World's Largest Neutrino Telescope Buried in Antarctic Ice

IceCube, the largest neutrino observatory on earth, covers one cubic kilometer of Antarctic ice. The detector is looking for high-energy neutrinos coming from deep space. Physicist Francis Halzen discusses the decision to build the telescope at the South Pole and how we can construct a map of the cosmos with neutrinos.

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IRA FLATOW, HOST:

This is SCIENCE FRIDAY, I'm Ira Flatow. We're broadcasting from the Wisconsin Institutes for Discovery in Madison, home this week of the Wisconsin Science Festival. Astronomers and astrophysicists have traditionally, for centuries, looked upwards to the sky to learn more about the universe. We've launched telescopes into space. We have sent probes beyond our solar system to study dark matter, colliding galaxies, how the planets formed.

But there is a group of scientists that is turning its eyes downward, and I mean way down. They have built a telescope a mile deep into the ice at the South Pole. Wow. What if you have to fix that? The frozen observatory is appropriately named IceCube and it's not looking for your typical star stuff, but the elusive neutrino.

What are neutrinos? Why build a telescope a mile deep in the Antarctic ice? Well, here's a man with all the answers. Francis Halzen is the Hilldale and Gregory Bright professor in the physics department, University of Wisconsin in Madison. Welcome to SCIENCE FRIDAY.

FRANCIS HALZEN: Glad to be here.

FLATOW: What are neutrinos and why are we so interested in them?

HALZEN: Well, I think your introduction was to the point. I mean, when we know of another way to look at the sky and at the universe, you know, we have never been able to resist it. And this was probably the hardest way to do it, and that made it a challenge. And so neutrinos, you can think of of as just another way, another form of flight and we discovered them in 1956 and as they have no electric charge, they make beams just like light beams.

And so we knew we could look at the sky with them. The only problem is that we collect light with our eyes, and with a mirror, neutrinos come as elementary particles and we know how to detect elementary particles. The only problem is that to actually look at the sky in an effective way, you have to build a kilometer cube detector.

And this had never been done before and so we took us about, since the idea to now, it took just half a century.

FLATOW: Just half a century.

HALZEN: Just half a century.

FLATOW: Why do you build it a mile down beneath the ice where it's absolutely dark, right?

HALZEN: Well, as I said, it's a particle detector. If we build it on the surface, in fact it's not impossible, but it would be much more complicated because the surface of the Earth is bombarded by cosmic rays in incredible numbers and you would have to find these neutrinos in this rain of cosmic rays and it would be very challenging. If you go a mile deep, then you have to only find one neutrino in a million events that impersonate a neutrino.

So your background is much lower. Most of the radiation that can impersonate a neutrino is shielded. Now of course, the best way to shield the radiation that impersonates neutrinos is to detect them through the Earth. So our favorite way to look at the universe at the South Pole is to look at the sky above Madison.

FLATOW: Oh. It's focused on Madison? The other end of the telescope looks at Madison from the South Pole?

HALZEN: That's our favorite direction.

FLATOW: Well, you've come to the right place then. Neutrinos are really tiny, aren't they? They go through just about everything, don't they?

HALZEN: They are just like light and they almost travel at the speed of light. They have a small mass, but that's irrelevant in this talk. And so they are identical except they go through walls.

FLATOW: Oh, except they go through walls. Light's not going through the wall, but then neutrino, like light, but it is dissimilar that it can go through walls.

HALZEN: Yes, and they go through the Earth and they go through our detector. They go through everything and it's only occasionally that you can catch one, and that's why you have to build this big device.

FLATOW: So you eliminate all the interference from the other particles, you hope you get to see a few neutrinos.

HALZEN: Yes.

FLATOW: Because they go through everything and they may occasionally go through your device?

HALZEN: That's about it.

FLATOW: And what is so special about them that we go through all this trouble to try to catch them? What do we want to know about them that we don't know?

HALZEN: Well, that's a different question. In fact, I'm a particle physicist. I don't know any astronomy. I learned very little, but I'm a particle physicist and of course my interest in this and of most of my colleagues who built this detector was a challenge of building a particle detector. In fact, after we spent 20 years on it, it came quite as a shock when it was finished that we actually had to do something with it.

FLATOW: You never expected to finish it.

HALZEN: In fact, I hope that the size is as exciting as building this thing.

FLATOW: So go ahead. As a particle physicist, you like to study the particles?

HALZEN: Well, yes. In fact, we have a big international collaboration and about one-third of my colleagues are only interested in the neutrinos themselves. They don't care where they come from in the sky.

FLATOW: Someone else's problem.

HALZEN: Yes, exactly. And these neutrinos have about a thousand times the energy of any neutrino we have ever produced with an accelerator. So this is a whole other aspect.

FLATOW: Another flavor of neutrinos? In other words, the ones you build in the accelerators, they're just puny compared to the ones you...

HALZEN: Exactly.

FLATOW: ...can detect coming from outer space?

HALZEN: Exactly.

FLATOW: And in fact, neutrinos are very mysterious for a while. There were a lot of different kinds of neutrinos?

HALZEN: Yeah. And so that's the particle physics side of this and they oscillate and do all kind of interesting things. And so if you look at particles in a different energy range, you usually discover something new about them, and that's of course what we are after also.

FLATOW: You know, they talk about all this dark energy things that inhabit most of the universe is made out of this dark energy. Could it be neutrinos?

HALZEN: No.

FLATOW: Okay, we've eliminated...

HALZEN: Actually this was an excellent idea that died about 20 years ago.

FLATOW: I know. I'm always up there on the times with the latest thinking.

HALZEN: On the other hand, you must by now have the idea that we just built this thing because nobody had built it before, and when you build something that nobody's built before, you usually discover something. And so it's basically a shot in the dark. But one aspect that's not a shot in the dark, which is related to your question, is that I'm now going to tell you what another third of my colleagues are doing. The last third is actually looking at the sky.

But one of the goals of this experiment, and in fact that's why we got our first meager funding 20 years ago, was because it could see dark matter.

FLATOW: Ah. So you went under the we're going to find dark matter.

HALZEN: That's right. And we are still looking, like everybody else. Of course, dark matter exists. It's real as the table.

FLATOW: We just can't see it though.

HALZEN: We just don't know what it is.

FLATOW: It holds those galaxies together.

HALZEN: So we have found the hardest way of seeing it. Dark matter, over time, as it interacts gravitationally, has been trapped in the sun and so the sun is actually a reservoir of dark matter.

FLATOW: No kidding?

HALZEN: And so the dark matter in the sun is concentrated in the center and when the dark matter particles collide, they make stuff that we know and some of this stuff will decay in neutrinos. So we are looking for neutrinos coming from the center of the sun that are made by dark matter and that would be our way to A) detect it and then B) maybe learn something new about it.

FLATOW: How long does it take a neutrino to get from the center of the sun to the outside of the sun and to Earth?

HALZEN: It comes immediately.

FLATOW: Immediately.

HALZEN: A photon, we see light, we see from the sun, has spent like 100 million years working it's way out.

FLATOW: It takes 100 million years for light to get from the center of the sun out?

HALZEN: It's something like that. It's a very long time.

FLATOW: And a neutrino pops out?

HALZEN: A neutrino comes at the speed of light.

FLATOW: Wow. No wonder you want to study this stuff. This is very mysterious. How many kinds of neutrinos are there?

HALZEN: There are three types.

FLATOW: Three types?

HALZEN: Yes. And so, but we take any type.

FLATOW: You build it, the neutrinos will come.

HALZEN: You must know that (technical difficulties) interested in for any of the subjects I have mentioned up to now, come about at a rate of 10 to 100 a year.

FLATOW: That's it?

HALZEN: Yes. So we know them all by name.

FLATOW: You've named all the neutrinos you've discovered?

HALZEN: Yes.

FLATOW: Give me a couple of names.

HALZEN: We named them after Sesame Street characters.

FLATOW: So you have a Big Bird and you have...

HALZEN: So after we ran the detector for a year, we looked at what we had and so we found two really interesting events and we called them Bert and Ernie.

FLATOW: Wow. Who else?

HALZEN: And we find the more energetic one, you know, what it will be called.

FLATOW: Cookie Monster?

HALZEN: No, Big Bird.

FLATOW: Big Bird. Have you got Cookie Monster in there too?

HALZEN: Already been used.

FLATOW: Yeah?

HALZEN: No, I've lost track, actually, but it's not funny because, you know, our block of ice in the South Pole has moved them 5,000 light senses in them and they also have each a name. And you would think you would number them one to five thousand. It's much easier when you work with a system like that. Occasionally we want to say it was, you know, sensor number 5120. It's much easier to remember them, we know them by name.

And it's the same for our neutrinos.

FLATOW: So this is a Bernie over here and Gilda there and it looks like Leo's not picking up anything. So you actually, you anthropomorphize, I guess, or whatever...

HALZEN: So we've got two of our post doc who named them and they promised me that they don't spend more than one hour a week on it.

FLATOW: On naming them.

HALZEN: Yeah.

FLATOW: This is fascinating. It's fascinating about neutrinos. Thank you very much. Thank you very much, Dr. Halzen, for taking the time to be with us today. More than you've ever wanted - and now can we name the neutrinos ourselves if we get new ones? Can we have a contest? We'll talk about it.

HALZEN: We'll arrange it.

FLATOW: We'll arrange for it. Okay. Francis Halzen is the Hilldale Gregory Bright professor in the physics department at the University of Madison in Wisconsin. We're going to come back after the break and talk about cheese and sausage. A lot of names for those, I'll bet, so stay with us. We'll be right back after this break.

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