muon and its predicted value in the Standard model, that could be a clue to a new field, a new particle that we haven't seen before. And since the early 2000s, there has been this very long standing anomaly in the magnetism of the muon. And so this was an experiment at Brookhaven on Long island where they basically had this big magnetic ring. They sent muons around it, they measured how magnetic they were, this very clever technique. And by the time they switched off, the anomaly was at what we call about three sigma. That means that it's about there was about a one in a thousand roughly chance that it was a cycle fluke, which is not enough to be confident that it's real, but is very intriguing. And so there was this huge effort to rebuild a new version of this experiment at Fermilab near Chicago. That experiment started taking data five or six years ago and it's been running ever since. And it's now made this really pristine, very precise measurement of the magnetism of the muon. And it aligns perfectly with what was measured at Brookhaven. So in principle this is really exciting because that seems like, well, okay, this is now very clearly in tension with the prediction. But the twist is that at the same time this measurement was being performed, theorists were re running or finding new ways to calculate the magnetism of the muon. And they found that using a different technique than had been used before, they got an answer that was much closer to the experimental measure. And this is just the standard model prediction. So in this case the theory goalpost shifted, if you like. And this is kind of potentially, it looks like the explanation for this anomaly unfortunately is not that there is some new particle to just be discovered. It is that we really didn't understand properly how to calculate the magnetism of the muon in the Standard model. And it's still not really resolved. We still don't really understand what went wrong. There's more work going on with that, but that seems to be the likely explanation now, which is a bit disappointing, I imagine. Was more than a bit disappointing if you spent 20 years of your life building this experiment. But nonetheless, this is how we learn. So we learn something about how to do these sorts of calculations properly and we learn something about the Standard Model. But in this case, it seems like it's probably not that we found dark matter or some new field that we'd not seen before. Yeah.

D (37:53)

I think you quote a colleague, Professor Davies, at Glasgow, saying, this isn't what I want to happen. We desperately want new physics. So I totally understand that you want to break the Standard Model. I did have two follow ups there. You talk about most people not being familiar with muons. They're probably not familiar with Paul Dirac as well. And you actually suggest he's second only to Einstein as a 20th century physicist. Could you give a brief synopsis of who he is and why he's so important?

C (38:22)

Yeah, I mean, Dirac is a really fascinating character, actually. If your readers are interested in reading more about him, there's a brilliant biography of him by Graham Farmeloe called the Strangest man, which I really won the Costa Book Prize some years ago, but it was really a great read. But Dirac was. He was a British physicist, a bit of a prodigy. You know, his. When I did my PhD, it had some very long, complicated title. His PhD that he did at Cambridge was simply titled Quantum Mechanics. So, I mean, that gives you a sense of like the impact of his PhD research. And basically he kind of reformulated quantum mechanics. So he kind of was coming up just in the mid to late 1920s at the time when Heisenberg and others were putting quantum mechanics together. And Dirac really made like some fundamental contributions to quantum mechanics. But the most sort of famous thing he's most famous for is this equation called the Dirac equation. And what Dirac was trying to do in the late 20s was to combine the two revolutionary theories that had upturned our view of physics in the 20th century at that point, which is quantum mechanics, this theory that describes the behavior of atoms and particles, and special relativity, which is Einstein's theory describing how space, time and light behave, particularly when you're going close to the speed of light. And Dirac was really the first person to come up with a successful synthesis combination of these two theories in order to understand the electron. So he wrote down this equation, which was the first relativistic quantum mechanical equation of the electron. And this was important for kind of what might sound like quite prosaic reasons. Basically, one of the key ways that quantum mechanics was tested was by looking at spectra of atoms. So these are the characteristic colors that atoms emit and absorb and so if you take a hydrogen atom, for example, and, and heat it up, hydrogen will emit particular colors and wavelengths of light. And this basically corresponds to electrons jumping between different energy levels in a hydrogen atom. These kind of orbits that we kind of often learn about in that surround the atom. But the measurements in those days were not agreeing perfectly with quantum mechanics. And Dirac's equation solved this problem. But the much more important thing that it did was that it predicted the existence of something called an anti electron. So it seemed that Dirac found when he combined relativity and quantum mechanics, you unavoidably ended up with a mirror image of the electron, which was exactly the same in every way, but had a positive rather than a negative charge. And when Dirac first found this solution to his equation, he was actually really dismayed because he thought, well, there's no such thing as a positively charged electron. No one's ever seen such a thing. So this, my theory must be wrong. But actually, you know, after years of trying to get rid of these things from his equation, he found that they couldn't be got rid of. They were intrinsically just. They come with relativity and quantum mechanics. And a year later, Carl Anderson discovered the antielectron in a cloud chamber experiment. So this is a kind of amazing example of theory predicting the existence. This is what we call antimatter. So this is a kind of mirror image of matter. We now know that every particle has an antiparticle. And so Dirac had predicted the existence of this whole other type of stuff, which no one had really even imagined up to that point and let alone seen in experiments. And then you go and look for it and you see it. And that's kind of. It's a great example, I think, of. Of what makes, I think, physics different from all other sciences, which is this incredible predictive power that based on sort of physical principles and mathematics, you can make these extraordinary hypotheses about what might exist. And then you find it. It's kind of this very rigid structure in that way. And so this is a kind of an amazing thing. And that sort of. He made many other contributions that there's too many to mention. But, but I think that was a kind of, in a way that was the foundation of what we call modern particle physics. It kind of begins with the Dirac equation and the work that he did in the late 20s.

D (42:27)

Great. And just one more follow up. I want to ask you about Feynman diagrams. I know him and Schwinger had a. I don't know if it was a debate, or they had different approaches, but eventually ended up at the same point. The particles, not fundamental at all, but vibrations in quantum fields. And can, you know, follow up to that? Are we all just vibrations at our very basic level?

C (42:48)

Yeah, we are. I mean, this kind of the. The apparent solidity of the world. We, you know, we look around us and, you know, things seem solid, but it's kind of an illusion, actually. And, you know, we are not solid. We are. Well, we are. We obviously are solid, but we're not made of, like, continuous solid things. We are. We are made of these vibrations in these invisible fields. That's what.

D (43:11)

What.

C (43:11)

At least that's what the theory says. That's the kind of model that we have of the world, and that's what all our experiments seem to confirm. So, deep down, nature is very, very different from how it appears. And I think that's why quantum mechanics and quantum field theory are so difficult, because they're so far away from our everyday experience of the world. The way that the world behaves down at these very, very short distances is really alien and very difficult to kind of comprehend, even for physicists. You know, this kind of very famous quote, I think, by Bohr, which is, you know, if you think you have understood quantum mechanics, you haven't understood it. You know, basically that this. This is a kind of a very, very different sort of world with different rules than the ones we're familiar with in our everyday lives. And so, yeah, the. The kind of world we live in is. Is a kind of an amazing illusion created by these very, very strange objects that live at very short distances.

D (44:10)

Awesome. And the Feynman diagrams, it seemed like he somehow managed to simplify some of the complexity around this.

C (44:16)

Yeah, I mean, Feynman was a kind of really original thinker. And when the quantum field theory was first being assembled, put together, really in the 40s, that you had these. Schwinger, who was Feynman and Schwinger both, actually, I think, from New York, both very, very gifted theoretical physicists. Schwinger approached quantum field theory with a very kind of formal, beautiful mathematical approach. Feynman famously stood up at this conference just after Schwinger had presented his formulation of quantum field theory and started drawing these strange hieroglyphic diagrams on the board. And everyone in the room was totally confused and baffled. No one knew what he was doing. People thought he'd lost his marbles, I think. But what Feynman had found was a kind of a pictorial way of simplifying very Complicated calculations in quantum field theory. So these Feynman diagrams, they are kind of representations of interactions between fundamental particles, so electrons interacting with photons, for example, particles of light. And Feynman found that you could draw one of these diagrams and then through certain rules, write down the prediction of how likely that process was to happen, rather than going through the great long mathematical derivation that Schringer had done. It turned out they were exactly analogues of each other. They're two different ways of looking at the same problem. But Feynman diagrams are something that physicists use, particle physicists use today to do calculations. And I often find myself drawing them to figure out what kind of process might work and how likely it is. So they're very, very useful, intuitive tools. And Feynman was very good at that sort of thing, that kind of coming up with a kind of more intuitive, pictorial way of thinking about the world that. That sometimes with the mathematics can get kind of obscured, I think.

D (46:04)

Yeah, no, it was a fascinating chapter. And at that conference, I think Dirac was there, it sounded like it was all over the place at one point. But I want to move on to the Large Hadron Collider and particularly your work there, and I believe it's called LHCB standing for ud. Could you talk about your work there and the Lepton Universality Anomaly? I think that chapter really encapsulates the entire theme of the book. I mean, it. I don't want to give it away, but at one point, your discovery, people were saying, nature is speaking to us. I had goosebumps. But the ending somehow ends up being a little different. Could you talk about that?

C (46:41)

Yeah, so. Well, first of all, lhcb, what is that? Well, it's one of the four big detectors at the Large Hadron Collider. So you have this 27 kilometer ring that collides, accelerates protons around to very, very close to speed of light, then collides them into each other. And at four points around the ring, these collisions happen. And there are these big detectors that surround the collision point and record what happens. LHCB is one of them. And it's this Overtooth is a large International Project, 2000 more than 2,000 scientists, I think, from all over the world working on it, of whom I am one. And what we, broadly speaking, do at LAUCB is studying particles called bottom quarks, also known as beauty quarks. There's the same two names for the same thing. And these particles are interesting because they're exotic, fundamental particles. They're things that Are predicted by the Standard Model or exist in the Standard Model, but they can tell us a lot about other things that might be out there. So in the same way that by measuring the magnetism of the muon, you can learn about new quantum fields by measuring how these bottom quarks behave, how they decay, in particular, they're unstable. They decay into other particles. The way they decay can tell you about other forces, other particles that we've not seen before. And for many years, since about 2014, so what? More than a decade now, we had been seeing evidence of something that could not be explained by the Standard Model, which is that bottom quarks, when they're produced, they live for about one and a half trillionths of a second, and then they decay into other particles. And they can decay into lots of different final states, as we call them. But we were particularly interested in looking at decays Where a bottom quark turns into either an electron and a positron, so an electron and an anti electron, Or a muon and an anti muon. And electrons and muons are partners of each other. They are kind of cousins in the Standard Model, and they interact with all the forces with the same strength. So that means a bottom quark ought to decay into electrons as often as it decays into muons. That's a kind of basic assumption of the Standard Model. And what we were seeing for many years Was that it was decaying into muons less often than electrons. And this could not be explained by any kind of Standard Model phenomena. If it was a real effect, it was a clear sign of new physics. And there were several measurements that seemed to be showing this effect. There were also other measurements of related processes to do with how often certain decays happened or the angles the particles come off at that. There were also anomalous that didn't agree with the Standard Model. So you had this whole suite of measurements that were all pointing in the same direction. And that got theorists very excited, because what they found they could do was you could take all these different measurements and you could construct a model, an extension of the Standard Model that would account for all of them at the same time. And it's not obvious that you should be able to do that if it was just a bunch of random quirks of the data. And so this kind of became a very, very interesting area. And it's an area that I moved into and started making my own measurements in as well. And in a. Around the sort of. The peak of all of this, I suppose, was around 20, 21. So just after the COVID pandemic, when a new result came out that seemed to confirm what we've been seeing, there was a lot of excitement, although people tried to remain cautious, I think about it, but there was a lot of excitement. But then, you know, the way this story turns out, and I suppose I am spoiling things a bit, but we discovered, much to our. To our horror, actually, as we went through the data in more detail, that there was a mistake in these measurements. Well, it was a. Basically, it was another type of particle process that was masquerading as electrons. So it was. Look, it looked like it was electrons, but it wasn't. And that shifted the balance between electrons and neurons and made it look like there was a deviation. And when this was corrected for, the anomalies disappeared, you know, went back to agree with the standard model. So this is a kind of good example of, you know, why you have to be so careful, because this was a very subtle effect. It wasn't, you know, these. The people that made these measurements, very, very meticulous, very careful. They were fully aware of the implications of what they were doing. These were very heavily scrutinized. But nonetheless, you know, this effect was missed. And now that we understand it, you know, it's. That's obviously a much better position to be in, but it was a very difficult process thing for many people to go through. That real sense that you're on the cusp of something and then for it to kind of evaporate. And, you know, the kind of addendum to all of this is that we learned a lot in that process, but there are actually, there are still anomalies in this data. So these lepton universality, as they're called, nominees have gone away, but some of the others that I mentioned are still there and we still don't understand what is causing them. So there's still a mystery to be unraveled there, and it may be maybe that will eventually turn into a discovery. More likely it's something else, a theoretical problem, experimental problem, we don't know, but that's the job. And you kind of keep pursuing these anomalies until you've rooted out what the cause is, because maybe it's the clue and you just have to keep going down the track.

D (51:52)

Interesting side note, where you work, LACB is in the town of Bernie Voltaire. And you actually quote the French philosopher Voltaire saying, love truth, impart an error, which I think nice theme throughout the book. I want to briefly move on to the Hubble tension. You Talk about that anomaly there. You spent some time with, I think, the two main proponents, Winter Friedman and Adam Reese. Could you briefly explain those two? And maybe, you know, is the James Webb telescope the answer to resolving that tension? How do you see that?

C (52:24)

So this anomaly is the one that I was talking about at the beginning of the conversation that I think is the most likely to turn into something big. And this is a cosmological anomaly. So it's anomaly to do with our understanding of the universe. And it's basically an anomaly in the rate of the universe's expansion. So the basic story of the universe is you have the Big Bang. Space expands rapidly at the beginning, and then it starts to slow down because of gravity. And then in the late 90s, there was this discovery that actually the expansion of the universe is now accelerating again. So it's speeding up. And that's driven by what we call dark energy, we believe. So that's the kind of basic picture. And for. Well, really since the expansion of the universe was discovered in the 1920s by Hubble, people have been making measurements of the expansion rates. And these are varied wildly, basically. So what. What Hubble originally found was he was able to measure the read. The expansionist was discovered through a comparison of the apparent speeds of different galaxies with their distance from the Earth or from. From the Milky Way. And what Hubble found was when you plot a graph with a distance of a distant galaxy on the x axis, on the horizontal axis, and the speed they're moving at on the Y axis, you get a straight line going up the way, basically saying that a galaxy that is further away is moving away from us faster. So whichever direction you look in the universe, pretty much every galaxy is moving away from us. And the further away they are, the faster they're moving away from us. And this is evidence for the idea that space itself is expanding. And Hubble measured the constant that relates the distance to the speeds. There's a simple equation which is basically speed equals this thing called the Hubble constant times the distance. So it's a very simple equation. And Hubble measured a value that was very large originally. And the thing that was strange about Hubble's measurement was when the expansion rate of the universe is important because it dictates how old the universe is, the faster it's expanding, the younger it is, because the more quickly you get back to the Big Bang, if you reverse the clock. And it turned out the age of the universe was less than the age of the Earth. So it was found that rocks on Earth seem to be older than the universe, according to Hubble's measurement.

D (54:40)

Measurement.

C (54:41)

And it was eventually discovered that there was a problem with the way that distances were being measured in the universe. And this is sort of at the heart of the whole issue. So one of the biggest challenges in astronomy, particularly in cosmology, is how you measure distance to an object in the sky. And it's this basic problem that it's very hard to know. When you see a star, for example, is that a very bright star that's a long way away, or is it a relatively dim star that's close? Both will appear the same brightness, and these are kind of point like sources. So it's very, very difficult. And the same is true with galaxies. It's hard to know if it's a big galaxy a long way away, or a small galaxy that's closer to us. So a lot of the work in astronomy in this area is trying to figure out how do you measure distances, how do you get a ruler that will let you measure how far away a galaxy is? And the primary way we've done that in the 20th century and the 21st are through these pulsating stars called Cepheid variables. So these are stars that were studied in detail by an American astronomer called Henrietta Swan Leavitt in the late 19th, early 20th century. And she discovered something extraordinary, which is that these Cepheid stars, they're basically very young, large stars that have unstable atmospheres that expand and contract with this kind of rhythmic pulsing, and that makes the star grow brighter and dimmer in the sky with a very characteristic period, like a heartbeat. And Leavitt discovered that the faster a Cepheid pulse is, the dimmer it is, or the other way around, the slower its heartbeat, the brighter it is. So what that meant was if you could calibrate the relationship between the pulse rate of a star and its brightness, then if you see a Cepheid in some very distant galaxy, all you need to do have to do is measure its pulse rate, and then you can read off how bright it is. And then from how bright it appears, you know how far away it is. Because if you know it's intrinsic brightness and you know that it looks, you know, and you're only receiving this much light, you can work out how far, how far away it is. So these are the styles that we basically use to measure distances, broadly speaking, in cosmology. And so what has emerged in the last decade or so, just a little bit more than a decade, is that there is a tension in the measurements of the expansion rate of the universe, which are primarily done through these measurements of distances, distant galaxies, and then their speeds versus what you would predict based on the Big bang and the standard cosmological model. So there are two ways of getting this expansion rate. One is you look at a load of galaxies in the sky, you measure their distances, you measure their speeds, you get the expansion rate straightforwardly like that. The other way is you look at the light left over from the Big bang. There's this faint microwave radiation that comes from the whole sky called the cosmic microwave background. And that is the light from the very early universe about 380,000 years after the Big Bang. And that light encodes within it a whole load of information about the very early universe. So what it was made of, how much dark matter, dark energy, radiation, atomic matter, et cetera, there was. And so you can use that data to figure out the properties of the early universe. You then plug that into your standard model of cosmology, and you predict what the expansion rate should be now. And when you compare that prediction with your measurement, they don't agree, and they now don't agree by about five standard deviations. So five errors. There's a really, really strong disagreement between these two things and this. You know, there's been a very active debate and attempt to understand what's happening here. And there is sort of broadly, there are two people sort of leading this whole argument. On one hand, you have Adam Rees, who is a Nobel prize winner. He won the Nobel prize with two others for the discovery of the accelerating expansion of the universe in the 90s. And he is leading the team that are making these measurements of the expansion rate that are in strong disagreement with the prediction. And he is fairly bullish about the idea that this anomaly is real and that there is really something missing from our understanding of the universe. On the other hand, you have a professor called Wendy Friedman who made the first really precise measurement of the expansion rate of the universe using the Hubble space telescope in the 1990s. So they both have their very strong pedigrees. Wendy Friedman is much more skeptical about the Hubble tension, as it's called. So you have these two kind of huge figures in cosmology with very different opinions who are trying to settle this debate one way or another, and they're endlessly putting out new papers. Wendy Friedman's team will put out a paper that seems to show that you can explain the tension through some problem with the way you deal with Cepheids. Adam Brees comes back and said, no, we've accounted for this and it's very solid. So you're in this kind of. It's a really a big, big debate and there's a lot at stake because if this effect is real, it's basically saying there is something missing from our understanding of cosmology. And that could be any number of things. It's very hard to know what it is. It could be sort of what I alluded to early on, which is that dark matter and dark energy are not what we think they are, are. They are more complicated, they're dynamic, they change with time. It could even be that Einstein's theory of gravity is missing something that's the kind of more extreme end of the possible explanations. But it's going to be something significant that will shift how we think about the universe and its history and what it's made from. But the problem is really figuring out is this real or not. And what everyone was waiting for was the James Webb Space Telescope that you mentioned. Because the reason people doubt these measurements is because these Cepheid stars, these pulsing stars that are used to calibrate distances in the universe, they are formed in the dust lanes of spiral galaxies. So these are kind of the star forming regions very rich in dust and gas. And that means there's a lot of stuff around them, dust and gas. So when you look at them with a telescope, there's stuff in between you and the star that can change their apparent brightness or color and that can mess up your measurement in principles. You have to account for that. But James Webb is most of these measurements until James, all of them, more or less, until James Webb have been made with a Hubble Space Telescope, which mostly looks at the universe in optical wavelengths. So in visible light, James Webb primarily looks at the universe in infrared. So the lower end of the spectrum, beyond outside the visible region and infrared, allows you to see through these dust lanes. So it allows you to measure these Cepheids much more directly without all this stuff in between you and the Cepheid. And so the idea was we'll use James Webb, we'll recalibrate these Cepheid stars, and then we'll measure the Hubble constant again and see if the tension is still there. And that's what Adam Rees's team have done and they find that James Webb confirms what they saw with Hubble. So that seems to suggest this is getting more solid. Some people still disagree though. So I think it's going to be. It's by no Means a settled issue yet, but I think it really is a, you know, is what I mean. Reese himself called this a crisis for cosmology, that it's, you know, a major, major problem that needs to be understood. And hopefully the answer is something really exciting and fundamental that we're going to learn about the makeup and the history of our universe. At worst, I guess we're going to learn something about Cepheid stars and how you measure distances. But, you know, both of which would be interesting. But I think that the former. More interesting than the latter, probably.

D (62:03)

No, I think everyone would agree with you. Harry. Last question. The next generation of telescopes, like the Extremely Large Telescope and the Square Kilometer Array, as well as the Vera Rubin Observatory, potentially could offer us even a better understanding of the universe. By the same token, we may end up with more anomalies. How do you see the next five or ten years developing there?

C (62:25)

I think it's so hard to predict. And, you know, whenever you switch on a new instrument, you see things you didn't expect. And James Webb has already seen all kinds of things that people didn't anticipate before they switched it on. So, you know, we're. One example is the discovery of extremely large black holes in the very early universe. So one of the epochs of the. Well, a key epoch of the universe that's being probed by James Webb, but also by. When it comes online, the Square Kilometer Array is this period called cosmic dawn, which is after the Big Bang, when the fireballs died down. You have this era known as the Dark Age, where the universe is full with this kind of hydrogen helium fog, but there are no stars yet, so it's dark, and then the first stars start to ignite. And this is a very interesting era, obviously, because it's kind of the beginning of the visible universe as we know it. And looking back to those very, very early epochs, people are starting to see objects that seem far too large to have formed in such an early time. And that's sort of challenging our understanding of how stars form, how black holes form. It's challenging, potentially even challenging, how we think about the history of the universe, depending on your point of view. So I think in terms of when these instruments come online, it's very hard to say what they will find, but you're almost guaranteed they're going to see things we didn't anticipate. And that's the kind of excitement. Whenever you get a really new instrument, you see new things. It's like when Galileo pointed his telescope at the sky. This new instrument was invented and suddenly you learn a whole load of stuff you didn't know before. And hopefully that's what these telescopes and also new particle colliders, new accelerators, new machines, laboratory machines on earth will also teach us new things.

D (64:13)

Yeah, it really does feel like we're on the precipice or the cusp of sort of an inflection point in our understanding. But maybe it just means, you know, again, more anomalies and trying to sort of grope in the dark. I know you use that analogy of looking for your keys under the light. That concludes our interview. I don't think it's any surprise that Harry Cliff is a wonderful public communicator for science. The book is Space Oddities, Mysterious Anomalies Challenging Our Understanding of the universe. Harry, thank you so much for your time and for such for writing such a thought provoking and really just fascinating book.

C (64:45)

Thanks Gregory. Pleasure talking to you.

D (64:47)

Likewise.