Heisenberg’s uncertain legacy

Editorial from Nature:

Nphys4330-i1The UK premiere of Simon Stephens's play Heisenberg: The Uncertainty Principle is a reminder that the cultural cachet of Werner Heisenberg's discovery 90 years ago1 is as strong as ever. Physics is in fact notable only by its absence in Stephens's play, which is about an unlikely relationship that sparks between the production's two sole characters in a starkly minimalist setting. But the rather tenuous evocation of this tenet of quantum mechanics illustrates how its interpretation in terms of the unpredictability of the world and its sensitivity to our intervention continues to offer an attractive metaphor for artists.

Physicists might rightly complain that this metaphor rests on a misconception. That there is an inherent unknowability about how the future will unfold, and that it might be shifted by almost imperceptible influences, seems far more aptly compared with chaos theory — a purely classical phenomenon, albeit with a quantum equivalent — than with the uncertainty principle. To suggest that Heisenberg's theorem proves we can't acquire perfect knowledge without disturbing that which we seek to understand is, in fact, rather to undersell, as well as to distort, the uncertainty principle.

A better way of looking at it is to say that certain pairs of quantum variables cannot meaningfully be said to have simultaneous values defined more tightly than Heisenberg's famous bound of ħ/2. Uncertainty is a misleading word for that, implying imperfect knowledge of a state of affairs rather than a fundamentally lacking definition of that state. Heisenberg of course expressed it in German in his 1927 paper, talking of both Ungenauigkeit and Unbestimmtheit; translation is inevitably approximate, but these might be reasonably rendered in English as inexactness and undeterminedness. The latter is closer to the mark; the origin of 'uncertainty' might be ascribed to Niels Bohr's preferred term Unsicherheit, which refers to doubtfulness or unsureness.

More here.