Transcripts:

**Francis Halzen:** 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 don’t underestimate yourself. Until I went on this crazy adventure of AMANDA and IceCube, 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.

**Brian Keating:** 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 back quite a ways, 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 was 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 he is one of my personal heroes. This is Francis Halzen, who is a Belgian particle physicist, although he’s been in America probably more than half of his life. He’s the Hilldale and Gregory Breit 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 IceCube 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 class. Please don’t tell my audience. I don’t want to.

**Francis Halzen:** I don’t think my bureaucracy, my administration is so good that I would be able to produce it.

**Brian Keating:** 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, it was such a treat.

## Judging books by their covers

**Brian Keating:** And I thought we’d start there, Francis. We just had on Martin Rees, Lord Martin Rees, 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 tenfold 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. But 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?

**Francis Halzen:** 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 realise now that this book was written a long time ago and that the W, the Z, the top quark, the Higgs, none of these things had been discovered. The whole book is kind of prediction of the standard model. And in fact, the book was in print when the W was discovered at CERN, the weak intermediate boson. And so the editor called me and said, you can still change things. I’ve read a newspaper. And I said, there’s nothing to change. And he was kind of worried that he had been part of some huge gamble. But the title is kind of embarrassing because, although I think the books were really written in parallel, Okun, famous Russian theorist, wrote the book *Leptons and Quarks* that came out around the same time. So it’s not that original.

**Brian Keating:** And the illustration that you have on the cover, what does that represent?

**Francis Halzen:** The only thing I remember is that I actually made it. And I remember, jets were something rather new then. Quark-gluon jets, the concept. And so I know that it represents a jet made in deep inelastic scattering or something like that. It’s amazing that I cannot remember.

**Brian Keating:** 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 IceCube 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 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.

## It is rumored that you knew Lemaître

**Brian Keating:** So I want to first take a step back. It is rumored that you knew Lemaître, the famous Belgian Catholic priest, the progenitor in some sense of the Big Bang. Is that true, Francis?

**Francis Halzen:** I barely knew him. He died when I was doing my undergraduate thesis. And so I could tell a number of stories for a long time. But I’ll only tell you two. Because of course every interaction I had with him is now legendary, right? But I only tell you two.

He had a computer he built himself to solve Friedmann’s equations. And it was a big room with vacuum tubes. And that computer, I mean, it was state of the art at the time, was only used during the day. And we, I mean another graduate student and I, Raymond Gastmans, we would actually break into the room and use the computer at night. And we used it to calculate Feynman diagrams. Veltman 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 Veltman discovered it.

The other story is that when I was an undergraduate, [inaudible] would arrive in a limousine at the physics building, and 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 Lemaître. I had never made the association. He was not very visible. He was retired and he came in very rarely.

Either 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 II, 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 at the time he died, as you know, when the microwave background was discovered.

**Brian Keating:** 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 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, Arcetri, 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 theologian?

**Francis Halzen:** Yeah, I cannot tell. 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. You read his bio, there are a couple of biographies of his which I’ve read, of course, and they clearly document that he didn’t make any connection to religion.

## The obligations of a scientist

**Brian Keating:** And when we were getting ready to start recording today, I wanted to kind of get your take on the obligations of a scientist in terms of what he or she should do, quote unquote, should do. And I think Lemaître certainly took that role seriously, but he had so many hats that he was wearing, 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 IceCube and 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?

**Francis Halzen:** Well, I think 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 given we are supported by the public to do our science, we owe some return to them. And I have never turned 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… Our expertise is science, and it’s actually, even within science, it’s a very limited expertise. And I think it’s to the advantage of scientists that they stick to their expertise. And that’s certainly something I always do.

**Brian Keating:** Yeah, I always say that the problem that we scientists deal with is that sometimes we become political scientists and delve into politics.

**Francis Halzen:** No, that’s not a good idea, and it also hurts science, right? Because then science begins to look like something that it shouldn’t be, and it’s not.

**Brian Keating:** 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.

## The whole story was a story of panic

**Brian Keating:** And I want to ask you, first, if we can go back to maybe a moment in your career that was a little bit perilous. I remember being in Madison, as you know, from ’97 till… Peter Timbie, 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 Badger State, Dairyland State. So I had nothing but fond memories of being there. When I had Peter on, we were kind of speculating on the notion as a scientist to deal with uncertainty. And I remember 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 IceCube? Did you ever have a moment of panic, that you just were so terrified the project might not move forward?

**Francis Halzen:** The whole story was a story of panic. You got the right word. From the beginning, when people thought that this was a very cute idea but that it wouldn’t work. Well, that’s what we thought 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 into a Cherenkov 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, among all the panic moments, the most exciting moment was not when we discovered cosmic neutrinos, but when we finally observed atmospheric neutrinos, because that, I felt, we’ve delivered. Now it’s up to nature. And even afterwards, I thought… the English like to gamble, right? [inaudible] wrote an article that gave us six to one to discover cosmic neutrinos. Against, of course. And I remember not feeling very well when I read this. But then I always thought, if we did 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 thought about is, how do we run this backwards if this happens not to be true? [inaudible] with that panic for quite a long time. In fact, it’s only now that we see sources, right, since 2017, that I sleep better at night.

**Brian Keating:** Well, that’s a thirty-year journey to…

**Francis Halzen:** This was a wild ride, right? This whole story of ups and downs, and yeah.

## What was the role of the South Pole?

**Brian Keating:** 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.

**Francis Halzen:** No, it’s a unique opportunity because you need clear ice. Clear ice exist elsewhere, but you have to build a sophisticated detector in a remote location. And without the South Pole stations, this would not have been possible. Remember, we started in a very modest way, and nobody would say… we always wanted to build a kilometer cube detector. Can you imagine someone saying, Halzen 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.

**Brian Keating:** 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 persuade. 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 is so hard nowadays, but then building a team. So it’s not enough to have the right idea. The idea is just kind of the table stakes, to use a gambling analogy. So when did that subsiding of that panic…

## Could IceCube have been done 50 years ago?

**Brian Keating:** What has been, I mean, of all the things that IceCube does, and we’re gonna cover it in 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 wanna ask you, of the technical breakthroughs, could IceCube have been done 50 years ago? Let’s say if there was a 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?

**Francis Halzen:** It depends. It’s a difficult question. It’s a question I’ve never answered, which is rare. One of the challenges, 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 don’t go to school and they teach you about hot water drills, right?

**Brian Keating:** Well, in Wisconsin they do, actually, right?

**Francis Halzen:** This was done by a group in Wisconsin of engineers, professors, graduate students, technicians, anyone who could contribute, contributed. And it was very exciting. But I think it’s car wash heaters that make the power. And the nozzle is very sophisticated. 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, right, that powers the photomultipliers. But the data acquisition of IceCube is a bit more sophisticated. We capture the signals in the ice, transform them to digital, and then transmit them. And so the chips that are used were 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 that technology looks like archaeology, right? But the Berkeley design was made in the late 1990s, because it was all in place when we submitted a proposal in ’99. So it probably could have been 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.

## Why do we conflate scientific ability with pedagogy ability?

**Brian Keating:** 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 for advanced undergraduates and graduate students on particle physics: why do we conflate scientific ability with educational or pedagogy ability? In other words, someone who’s a pilot may be extremely skilled at flying, but we don’t ask him to teach flying necessarily, although 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, 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?

**Francis Halzen:** Well, this is one of the things I feel passionately about and never had the 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.

**Brian Keating:** No.

**Francis Halzen:** So 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.

**Brian Keating:** 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 IceCube, and you’ve been kind enough to prepare some slides. So I’ll let you get set up to share the screen.

## We want to do astronomy with neutrinos

**Francis Halzen:** Okay, so 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 beautiful experiment is that the theorists in their wisdom told us, and by the way, I’m not blaming anyone, I was one of them, told us that this experiment is ten thousand times too small to do astronomy and catch the tiny flux of neutrinos that reach us from the universe.

So to make a long story short, we build a detector that, instead of made out of water, it’s 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 if you go under the South Pole… 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 there are eighty-six of them. So the light sensors between strings are seventeen meters apart, and the strings are one hundred twenty-five meters apart, and eighty-six of these strings fill a volume of ice with light sensors. This idea, by the way, was born in 1960. A 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 hundreds and sometimes hundreds of meters. So this is compacted snow of 70,000 years ago. It’s ultra pure, and even in a lab, you cannot build a block that transparent to blue light. So 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 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. Occasionally, like one in a million will hit a 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 Cherenkov radiation. And if this is a muon neutrino, then it makes a particle called a 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 speed boat that outruns its waves, and so you get a shock wave, like the bow wave of a boat, 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 eleven degrees below the horizon and streams through the detector. It deposits inside the detector two thousand six hundred TeV of energy. So you must remember the highest energy beam at CERN in Geneva is fourteen TeV. So this particle deposits 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 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 collects in neutrinos from the cosmos, 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 is the flux measured 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.

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 a hundred [inaudible]. Then we get more and more background in atmospheric neutrinos. And when we reach someplace like tens of TeV, 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 ten sigma, and we have seen atmospheric neutrinos in at least four totally different ways. That’s the status of IceCube. I have a few slides to show what our results are, if… In fact, one of our most important results.

**Brian Keating:** Please do. Please do.

**Francis Halzen:** 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 nu F nu. And of course, we detect fluxes of gamma rays. Here you see this peak, this huge peak, that’s the CMB. Very familiar to my host. It dominates the universe. Then you go to different wavelengths of light. This is ultraviolet light, X-rays, gamma rays. But then the universe turns opaque to gamma rays. They turns opaque to gamma rays because 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.

**Brian Keating:** Right. And actually, Francis, I have to stop you there. Did you know that the namesake of your professorship, Breit, the Breit-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 really exciting, right? Anyway, please go on. Yeah, the pair production. Yep.

**Francis Halzen:** Pair production. So these photons pair produce, make e plus e minus pairs interacting with the microwave photons, and so they lose energy. So the flux disappears. And that, of course, is the raison d’être 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.

## Now we have to find where these neutrinos come from

**Francis Halzen:** Now we have to find where these neutrinos come from. So here is a picture of one year of neutrino data in galactic coordinates. So that’s the sky we see in 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. Two hundred of them are actually cosmic. But you also saw in 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 looks 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. So there are 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 looked 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 these particles come from? 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 the source in a multi-messenger campaign in 2017. It was an active galaxy. And I’m going to tell you what happened since then. We looked at our 10-year map and we published a paper a year ago, and you are looking at what’s in this paper. And forget the red, just look at the blue. This is the upper limit on neutrino sources. We didn’t see any by analyzing 10 years of data.

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 ten years was not quite symmetric, isotropic, anymore, and it was due to these four sources that we detected at almost five sigma discovery. But that’s before the look-elsewhere effect. After you apply the look-elsewhere effect, the asymmetry in the map was still real, but the evidence for the sources not so great. NGC 1068 almost reached three sigma with all trials taken into account.

So the question was, were these limits and fluctuations? 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 learning. 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 posed. And here you see NGC 1068 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 location. You see how, with more statistics, we move towards the source, we point better at the source, and our final evidence is at a level of 4.2 sigma.

Now, this is the result of a blind analysis where we mathematically can compute all our trials. So 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 1068. It has this very active hot corona surrounding the central black hole. And here you see the simplest way to look at a signal. You see, this is the direction of NGC 1068. 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 the 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 five sigma discovery, which I showed before. And you see this is pre-trial, you see NGC sticking out of it. You see a source, TXS 0506. That’s the source we discovered in 2017 because it produced a neutrino of enormous energy. The telescopes in a multi-messenger campaign pointed back at this source. And a source, PKS 1424+240. So the top three sources reappear with improved significance. Interestingly, one of these sources here on the fourth row in this analysis is NGC 4151. So here we are: in 1943, Seyfert discovered broad spectral lines from these two sources, published in this paper. So we reproduce these two sources in neutrinos.

And now I could go on forever. But 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. 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. So this is a NASA picture of NGC 1068. Sorry, this is it.

## A limit on quantum gravity

**Brian Keating:** 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? Okay, great.

**Francis Halzen:** Yes, I do. I do. And I want to emphasize that we are about 350 people analyzing IceCube data. The astronomy I presented, 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 Fermilab, with a different part of the detector that I didn’t emphasize here. But one of the things we have been doing, given the opportunity 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 married to the alternative, which is looking for violations of special relativity. They are in bed together, as you will see. But it’s easy to explain. 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 irrelevant here. Our neutrinos are like photons. And so 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 mu neutrinos 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 mechanics and gravity, the sheet that matter transforms that Wheeler always talks about, that sheet now has quantum fluctuations. And so this is a picture of what a vacuum looks like, our daily vacuum we live in now, what it looks like when it has quantum properties and quantum fluctuations, 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’re tiny. And if I transform the Planck scale into a distance, they’re ten to the minus thirty-three 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, but…

**Brian Keating:** Yeah, I mean, the audience…

**Francis Halzen:** Details. How do we look for this? I mean, clearly our experiment is ideal to look at these particles traveling through these 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 the classic special relativity dispersion relation, E square equal P square plus m square. And that’s the property of these neutrinos when they travel 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 feel this quantum fluctuation, and that we parametrize as a power series, a sum of n equal one, two, three, and so on, a distortion in energy that’s characteristic of the 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 this E square equal P square relation. And so the particles with higher energy get a time delay proportion to particles with lower 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’re investigating, this power series. We typically look for the n equal two term, and so we look for 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 a very small effect, if you build it up over long enough distances, it can become observable, and it does in the case of IceCube. 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 show 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 on 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 are sensitive to variations of order 10 to the minus 28 when the coupling to the gravitational field is strong enough. It’s given by this angle, which 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 explained this. So you have these wave packages of three flavors traveling through these 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 can violate the prediction I made two minutes ago that we have wave packages of equal flavor arriving. And so what IceCube now does, and the paper we recently published was one version of this… So this is the number of tau, mu 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: Lorentz violation, quantum gravity, and you can interpret it any way you want.

And so our present… this red dot, with our present measurements of the three-flavour scenario, has a systematic error which is given by this butterfly. But we can establish violations of these principles, new physics, by ending up 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 than 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 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 a violation of relativity many years ago.

**Brian Keating:** Was actually the year before the BICEP2 results came out, which made us very nervous that we made a blunder.

**Francis Halzen:** 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 a speed of light c prime, which is bigger than the speed of light, they carry an excess energy, c prime square minus c square, and that extra energy they can radiate away. And that radiation, for instance, can come out as e plus e minus pairs. But when they radiate away their energy, it means they lose energy. And at the time, Glashow and Floyd Stecker pointed out that the highest energy neutrinos had actually too high energy and were inconsistent with the OPERA observation. Of course, I don’t know whether the OPERA 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’re doing something real in a very concrete way, right? That’s my last.

**Brian Keating:** Actually, yes, I didn’t know about 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…

**Francis Halzen:** Yeah, I mean, how do you pick your subjects, right?

**Brian Keating:** 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 or do not rotate, and you can look for birefringence types effects, as you talked about with the ice.

**Francis Halzen:** By the way, it’s a clear quantum… but what I was explaining before is the analogous of gamma rays traveling through 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.

**Brian Keating:** Yeah, it’s very interesting. And I guess the only kind of 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 specific to a particular class of… It’s like they say about non-Gaussianity, 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 kind of…

**Francis Halzen:** Well, my answer is that this is the least model dependent you can get, right? If you have a model, you can always reinterpret it in terms of this power expansion. This is actually a standard method that was pioneered by Weinberg, right? You have a theory and you parametrize the deviations in terms of operators of different dimensions, and n are the dimension of these operators.

**Brian Keating:** That’s amazing. So I think that’s the most mod…

**Francis Halzen:** So it’s the most model independent you can get. Any model we can reinterpret in terms of this result.

## What comes next for IceCube

**Brian Keating:** 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 IceCube. There’s rumors of advanced IceCube structure that will come about. Can you talk a little bit about the status of… Do you have any slides on that?

**Francis Halzen:** No. That’s fine. But it’s very simple. I mean, if I had given a one hour talk, and I often do on IceCube, 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.

Fortunately, we discovered… when we build IceCube, we only had vague understanding of how large the absorption length actually was. Because AMANDA was much smaller, right? So we had never seen light propagating in ice over kilometer distance, which was necessary to measure an absorption length, which at the bottom of the detector is 250 meters for Cherenkov light. So it’s just incredible. So we were conservative, but now we know we can space our photomultipliers further. And so this 125 meter distance between strings of photomultipliers, we’re going to increase to 250 meters. And then we can instrument eight to ten times the volume with the same number of photomultipliers. So ten times the volume for the price of IceCube.

And of course, all 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 have designed this detector and submitted the design to the Decadal Review, who endorsed it. So it’s not a dream anymore. And at least, unlike the first dream, at this point we know what we’re doing. It’s not a gamble anymore.

**Brian Keating:** That’s wonderful. Well, congratulations with that. And of course, it’s just a phenomenal treat to see not only the success of your projects, 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, though.

**Francis Halzen:** I always show a picture of the collaboration at the end of my talk. But it’s actually a picture, and it’s incredible the age of our collaboration. I have theories why this is, but I won’t go into them. It doesn’t matter. The other thing I have to say is, and this was always emphasized by the late Trevor Weekes, who developed gamma ray astronomy, Earth-based gamma ray astronomy. Trevor Weekes 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 I think the biggest compliment to IceCube 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. So there are many, and many more to come, I hope.

**Brian Keating:** It’s truly breathtaking and inspiring to me, as one of the five or so co-leaders of the Simons Observatory, which is equally sized and is also hoping to shed some light, no pun intended, on neutrinos and their role as dark matter and as a very important energy budget constituent in the cosmos.

## The final four

**Brian Keating:** 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 call the existential questions, or the final four. And I’d be honored if you’d answer some of them, if you don’t mind, right now.

**Francis Halzen:** I am fearless, but I’m probably not a good, or certainly not a typical, subject for this, but let’s go.

**Brian Keating:** There’s no difficult subject. So yes, let’s go into the impossible. The final four questions I ask. 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-grandchildren or many, many generations down the line. What piece of wisdom guides you, if any?

**Francis Halzen:** 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 Lemaître’s institute writing my undergraduate thesis on quarks. And imagine, this was 1967, ’68. 1967, actually. So I alluded to the fact that if you worked on cosmology at the time, you were kind of a crackpot, right? Lemaître was not a famous person, except for the fact he was a cardinal and he had been the president of the papal academy. But 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… 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 the 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 [inaudible] in a different direction. But I’m sure it will continue. Science will always be exciting.

**Brian Keating:** Yes, absolutely. As you’ll hear in just a second, I’ll say what my next quote comes from: Sir Arthur C. Clarke, 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, one sentence or the shortest amount of words that you could possibly describe, the thing that humanity should be most proud of?

**Francis Halzen:** Well, as 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.

**Brian Keating:** I agree. My late great colleague Hans Paar, did you ever know Hans? Did you ever know him? Yes. So he was a great influence on me. He said that 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.

**Francis Halzen:** And people who are really obsessed with what they’re doing, those are the ones you are looking for.

## What have you been wrong about?

**Brian Keating:** That’s right. Next question, before the second to last question, is another quote from Sir Arthur C. Clarke, 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?

**Francis Halzen:** Many, many things. In fact, let’s go back to my career. My career was half astroparticle physics and half particle physics, right? I lived around accelerators. And when we were writing Halzen and Martin… Halzen and Martin originated because I took a semester leave at the University of Hawaii, because I was working with Sandip Pakvasa. By the way, that was the time they were developing DUMAND. And I had no idea what DUMAND was, but I’m sure it 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 last week. I walked into the room and the whole faculty showed up. And so I decided… the Z0 had just been discovered, the 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 develop what we now recognize as the standard model, as lectures. And so that was the beginning of Halzen and Martin.

So there’s something called the GIM 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 GIM mechanism. The GIM mechanism fixed the standard model by postulating that there was a charm quark in the theory. And so it made a big impression on me, in the sense that 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.

**Brian Keating:** No, absolutely not. But I think the mark of a good scientist is to recognize, learn from it, own up and…

**Francis Halzen:** I learned my lesson from that. And unless something is demonstrably wrong, I accept everything, basically. I mean, you have to have a very open mind when you approach a problem. It’s not good to be biased. That’s one thing I learned here.

## Go into the impossible

**Brian Keating:** Well, that really does dovetail into the last question, which is from Arthur C. Clarke’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 advice to your former self. Maybe if you were getting into this career or in 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 Halzen to go and do, to do as you’ve done, to go into the impossible?

**Francis Halzen:** 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, I always lived with… don’t underestimate yourself. I totally lived with, until I went on this crazy adventure of AMANDA and IceCube, 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 go on a wild ride like we did with neutrino astronomy, then you just surfing the waves.

**Brian Keating:** Right. It reminds me…

**Francis Halzen:** At many times of my career, I think I missed opportunities because I was insecure. And so it are not always the smart people who make the breakthroughs. That’s a very parochial way of thinking about it. Imagination can trump intelligence. There are many angles to doing research. And so just go for it.

**Brian Keating:** Wow. 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. I said, 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 Gell-Mann at his home institution of Caltech. And he goes farther back, he sees Maria Goeppert Mayer, maybe here at San Diego, and then he goes back to 1922 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? Einstein had the imposter syndrome. 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, you gotta 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 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 foreword to my second book, which has his interviews in it. And I hope someday that…

**Francis Halzen:** I actually read it, either on your web page or somewhere, yeah. I don’t think it’s unusual. I don’t think it’s a problem. It’s only a problem when it prevents you from doing things.

**Brian Keating:** That’s right. That’s exactly right. And I think you’re right to have the courage and, 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.

**Francis Halzen:** It was a pleasure.