Celebrating 100 years of Eddington's eclipse

May the 29th marked an important Centenary for the world of physics: on that day in 1919, Cambridge Astronomer, Arthur Eddington, led teams to two continents to take what are now some of the most famous photographs we have. The results sent the scientific world into turmoil. Newton’s laws of gravity, that had stood unshaken for hundreds of years, were overturned by a young German scientist called Albert Einstein. To hear how it happened, Izzie Clarke headed over to the Institute of Astronomy at the University of Cambridge, to see space scientist Carolin Crawford.

Carolin - The dominant law of gravity was, of course, Newton's laws of gravity - which he had devised, and which proved a very accurate description of the way objects moved on the earth, and how the planets move round the sun. It was only during the middle part of the 19th century that it was clear that there was one thing it didn't quite account for, and that was the way that Mercury's orbit moved round the sun. But Einstein was the first one that could account for everything that Newton could account for - that we saw in space and on Earth - but could also justify what was happening to Mercury, due to an extra curvature of space-time in the proximity of the sun.

Izzie - This idea of space-time was at the heart of Einstein's theory: that space and time can be considered as one entity. I know, it's quite a lot to get your head around, but bear with me. Say you need to pop to the shops. You could say that they're 10 minutes away - or equally a few kilometres away. You can describe that journey in distance and time, because you know how fast you walk. There's a similar thing with space-time: that both space and time are interchangeable because you know the speed of light, and the speed of light is the same everywhere. And what Einstein then said that is so different from Newton is that this space-time could be distorted by massive objects like our sun - that they bend the shape of space, which creates that key difference in Einstein's theory of gravity.

Carolin - Light likes to travel in a straight line. But if you had the light from a distant star, a light ray, and it just grazed the surface of the sun - because the sun is the nearest large mass we have around - it would just deflect that light a little bit and cause a tiny shift in the apparent position of the stars, but it was so small it wasn't practically observable. The difference was that Einstein made a prediction that when you take into account the curvature of space you actually double the amount of that deflection, which brings it into the realms of observability; and it also provides a very nice discrimination between what Newton says and what Einstein says, if you could measure this deflection.

Izzie - But how can you measure the deflection of light from a distant object if your own giant fireball, i.e. our sun, is in the way? It would be impossible to distinguish the light from the two sources. The idea was proposed that pictures of distant galaxies could be taken during an eclipse, where the Moon blocks the light from our sun.

Carolin - So it wasn't a new idea to make this measurement, but the exciting thing was there was a particularly good eclipse coming up on 29th May 1919. It was good because it was of long duration, about six minutes or so, which gives you plenty of time to take your images. And also, quite unusually, the sun would be right in front of a very bright nearby star cluster - it's called the Hyadas, is in the constellation of Taurus - which meant that during the eclipse the sun would lie in a region surrounded by fairly bright stars, which would enable the observations. So Arthur Eddington, who was a director of the observatories here at Cambridge at the time, he was one of the few people who fully understood the theory to study Einstein's ideas. And it was Arthur Eddington and also particularly Frank Dyson, who was the Astronomer Royal at the time, who realised that this was a particularly momentous eclipse for doing this. What they decided to do was to make two expeditions. There was one that was led by Andrew Crommelin from the Royal Greenwich Observatory which took equipment to Sobral in northern Brazil, and they would catch the start of the eclipse. And then the path would move right across the Atlantic Ocean and on the other side you'd have Sir Arthur Eddington and his small team who would do the same observations on an island off the coast of West Africa. They're carrying out the same experiment in both locations, and the ideal thing about having two locations, of course, is that you're never quite sure of the weather; and indeed, both expeditions had problems. In Principe, off the west coast of Africa, Eddington had terrible weather; and so they took plenty of images, but in most of them there’s too much cloud in the way. And in Sobral, in Brazil, they had problems with the equipment - there was vibrations, which just ended up blurring some of the images. And in fact the really important data from Brazil were from a sort of backup telescope they'd just taken as a spare. But the true and precise measurements don't happen until they come back to the UK, and then the results are announced in November in 1919 at a very special occasion at the Royal Society in London.

Izzie - And what did they find?

Carolin - They found that their results vindicated Einstein's predictions of what should happen under his theory of gravity, rather than Newton’s.

Izzie - And how important was that finding, and what did that do for the field of physics?

Carolin - Well, it has revolutionised physics. I mean, at the time it was hugely important because very few people had really taken notice of Einstein's theories, and this idea of the whole curvature of light is quite a conceptual leap. And, to be quite honest, for a lot of scientists - and we have this in notes and letters - there is a resistance to having to use a more complicated theory. You know, if Newton's laws were sort of good enough, why not use those? But the point is, once this announcement is made, Einstein's relativity is proven as the better description of what happens on earth and in space - and at that point Einstein becomes the celebrity genius.

Izzie - And so Einstein's theory of general relativity was accepted: that what we perceive as the force of gravity in fact arises from that all-important curvature of space and time.

Carolin - Relativity is part of our general understanding of physics, so it's crucial to how we use physics. Now it may not make much difference here on earth, because Newton's laws are good enough. Where it becomes important is in more extreme situations - and so the closest thing to earth that people might run into everyday is of course your GPS satellites. If you didn't take into account the relativistic corrections for the fact that they're travelling in a reduced gravity field - and also faster compared to the surface of the earth - they would start to give you inaccurate results. Within a couple of minutes they'd be 10 km out per day. That's an immediate result that people might be able to relate to. I will say though, it's incredibly important for astronomers, because relativity gives the only good description of what happens where you have very large masses - and in astronomy, of course, you're involving the largest masses possible.