-  All right, good evening and welcome to a guest presentation at Space Place. Tonight, I'm very pleased to introduce Professor Ellen Zweibel, who is in the UW physics and astronomy departments. She's a plasma physicist, among other things, and won a major award last year, the Maxwell Prize of the American Physical Society. So I have imposed on her to come talk to us tonight, and her title is "The Fastest Particles in the Universe." So, Ellen, thanks for coming.   (applause)    -  Thanks, Jim. I'm not new to speaking, but I am new to television. So I apologize in advance for any awkwardness. The story of the fastest particles in the universe, or cosmic rays, is really a story of curiosity and where it goes, but, where it goes, as I said, but before I launch into the main part, I just want to acknowledge my wonderful colleagues here at the UW. I couldn't work without them. So everybody from undergrads,   (inaudible)   and Cory Cotter, to the most senior faculty, yes, and in between, our graduate students and postdocs. And they really do so much to make life a joy. So the story really begins back in the 18th century when Coulomb discovered that air is partially ionized. So I've indicated that schematically here with some little pluses and minuses, and everything else is neutral atoms and molecules. And we expect hot gases to be ionized, but we don't expect air to be ionized. So part of the art to science, and there really is a lot of art in science, is knowing when something is interesting and really worth thinking about or whether it's just a detail that you can discard. Just kind of an extraneous detail. And, in this case, they made the right decision, that this was really something worth thinking about. In 1896, radioactivity was discovered, and so this led to the hypothesis that the ionization of the air is caused by high energy particles emitted by radioactive elements in the Earth. So that was the hypothesis, but then, in 1911-1912, an Austrian physicist, Victor Hess, went up in a balloon to test this idea. So if the ionization is produced by particles coming from within the Earth's crust, it should drop off with height. But what he showed instead is that it increased with height. And then he thought, well, maybe these are particles coming from the sun. So, fortunately there was a solar eclipse. Sometimes you have to wait a long time, although there will be one this summer, and he went up during a solar eclipse, and he still found that the ionization dropped off with height. And so he concluded that whatever was doing the ionizing, it came from the sky; it didn't come from the ground. Now, another sort of universal thing in science is that when you think of something, someone else has thought of it long before. And, in fact, this idea had been tried previously with the Eiffel Tower. But the Eiffel Tower is not tall enough to show this effect. You really have to go up several kilometers. And Victor Hess got the Nobel Prize in physics in 1936 for this work. So the name that he gave this ionizing radiation is one of those sort of descriptive names that you give something when you don't really know what it is, like adding -itis to a body part.   (laughter)   So, okay, cosmic ray, and so the question is: is this high energy light or is it charged particles? And to answer this question, studies beginning in the 1920s showed that cosmic rays arrive preferentially from the west. And so the idea is something like this. Here's the Earth. And here is its magnetic field, like the magnetic field of a bar magnet. And if we kind of zoom in close up to these field lines, charged particles are gyrating around them in kind of helical orbits. And the size of the orbit depends on the energy of the particle, and the direction of circulation depends on whether the particle is positively or negatively charged. Now, I put this all on one slide, but it did not come easily. There was a 10-year effort on the part of Arthur Holly Compton really to go all over the world and to work at sea level, to work on mountaintops. He rounded Cape Ho  A couple of decades ago, we developed, really, a new window on cosmic rays with the ability to detect cosmic gamma rays. And so it turns out-- So this is a gamma ray view of a supernova remnant, and it turns out that supernovae remnants are just lit up in gamma rays. Here's the Large Magellanic Cloud, the small dwarf galaxy that you can only see from the southern hemisphere, and here's a region where a lot of stars are forming and supernovae are going off and it's all lit up in gamma rays too. And what this means is that 70-80 years after Baade and Zwicky made their intuitive suggestion, we've shown, really, that they're at least partly right. Cosmic rays do have something to do with supernovae remnants, the origin of cosmic rays. So another way in which we can detect cosmic rays relies on a sort of sonic boom phenomenon in cosmic ray, in light that we call Cherenkov radiation. So here's an airplane breaking the sound barrier, a very sleek-looking airplane. And what Cherenkov radiation is about is that the speed of light in water is slower than the speed of light in the vacuum. And so if a cosmic ray goes tearing through a tank of water, it's not going faster than light in a vacuum but it's going faster than light in water. And so the radiation that it emits can't spread out in all directions from it the way it can for a normal particle. It gets concentrated in this cone. And this is called Cherenkov Radiation, and it's similar to a sonic boom. And so the UW is a partner in a high energy cosmic ray detector called HAWC is a Cherenkov telescope. It's being built in partnership with many institutions, primarily Mexico. So it's near Puebla. And everything you see here is a giant water tank. And when cosmic rays break up in the atmosphere above HAWC, their particles come sort of streaming down and they set off Cherenkov Radiation in all these tanks. And by looking at the timing when the Cherenkov Radiation is seen in the tanks, you can estimate how much energy the particle had and where it came from. And I'll be showing you some results from that a little bit later on. There's also theory. That's what I do. Again, many, many players. This is a former graduate student of mine, Tova Yoast-Hull. And I just love this picture because look at this blackboard and she looks so exhausted. But she did get her PhD, and she's now a postdoc at the Canadian Institute for Theoretical Astrophysics in Canada. And this is the kind of thing we calculate. So I've mentioned these unique facilities. This is the largest HAWC-like telescope, Cherenkov telescope. This is called the Pierre Auger Observatory. It's 1660 water tanks spread out over this large area of western Argentina. And this is an artist's conception of particles from an air shower as a cosmic ray breaks up, setting off these different water tanks across 3,000 square kilometers. So this is the largest cosmic ray detector ever built. But, in addition to these unique facilities, there are some not unique facilities. You can turn your cell phone into a cosmic ray detector. So my colleague in the physics department, Justin Vandenbroucke, developed this project called DECO, Distributed Electronic Cosmic Ray Observatory. Your cell phone camera, so you have to leave you cell phone on, and it can detect the particles produced by air showers. And if you download two apps, the data will be automatically uploaded and sent for analysis, and you can become part of a high energy cosmic ray detection project. All you need is a smartphone, which I ironically do not have.   (laughter)   Okay, so now let's talk about where the highest energy cosmic rays come from. And this is going to be-- We're not going to-- We may not get to the answer, but we'll go through kind of a process of elimination. So here's a view again of what our galaxy probably looks like. So this is, of course, not our galaxy, but it's the nearest galaxy similar to ours, the great nebula in Andromeda. So a spiral galaxy. This is how we see the galaxy from where we are, namely inside it. So the galactic plane, so most of the matter  So we can go a little further. So here's another international project in which Wisconsin has an important role. In fact, we're leading it. This is the IceCube Observatory at the South Pole. And these are, this is again an artist's conception of deep holes that are drilled in the Antarctic ice that are lit up when high-energy particles come through. So the IceCube Observatory has seen a lot. It's detected a lot of cosmic rays, and it's detected high energy neutrinos. And this is what the sky looks like in high energy neutrinos. So purple is a neutrino. There are so few neutrons that they can be given individual numbers, although they haven't been given names. Are the sources of these neutrinos the same as the sources of these UHECR? Well, they share the same list of "nots." They're not concentrated to the galactic plane. They're not preferentially coming from active galaxies. And for now, that's all that we can say. So the study of cosmic rays began with the observation that air was ionized. So if we kind of bring it all back home and say, okay, there are these high energy particles that are impinging on the atmosphere and breaking up into this cascade of other energetic particles. Do they do anything? Do they do anything noticeable to the atmosphere of the Earth?  And there's pretty good evidence that they can trigger lightning strikes by kind of pre-ionizing the air. So the same ionization of the air that Culham discovered back in 1785 is sort of providing a conduit for lightning strikes. And here's a dramatic lightning-- This is the best picture I found. This is actually a little town in Belgium. But lightning is the same all over the Earth. And so a fair question would be: Do cosmic rays affect the chemistry of the atmosphere? Do they affect the climate? And this has been thoroughly studied because cosmic rays are always there, and the answer seems to be a little bit but not very much. So they are primarily a cosmic phenomenon, and that's, I think, where they're going to stay. So, what's the summary? So cosmic rays are an extreme form of matter. The highest energy particles that we've ever detected. Much higher energy than we can make in any accelerator on Earth. And there's no single person who made a decisive contribution to the study of cosmic rays. It's just been this vast web of human effort. And it's been both pure science and applied science. The creation of new plastics. The creation of counters to measure to faint Cherenkov Radiation. Contributions from all sides that have allowed us to understand them as well as we do. But there's so much more work to be done.  So, that is the end. This is how you can find me. And thank you.   (applause)