Stephen Hawking on falling into a black hole - audio

Falling into a black hole is a bit like going over Niagara Falls in a canoe - there is no way back, says Professor Stephen Hawking. He is giving this year's BBC Reith Lecture exploring these collapsed stars. They challenge the very nature of space and time, as they contain a singularity - a phenomenon where the normal rules of the universe break down. Photograph: Philip Toscano/PA Wire

• Most threats to humans come from science and technology, warns Hawking