I have thee not, and yet I see thee still.' Shakespeare buffs will recognise this from the famous scene in Macbeth in which the guilt-ridden Scottish king attempts and fails to grasp a midair dagger.

Dark powers guide dagger-sharp ‘Macbeth’ at Washington National Opera© Scott Suchman/Kennedy Center Members of the Washington National Opera in “Macbeth.”© Scott Suchman/Kennedy Center ...