# General relativity, 100 years on...

## Interview with

One hundred years ago, on the 25th of November, Albert Einstein presented his theory of general relativity to the world. This theory fundamentally changed aspects of physics that hadn't altered since Isaac Newton first came up with them, hundreds of years earlier. Over night, Einstein became a celebrity - in fact the first scientific celebrity. Connie Orbach has been hearing how, beginning with a whistlestop tour, thanks to the UK's Science and Technology Facilities Council and David Tenant, of what general relativity is all about...

David - At 26, he figured out nothing less than a new theory of space and time. It led to a nifty way of simplifying physics by treating space and time as one thing - spacetime. But Albert was just warming up. He wasn't happy with Isaac Newton's mysterious force of gravity. Naturally he started work on his own theory and sure enough, he cracked it.

Mass causes space time to curve. The natural motion of thing is to follow the simplest path through space time but, since objects with mass curve space time, stuff moves towards the most massive object - that's what you feel as gravity. It's warped space and time that's keeping your feet on the ground.

Connie - So that general relativity - a theory to end all theories. But how did Einstein himself become so famous. I caught up with Professor Gerry Gilmore at Cambridge University's Institute of Astronomy to find out...

Gerry - We're going down to this end of the building which is where the Director used to live and going into the room that used to be the Director's sitting room where the great people would have worked, and there's a whole string of famous people lived in this half of the building. The one that's particularly relevant for this current centenary is Sir Arthur Eddington, who in 1919, showed that light is bent by the sun in just exactly the way that Einstein's general relativity, which is 100 years old today, predicted. That was the event - the public announcement of that - which was in November, 1919 was what made Einstein famous. That's when Einstein appeared on the front page of the New York Times and London Times and became a celebrity. That was the beginning of scientific celebrity and it changed our view of scientists completely.

Connie - Early A-lister...

Gerry - Absolutely.

Connie - An A-lister indeed. And if you ask many people today, Einstein will still be on their dream dinner party list. Let's go back 100 years to find out how it all came about...

Gerry - Okay, so this was, of course, right in the middle of the darkest days of the First World War but, in spite of that, mathematical physics in Germany was advancing at a spectacularly fast rate, so there were several people racing towards what became general relativity - Einstein got there first - but none of this was known in the West. The only connection between German science and the outside world was via The Netherlands - a neutral country at the time - they were communicating both with Einstein and with Eddington and so the people in Leiden realised the importance of what Einstein was doing and sent the work to England to be published by Eddington. Eddington also independently realised the enormous value of this and further developed the work himself, and took it upon himself to market and explain relativity to Western scientists. And so he wrote books and articles which were hugely popular, hugely influential. Whereas Einstein's own work marketing the theory didn't actually appear in English until the 1920s, so Eddington had already done it.

Connie - Since then, we're now a 100 years on from that this theory still stands.

Gerry - That's right, yes. It turns out that general relativity has been tested to astonishing precision and every single test we've done of it over the last 100 years, it turns out to be dead right. It's truly remarkable. So the whole subject of black holes and space, all these wonderful things we take for granted, X-ray astronomy, massive exotic events, quasars; these are all extreme version of general realistic phenomena, none of which happened under Newton. And every single day we astronomers are studying things that just exactly follow general relativity. So we know it's accurate because the description of the solar system around us to much better than one part in a million.

Connie - Einstein's theory has inspired scores of scientists with his work but, beyond that, he has also become a figure of popular culture in literature, art and, on this centenary, also in music and Sam Genders has written a song to commemorate the anniversary with his band, Diagrams. I got in touch with Sam to see what inspired him...

Sam - He was obviously a really interesting person and came up with these incredible ideas and, for me, there's something so inspiring about the fact that so much, so much of the work he did just came from pure maths and thought experiments. And so much science is done through technology these days that I find that really exciting.

Connie - The bending of space and time is something which can quite easily be kind of turned into music in a way. It's got quite a romantic element to it...

Sam - Yes, it's sort of metaphorically ripe for kind of slightly ambiguous lyrics that could be about science, or could be about relationships. Einstein himself was a, as far as relationships went, especially romantic ones, was a fairly complicated human being and so, within the song, there are sort of these threads of relationship and playing on the words a little bit of general relativity about how we do all see things from certain different perspectives. So yes, it inspired me anyway.

Connie - Sam joins the pile of people inspired by Einstein - science's first A-lister, but he couldn't have got there alone. Work on relativity made him a great scientist, but Sir Arthur Eddington made him famous..,

Gerry - Yes, Eddington deliberately made Einstein a superstar. It was part of a conscious programme, for a small number of people lead by Eddington, to try and rebuild scientific connections between Germany and the rest of the world after the war. But Einstein was all over the front page of the New York Times and the front page of the London times - a world shaken by a new theory; Newton wrong; the stars are not where they ought to be - all this sort of stuff, and so this started the cult of celebrity science.

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