Ninety Years On…

The 29th May 2009 is a very special day that should be marked by anyone interested in the theory of relativity as it is the 90th anniversary of one of the most famous experiments of all time.

On 29th May 1919, measurements were made during total eclipse of the Sun that have gone down in history as vindicating Einstein’s (then) new general theory of relativity. I’ve written quite a lot about this in past years, including a little book and a slightly more technical paper. I decided, though, to post this little piece that is based on an article I wrote for Firstscience.

The Eclipse that Changed the Universe

A total eclipse of the Sun is a moment of magic: a scant few minutes when our perceptions of the whole Universe are turned on their heads. The Sun’s blinding disc is replaced by ghostly pale tentacles surrounding a black heart – an eerie experience witnessed by hundreds of millions of people throughout Europe and the Near East last August.

But one particular eclipse of the Sun, eighty years ago, challenged not only people’s emotional world. It was set to turn the science of the Universe on its head. For over two centuries, scientists had believed Sir Isaac Newton’s view of the Universe. Now his ideas had been challenged by a young German-Swiss scientist, called Albert Einstein. The showdown – Newton vs Einstein – would be the total eclipse of 29 May 1919.

Newton’s position was set out in his monumental Philosophiae Naturalis Principia Mathematica, published in 1687. The Principia – as it’s familiarly known – laid down a set of mathematical laws that described all forms of motion in the Universe. These rules applied as much to the motion of planets around the Sun as to more mundane objects like apples falling from trees.

At the heart of Newton’s concept of the Universe were his ideas about space and time. Space was inflexible, laid out in a way that had been described by the ancient Greek mathematician Euclid in his laws of geometry. To Newton, space was the immovable and unyielding stage on which bodies acted out their motions. Time was also absolute, ticking away inexorably at the same rate for everyone in the Universe.

Sir Isaac Newton
Sir Isaac Newton by Sir Godfrey Kneller
Courtesy of the National Portrait Gallery, London Sir Isaac Newton proposed the first theory of gravity.

For over 200 years, scientists saw the Cosmos through Newton’s eyes. It was a vast clockwork machine, evolving by predetermined rules through regular space, against the beat of an absolute clock. This edifice totally dominated scientific thought, until it was challenged by Albert Einstein.

In 1905, Einstein dispensed with Newton’s absolute nature of space and time. Although born in Germany, during this period of his life he was working as a patent clerk in Berne, Switzerland. He encapsulated his new ideas on motion, space and time in his special theory of relativity. But it took another ten years for Einstein to work out the full consequences of his ideas, including gravity. The general theory of relativity, first aired in 1915, was as complete a description of motion as Newton had prescribed in his Principia. But Einstein’s description of gravity required space to be curved. Whereas for Newton space was an inflexible backdrop, for Einstein it had to bend and flex near massive bodies. This warping of space, in turn, would be responsible for guiding objects such as planets along their orbits.

Einstein and Eddington
Royal Observatory Greenwich Albert Einstein and Arthur Eddington: the father of relativity and the man who proved him right.

By the time he developed his general theory, Einstein was back in Germany, working in Berlin. But a copy of his general theory of relativity was soon smuggled through war-torn Europe to Cambridge. There it was read by Arthur Stanley Eddington, Britain’s leading astrophysicist. Eddington realised that Einstein’s theory could be tested. If space really was distorted by gravity, then light passing through it would not travel in a straight line, but would follow a curved path. The stronger the force of gravity, the more the light would be bent. The bending would be largest for light passing very close to a very massive body, such as the Sun.

Unfortunately, the most massive objects known to astronomers at the time were also very bright. This was before black holes were seriously considered, and stars provided the strongest gravitational fields known. The Sun was particularly useful, being a star right on our doorstep. But it is impossible to see how the light from faint background stars might be bent by the Sun’s gravity, because the Sun’s light is so bright it completely swamps the light from objects beyond it.

Click here for enlarged version
Royal Observatory Greenwich Scientist’s sketch of the path of the vital 1919 eclipse.

Eddington realised the solution. Observe during a total eclipse, when the Sun’s light is blotted out for a few minutes, and you can see distant stars that appear close to the Sun in the sky. If Einstein was right, the Sun’s gravity would shift these stars to slightly different positions, compared to where they are seen in the night sky at other times of the year when the Sun far away from them. The closer the star appears to the Sun during totality, the bigger the shift would be.

Eddington began to put pressure on the British scientific establishment to organise an experiment. The Astronomer Royal of the time, Sir Frank Watson Dyson, realised that the 1919 eclipse was ideal. Not only was totality unusually long (around six minutes, compared with the two minutes we experienced in 1999) but during totality the Sun would be right in front of the Hyades, a cluster of bright stars.

But at this point the story took a twist. Eddington was a Quaker and, as such, a pacifist. In 1917, after disastrous losses during the Somme offensive, the British government introduced conscription to the armed forces. Eddington refused the draft and was threatened with imprisonment. In the end, Dyson’s intervention was crucial persuading the government to spare Eddington. His conscription was postponed under the condition that, if the war had finished by 1919, Eddington himself would lead an expedition to measure the bending of light by the Sun. The rest, as they say, is history.

The path of totality of the 1919 eclipse passed from northern Brazil, across the Atlantic Ocean to West Africa. In case of bad weather (amongst other reasons) two expeditions were organised: one to Sobral, in Brazil, and the other to the island of Principe, in the Gulf of Guinea close to the West African coast. Eddington himself went to Principe; the expedition to Sobral was led by Andrew Crommelin from the Royal Observatory at Greenwich.

Click for enlarged version
Royal Observatory Greenwich British scientists in the field at Sobral in 1919.

The expeditions did not go entirely according to plan. When the day of the eclipse (29 May) dawned on Principe, Eddington was greeted with a thunderstorm and torrential rain. By mid-afternoon the skies had partly cleared and he took some pictures through cloud.

Meanwhile, at Sobral, Crommelin had much better weather – but he had made serious errors in setting up his equipment. He focused his main telescope the night before the eclipse, but did not allow for the distortions that would take place as the temperature climbed during the day. Luckily, he had taken a backup telescope along, and this in the end provided the best results of all.

After the eclipse, Eddington himself carefully measured the positions of the stars that appeared near the Sun’s eclipsed image, on the photographic plates exposed at both Sobral and Principe. He then compared them with reference positions taken previously when the Hyades were visible in the night sky. The measurements had to be incredibly accurate, not only because the expected deflections were small. The images of the stars were also quite blurred, because of problems with the telescopes and because they were seen through the light of the Sun’s glowing atmosphere, the solar corona.

Before long the results were ready. Britain’s premier scientific body, the Royal Society, called a special meeting in London on 6 November. Dyson, as Astronomer Royal took the floor, and announced that the measurements did not support Newton’s long-accepted theory of gravity. Instead, they agreed with the predictions of Einstein’s new theory.

Image from Sobral
Royal Observatory Greenwich The final proof: the small red line shows how far the position of the star has been shifted by the Sun’s gravity.

The press reaction was extraordinary. Einstein was immediately propelled onto the front pages of the world’s media and, almost overnight, became a household name. There was more to this than purely the scientific content of his theory. After years of war, the public embraced a moment that moved mankind from the horrors of destruction to the sublimity of the human mind laying bare the secrets of the Cosmos. The two pacifists in the limelight – the British Eddington and the German-born Einstein – were particularly pleased at the reconciliation between their nations brought about by the results.

But the popular perception of the eclipse results differed quite significantly from the way they were viewed in the scientific establishment. Physicists of the day were justifiably cautious. Eddington had needed to make significant corrections to some of the measurements, for various technical reasons, and in the end decided to leave some of the Sobral data out of the calculation entirely. Many scientists were suspicious that he had cooked the books. Although the suspicion lingered for years in some quarters, in the end the results were confirmed at eclipse after eclipse with higher and higher precision.

Image from Hubble

NASA In this cosmic ‘gravitational lens,’ a huge cluster of galaxies distorts the light from more distant galaxies into a pattern of giant arcs.

Nowadays astronomers are so confident of Einstein’s theory that they rely on the bending of light by gravity to make telescopes almost as big as the Universe. When the conditions are right, gravity can shift an object’s position by far more than a microscopic amount. The ideal situation is when we look far out into space, and centre our view not on an individual star like the Sun, but on a cluster of hundreds of galaxies – with a total mass of perhaps 100 million million suns. The space-curvature of this immense ‘gravitational lens’ can gather the light from more remote objects, and focus them into brilliant curved arcs in the sky. From the size of the arcs, astronomers can ‘weigh’ the cluster of galaxies.

Einstein didn’t live long enough to see through a gravitational lens, but if he had he would definitely have approved….

19 Responses to “Ninety Years On…”

  1. Thomas D Says:

    Have you read ‘Fabulous Science’ (John Waller)? Makes a strong case that Eddington’s data were essentially useless and – given the flexible attitude to statistical errors at the time – susceptible to any sort of interpretation he wanted to put on them. In particular by throwing away much of the technically better data.

    The theory was eventually shown to be correct, but that doesn’t mean Eddington wasn’t cooking the books. He just had the luck to cook them in the ‘right’ direction. The observation is famous not because it was a real test of the theory but because of the publicity effort surrounding it…

    • telescoper Says:

      What you say is partially correct. In fact there were two expeditions, one to Sobral and one to Principe; Eddington went to the latter location but the weather was poor and his own results were indeed fairly useless. There is no question, however, that the results from one of the two Sobral telescopes gave beautiful results that definitely prefer the Einstein value at quite high significance.

      Incidentally, although the plates haven’t survived the measurements are all tabulated in the Phil Trans paper. It’s quite a straightforward exercise to do the statistics yourself. I have an undergraduate project where the students do precisely that….

      I don’t think Eddington cooked the books, but I don’t think he helped his case by insisting on including his own poor quality data instead of just using the good stuff.

  2. Anton Garrett Says:

    Great story, great theory, great anniversary!

  3. “Einstein came out with the general theory of relativity (GRT)in its final form in 1915. By that time he had proposed his three famous (”critical”) effects to be used for verification of the theory: gravitational displacement of spectral lines, light deflection in the gravitational field of the Sun and displacement of prehelion of Mercury. More than half a century has passed but the problem of the experimental verification of GRT is still as urgent as ever.

    All the Einstein’s effects have been observed but the experimental accuracy is still low. The error of measurement of the gravitational displacement of spectral lines is about 1%. The deflection of light rays in the field of the Sun has been measured to an accuracy of about 10$ and so on. (From Key Problems of Physics and Astrophysics by V.L.Ginzburg, Published by Mir Publishers, Moscow, 1976)”

    I don’t know what is the current status of experimental verification ? Would you like to enlighten me on this topic?

    Kindly also let me know how the age of Sun has been determined?

    Warm regards


  4. telescoper Says:

    Measurements of the bending of optical light during an eclipse are extremely difficult and their accuracy never improved all that much from 1919, with an error of about 0.2 arcseconds being about the best one can do owing to the inherent stability problems. However, radio observations of occultations have the advantage of not requiring an eclipse. Measurements made that way have reached millisecond accuracy, i.e. at least a few hundred times better accuracy. Best results now give an error of 0.04%.

    Gravitational redshifts are more difficult to measure, but the accuracy of local experiments is indeed about 1%. You might be interested to know that general relativistic corrections at this level were needed in the design of the GPS system.

    The source you quote is over thirty years old and is a bit out of date. You can find much more up-to-date material on wikipedia.

  5. telescoper Says:

    It’s quite interesting that the “automatically generated” links include a page about Cher. I don’t know what the connection is between her and the events of 1919, but it can’t be very strong. She was only a teenager then.

  6. Anton Garrett Says:

    That’s the distortion of spacetime to which you refer!

  7. Gwen Jackson Says:

    Extraordinary. you have given me the best answers to do my assignment on Eddington’s solar eclipse and how it related to Einstein. Thanks

  8. […] example, go back about 90 years to one of the most famous astronomical studies of all time, Eddington’s measurement of the […]

  9. […] degree of experimental confirmation of Einstein’s general theory of relativity and which I blogged about at some length last year, on its 90th […]

  10. […] One pair, for example, is reanalysing the measurements made at the 1919 Eclipse expedition that I blogged about here, which is not only interesting from a historical point of view but which also poses an […]

  11. […] the bending of light by the Sun as a test of Einstein’s general theory of relativity; I blogged about this on its ninetieth anniversary, by the way, in case anyone wants to read any more about […]

  12. Just a small linguistic point. Einstein was not a German-Swiss. A German-Swiss is a German speaking native born Swiss as opposed to a French or Italian speaking one. Einstein was a native born German living in Switzerland. 😉

    • telescoper Says:

      A person born Swiss and speaking German would be a Swiss-German, not a German-Swiss… n’est-ce pas? Einstein was born in Germany but took Swiss nationality…

  13. “A person born Swiss and speaking German would be a Swiss-German, not a German-Swiss… ”

    Wrong! He or she is as I stated a German-Swiss 😉

    • telescoper Says:

      In that case you had better write in complaint to the Oxford English Dictionary. They define “German-Swiss” as “a Swiss person of German ancestry”, whereas “Swiss-German” is “the dialect of French or German spoken in Switzerland, or a speaker of this”….

  14. […] degree of experimental confirmation of Einstein’s general theory of relativity and which I blogged about at some length last year, on its 90th […]

  15. […] example, go back about 90 years to one of the most famous astronomical studies of all time, Eddington‘s measurement of the […]

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