The 'grandfather paradox' of time travel has been puzzling philosophers, quantum physicists and novelists for years. Now there's an answer as Cathal O'Connell reports. It is more than 120 years since H.G. Wells published The Time Machine, the novel that was to popularise the concept of time travel and lead to myriad stories on the theme. But it is only now that we have finally developed a plotline for time travel that makes logical sense – and it has been penned by a machine. The breakthrough involves the grandfather paradox – that favourite plaything of philosophers where somebody travels into the past and kills their own grandfather, preventing the existence of one of their parents, and therefore their own. But the problem is, if the protagonist doesn't exist, then how could they go back in time to set off the chain of events in the first place? The paradox is often extended, in various guises, to regard any action that alters the past – such as Marty McFly avoiding the amorous