There are many theories in particle physics that cannot be tested in experiments. Does this make it unscientific? This debate, featuring one of our favourite theoretical physicists, David Tong, explores the question.
"[String theory] has led us in strange directions that we couldn't conceive of; it's revealed depths of mathematical structure that we couldn't have anticipated". Watch a video interview with Professor Michael Green, winner of the 2014 Fundamental Physics Prize.