Nobel Prize in Physics 2026 Explained: IceCube & Neutrinos

Hunting ghost particles beneath Antarctica

0:02A 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, IceCube was complete.

0:44A 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 and Nobel Prize

1:16Hello internet. This is your captain speaking. Lester Nare joined as always by my co-host and our resident PhD, Krishna Choudhary. 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 IceCube Neutrino Observatory in Antarctica and the quest to find ghostly messengers from space. As always, we are gonna talk about the science from the ground up today, starting with who won?

2:02That'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 IceCube Neutrino Observatory and the discovery of high energy neutrinos of astrophysical origin." Now we've covered IceCube 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.

2:47Instead, 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.

3:28That's what he's winning this award for. And you know, it came kind of out of left field for us. I don't know if I'm on my radar at all, but in 2014, the ingenuity awards that are awarded by the Smithsonian, one was given to Francis Halzen, and they had a accompanying comic book thing. Neutrino Man. And he was called Neutrino Man, and on the right-hand side over there, at the very last sort of comic strip, it says it's on the cover of "Science," "The Times," "Nobel Prize," question mark. So they were already flirting with that idea that perhaps Francis Halzen would get the Nobel Prize. It's like the Simpsons meme, where they predict everything in the future in the Simpsons.

4:10Yeah 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 extra galactic origin? It's not easy, because you need to distinguish between the cosmic neutrinos that come from these

4:51particle accelerators out in deep space, and all of the neutrinos that we get bathed 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've gotta be able to sift through all of that and find the ones that are coming from way out there. That's what this is about. And so the first place that we need to start to understand the IceCube Neutrino Observatory is probably the middle word in the name.

What are neutrinos?

5:30People might know IceCube, not the rapper from California or the actor, if you might know. People kind of know what an observatory is. But what are neutrinos? 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? If you look at the data, energy was being lost, okay? This was a huge conundrum. People were like, where's the energy going? Niels Bohr very famously said, maybe energy is not conserved.

6:13Everyone else was like, what are you talking about? Oh, because we have to have conservation of energy. 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 average. Wolfgang Pauli, on the other hand, was like, no. Okay, the conservation of energy is up there in 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.

6:58And 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 antineutrinos from a nuclear reactor. This was by C. Cowan and F. Reines. They both won, Reines 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. Cowan had passed away in 1974. So he never got to share that Nobel Prize.

7:39And 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.

8:21For the electron, we've got a electron neutrino. For the tau, we've got a tau neutrino, so on and so forth. So these are sort of the fundamental building blocks for everything we see in the world around us. Yes, everything except for gravity is contained in that representation right there. This is like the periodic table distilled down to its absolute essentials. And just to clarify, these absolute essentials all have a dance partner, which is a neutrino version of themselves. Yeah, no, 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.

9:03So lepton neutrinos are only in this lepton category. 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. 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 and his initial questions, because we were seeing the loss of energy and we're like that can't happen. Right, that rule can't be broken. That rule cannot be broken, okay? So that's the neutrino. Okay, so this makes sense. So now we kind of understand, what is the thing that this Nobel Prize is sort of looking to

9:48discover or detect. But I think as you mentioned earlier, they're already here on Earth. Yeah, we've detected them. Because they make up stuff around us.

Neutrinos as cosmic messengers

10:00And they were originally detected using decay of radioactive material here on Earth. Right, so who cares? Or why does it matter that we detect neutrinos that are coming from space? This is the big leap that is central to this particular Nobel Prize. So right after Cowan and Reines 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. It's weak. To be perfectly honest, because the neutrino especially, it's weak and the neutrino is like near

10:44massless. So it's just gonna like go through. I mean, it can still move around because of the curvature of space-time, but 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. Okay, that's interesting. 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 to detected cosmic neutrinos. What they did was detect neutrinos from a very significant event, Supernova

11:291987A. 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. 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. That is a new star that wasn't there like a week before. That's so unbelievable. 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

12:13this thing with my naked eye. I mean, this is incredible. 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. 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 universe around us. Exactly. Is that kind of the right way to think about it? Okay. That's exactly right. And those particular neutrinos from the Supernova 1987A, those were on the order of mega

12:58electron 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. Pretty big. Yeah. 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 tera electron volt is a trillion electron volts. A peta electron volt is a thousand trillion electron volts. This thing is insane, and it's only possible because of an observatory like IceCube, okay? The reason to pursue these particles comes from a century-old mystery because we know they

13:39exist. 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 got on a balloon, literally, and detected an excess of charged particles in the upper atmosphere coming from extraterrestrial origin. Dude, physicists back in the day-- Right, we're really putting their, it's gumshoe work out in the field. That's fun. Isn't that hilarious? Yeah, that's quite fun. Right, and so these cosmic rays, we'd figured out they're mostly protons and charged atomic nuclei that are arriving from space.

14:20And 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. Right, not it. And it's like hard to do that, okay?

The Oh-My-God particle

15:0210 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. It was discovered in one of the salt flats in Utah. And what they figured out was that this thing must have been traveling at 99.999999 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?

15:43Right, because the point is the source of where it came 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. 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 particle, right? It's insane the amount of energy that was packed and it's very non-trivial how you even do that, right?

Cosmic-ray energies

16:21Let'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 gonna 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. 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. Now, the ones that are low energy on the left hand side, those things are pretty common. You'll get like one particle per square meter per second.

17:01That's the flux. So if you had like a detector, those one square meter, every second you'd get a single particle that's like, I don't know, 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. Then there is a knee in this plot. The feature there is called a knee because up to this energy, which is about like 10 to the 15 electron volts, all of the stuff before then is stuff that probably originates within the Milky Way.

17:43There's almost a ceiling to how far it could have been from. Yeah, but then there's a little kink and that's telling us that there is another source. And that other source is probably extra galactic. This is where we're getting to like one particle per square meter per year. The probability of it coming straight at Earth in our little square meter detector is getting lower and lower. 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 in increasingly smaller. Yeah, the source itself is not, I mean, there's not that many sources that are doing this.

18:24So as it spreads out, 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. Those are extra, extra galactic. 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 extra galactic, the ones that we don't like what, that's the oh my God particle. So there's these three sources that we're trying to make sense of. In order to find the ones that are really, really energetic,

19:05we need to do a lot of work.

Cosmic particle accelerators

19:07Now, before we get into that, let's just try to hypothesize what could even do this. It's of extra galactic origin that we're talking about. What kind of particle accelerator would actually do this? 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. The gyro radius of a proton that is traveling that fast exceeds galactic scales. You need a particle accelerator larger than a galaxy.

19:48I don't like it. If you just do the fundamental physics. I don't like it. It's weird. That's quite, and I'm thinking in my head one thing, but let's see if it's going to go in that direction. It's ridiculous, right? One proposed accelerator is the active galactic nucleus. 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 homogenous, they're not constant everywhere, and so you get like these-- Which we've talked about.

20:31Yeah. 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 gonna 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. We'd like to know.

21:12Where are these ghostly messengers coming from? Yeah, it's pretty insane that 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? So if you wanna do that, you wanna figure out where these particles came from, right? Now, for the oh my god particle, right? 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

21:53galaxy. There's a faint blue trace that you're seeing. 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. 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. Yeah, yeah, the line of sight could be this way, right? Even though the mountain is to the right. So instead, we wouldn't like something that has a clear line of sight.

Why look for neutrinos?

22:28And that is where we start thinking about what else could we see, right? There's a particle accelerator that is 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, at that electric core. At the origin point. 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,

23:12and neutrinos. Those are the three that we could kind of look for. 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. So, and let me just take a quick step. So the idea is 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. And we've just gone through a list of the things we could detect that are not neutrinos, that

23:53they shoot across the universe, and why it doesn't really work. Yeah, it doesn't really work. Protons, which is the actual thing, 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 gotta put a telescope in space in order to actually see this thing. How big of a detector are you gonna make in space? So there's an engineering problem. 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. But detecting them is kind of a hassle.

24:35Neutrinos, 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. This makes them kind of a nice target to look for. 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 the neutrino. So we can reverse engineer back to the total initial energy starting point. Yeah, we can 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.

25:19If 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. So 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. In our quest to again now find a way to figure out these highly energetic objects that are extra, extra galactic,

How to detect a neutrino

25:52we've figured out the signature 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. We know what we're looking for and why we are looking for it. And so then the next obvious question is, okay, how do we detect a neutrino? That's right. It sounds very promising. It sounds very promising, but the same reason why the neutrino is a great candidate for extra galactic astronomy is also a reason why they're notoriously hard to detect. Namely, they don't interact with a lot of things.

26:35That's the whole point of trying to detect them. But at the end of the day, if they don't interact with stuff, how are you going to see them? 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, every second, 65 billion neutrinos from the sun blow through my fingernail. I'm not feeling anything. Unbelievable. 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.

27:19Nucleons 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? 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

28:01type of neutrino coming out, okay? On the right-hand side, this is the first ever neutrino that was detected as a photograph, okay? This is in a hydrogen bubble chamber. You see on the left there, there's just 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 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.

28:42It has a fingerprint. 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. So would it be the case that because they're moving so quickly, we just would see this interaction kind of pop out of nowhere? Mm-hmm, yeah, because the neutrino, we can't detect. We can't detect it. We can only see this, like, yeah, okay. Once the neutrino interacts with something, we can detect the byproducts because the byproducts are charged, so they're gonna be interacting with the electromagnetic field, which then creates light.

29:22What type of light?

Cherenkov light

29:23This is Cherenkov radiation. This is the very famous blue light that is emitted in particle, actually in nuclear reactors. When you have 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 did our IQ-- Speed of the light in ice. Yeah exactly. So, oh, geez, we'll remember that meeting. Yeah, it was one of our first viral videos, actually. The idea is the following.

30:04So, 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 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. So if a charged particle moves faster than the speed of light in the medium, but obviously

30:48slower 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. The light boom! There you go, yeah. You get a light boom. It's a shockwave in the electromagnetic field. A photonic boom. A photonic boom, and 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

31:31was, like how big the neutrino was in terms of energy and where it came from. Now, the probability is extremely tiny, okay? At these petaelectron volt energies, the neutrino nucleon cross-section, meaning like how often does the neutrino interact with quarks? 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 target that the neutrino has to hit on the quark to like get an interaction. So extremely small. On top of that, the flux itself is very small, and so you need

32:12a giant, giant detector to be able to achieve any hope of detecting these things, even though there's many of them coming through at the level that we wanna see, like the-- Yeah, the energies that we wanna see, there's not that many. There's not that many. When I say 60 billion coming through my fingernail, no, that's like 10 to the five, 10 to the six electron volts. Those are the earthly ones. Those are the earthly ones, the solar system-y ones. We want the extra galactic. We want the extra galactic ones that are like 10 orders of magnitude above the stuff that's going through my fingernail. And so, again, so now we know what a neutrino is, like how you would detect it because of this Cherenkov radiation.

32:52It 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. Yeah, yeah, yeah, no, Einstein is still happy. Which one'd you know we're here?

Building a neutrino observatory

33:13So now, the problems that is clear, this is like a highly improbable event to capture. 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. That's right, because in Hubble and all of these traditional observatories, you just look for 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'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.

33:58The first sort of attempt at doing this was do-mand, do-mand 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. What they wanted to do was basically on the ocean floor, rest a bunch of these detectors, like in this case, there's 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

34:42like on steroids, like a single photon. We'll set this thing off, right? 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 big thing was measurement backgrounds from ocean bioluminescence. Yeah, I was going to say. 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 removes a whole chunk of stuff that we would be otherwise seeing.

35:24The 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. Right. Like the water itself has dissolved radioactive potassium. And so you get this massive background, which is just super annoying. And you're just going to spend all your time filtering out. Right yeah. So the project was canceled in 1995. 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 much bigger sense.

36:05My favorite one is actually the Baikal Neutrino Telescope. This is in Lake Baikal, which is the largest body of fresh water on the planet. It has something like 25% of the world's freshwater in a single lake in Russia. And they've got a very similar detector from Dumond, where in the bottom, they've got these photomultiplier tubes that are arranged in a grid, and they're trying to find neutrinos. So water-based detection is working, right? But there are problems with trying to get it to detect the really, really high energy stuff, okay? Again, because of some of the background, some of that radiation stuff, so on and so forth.

36:48So this is where our Nobel Laureate comes in. I always love talking about this story because it's one of those, there's so much deeper than what you see on the headline. People take away Antarctica and neutrino, not knowing and understanding the importance of neutrino in terms of this mapping piece. How can an observatory be under the ice? Yeah.

From Antarctic ice to AMANDA

37:17How can you move faster than the speed of light through ice? (Laughs) There's a lot of working parts, right? And this is where Francis Halzen comes in. 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

37:59detection threshold would be too high for the petaelectron volt neutrinos that we really wanna see. Right. 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 Learned, 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.

38:41The Earth there is upside down. Antarctica is at the top. What they're proposing is to look downward into the Earth. 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 gonna get solar, we're gonna get atmospheric neutrinos, we're gonna get all sorts of stuff. But the really high-energy ones don't even care that the Earth was there. It's built-in filtering. Yeah. That's very different than the DUMAND problem. You're using the entire planet as a filter. It's pretty amazing. These two came up with this idea in 1988. Halzen takes this idea a little bit further.

39:23It'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. There's a chance this could actually happen. You're saying there's a chance. Right yeah. So now we could actually happen. The Amanda project is proposed. Antarctic muon and neutrino detection array.

40:06And the Amanda project in Antarctica subsequently demonstrates the instrumentation that leads to IceCube. There's photomultiplier tubes that go down into these boreholes. 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 gonna put all these detectors into the ice. Amanda was constructed in phases from 1993 and 2000 and Halsin was a driving force in Amanda. And in those early deployments, they run into problems. Namely,

40:46the 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 have a lot of air because the air starts escaping because of all of that

41:30high pressure. So the deeper I go into the Antarctic ice sheet, the clearer the ice gets, okay? That was a big finding of the Amanda project. And there's a driving force behind now understanding at what depth do we need to go to in order to basically have a clean lens as an analogy. 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 definitely works.

42:10We just need the money. We've got a plan. We need the money. Show me the money. NSF comes in, the National Science Foundation and they make a bet.

Building IceCube

42:21They say, all right, we're gonna give you a lot of money now in order for you to make the IceCube project. Construction completes in 2011. IceCube is now a cubic kilometer worth of detectors. The building that everyone sees, right? That's the tiny dot at the very top. This is such a massive infrastructure project. It's like really actually hard to, especially in an environment that's not very conducive. No, it's in Antarctica. They're building this thing in Antarctica. That means that their construction timeline is only six months. Right, because you can't do that. Because you can't go in the winter to construct because everything's dark, right?

43:03So 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 borehole at a time. Slowly but surely, they started getting this thing together. And just to reiterate, the point of why is it so deep and why is it so wide? It's so deep because we can't detect these extra galactic, highly energetic neutrinos too close to the surface. Yeah, we can't detect any neutrinos close to the surface because light just, it starts and ends, starts and ends. So if I'm trying to detect light from like a meter or two meters away-- Even if it happened over there, it wouldn't even, the light would never make it to your-- Yeah, it would just like interact with the air and then you're done.

43:45And then the reason it needs to have such this large area is because the thing we're looking for, the extra galactic, high energy stuff is so infrequent. We need to create the biggest target possible and hope we get lucky. And hope we get lucky. The other thing for why it can be this big is that if we do catch it, we can really track the whole thing. And then we have a good idea about what direction it came from. It's absolutely incredible. What they've been able to do. And this is what a detector looks like. These are the photomultiplier tube type thingies that go in.

44:27This is called the deep core detector. It's a densely instrumented set of photomultiplier tubes. It's a fancy CCD. Yeah right. And there's a bunch of these all the way up and down into the thing. Okay. Like just like, so it's basically like a ski ball when you have all the balls next to each other. It's just like a long ski ball thing. Yeah yeah. It's a pressure resistant glass sphere because you gotta get down to really high pressure. It's got a bunch of electronics and it's absolutely amazing.

Reading tracks and cascades

44:59And so now let's look at what an event looks like. Here on neutrinos coming in, it interacts somewhere. Like there's either it spits out a muon. In this case, it's a muon 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. Yeah, that's that secondary signal that we then use to then back into the red line, which is the path of the neutrino. Because crucially, we can tell what photomultiplier reacted first and what photomultiplier

45:43reacted 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 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. 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 noticed.

46:24There, 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 and then it's 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. Okay, so we have also different problems. 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 how big the event was, but we know exactly sort of where it came from within like

47:08even like a one degree of the sky. And this is the first time that we can really do this. Yeah, yeah, and it's so interesting that when you start looking at these results, the way you just classify these two is like an important, we're still trying to get back to this mapping piece. And if we had made this 50 kilometers, right, we would have-- I mean, the way it'd have multiple events and we'd track a whole event the whole time. And so partly this is a limitation of the scale of the system. Yeah, I mean, this is a cubic kilometer and it's already like, we're getting up to the limits of it. But it's still incredible because for the first time, we're able to see it.

47:50100% no 100%. It just, I think, speaks to the complexity of the problem. Yeah, it's absolutely amazing. A few things that people don't really think about is like when you make this detector, 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 gonna 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.

48:34And so you need to calibrate for that. You need to be able to get rid of your air. All of these things have to be done by this collaboration. It's something that you don't really think about, but 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 ground truth. This dovetails with our story on Black Marble and the way they were doing nighttime satellite detection from what was originally a weather satellite and the need to do a lot of this data cleaning and filtering process. And this has similar but very different challenges because it's not a Carl Zeiss lens

49:19or something. Yeah, it's not something that we've made. This is nature itself. And so there's gonna be imperfections. On top of that, even if we've taken care of all of those systematic irregularities,

Backgrounds and the 2013 discovery

49:30the background neutrinos are a challenge. There's so many neutrinos that are coming in. 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 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. 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, these high energy ones definitely

50:11have to be outside. And so in 2013, IceCube reported two events with deposited energies of one petaelectron 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 IceCube. And so it was really a big triumph. It was only two or three years after IceCube had gone online. And that kind of matches the one event per square kilometer per year. The math actually ended up being in practice. 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

50:56seeing 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 extra galactic origin. And what we're seeing is 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. Modeling matters. Modeling matters, especially in particle physics. They do a lot of it. Yeah, and it's great when your tools map to see.

51:37This goes back to this not a black box thing we talk about often. 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 within two years after coming online. Which is, it's better than what they were trying to do with Higgs 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 in the word observatory is to try to understand and map everything around us. And we landed on a limitation, which still leaves me with the question, where are they coming

52:20from? 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 detect it, it's in that peta volt, peta- Peta electron volt. Peta electron volt scale. Which we've seen like the two we just discussed. That's right, and that was the whole point, right? Yeah. Now with the 2013, the two that were detected there, right?

Tracing cosmic neutrino sources

52:46We 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. 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. And you can imagine the galaxy's jet is looking straight at us, which is why it's so bright for

53:30how far away it is. The galaxy also brightened in gamma rays. We have gamma ray observatories that had coincident detection. We like coincident detection. 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, they travel at about the same speed 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 neutrinos just on its way from whatever event. And so in 2017, we got kind of an indication that actually it could be, in fact, these extra

54:14galactic jets.

Mapping the Milky Way

54:16IceCube 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 if you go to Joshua Tree or even better if you go to the 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, right? Using a cubic kilometer worth of ice.

54:58We've created 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. 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.

55:38But as we got better over time, the resolution just became more well-defined and we kind of see it. Maybe we will get to a level that looks similar with neutrinos, but you gotta start somewhere. And because of the difficulty level as compared to the other three types, we can put stuff in space for the gamma rays. We can put the CCD on camera, on the ground and in space. No problem. But we need a massive thing for the neutrinos. We need a whole IceCube that is one kilometer by one kilometer by one kilometer. Big IceCube. Yeah, it's insane. One of the most important things that IceCube has done is actually falsify certain theories.

56:20Science 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 things that people thought could be these extragalactic sources 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 were talking smack about, oh, it's obviously gamma rays. What's the big deal? Well, where are they? We see gamma rays from gamma ray bursts, but not that many neutrinos of this really high energy.

57:06There's no, like, because we would be able to do coincident detection with this as well. Exactly yeah. Not there. It's not there. I don't know what to tell you. I don't know what to tell you, right? So the achievement is recognized by this price. Yeah. This price is recognizing that exact achievement. And this is the Nobel committee drawing. Which we can now understand very well. Which we can now understand very well. It's showing a proton, which is in red, or a charged particle. It's sad because it's moving around because of extra galactic magnetic fields and 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.

57:46But those things start interacting with the matter and they get stopped. The neutrino on the other hand, it's like, 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. This is so funny. And then it's got the little sensors inside the ice with the circles, right? Like the similar, like, shape for it. This is actually quite-- This one's pretty good. Yeah, this is quite nice. Yeah, this one's pretty good. I quite like that one.

IceCube collaboration and Gen2

58:23So, Halzen, who is the Nobel Prize winner this year, Francis Halzen, he's been instrumental with IceCube. Okay, 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. A majority of that came from the National Science Foundation over several decades of work.

59:08And they're gonna be upgrading to IceCube gen two. Gen two. 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 on a roll. They're doing quite well. Maybe we'll put up our Nobel Prize top 10 power rankings. Yeah. So we know who's ballin'. Exactly. Because that's a lot. That's a lot, right? Yeah so these detectors, they're gonna be absolutely amazing. I am myself looking forward to IceCube gen two.

59:50Gen two's gonna be great. 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. We can get a statistics about, oh, this many blazars, this many this, this many that. It's gonna be really, really nice. I just, I really want to... As we end on this image, 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, 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.

1:00:35It'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. It's been a while. And they're not that frequent. And I know we've touched on this a couple times before anything with space for longtime listeners. You guys know this is one of my favorite things to discuss.

Closing and Nobel week

1:01:05I think the physics ones are always 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, yesterdays and tomorrow's episodes. Socials, FFPpod, we have video available. Watch the video, it's great. We do all these overlays that are really important to understand this.

1:01:47My name is Lester Nare, 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.