At the turn of the 20th Century, Einstein’s theory of relativity stunned the physics world, but the experimental evidence needed to be found. And so, in 1919, another respected astronomer, Arthur Eddington, observed the deflection of stars by the gravity of the Sun during a solar eclipse. Here’s the story of that famous experiment.
1919 Eclipse and General Relativity by Simon Singh
Illuminating relativity: Experimenting with the stars
How Eddington demonstrated that Einstein was right by Kash Farooq
Not Only Because of Theory: Dyson, Eddington and the Competing Myths of the 1919 Eclipse Expedition by Daniel Kennefick
Space, Time and Gravitation by Sir Arthur Stanley Eddington
Works by Sir Arthur Stanley Eddington available on the Internet Archive
Additional works by Eddington
Transcription services provided by: GMR Transcription
Announcer: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest running online Astronomy degree program. Visit astronomy.swin.edu.au for more information.
Fraser Cain: Astronomy Cast, Episode 371. The Eddington Eclipse Experiment. Welcome to Astronomy Cast, our weekly facts based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. My name is Fraser Cain:, I’m the publisher of Universe Today and with me is Dr. Pamela Gay:, a professor at Southern Illinois University, Edwardsville and the Director of Cosmo Quest. Pamela, how are you doing?
Dr. Pamela Gay: I’m doing well, how are you doing Fraser?
Fraser Cain: Doing great. I apologize; I’m a little throaty today. It’s allergy season and I’ve got a bunch of pollen clogged in my throat, I think.
Dr. Pamela Gay: That is sadness.
Fraser Cain: Yes. But I’ll tough through it. Do you have some kind of announcement? Some news? Updates on the Hangout-A-Thon?
Dr. Pamela Gay: I do. So, we are gearing up for the 2015 Cosmo Quest Hangout-A-Thon. It will be April 25th, 26th. We know have the event pages up on Google and I’m for one very grateful that we can now record eight hours at a time. We’re partnered up with the Astronomers Without Borders, Global Astronomy Month, Global Star Party, so we will be bringing you live coverage of star parties around the world, following the night as it passes from nation to nation.
Fraser Cain: How cool is that! It just gets bigger and more insane every year and we watch you go crazier online, 36 straight hours of Pamela broadcasting.
Dr. Pamela Gay: Yeah, if there was an easier way to fund myself, I would, but this is the most reliable. Yeah, I trust you guys to do what’s right more then I trust referees who may not have actually read my grant proposal to do what’s right.
Fraser Cain: Right. Oh yeah, so the event page is already set up, so if you’re listening to this, how could people navigate to the event page to click yes, so that they then get all of the updates and recommendations that will go into your calendar?
Dr. Pamela Gay: So we are +cosmicquest on Google+, that sounds redundant and just look up all of our events, they should be the most recent four posts on our feed, and I will be putting all those links on our Google Hangout Page, that’s over on Cosmo Quest and I have course have forgotten the bitly link and the time it takes me to open up and get to the show.
Fraser Cain: We’ll get that next week. We still have a little time.
Dr. Pamela Gay: We do.
Fraser Cain: All right, well let’s get cracking then.
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Fraser Cain: At the turn of the 20th century, Einstein’s theory of relativity stunned the physics world, but the experimental evidence needed to be found. And so, in 1919 another respected astronomer, Arthur Eddington, observed the deflection of stars by the gravity of the sun during a solar eclipse. Here is the story of that famous experiment.
So we’re continuing our sort of series on experiments and they all seem to be hovering around relativity, which is kind of great, like Einstein was right, as we’ve said in a previous episode, and these are the experiments that showed that he was right. Now this one of course is great because Arthur Eddington looked exactly like David Tennant if I recall.
Dr. Pamela Gay: So, I’m not the only one who found that while preparing for the show. Yeah, there was apparently –
Fraser Cain: Well, you know we did a show. They did a documentary.
Dr. Pamela Gay: The BBC show, yes.
Fraser Cain: Did a dramatization of the Arthur –
Dr. Pamela Gay: Have you seen it?
Fraser Cain: Yes! Yeah.
Dr. Pamela Gay: I love you with hate or hate you with love. I’m not sure which but I tried frantically to find that show because it’s two awesome people, David Tennant, Andy –
Fraser Cain: Serkis?
Dr. Pamela Gay: Yes, Gollum.
Fraser Cain: Yes.
Dr. Pamela Gay: They did a show where they played – David Tennant was Eddington and Andy of Gollum fame was Einstein and it was apparently quite excellent.
Fraser Cain: It was wonderful.
Dr. Pamela Gay: And it was impossible to get in the U.S.
Fraser Cain: Yeah, I guess as a Canadian we get access to the BBC stuff and I remember watching it, like a long time ago, like it must have been six –
Dr. Pamela Gay: Came out in 2008 and HBO ran it in 2010, but I don’t have HBO, so –
Fraser Cain: There’s got to be some place on the Internet, so email Pamela with a link to where she can find it and she will get a chance to watch it as well. But yeah, absolutely terrific, and so it’s great to have it dramatized and so I saw this just wonderful relationship with Eddington and Einstein as they sort of – Eddington went out and helped find the experimental evidence to support these really revolutionary theories. So, what happened in reality?
Dr. Pamela Gay: So, from what I have been able to piece together, the two of them did actually work together on this in so far as Eddington was very much a, “I want to figure things out by the books” kind of man. He did do some stupid things now and then; he did not believe in white dwarfs, it was a belief system, he and Chandrasekhar fought for years and years and years. But at the same time he was also someone that was very interested in trying to figure out things like, why does the sun work? Why do we see what we see? And he was the person who figured out why actually the sun has been glowing for longer then fuels like coal would allow. But one of the other things that just kind of itched at the back of his brain, is this constant annoyance, was our inability to explain the orbit of the planet Mercury.
It was off by just enough, that you couldn’t wipe it away by saying it was observational error, and when Einstein started publishing his theories, Eddington engaged him in conversation basically trying to understand what are all the observational problems this is now going to explain away. And once he wrapped his head around the idea of relativity, he looked up when is the next eclipse that one can get to that is a full eclipse and long enough to set up cameras of the time, and it was the 1919 eclipse that passed over Africa and the island of Principe off the coast of Africa, which is where I think Eddington went.
Fraser Cain: That’s right and so the experiment that he was trying to do here was as predicted by Einstein, I guess gravitational masses will be warping space time and so light should – the sun should warp the light that’s moving towards us, but that’s a really hard thing to test because the sun is so bright, and so the only time you’re going to be able to see that is during an eclipse. So what was sort of the experiment set-up that they were looking to do?
Dr. Pamela Gay: Well, conveniently the sun was affecting the light from the Hyades cluster, a nearby cluster of stars that many dozen million years old, actually I think it’s over 200 million years old, and these stars are spread out a far amount. They’re sort of scattered all over the head of Taurus the bull and the sunlight depending on how far the sun was from the light of a given star, would bend the positions that the stars appeared to be at, because the light rather than shining straight towards us, well, it would get bent, so the light that we would normally see would get bent out of our field of view and the light that would normally shoot above, or below, or beside the planet Earth would instead get bent to actually reach us here on the planet Earth.
This bending changes the apparent positions of the stars. Now Newton’s theory of gravity also predicted that gravity would have the ability to bend light, but because Einstein’s theory didn’t actually bend space-time, his theory was off by an entire factor of two.
Fraser Cain: So the measurements that Eddington was trying to make of the positions of the stars next to the sun, would show whether Newton’s ideas of this how should work or how Einstein’s should work, which one was more accurate and so the precision of the experiment was really important. Now, I know that while Eddington was in Africa, other teams were in South America, because the eclipse was going to do this big long path, right? And so this was a part of it as well, was they wanted to test this out from different positions on the Earth as well.
Dr. Pamela Gay: And so we had astronomers that were in Brazil, catching one part of the eclipse path. We had Eddington on Principe, which is actually in island off the coast of Africa and they were all basically doing the, if enough of us look up, someone’s got to be without cloud cover, and this actually meant that the observations were more complex then one might wish.
So, you have the problem that when you look at something like a cluster of stars, you’re capturing that light through an optical system that may have all sorts of distortions in it, and we’ve done several different shows on this. So, you have to, as best you can, use the exact same optics and the same optical alignment, but you can’t always do that.
So, they did the best they could. They did fancy plate solutions to try and correct for everything that was wrong with their optics and they took images first when they Hyades cluster was in the night sky far from the sun, and then they captured pictures of it during essentially the daylight, but during an eclipsed daylight moment.
And in 1919, photographic technology wasn’t exactly what is was today, so they’re in a situation where they’re trying to not destroy their optics, because well, the sun’s corona is still there, it’s not entirely dark, if you’ve experienced an eclipse as many of you’ve experienced on Friday, there’s still a whole lot of daylight to deal with. And so, they’re initial measurements weren’t great, but they were good enough.
Fraser Cain: They were good enough, I mean, if I understand – there was a lot of controversy after they even made those calculations. I think as – if in the show, which is the way I’m going to base all of this, television taught me so much, but in the show, it was like it was a slam. “We’ve seen it! There it is in the telescope!” You know? “Einstein was right! Have you got this Einstein?” So, but if you know, it was photographic plates, they brought them back, they had to study them, and there was enough sort of discrepancy in the way they did their calculations that a lot of astronomers weren’t convinced.
Dr. Pamela Gay: And it was the basic problem of, you’re trying to take a photo very quickly, you’ve pretty much got a shot, and then you’re making all of the measurements by hand on a system where all sorts of distortions could have come in to play at various steps. So, there’s actually pictures on the Internet that we’ll try to find and put up in our show notes of the photographic evidence that Eddington took. And you can see the little marks identifying which of the stars they’re measuring, where the sun has essentially been blocked out by the moon, and there is not a whole lot of stars to go by.
And they’re not all snuggled right up next to the sun, so the amount of deflection that we’re looking at, is the amount of a few periods of newsprint on the glass plate. It’s not a lot to work with. And there was probably a whole lot of confirmation bias involved in Eddington’s discovery, but the thing was even if his initial results were, for lack of a better phrase, kind of crap, they got people thinking in the right direction. Had he come out and said, “No! Einstein is wrong!” as so many cranks wish he had, there might not have been all the follow-up observations that were made.
Instead, thanks to confirmation bias and the determination that the results would be correct, there was first of all cover stories in things like The London Times, and there was experiment, after experiment, after experiment knocking down the error and showing that while those initial results…had a lot of error, had systematic bias, had confirmation bias that confirmed Einstein’s theory. Well, that confirmation bias was based in luck but led to the actual correct answer.
Fraser Cain: Well, and the other observations that were done in Brazil, were more – gave more evidence towards the Newtonian findings, so you can see why there was a lot of controversy. But still – and apparently like 1979, they did a lot more research and used modern computers, modernish Apollo era computers, to reanalyze the data and got a much better result out of it.
Dr. Pamela Gay: Well, and luckily there are eclipses on a regular basis, so we didn’t actually have to wait all the way up until 1973, which is lucky, because now you are starting to get into things that happened in our lifetime, but in 1922 Lick Observatory was able to have it’s astronomers make a series of observations that agreed with the 1919, admittly biased and highly errored results and confirmed yes, this does appear to be true.
And it’s been major eclipse after major eclipse since then, where we’ve continued to improve our results.
Fraser Cain: Very cool. So, how did it kind of go after, with Eddington’s work? How did this sort of affect his relationship with relativity as a theory and sort of the thinking and the science that he did? As you said, he was so sort of – he was the person who figured out how the sun works.
Dr. Pamela Gay: Right.
Fraser Cain: And a big chunk of that is relativity, right?
Dr. Pamela Gay: No.
Fraser Cain: And the sun is, you know! I just think of it as in terms of like there’s the quantum processes that’s going on in the sun, a lot of this is all connected.
Dr. Pamela Gay: No.
Fraser Cain: Now I sound like a woo-woo [inaudible], don’t I?
Dr. Pamela Gay: So, I think he was someone who’s deeply passionate about understanding the observable universe and trying to piece together enigmas and the sun was an enigma. When he started doing his research at the turn of the last century, people couldn’t figure out how to bring the geologic record and the solar record together because the geologic record said Earth is older than the sun should have existed, even if we are going with the few thousand year model folks. And that’s kind of depressing to think about but they’re trying to figure out how long the sun had been burning based on observed output and the most efficient energy sources we have on Earth, which are things like coal and the sun as a coal burning object just couldn’t have lasted that long.
And it was Eddington who sat down and started working through the quantum mechanics, which is in absolutely no way unified with general relativity. We haven’t got there yet.
Fraser Cain: We still haven’t unified those together.
Dr. Pamela Gay: Right. So, he was deeply engaged in trying to piece things together, looking at nuclear processes, looking at the results coming out of many other different individuals. There’s J.J. Thompson’s work that just got us to start thinking about things in quantist ways and so many other people, and we’re going to get to these in future episodes. But, there were other enigmas out there that were bothering Eddington as they bothered so many other scientists, and Mercury’s orbit was one of them and being able to use the theory of relativity to start erasing these enigmas was a powerful thing.
That he was in a position to go out and say, “Yeah, this theory actually works.” But when it comes to his work with the sun, this was just a side foray into a different part of science, but what was important is we were able to go from, hmmm, light gets bent, we know that from Newton. Light gets bent more, we know that from Einstein to now we know that there’s gravitational lensing that we can use to see multiple replays of the same quasar as the light takes multiple journeys to reach us.
We can use it to detect dark matter because gravitational lensing, well, dark matter that we can’t see any other way bends light as it passes through the universe and we can basically figure out from all of those bends where the dark matter has to be. Eddington’s observations were the first step in a revolution in how we understand the path of light and it’s gotten us all sorts of different places.
Fraser Cain: So, you know we discussed this. That episode about Einstein being right and we sort of talked about all the different kinds of observational tests, some were done very early, like this one. This was really one of the big, first observational tests – the precision of Mercury was the other one, but some have still yet to be done, even to this day, right? Some of the – like the direct observation of gravitational waves, has still yet to be done.
Dr. Pamela Gay: Right, so we’re still missing gravitational waves and it’s been a steady progress. Einstein’s theory of relativity started out from a place of we don’t get Mercury, oh wait now we get Mercury to solid predictions where one of the quotes I found was, Einstein famously made the quip, “Then I would feel sorry for the dear Lord, the theory is correct anyways.” Was his response to Dyson on how we’d feel if Eddington didn’t observe the correct amount of bending of light.
Fraser Cain: Yeah, God is messing up moving the light rays, because I’m right, not God.
Dr. Pamela Gay: Exactly, so we went from he solved the problem with Mercury. Done. Out the door. To predicting how the light would be bent, and they had the technology at the time, be it with a lot of errors, to prove that his idea for a test actually confirmed his results. Then there were the time dilation issues which we needed more precise clocks, but atomic clocks came along. Jet planes came along. And we’ve proven the ideas behind time dilation. The muon which we figured out how to detect proves the idea of time dilation as well with its journey from the upper atmosphere to the Earth without destroying itself.
Gravity waves are a problem. Simply because they’re really hard to observe. We have spent billions of dollars here in the United States trying to detect them with LIGO, which is Long Baseline and Infromatory System that has multiple set-ups across the Earth, where they’ve giant – essentially letter L’s, different perpendicular tracks that they look for the length of the tracks to change as a gravity wave passes over the planet Earth.
The problem is that these set-ups also detect things like; oh that mountain actually weighs more because it’s absorbed a lot of water from that rainstorm and there was this small seismic thing. Yeah, we can’t do it from the surface.
Fraser Cain: But the Eddington eclipse experiment is just like – that is in with relativity. I mean, that is classic perfect process here, right? That Einstein does the calculations and explains something that was already an anomaly with Mercury and then makes a prediction about what experimenters should find if they try to do something. And so then the experimenters go – and he says, “Hey, I know this has never occurred to you, but why don’t you go and see if stars are bent by the gravity of the sun.” And people are like, “What? Okay!” And then they go and they see that gravity is bent, you know, bends the light from the sun.
And then the prediction holds and then they – like you couldn’t ask for a better scientific theory really than that. And the fact that he then made –he’s like a Nostradamus, he made all these predictions about what we should see in the universe and then the experimenters went out and they were true!
Dr. Pamela Gay: Except you can recreate how he made his predictions; you can’t recreate how Nostradamus made his predictions.
Fraser Cain: Well, sure you can. You just make stuff up and it’s all wrong. Right? That’s how you do it.
Dr. Pamela Gay: Yeah, then there’s nothing the same between those two.
Fraser Cain: Right. So, all I’m saying is that if you are a want to be physicist scientist out there and you want to really perfect your craft and you want to be remembered through history, be more like Einstein, that’s all I’m saying, right?
Dr. Pamela Gay: And this is where there’s so many areas of science where we get questions and sometimes we have to say, “Mm, not actually sciency right now.” What caused the big bang? Don’t know, can’t get there from here, not testable. What is the string theory thing? Don’t know, can’t get there from here, not testable. Give me a theory that has tests, and I will happily read your maths, but if you present me something that has no testability, I’m not sure that’s worth my time right now. Life is too short for untestable theories.
Fraser Cain: Right. Now were we going to tackle some of the other experiments as part of this – my concern is that this is going to turn into a series that is going to run for seven years, so…there’s just so many great experiments. And I mean, this is – we are in the nitty gritty of how these experiments were set up and how they worked and what the outcome was and how it really helped push the predictions forward and as you said, there’s the time dilation, there’s indirect detection of gravitational waves, there’s the procession of Mercury, there’s all of these other experiments so –
Dr. Pamela Gay: Well, we’ve already talked about the procession of Mercury in another episode where I got the number wrong. That’s why I remember we did it. We are going to handle some more of these experiments; we’re not going to spend seven years on it. Although we could! I had the terrifying realization this morning that I’ve now been podcasting for more than ten years and so clearly we could keep going, but we will be getting back to straight astronomy in the not to distant future. I just want to do a little bit more to explore the atom before we get there.
Fraser Cain: Sure, why don’t we – I don’t know, we’ll put a limit on the number of these experimental type shows we’re going to do and then we’ll move on to other topics in space and astronomy, because there’s other stuff too. We know you want to hear about it.
Dr. Pamela Gay: And the reason that I added this one is we’re gearing up in 2017 for an eclipse that’s amazingly viewable. The one that Eddington dealt with in 1919 started over South America, great wonderful populated cities, but it started there; it didn’t have a lot of track over South America. Then it spent a vast amount of time crossing the Atlantic, went over the Principe islands, went over Africa, but by in large, this wasn’t an eclipse that was observed by millions and millions of people. In 2017, we have an eclipse that basically is going on a music tour of the United States, starting at Seattle, crossing through St. Louis, Nashville, pick a city with the music style you like, go there, watch the eclipse.
Fraser Cain: Yeah, and I will absolutely be visiting the United States during that time period. We are about 900 days away I think at this point.
Dr. Pamela Gay: And people are starting to gear up. I’m on a task force with the American Astronomical Society, where we’re starting to gear up on giving the word out on how to observe it safely. We’ll be doing something here; the eclipse is at maximum 30 miles south of where I live. It’s going to be a great event and we’re going to be able to, if we feel like it, recreate what Eddington did, except better. And it’s always fun to recreate things better.
Fraser Cain: So, by all means, absolutely clear your schedule. 2017. August. You’re going to want to visit the United States and you’re going to want to see this eclipse, because it couldn’t be a better time of the year, it couldn’t be moving through in a more easily accessible part of the United States, it’s going to be mind bending.
Dr. Pamela Gay: And every time someone says, “Where’s the best place to go?” All I can say is pick the type of music you like and go.
Fraser Cain: I’m going to look at past weather forecasts and find a place that typically gets a lot of sun during August, but that’s just me.
Dr. Pamela Gay: Yeah, it’s August everyone. Yeah, that’s a good idea.
Fraser Cain: I was thinking eastern Oregon.
Dr. Pamela Gay: One of the rainiest places in the U.S.?
Fraser Cain: Eastern Oregon as [inaudible].
Dr. Pamela Gay: Oh, right.
Fraser Cain: Cool. Well thanks, Pamela. We’ll see you next week.
Dr. Pamela Gay: Thank you.
Announcer: Thanks for listening to Astronomy Cast, a non-profit resource provided by Astrosphere New Media Association, Fraser Cain and Dr. Pamela Gay. You can find show notes and transcripts for every episode at astronomycast.com. You can email us at firstname.lastname@example.org, tweet us @astronomycast, like us on Facebook or circle us on Google+. We record our show live on Google+ every Monday at 12:00p.m. Pacific, 3:00 p.m. Eastern or 20:00 GMT. If you miss the live event, you can always catch up over at cosmoquest.org.
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Duration: 29 minutes