 We're going to start this off with a quiz, and if you know the answer, you can yell it out. You can keep it to yourself, whatever you like. But I'm going to, I'm going to, I don't think I need this. If you can't hear me, be sure to tell me. But I think I can just show you that. That was the same thing that David said, that's right. Okay, yes, thank you. I'll give you three seconds to come up with the answer to this question, okay? And I know some of you will have seen this before in one of my classes, but, and we're going to point it up there, I think, yeah. What's the car doing? Two, one, zero, turning. You're saying turning? Any other guesses? If you're guessing, if you're guessing that that balloon's leaning forward, if you're guessing that the car is slowing down, because everything slides forward when you slow down. No, it's not. You can tell from the vertical string, the stretch string right there, that that balloon is lighter than air, right? The heavier air is being pulled down by gravity, and that's forcing the balloon upward. We call that buoyancy, right? That's pretty, pretty normal stuff. You can also tell that that string is leaning forward a little bit. So that means that the balloon is being pushed forward because it's lighter than air, and that means the heavier air is sliding back, because the car is, in fact, accelerating in this case. All the massive stuff slides to the back. Anything lighter than air is going to slide forward. If that car takes a turn, I think you were getting to this, if that car were to turn right, all the heavy stuff slides left, as we all know, toward the outside of the curve, that balloon would lean into the curve. Next time you're behind a birthday car, you know, kind of fun to watch that. The next question is really easy. Why is the balloon lighter than air? It's filled with helium. I think we all know that. It's filled with helium. Could be filled with hydrogen. We learned that lesson a long time ago. It's pretty dangerous stuff, so we fill them with helium, and that makes it lighter than air. So helium, it's the second most abundant element, chemical element in the universe. It's been here since the very beginning of the universe. The Big Bang produced some helium. It is the, let's do this one. It is the second lightest and second simplest atom on the atomic scale, on the chemical scale of the elements. Hydrogen up there is the very lightest and simplest. Lithium is third, and there's helium up there. So you think, okay, most abundant element in the universe, been around forever, literally forever. So we know all about helium, right? Probably known for thousands of years all about helium. Well, take a look at that chemical chart of the elements from about 150 years ago. So there is hydrogen right up there. Make sure I press the right button, and it skips right to lithium. You know, helium is not there. A lot of elements aren't there, but helium, really helium, is not on that chemical chart from about 1870. Hmm, that's weird. So maybe nature is urging you now to ask a question. How do we identify these elements anyway? You have a glass tube, maybe filled with gas, and you want to know what's in the gas. What chemical elements is it made of? Or you have a glass box or something like that. One of the easiest ways to tell what it's made of is to heat it up, make sure it's in the gaseous state. Wrong one. And heat it up, send that light until it glows, basically. It's called thermal emission of light. Tell it glows because it's so hot. And that light comes out. I can't use these buttons either. I'm not used to these things. The light comes out. Send it through something like a prism to spread it out into a spectrum. And what you'll find, if it's in the gaseous state, whatever's in there glowing, is in the gaseous state, it only emits light in certain colors. That's the characteristic emission spectrum as we say of sodium right up there. Only certain colors come out of the whole spectrum. Hydrogen's right below a calcium and so forth. So you can look at those particular colors that are emitted and identify what's inside that glass tube or whatever and inside what's in your gas that you're talking about. We call that an emission spectrum and we call those emission spectral lines. You could also take that gas and cool it down and send a full spectrum of light through it. So let's get to that one. So send a full spectrum through the gas in a cooler state. Send it through a prism again to spread it out. And this time what you'll see is the original spectrum that you sent in there with certain colors missing, absorbed by the atoms in the gas. They're the same colors that are emitted when it's hot. Same ones are absorbed when it's cold. Now I know most of you know this stuff already. I prepared this for people that came in from BCA and stuff like that, Burlington City Arts. Anyway, so once again, you can look at a coal gas and send light through it and identify what's in the gas or you can heat it up and identify what's in the gas. In particular, look at a couple of spectral lines that we'll be mentioning later. Two spectral lines very close together of sodium right up there. They're called the D lines. Those are the original what are called Kirchhoff designations of spectral lines. Capital letter D was for the sodium lines up here. And the hydrogen alpha line, which we see all over the universe because there's hydrogen all over the universe basically, right there, that red line right there. I got a circle around it which is kind of hard to make out with this lighting. Will that go totally off or not to that thing up top? It'd be great if we could just, yeah, really kill it. You can kind of make out that circle now. So the hydrogen alpha line and red, we see it all over the place also. And of course, as I think all of you know, I don't have to say it, but the color is controlled by the wavelength of the light, the longer wavelength light we perceive as red and shorter wavelength light we perceive as blue. So what we're really doing is measuring the wavelengths peak to peak of those light waves. Stellar spectra, stars that are living at least like the sun, they give us an absorption spectrum. Like that right there. They give us an absorption spectrum, a full spectrum with certain colors absorbed or missing certain wavelengths absorbed out that hopefully that'll seem weird to you because what I just said was when you heat up a gas you get an emission spectrum. You get that kind of a spectrum. How come we get an absorption spectrum from stars which we know now? They didn't know this 150 years ago, which we're going to be talking about, but we know now they're big balls of hot gas. Why don't they emit emission spectrums? The interior of the star right in here, the inner part, about 90% of the inner part of the whole star, all the inner region, the conditions are so extreme, the density, the temperature, the pressure, gas pressure are so extreme that a continuous spectrum is emitted from that interior region. But that light has to pass through a thin layer above it, which we call the chromosphere, and there the conditions are a lot cooler. Here's the chromosphere in orange right here. It's cooler, lower density, lower pressure, and so that's where those particular spectral lines, those particular wavelengths of light are absorbed out of that full spectrum. So what we get when we look at the sun, or almost any star really, what we see is a spectrum like that. It's an absorption spectrum, as I just said a minute ago. This in fact is the spectrum of the sun right here that you're looking at. Enough background physics. Let's look a little bit, I think, next. A little bit of the history of eclipses, not a whole lot. Really, you can take this back to 3000 B.C., if you want. Even further, the history gets a little more variable depending on where you look for it. So I'm going to start in 71 A.D. when apparently, I think, the first real report of a glowing region around the sun during an eclipse was recorded, a report was recorded by Plutarch in 1781 that when the sun was completely eclipsed, you could see this bright region around it. A couple of hundred years later, Julius Maternis first described prominences. These, oops, yeah, these bright, right there. These little bright spritz of red light that rise up off the visible surface of the sun during an eclipse, when you're looking at an eclipse, you can see those without an eclipse, but it's really hard. I'm sort of wondering, in fact, about that report at all from so early, usually you need a telescope to see those, but apparently he didn't, there were no telescopes back then. And then finally, early in the 18th century, a Spanish astronomer gave that glow, that glowing region around the sun a name, the crown of the sun, the corona of the sun. And nobody knew then whether that was part of the sun or whether that was part of something between us and the sun and the space between us or whether that was in our own Earth's atmosphere. We didn't know. We know now that that corona is always there. Geez. Gotta get used to this thing. They've got a little teeny thing here for the laser pointer. I should have brought my own. We know now that the corona is always there. I think you all know this. It's just that it's much dimmer than the daytime sky or the disk of the sun itself. You have to block that disk out, make the sky dark during a total eclipse, and then you get to see the corona. Hopefully we'll get to see the corona on April 8th and maybe we'll see some solar flares. The sun is pretty close to maximum right now, so good chance to see some of those. They're called flares or prominences. I'm going to use the word flares just because it's a lot easier to say than prominence every time. So we're going to skip up now, as you just saw a minute ago when I hit the wrong button, to 1867. 1867. And at 1867, nobody was photographing spectra of stars yet, even the sun. So everybody, every astronomer that made, that looked at a spectrum of the sun through a telescope and then through a prism. They used prisms back then. They would see something like this. They'd see all the absorption lines against the continuous spectrum of the sun, and they would simply draw what they saw. They wouldn't try to photograph it. That involved not only a lot of knowledge about astronomy and spectroscopy, but it involved a lot of knowledge about photography. You had to coat your own glass plates and all this kind of stuff. So here's a typical absorption spectrum of the sun is drawn out around 1967. A lot of those astronomers, of course, they all knew really about the corona. Everybody knew about the corona then during a total eclipse, and they knew about these little red spresses or flares that we're going to call them rising off the surface. They knew about that too. And they thought to themselves very cleverly, we can use this spectral analysis to figure out what the sun is made of, the disk of the sun. What about the corona? Again, maybe part of the sun, might be part of the Earth's atmosphere, we don't know. Let's get a spectrum of the corona or at least of these little red flares sticking up. Let's try to get a spectrum of those and analyze that to see what they're made of. 1867 or so. Of course, sky's too bright, sun's too bright during the daytime. Otherwise, the sun is down and not even in the sky. So they knew they needed a total solar eclipse in order to do this. I keep trying to remind myself to lift this mic, okay. Three of those astronomers were particularly determined to do this in 1967. They said, okay, time's up, we're going to do it all separately. They were not working together. Here's where the story usually is gotten wrong by history. So if you already have an impression of who did this work, you might have it wrong. Of course, I might have it wrong, but I think I got it right. I'm going to introduce them to you. Here's the first one, Norman Lock here, a British public servant actually who is an amateur astronomer really, very experienced with a telescope and very experienced with a spectroscopy that's best known for being the founder of Nature magazine. There's page one of the first issue right there which he wrote, I believe. And of course, that's this month's issue right up here. Nature is still going strong, very good journal of science. He was the editor for the first 50 years. Think about that. 50 years. Even to me, that sounds scary to be the editor of any magazine for that part of your life. So that's Norman Locker. He's British. He was able to live just outside London, West Hampstead, and we'll get back to him in a minute. Then there is, oops, sorry, I've got to click it up there and do it once. Jules Jansen, a French astronomer, a member of the French Academy of Sciences. Usually you find this wonderful photograph of him over here years later. This is a photograph of him more in that time period when this work was being done. I think around 1870, so maybe three years before this when his hair wasn't quite so great, something like that. We'll move on. The third one is Norman Pogson, British also. And Norman Pogson at that time was the director of the Madras Observatory in that condition when he inherited it basically, the control of it. The telescopes were a wreck. They didn't have much equipment. It was really kind of a miserable place to be. But he had to talk his way into being that director because nobody liked him basically. He was a very unpopular guy and that's because he was brash. He was insulting to people, constantly insulting to people. He was racist. He would speak very disparagingly to the native population of India where he was living. Really awful remarks and maybe more important than just the personal feelings was a lot of professional astronomers would tend to push him to the back of the room when it came to getting recognition for one thing or another. Well, they all wanted to do this. They all wanted to look at an eclipse and they all had sort of the same rough idea that maybe during an eclipse you'd be able to zero in with your telescope and with the spectroscope on the back of it to spread the light out and they're all using a prism for that to spread the light out. Maybe if you could just get the spectrum of one of these flares or maybe the inner hemisphere, I'm sorry, the inner corona you might be able to learn what they're made of. Is that really made of the same stuff that the rest of the sun is made of? The sun are not. And they thought, well, that light won't have passed through those upper layers, cooler layers of the sun. They were kind of guessing here. We know this now, but they didn't know this very well back then. Maybe that light would come straight to us without passing through the chromosphere. They didn't know about the chromosphere, but they guessed pretty well. And maybe what we'd see is the spectrum of a hot gas and emission spectrum. You'd be watching the spectrum coming out of your telescope and spectroscope and you'd see this absorption spectrum, both I've showed you before. And then just as the eclipse goes total, you would see that just transitioning into an emission spectrum. That background, the bright background spectrum would disappear as the disk of the sun disappears behind the moon and what would be left would just be these bright emission lines coming through. Pretty beautiful thing to see. It's transitioning forward, switch, slowly shift back again when the eclipse ends. So they had that thought. So they needed an eclipse, a total solar eclipse. And I'm going to point it up here. And here were some of the eclipses available to them, total solar eclipses from 1867, 68, 69, 70. They didn't want to wait that long, frankly. And look at where they had to go anyway. They weren't going to go to Patagonia and Antarctica in 1867. They didn't particularly want to go to Siberia or Canada or New England either. And they didn't want to wait that long. So they all chose this one, 1868, August 18th, 1868 is the eclipse they're going to zero in on. Take a look at one number that's up here. That one right there, duration six minutes and 47 seconds is the time of duration. That's over twice as long as we're going to get to the eighteenth weather or no weather. So they knew this was a good eclipse to look at. This was going to be a really good one with lots of time to figure out what's going on. I know I'm pressing the right button. But it's not react. There we go. There was another amateur astronomer, a very active astronomer in India at that time, but he was a military man. He was a British military man and he was on the East Coast of India. Once again, once again, my fault, east coast of India, and he was there with his telescope and he didn't bother with the spectrum of the sun. He didn't spend any time trying to get a spectrum of the sun. He just wanted to photograph it, and he was pretty skilled at that, and this is what he got right there. There's the photographic eclipse that he got. There's the moon blocking the sun and look at all the activity, all the activity around, above the visible surface of the sun, lots of flares, lots and lots of flares. Pretty neat. Pogson, Norman Pogson, Norman Pogson, try it again. Yeah. He cobbled together a team of railroad workers to help him get his equipment down to the coast of India, down to the east coast of India. So he also went to a very similar place where a tenant was, right down in here, east coast of India. He went to Mazulupadnam. That's the original name of it. It's been changed a little bit today. And he set up his telescopes, maybe three or four of them, all of them kind of broken down. He really didn't have much to work with, and he didn't have a spectroscope. He had to borrow a spectroscope from England, had to be shipped to him in order to use for this eclipse. Somehow he talked somebody into doing that. And this is what he saw. He saw the usual absorption line spectrum of the sun that we talked about before, before totality, and after totality. He saw that spectrum, which we all expected to get. Those again, those of you who are in physics, just about everybody in here, those of the Fraunhofer designations of the known spectral lines at that time. He did then see that bright spectrum, background spectrum, disappear as totality took place. That all went away, and he saw basically some emission lines, some bright emission lines. Whoa, very happy guy. You know, they all hoped this is what they were going to see. And in fact, they did. He also, I've got to point it up here, he noticed in particular that spectral line right there in yellow. It's just a little bit toward the blue end of the spectrum from the D lines of sodium. He noticed that line. He said, I don't recognize that line. It's very close to the sodium D lines that we talked about before, but it's just a little bit off. He said, I don't think it's a sodium D line. Maybe it's a third line of sodium that we didn't know anything about it. He wasn't so sure. So he talked that up. My understanding of this is he talked that up verbally that day with people that were around his telescopes and stuff. He said, I've seen this weird yellow line. I don't know. I don't know what it is. And he wrote it up later in a report using those words, but he didn't report this officially to a broad audience until a couple of years later. He took the news back to his observatory in Madras, but nobody really cared much about what he was talking about, probably because, as I said, they didn't like him so much. So he saw that line. Now, Jules Janssen from France, he got funding from the French Academy of Sciences to go to India to get a spectrum of that eclipse, and he did. And he saw it the same thing. I mean, he saw a usual absorption spectrum of the sun slowly transitioning to that emission spectrum when totality began of the sun from one of the flares rising off the sun. But he didn't see anything weird. He thought, well, I know these spectral lines. No big deal. And so he didn't pay any attention. He didn't notice that yellow line, which didn't really belong there that nobody could recognize. I believe, though, that he heard, this is only about 40 miles away, Mazullah Panam and Guntur, where Pogson set up, aren't very far away. And I'm pretty sure that Janssen heard from word of mouth that this strange yellow line had been seen and nobody knew what to make of it. So he thought to himself, I'll bet that I can focus my telescope down without an eclipse just in normal conditions. I bet I can focus my telescope down just onto a solar flare. There are so many right now around the sun. He had seen them the day before. I bet I can focus down one of those flares and get a spectrum just of that flare, at least a bright enough one from the flare that I'll be able to make out those bright spectral lines again the way I did the day before during the eclipse. He did the very next day. He went out to the telescope again. There's a sun up in the sky, no eclipse. Focused in on one of those flares and saw the emission spectrum. And he saw the strange yellow line that he had heard about, the unidentified yellow spectral line. So he put together a note, a report, really, to send back to the French Academy of Sciences in Paris. And he had a friend post it for him. He went on right away the next day to the Middle East because he had some other experiments he wanted to do there. So he didn't stick around in India. And he had a friend ship off that report to the French Academy of Sciences. And I do mean ship. He sent it by ship from India to France. Our friend Norman Lock here, he didn't have the funding to go to India. He didn't have the inclination to go to India. He wanted to stay home in England. He had a job after all, and it wasn't astronomy. That was really his sort of side thing. But he and a friend had been all summer of 1968, had been working our way. They thought maybe they'd be able to see the emission lines coming from around the sun if there were such things. And it involved, what it involved was spreading the spectrum, the background, bright background spectrum of the sun, spread it out much, much further. This is called spectral dispersion. So from the red end to the blue end, instead of being like that, it would be like that, much, much wider. That means that that bright background spectrum is going to get dimmer and dimmer and dimmer, as you spread the light out more and more. And he reasoned that then, if there are bright lines in there, we'll be able to see them more easily. I thought about this. I'm not sure the reasoning was very sound. I don't know. But it worked. And his friend, the instrument that he needed that they were designing, was a kind of super spectroscope. The light comes in from up here and hits this prism. And that prism bends the light a little bit, as prisms do, and spreads it out a little bit into a fairly narrow spectrum. You can't get much spectral dispersion, much spreading of the spectrum out of a single prism. You just can't do it. It's the angles of the prism. You can't do it. We use something else. These days, called a grating. In a grating, you can really control the dispersion. Grating is basically a the simplest possible hologram to spreads the light into a spectrum. You can control that spectrum really well. So the light comes in. Sorry about that. There he is on his, kind of using all this stuff. The light comes in. And goes through the first prism, gets bent and spread out a little bit. Second prism spreads it out more, bends it some more, more, more, more, more, more. So each time it goes through a prism, that spectrum gets spread out further and further and further. And it's going to go around a spiral as it does that. And then finally, the light comes out in this direction. This is the viewing tube over here, which normally would be rotated all the way around to there. I think they put it here to fit it into the photograph, basically. So his friend was building this for him in the summer of 1868. He was making this seven prism spectroscope. It's a beautiful thing, really. And all this brassware and everything like that. The friend died late in the summer in mid-construction of the spectroscope. So somebody else had to finish it. Somebody else had to make this thing, get it done, basically, and ship it off to Lockhear, deliver it to him in West Hempstead. And they did. They delivered it on October 18th of 1868. He got it. There he is. That's Lockhear. There's his, I think it's a six inch, four and a half inch aperture telescope. Light comes down here, goes into that input tube, goes around the prisms, comes out on the viewing tube right here. And he has a way to rotate that rig just very slightly to make the spectrum move back and forth a little bit so he can measure the exact wavelengths of the spectral lines that he sees. He wasn't planning to use this in an eclipse. They didn't have any eclipse then in England. It was in India, half a world away. So he wasn't planning to use it. And it wasn't a problem for him that it came late. But two days later, after he received it, he got it on October 18th. And two days later, he's got it mounted to the back of his telescope. He's got it out in his backyard. It's a clear day, miracle in England, but it's a clear day. And sure enough, it works. He gets a spectrum of the sun and draws, once again, not photographs, but draws what he is seeing. And what he draws is the normal absorption spectrum of the sun that he saw before and after the eclipse. There are the usual suspects here, hydrogen lines, hydrogen lines, a couple of magnesium lines, sodium D lines are right there. And we're at another hydrogen line out here and there. And then above that is the emission spectrum that he was able to make out just from that hot solar flare rising up off the surface. And he was able to measure those wavelengths. And sure enough, he saw that the two, the D lines of sodium in nanometers, their wavelengths are 589 and 589.6. They're really close together. It's hard to make them apart. Tell them apart. And here's that mystery line of 587.5. Fairly different, really, from those. He noticed that. The next year, he wrote it right up in Nature magazine. I think the very first issue of Nature magazine, he wrote it up as an article. But he said, man, that's interesting. That mystery line, I don't think he had heard about it from the India two months before the India eclipse. I'm not sure about that. I don't think he had gotten any word of it. I don't know how he could have, really. And he said, that really is big stuff. So he wrote a report to the French Academy of Sciences. French Academy of Sciences was the place to report things, and that was it. So he wrote a report. What actually happened, I think, was a friend of his who knew about astronomy visited him that very day, later in the day, after he had seen that line. And he told them all about it. And he said, we've got this weird spectral line. Let me jot down some notes. If you would write them into report, take them over to Paris for me. Great. And hand it over to the, deliver them to the French Academy of Sciences. I suspect that his friend spoke better French, maybe. Maybe Locker didn't speak any French. I'm not sure why he had this friend of his send the report over. So two years later, three years later, I guess it is, he was the one to suggest this line had been seen a number of times then. And he was the one, Locker, to suggest that this might represent a new element, a new atomic element that we don't know anything about. Maybe it only exists in the sun. Maybe it exists everywhere. We just don't know. But we've seen it only in the sun. And therefore, he named it Helium after Helios for the sun. And that's why it's called Helium. Keep in mind to point this in different directions. So the final history here is Poxton saw it first. But nobody cared. Nobody paid any attention. He didn't really report it. Two years later, I think it was two years later, he finally wrote a report sent it to the British, the Royal Academy of Sciences. And they published it. The Royal Academy published Poxton's report two years too late. And they printed three copies of his report. There's no internet. Nothing like that. They printed three copies. That was it. Poxton was furious, as you might imagine. So that was too bad. So he maybe should get credit for seeing it first. Maybe, maybe not. Janssen's report was delayed two months getting to Paris by ship. And it arrived just after Locker's. I think it was one day later that it arrived. Locker's report just had to cross the channel. No big deal. And Janssen's came in a little later. So the French Academy of Sciences at the next meeting, which was October 26th, just a few days later, really, 1868, there are the two pages of the proceedings. And you can see right here is the friend of Locker's right here reported what he saw in that emission spectrum, different lines at the Frownhofer sea line, the F line, and then this weird line that doesn't quite fit. Now, my friends are getting pretty bad these days. I did struggle through this, but I think I got it right. And here is Locker's statement that, in fact, he gave all that information to his friend that day, and he did report it correctly. And Locker, I think, drew that diagram of the surface of the sun, the so-called photosphere, with that flare that he got the spectrum of sticking up off of it. And then it's followed pretty much immediately by Janssen's report. So they read out Locker's first, they read out Janssen's second, even though Janssen Locker was the last one to actually see the spectral line for what it's worth. And that's kind of what the French Academy said. They said, for what it's worth, they gave credit for seeing the discovery of that spectral line. They gave credit to both Locker and Janssen, both of them. So now history has it that both Locker and Janssen get credit for the discovery of helium. That's kind of a stretch. Janssen didn't go that far at all. In fact, when Locker suggested that maybe this is a new element, a couple of years later, he was basically laughed out of the room. Physicists and astronomers just said it's nonsense. But it turned out he was right. So they both got credit. They became good friends later. They hadn't met before. They met some years later, became great friends for life, really. They saw each other often. And it wasn't for another 17 years that helium, that spectral line, really, was detected on Earth. It's only produced, helium's only produced below the crust of the Earth. Nuclear transmutations taking a place under the crust of the Earth. One of the side effects of those, one of the byproducts is helium. So helium's only produced deep in the Earth. It escapes in volcanic fumes into the atmosphere. And because it's so light, such a light element, low mass, it escapes our gravity pretty much instantaneously. It just goes right up and escapes. Hydrogen escapes even faster. But we have oceans full of hydrogen, right? Playing that around. We just don't have much of a source of helium. So the moral of the story today is don't waste it. Basically, I mean, we all love helium balloons and we love talking like Donald Duck, you know, and stuff like that when you inhale it. But you're basically wasting helium. It's very important to industry. It's very important to medicine, helium is. So that's that. Let's jump forward a little bit into the 20th century and there's our friend Albert. And the weirdness of relativity theory. Most of you will be familiar with this already. Again, I was kind of prepared to talk to people who didn't know anything about it. But some of the weirdness, time is one of four dimensions, x, y, z, and t. We just don't observe it the same way. We can't see it the way we see x, y, and z. They're all flexible. They can be, they can stretch, they can contract. Therefore, they can curve around each other and jumping down a little bit. It's mass that causes that curvature of space and time. It's really energy. But the biggest, the biggest energy bang for your buck is in the form of mass and c squared is the equivalent amount of energy. And that sees a big number. c squared is even bigger, of course. Mass and energy are the same thing. I kind of skipped over that, I guess. But you can convert energy to mass to energy. We didn't know that until relativity theory. That seemed kind of strange. And so basically reality, however you want to define that really depends on the viewer. Just by moving, you're living in a different scale of space and time than the people around you. If you move relative to them, if you accelerate relative to them, it gets even weirder. So that certainly is part of your impression of space and time is, or of reality. I'm sorry, how you observe the scale of space and time that you observe. Weird. Okay, really weird. Back then in 1915, and the question was, who was really prepared to believe this? Artists were definitely prepared. They loved it. It's the best thing since sliced bread. But about half of physicists were not prepared to believe it. They said this nonsense, sort of like proposing that helium is real. So how could we confirm it? That's the question. How could we confirm it to observationally to try to convince people? And basically in relativity, you don't have gravity. You don't have gravitational fields. Gravitational fields were kind of proposed by Newton hundreds of years before to make things make sense. But to gravitate this field in space, we all bought it for 400 years or so. We still do. It works for everyday things. But in relativity, you don't talk about, use the word, but you don't consider a gravitational field. You consider the curvature of spacetime kind of represented by that so-called rubber sheet diagram right there. It's pretty good, not really accurate. And another, like there's the curvature of spacetime produced by the mass of the sun. And here comes another massive object into that curved spacetime, and it will curve around the sun not because of gravity. The gravitational pull between the two masses, not in relativity theory, it's because of the curvature of that spacetime caused by the mass of the sun. That object is trying to move in the straightest line it can, but it's moving through curved space. So it follows a curve basically. I'm kind of brutalizing this a little bit, but that's all right. So because it's not the mass, it's not gravity that's causing that curve. It's the curvature of spacetime caused by, in this case, the sun right here. Then anything is going to follow a curved path as it goes around. It doesn't have to respond to gravity. It doesn't have to have mass. What doesn't have mass? Light doesn't have mass. It has no mass. So light, as it travels close to the sun, will be curved by that curvature of spacetime. Einstein predicted this in 1911. He didn't publish it until 1915 with his theory of general relativity, but the big theory of relativity basically. We call it gravitational lensing today. And here's what he predicted. That a star that is really here, the light goes off in all directions, but some of that light will skirt really close to the sun. It'll curve around the sun and then comes straight to us, comes back out into flat space again, comes to us along this direction. So we look at that light, we look at that star, and we're looking in this direction and it appears that it's really here. Looks to us like it's here. That's the apparent position, but that's the real position of the star. Gravitational lensing. And he said, come on, astronomers, get going. Let's look for this thing. A lot of astronomers read his early drafts of that paper before 1915. They knew he was going to publish this, so they already knew long before 1915. We got to look for this. We got to look for this curvature of space around the sun. Two things made it really hard, really hard. And Einstein knew this and every astronomer knew this, too. It was going to be very difficult to do. And one of those reasons is that angle, let's go try going back. Yeah, that angle right there, it's about a millionth of the angle that I've drawn there. It's about a million times smaller than the angle that I drew. I just drew it that big so you could see it, basically. But it's really less than two seconds of arc, which is a teeny, teeny, teeny angle. It's like standing in Burlington and looking across the lake to the New York side and resolving a baseball, not a softball, but a hardball on the other side of the lake, that tiny an angle. So that's going to make it hard. Very, very accurate measurement of apparent stellar positions is going to be required right around the sun, near the sun. The other one is near the sun, meaning you got to make this, you got to, you got to measure the positions of those stars in the daytime when the sun's up. And when the sun's up, as we've said before, it's bright. You don't see any stars around the sun, unless it's an eclipse. You'd never see stars close to the sun, unless there's a total eclipse. The sky's too bright, the sun's too bright. So they knew they needed a total solar eclipse. Einstein knew that, too. So he said, come on, Mr. Honors, you've got some eclipses coming up. Get going. They did. There were two eclipses coming up right away after 1911. 1912, there was an eclipse. 1914, there was an eclipse. Those are the total eclipse paths. These are the lines of less and less partial eclipse taking place along those lines. I didn't describe that earlier, I should have. This went right through Brazil and some astronomical teams went to Brazil to photograph the eclipsed sun with the stars around it. Had bad weather. Bad weather. We won't go there. And then this one in 1914 went right through, Northern Greenland was not too popular back then with them, but it went right through Norway and Sweden and Finland and Russia right there, a little piece of it, the center of it actually in Russia. And again, bad weather plagued most of those attempts to photograph it then. But one team of German astronomers went to Russia to photograph that eclipse and were arrested as war spies. So they were released, but they missed the eclipse. So World War I kind of got in the way of that one too. I think there's a board game about that actually. I don't know anything about it. So the next one then that was really going to be any good was actually predicted by an astronomer named Dyson in England. Not no relation to Freeman Dyson these days that anybody knows of at least. But he predicted, he said on May 29, 1919, there's going to be another solar eclipse. Guys, get ready for this and it's going to be a really good one. Look at the time of totality up here. Six minutes and 51 seconds. The longest period of totality in history, in recorded history. A long time to fiddle with your cameras and telescopes to get this thing going. So Sir Arthur Eddington, British astronomer, very well known as bird astronomer at that time. There he is. He took a team down to Princepe, island off the west coast of Africa right here, right in the middle of the eclipse path. He thought, good chance I'm going to have bad weather as we all know. So he sent another team to Brazil to observe it to double his chances of getting a good picture of the stars around the sun during the eclipse when the sun is blocked out. The camera, remember this is a big black box with a glass plate holder in it. The glass plate's about, there were 17 by 20 inches maybe, with a motion coated on the glass. This is a big deal to have this thing mounted on the back of the camera. It's not just, it's not like a Pentax digital camera. And then you've got to process the thing and all that, wet process it. Well, it turns out he had good weather. And don't go yet. Okay. There's the photograph he got. Whoa, man. Look at that. There is the eclipse. There's the moon. There's the corona of the sun around it. That's a scratch on the emulsion right there. Forget that. And there's a star there, there, there. Don't know what died here. Kind of hard to see in this light. There is one there and another one. Can't see that one either. But what you normally do, you draw a straight line right through the star with a ruler and a pen and leave a gap in the middle where the star image is so you can identify where they are. That's pretty standard stuff. So they compared this photograph to another photograph of those same stars taken when the sun was not in that region. Had to be taken at night then, of course. So later or earlier in the year. And they compared the exact apparent positions of these stars to where they normally are, where they should be in the sky. And sure enough, they're just slightly out. Just a fraction of a second of arc off of where they should have been, exactly as predicted by relativity. Amazing. Really a kind of amazing piece of work. That, by the way, you're looking at, is a positive image, a bright corona around a black moon. You're looking at the dark side of the moon here, of course. It looks really dark. What they did all their measurements on was the original negative. It's just a molten coated on a glass plate. You process it into a negative image and don't bother to print it into a positive. Who cares? You just look at the maker measurements on the negative. This one, somebody along the line, reversed it into a positive just to make it look more realistic, I guess, and easier to figure out what's what in it. So, again, a lot of you will be familiar with this from my courses, but I won't have to describe this maybe to most of you. I will. What you see first over here on the left, I was going to say this first, but what you see first over on the left is a photograph of the nearby galaxy. The mass distribution of that galaxy is used to calculate the bending of light, the gravitational lensing of two quasars that pass right behind that galaxy. Unfortunately, they fade the galaxy out, so you don't see it anymore. I wish I had left it in, but so there's, well, I missed it, there on the left is without, is calculating what that should look like. Those two very distant quasars, what they should look like without gravitational lensing, and over here is with gravitational lensing. And I will, since I kind of missed the beginning of it, I'll let it run one more time, and then we're going to stop it midway. And there it goes. There's the galaxy used for the mass distribution, and there come a double quasar behind it without any gravitational lensing on the left and width on the right, and I'm going to stop it right there, and there's a Hubble photograph showing a nearby galaxy right here, and the light from a much more distant light source, a distant quasar, about 10 times further away than that nearby galaxy. So all that light came from right back there, you know, behind that galaxy, and got curved around it by the curvature of spacetime, very much like light bending around the sun, just more extreme in this case because there's a lot more mass in there, by measuring, by knowing the distance to that galaxy and where that light, how far that was. We can get that from the redshift of the light, the Hubble law, and knowing the angular diameter of that so-called Einstein ring right there, you measure that, get the distance to both those light sources, you can calculate how much mass was required in there to bend the light that much. The more mass that's in there, the bigger that arc is going to be, that so-called Einstein ring. What you find is there's about five times more mass in there, five or six times more mass than the brightness of that galaxy would indicate dark matter, we call it. A lot of mass in there that we're not seeing, and we don't know why, we don't know what it is, we don't know why. We've got lots of examples of this now, they're all over the place, sometimes we see a pretty full Einstein ring, if the alignment is just right, but we see a lot of them. We can calculate, just about every single case, we can calculate how much mass was in there to curve it that much, and we always get way too much, way more than we're seeing. Here's some more, sometimes we just get a few dots of light instead of an arc. Both of those dots came from the same source, you know, much, much further away, and many, many more, you can barely see this with this light, but lots and lots of rings and dots and Einstein rings around these gold colored nearby galaxies. This is the slide I thought I was going to come up with. Lots of confirmations now of relativity theory, I used this slide in the introductory course where we talk about relativity, and the very first one up here was explaining some oddnesses about Mercury's orbit around the Sun. It isn't quite right, and we couldn't quite explain all that until 1915 when Einstein said it's the curvature of space-time around the Sun. It's warping that orbit ever so slightly, and it exactly fit. This is the one we've just been talking about right here, and the only one that I'll really mention here are lots and lots of confirmations of relativity theory. The only one I'll mention is GPS clocks. You probably heard me say this too after you're one of my classes, but we have to correct the clocks in GPS satellites continually, many times a day. We have to advance them because they're moving by us quickly, and we have to set them back in time, retard them, because they're further from the center of the Earth, the center of mass of the Earth, less curvature of space out there than there is down here where your cell phone is. If we didn't, we know all about those corrections, and we do them continually, and maybe, I don't know, 10 times a day or something like that. If we didn't make those corrections to GPS clocks up in the satellites for maybe a couple of weeks, your cell phone would drive right down over into the lake, I don't think because of relativity probably, but probably they just entered the wrong thing into their GPS or into their satellite. So lots of confirmations. One more thing I want to mention, and it'll just take about five minutes here at most. Here's our path of totality coming up right here. It starts down there, actually starts down in the south Pacific and goes right up through the United States and out over northern Maine and out into the ocean. The CATE project, C-A-T-E, I don't know if anybody in here has heard of it. This is what it stands for, Continental America Telescopic Eclipse. None of the people involved in it are in the room right now, but we'll get to that in a minute. They're going to set up 35 solar telescopes from San Antonio to northern Maine, and the reason they're going to do that is they can watch this eclipse taking place for over 90 minutes. They can, you know, this telescope down here, when we see it for three minutes, and then this one sees it for the next three minutes, and this one this one. They even overlap in time of totality so that if one of them has bad weather, either the one after or the one before might get the images that it missed, they're also going to have polarizing filters on the telescopes that they can rotate. So they can measure the degree of polarization of the light coming from the corona, and they can measure the angle of the polarization of the light coming from the corona. And that's going to tell them a great deal about the magnetic field, the solar magnetic field of the sun, in the corona in that region. So what there are three basic questions they're trying to answer. One of them is we know that magnetic field lines above the photosphere of the sun in the corona, they get all distorted and bent out of shape, they rise way up in big loops. That's what causes those solar flares, and they can break the magnetic field line, can just sever. And then all those electrically charged high energy particles are spit out into space and go flying out by all the planets, coronal mass ejection, we call that, and it's very dangerous. It's dangerous to earth, especially communications. Military always worries about this a great deal, coronal mass ejection, so we're trying to predict them a little better. But in particular those magnetic field lines can sever and then they can reconnect later. They can rejoin again, and we don't understand how they do that. Physicists are sort of like, how does the sun do this, you know? So they're hoping to get visual evidence of that happening and try to explain it a little bit better. We also know the solar wind particles coming out from the sun are very high, very fast, very high energy, they get to earth in about three days, very high energy. So how do they get accelerated in the corona? We don't know what gets them going that fast. And we know that the corona around the sun is actually much, much hotter than the surface of the sun itself. It's much hotter. And we don't know why. We don't know what the heating mechanism is for that corona around the sun. They're trying to get, again, visual evidence to try to explain some of those. And the reason I'm particularly bringing this up is UVM, we have, whoops, well lost it here. Let's try going back again. There we go. We have two current and two recently graduated students who are directing eight of those teams in the Cape Project. All the ones in Vermont and New Hampshire, all within this region right up in here. One of them, well known to all of my students here, I think, was actually, I told her about that. I heard about it last summer and I told her about it. I knew she was graduating, so she would be available to do that this spring. She said, got to do it. So she applied, got the job to direct those teams. They sent her down to San Antonio to be trained back in the fall. She came back and picked out some other students and trained them. They are now training the other seven teams in Vermont and New Hampshire. Her team is going to be located up on Grand Isle on Eclipse Day. They're not going to tell anybody where, I don't think. They don't want to be bothered, basically, when they get their work done. But we're very, very pleased with that. She got this job, introduced some other UVM students into it, and that they're doing this thing. It was done also in 2017. They did a big eclipse that went from the Northwest down to the Southeast in 2017, total eclipse. They did the same thing with multiple telescopes and took all the photographs of the corona and solar flares, but they didn't have polarizing filters then. I don't believe. They weren't particularly looking for polarization. So they were able to take all those images that were recorded from up here to down there over a period of, I think, over an hour in that case, and see, watch the corona changing for a full hour, more than an hour, and watch the solar flares coming and going for a full hour. They stitched all these still photos together into a video, basically, kind of a choppy video. I'm going to show it to you. It might not play on the TV recording because I had to give them a PDF of this, but here it comes. There might give you headache, too. Oh, that's a slow computer. Oh, dear. When I play this at home, it goes, anyway, maybe this is better. Won't give you headache. You're seeing the moon, of course, the sun, the corona of the sun out here, solar flares coming and going, changing shape from one time in the totality tip to the next time along the path. And you can see the corona changing like crazy down here, changing its shape. Those lines, bright lines in the corona, pretty much mapping out the magnetic field of the sun. So we're seeing how the magnetic field is changing over time. It's kind of fun to watch this thing go at normal speed because it really, it just sort of blows your brains out. I will get back off this. Well, maybe I'll let that slide play. So the last thing I'll say to you is we're proud of those students and you should be proud of our UVM astronomy students. Really, they go out and do great things. I'm done. How about questions? Any questions? I know a lot of this material, a lot of you already knew because you've seen them in my classes, but anything at all? We had a visiting candidate for the next astronomy lecturer today, and you were there, and you were there, and I don't know if it was, maybe not. And he would ask a question and then just wait. It was great. Just wait. Yeah, for the... What is the K team expecting to find with the polarization filters looking at the magnetic field? Again, they're keeping their cards fairly close to the chest right now. I've asked a lot of questions and not getting a lot of answers yet. I'm pleased with that. They're in charge. I am not. I want them to tell me as much as they're allowed. I think the polarizing filters are going to increase the contrast of all of the structure that you see in the corona here. It's going to increase the contrast a lot and probably increase the rate of change of the shape of the corona because they'll be seeing just... A lot of that light is polarized light and the light coming off the flares is highly polarized also. I think that's right. And so when they turn the polarizing filters, they'll see some of these lines come, some of these lines go, and they might see more activity, I think. I wish I could answer you more on that, but I don't know. They have a website. Kate has a website up there, but I don't think you'll find much more than I've already told you about it. They're also going to have some F... Not F-35s. They're going to have some fighter jets flying along the eclipse path also making observations, trying to keep up with it. That's pretty fast. I mean an hour and a half from San Antonio to northern Maine, you're moving right along. That path of totality will approach us across the lake at over a thousand miles an hour because of the moon's orbital velocity. And they've got to keep up with that, those fighter planes. They're moving. Any other questions? No? I thought I saw a hint. Okay, well let's... I gotta go meet the candidate for dinner actually now after this right around the corner. Thanks for coming. Where? I think I will volunteer for one of the campus. We're gonna have five solar telescopes set up on campus with teams of volunteers to help with the telescopes that answer questions and stuff like that, keep people from passing out. And I think I will volunteer for one of those. I haven't done it yet. Come to your... I'll just be a sign, you know. You're at Harris-Millis. Yeah. Yeah. Do you have... Does Alan have a full team on that one yet? Do you know? Oh really? Oh, that'd be fun. Yeah. Yeah. Yeah. All good. Enjoy the eclipse.