Nobel Prize in Physics 2026 Explained: Neutrinos and the IceCube Observatory

Hi and welcome to Azim Premji University's Nobel Prize explainers. We're here now to discuss the 2026 physics prize that has gone to Francis Halzen for decisive contributions to the IceCube neutrino observatory and the discovery of high energy neutrinos of astrophysical origin. Halzen by the way is the first in over 30 years to be a solo winner of the physics prize. And the crux, really, of this year's prize is the subatomic particle called neutrinos that is hardly a stranger to Nobel Prizes. Most recently, discoveries concerning this particle won a Nobel in 2015 and before that in 2002. To figure out what... What is new in the area and how this particular prize stands out and also to hear more about this giant neutrino detecting IceCube in the South Pole, I have with me theoretical physicist Sridhar K who also teaches here at the university. Hi Sridhar, thanks for joining us. So, I want to start at the very basic, Sridhar, can you tell us what are neutrinos and what is it about them that's fascinating? Can I even backtrack a little more? Go ahead. Start from some point where, you know, we all know, for example, the structure of an atom, which was established in the 1920s, which is a very simple structure, it has positively electrically charged protons, neutrons. Which are neutral inside a nucleus and then there are electrons orbiting around it. So it was a very simple structure and it explained all the elements of chemistry. Therefore all the all matter that as we knew it could be explained in terms of three particles. It was a physicist's dream to be able to explain everything in terms of just three particles. But, The problem was of course the nucleus is made up of just neutral neutrons and positive protons, so how does it stay together, you know, what holds the nucleus together. So people already knew when they drew that picture of the atom that there should be something more and indeed there was more because before the 1920s there were only two kinds of Fundamental forces that we knew and those are electromagnetic force which is responsible for interaction of charged particles and also that radiation that's coming out of electromagnetism is light. So light is a form of electromagnetic radiation Thank you very much. and there is a gravitational force which you know of for a long time. But when they started thinking on the nucleus to cut a long story short they discovered two more interactions and the reason we hadn't seen them before is because they operate only within the domain of the nucleus which is a very very tiny size i mean it's 10 raised to minus 15 meters which means it is one million one thousandth of a millionth of a millionth of a meter okay okay it's that tiny and And so, Those forces are seen only at those tiny scales. So there are two new forces. One is called the strong nuclear force which holds the nucleus together. And one is called the weak nuclear force, which is responsible for the disintegration of the nucleus because some nuclei are not stable. Stable meaning they don't exist forever in one state. So it is energetically more favourable for the nucleus. To convert a neutron inside a nucleus into a proton or a proton into a neutron depending on what nucleus it is and emit an electron or an antiparticle of an electron, a positron. So it becomes favorable for nuclei to go through these transitions where a neutron gets converted to a proton or a proton gets converted to a neutron. These are called beta decays in nuclear physics. And people notice that the Conservation laws which we hold so sacred in physics, energy conservation, momentum conservation and so on, those get violated in beta decay. I see. So basically, to put it differently, when a nucleus changes to another nucleus by converting a proton to a neutron, let's say, and in the process it emits a positron. Yeah. If you add up the energy changes that has happened in this reaction, it doesn't all add up together, so right and left hand sides don't match. So it was Pauli, the very famous physicist from Zurich who proposed that there must be another particle and that particle is taking away the rest of the energy. The question is then why don't we see that particle? Turns out that this particle is a strange thing, it is the only particle we know as such which does not have a charge so it does not take part in electromagnetic interactions okay it does not have a mass so almost does not have a very very small mass so it does not have gravitational is it as small as the electron is it as small as the electron it is tiny as the electron it's like a fundamental particle has no size but the electron has charge so it can take part in it but this does not have a charge So the only interaction that it takes part in is what is called the weak interaction. The weak interaction is very feeble. It's one of the, I told you, the weak interaction actually mediates the decays of these nuclei So weak interaction is actually very feeble. It's like one thousandth of the strength of electromagnetic interaction so because of that the neutrino hardly interacts with anything it can it does not interact with matter it can pass through tons of matter in fact it can enter the earth from one side and exit from the other it does not get absorbed okay so so yeah so these are the It is a particle called neutrinos, very difficult to detect, so people had to devise very interesting ways to detect it. So I mean can you tell me about the IceCube Observatory and I mean I understand that it's quite literally a giant cube of ice deep underneath the South Pole so why was it built and what was Halzen's role and what did he finally discover? So again going back a little yeah the this is the one kind of experiment that we have High energy physics are accelerator based experiments which we do in our labs like in big labs like CERN or Fermilab. So where you accelerate a particle to very high energies. In CERN right now they are accelerating two protons to very very high energies. They collide them and in the collision they try to see what new particles. Because you can only detect it when they collide. Yeah, you look at the products of collision and from there infer what could have been produced. The idea of going to high energies is that many of these particles are Quite massive, so you will see those effects only at higher energies. The other kind of thing which experiments which are done in high energy physics are those which do not have accelerators. So it is basically the idea is to wait for the heavens to shower. So there are these kind of very highly energetic particles. Interactions taking place, collisions taking place in our galaxy, outside our galaxy and so on. Some of it we don't know even where they come from, some we have some idea. But these particles, highly energetic particles come into the Earth's atmosphere, they shower in the atmosphere and they produce a large number of particles. So what we are saying is that there is an accelerator given to us by nature and that produces particles by showering in the atmosphere. So all we have to do is go to a higher altitude. sit with our detectors there and wait for the heavens to shower literally and in that in fact in the 50s this was a very very important way of discovering new particles. A lot of particle physics actually got established through experiments of this kind. These are called cosmic ray experiments. And India had a lead in these experiments, very close to where we are sitting. In fact, in Kolar, deep under the mines, the gold mines in Kolar, they had set up these Kolar gold field experiments, which actually discovered some very important things in 1965. They discovered What is called the atmospheric neutrino in 1965. So, neutrinos come to us from various sources. For example, you have nuclear interactions and the Sun is fueled by nuclear interactions. And in those nuclear interactions, of course, you produce a lot of neutrinos. And those neutrinos, unlike light, which takes a lot of time to come out of the Sun and reach us. The neutrinos just, because they don't interact, they just come to the earth, but the thing is detecting them also is difficult, But solar neutrinos have been observed, they are at somewhat lower energies. higher energies than those are the ones, I told you, these cosmic rays enter the atmosphere, like a lot of high energy radiation entering the atmosphere. And by cosmic we mean they are coming from far away, like way beyond. We have to go deep underground to observe because what you want to do is go underground so that the rock absorbs all the other particles and essentially you see only a neutrino signal. That was the discovery made by the Indian physicist T. R. R. Rathis in 1965. Could be thought of as a precursor to the IceCube experiment. But those are higher energies than the solar neutrinos. But the question is if you want to look at really high energies, What do you do? And so that is why they thought of, I should say they, okay, Francis Halzen and John Learned, these other people who conceived of this idea of doing an experiment, you know, where you, so they needed a large mass of water essentially. Okay. The idea of using water, what we do is, Okay, so neutrinos cannot, because they are so weakly interacting, you can't just detect them. For charged particles like electrons or their heavier cousins, you can detect them easily in experiments. So neutrinos are detected indirectly. To each of the charged particles like electron has its own neutrino. There's another particle like the electron which is 200 times heavier that also has its own neutrino. So there are three of these charged particles electron, muon and tau and each of them has the flavors. There are three neutrinos so three flavors of neutrinos. So they are called electron, muon, tau. They are But they are associated with these particles. So if you produce an... Electron, then you can only produce an electron neutrino, you cannot produce a muon, so there's some kind of conservation law going on there. So to detect the neutrino, what you do is the reverse thing, like you look at the neutrino, you know colliding with the atomic nucleus, interacting with the proton and neutron there. and in that transition the electron neutrino will get converted to an electron and the electron is something you can detect it's a difficult experiment but so you detect but this has a higher chance of happening on in water no so this is the thing so you produce the charged particle Now if you have a very very high energy neutrino, this very high energy neutrino is coming from outer space, I mean literally from outside our galaxy, I'll tell you why we know that, so this very high energy neutrino comes in, doesn't interact at all with anything, reaches your detectors, it collides, its collision first takes place. The nuclei in the water molecule and then in that it gets converted to from a neutrino it becomes a charged particle like so the electron neutrino will get converted to electron muon neutrino will get converted to muon. So as you produce the electron or muon, it Because the neutrino is highly energetic, this charged particle is also very energetic. So it's moving at a very very fast speed. But nothing, no matter we know from special relativity can move faster than the speed of light. We know that. But that is only Absolutely. the speed of light is some 300,000 kilometers a second in vacuum, in empty space. But Light actually becomes slower when it is in a medium. So you know medium like water, light travels only at three-fourths speed. But this charged particle can still travel quite fast and because it's a very fast charged particle. You have created conditions inside the ice for the charged particle to travel faster than the speed light travels in that medium. So you get something like... It's similar to a supersonic jet which travels faster than the speed of sound so you get a sonic boom. Here you get something equivalent which is called a Cherenkov radiation named after a Russian physicist. So this radiation, the light coming from there is what is detected by detectors which are So, they have stacked up these detectors under ice and it's a huge amount of ice. You could have asked why didn't they do this with water, earlier experiments have done it with water. But to do this kind of experiment with very high energy, you need a very large volume, it's a 1 km by 1 km array of, and you have detectors all over that. Normally, to get that kind of water would have been really exorbitantly expensive to, you know, create that much of a... And it has to be very pure water, I'm guessing? That also, and also there are other things because of the compression due to ice, you avoid air bubbles and so on. So, there are lots of good features. So, South Pole was the best place? Yeah, in South Pole, also because in the North Pole, there's apparently more. You know, melting of ice in South Pole is more protected by that. It's also protected from seismic disturbances, so that's how they converged on the ice. So now we have successfully detected these high-energy cosmic neutrinos. That is the success of this thing and you know, relatively short time. I mean, they did one experiment earlier called AMANDA, which was an experiment that was done experiment to check whether a thing something like the ice cube will actually perform to its best and in ice cube they have within less than 10 years detected this kind of a high energy neutrino. So we know one that it's a very high energy neutrino. Crazy amount, it's a big number, so I can't even, you have to put in several billions to say how much the energy is. So it's a very high energy thing, but it's also clear that it's coming from outside our galaxy, it's extra-galactic. Because if it's from within the galaxy, if the source is in the galaxy, then you would see it. In our galactic plane, but this is not so, it is actually distributed evenly all over. If that is the case, it means that the source is not within the galaxy but it's from outside. So having seen this event, it opens up a completely new area of astronomy. Yeah, because the Nobel Committee said that this discovery can pave the way to a new kind of astronomy. Yeah, so on that early astronomy, as we know, was done with telescopes. In telescopes, you use light. You look at light coming from objects in the sky and analyze that. Usually you analyze it in terms of a spectrum. You put it through a spectrometer and you look at the spectrum and then you understand what the constitution of that object that you are looking at is. But I told you, In the 19th century, thanks to someone called James Clerk Maxwell, it was realized that light itself is what we call light is only one part of a huge spectrum. And that huge spectrum is called electromagnetic radiation. So electricity and magnetism sort of conspired to give us light. In fact, and so you have and we all know now even very commonly that there is visible light, but there's also infrared light. We use it for light Like in our homes, for example, the mobile phone or, you know, in our microwave ovens and so on. And ultraviolet, which is also something familiar to all of us. Because, you know, the sun is showering us with ultraviolet. And so when we go out to the beach, we make sure we have our UV cream on. And also x-rays and so on, which are even higher in energy than what we normally call as ultraviolet. So there's an entire spectrum of radiation and people have now gone and done astronomy in all these different wavelengths. So what you can do is instead of looking at light, visible light, you can go and set up a radio receiver and look at the radio waves coming from outer space. So you get Complementary information, what light tells you, what visible light tells you and what radio telescope can tell you is different. It gives you therefore complementary and more information about the object that you're studying. But like how does how's that connecting with? But now you have a new source of Information on something that is happening far out in space. Okay, so electromagnetic radiation is all light. Yeah, so light, light it will interact with charged particles all very much. Some light coming from a very, very far away source, then that light is bound to interact and get scattered by all the material that is there in between you and this very far away source. Neutrinos don't interact with anything, so they just come. So it is carrying information of a very, very distant source in a very pure way. If you find a nice way of detecting that neutrino, then you can actually use that information Right. Where it's coming from. And first of all, if they are such high-energy neutrinos, they are probably happening in very exotic things like, you know, not some run-of-the-mill astrophysical process that's producing it, but it's happening in like two huge black holes merging together and in that Collision, you are producing these high energy particles. So you can actually get information about things which are literally happening at what we now know as the edge of the universe. And so neutrinos open up a new way of doing astronomy. It's not the familiar astronomy that we know, but it's a new way. So that is actually the success of IceCube. But it's been... I would think of this Nobel Prize as a Nobel Prize to the IceCube experiment and of course for Francis Halzen for thinking about it first and seeing it through, persevering with it. Alright, thank you so much Sridhar for doing this explainer with us and thanks all of you for watching this video and make sure that you also keep an eye out for our chemistry and our medicine prize explainers.