The prelude is set on Eddington’s expedition to observe the solar eclipse in 1919, before moving back in time to 1914. At the outbreak of the First World War Eddington is appointed chief astronomer at Cambridge by Sir Oliver Lodge and instructed to research Einstein’s work and defend the Newtonian status quo, whilst Einstein is tempted back from Zurich to the Prussian Academy of Sciences in Berlin in an attempt to aid the war effort by embarrassing Britain in disproving the work of its great scientist Isaac Newton. In Berlin, with his marriage already under tension, Einstein falls in love with his cousinElsa.
A Quaker and therefore unable to go to war, Eddington also says farewell to his friend William Marston as the latter goes off to war as an officer in the Cambridgeshire Regiment, but just misses William’s train as he goes to say goodbye to him before he departs. (There is some artistic licence here. Unless Marston were a regular or reserve officer, he would have undergone at least six months’ training (rather than seven days) before going to France – the first units of the New Armies did not land there until well into 1915.) He then presents his lecture to his fellow astronomers at the university — defending Newton but still thinking Einstein might be right — and takes the German Müller family into his home after saving them from a violent anti-German mob. When Einstein’s wife arrives in Berlin, she discovers Einstein’s affair and leaves him, whilst Eddington faces down protesters who despise his status as a conscientious objector. Einstein arrives late at a demonstration of Fritz Haber’s poison gas and is so disgusted by this application of science to murder that he rejects an offer to convert his citizenship back from Swiss to German and refuses to sign a “Manifesto to the Civilized World”, a list of prominent German scientists, artists and academics supporting the war.
Eddington finds his research into Einstein obstructed by a British ban on the circulation of German scientific literature. Realising thatMercury’s orbit is precessing slightly less than it should be according to Newton’s laws, he writes to Einstein despite the ban to inquire into his view on the problem. Einstein’s relationship with Elsa deepens and on receiving Eddington’s letter he starts work on this new avenue with Max Planck, whilst consoling colleague Planck on the loss of his son in the war despite Einstein’s lack of belief in a human-like God or an afterlife. They find that Einstein’s work agrees with Mercury’s orbit where Newton’s does not, and send this reply back to Eddington. At the same time Eddington receives news and grieves for the death of Marston among the 15,000 killed by German use ofchlorine gas at the Second Battle of Ypres, causing doubts in his faith but leading him to fight all the more loudly against an expulsion of German scientists from the Royal Society. The expulsion has been initiated by Lodge, whose son was also among the killed and who clings to Newton as a consolation of “order in the universe”, but Eddington is unable to admit to Lodge that he too is grieving for a loved one lost.
News of the gas attack also leads Einstein to an outburst against his fellow scientists, which leads to his being cut off from the university, and — overworking — he falls sick and Elsa leaves him. Even so, he manages to complete his work on general relativity and on space being shaped by the presence of mass and gets this result through to Eddington via Planck. Eddington realises he can prove that space and light is being bent by observing the solar eclipse of 29 May 1919 on the west African island of Principe and with Dyson as an ally manages to gain funding for his expedition, despite Lodge’s initial opposition. As the end of the war comes, Eddington’s sister and housekeeper Winifred sets off to help the Quaker relief effort in war-shattered Germany despite her fears as to Eddington’s waning faith.
The action returns to the Principe expedition, prevented by bad weather until the very last moment, while Einstein briefly returns to his ex-wife and children. Bringing back two photographs from the eclipse to compare to photographs of the night sky in normal conditions, Eddington compares them in public with Lodge and Winifred in attendance and not only proves Einstein right but also finds this confirmation reaffirming his faith — as he states, “I can hear God, thinking”. News of his vindication reaches Einstein, and crowds of press arrive at his door just as Elsa returns to him. A year later, in the closing scene, Einstein visits Cambridge and meets Eddington, and closing credits remark on both scientists’ later work, Einstein’s celebrity and Eddington’s obscurity.
This final assessment of Eddington’s work is perhaps a bit unfair, as a number of key results obtained by Eddington, apart from the observational tests of general relativity considered in the film, are well known among astrophysicists, most notably the “Eddington limit” concerning the maximum possible luminosity of a star or galactic nucleus. Eddington also remains a well known figure among Quaker circles to this day.