to illustrate his theories. Mallett was one of the first African-Americans to obtain a PhD in theoretical physics. He was inspired to do so because he wished to travel back in time to visit his dead father, who died when Mallett was ten. HG Wells did it by means of an armchair and a cut lead crystal handle. The Starship Enterprise does it by unfeasibly powerful engines. And Doctor Who managed it in a 1960s police box, of all things. But just what is the truth behind time travel -- does ...