It has been more than a century since the groundwork of quantum physics was first formulated and yet the consequences of the theory still elude both scientists and philosophers. Why does light sometimes behave as a wave, and other times as a particle? Why does the outcome of an experiment apparently depend on whether the particles are being observed or not? In the first of two episodes, Ian Sample sits down with the physicist Carlo Rovelli to discuss the strange consequences of quantum theory and the explanation he sets out in his book Helgoland. Help support our independent journalism at theguardian.com/sciencepod