What We Cannot Know: Explorations at the Edge of Knowledge

by Marcus du Sautoy

My children are obsessed with the science fiction TV series Doctor Who, just as I was as a kid. The aliens we find scariest are the Weeping Angels, stone figurines like those in our local cemetery that don’t move provided you don’t take your eyes off them. But blink and they can move. The theory says that the pot of uranium on my desk is a bit like a Weeping Angel. If I keep observing the uranium, which obviously means a little more than just keeping my eyes on the pot on my desk, I can freeze it in such a way that it stops emitting radiation. Although Turing first suggested the idea as a theoretical consequence of the mathematics, it turns out that it is not just mathematical fiction. There have been some experiments in the last decade that have demonstrated the real possibility of using observation to inhibit the progress of a quantum system.