Sir

In his Essay 'Lifting the fog from the north' (Nature 453, 39; 2008), Maximilian Schlosshauer describes how the process of decoherence can explain the famous double-slit experiment. An electron interacting with innumerable quanta in the photographic plate (and its environment) becomes entangled with all of them — and the resulting collective wavefunction is so narrow that it appears particle-like.

But the question remains as to why the wavefunction narrows in precisely the location where it does, or — as Schlosshauer puts it — “Why is a single spot here and not there?”

The author's somewhat 'foggy' answer is suggestive of a version of Everett's 'many worlds' idea (see Nature 448, 15–17; 2007), in which all possible branches of the wavefunction continue to exist autonomously. But this interpretation merely shifts the question to “Why do I find myself experiencing the branch/world with the spot here and not the branch/world with the spot there?”

We still have no answer and, if there is one, decoherence is at best only part of it (S. L. Adler Stud. Hist. Philos. Sci. 34, 135–142; 2003). As Joos and Zeh remarked on decoherence as a source of spatial localization: “Of course no unitary treatment of the time dependence can explain why only one of these dynamically independent components is experienced.” (E. Joos and H. D. Zeh Zeitschrift Phys. B 59, 223–243; 1985).

We are still left with a dichotomy: on the one hand, infinitely many continuously distributed potentialities, and on the other, one narrow, irreversibly realized actuality. Contrary to Schlosshauer's conclusions, complementary (mutually incompatible) descriptions are necessary to describe the landscape we are currently experiencing, even as the fog is lifting.