Image copyright Science Photo Library Image caption Einstein (left) and Eddington (right) only met for the first time years after the end of World War One

It is hard to imagine a time when Albert Einstein's name was not recognised around the world.

But even after he finished his theory of relativity in 1915, he was nearly unknown outside Germany - until British astronomer Arthur Stanley Eddington became involved.

Einstein's ideas were trapped by the blockades of the Great War, and even more by the vicious nationalism that made "enemy" science unwelcome in the UK.

But Einstein, a socialist, and Eddington, a Quaker, both believed that science should transcend the divisions of the war.

It was their partnership that allowed relativity to leap the trenches and make Einstein one of the most famous people on the globe.

Einstein and Eddington did not meet during the war, or even send direct messages. Instead, a mutual friend in the neutral Netherlands decided to spread the new theory of relativity to Britain.

Einstein was very, very lucky that it was Eddington, the Plumian Professor at Cambridge and officer of the Royal Astronomical Society, who received that letter.

Not only did he understand the theory's complicated mathematics, as a pacifist he was one of the few British scientists willing to even think about German science.

He dedicated himself to championing Einstein to both revolutionise the foundations of science and restore internationalism to scientists themselves.

Einstein was the perfect symbol for this - a brilliant, peaceful German who refuted every wartime stereotype while challenging the deepest truths of Newton himself.

Desperate fight to test Einstein's theory

So, as Einstein was trapped in Berlin, starving behind the blockade and living under government surveillance for his political views, Eddington tried to convince a hostile English-speaking world that an enemy scientist was worthy of their attention.

He wrote the first books on relativity, gave popular lectures on Einstein, and became one of the great science communicators of the 20th Century.

His books stayed on the bestseller lists for decades, he was a constant presence on BBC radio, and was eventually knighted for this work.

Image copyright Science Photo Library Image caption Eddington photographed the solar eclipse on 29 May 1919

It was hard to convince the UK to care about space-time and gravity as the U-boats were sinking food transports, and thousands of young lives were lost for meagre gains in Flanders, Belgium.

Just Einstein's ideas were not enough. Relativity is strange, with twins aging differently and planets trapped by warped space.

Eddington needed a definitive demonstration that relativity was true and Einstein was right, and that only his international approach could revolutionise science.

His best option was to test a bizarre prediction of Einstein's theory of general relativity.

When light passed near a massive body like the Sun, Einstein said, gravity would bend the rays ever so slightly.

This meant the image of a distant star would be shifted a small amount - the star would seem to be in the wrong place.

Einstein predicted a specific number for that shift (1.7 arc-seconds, or about 1/60 millimetre on a photograph). An astronomer would find this challenging to measure, but it could be done.

Unfortunately, it is normally impossible to see stars during daytime, so one would have to wait until a total solar eclipse to make the observation.

Total eclipses are rare, short, and often located in inconvenient places requiring extensive travel for European astronomers. Einstein had been trying for years to have this prediction tested, with no success.

Eddington, though, thought he might be able to make it happen at an upcoming eclipse in May 1919, visible in the southern hemisphere.

Even with the U-boat threat, no country was better positioned than Britain to undertake an expedition to test Einstein's prediction.

Image copyright Science Photo Library Image caption Frank W Dyson (left) secured funding for Eddington's expedition

Eddington needed a great deal of support for this.

Fortunately, he was close friends with Frank W Dyson, the Astronomer Royal. Dyson secured funding, although even with the money the war made it difficult to procure needed equipment.

Even worse, it was possible that Eddington would not be able to go on the expedition - because he might be in prison.

As a Quaker, Eddington was a conscientious objector to the war and refused to participate in conscription. Many other Quakers ended up jailed or performing hard labour.

After many failed appeals it seemed that Eddington would be arrested, but at the last moment he received an exemption (no doubt engineered by his politically savvy friend the astronomer royal).

Amazingly, it was given on the condition that he carry out the expedition to test Einstein's theory.

'Greatest moment in life'

The armistice in November 1918 meant that the expedition could go ahead.

Eddington wanted to make sure the results of the expedition, whatever they were, brought Einstein to the attention of the world.

So he and Dyson started a public relations campaign to get both the scientific community and ordinary people excited for the results.

The newspapers were primed and ready to report on what Eddington presented as an epic battle between Britain's own Newton and the upstart Einstein.

Einstein, seriously ill from wartime starvation and trying to navigate revolution-torn Berlin, knew little of this.

Instead, Eddington and his colleagues had to test Einstein's prediction almost completely on their own.

Two teams were sent to observe the eclipse: one to Brazil, and one - led by Eddington - to the island of Principe in West Africa.

Image copyright Science Photo Library Image caption Eddington's 'comparator' measured changes in the positions of stars, seen on telescope glass plates mounted below the movable microscopes

On 29 May 1919 - 100 years ago - those astronomers watched the darkened sky for six minutes to catch the smallest change in the stars to reveal the greatest change in our understanding of the universe.

Nearly ruined by weather, equipment malfunctions, and steamship strikes, the expeditions brought back photographs that hopefully showed stars displaced by the Sun's gravity.

After months of intense measurement and mathematics, Eddington had a positive result.

He called this the greatest moment of his life: "I knew that Einstein's theory had stood the test and the new outlook of scientific thought must prevail."

He presented the results to a room at the Royal Society packed with scientists and reporters eager to hear who had triumphed, Einstein or Newton (even as the portrait of Newton gazed over the proceedings).

The announcement created an enormous stir. The president of the Royal Society declared this "one of the highest achievements in human thought".

The Times headline the next day read "Revolution in Science".

Eddington had planned the event perfectly. Einstein, virtually overnight, went from an obscure academic to a sage everyone wanted to know more about.

And Eddington gave the public what they wanted. As the chief apostle of relativity in the Anglophone world, he was the one every newspaper and magazine went to.

His lectures had to turn away hundreds of people. Those who made it in not only learned about the strange physics of relativity, but also about Einstein as the symbol of international science, able to rise above the hatred and chaos of war.

Einstein himself could barely rise from his sickbed. He heard about the results from a telegram via the Netherlands.

He was delighted that his theory had been verified even as he was baffled by the media firestorm that suddenly enveloped his life.

Never again would he be able to venture through his front door without being accosted by reporters.

Without Eddington, relativity would have gone unproven, and Einstein would have never become the icon of genius.

Eddington was Einstein's most essential ally, though they did not meet until years after the war's end.

Their collaboration was crucial not only to the birth of modern physics, but to the survival of science as an international community through the darkest days of World War One.

Matthew Stanley is the author of Einstein's War: How Relativity Conquered Nationalism and Shook the World