Into the Impossible With Brian Keating - "I Thought It Would Fail!" Francis Halzen: Winner of the 2026 Nobel Prize In Physics [Reupload from 2022]

Episode Date: October 7, 2026

The physicist who turned a cubic kilometer of Antarctic ice into a telescope says the most exciting moment of his career was not the discovery. It was the first atmospheric neutrino. Before that, he was deep in debt to NSF and his own university with no neutrino to show for it. Francis Halzen is principal investigator of the IceCube Neutrino Observatory at the South Pole and Hilldale and Gregory Breit Distinguished Professor of Physics at the University of Wisconsin-Madison. We cover: why ice under the South Pole is clearer than anything a lab can make, the 2,600 TeV event that dwarfs the LHC, why IceCube sees more energy in cosmic neutrinos than in gamma rays, how neutrinos crossing the universe test quantum gravity, and the quark he refused to believe in. Physics is a miserable career unless you are obsessed by it. Recorded in 2022, before IceCube detected neutrinos from our own galaxy. Learn from Nobel Prize winners, and you could win a meteorite: Please join my mailing list here 👉 briankeating.com/x Chapters 00:00 Physics is a miserable career, unless 02:54 A textbook written before the W was found 06:11 Breaking into Lemaître's computer at night 10:57 Do scientists owe the public anything? 13:55 Deep in debt with zero neutrinos 19:40 The persuasion nobody teaches scientists 20:15 Why it could only be the South Pole 23:20 Learn physics by doing, not by courses 26:00 How to turn Antarctic ice into a telescope 31:30 2,600 TeV: far beyond the LHC 35:00 More energy in neutrinos than gamma rays 40:00 Tracing ghost particles to NGC 1068 46:00 Testing quantum gravity with neutrinos 1:01:00 Ten times the ice for the same price 1:07:08 What he would leave his descendants 1:11:19 What his generation should be proud of 1:12:51 The quark he refused to believe in 1:17:09 Imagination can trump intelligence 📺 Watch my most popular videos: Neil Turok Believes Physics Is In Crisis Nobel Prizewinner Frank Wilczek Stephen Wolfram vs. Eric Weinstein Sir Roger Penrose: Nobel Prize in Physics, Black Holes and More Noam Chomsky: Consciousness, Elon Musk, and More Sabine Hossenfelder Avi Loeb: UFOs and UAPs Neil deGrasse Tyson Follow me to ask questions of my guests: 🏄‍♂️ Twitter: @DrBrianKeating 📝 Mailing list: briankeating.com/mailing_list ✍️ Detailed blog posts: briankeating.com/blog 🎙️ Listen on audio-only platforms: briankeating.com/podcast A production of imagination.ucsd.edu Executive Producer: Brian Keating Producer P.G.A: Stuart Volkow Editing: Lucas Scheiblich Learn more about your ad choices. Visit megaphone.fm/adchoices

Being a physicist, from all objective point of views, is a miserable career. Yes. And I don't have to state the reasons. The only reason it's worse doing is you just enjoy this. You're obsessed by it. You love what you're doing. And so if you do it as a job, it's going to be very, very disappointing, I think. So don't underestimate yourself until...

I went on this crazy adventure of Amanda and Ice Cube. I always live with the insecurity that I didn't belong to the circles I moved in, which I think must be true for almost every graduate student. Just get over that. Any sufficiently advanced technology is indistinguishable from magic. Well, it's not every day you get to talk to a hero of physics, of modern physics. someone whose roots go go back quite a way, someone who's been a leader in the field of astrophysics, of particle physics, who's an educator and a communicator of science, but also who is actually,

I think today's guest is my first ever guest who is a professor of mine, although I took his class as an adjunct student. Well, I was at the University of Wisconsin, and it is one of my personal heroes. This is Francis Halzin, who is a Belgian particle physicist, although he's been in America, probably more than half of his life. He's the Hildale and Gregory Bright, distinguished professor at the University of Wisconsin-Madison. He's a director of the Institute for Elementary Particle Physics, and he's the principal investigator of the Ice Cube Neutrino Observatory, which is operated at the South Pole Antarctica since 2010.

Before that, there was Amanda. Francis, it's a certain treat to be here for me to host you here, rather. And I just hope you won't share the grade that I got in your club. Please don't tell my audio. I don't think my bureaucracy, my administration is so good that I would be able to produce it. Well, it was one of my favorite classes of all time. Not to mention the fact that back then it was rare that professors wrote the textbook that was used for the class. I think I had that only with Leon Cooper at Brown.

with one of his quantum mechanics books. But to take this course on particle physics with you, and it was such a treat. And I thought we'd start there, Francis. We always start, we have on, you know, we just had on Martin Reese, Lord Martin Reese last week, and he spoke about his book. And we always do a scenario called judging books by their covers,

where we look at the cover of your book and we judge it and how did you come up with the name and hopefully entice a 10, endfold increase in book sales instantaneously. So the book we're talking about is called Quarks and Leptons. It was a wonderful, wonderful book that I had there. We'll show it on the screen. It's written with your co-author, Alan Martin. I don't recall much about him, but certainly taking it from you was quite a treat. So, Francis, can you help us judge Quarks and Leptons by its cover? How'd you come up with the title, the subtitle, and the cover illustration?

The title was actually, first of all, I must say this was a long time ago. And that's the amazing thing about the book, right? People don't realize now that this book was written a long time ago and that the W, the Z, the top quark, none of these things, the Higgs, none of these things had been discovered. you know like the whole book is kind of prediction of the standard model and in fact it I the book was in print when the W was discovered at Sun the weak intermediate boson wow and so the editor called me and said you can still change things you know I've read a newspaper

and I said there's nothing to change and he was kind of of worried that he had been part of some huge gamble. So, but the title is kind of embarrassing because although I think it was really, the books were really written in parallel, O'Koon, famous Russian theorists wrote a book, Leptons and Quarks, that came out around the same time. So it's not that original. And the illustration that you have on the cover, what does that represent? The only thing I remember is that I actually made it.

And it remembers, you know, jets were something rather new than, you know, quark glue on jets, the concept. And so I know that it represents like a jet made in deep in elastic. scattering or something like that. It's amazing that I cannot remember. No, I think that rings true. And actually, we have had on past guest Frank Wilczek many times. And he cites that, the discovery of jets at least, not by Ice Cube or anything personally. But he cites that as the kind of signature achievement of his career and of work that

particle physics as a community has led to. And I wonder, you know, if we could pivot from this wonderful book, which I learned a great deal from and I still make reference from when I have the task of teaching to young people as well, as well as your style. So I want to first take a step back. It is rumored that you knew La Maitre, the famous Belgian Catholic priest, the progenitor in some sense of the Big Bang. Is that true, Francis? I barely knew him. He died. he died when I was doing my undergraduate thesis.

And so I could tell a mattress stories for a long time. But I'll only tell you two. Because, of course, every interaction I had to resume is now legendary. But I'll only tell you too. He had a computer. He built himself to solve Friedman's equations. And it was a big room with vacucus. tubes.

And that computer, I mean, it was state of the art at the time, was only used during the day. And we actually, we mean another graduate student and I, Remy Gassmann's, we would actually break into the room and

use the computer at night. And we used it to calculate Feynman diagrams. Feldman had just discovered the methods to calculate Feynman diagrams by computer and so I think we may have been the first people to do this after Feldman discovered it. The other story is that when I

was an undergradger, the matter would arrive in a limousine at a physics building and, you know, it was an important person, and he couldn't walk anymore, barely walk. And we put him on a chair and then carried him to his office on the second floor. And it's only later that someone else who was part of the institute told me that that was actually Le Maitre. I had never made the association. Wow. He was not very visible.

He was retired. than, you know, he came in very rarely. And by the way, he was not famous, right? As you can imagine. He was famous for being a cardinal and for his role, maybe his role in Vatican 2, but I don't think so, actually. So he was, I mean, you know, the status of cosmology at that time, right? So it was just a.

at the time, he died, as you know, when the microwave background was discovered. That's right. And one of the things that's always impressed me about him and many others is that he was an advocate that the Pope, was it Gregory? I forget who it was in Vatican. But they attempted, the Vatican attempted to utilize the predictions of the Big Bang, at least, as a motivation for Genesis. And I was actually just in Galileo's, house in Florence, or Chetri, Italy. And of course, he was refuted by the

Catholic Church and repudiated his own ideas under penalty of death, basically. But did you ever get the sense, or was there anything in the Belgian physics community, perhaps, that spoke of what was his true emotional investment

in the Big Bang? Did he have one? Or was he able to detach being a scientist from being a theological? Yeah, I cannot tell. I was not, you know, I never got any class. He basically was at the end of his life. And so we had really no interactions scientifically with him. But the fact that he denied that the Big Bang had any connections to creation, that's, I think, well documented. Yeah. You read this bio, there are a couple of biographies of his which I've read, of course, and they clearly document that he

didn't see, he didn't make any connection to religion. And when we were getting ready to start recording today, I wanted to kind of get your, you know, take on the obligations of a scientist in terms of what he or she should do, quote unquote, should do. And I think, you know, The Matrix certainly took that role seriously, but, you know, he had so many hats that he was wearing, you know, pontifical hats maybe and science hats. But what's your philosophy? We'll come to more kind of big picture questions later after we review Ice Cube in your career. But what is your view of what the obligation, if any, that a scientist has to explain things to the public? And then do they have to kind of be careful how they explain things,

because maybe the public doesn't understand what they're doing. Well, I think I, from this point of view, I am a very practical person. I don't think I'm on a mission of anything. And on the other hand, I think that even we are supported by the public to do our science, we owe some return to them. And, you know, I have never done an invitation to talk about my work to anyone. At any level, any student of any age, I always react positively to an invitation to discuss my work. And that's, I think, an obligation we have.

Beyond that, I think it's very dangerous to, you know, our expertise, is science. And it's actually, even within science, it's a very limited expertise. And I think it's to the advances of scientists that they stick to their expertise. And that's certainly something I always do. Yeah, I always say that, you know, the problem that we are scientists deal with is that sometimes we become political scientists and you start to delve into politics. No, that's not a good idea. And it also, it hurts science, right? Because then science begin to look like something that it shouldn't be, and it's not.

Right. And there's a danger in what we call audience capture that you have to only do what your audience wants you to do. So on this channel, a lot of times people want me to only have people that won the Nobel Prize or only talk about aliens or only talk about dark matter. And I have to be very careful because I get a lot of attention from the videos that are popular, but that isn't necessarily the totality of what I think a scientist should be interested in. So anyway, I think you're absolutely right. And I want to ask you, you know, first we can go back to maybe a moment in your career that was a little bit, you know, maybe perilous.

I remember being in Madison, as you know, from 97 till Peter Timby, my wonderful advisor, who's been a guest on the channel. So you're my second guest from the University of Wisconsin-Madison, the great Badgers, cheese state, Dairyland State. So I had nothing but fond memories of being there. When I had Peter on, we were kind of, you know, speculating on the notion as a scientist to deal with uncertainty. And I remember you building, you preparing to build and starting to build up Amanda. And it wasn't just the technology.

It wasn't just the theory. It wasn't just the science. It was the people. It was the team that you built, which is unrivaled. And it seems to me you've been working on this for so long. Was there ever a time that you had doubts in yourself, that the funding would come about, that you might not succeed as you've done so spectacularly well with Ice Cube? Did you ever have a moment of panic that you just were so terrified that project might not move forward?

The whole story was a story of panic. you got a right work. You know, from the beginning, when people thought that this was a very cute idea, but that it wouldn't work. Well, that's what we taught too. But it kind of started as a game. It didn't cost that much.

we were piggybacking on the South Pole infrastructure that already existed to a large extent. But then NSF started to fund the project. I actually borrowed a lot of money from the University

of Wisconsin. And I was deep in debt to both NSF and the university. And we had never seen a neutrino, not an atmospheric neutrino. And of course it's because we really didn't know whether turning ice in a Schrenkoff detector was possible. It took us like a decade to figure out what the ice was like.

We couldn't put it in a test beam at Fermilab. And so, you know, the most exciting, among all the panic moments, the most exciting moment was not when we discovered cosmic neutrin. but when we finally observed atmospheric neutrinos, because that I felt we have delivered. Now it's up to nature. And even afterwards, I thought, you know, if scientists, the English liked to gamble, right?

Scientists wrote an article that gave us 6 to 1 to discover cosmic neutrinos, against, of course. And I remember not feeling very very, well when I read this. But then I always thought if we build something that unusual, we'll do something interesting. The big surprise actually is that we are doing neutrino astronomy and not something totally different. We do different things as well. But the other moment of panic is when we finally got the courage to publish in science. that we had detected cosmic neutrinos, and weeks later,

we were declared breakthrough of the year in 2013. And I must say that was also a moment of panic. And I remember the press conference, and the only thing I told about is, how do we run this backwards if this happens not to be true? Right. And I lived with that. I lived with that panic for quite a long time.

In fact, it's only now that we see sources, right, since 2017, that, you know, I sleep better at night. Well, that's a, you know, a 30-year journey to... But this was a wild ride, right, right? This whole story of ups and downs and, yeah. So what was the role of the South Pole in this? could it have been done somewhere else? There are alternative approaches, but none even closely related in success to what you've achieved.

No, that was, it's a unique opportunity because you need clear eyes, clear eyes exist elsewhere, but you have to build a sophisticated detector in a remote location. And without the South Pole stations, I mean, this would not have been possible. Remember, we start, in a very modest way. And nobody would say,

oh, we always wanted to build a kilometer-cube detector. Can you imagine someone saying, Halseman wants to build a kilometer-cube detector? Let's build a station somewhere above the glacier in Greenland and give him the opportunity. It was pure serendipity that these two things came together, the clear ice at the South Pole, and the station being there. Nobody would have built it for us.

No, and it's such a unique location. I've been there twice. I actually prefer going to McMurdo than going to the South Pole, but that's just me. A lot of people love going to the South Pole more than anything. And the results that you've garnered, yeah, it is serendipitous in a way, but it also took a lot of foresight by you and your team to recognize and pursue. I think one of the least appreciated aspects of a scientist is his or her ability to persuade. And that includes persuading an undergraduate to work in his or her lab or a funding agency or to get tenure or to get even a professor job. It's so hard nowadays.

But then building a team. So it's not enough to have the right idea. Like the idea is just kind of the table stakes, you know, to use a gambling analogy. So when did the kind of that subsiding? of that panic, what has been, I mean, of all the things that Ice Cube does, and we're going to cover it a little bit of technical detail because my audience is the most sophisticated in the known regions of the multiverse that we inhabit. But I want to ask you of the technical breakthroughs, could Ice Cube have been done, you know, 50 years ago, let's say if there was a, you know,

very, very young Francis Halzen as a graduate student, could you have done it 50 years ago, or were there technological breakthroughs that you and your colleagues and understanding of the ice has made? Yeah. It depends. It's a difficult question. It's a question I've never answered, which is rare. One of the challenges, I mean, maybe the biggest challenge was to build a hot water drill. And that was such an unusual problem.

It was also an exciting problem because, you know, you don't go to school and they teach you about hot water drills, right? No. So in Wisconsin they do actually, right? This was done by a group in Wisconsin of engineers, professors, graduate student technicians, anyone who could contribute contributed. And it was very exciting. But I think, you know, it's car wash eaters that make the power. and the nozzle is very sophisticated that it was designed by an engineer

but it could have been done before no doubt about it I am not so sure about the data acquisition because of course we could have used the Amanda technique of just sending analog signals over the cable

that brings down the high voltage that powers the photomultipliers But the data acquisition of Ice Cube is a bit more sophisticated. We capture the signals in the ice, transform them to digital and then transmit them. And so that was, in fact, the chips that are used, where the PhD thesis of a student of David Nygren at Berkeley. So that was really.

a bit beyond state of the art at the time. Of course, now, uh, uh, that technology is, is, looks like archaeology, right?

But, uh, this was, uh, the, the, the Berkeley design was made in, um,

in the late 1990s, because it was all in place when we submitted a proposal in 99. Right. Wow. So it could have been done. It probably could have been done. done earlier, but in its present form, and for the discoveries we make, the electronics, I don't think we could have done it with Amanda analog signals. It's pretty essential to

have the technology that Nygren and the Berkeley group developed for the data acquisition. I want to ask you, because I knew you as a student of yours, albeit not as a graduate student. I think that would be a great treat. But as a student in a class about for advanced, you know, undergraduates and graduate students on particle physics, why do we conflate scientific ability with educational or pedagogy ability? In other words, you know, someone who's a pilot may be extremely skilled at flying, but we don't ask, you know, him to like teach flying necessarily. Although you, and you can actually teach flying with, I'm a pilot in my side hustle to pay the bills here at the state. University of California. But I want to ask you, why is it that we do that? Do they do that in Europe? Is that common, you know, that we have an expectation that because Francis or Brian are good at

scientific research, that they're also good at teaching? Or is that a mistake? Well, I, this is one of the things I feel passionately about and never had a time to do much about it. But I think that the graduate school that exists today at American universities is in a sense

unfortunate. It still lives in the days that you take courses and learn all of physics and then do research. First of all, you cannot learn all of physics anymore. You cannot even learn all about some specialized

subject in physics. No. So the way I have everything I know and the Belgian system at the time allowed you to do this. Everything I know I've learned

by doing research. You get interested in a problem. You have to solve it. You learn everything you need to solve that problem. And that's how you learn. And you remember, by the way. And I think that's the ideal way

trying to get a background of all of physics before you start doing research is an illusion which we are still living in in our present system of graduate schools. Yes, yes, and it seems to be sort of strange how the European system influenced the American system,

the Germanic kind of emphasis on research. And now most of the Ludwig Boltzmann or Max Planck Institute, they don't teach, as I understand it. So it's kind of ironic that the educators of the American system, they don't do any education of their own anymore. So I wanted to talk a little bit about the technical details and the theoretical underpinnings of Ice Cube. And you've been kind enough to prepare some slides. So I'll let you get set up to share the screen. Okay. So, So, you know, we want to detect neutrinos from space because we want to do astronomy with neutrinos rather than with photons. And we know how to detect neutrinos.

This is an experiment in Japan, in a deep mine, and it's a huge tank of water and light sensors. And that's how you detect neutrinos. The problem with this is a beautiful experiment. is that the theorists in their wisdom told us that, and by the way, I'm not blaming anyone, was one of them, told us that this experiment is 10,000 times too small to do astronomy and catch the tiny flux of neutrinos

that reaches from the universe. So to make a long story short, we build a detector that instead of water, made out of water, is made out of ice. And the phototubes are distributed throughout the ice. And according to our wisdom, as theorists, it probably had a chance to detect cosmic neutrinos, but no guarantees were given.

And so what you're looking at, of course, is not a real picture. But so if you go under the South Pole, you stand, the South Pole station stands on three kilometers of ice. And you go one and a half kilometer deep. There are strings with the light sensors that you saw on the walls of the Japanese experiment. And so the strings are a kilometer long and they are 86 of them. So the light sensors between strings are 17 meters a point. and the strings are 125 meters apart.

And 86 of these strings fill a volume of ice with light sensors. This idea, by the way, was born in 1960. Russian physicist Markov came up with this idea. So the idea is certainly not new. So the challenge was to melt these sensors into the ice. And so we developed, that was the first big challenge. We developed technology at Wisconsin to do this. And the second challenge was, was this ice good enough to act as a particle detector?

And that's a long story. And the story is told in this slide. And I would spend the rest of your time if I went through this slide. But we made our major breakthrough when we realized that the light, blue light in ice, travels over hundred and sometimes hundreds of meter. So this is a compacted snow of 70,000 years ago. It's ultra pure. And even in a lab, you cannot build a block that transparent, the blue light. So that we didn't know that. That was pure serendipity.

So these two things being solved, we built it and took data. And the idea is you can imagine your block of ice here. And you can imagine that it's like one and a half kilometer deep. It's dark. So what we do is we look for particles coming through the earth. And only a neutrino can come through the earth, nothing else. and that neutrino not only comes through the earth, it comes through your detector. But occasionally, like one in a million, will hit the nucleus in the ice,

a nucleus of hydrogen or oxygen, and make particles. The particles it makes in the nuclear interaction, they are charged, and they make the light glow. That's called Schrenkoff radiation. And if this is a muon neutrino, then it makes a particle called the muon, and it travels for kilometers in the ice. So that muon, you can detect from 50 meters to at high energy,

50 kilometers away. That's not quite practical. But you get the idea that your detector is, bigger than what you instrument. And the other thing is you have a telescope because you not only detect a muon, you can point it back because the muon travels at the speed of light almost.

And the light in ice travels at three quarters of the speed of light. So it's like a speedboat that outruns its waves. And so you get a shock wave, like the bow wave of a bird. and it points back where that muon comes from and where the neutrino comes from. So we built a detector and here you see an event. This is a neutrino that comes 11 degrees below the horizon and streams through the detector.

It deposited inside the detector 2,600 TV of energy. So you must remember the highest energy beam at, Saar in Geneva, it's 14 TV. So this particle deposit just inside the detector, 2,600. And by the way, I mentioned atmospheric neutrinos, which are background. This event is much too heavy to be produced in the atmosphere, much too energetic. And we can do astronomy these days with about 0.3 degree resolution. If you didn't get a picture here you see on our online display, you see the muon going through the detector with the speed of light slowed down by the computer.

And so that's the method. And so what we did is we measured neutrinos from the cosmos. And you see here this is the energy that the detector collect the neutrinos from the cosmos from the cosmos and as a function of the energy of the neutrinos. And the data points you are looking at are electron and tau neutrinos. And this actually is the flux of the muon neutrinos measured the pink band. It's the flux measure with the method I just displayed. The other data points we measure electron and tau neutrinos with a different method,

but you see they agree. And so this is the muon flux through the earth. And you see how we measure the flux from the atmosphere when we reach the threshold of the detector, which is 100 GV. Then we get more and more background in atmospheric neutrinos. And when we reach someplace like tens of TV, then you see the atmosphere turns off.

and we see the excess flux that was shown on the previous picture. This is a measurement now close to 10 sigma, and we have seen atmospheric neutrinos in at least four totally different ways. That's the status of Ice Cube. I have a few slides to show what our results are. Yeah, please do. In fact, one, our most important result, we actually never advertise,

but I'll do today, and I always personally do. I showed you the flux that we receive from the universe, the energy flux in neutrinos. It's what astronomers call New F Nu. And of course, we receive, we detect fluxes of gamma rays. Here you see this peak, this huge peak,

that's the CMB, very familiar. to my host. And then you go through, it dominates the universe, then you go to different waves, legs of light, this is ultraviolet light, x-rays, gamma rays, but then the universe turns opaque to gamma rays. It turns opaque to gamma rays because it cannot make,

they cannot make their way through the microwave background. The microwave background filled the universe, stops them from reaching us except from our own galaxy. Right. And actually, Francis, I have to stop you there. Yeah, go ahead. Did you know that the namesake of your professorship, Bright, the Bright-wheeler phenomenon, which is the interconversion of photons to

positron-electron pairs, that was only recently detected. And actually, I made a video about that. I'll have a link to it in here. But that was- Thank you. Yeah. That was really exciting, right? Anyway, please go on.

Yeah, the pair production. Yep. Per production. So these photons pair produce, make E plus and E minus pairs interacting with the microwave photons, and so they lose energy. So the flux disappears. And that, of course, is the resonant data of neutrino astronomy.

You see here, once you are beyond this demarcation line, you can only do astronomy with. neutrinos. But the other interesting thing is, if you look at the flux of the gamma rays, the flux in neutrinos we observe is actually larger than the flux in gamma rays. And nobody had ever expected that. And that's also why we found it with only two years of data. And so this is an interesting fact, which still requires some thought and explanation.

nation. So now we have to find where this neutrinos come from. So here is a picture of one year of neutrino data in galactic coordinates. So that's the sky. We see

neutrinos. We have 10 of these pictures for 10 years of data. More now. And we know from the flux we just measured that in this map there are 138,000 plus atmospheric neutrinos, 200 of them are actually cosmic.

But you also saw what I explained. If you go to very high energy, the atmosphere cannot deliver neutrinos anymore, and you are in business. So if you look at the very highest energies, it look like this. These are various methods with which we collected these cosmic neutrinos.

But notice one thing, we don't see our, own galaxy. Now, in any other wavelength of light or any other way of doing astronomy, the first thing you see are the sources in your own galaxy. We don't see those. We see flux at the 10% level of the flux that reaches us from the cosmos. So there are galaxies outside or sources outside our own galaxy that overpower the flux from the nearby sources that produce neutrinos in our own galaxy. We have a paper that's under review in science that will address this question. But just to remind you, that's what the galaxy looks like in visible light,

and we don't see the galactic plane in neutrinos. Now, for many years, we made the cosmic neutrinos were discovered in 2013. Of course, the game was where do this particle come from. You know, we know their direction now to 0.3 degrees. And we did all kinds of things. But I'm going to just tell you about the latest. We actually discovered a source in a multi-messinger company. in 2017, it was an active galaxy.

And I'm going to tell you what happened since then. We collected, we looked at our 10-year map, and we published a paper a year ago, and you are looking what's in this paper. And forget the red, just look at the blue. The blue, this is the upper limit on neutrino sources. We didn't see any by analyzing 10 years of data. And you see the dashed line is our sensitivity to sources as a function of where they are in the sky. The northern hemisphere is on the right, zero is the horizon, and the left is the southern hemisphere.

And we discovered, however, that our map after 10 years was not quite symmetric, isotropic anymore. And it was due to these four sources that we detected at almost 5-Sigma discovery. But that's before the look elsewhere effect. After you imply the look-elfare effect, the asymmetry in the map was still real, but the evidence for the sources not so great. NGC,068 almost reached 3-Sigma, with all trials taken into account. So the question was for these limits on fluctuations. And I'm not going to, I'll just give you the answer.

They are not fluctuations. We went to a campaign to calibrate the detector better to do improved reconstructions using machine language. We improved the point spread function of the telescope, abandoning Gaussian approximations. And with the conspiracy of all these improvements, we answered the question I just post. And here you see NGC, 168 before the improvements. And then you see we calibrated the detector better, including calibrating each phototube individually.

and you see how this is the source, the astronomical occasion, you see how with more statistics, we move towards the source, we point better at the source, and our final evidence is at the level of 4.2 sigma. Now, this is the result of a blind analysis where we mathematically can compute all,

our trial. So this is a real, this is not a social construct, it's a real probability. And so I can sleep well at night with 4.2 Sigma. So here, another way of looking at it, this is a picture of NGC,068. It has this very active hot corona surrounding the central black hole. And here you see the simplest way to look at the signal. You see this is the direction of NGC,068. The oranges are background from atmospheric neutrinos, and you see if you approach the source,

the number of neutrinos increases. So we see 88 neutrinos pointing at this source. with on average higher energy than the energy of the background, which you cannot see on this picture. So that was certainly very exciting. Here you see, let me just look at the top one. These are three analyses that confirm each other. They are slightly different.

They actually assume different spectral indices. But you see here the 5 Sigma discovery, which I showed before, And you see, this is pre-trial, you see NGC sticking out of it. You see a source TXS0506. That's the source we discovered in 2017, because it produced a neutrino of enormous energy that telescopes in a multi-messenger campaign pointed back at this source. N of source PKS-1424 plus 240. So the top three sources reappear with improved.

significant. Interestingly, one of these sources here on the fourth row in this analysis is NGC 4151. So here we are in

in 1943 Sefer discovered broad spectral lines from these two sources published in this paper. So we reproduced

these two sources in neutrinos. And now I could go forever on forever again. But so all these sources are sources that are actually active in radio, so they have an active core. And the core is opaque to gamma rays. So none of the stuff we see is accompanied by gamma rays. The gamma rays are shifted in the dense target that produces the neutrinos. They are shifted to MEV energy, X-ray or below.

For 50 years, Canadians have wondered what the names of IKEA furniture mean. Like Natopa, the guardrail for children's beds. Natopa means giving your favorite person room to grow. It means their safety and your piece of... No. In Swedish, Natopa means night monkey. Mom, he's back. IKEA, 50 years of Swedishness at home.

Look, he's wearing little pajamas. So cute. And so this is where we stand. So, in fact, this is multi-messenger astronomy plan B. How do you look for sources that don't emit gamma rays? That's not so simple, actually. So I'm waiting for ideas. I think that's the end of this.

Yes, so this is NASA picture of... NGC,068. Yeah, sorry, this is it. And thank you so much, Francis. So the other thing I wanted to pivot to was a recent paper that your team had in nature about a limit on quantum gravity. And I wonder, first of all, do you have any slides about that? Yes, I do. Okay, great.

I do. And I, you know, I want to emphasize. I want to emphasize. that we are about 350 people analyzing Ice Cube data. What I presented, the astronomy I presented, is probably only about one-third of us are doing this. I don't know the exact number. But people are using this detector, of course, to do glaciology.

There was a time in my life I published more glaciology papers than physics papers. I'm very proud of it. And a large fraction is looking for dark matter. That has been a long haul. Amanda was actually originally very much motivated by the fact that we would have discovered dark matter if the Wimp miracle had happened. And unfortunately it didn't.

But we are still looking. And people use the neutrinos. to do tomography of the earth, geology. And it goes on and on. We do neutrino oscillations competing with Fermilap, with a different part of the detector that I didn't emphasize here. But one of the things we have been doing is, given the opportunity is so unique,

we have been looking for quantum gravity. And this is a very old story. There is actually a whole community in physics that involves looking for quantum gravity, which is kind of merit to the alternative which is looking for violations of special relativity. It's they are in bed together, as you will see. It's easy to explain, you know, our neutrinos have a mass. We detect them over cosmic distances with enormous energies compared to neutrinos we make at accelerators. So the fact that they have these tiny masses is very important for physics, but it's totally relevant here.

Our neutrinos are like photons. And so if they travel through the vacuum, from the sources far away to here. And even though they have a flavor and oscillate into each other, by the time they arrive here, they arrive as packages of electron, Tau and Mune neutrinos with in equal numbers. So this is boring.

Unless this thing happens, and I wrote an article two decades ago, and a German artist made this picture. and what does it show? Well, it shows what the vacuum that our neutrinos travel through looks like if you marry quantum mechanics and gravity. And so if you marry quantum gravity and quantum mechanics and gravity, you know, the sheet that matter transforms that Wheeler always talks about,

that sheet now has quantum fluctuations. Yes. And so this is a picture of what the vacuum looks like. Our daily vacuum we live in now, what it looks like when it has quantum properties and quantum fluctuation, which it does have to have. The problem is that in our present vacuum we live in, they are suppressed by the Planck scale.

So they are tiny. And if I transform the Planck scale in a distance, they're 10 to the minus 33 centimeter. So you need neutrinos or photons with wavelengths of that magnitude to become sensitive to these fluctuations. And we do. And we already were aware of this and set the first limits with Amanda. I don't know if we want to go into that much detail. We should.

We should. Yeah, I mean, the audience loves to hear the details. How do we look for this? I mean, clearly, our experiment is ideal to look at these particles traveling through this quantum fluctuations over cosmic distances. That's how we get sensitivity. This effect is very small, but we can build it up by having the neutrinos travel for a very long way. And so what you basically do is you test whether the particles satisfy this,

the classic special relativity dispersal relation, E square equal p square plus M square. And that's the property of these neutrinos when they travel through, in absence, through just vacuum. The M square doesn't really matter at our energy. But this relation between energy and momentum is modified when the neutrinos start to interact with the, they start to feel this quantum fluctuation. And that we parameterize as a power series, you know, a sum of n equal 1, 2, 3 and so on, a distortion in energy that's characteristic of the power. Planck scale, so the quantum gravity scale or the scale of the violation of relativity, because

you see, E-square not equal to P-square can also be caused by violating special relativity. So we look for terms like that when the neutrinos propagate. And so it's a few-line calculation. So what actually happens? What happens is that particles with different energy now begin to propagate, violating to a different extent is E-square equal P-square relation. And so the particles with higher energy get a time delay proportion to particles with slower energy. The ones with high energy interact more, so they get delayed.

And the time delay is given by the power of the term you are investigating this power series. You know, we typically look for an equal to term. And then so we look for a term of time delays that are proportional to the energy square. But the time delay builds up the larger distance you look to the source. So even though this is very small effect, if you build it up over long enough distances, you can become observable. And it does in the case of Ice Cube.

So as it says here, integrate over very long distances. So here is an example of one version of, this analysis. And I always like to quote to show it with a quote from Michael Turner who said that unlike Newtonian physics, general relativity

will not last 200 years. So we're trying to prove him right. But you see here, we can put limits of delta C over C, a change in speed of light, or an influence

of the gravitational field phi on the propagation of the neutrinos. And you see, we're sensitive to the variation of order 10 to the minus 28. When the coupling to the gravitational field is strong enough, it's given by this angle, which is, I won't explain. So we are putting limits on violations of relativity of 10 to the minus 28. Now, if you listen to how I explain this, so you have these wave packages of three flavors

traveling through this quantum fluctuations. So the different flavors can undergo a phase shift when they interact with these gravitational fluctuations. And that creates flavor oscillations made by the interaction with this field, with this quantized field. And that violates, can violate the prediction I made two minutes ago

that we have wave package of equal flavor arriving. And so what Ice Cube now does is it, and that the paper we recently published was one version of this. So this is the number of, Tau, new, and electron neutrinos in the beam. So we have to

discover this red dot. If we are anywhere else in this plot, we have discovered something. Lorenz violation, quantum gravity, you can interpret it any way you want. And so, our present

this red dot with our present measurements of the three flavor scenario as a systematic error which is given by this butterfly. But so if we can establish violations of these principles, new physics, by ending up somewhere, anywhere else in this diagram. And so this is our present measurement. This is the error bar at a 68% level.

And we are doing a dedicated effort similar to, or even bigger, to what I explained for finding the sources of the cosmic neutrinos to squeeze these error bars. We are actually going to put in new strings upgrading the detector in the season 25-26, which among other things will exactly focus on making this a story. small ellipse around the red dot, and hopefully, maybe not. So we will see. But this is a high priority of the experiment. I have two more slides to remind you of a fun fact.

You may remember that the opera experiment discovered the violation of relativity many years ago. Yeah, it was actually the year before the Bicep 2 results came out, which made us very nervous that we made a wonder. Yeah, exactly. And you can see on this slide, there is another way to test relativity, which was pointed out by Glashow. That is if your particles move at the speed of light C prime, which is bigger than the speed of light, they carry an excess energy, MC prime square minus MC square, and that extra energy, they can radiate away.

And that radiation, for instance, can come out as E plus and E minus pairs. But when they radiate away their energy, it means they lose energy. And at the time, Glasho and Floyd Stecker pointed out that the highest energy neutrinos at actually too high energy

and were inconsistent with the opera observation. Of course, I don't know whether the opera has observation disappeared before the paper was published, but our highest energy events seen at the time were inconsistent with that violation of relativity. So it shows we are doing something real in a very concrete way, right? Actually, yes, I didn't know about it.

that and Sheldon was a guest on the podcast about two years ago and I neglected to ask him about that event well there's so much to talk to Sheldon about as there is a view. Oh yeah I mean how do you pick your subjects right? Yeah we could go for hours the only thing I want to say about the Lorentz invariance and we're looking for it in the polarization signals of distant objects as they rotate yes do not rotate and you can look for biorefringence types of facts as you talked about the ice. By the way, you know, it's a clear quantum, but what I was explaining before is the analogous of gamma rays traveling to a crystal, right?

And they can exchange their polarizations. So it's a pure quantum effect. But it's very straightforward to look for it. For 50 years, Canadians have wondered what the names of IKEA furniture mean, like Natopa, the guardrail for children's bed. Natopa means giving your favorite person room to grow. It means their safety and your piece of... No. In Swedish, Natapa means night monkey.

Mom, he's back. IKEA, 50 years of Swedishness at home. Look, he's wearing little pajamas. So cute. Yeah, it's very interesting. And I guess the only kind of, you know, kind of, you know, critique that I've heard is that the results are somewhat model dependent. I mean, this SME framework, which I've used in my papers before as well, is very, it's very specific to a particular class of, you know, it's like they say about

non-Galcianity. It's like saying it's a non-dog animal. Well, there's a lot of non-dog animals in the world, right? So how do you address those, you know, kind of... Well, my answer is that this is the least model dependent you can get, right? If you have... If you have a model, you can always reinterpret it in terms of this power expansion. That's right. This is actually a standard method that was pioneered by Weinberg, right? You have a theory and you parameterize the deviations in terms of operators of different dimensions. And N are the dimension of these operators.

That's amazing. So I think that's the most model. So it's the most model independent you can get that. And any model we can reinterpret in terms of these results. That's right. That's right. Well, thank you so much for that, Francis. I guess the topics I'd now like to turn to is what comes next for Ice Cube. There's rumors of advanced Ice Cube structure that will come about.

Can you talk a little bit about the status of? Do you have any slides on that? No, I don't have any slides on that. That's fine, yeah. But it's very simple. I mean, if I had given a one hour talk, and I often do on Ice Cube, the only conclusion any reasonable listener can come to is we need better angle resolution, we need more statistics, we just don't want to deal with a.

few sources, we need more sources, and what do you do? You have to build a bigger detector. Yes. Fortunately, we discovered, when we build and design, when we build ice cube, we only had vague understanding of how large the absorption length actually was. because Amanda was much smaller, right? So we had never seen ice and light propagating ice over kilometer distance, which was necessary to measure an absorption length, which at the bottom of the detector is 250 meter for shrink of life. So it's just incredible.

And so we were conservative, but now we know we can space our photomultipliers further. and so this 125 meter distance between strings of photo multipliers, we're going to increase to 250 meters. And then we can instrument 8 to 10 times the volume with the same number of photomultipliers. So 10 times the volume for the price of ice cube. and of course you know all the

the data acquisition which I discussed at the beginning all that is improved and much cheaper and so we really think we can build the next generation detector for the same

cost as the original one. We actually have we have designed this detector and submitted the design to the Decadal Review, who endorsed it. So it's not a dream anymore. At least unlike the first dream, at this point we know what we're doing. It's not a gamble anymore.

That's wonderful. Well, congratulations with that. And of course, you know, it's this phenomenal treat to see not only the success of your projects and the leaders under your leadership, but all the young people that have come out of it. My friend Nathan Whitehorn at UCLA and of course sharing the space with you and your team down at the South Pole with so many young people. It's always so energizing to be down there and not to mention the hot tubs, which were quite great. I never got to go swimming in one of the hot tubs.

Yeah, I always show a picture of the collaboration at the end of my talk. Yeah. But it's actually a picture. It's incredible, the age of our collaboration. I have theories why this is, but I won't go into it. It doesn't matter. The other thing I have to say is, and this was always emphasized by the late Trevor Weeks who discovered, who developed gamma ray astronomy, earth-based gamma-ray astronomy.

Trevor Weeks used to say astronomy is not like particle physics. You need many telescopes. It's not like discovering the Higgs. In principle, one detector could maybe have done it, but you need two. Even there you need two. And so astronomy, the sky is big. There are many problems. So with a smaller telescope and the right idea you can contribute.

And so people pick, I think the biggest complement to Ice Cube is that people are now planning and building detectors, building in Lake Baikal, building in the Mediterranean, building off the coast of Vancouver in Canada, and planning a detector in China. Yeah. So there are many more to come, I hope. It's truly breathtaking and inspiring to me as one of the five or so coldly. of the Simon's Observatory, which is equally sized and is also hoping to shed some light, no pun intended, on neutrinos, and their role is dark matter and as a very important

budget and energy budget constituent in the cosmos. So, Francis, we speak a lot on this channel to young people, graduate students, postdocs, undergraduates, and one of the favorite parts of the show is when I ask kind of non-scientific questions, which I I call the existential questions are the final four. And I'd be honored if you'd answer some of them, if you don't mind right now. I am fearless, but I'm probably not a good or certainly not a typical subject for this, but let's go. There's no difficult subject. So yes, let's go into the impossible with the final four questions I ask. So they typically have to do with the distant future, of both yourself and of humanity, perhaps, and then advice to your former self as a young person,

because as I said, we have a lot of mostly men, I'm trying to boost that up, but mostly young men, interested in science and technology, but we are open for all. So the very first question that I always ask is, what would you put in what's called your ethical will? In other words, what would you like to impart as wisdom that you've learned? not scientific necessarily. It could be anything. It could be to your children or great, you know, grandchildren or many, many generations down the line. What piece of wisdom guides you, if any? Well, I think with the advantage of not just looking at the future, but looking back,

I want to come back to my years that I was at Lemaitres Institute writing my undergraduate thesis on Quarks. And imagine this was 1967, 8, 1967, actually. And so I alluded to the fact that if you worked on cosmology at the time, you were kind of a crack portrait. Le Maitre was not a famous person except for the fact it was a cardinal. and he had been the president of the papal academy and so. But I actually, my undergraduate thesis was on quarks, another topic that nobody would touch at the time.

And so first of all, the first message is just do what you're interested in. You cannot be successful if you do physics as a job. And I'm kind of at the edge of the generation where actually physics wasn't a job. And that was wonderful. I'm afraid to say the time is gone and it's rightly gone. So that's one message. The other message is how to, you know, I don't have to predict or be a visionary. Look how exciting science is.

Can you imagine that in 67, you worked on quarks and jets and on cosmology and the Big Bang? You were not taken seriously. In fact, you couldn't get a job, even if it was a job. And look where we are now. I mean, a black hole and a big bang and a quark jet are as real as this cup of coffee on my desk, right? Nobody questions this anymore. And this happens in my career in one generation of a physicist. And I have no doubt that this will continue.

It may not continue in the direction that we are now talking about. It may be off in a different direction. But I'm sure it will continue. Science will always be exciting. Yes, absolutely. It's the, as you'll hear in just a second, I'll say what my next, my next quote comes from Sir Arthur C. Clark, who said that any sufficiently advanced technology is indistinguishable from magic. And I want to ask you, what knowledge,

either from your personal career or from all of science, or maybe, I don't know, anything from philosophy, theology, who knows? What piece of one sentence or the shortest amount of words that you could possibly describe the thing that humanity should be most proud of. Well, I always limit myself to the expertise I have. And not just humanity, my generation should be proud of what we just talked about. And that's something to be very proud of, I think. I agree. Yeah, my late great colleague Hans Parr, did you ever know, Han?

Did you hear you know? Yes. Yes. So he was a great influence on me. He said that the general relativity, which is part of some of what we've talked about, was the pinnacle not of just science, but of all of Western civilization. And so I agree. A lot had to go into that.

You had to have cooperation, language, there's a beauty, there's artistry, and there's also hard technology. And the most important thing, which you keep emphasizing so rightfully is people. And the culture of an experiment is so important. Okay. And people who are really obsessed with what they're doing. Those are the ones you are looking for. That's right.

Next question, the second to last question, is another quote from Sir Arthur C. Clark. And he said, when a distinguished but elderly scientist says that something is possible, he is almost certainly right. But when he states something is impossible, he is probably wrong. So I want to ask you, Francis, what have you been, wrong about, if anything? Oh, many, many things. In fact, let's go back to my career,

career was half astro-particle physics and half particle physics, right? I lived around accelerators. And when we were writing Halsen and Martin, I actually gave Halsey, and Martin originated because I took a semester leave at the University of Hawaii because I was working with San Di Pakvasa. By the way, that was the time they were developing Dumont. And I had no idea what Dumont was, but I'm sure it's stuck in my brain somewhere. But at the time,

they asked me to give a course and they say, you just come and talk what you did the last week. I thought I walked into the room and the whole faculty showed up wow and so I decided the set

the C-0 had just been discovered a weak intermediate boson and I talked the first lecture about what a breakthrough this was and so then I actually decided over the semester to to develop what we now

recognize as the standard model last lectures. So that was the beginning of Halton and Martin. So I think I never, there's something called the gym mechanism and I couldn't believe that you could just invent another quark

to fix a deep problem of the theory. And I thought that was incredibly naive. And it turned out to be right. And I remember going through this history of me being wrong not believing the gym mechanism. You know, gym mechanism

fixed a standard model with postulating that there was a charm quark in the theory. And so it made a big impression on me in the sense that you know, sometimes simple, you

shouldn't hold it against simple ideas. the simpler an idea, the more elegant, this was in a sense elegant. And my reaction was totally wrong that something simple cannot be right in nature. That simple. And so I learned a lot from that. It made a big impression on me too because it doesn't feel good being totally wrong about something. No, absolutely not.

take the mark of a good scientist is to recognize, learn from it. Yeah. And I learned my lesson from that. Absolutely. And I, you know, I never have, unless something is demonstrably wrong, I have very, I accept everything, basically. I mean, if you, you have to have a very open mind when you approach a problem. It's not good to be biased. That's the thing I learned here. Yeah. Well, that really does dovetail into. the last question, which is from Arthur C. Clark's third law, which states the only way of discovering the limits of the possible is to venture a little way into the impossible. And maybe I'll pivot because

you've said so much about, you know, kind of advice to your former self. Maybe if you were getting into this career or any kind of career, you're 20 years old, you're at the University of Wisconsin, Madison, you're at UC San Diego or wherever, what would you go into? What would you advise a young Francis Halzin to go and do? to do as you've done to go into the impossible. I already gave the answer to that. You go what you're interested in, unless you're obsessed by the subject. I mean, being a physicist from all objective point of views is a miserable career.

And I don't have to state the reasons. The only reason it's worth doing is you just enjoy this. You're obsessed by it. You love what you're doing. And so if you do it as a job, it's going to be very, very disappointing, I think. So I think this is just, and the other thing is, you know, I always lived with, don't underestimate yourself. I totally lived with until I went on this crazy adventure of, Amanda and Ice Cube.

I always live with the insecurity that I didn't belong to the circles I moved in, which I think must be true for almost every graduate student. Just get over that. And I don't know how to do this. I never did it until, you know, you go on a wild ride like we did with neutrino astronomy. Then you're just, you know, surfing the way.

Right. But at many times of my career, I think I missed opportunities because I was insecure. And so, you know, it's, it are not always the smart people who make the breakthroughs. That's another way. That's a very parochial way of thinking about it. You know, imagination can trump intelligence. There are many angles to doing research. And so just go for it. It's very, very wonderful that you say that and we'll close with this. So when I interviewed Barry Barish two or three years ago now for this podcast, I asked him that same kind of a question in the form of,

have you ever experienced the imposter syndrome, which is this feeling of insecurity that you just spoke about? And he said, yes, I have it worse than ever now. What are you talking about? You have the Nobel Prize. He said, no, no, no, Brian. When you win a Nobel Prize, you have to go and sign this logbook in Sweden that says that I got my Nobel Prize.

So Barry's incredibly curious, very imaginative. And he goes back and he looks through the previous pages and he sees Feynman and he sees a galman at his home institution of Caltech. And he goes farther back. He sees Maria Gepert-Mayer maybe here at San Diego. And then he goes back to 1921. and sees Albert Einstein. He sees this guy's signature here, my favorite little sock puppet.

And he says, I'm not worthy. I'm not as smart as Einstein. And I say, Barry, guess what? You know, Einstein had the imposter syndrome. You know, he wasn't always Einstein. He said, what are you talking about? He said, Einstein felt that Newton did more for the human culture and civilization than any person before or since.

And he said, wow, I didn't know that. I said, Barry, guess what? It gets better. Newton had the imposter syndrome. He's like, ah, you got to be kidding me. I said, no, Barry. Isaac Newton wrote, he said, I live in shame that I never lived up to the ideals of Jesus Christ. So we all have our imposter, you know, kind of moments when we feel we're inadequate.

And then he ended up writing, I don't know if you can read this, but I'll send you a copy of this book next time I'm out there. But Barry wrote the forward to my second book, which has his. interviews and I hope someday. I actually read it either on your webpage or somewhere. Yes. Yes. I don't think it's unusual. I think it's also, I don't think it's

a problem. It's only a problem when it prevents you from doing things. That's right. That's exactly right. And I think you're right to have the courage. As you said, just as you said, just do it. Well, Francis, you've been an inspiration to me as a scientist, as a teacher, as a leader,

and as an author. And I'm just so glad and honored and thrilled that you were able to spend so much of your valuable time with us. Thank you so much. I hope we can see each other again. Maybe you'll come here in January and I'll go there in June and we'll go and get some cheese curds on the Capitol together. It was a pleasure.