Subtitles section Play video Print subtitles About every 18 months, the Moon passes directly between the Sun and the Earth for a total solar eclipse. This alignment casts a brief shadow on Earth that we can track … and its location changes each time. This year’s eclipse swept across South America, stretching over Chile and Argentina. It was visible over the Atacama desert, hovering right above some of the world’s most famous telescopes, and gave a spectacular 2 minute show. Eclipses are grand celestial events, a chance to witness the mechanics of our solar system in action and ponder our place in it. But they also play an important role in the timeline of scientific discovery. Over a hundred years ago, a relatively unknown German scientist introduced a new theory that would completely shift our understanding of space, time, and motion. And if you’re someone who doesn’t really understand this theory today, well, neither did many scientists at the time. It took two intrepid astronomers, and an ambition solar eclipse expedition in the midst of a world war, to prove it and turn Einstein into a household name. So it’s November 25, 1915, and Albert Einstein presents his famous field equations to the world. He challenged the reigning genius, who saw space as flat and unchanging, and defined gravity as a force that objects exert upon each other. Now, space and time are a four dimensional thing called spacetime, and it can be warped by mass and energy. That warpage creates the effect of gravity. This was just wild new territory for scientists. If you try to do calculations in Newton's theory with the new rules of relativity, it actually gets you a different answer depending on who's doing the measurement or the calculation. Of course that's uncomfortable for a physicist, how do I know which answer is the right one? Einstein himself was very conscious of the need to test the theory. And so, Einstein quickly realized, "Well, what I propose is that astronomers could go to an eclipse, take a photograph of the stars close to the sun...and compare that to a photograph of the same stars at night when the sun isn't there.” An eclipse gives scientists a unique opportunity to study light as it passes near the sun. Einstein’s theory predicted that the sun’s gravitational field would bend the starlight’s path, making the stars appear slightly out of place in a photo, by 1.75 arcseconds to be exact. If Newton was correct, it’d only be by half as much. Einstein himself really promoted it. He got a German Astronomer colleague called Erwin Freundlich to try to observe an eclipse in Russia.Freundlich was German, so the Russians arrested him as an enemy alien. He never saw the eclipse. We have this picture of Einstein furrowing a kind of a lonely path, trying to persuade astronomers that this is something they should do and finding that it's difficult and not many astronomers are interested. And that’s partly because the backdrop on all of this is World War I. You couldn't get German journals delivered to you in Britain during the war. The telegraph cables, which would have been the internet of the day, were cut physically so that you couldn't send a telegraph and the astronomers really depended on these telegraphs. It really was a very clean break in scientific relations and furthermore, the nature of the First World War soured relations between the scientists themselves. But Einstein’s theory did grab the attention of a British astronomer. The sheer intrinsic scientific excitement and potential of the theory were recognized by important people like Astrophysicist Arthur Stanley Eddington. Eddington was really very impressed. He realized, "Boy, this is important. We really ought to look into this." And it's probable that he went to Dyson, who was a close friend of his. Dyson said, "Well, you know, if you were going to test this theory, absolutely the best time to do it would be in 1919. That eclipse will take place when the sun is in a special place in the sky, right in the middle of the Hyades star cluster. That is the closest star cluster to the earth and therefore there will be an unusual number of bright stars close to the sun. So, that really lit a fire under them The eclipse prep started with maps. They needed to track the path of totality and pinpoint the best location for the expedition. A good part of the track was the Mid-Atlantic Ocean, that's not much good. You can't do this experiment from a ship. The rest of the track passed through the world’s two great jungle basins, the Amazon in South America and the Congo in Africa.You need a railway or a port and the middle of the jungle is not the place. Sobral, a city in Northeastern Brazil, quickly emerged as the best contender. And the Island of Principe off the coast of West Africa was one of the few places that seemed convenient. They decided that they should go to different locations because that would give them a better chance of at least one of them avoiding bad weather and to some extent, the two expeditions were quite independent of each other. They needed telescopes to take images of the star field during the eclipse, but they weren't in a position to take the complete telescope with them. They took the lens the most important part of the telescope out of the telescope. Then they brought with them what's called a coelostat mirror. You set it up so that the mirror turns and cancels out the motion of the earth and the lens and the telescope tube doesn't have to turn at all. The plates obviously were critical to the whole enterprise because it was only by comparing the positions of stars that you could actually make this tiny measurement. Using photography to measure celestial objects was a pretty cutting edge technique at the time. They had exactly the right expertise for this problem. It was a very, very delicate measurement. You could easily mess it up. There’s no question about that. And as a matter of fact, we have a perfect example in the case of this eclipse. After photos were taken and analysis completed, the teams had three sets of data in front of them, with bad weather and other complications to consider. One of the two instruments had a faulty mirror. Now you have a real problem and it's the kind of problem that comes up all the time in science. You have three instruments, which gives you three measurements. Two of them agree with Einstein, one agrees with Newton. Now what do you do? They deliberated and ultimately determined, we don't trust this instrument. So we're going to throw that data away. Then that leaves us just two instruments agreeing with Einstein, so we think Einstein is right. That's what they announced at this famous joint-meeting of the Royal Astronomical Society in November, 1919, a century ago. And, it launched Einstein into the spotlight. It's remarkable how it caught the public imagination. Here is this English team proving the theory of a German scientist. The idea that science could in the end, rise above nationalistic differences turned out to be quite appealing. I'm sure a big part of it is simply that if what most of us know about science, even if we've just had one physics class ever in our lives, we know about Newton's theory of gravity. Here was that famous theory being overthrown by this new guy and this new theory, which nobody was able to explain because it was so difficult. That certainly seems to have caught people's attention. A scientific theory isn't just about writing down a set of equations or coming up with a big idea. It has to be capable of predicting new things coming up with new explanations for things that are puzzling us. Well certainly this theory has come through in a really impressive way. Black holes, gravitational waves, gravitational lensing, the very thing that they went to measure, the fact that light is bent by gravity means that really large objects like galaxies and black holes can actually focus light towards us. In many, many ways it's revolutionized our view of the world around us, our understanding of the universe. But I had to ask Daniel, is there a theory out there that could upend Einstein? It’s been an essential theme in our coverage here at Seeker and a puzzle for scientists today. General relativity has passed every test, but it’s incompatible with the quantum view of nature. There's no doubt that we're in some ways in a very similar situation to the one that confronted scientists 100 years ago. Einstein's theory actually presumes that nature is continuous and quantum theory says, "No, no it comes in little chunks." How do we reconcile these two theories? There are many candidate theories, string theory, loop quantum gravity and there are others. Well, I once asked exactly your question to a very famous experimental physicist He said, if you look back there were many theories that were candidates to be the next theory. Einstein's theory came after 1900, it was that theory which reconciled everything and nobody saw it coming. It could be that we will have a new theory but that theory hasn't even been born in the mind of its creator yet. That none of the theories that we are working on today will be the one that will be the successor theory. Or perhaps, there is a theory out there today that is it. We might just have to go on a new kind of expedition to prove it.