Nobel Prize in Physics 2026 Explained: IceCube & Neutrinos (EP 62)

Why build a telescope inside a billion tons of Antarctic ice? The 2026 Nobel Prize in Physics recognizes Francis Halzen's work on IceCube and the discovery of high-energy neutrinos from the cosmos. In Episode 62 of From First Principles, Lester Nare and Krishna Choudhary explain neutrinos from the ground up: why these elusive particles make powerful cosmic messengers, how faint flashes of Cherenkov light reveal their interactions, and why detecting them requires an observatory buried deep beneath the South Pole. We follow the path from beta decay and the first neutrino experiments to AMANDA, IceCube's construction, the 2013 astrophysical breakthrough, a distant blazar, and a neutrino map of the Milky Way. Along the way: cosmic rays, the Oh-My-God particle, tracks versus cascades, and the international collaboration behind the discovery. CHAPTERS 00:00 Hunting ghost particles beneath Antarctica 01:16 Hello Internet and Nobel Prize 05:30 What are neutrinos? 10:00 Neutrinos as cosmic messengers 15:02 The Oh-My-God particle 16:21 Cosmic-ray energies 19:07 Cosmic particle accelerators 22:28 Why look for neutrinos? 25:52 How to detect a neutrino 29:23 Cherenkov light 33:13 Building a neutrino observatory 37:17 From Antarctic ice to AMANDA 42:21 Building IceCube 44:59 Reading tracks and cascades 49:30 Backgrounds and the 2013 discovery 52:46 Tracing cosmic neutrino sources 54:16 Mapping the Milky Way 58:23 IceCube collaboration and Gen2 1:01:05 Closing and Nobel week RESEARCH & FURTHER READING AMANDA in Antarctic ice (2001): https://doi.org/10.1038/35068509 IceCube detector and instrumentation (2017): https://doi.org/10.1088/1748-0221/12/03/P03012 First PeV neutrinos (2013): https://doi.org/10.1103/PhysRevLett.111.021103 Astrophysical neutrino evidence (2013): https://doi.org/10.1126/science.1242856 Blazar TXS 0506+056 (2018): https://doi.org/10.1126/science.aat1378 Archival blazar neutrino emission (2018): https://doi.org/10.1126/science.aat2890 Milky Way neutrino map (2023): https://doi.org/10.1126/science.adc9818 Gamma-ray burst constraints (2012): https://doi.org/10.1038/nature11068 IceCube overview: https://icecube.wisc.edu/science/icecube/ EDITORIAL NOTES Intro: the 2013 breakthrough was high-energy astrophysical neutrinos. Lower-energy supernova neutrinos were detected in 1987. On-screen clarifications: 06:45 Beta-minus decay produces a proton, electron and electron antineutrino. 11:10 Davis studied solar neutrinos; Koshiba's team detected SN 1987A neutrinos. 17:50 The cosmic-ray knee and ankle are not fixed distance boundaries. 19:38 Required accelerator size depends on magnetic-field strength. 24:18 Ground-based telescopes also detect gamma rays through air showers. 27:48 W interactions produce charged leptons; Z scattering preserves neutrino flavor. 34:06 The underwater concept dates to 1960; DUMAND developed in the 1970s. 36:09 Baikal holds about one-fifth of unfrozen surface freshwater. 38:53 Earth filters muons but also absorbs many very-high-energy neutrinos. 41:26 Pressure converts air bubbles into clathrates, reducing light scattering. 42:28 Construction finished in December 2010; full operations began in May 2011. 44:27 Sensors are DOMs; DeepCore is a densely instrumented detector region. 45:47 Timing gives direction; light yield and pattern help estimate energy. 49:44 Upgoing events can still be atmospheric neutrinos. 53:12 TXS 0506+056 is about 3.7 billion light-years away. 56:38 Long GRBs often involve collapsing stars; short GRBs often involve mergers. WATCH & FOLLOW Full video: https://youtu.be/eMahxeBj5k0 Episode notes: https://ffppod.com/episodes/ep62 Medicine Nobel explained: https://youtu.be/PKAYqhy8xf8 Follow @FFPPod on Instagram, TikTok, X and Facebook. From First Principles: Breaking down science news so it makes sense to curious people everywhere.

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:02Lester Nare?

  A particle physicist from Belgium who spent decades turning the South Pole into a telescope. The target? The neutrino, the shyest particle in the universe. 65 billion from the sun pass through your fingernail every second. Almost none ever stop. In 1988, Francis Halzen proposed a wild idea. When a neutrino does hit an atom in ice, it leaves a faint flash of blue light. So, fill the Antarctic ice with sensors and watch. By 2011, Ice Cube was complete.

• 0:44Lester Nare?

  A cubic kilometer of ice and more than 5,000 light sensors frozen up to 2.5 kilometers deep. In 2013, it found the first evidence of neutrinos from beyond our solar system. Messengers that fly straight from the most violent places in the cosmos. A new kind of astronomy. And this is the 2026 Nobel Prize in Physics. Hello, Internet. This is your captain speaking, Lester Nare, joined as always by my co-host and our resident PhD, Krishna Choudhary.

• 1:25Lester Nare?

  We are back for day two of the Nobel Prizes, which is the first solo winner, which also happens to be the first solo Physics Nobel Prize winner since 1992, when Georges Charpak won for the invention of a major class of electronic particle detectors. Today, we are covering a unique telescope called the Ice Cube Neutrino Observatory in Antarctica in the quest to find ghostly messengers from space. As always, we are going to talk about the science from the ground up today, starting with who won?

• 2:02Krishna Choudhary?

  That's right. 2026 Nobel Prize in Physics has been awarded to Francis Halzen for laying the foundations for an entirely new type of astronomy. The official tagline says, "For decisive contributions to the Ice Cube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin." Now, we've covered Ice Cube before on this podcast. It is simply said, "A triumph of human ingenuity and sheer will." It's located at the South Pole. This is a telescope that is buried in a kilometer of ice on the South Pole. It has no mirrors, it has no lenses, and it doesn't collect the light from distant stars.

• 2:47Krishna Choudhary?

  Instead, it collects light coming out of the ice itself. It's like a telescope that's pointed downward in some sense. And what it's looking for is the light from neutrinos that interact with the ice. There's interesting interactions that happen, and the light that comes out of that is the data that this thing is collecting. Those flashes will then give us some idea about the origin of those neutrinos, and that's the central question. This observatory is Francis Halzen's baby. It's recent success at finding high-energy neutrinos of astrophysical origin, and I'm talking about outside the galaxy. That's what he's winning this award for.

• 3:30Krishna Choudhary?

  And, you know, it came kind of out of left field for us. It was not on my radar at all. But, you know, in 2014, the Ingenuity Awards that are awarded by the Smithsonian, one was given to Francis Halzen, and they had an accompanying comic book.

• 3:47Lester Nare?

  This is great.

• 3:48Krishna Choudhary?

  Thing.

• 3:49Lester Nare?

  Neutrino Man.

• 3:50Krishna Choudhary?

  And he was called Neutrino Man. And on the right-hand side over there, at the very last sort of comic strip, it says, you know, it's on the cover of Science, The Times, Nobel Prize? Question mark. So, they were already, you know, flirting with that idea that perhaps Francis Halzen would get the Nobel Prize.

• 4:07Lester Nare?

  It's like the Simpsons meme where they predict everything in the future in The Simpsons.

• 4:10Krishna Choudhary?

  Yeah, exactly. This time it was the Smithsonian. So, to understand this discovery, we need to understand neutrinos, and we need to understand why they are uniquely suited to tell us more about how the largest and most powerful particle accelerators in our universe work. These are accelerators that are out in deep space, and they make CERN look like a hula hoop for ants. Honestly, they're insane. And it's not an easy question to answer. This is the fundamental question. How do these particle accelerators work of extragalactic origin? It's not easy, because you need to distinguish between the cosmic neutrinos that come from these particle accelerators out in deep space and all of the neutrinos that we get bathed

• 4:55Krishna Choudhary?

  by on a daily basis that are coming from the Earth's atmosphere, that are coming from the sun. There's something like 60 billion neutrinos that are passing through my fingernail right now as we tape this episode. You know, you've got to be able to sift through all of that and find the ones that are coming from way out there, right? That's what this is about.

• 5:16Lester Nare?

  And so, the first place that we need to start to understand the Ice Cube Neutrino Observatory is probably the middle word in the name. People might know Ice Cube, not the rapper from California or the actor, if you might know. People kind of know what an observatory is.

• 5:37Krishna Choudhary?

  Yeah.

• 5:38Lester Nare?

  But what are neutrinos?

• 5:41Krishna Choudhary?

  So, neutrinos are an enigmatic particle. They entered physics in 1930 because people were observing radioactive beta decay. You need to conserve charge. So, a proton comes out, that's the positive charge, but we started with a neutron, so an electron has to come out so that it balances out. And people thought, okay, that's enough, right? But, if you look at the data, energy was being lost. Okay? This was a huge conundrum. People were like, where is the energy going? Niels Bohr very famously said, "Maybe energy is not conserved." Everyone else was like, what are you talking about?

• 6:14Lester Nare?

  Oh, because we have to have conservation of energy.

• 6:16Krishna Choudhary?

  Yeah, right. And because of conservation of energy, I mean, that's like the central tenet, right? And Niels Bohr said, "Well, this experiment shows maybe it's only averaged." Wolfgang Pauli, on the other hand, was like, "No." Okay? The conservation of energy is up there on the pedestal. The universe will have created an accounting trick in order to make it work. And he wrote this very famous letter to a conference in Tübingen, Germany. He starts out with, "Dear radioactive ladies and gentlemen." And he postulates that there is a particle that is neutral, that has the mass of probably zero, and that has a spin of one-half to conserve angular momentum.

• 6:58Krishna Choudhary?

  And this thing he called the neutrino. Actually, Enrico Fermi coined that term because it's neutral, but it's a small one, so it's the little one. So, hence the neutrino. Enrico Fermi comes up with this name. It's postulated. Finally, they are detected experimentally in 1956 using anti-neutrinos from a nuclear reactor. This was by C. Cohen and F. Rines. They both won. Rines actually won the Nobel Prize in Physics in 1995. He was at the University of California in Irvine. That was UCI's first Nobel Prize in Physics. Cohen had passed away in 1974, so he never got to share that Nobel Prize.

• 7:40Krishna Choudhary?

  And so now, neutrinos have become part of a much larger particle zoo that we call the standard model. All the way from the Higgs, the God particle at the very center, you've got the bosons, which are the force carriers on the outside. That's the photon, the gluon, and the W and Z bosons. And then you've got the particles on the outside. On the top are the quarks. Those are the things that make up protons and neutrons, the nucleus of the atom. And on the bottom are leptons. Leptons are the electron, the muon, and the tau. Those are the negatively charged, smallish particles that don't interact with the strong nuclear force. And their partners are the neutrinos. For the electron, we've got an electron neutrino.

• 8:23Krishna Choudhary?

  For the tau, we've got a tau neutrino, so on and so forth.

• 8:26Lester Nare?

  So, these are sort of the fundamental building blocks for everything we see in the world around us.

• 8:33Krishna Choudhary?

  Yes. Everything except for gravity is contained in that.

• 8:36Lester Nare?

  Represented here.

• 8:37Krishna Choudhary?

  Representation right there. This is like the periodic table distilled down to its absolute essentials.

• 8:44Lester Nare?

  And just to clarify, these absolute essentials all have a dance partner, which is a neutrino version of themselves.

• 8:50Krishna Choudhary?

  Yeah, no, the, well, on the bottom, the three are the tau, muon, and electron. Those guys have the dance partner on the right-hand side. That's the V-looking thing. So, the electron neutrino, the tau neutrino, and the muon neutrino.

• 9:03Lester Nare?

  So, leptons, neutrinos are only in this lepton category?

• 9:06Krishna Choudhary?

  Yes. And neutrinos are leptons, just like the quarks have their own dance partner, like the up has a down, the strange has a charm. Similarly, the electrons have their own dance partners.

• 9:16Lester Nare?

  We just happened to discover neutrinos as the dance partners for the other leptons by way of this kind of entry point with Pauli and Enrico Fermi.

• 9:28Krishna Choudhary?

  Yeah.

• 9:29Lester Nare?

  Initial questions because we were seeing the loss of.

• 9:31Krishna Choudhary?

  Energy.

• 9:32Lester Nare?

  And we're like, that can't happen.

• 9:34Krishna Choudhary?

  And beta decay, right?

• 9:35Lester Nare?

  That rule can't be.

• 9:36Krishna Choudhary?

  That rule, that rule cannot be broken. Okay? So, that's the neutrino.

• 9:41Lester Nare?

  Okay. So, this makes sense. So, now we kind of understand, you know, what is the thing that this Nobel Prize is sort of looking to discover or detect. But I think, as you mentioned earlier, they're already here on Earth.

• 9:55Krishna Choudhary?

  Yeah, we've detected them.

• 9:57Lester Nare?

  Because they make up stuff.

• 9:58Krishna Choudhary?

  Yeah.

• 9:59Lester Nare?

  Around us.

• 10:00Krishna Choudhary?

  And they were originally detected using decay of radioactive material here on Earth.

• 10:05Lester Nare?

  Right. So, who cares? Or why does it matter that we detect neutrinos that are coming from space?

• 10:11Krishna Choudhary?

  Yeah. This is the big leap that is central to this particular Nobel Prize. So, right after Cohen and Rines detected the neutrino, it was immediately understood that it would be an ideal astronomical messenger because it's electrically neutral, so it doesn't get messed around by magnetic fields, and it's only subject to weak interactions or gravitational interactions. Gravity doesn't really move stuff around all that much.

• 10:39Lester Nare?

  It's weak.

• 10:40Krishna Choudhary?

  To be perfectly honest, because the neutrino especially, it's weak, and the neutrino is like near-massless. So, it's just going to like go through. I mean, it can still move around because of the curvature of spacetime, but, you know, not by a lot. And it's not really interacting with other types of matter. And so, it could give us a sense of what's happening in astronomical sources because it's a direct line of sight.

• 11:01Lester Nare?

  Oh, okay. That's interesting.

• 11:04Krishna Choudhary?

  Yeah. And the first time that this was actually discovered was in 1987 by two Nobel Prize winners, Davis and Koshiba. They were awarded the Nobel Prize in 2002 for pioneering contributions to astrophysics, in particular, the detection of cosmic neutrinos. These guys are the first who detected cosmic neutrinos. What they did was. Detect neutrinos from a very significant event supernova 1987 A. On the right hand side, that's a picture of the Magellanic Cloud, which is a kind of dwarf galaxy that's orbiting the Milky Way. In 1987, a star went supernova in that cloud. That's really nice because it's really close to us. It's not in the Milky Way. If it was too close, we'd be freaked out.

• 11:48Krishna Choudhary?

  But this is just far enough that you see that central really bright star. That's like outshining almost the entire galactic core over there.

• 11:57Lester Nare?

  Yes.

• 11:57Krishna Choudhary?

  That is a new star that wasn't there like a week before.

• 12:00Lester Nare?

  That's so unbelievable.

• 12:01Krishna Choudhary?

  It was, you could see it with your naked eye. It was a new star that had formed. People were traveling to the southern hemisphere to see this thing. And believe me, had I been alive and had the ability, I would have gotten on a flight to see this thing with my naked eye. I mean, this is incredible, right? That's a supernova. That's the light coming from it. These two detected the neutrinos that came from that explosion. And that was the first time that we were like, oh, so we can actually do this. We can do neutrino astronomy, right?

• 12:30Lester Nare?

  It's a pathway by which to observe. Usually we use photons to observe or other aspects of the electromagnetic spectrum to observe what's happening in space, to create a map. What we're saying is there's another way to do so. And it happens to be that neutrinos are this other vector for mapping the world, the universe around us.

• 12:50Krishna Choudhary?

  Exactly.

• 12:50Lester Nare?

  Is that kind of the right way to think about it?

• 12:52Krishna Choudhary?

  That's exactly right. And those particular neutrinos from the supernova 1987 A, those were on the order of mega electron volts. So that's 10 to the six, like a billion, or no, a million electron volts. Mega is a million, a million electron volts.

• 13:07Lester Nare?

  Okay.

• 13:07Krishna Choudhary?

  Pretty, pretty big.

• 13:08Lester Nare?

  Yeah.

• 13:09Krishna Choudhary?

  This year, the Nobel Prize recognizes the discovery of peta electron volt energies, 10 to the 15. Okay, 10 to the 15 electron volts. A terra electron volt is a trillion electron volts. A peta electron volt is a thousand trillion electron volts. Okay, this, this thing is insane. And it's only possible because of an observatory like Ice Cube. Okay? The reason to pursue these particles comes from a century old mystery because we know they exist. These high energy events exist. It's about cosmic rays. Cosmic rays were first discovered in 1912 by Hess. He was the Nobel Prize winner in 1936. He, he got on a balloon, literally,

• 13:53Krishna Choudhary?

  and like detected an excess of charged particles in the upper atmosphere coming from like extraterrestrial origin. Dude, physicists back in the day.

• 14:05Lester Nare?

  Right, we're really putting there, it's a gumshoe work out in the field. That's, that's fun.

• 14:12Krishna Choudhary?

  Isn't that, isn't that hilarious?

• 14:13Lester Nare?

  That's quite fun.

• 14:13Krishna Choudhary?

  Right, and so these cosmic rays, we'd figured out they're mostly protons and charged atomic nuclei that are arriving from space. And some of these cosmic rays reach energies of 10 to the 20 electron volts. Okay, that's far beyond what anything on Earth that we've built can do. Okay, CERN is at about 10 to the 12. This is 10 to the 20. That's eight orders of magnitude above. So that's like 10, no, 100 million times more energy per particle than CERN. Okay, that's not a joke because CERN is like really kind of the limit of what we can do right now. Okay, there are proposals for larger ones, but even then you're adding like maybe an order of magnitude.

• 14:58Lester Nare?

  Right, not eight.

• 14:59Krishna Choudhary?

  And it's like hard to do that, okay? 10 to the 20 electron volts is an insane amount. The very famous, oh my God, particle, which we have covered on this podcast before. This was named because researchers shouted, oh my God, when they saw this particle. It was in 1991, it was discovered in Utah. There was an array in one of the salt flats in Utah. And what they figured out was that this thing must have been traveling at 99.99999 with like 15 nines, the speed of light. This was the fastest thing, this is currently still. This is the fastest thing that we have ever observed in our universe. And it begs the question, like, what is actually making it that fast?

• 15:43Lester Nare?

  Right, because the point is the source of where it came from is an interesting thing to understand because it is basically able to get the closest to the speed of light that we've seen of any that.

• 15:55Krishna Choudhary?

  Of any massive particle, right? Like this thing had enough energy, a single particle, like could be a proton, could be maybe a atomic nucleus. This thing had the same amount of energy as like Yamamoto from the Dodgers throwing a baseball, a single, a single particle, right? It's insane, the amount of energy that was packed into this thing. And it's very non-trivial how you even do that, right? Let's first talk about like the energy spectrum of cosmic rays, right? Because you could be like, oh, that's a fluke, right? It's like, I don't know. Well, the more and more that we detect, we can actually now trace what's the probability that we're going to get a certain cosmic ray of a certain energy. And that's called the energy spectrum. On the left hand side, that's the plot.

• 16:41Krishna Choudhary?

  On, on the Y axis is the flux of like how much we're getting. And on the X axis is in the amount of energy. Both are in log scale, crucially. So you're getting a power law.

• 16:52Lester Nare?

  Yep.

• 16:52Krishna Choudhary?

  Now, the, the ones that are low energy on the left hand side, that, those things are pretty common. You'll get like one particle per square meter per second. That's the flux. So if you had like a detector, there was one square meter. Every second you'd get a single particle that's like, you know, I don't know, 10 to the, 10 to the 11, 10 to the 12, like the stuff at CERN. The universe is giving us the stuff at CERN at about one per second per square meter. Okay, then there is a knee in this, in, in this plot. Okay, the, the feature there is called a knee because up to this energy, which is about like 10 to the, 10 to the 15 electron volts,

• 17:36Krishna Choudhary?

  all, all of the stuff before then is stuff that probably originates within the Milky Way.

• 17:42Lester Nare?

  Hmm, there's almost a ceiling.

• 17:44Krishna Choudhary?

  Yeah.

• 17:45Lester Nare?

  To how far it could have been from.

• 17:46Krishna Choudhary?

  Yeah, but then there's a little kink and that's telling us that there is another source, okay? And that other source is probably extragalactic. This is where we're getting to like one particle per square meter per year.

• 18:00Lester Nare?

  Okay.

• 18:00Krishna Choudhary?

  Okay, the, the, the probability of it coming straight at Earth, right, in, in our little square meter detector is getting lower and lower.

• 18:09Lester Nare?

  Partly because in every direction, the greater the distance, the easier it is as it travels over that long distance for it to miss Earth because we come an increasingly smaller target.

• 18:19Krishna Choudhary?

  The, the, the source itself is not, I mean, there's not that many sources that are doing this. And so as it spreads out, like, you know, the probability is very, very low. And then finally, there's an ankle and that's one particle per square kilometer per year. That's at around 10 to the 18, 10 to the 19. Those are the ones that we want, okay? Those are extra, extragalactic, right? There's the, there's the ones that happen within our solar system. Then there's the one that happens within the galaxy. And then there's the extragalactic, the ones that we don't like what? That's the oh my God particle, right?

• 18:54Lester Nare?

  Yep.

• 18:54Krishna Choudhary?

  So there's these three sources that we're trying to make sense of, right? In order to find the ones that are really, really energetic, we need to do a lot of work, right? Now, before we get into that, let's just try to hypothesize what could even do this. Right, it's of extragalactic origin that we're talking about. What, what kind of particle accelerator would actually do this, right? If you do the calculation, you actually figure out that where that ankle was, that marks where the gyro radius of a proton, meaning like how big of a circle would be needed for a magnetic field to accelerate that thing, that exceeds the galaxy.

• 19:39Krishna Choudhary?

  The gyro radius of a proton that is traveling that fast exceeds galactic scales. You need, you need a particle accelerator larger than a galaxy. If you just do the, the, the fundamental physics.

• 19:51Lester Nare?

  I don't like it.

• 19:52Krishna Choudhary?

  It's weird.

• 19:53Lester Nare?

  That's, that's quite, and there, I'm thinking in my head one thing, but let's see if it's going to go in that direction.

• 19:59Krishna Choudhary?

  It's ridiculous, right? So one proposed accelerator is the active galactic nucleus. I mean, this is where you've got a galaxy that's super energetic in the central region, probably a supermassive black hole that is accreting matter. And it launches these powerful jets in both directions. Those jets, when they encounter their surroundings, they create shock waves and those shock waves can start accelerating particles. Because the magnetic fields there are not homogeneous. They're not like constant everywhere. And so you get like these.

• 20:31Lester Nare?

  Which we've talked about.

• 20:31Krishna Choudhary?

  Yeah, and so you get these massive, massive particle jets, right? So in a simplified picture, the particle would repeatedly create a shock and it would scatter off these magnetic irregularities. And the particle has to be held in this magnetic field for long enough to gain that kind of energy. Again, it's not really trivial how this would work, right? Another source could be clusters of galaxies. As cold gas accretes and falls into the hot medium on the inside, that's going to give rise to shock waves. And those shock waves at the outskirts of this cluster could be powerful enough to give off this kind of acceleration. But the point is, we don't know and we'd like to know.

• 21:11Lester Nare?

  We'd like to know. Where are these ghostly messengers coming from?

• 21:14Krishna Choudhary?

  Yeah, it's, it's pretty insane that like you can create a particle that is that. Energetic, it just doesn't make sense how you pack so much energy into such a tiny amount of matter, right? Um, if you want to do that, you want to figure out where these particles came from, right? Now, for the oh my god particle, right? You could, you could figure out, oh, it came from that direction, but that's not really a good indication of where it came from because charged particles can interact with the intergalactic medium and they can interact with the magnetic fields there to start moving around. For example, in this particular example, you've got a supernova that's at the edge of our galaxy. There's a faint blue trace that you're seeing.

• 21:58Krishna Choudhary?

  That's what a charged particle would do because there's so much magnetic field anomalies in the galaxy that as it's traveling towards us, it's gonna start moving around and bending. So even though the particle came from this direction, the actual source could be somewhere else.

• 22:13Lester Nare?

  It's like a lazy river. It's like you can point and say the top of the lazy river is there, but when you're coming down it, it's not just like the line of sight.

• 22:19Krishna Choudhary?

  Yeah, yeah, the line of sight could be this way, right? Even though the mountain is to the, to the, to the right. So instead, we wouldn't like something that has a clear line of sight. And that is where we start thinking about what else could we see, right? There's a particle accelerator that is like flinging these protons and these charged particles at extremely high velocities. What else could we see in that region that could come at us? There's two possibilities. This is a diagram showing some of the interactions that could be happening where the particle is accelerating, like at that galactic core.

• 22:54Lester Nare?

  At the origin.

• 22:55Krishna Choudhary?

  Yeah, at the origin point, right? This is a proton that's of cosmic ray origin. It interacts with, let's say, a gamma ray that's nearby, like all of the light and all of the random stuff that's going on. And it creates a particle of showers, pions, those pions then decay into electrons, gamma rays, and neutrinos. Those are the three that we could kind of look for.

• 23:16Lester Nare?

  Okay.

• 23:17Krishna Choudhary?

  Okay? Electrons have the same problem. They're charged, so they're gonna be moving around as well. Gamma rays are interesting, right? Because gamma rays, they are uncharged, so they could be coming straight. And neutrinos are interesting because they're uncharged. The problem with gamma rays, though, is they're light.

• 23:35Lester Nare?

  So, and let me just take a quick step. So the idea is we're, we're trying to figure out what these giant particle accelerators, like, what are they? We're trying to understand what they are. And so we can detect them through the things they shoot across the universe.

• 23:49Krishna Choudhary?

  Yeah.

• 23:50Lester Nare?

  And we've just gone through a list of the things we could detect.

• 23:52Krishna Choudhary?

  Yeah.

• 23:53Lester Nare?

  That are not neutrinos, that they shoot across the universe.

• 23:54Krishna Choudhary?

  Yeah.

• 23:55Lester Nare?

  And why it doesn't really work.

• 23:56Krishna Choudhary?

  Yeah, it doesn't really work. Protons, which is the actual thing, if, if they came without interacting with anything, that doesn't really work because they're moving around through the galaxy. Gamma rays could work because at least they're coming straight at us, but again, they kind of get slowed down sometimes by the intergalactic medium. They're light, so they interact with the intergalactic medium. And you, you gotta put a telescope in space in order to actually see this thing. How big of a detector are you gonna make in space?

• 24:24Lester Nare?

  Space.

• 24:25Krishna Choudhary?

  Right?

• 24:26Lester Nare?

  So there's an engineering problem.

• 24:27Krishna Choudhary?

  There's an engineering problem there as well because the Earth's atmosphere doesn't like gamma rays. Very good for us here on Earth because we don't want gamma rays.

• 24:32Lester Nare?

  We don't want gamma rays.

• 24:33Krishna Choudhary?

  But like, detecting them is kind of a hassle. Neutrinos, on the other hand, they're gonna shoot right through, not just the Earth's atmosphere, not just all of intergalactic medium, but the Earth itself, okay? This makes them kind of a nice target to look for, right? There's also an approximate energy connection because if the proton itself is at 10 to the 20 electron volts, in that interaction that I showed you with those particles, about 5% of the energy gets deposited into, into the neutrino.

• 25:04Lester Nare?

  So we can reverse engineer back to the total initial energy starting point?

• 25:08Krishna Choudhary?

  Yeah, we can like probabilistically say that the original event had some amount of energy. And so this is where we get into the kinds of neutrinos that we're trying to look at. If we want something that's 10 to the 20, then maybe we'll look at something that's around 10 to the 18, 10 to the 19 electron volts for the neutrinos. And that's kind of our target.

• 25:29Lester Nare?

  Okay.

• 25:30Krishna Choudhary?

  So we know, we now have a flux, which is one per square kilometer per year, and we have a kind of target energy that we wanna take a look at.

• 25:41Lester Nare?

  In our quest to, again, now, uh, find a way to figure out these highly energetic objects that are extra, extra galactic, um, we've figured out the signature that has the least, that's least impacted by the sort of intergalactic medium on its journey, which allows us then to make the most direct correlation back to that initial energy starting point. So like, we know what we're looking for.

• 26:10Krishna Choudhary?

  Yeah.

• 26:11Lester Nare?

  And why we are looking for it. Uh, and so then the next obvious question is, okay, how do we, how do we detect a neutrino?

• 26:19Krishna Choudhary?

  That's right.

• 26:20Lester Nare?

  This sounds very promising.

• 26:21Krishna Choudhary?

  It sounds very promising, but the same reason why the neutrino is a great candidate for extragalactic astronomy is also a reason why they're notoriously hard to detect. Namely, they don't interact with a lot of things. That's the whole point of trying to detect them.

• 26:37Lester Nare?

  Right.

• 26:38Krishna Choudhary?

  But at the end of the day, if they don't interact with stuff, how are you going to, like, see them, right? The reason why I can see light is because the light is readily interacting with the rods and cones in my eye, which we talked about yesterday in yesterday's Nobel Prize. So neutrinos don't do this. I mean, as I said, right? Every second, 65 billion neutrinos from the sun blow through my fingernail. I'm not feeling anything.

• 27:01Lester Nare?

  Unbelievable.

• 27:02Krishna Choudhary?

  Right? There are a few ways, there are a few ways that the neutrinos do interact, and here's how it would happen. On the left, you've got sort of a particle picture of how a neutrino would do something. The neutrino is coming from the upper left, and it is interacting with a nucleon. Nucleons are full of quarks, right? The up and down quarks, there's three of them, that's why there's two that are just sort of skating by. And one of the quarks is going to interact with this neutrino using the W or Z bosons, which is the weak nuclear force. When that interaction happens, we have either one of two choices, okay?

• 27:43Krishna Choudhary?

  Either an electron is going to be born that sort of moves off and carries some of the energy from that initial neutrino, or another type of neutrino is going to be born. Depending on what the interaction was, you can have either an electron coming out or a different type of neutrino coming out, okay? On the right-hand side, this is the first ever neutrino that was detected, um, as a, as a photograph.

• 28:11Lester Nare?

  Okay.

• 28:12Krishna Choudhary?

  Okay? This is in a hydrogen bubble chamber. You see on the left there, there's just like kind of a source where three things are coming out. That's where the neutrino interaction happened, okay? We're not seeing the neutrino come from the right-hand side of the page because not, because the neutrino's just coming and then boom, it hits something over there, which caused a spray of particles. That's how we see if a neutrino detection happens. If randomly a spray of particles comes out and it's the exact type of particle that we expect.

• 28:42Lester Nare?

  It has a fingerprint.

• 28:43Krishna Choudhary?

  Mm-hmm, it has a fingerprint. In this case, there's a proton coming out, there's a collision with a meson, and then there's also a mu meson that's coming out. And those things are exactly what we would expect if the neutrino interacted with the hydrogen that's in this bubble chamber.

• 28:58Lester Nare?

  So would it be the case that because they're moving so quickly, it just like, we just would see this interaction kind of pop out of nowhere?

• 29:04Krishna Choudhary?

  Mm-hmm. Yeah, we'd, I mean, because the neutrino, we can't detect.

• 29:08Lester Nare?

  We can't detect it. We can only see this, like, yeah, okay.

• 29:11Krishna Choudhary?

  Once, once the neutrino interacts with something, we can detect the byproducts because the byproducts are charged.

• 29:17Lester Nare?

  Yep.

• 29:18Krishna Choudhary?

  So they're gonna be interacting with the electromagnetic field, which then creates light. Okay? What type of light? This is Cherenkov radiation. This is the very famous blue light that is emitted in particle, um, in, actually in nuclear reactors. When, when you have like nuclear reactor rods that are covered with water, those nuclear rods are emitting a bunch of particles that are moving at extremely high energies, right? Some of those energies lead to speeds that are faster than the speed of light in water. This is one of the, way, way back, we did a, when we, when we did our.

• 29:55Lester Nare?

  Speed of light in ice.

• 29:56Krishna Choudhary?

  Yeah, exactly.

• 29:57Lester Nare?

  Oh geez, we'll remember that meme.

• 29:59Krishna Choudhary?

  Yeah, it's, it was one of our first viral videos, actually. Um, the idea is the following, so the speed of light in a vacuum is the absolute speed limit of the universe. But light slows down in stuff because there's stuff that interacts with light. So as light bounces from one particle to the other, the phase velocity of light is actually slower than the actual velocity of light because of all of these interactions, right? In ice and in water, it can be as much as 20 to 30% slower. That gap is now accessible to particles because the, the speed of light in a vacuum is the absolute limit, but this is what the speed of light is doing in all of this media.

• 30:42Krishna Choudhary?

  So if, if a charged particle moves faster than the speed of light in the medium, but obviously slower than the absolute limit imposed by Einstein's relativity, you get something called Cherenkov radiation. In the same way that a fighter jet produces a sonic boom when it travels faster than the speed of sound, in this case, you're getting a light boom.

• 31:04Lester Nare?

  The light boom!

• 31:06Krishna Choudhary?

  There you go, yeah. You get a light boom, it's, it's a, it's a shock wave in the electromagnetic field.

• 31:13Lester Nare?

  A photonic boom.

• 31:14Krishna Choudhary?

  A photonic boom, and that's, that's the radiation that we are trying to catch. If we can catch Cherenkov light, then we can reconstruct what the particle trajectories were, and then from that, then reconstruct what the neutrino.

• 31:30Krishna Choudhary?

  Was like how how big the neutrino was in terms of energy and where it came from. Now, the probability is extremely tiny. Okay, at these pet electron volt energies, the neutrino nucleon cross section, meaning like, how often does the neutrino interact with quirks? That cross section is on the order of 10 to the minus 33 square centimeters. That's like, you can imagine this is like the, the, the, the target that the neutrino has to hit on the quark to, to like, get an interaction. So extremely small. On top of that, the, the flux itself is very small.

• 32:09Krishna Choudhary?

  And so you need, you need a giant, giant detector to be able to achieve any hope of detecting these things.

• 32:18Lester Nare?

  Even though there's many of them coming through at the level that we want to see, like the

• 32:24Krishna Choudhary?

  Yeah, the energies that we want to see, there's not that many.

• 32:26Lester Nare?

  Not that many.

• 32:27Krishna Choudhary?

  When I, when I say 60 billion coming through my fingernail, no, that's like 10 to the five, 10 to the six electron volts.

• 32:32Lester Nare?

  Those are the earthly ones.

• 32:33Krishna Choudhary?

  Those are the earthly ones, the solar systemy ones.

• 32:35Lester Nare?

  We want the extragalactic.

• 32:37Krishna Choudhary?

  We want the extragalactic ones that are like 10 orders of magnitude above, above the stuff that's going through my fingernail.

• 32:44Lester Nare?

  And so, and this, so again, so now we know what a neutrino is, um, like how you would detect it because of this Cherenkov radiation. It kind of accesses this gap when the speed of light through a medium like water or ice is lower than the absolute limit. And so there's sort of a signal that goes off. There's a little blue, oh no, don't worry, we're not violating the laws of physics.

• 33:07Krishna Choudhary?

  Yeah, yeah, yeah, no, Einstein is still happy.

• 33:09Lester Nare?

  We just want you to know we're here.

• 33:12Lester Nare?

  So, so now, because the, the problem set is clear. This is like a highly improbable event to capture.

• 33:19Krishna Choudhary?

  Yeah.

• 33:20Lester Nare?

  So how would we make a neutrino observatory? Because this is very different than Hubble and JWST and the other types of observatories that many folks would think about when they hear that word.

• 33:34Krishna Choudhary?

  That's right, because in, in Hubble and all of these traditional observatories, you just look for the light and light is, and then you capture the light and you're good to go. Here, we're looking for like a second messenger, right? We, we're not detecting the neutrino, we're detecting how the neutrino interacts with the stuff that we then look at. Okay. And we need a lot of it. We need a lot of stuff because the flux of these events is so small. The first sort of attempt at doing this was Dumand, Dumand, maybe, Deep Underwater Muon and Neutrino Detector Project. This was way back in 1960. They were trying to create a deep ocean detection off the coast of Hawaii, off the main island. This was spearheaded by the University of Hawaii.

• 34:18Krishna Choudhary?

  What they wanted to do was basically on the ocean floor, rest a bunch of these detectors, like in this case, there are six that are coming out of the ocean floor. Each of these detectors is going to detect this light. It's a bunch of photomultiplier tubes in some sense, which just, once a photon comes in, that's going to be converted into an electrical signal, very much like a CCD that's in our camera, but like on steroids, like a single photon will, will set this thing off, right? Um, and it was right off the coast of Hawaii. There were a lot of problems, though. The first time you try something as crazy as this, there's going to be problems. A, a big thing was measurement backgrounds from ocean bioluminescence.

• 35:00Lester Nare?

  Yeah.

• 35:01Krishna Choudhary?

  Because like the living things in the ocean themselves produce light. And if these things are that sensitive, they're just going to go off on the ocean bioluminescence. Now, all of that stuff is in the visible range, but that means that we have to tune our detectors for higher frequency light. And that, you know, removes a whole chunk of stuff that we would be otherwise seeing. The other thing was, the ocean is full of potassium 40, which is a radioactive isotope of potassium. So there's radiation in the ocean itself.

• 35:37Lester Nare?

  Right.

• 35:37Krishna Choudhary?

  Like the water itself has dissolved radioactive potassium. And so you get this massive background, which is just super annoying.

• 35:45Lester Nare?

  And you're just going to spend all your time filtering out.

• 35:47Krishna Choudhary?

  Right, yeah. So the project was canceled in 1995. It was, it had serious engineering problems, but we learned a lot because that's, we learn a lot whenever we try new things. And now there are new detectors in bodies of water that are actually trying to do the same thing. But in a, in a much bigger sense, the, my favorite one is actually the Baikal Neutrino Telescope. This is in Lake Baikal, which is the, um, largest body of fresh water on the planet. It has something like 25% of the world's fresh water in a single lake in Russia. And they've got a very similar detector from Dumand, where in the bottom, they've got these photomultiplier tubes that are in, arranged in a grid, and they're trying to find neutrinos.

• 36:33Krishna Choudhary?

  So water-based detection is working, right? But there are problems with trying to get it to detect the really, really high, high energy stuff. Okay. Again, because of some of the background, some of that radiation stuff, so on and so forth. So this is where our Nobel Laureate comes in.

• 36:54Lester Nare?

  This is, this is, it's, I, I, I always love talking about this story because it's one of those, um, there's so much deeper than what you see on the headline.

• 37:06Lester Nare?

  You know, people take away Antarctica and neutrino, not knowing and understanding the importance of neutrino in terms of this mapping.

• 37:13Krishna Choudhary?

  Yeah.

• 37:13Lester Nare?

  Piece, how can an observatory be under the ice?

• 37:16Krishna Choudhary?

  Yeah.

• 37:17Lester Nare?

  How can, how can you move faster than the speed of light through ice?

• 37:21Krishna Choudhary?

  There's a lot, a lot of working parts, right?

• 37:23Lester Nare?

  Yeah.

• 37:23Krishna Choudhary?

  And this is where Francis Halzen comes in. 1987, he's giving a talk at the University of Kansas about the possibility of detecting neutrinos using water. And a glaciologist who was in the audience, he informs Halzen that there are Russian plans to detect radio emissions of charged particles produced by neutrino interactions in the Antarctic ice. They're using radio detectors. Okay. Now, the idea here is that the neutrino collides with an atomic nucleus in the ice. That creates an electric spark and that produces a radio wave. He quickly does some calculations with these colleagues and he figures out that actually the detection threshold would be too high for the pet electron volt neutrinos that we really want to see.

• 38:06Lester Nare?

  Right.

• 38:07Krishna Choudhary?

  Okay. So Halzen's idea is, wait, ice is interesting, though. Okay. He's like, instead of radio, what if we used Cherenkov radiation? And him and his research colleague, John Larned, who was a big part of the Dumand Project, he was actually at the University of Hawaii, they come out with this proposal at a conference in Poland in 1988. This is the proposal for an ice cube-like observatory. On the right-hand side, you can already see somewhat their idea. The earth there is upside down. Antarctica is at the top. What they're proposing is to look downward into the earth.

• 38:50Krishna Choudhary?

  Okay, so if we were in Antarctica, our detector would be pointing downwards. Why? Because we want to look for neutrinos that have made it all the way through the earth. If we look up, we're going to get solar, we're going to get atmospheric neutrinos, we're going to get all sorts of stuff. But the really high energy ones don't even care that the earth was there.

• 39:09Lester Nare?

  It's built in filtering that.

• 39:10Krishna Choudhary?

  Yeah.

• 39:10Lester Nare?

  It's very different than the Dumand problem.

• 39:12Krishna Choudhary?

  You're using the entire planet as a filter. It's, it's pretty amazing. These two came up with this idea in 1988. Halzen takes this idea a little bit further. It's, it's kind of just an idea. And then in Greenland, it works for the first time. So Greenland publishes, the Greenland detector publishes in 1991, an observation of muons using polar ice caps as a Cherenkov detector. They have a single sort of tube of photomultiplier tubes and they report using ice to detect Cherenkov light. Once this happens, people are like, okay, so there's a chance.

• 39:53Lester Nare?

  Yeah, yeah.

• 39:53Krishna Choudhary?

  There's a chance this could actually happen.

• 39:55Lester Nare?

  You're saying there's a chance.

• 39:56Krishna Choudhary?

  Right, yeah. So now we could actually happen. The Amanda Project is proposed. This is the Antarctic Muon and Neutrino Detection Array. And the Amanda Project in Antarctica subsequently demonstrates the instrumentation that leads to ice cube. The, there's photomultiplier tubes that go down into these bore holes. The holes are drilled using high-pressure hot water systems that are originally pioneered for glaciology. The physicists just borrowed that and said, hey, you guys have been digging for like climate change and glaciology, let's do the same kind of tunneling, but now instead, I'm going to put all these detectors into the ice. Amanda was constructed in phases from 1993 and 2000, and Halzen was a driving force

• 40:41Krishna Choudhary?

  in Amanda. And in those early deployments, they run into problems. Namely, the ice that's at the very top, it's very white, right? The reason why it's white is because there's a lot of air that's trapped in the ice. That's what's causing the diffraction, right? And you can't really see that far into the ice. Well, just the same reason why we can't see into the ice all that far, or the light doesn't penetrate, well, that means that the Cherenkov light that I'm trying to see also doesn't penetrate through the ice because of all the trapped air particles, right? They went deeper and they found that as the ice gets packed, the deeper layers don't

• 41:25Krishna Choudhary?

  have a lot of air because the air starts escaping because of all of that high pressure. So the deeper I go into the, into the Antarctic ice sheet, the clearer the ice gets. Okay. That was a big finding of the Amanda Project.

• 41:40Lester Nare?

  And there's a driving force behind, you know, now understanding at what depth do we need to go to, um, in order to basically have a clean lens.

• 41:50Krishna Choudhary?

  Yeah.

• 41:51Lester Nare?

  As an analogy.

• 41:51Krishna Choudhary?

  As an, yeah, that's a very good analogy, actually. And so all of this was because of Amanda, and Amanda finally detected atmospheric neutrinos and demonstrated that the method would actually work. They're able to observe high-energy neutrinos, but these are still from the atmosphere and sort of solar system sources. But they're saying that now this, this definitely works. We just need the money. We've got a plan, we need the money.

• 42:14Lester Nare?

  Show me the money.

• 42:15Krishna Choudhary?

  NSF comes in, of course, the National Science Foundation, and they make a bet.

• 42:21Krishna Choudhary?

  They say, all right, we're gonna give you a lot of money now. In order to for you to make the Ice Cube project. Construction completes in 2011. Ice Cube is now a cubic kilometer worth of detectors. The the building that everyone sees, right? That's the tiny dot at the very top.

• 42:44Lester Nare?

  This is such a massive infrastructure project.

• 42:46Krishna Choudhary?

  Yeah.

• 42:47Lester Nare?

  It's like really actually hard to especially in an environment that's not very conducive.

• 42:51Krishna Choudhary?

  No, it's in Antarctica. They're building this thing in Antarctica. That means that their construction timeline is only six months.

• 42:58Lester Nare?

  Right, because.

• 42:59Krishna Choudhary?

  Right, because you can't go in the winter to construct because everything's dark. Right? So you have to go in the summer. And you have to try and construct this thing. They did it one at a time, you know, one bore hole at a time. Slowly but surely, they started getting this thing together.

• 43:16Lester Nare?

  And just to reiterate, the point of like, why is it so deep? And why is it so wide? It's so deep because we can't detect these extragalactic, highly energetic neutrinos too close to the surface.

• 43:29Krishna Choudhary?

  Yeah, we can't detect any neutrinos close to the surface because like just it it starts and it ends, starts and ends. So if if I'm trying to detect light from like a meter or two meters away.

• 43:38Lester Nare?

  Even if it happened over there, it wouldn't even. Yeah, it would never make it to your.

• 43:41Krishna Choudhary?

  Yeah, yeah, it would just like interact with the the air and then you're done.

• 43:45Lester Nare?

  And then the reason it needs to have such this large surface area is because the in the the thing we're looking for, the extragalactic high energy stuff is so infrequent. We need to create the biggest target possible.

• 43:59Krishna Choudhary?

  Yeah.

• 44:00Lester Nare?

  And hope we get lucky.

• 44:02Krishna Choudhary?

  And hope we get lucky. The other thing for for why it can be this big is that if we do catch it, we can really track.

• 44:09Lester Nare?

  Yeah.

• 44:09Krishna Choudhary?

  The whole thing.

• 44:10Lester Nare?

  Yeah.

• 44:10Krishna Choudhary?

  Right, and then we have a good idea about what direction it came from. It's it's absolutely incredible. What what they've been able to do. And this is what a detector looks like. These are the, you know, photomultiplier tube type thingies that that go in. This called the deep core detector. It's a densely instrumented set of photomultiplier tubes. It's a fancy CCD.

• 44:36Lester Nare?

  Yeah, yeah, right.

• 44:37Krishna Choudhary?

  And there's a bunch of these all the way up and down into the thing.

• 44:41Lester Nare?

  Okay. Right, like just like so it's basically like a, you know, in Ski Ball when you have all the balls next to each other. Like it's just like a long Ski Ball thing.

• 44:48Krishna Choudhary?

  Yeah, yeah, it's it's a pressure resistant glass sphere. Because, you know, it's got you got to get down to really high pressure.

• 44:55Lester Nare?

  Yeah.

• 44:55Krishna Choudhary?

  Um, it's got a bunch of electronics and it's it's absolutely amazing. And so now let's look at what an event looks like. Here a neutrino is coming in. It interacts somewhere, right? Like there's either either it spits out a muon. In this case, it's a muon that that gets spit out. That muon then travels through the detector and sets off all of these detectors, all of these photomultiplier tubes with that Cherenkov radiation, the blue glow that you see. That's the Cherenkov radiation that is coming out.

• 45:27Lester Nare?

  Yeah, that's that secondary signal that we then use to then back in the back into the red line, which is the path.

• 45:35Krishna Choudhary?

  Yeah, because you know, because crucially, we can tell what did what photomultiplier reacted first.

• 45:42Lester Nare?

  Yep.

• 45:42Krishna Choudhary?

  And what photomultiplier reacted next and next. And so we can retrace with timing. What was the path that it came from? Also with the timing, we can figure out what the speed is. And so that tells us a lot about, um, at least the speed of the muon. We don't maybe know what the speed of the original neutrino was because crucially, when a muon is created, other particles are created that are lost in the signal, but we have a really good idea of where it came from.

• 46:11Lester Nare?

  Understand.

• 46:12Krishna Choudhary?

  Okay, and we've got a probability estimate of sort of how big the original neutrino was. Now, that brings me to my second point, which is there's two types of events. On the right hand side, that's the muon type of event that we've we've noticed. There, um, you know, the muon crosses the detector. It deposits all of these signatures, and that tells us where it came from. On the left, you can have a cascade, which is a particle shower that develops over a distance that's really small compared to the detector. So the entire event is constrained inside of the detector. And we've got a really good estimate of the total energy. Just not where it came from.

• 46:50Lester Nare?

  Right, okay, so that we have also different problems.

• 46:53Krishna Choudhary?

  Yeah, we have different problems. On the left, everything is inside the detector. And so we've got a really good estimate of energy. On the right, there's some events that happened outside the detector. We don't know, you know, how big the event was, but we know exactly sort of where it came from within like even like a one degree of the sky. And this is the first time that we can really do this.

• 47:13Lester Nare?

  Yeah, yeah, this and it's it's so interesting that, um, when you start looking at these results, the way you just classify these two is like an important. We're we're still trying to get back to this mapping piece. And, you know, if we had made this 50 kilometers, right? We would, we would have.

• 47:33Krishna Choudhary?

  I mean, we'd have multiple events and we'd have we'd track a whole event the whole time.

• 47:38Lester Nare?

  And so partly, this is a limitation of the scale of the system.

• 47:41Krishna Choudhary?

  Yeah, I mean, this is a cubic kilometer and it's already like we're we're getting up to the limits of it.

• 47:47Lester Nare?

  Yeah.

• 47:47Krishna Choudhary?

  But it's still incredible because for the first time, we're we're.

• 47:50Lester Nare?

  Hundred percent, no, hundred percent. Um, it just, I think, speaks to the complexity of the problem.

• 47:54Krishna Choudhary?

  Yeah, it's it's absolutely amazing. A few things that, you know, people don't really think about is like when you make this detector, it's a it's a cubic kilometer in ice. The instrument samples light at all these separate locations and then tries to reconstruct them. You need to calibrate the signal, right? Because there's like background that you need to account for. There's dust scattering, there's layers in the ice itself. The ice is not a perfectly human engineered thing. This is we're borrowing from nature itself. So there's going to be folds in the ice. And so a clean neutrino comes through, creates a muon that creates the shower. Well, the ice to the left of it could have a different density than the ice to the right of it. And so you need to calibrate for that. So that's not part of your error.

• 48:38Krishna Choudhary?

  All of these things have to be done by this collaboration. It's something that you don't really think about, but like in order to really answer the question, you have to be so very, very careful of the nature of your detector and what is the actual, you know, ground truth.

• 48:53Lester Nare?

  This dovetails with our story on black marble and the way they were doing nighttime satellite detection from what was originally out weather satellite.

• 49:03Krishna Choudhary?

  Yeah.

• 49:04Lester Nare?

  And the need to do a lot of this data.

• 49:09Krishna Choudhary?

  Cleaning.

• 49:09Lester Nare?

  Cleaning.

• 49:10Krishna Choudhary?

  Yeah.

• 49:10Lester Nare?

  Uh, and filtering process. And this has similar but very different challenges.

• 49:16Krishna Choudhary?

  Yeah.

• 49:16Lester Nare?

  Because it's not a Carl Zeiss lens, you know, or something.

• 49:20Krishna Choudhary?

  Yeah, it's not something that we've made. This is nature itself. And so there's going to be imperfections. On top of that, even if we've taken care of all of those systematic irregularities, the background neutrinos are a challenge. There's so many neutrinos that are coming in, right? So how do we filter out? Well, there's three ways that we can do this. One is direction. Just look for the ones that are coming up from the ground, right? Because they had to go through the earth. And so at least we know that those mostly aren't atmospheric. Another one is containment. We require most of the event to be inside the detector. It needs to start inside the detector. And hopefully, most of it is contained inside. And the third is energy.

• 50:02Krishna Choudhary?

  We can just filter out the low energy ones and be like, these high energy ones are definitely not just because based on our power spectrum that we had earlier.

• 50:10Lester Nare?

  Right.

• 50:10Krishna Choudhary?

  These high energy ones definitely have to be outside. And so in 2013, Ice Cube reported two events with deposited energies of one pet electron volt. That was huge for the world because this was the first time that we were having access to neutrino detections at that scale. That was the whole point of Ice Cube.

• 50:30Lester Nare?

  Yes.

• 50:31Krishna Choudhary?

  And so it was it was really a big triumph. It was only two or three years after Ice Cube had gone online. And that kind of matches the one event per square kilometer per year.

• 50:43Lester Nare?

  The math actually ended up being in in practice.

• 50:47Krishna Choudhary?

  Yeah, it's like, oh, we detected the two events that we would have expected. And on the right hand side, you can actually match the distribution of like how many we were seeing at each of these different energies. The blue is our model for what the atmosphere and the smaller neutrinos background would look like. The red is how we're fitting the data for the extragalactic origin. And what we're seeing is the the the data is matching the sum of these two curves, right? The blue for the smaller energies and the red for the larger energies. Because the larger energies are what we actually care about.

• 51:28Lester Nare?

  Modeling matters.

• 51:29Krishna Choudhary?

  Modeling matters, especially in particle physics. They do a lot of it.

• 51:32Lester Nare?

  Yeah. And it's great when your tools map to can see, this goes back to this not a black box thing we talk about often.

• 51:40Krishna Choudhary?

  Yeah.

• 51:40Lester Nare?

  So we now understand what the sort of end product of this Nobel Prize was given for, which is this neutrino observatory. It made these landmark detections, uh, within two, within two years after coming online.

• 51:57Krishna Choudhary?

  Yeah.

• 51:57Lester Nare?

  Which is, you know, it would it's better than what they were trying to do with X Boson, but that's a whole nother story. And I still think we come back to where we started, which is the whole purpose of this and the word observatory is to try to understand and map everything around us. And we we landed on a limitation, which still leaves me with the question, where are they coming from? Because what we want to know is what is this object that's generating these energies? That's generating this amount of speed such that when we detected it's in that petavolt, uh, peta.

• 52:33Krishna Choudhary?

  Peta electron volt.

• 52:34Lester Nare?

  Peta electron volt scale.

• 52:36Krishna Choudhary?

  Yeah.

• 52:36Lester Nare?

  Which we've seen like the two we just discussed.

• 52:38Krishna Choudhary?

  That's right, and that was the whole point, right?

• 52:40Lester Nare?

  Yeah.

• 52:40Krishna Choudhary?

  Now, with the 2013, the two the two that were detected there, right?

• 52:46Krishna Choudhary?

  Those, we had a good estimate maybe of the energies, but maybe not like the direction because of those two types of events that I was telling you about. But certain events, we could actually pinpoint. And in September 2017, IceCube detected a high-energy neutrino with a direction that was consistent with a blazar.

• 53:07Krishna Choudhary?

  The blazar is called TXS 0506 plus 056. This is at the edge of our universe. It's extremely tiny. This is not anywhere near us, okay? Billions of light-years away. A blazar is an active galaxy with a jet that's directed towards us. So you can imagine the galaxy's jet is looking straight at us, which is why it's so bright for how far away it is. The galaxy also brightened in gamma rays. We have gamma ray observatories that had coincident detection.

• 53:40Lester Nare?

  We like coincident detection.

• 53:41Krishna Choudhary?

  We like coincident detection because that means something happened in this blazar, right? There was some event that released a bunch of gamma rays and a bunch of neutrinos. Neutrinos, because they're so light, they're like near zero mass.

• 53:55Krishna Choudhary?

  They travel at about the same speed as, as light. Sometimes they even arrive before light because the light interacts with a bunch of stuff and gets delayed and then comes to us. The neutrino is just on its way from whatever event. And so in 2017, we got, we got kind of an indication that actually it could be, in fact, these extragalactic jets. IceCube also mapped out the Milky Way with diffuse high-energy neutrinos. And this, I think, is absolutely incredible. So here we're seeing maps of our own Milky Way in different bands. The first is radio, optical. This is what we see with our naked eye.

• 54:38Krishna Choudhary?

  If you go to Joshua Tree, or even better, if you go to like the middle of, middle of nowhere, and gamma rays. And neutrinos are on the bottom, very blobby. But you don't understand just how hard it is to catch neutrinos and create a picture at that.

• 54:54Lester Nare?

  Yeah.

• 54:54Krishna Choudhary?

  Right? Using a cubic kilometer worth of ice. We've created a, a map of the Milky Way and it kind of matches, right? The central blob in the center is the galactic nucleus, where the supermassive black hole Sagittarius A star is, along with a bunch of other star-forming regions, all sorts of random nonsense that's happening. And that is the region that is brighter in the neutrino band as well. This is a map of our Milky Way using neutrinos. I think that's absolutely incredible.

• 55:24Lester Nare?

  It, it makes me think of the history of other sort of astronomical imaging and the progress when you start. Like it goes from blobby to resolve. Like I think like the cosmic microwave background kind of went from blobby to more resolved, but it kept its same structure. But as we got better over time, you know, the resolution just became more well-defined and it's, we kind of see it. Maybe, maybe we will get to a level that looks similar with neutrinos, but you gotta start somewhere. And this is because of the difficulty level as compared to the other three types. We can put stuff in space for the gamma rays.

• 55:58Krishna Choudhary?

  Yeah.

• 55:59Lester Nare?

  We can put the CCD on camera on the ground and in space.

• 56:03Krishna Choudhary?

  Yeah.

• 56:03Lester Nare?

  No problem.

• 56:04Krishna Choudhary?

  Yeah, but we need, we need, we need a massive thing for the neutrinos. We need, we need a, a whole ice cube that is one kilometer by one kilometer by one kilometer.

• 56:13Lester Nare?

  Big ice cube.

• 56:14Krishna Choudhary?

  Yeah, it's insane. One of the most important things that IceCube has done is actually falsify certain theories. Science is all about falsification, right? A theory is testable and if we can falsify it, we are making progress. One of the proposed sources, one of the, one of the things that people thought could be these, you know, extragalactic sources that of, of really high-energy particles was gamma ray bursts. We don't really know what gamma ray bursts are, but there's a burst of gamma rays from singular sources. We have found no significant association in the samples that we've discussed. And so those absences constrain the models because the people who are, you know, talking smack about, oh, it's, it's obviously gamma rays. You know, what's the big deal? Well, where are they? Right?

• 56:59Lester Nare?

  That's tough.

• 57:00Krishna Choudhary?

  We see gamma rays from gamma ray bursts, but not that many neutrinos of this really high energy.

• 57:06Lester Nare?

  There's like, because we can, we would be able to do coincident detection with this as well.

• 57:09Krishna Choudhary?

  Exactly.

• 57:10Lester Nare?

  Not there.

• 57:11Krishna Choudhary?

  Right? It's not there.

• 57:12Lester Nare?

  I don't know what to tell you.

• 57:12Krishna Choudhary?

  I don't know what to tell you, right? So the achievement is recognized by this prize.

• 57:17Lester Nare?

  Yeah.

• 57:17Krishna Choudhary?

  Right? This prize is recognizing that exact achievement. And this is the Nobel Committee drawing.

• 57:23Lester Nare?

  Which we can now understand very well.

• 57:25Krishna Choudhary?

  Which we can now understand very well. It's showing a proton, which is in red, and/or a charged particle. It's sad because it's moving around because of extragalactic magnetic fields and the magnetic fields in the Milky Way. There's a gamma ray that's also coming from these particle accelerations that are happening outside of our galaxy. But those things start interacting with the matter and they get stopped. The neutrino, on the other hand, it's like the, they use the V as kind of an angry eyebrow. You see that? And this thing is just coming at us. Yeah, it's coming straight at us into the IceCube detector and it's gonna create the detection that we want.

• 58:07Lester Nare?

  This is so funny. And then it's got the, the little sensors inside the ice with the circles, right? Like the similar like.

• 58:14Krishna Choudhary?

  Yeah, yeah, yeah.

• 58:15Lester Nare?

  Shape for this. That's, this is actually quite, quite.

• 58:18Krishna Choudhary?

  This one, this one's pretty good.

• 58:19Lester Nare?

  Yeah, this is quite nice.

• 58:20Krishna Choudhary?

  Yeah. This one's pretty good. I quite like that one. So Halzen, who is the Nobel Prize winner this year, Francis Halzen, he's been instrumental with IceCube. Okay, he, he helped carry the idea from a conversation about Antarctic ice into a fully-built observatory. The University of Wisconsin is the main backing institution for IceCube. But IceCube is a massive, massive enterprise. It involves 450 scientists across 58 research institutions, across 14 different nations. It cost $280 million.

• 59:02Krishna Choudhary?

  A majority of that came from the National Science Foundation over several decades of work. And they're gonna be upgrading to IceCube Gen 2.

• 59:12Lester Nare?

  Gen 2.

• 59:13Krishna Choudhary?

  Which is gonna make an even bigger detector for even more frequent events and even more precise determination of energy and origin. This is University of Wisconsin's 20th Nobel Prize affiliation in the three sciences. So they're, they're, they're on a roll.

• 59:30Lester Nare?

  Yeah.

• 59:31Krishna Choudhary?

  They're doing quite well.

• 59:32Lester Nare?

  Maybe we'll put up our Nobel Prize, uh, uh, top 10 power rankings.

• 59:37Krishna Choudhary?

  Yeah.

• 59:38Lester Nare?

  So we know who's, who's ballin'.

• 59:40Krishna Choudhary?

  Exactly.

• 59:40Lester Nare?

  Because that's, that's a lot.

• 59:41Krishna Choudhary?

  That's a lot, right? Yeah, so, um, these detectors, they're gonna be, they're gonna be absolutely amazing. I am myself looking forward to IceCube Gen 2.

• 59:50Lester Nare?

  Gen 2 is gonna be great.

• 59:51Krishna Choudhary?

  Because that's gonna collect even more events. It's gonna improve the source picture. It's gonna account for which objects actually produce it. You know, we can get a statistics about, oh, this many blazars, this many this, this many that. It's gonna be really, really nice.

• 1:00:08Lester Nare?

  I, I just, I really want to, as we end on this image, you know, our curiosity as a species about the universe around us is so interesting and we continue to try to find ways to learn more. And this is a very, very clever.

• 1:00:22Krishna Choudhary?

  Yeah.

• 1:00:25Lester Nare?

  Somewhat non-intuitive unless you kind of have an understanding of the space way to think about it. It's in the ice, it's looking down, it's looking for this sort of secondary event. It's not a primary detection. These things that we're looking for that these neutrinos come from are still a mystery. And certainly Nobel-worthy. Again, solo Nobel, it's been a while.

• 1:00:50Krishna Choudhary?

  It's been a while.

• 1:00:50Lester Nare?

  And they're not that frequent.

• 1:00:54Lester Nare?

  And I know we've touched on this a couple of times before. Anything with space, for long-time listeners, you guys know this is one of my favorite things to discuss. We, I think the physics ones are always the, some of the most interesting. We are gonna be back tomorrow for the last day of our coverage for the Nobel Prizes, which will be in chemistry. If you have not checked out our coverage of the Physiology and Medicine Nobel, we did do that yesterday. It's quite good. Many of you liked that coverage as well. You can check out all of our past episodes at FFPpod.com, as well as all the research papers that we covered in this and last, yesterday's and tomorrow's episodes.

• 1:01:40Lester Nare?

  Socials, FFPod, we have video available. Watch the video, it's great. We do all these overlays that are really important to understand this. My name is Lester Naray, joined as always by my co-host and our resident PhD, Krishna Choudhary. Our last day of Nobel coverage will be upon us, our favorite time of the year. We will see you all tomorrow.