General summary: The theory of physics which scientists since the 1920s believe describes all of Nature, quantum mechanics, says profound things about how we should look at the world. It says that everything is uncertain until it is observed, and that observation inevitably alters reality. For nearly a century, scientists and philosophers have tried to come to grips with what this really means. In the 1990s, Lucien Hardy proposed a paradox which seemed to support the idea that one could never reliably make inferences about past events which hadn't been directly observed. Quite independently, from the late 1980s up until the present day, Yakir Aharonov's group had been developing a new way of thinking about measurements that suggested just the opposite. Until recently, it seemed impossible to carry out Hardy's proposal in practice, let alone to confirm or resolve the paradox. We have finally been able to do so, and to apply Aharonov's methods to the problem, showing that there is a way, even in quantum mechanics, in which one can quite consistently discuss past events even after they are over and done. |