Host: Kerri Smith

This is the Nature Pastcast, each month raiding Nature’s archive and looking at key moments in science. The First World War is drawing to an end, and a team of astronomers hatch a plan to chase a solar eclipse across the globe in order to test Einstein’s bold new theory.

Music: Organ Symphony No. 3, by Camille Saint-Saens

Voice of Nature: John Howes

Nature, November 13th 1919, Results of the Solar Eclipse of May 29 and the Relativity Theory.

Matthew Stanley

The May 1919 eclipse was one of the first opportunities anyone would have to really check and see whether Einstein’s ideas were correct.

Music: Organ Symphony No. 3, by Camille Saint-Saens

David Kaiser

Just on the heels of what had been this horrible, horrible conflict between Britain and Germany among others, here was a British team led by a Quaker pacifist, internationalist, mathematical physicist eager to restore what he called the brotherhood of science.

Matthew Stanley

Einstein becomes famous essentially overnight because the expedition was so dramatic and so exciting and such an extraordinary way to think about science being done.

Music: Organ Symphony No. 3, by Camille Saint-Saens

David Kaiser

My name is David Kaiser. I’m a historian of science and a physicist and I teach at the Massachusetts Institute of Technology.

Voice of Nature: John Howes

1918. From a speech by Arthur Eddington to the Royal Institution: ‘Gravitation and the Principle of Relativity.’

David Kaiser

Arthur Eddington was a remarkably gifted astronomer and mathematical physicist in a way that was not always the case – people by that point had often specialised in one or the other and Eddington was remarkably talented across the board. And he was very deeply interested – earlier than almost anyone certainly in Britain – deeply interested in this new, not-well-understood work by Albert Einstein on what came to be known as general relativity, so a generalisation ten years on from Einstein’s special relativity. So, Eddington had a special and unusual interest in Einstein’s work. It was also incredibly difficult for Eddington to learn about it in any direct way because it was all happening in the midst of World War I.

Matthew Stanley

His family and personal background was unusual too in that he was a Quaker. I’m Matthew Stanley. I’m an associate professor at New York University. So, after mass conscription was introduced in Britain, Eddington, like most men his age, were expected to register for conscription and fight for the war if selected, but as a pacifist he refused. He engaged what was called the conscientious objector clause in the Conscription Act, and this was legally allowed but extremely despised. Conscientious objectors were often forced to wear special armbands, they could be sent to prison and in the end, Eddington was only saved by the fact that he was good friends with Frank Dyson who was the Astronomer Royal at the time. It’s unclear precisely what happened but it seems likely that Dyson intervened with high levels of government to get Eddington exemption from conscription on the condition that he conduct this experiment, that is this observation of Einstein’s prediction.

Music: Organ Symphony No. 3, by Camille Saint-Saens

Voice of Nature: John Howes

Gravitation and the principal of relativity.

David Kaiser

So, in brief, Einstein’s work on general relativity was predicated on the notion that there’s no force of gravity. Einstein said maybe there’s no force of gravity at all and that what appears as the phenomena of gravity – an apple falling on Newton’s head and near his orchard and the Moon falling in its orbit around Earth or all of these gravitational effects that had been so well studied – what if they arose because space and time are as wobbly as a trampoline, that space time is plastic, it’s malleable, it can be warped or distended or it can be pushed out into a funny shape, and then everything else has to move as straight as it can on a warped surface.

Music: Organ Symphony No. 3, by Camille Saint-Saens

David Kaiser

That’s a remarkable shift in what we think gravity is all about, and then Einstein again in the years he was thinking about this during World War I, so cut off from many colleagues, tried to think of ways to test that idea. So, if this preposterous sounding notion is true – that there’s no force of gravity and that Newton was wrong, space is like a wobbly trampoline – then what affects would show that? How would we see, say, the curvature of space and time? And one way he realised was to think about the effect that this warped, bent surface would have on objects moving through it, objects like light beams.

Music: Organ Symphony No. 3, by Camille Saint-Saens

Voice of Nature: John Howes

From a speech by Arthur Eddington to the Royal Institution: ‘If a beam of light passes an object which exerts a gravitational attraction, it must drop a little towards the object.’

Matthew Stanley

Light passing near the edge of a massive body like the Sun would be slightly deflected from the straight line that we expect like to travel in, and instead it would be curved by the Sun’s gravity in exactly the same way that the trajectory of a baseball is bent by gravity as you throw it.

Voice of Nature: John Howes

Just as the trajectory of a rifle bullet is curved.

David Kaiser

Well, ordinarily we wouldn’t see the light from very dim or distant stars when the Sun is shining in our eyes – the Sun is so much brighter. So, Einstein realised that if you could conduct, if you could photograph the field of stars during a total eclipse when the Moon has conveniently moved between us and the Sun, blocking the glare of the Sun, and in that sudden darkness of the eclipse, seeing the little pin pricks of light from the surrounding stars that otherwise we would be blind to.

Voice of Nature: John Howes

During a total eclipse, stars have occasionally been photographed fairly close to the Sun and with care it should be possible to observe this effect. There is a magnificent opportunity next year when a total eclipse of the Sun takes place right in the midst of a field of bright stars. This is the best opportunity for some generations and it is hoped to send out expeditions.

Matthew Stanley

So, Einstein was trapped in Germany and couldn’t conduct any of these experiments himself, and there was one German astronomer named Erwin Freundlich who did try to observe this in August 1914, but had the misfortune of trying to observe an eclipse that was in Russian territory just as World War I broke out, so he and his colleagues were actually arrested as enemy aliens and his equipment was impounded and it took some years for him to get home. So, by the end of World War I, Einstein’s theory had sat for some years without any kind of clear, empirical evidence one way or the other.

Music: Organ Symphony No. 3, by Camille Saint-Saens

Voice of Nature: John Howes

February 1919. Advantage is being taken of the cessation of hostilities to arrange for the occupation of two stations in the eclipse of next May.

Matthew Stanley

One of the problems that the observers had at the time was that the path of the eclipse was across the southern half of the planet – not very convenient for European astronomers. So, if Eddington was going to take a crew to observe this, they were going to have to travel to places that were not ways to get to, especially so soon after the end of the war. In the end, they decided to send two teams.

Voice of Nature: John HowesThe selected Brazilian station is Sobral, about 80 miles inland, connected by railway with Camocim, which is reached by steamer from Pará. Professor Eddington and Mr Cottingham will occupy the Portuguese island of Principe, 110 miles distant from the African coast, which is reached by fortnightly steamer from Lisbon.

Matthew Stanley

So, Eddington went to the observing location of the coast of West Africa, which was a Portuguese colony, a sugar plantation. One of the things he discovered after he managed to get there was that the island had terrible weather, so the odds of them getting a good observation were very low, and it’s sort of hard to imagine in this day of satellite observations and ubiquitous weather forecasts that he couldn’t know what the weather was going to be like on this island before he actually got there, but he didn’t. So, once they got there, they had to not only find a place where they could set up out of the rain, but it was very near the edge of the forest, so one of the problems they had was that monkeys living in the forest kept coming down and sabotaging their equipment structures. So, as they would set something up, the monkeys would run out of the forest, grab pieces of equipment and then chase down the monkeys, and eventually they had to get labourers from the plantation to come help them hunt down the local wildlife to help keep them from interfering. Brazil actually had it a little easier because they were closer to what we might think of as a developed infrastructure. So, the team in Brazil had access not only to Brazil’s first car, but also to a local ice-making machine with they were able to use to help stabilise the temperature for the development of the photographs. In Brazil, there were two telescopes at work, one of them the so-called ‘Astrographic’ was the primary, more sophisticated telescope, but its mirror deformed slightly underneath the heat of the Sun and distorted the results.

Music: Organ Symphony No. 3, by Camille Saint-Saens

David Kaiser

It was a hard, hard observation under difficult circumstances. Not a perfect experiment – no experiment ever is – but it was also certainly not a very bad one.

Music: Organ Symphony No. 3, by Camille Saint-Saens

Voice of Nature: John Howes

November 13th 1919. Results of the Solar Eclipse of May 29th and the Relativity Theory. The results obtained at the total solar eclipse of May 29th last were reported at the joint meeting of the Royal Astronomical Societies.

Matthew Stanley

The results from Africa had a handful of the stars that were needed and gave good results that were largely in agreement with Einstein’s prediction. The results from Brazil, however, were somewhat different. The smaller backup telescope captured a number of excellent stars that showed the deflection very close to Einstein’s prediction. Unfortunately, the larger, distorted telescope also captured a known number of stars but because of the distortion, gave a number about half what Einstein had predicted.

Voice of Nature: John Howes

The explanation appears to be a change of figure of the coelostat mirror due to the heat of the Sun.

David KaiserNonetheless, when they boiled down all their numbers, it looked like Einstein’s predictions were right and that gravity was not as Newton had said, that Einstein’s theory of general relativity, with its very strange sounding warping of space and time, its very abstract and difficult mathematics, maybe that had captured a truth of the world.

Music: Organ Symphony No. 3, by Camille Saint-Saens

Matthew StanleyIt’s really pretty remarkable how quickly a scent gathered behind the idea that Einstein’s prediction had been verified, and partially that’s because the actual numerical prediction – that is 1.75 arcseconds – was not really the important feature. The important feature was simply the claim that light was deflected by gravity at all. For most people, they felt that any significant deviation from the straight line propagation of light was evidence for the kind of deflection that Einstein was talking about.

Music: Organ Symphony No. 3, by Camille Saint-Saens

David KaiserThere’s a controversy that I think has been settled but it still flares up from time to time – in the extreme form people have claimed that Eddington cooked the books in favour of Einstein – was there too much massaging of data? And I think for my money, that it has been overblown and the results have been tested many, many times since and Einstein’s predictions have been held up now to extraordinary accuracy and so Eddington’s early and quite astonishing attempts have stood the test of time.

Music: Organ Symphony No. 3, by Camille Saint-Saens

Voice of Nature: John HowesSir J. J. Thompson who presided spoke of the verification as epoch-making.

Matthew StanleyEinstein’s theory in having these clear predictions that can be – at least conceptually –

easily tested in fact becomes kind of a model for how people think about theoretical science in the 20th century, particularly the very influential philosopher of science Karl Popper points to the 1919 expedition of the classic example of how science is supposed to work.

Music: Organ Symphony No. 3, by Camille Saint-Saens

Voice of Nature: John HowesSir J. J. Thompson who presided spoke of the verification as epoch-making, but he regretted the very complicated form in which Einstein expressed his theory and hoped it might be possible to put it into a form in which it would be more generally comprehensible and useful.

Matthew StanleyThis is really something that people say a lot at the time. When people are first encountering relativity and Einstein’s ideas, there’s this common idea that many people voice that Einstein as a German was being overly thorough and making things more complicated than they needed to be, but Eddington was a really gifted populariser of science, so he was out there on the street and in the newspapers and in the magazines, writing essays and giving lectures about what relativity meant and why people should care about it. The drama of going to these far-flung corners of the world to test this esoteric prediction literally as the armistice is being signed in Europe caught people’s imagination, and to a large part we can think of Einstein’s fame as coming out of exactly that kind of public interest.

Music: Organ Symphony No. 3, by Camille Saint-Saens

Kerri Smith

You’ve been listening to the Nature Pastcast. This month’s contributors were historians Matthew Stanley of Columbia University in New York and David Kaiser at the Massachusetts Institute of Technology in Boston. The music was Saint-Saens’ Organ Symphony No. 3. Saint-Saens was a keen amateur astronomer – he owned a telescope and scheduled concerts to coincide with solar eclipses.