The Life Scientific returns with a special episode from the USA; Princeton, New Jersey, to be precise. Here, the Institute for Advanced Study has hosted some of the greatest scientific minds of our time - Einstein was one of its first professors, J. Robert Oppenheimer its longest-serving director - and today's guest counts among them. Edward Witten is professor emeritus at the institute and the physicist behind M-Theory, a leading contender for what is commonly referred to as ‘the theory of everything’, uniting quantum mechanics and Einstein’s theory of gravity. He talks to Jim al-Khalili about a career that’s spanned some of the most exciting periods in modern theoretical physics - and about one particular problem that has obsessed and eluded him since his days as a student.Producer: Lucy Taylor

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