Tracing A Neutrino’s 4 Billion Light-Year Journey

17:13 minutes

an isolated station out in a snowy field of antarctica at sunset
IceCube Lab in the Antarctic. Credit: Martin Wolf, IceCube/NSF

Neutrinos are particles that are constantly raining down in the universe. They are created from nuclear reactions in places like our sun, distant stars, and even on Earth. But the source of higher-energy cosmic neutrinos formed deeper in the universe is still a mystery.

Researchers have built telescopes to detect these low and high energy neutrinos as they pass through the Earth. One of these telescopes is IceCube, which is buried deep beneath the ice in the Antarctic. In September, IceCube detected one of these cosmic neutrinos and alerted the Fermi Gamma-Ray Space Telescope and other observatories. These telescopes were able to trace the source of the neutrino to a flare up in a blazar—a black hole at the center of a galaxy—4 billion light-years away.

[The mystery of the Namibian fairy circles.]

This week, the team of scientists published their results in two studies in the journal Science.

Physicist Chad Finley, who is part of the IceCube collaboration and an author on one of those studies, discusses what this finding tells us about cosmic neutrinos and how this network of telescopes could be used a new tool in astronomy. Watch animation of the IceCube neutrino’s trace of blue light.  

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Segment Guests

Chad Finley

Chad Finley is former leader for IceCube’s Point-Source Analysis Working Group and an associate professor of Physics at Stockholm University in Stockholm, Sweden.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. I know you’ve probably heard about neutrinos. We’ve talked about them over the years. They have been called the ghost particle created by nuclear reactions in the sun, and they are particles that pass through just about anything without leaving a trace. And because there are so many of them, they’re raining down on us all the time.

But there are even higher energy neutrinos coming from way out in the universe. These are even harder to detect. Figuring out where they come from is a bit of a mystery. Well, this week, scientists came a little closer to solving that puzzle, and all it took was a giant collaboration of scientists and a network of telescopes scattered around the world, an observatory called cube buried a mile deep in the Antarctic ice. All of this to detect one neutrino.

One neutrino made all the difference. And so we want to know what made this single neutrino so important. Well, we’re going to talk about it. Scientists were able to follow it back to its source. This is first time they could take a single neutrino and follow it four billion light years it traveled, followed it back to its source.

They traced this one way back. You know, and the source was something called a blazar. You know, what a quasar is? It’s a cousin to a blazar. How did this all work? What does it mean for the understanding of the universe and astronomy? Good question. A massive team of scientists published their results this week in the Journal of Science.

My next guest is an author on one of these studies. And he’s here to answer all those questions. Chad Finley, an associate professor of physics at Stockholm University. He’s also the former leader of IceCube’s point source analysis working group. Welcome to Science Friday.

CHAD FINLEY: Thank you very much. It’s nice to be here.

IRA FLATOW: Nice to have you. I’ll tell our listeners if they have any questions, they can phone them in at 844-724-8255, also Sci Talk, and also you can tweet us @SciFri.

IceCube is a neutrino detector. Give us a number 101 lesson, like a neutrino 101 about why this is so important, what IceCube is, how this all worked.

CHAD FINLEY: So, yeah, I mean, the goal of IceCube as this neutrino observatory is to initiate the field of neutrino astronomy. And the goal of neutrino astronomy is to do astronomy but with something different than light, to do it with these neutrino particles. And it’s in the same vein as we’re doing now in astronomy with gravitational waves, which I’m sure, you know, people have heard about before as well.

And I mean, each of these things, already starting with light, when people started to be able to do astronomy with gamma rays and x-rays and radio waves, each of those opened up new windows, and we learned so much more about the cosmos, because we could see things that you couldn’t see, you know, with optical light. And the idea with neutrino astronomy is similar, that you can learn things about the universe. You can see things. It’s like another diagnostic tool that a doctor might have to look into a patient. You know, you can investigate the universe with this different messenger.

But the problem, I mean, what makes this challenging and why we’re just starting to see things now is, what you said, that the neutrino is this ghost particle. It hardly ever interacts at all with ordinary matter. Most neutrinos will just fly through the Earth, and they don’t leave a trace. So that’s the challenge that IceCube neutrino observatory has tried to tackle is finding a way to detect a few of these, and be able to trace back where they came from.

IRA FLATOW: And so this is the first time you could do that detect one and trace it way back. And you traced it back to something called the blazar, four billion light years away. Tell us what that is.

CHAD FINLEY: That’s right. So I mean, what we’ve seen is as an association. It started with this first neutrino, and that kicked things off. And we’ve been able to see more neutrinos since then, which made us more convinced that this really seems to be the first neutrino source that we’re pinpointing in the sky.

Because we have the directions of the neutrinos. But if you look back in the sky, out the edge of the universe, there’s actually quite a number of objects that could be in the same field of view as that neutrino. So it takes a lot of work to really make a convincing association between the neutrino and the object.

And what happened here was we started about a year or two years ago from now, but about a year before this event happened in September of 2017. We had started an alert program. So that when we get this IceCube observatory, our telescope is built into the ice, into the glacier at the South Pole in Antarctica. It’s a crazy place to make a telescope. And it’s a crazy thing that it actually sits in the ice about three kilometers deep and watches for neutrino interactions that can happen in the ice.

And when we get a really high energy neutrino, the kind that we might only see two or three times a year, we send out an alert. It’s a new system that we had started. And we could send out an alert within about 30 seconds from when that neutrino interacted in the ice. It’s a public alert that goes to all observatories. In fact, anyone who wants to track these alerts can receive them and can find them on the web.

And that means that telescopes can then go point and look and see what’s happening in that spot in the sky. And when this one went out last September 22nd, the Fermi satellite, gamma ray satellite, made a connection in the same direction in the sky. They had noticed that there was a blazar that was in a flaring state. It was in an enhanced state of gamma ray emission.

And what the blazar is, we’ve known about blazars for some time, they’re one of the craziest objects in the universe. Every galaxy, we believe, has a massive black hole at its center. Our galaxy has one several million times the mass of the sun. And some galaxies have them that are billion times the mass of the sun. And most of those are relatively quiet.

But in some galaxies, those supermassive black holes are busy eating up gas that’s around them. And when they do that, they shoot out these jets of particles, these extreme jets of particles that go out to axis, sort of the top and bottom axis of the black hole.

Those jets shoot out of the galaxy. And if one of those galaxies happens to just be lined up so that that jet’s pointing at us, so it’s sort of like you’re looking into the beam of a lighthouse, then we get this intense blast. And so we can see these things out to the edges of the universe, because they’re so bright when we happen to be looking at them.

So the blazar is a special case where not only do you have a supermassive black hole that’s eating matter and devouring and shooting out these jets, but the jet’s also pointed right at us. And so this blazar had perked up a few months before, and had been in a much higher state of gamma ray emission.

And so this connection between having such a high energy neutrino and having it just lined up perfectly with this blazar, this started to make a convincing case that there’s an association that these blazars can be the source of the high neutrinos and also the high energy cosmic rays, which has been a mystery going back to 1912 when cosmic rays were first discovered. Where in the universe do high energy cosmic rays get created?

IRA FLATOW: So this is sort of this is just a quasar that’s pointing at us with those beams is a blazar.

CHAD FINLEY: That’s right. I mean, if it was pointed somewhere else, it’s still possible to see them. But it wouldn’t be as bright, and probably the particles and the neutrinos it would be creating in some of those jets would, you know, point off into some other part of the universe.

IRA FLATOW: So how many of these do you need to discover before they become useful for your, what do you call, is it astronomy? Do we still call it astronomy?

CHAD FINLEY: Yeah. Astronomy. Astrophysics. Astroparticle physics. There’s a lot of different names that people use it for.

So I mean, what we’ve seen so far, you know, there’s sort of two. Where we’ve just arrived now, this is a milestone for neutrino astronomy to be able to start to pinpoint one source is we’re hopefully closing the door, beginning to close the door on this old question of where nature’s most powerful particle accelerators, millions of times more powerful than the Large Hadron Collider or the Fermilab accelerator. Somewhere in the universe, these particles are being accelerated to enormous energies.

And the neutrinos are our tracer of that. If we find the high energy neutrinos, we can say, this is where it’s happening. And if we start to be able to answer that question, it’s blazars. That’s at least one of the sources. There may be other sources, but at least we start to have one that we can study in detail.

But that’s the flip side is that now we want to understand, now it’s the diagnostic. People have been studying blazars with gamma rays, with x-rays, with radio waves. And they’re really fascinating objects. They’re extremely variable despite being so massive. These massive black holes, they can sometimes flare up and turn off in a matter of minutes. Sometimes they flare for months. It’s unpredictable. No one really knows when they’re going to turn on or off.

And so neutrinos will give us a new window into what’s going on. The neutrinos that we see are actually about 1,000 times higher in energy than the highest energy gamma rays that people can see from these blazars. So we’re looking at a much higher energy, and we’re using these particles that are created in different processes.

And already we’re seeing some strangeness in the fact that we have this one neutrino associated with this flare, but we also looked back on our data and we saw more neutrinos coming from this source, which were hard to find before because we didn’t know how to do the analysis right at the spot. We didn’t have a reason to look exactly at this blazar and do this deep analysis. But we see that there was a neutrinos being emitted before in 2014, 2015 over a period of months.

And there’s not the same enhancement of gamma reactivity at that time. So you know, maybe there’s something different going on in these two periods of time. Maybe sometimes you see an enhancement with gamma rays, and sometimes you don’t. So we don’t know where this is going to take us, but this is a lot of new data. And this starts to become a new window into the extreme physics that we can study without leaving our comfortable planet.

And you know, we can’t do these kinds of physics experiments on Earth. We wouldn’t want a supermassive black hole in our laboratory. But we can study it where this happens in the universe. And so it’s also a way of understanding physics, understanding extreme states of matter and energy.

IRA FLATOW: Let’s go to the phones. Let’s go to Los Angeles. Stuart, hi. Welcome to Science Friday.

STUART: Hi. Yes. This one might be a little heady. But I’m curious. Can you explain the substantive nature of the neutrino maybe in relation to gamma rays and radio waves and also in relation to like quarks or smaller particles of matter?

IRA FLATOW: Good question.

CHAD FINLEY: Yeah. That’s, I mean, that’s probably the easier way. They’re fundamental particles. So if you’re familiar with quarks that make up protons and neutrons, and then you have also the electrons, a fundamental particle, and so those families of quarks and electron are the familiar ones.

And then the cousin of the electron, I would call it that, it’s sort of electrons kin, is the neutrino. So it’s also a fundamental particle, but the difference is that it doesn’t have any charge. And so it doesn’t interact. It doesn’t have any of the interactions the other particles. It doesn’t have the electric interaction. It doesn’t have the strong force interaction that holds the quarks together.

It only has this thing called the weak force interaction. And as you guessed from the name, that means it interacts very weakly. So they, most of the time, just tend to they’re created, and then they just zip off, and they don’t ever interact again. But they’re in that same level of fundamental particle as the quarks and the electrons.

And they should be created. And we know they’re created in the nuclear fusion process in the sun. They’re created in nuclear reactors. And they should be created in the high energy particle collisions where cosmic rays are born. So that’s why we can use them to do astronomy, because they’re created in other places in space, and we can look and see where they’re coming from.

IRA FLATOW: Now you say you’ve already looked into your data and found other instances.


IRA FLATOW: The more you look, do you think the more you’ll find?

CHAD FINLEY: That’s yeah. I mean, that’s what we’re hoping for is that now with this lead. We actually have an immense amount of data. But the troublesome thing is that we have an enormous number of neutrinos. We actually have close to a million neutrinos recorded, but most of those are background neutrinos.

They’re created, oddly enough, they’re created by cosmic rays hitting our atmosphere. And the cosmic rays, when they hit our atmosphere, they make neutrinos. And we detect those as well. So fishing out, you know, the rare neutrinos that are actually from space, from deep space, from billions of light years away, from this background is what’s a very big data analysis challenge.

And once you have a lead, and that started with that first neutrino that we sent the alert out from, once you have a lead, then you have an idea how to sharpen your analysis. So we went to look at other blazars we have in our data and see if we can find more evidence of these kinds of emissions.

IRA FLATOW: I’m Ira Flatow. This is Science Friday from WNYC Studios, talking with Dr. Chad Finley, associate professor of physics at Stockholm University in Stockholm, Sweden, about this new finding of a neutrino. OK.

So we have all kinds of tools in astronomy now. We have telescopes that can see infrared, ultraviolet, x-ray, gamma ray, gravitational waves, and now we’ve got neutrinos.


IRA FLATOW: How do you put all that together to get a better picture of what the universe looks like or should be or what it’s made out of?

CHAD FINLEY: Yeah, sure. I mean, that would be the most amazing thing is if you eventually will see something in every single one of those messengers. And so that hasn’t happened yet. We have the gravitational waves were first seen sort of by themselves, and then they were seen last year with where there was electromagnetic radiation at the same. So people saw light and gamma rays at the same time. But we didn’t see neutrinos at that time.

Seeing all of those things from one object may be, it would be fantastic, but it may be asking too much. But what’s happening right now is that a lot of these tools are for each sort of question you want to ask, there’s a subset of these tools that work the best. So there’s a good connection between, let’s say, looking for gamma rays and looking for gravitational waves. There’s a good connection.

If you went from this happens usually, the gravitational waves will come from things that merge. Two black holes slam together, two neutron stars slam together, that’s the kind of thing that makes a very intense gravitational wave emission. And so that’s what gravitational wave detectors are good for.

For these blazars, as far as we know, you don’t have the same sort of dynamics of two masses coming together. So it’s not expected that you would get strong gravitational wave signal from blazars. So this is where neutrinos would come in. Neutrinos together with gamma rays and x-rays and radio waves.

IRA FLATOW: Interesting. I know you’ve worked on this for a decade. Right? Did you imagine you’d ever get to this point? What does it feel like now?

CHAD FINLEY: Yeah. Well, you do and you don’t. Yeah. I mean, you certainly start with optimism, and then at a certain point, you’re wondering if you are ever going to see anything.

So I mean, there’s the moment where these things start to connect, and I think myself and others, you know, found it hard to believe. We’d never seen something like that before where things seem to fall into place. So there’s, I mean, there’s a lot of going back and checking and rechecking and trying to make sure that you really have all the evidence solid and fitting together. But I mean, it’s tremendously exciting to get to this point and start to feel that with this lead, you actually know where to go next.

IRA FLATOW: Yeah. And which is?

CHAD FINLEY: To go back. The first thing, I mean, the thing we’re doing now is going back in our data and looking for more evidence than we already have. And then the next steps are there’s a plan. We have an idea of how we can improve already the data we have with an upgrade. The main thing we’ve learned running the telescope for 10 years is how much better we can make the telescope.

IRA FLATOW: That’s always better. You can upgrade it, and see where you go from there.

Unfortunately, we’ve run out of time. But please come back and talk more with us Dr. Finley.

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