Robert P Crease writes about the life and legacy of Neils Bohr in this piece at Physics World:
Bohr practised physics as if he were on a quest. The grail was to fully express the quantum world in a framework of ordinary language and classical concepts. “[I]n the end,” as Michael Frayn has Bohr’s character say in the play Copenhagen, “we have to be able to explain it all to Margrethe” — his wife and amanuensis who serves as the onstage stand-in for the ordinary (i.e. classically thinking) person.
Many physicists, finding the quest irrelevant or impossible, were satisfied with partial explanations — and Heisenberg argued that the mathematics works: that’s enough! Bohr rejected such dodges, and rubbed physicists’ noses in what they did not understand or tried to hide. However, he did not have an answer himself — and he knew it — but had no reason to think one could not be found. His closest approximation was the doctrine of complementarity. While this provoked debate among physicists on the “meaning” of quantum mechanics, the doctrine — and discussion — soon all but vanished.
Why? The best explanation I have heard is advanced by the physicist John H Marburger, who is currently science advisor to US President George Bush. By 1930, Marburger points out, physicists had found a perfectly adequate way of representing classical concepts within the quantum framework using Hilbert (infinite-dimensional) space. Quantum systems, he says, “live” in Hilbert space, and the concepts of position and momentum, for instance, are associated with different sets of coordinate axes that do not line up with each other, thereby resulting in the situation captured in ordinary-language terms by complementarity.
“It’s a clear, logical and consistent way of framing the complementarity issue,” Marburger explained to me. “It clarifies how quantum phenomena are represented in alternative classical ‘pictures’, and it fits in beautifully with the rest of physics. The clarity of this scheme removes much of the mysticism surrounding complementarity. What happened was like a gestalt-switch, from a struggle to view microscopic nature from a classical point of view to an acceptance of the Hilbert-space picture, from which classical concepts emerged naturally. Bohr brokered that transition.”
Thus while Bohr used the notion of complementarity to say that quantum phenomena are both particles and waves — somewhat confusingly, and in ordinary-language terms — the notion of Hilbert space provided an alternate and much more precise framework in which to say that they are neither. Yet the language is abstract, and the closest outsiders can come to grasping it is Bohr’s awkward and imperfect notion.
A must-read piece; link via Chad.