From the classical point of view, a real photon travels between the light source and the screen. The odds are 50–50 that it will go to slit one and 50–50 that it will go to slit two. From the point of view of quantum mechanics, there is no photon until a detector fires. There is only a developing potentiality in which a photon goes to slit one and to slit two. This is Heisenberg’s “strange kind of physical reality just in the middle between possibility and reality.”5
