It is common knowledge that surveillance technologies keep us safe. These technologies can uncover where a would-be bomber has fled or reveal that a suspect is planning an attack on the New York City subways. Such successes, and the belief - and business - in them has fueled a vast expansion of CCTV, wiretapping and data mining since 9/11, the Madrid bombings in 2004 and the attacks on London's public transport system in 2005. But this common knowledge is based, at best, on shaky evidence. In…