- Decoherence doesn’t completely neutralise the puzzle of quantum mechanics. Most importantly, although it shows how the probabilities inherent in the quantum wave function get pared down to classical-like particulars, it does not explain the issue of uniqueness: why, out of the possible outcomes of a measurement that survive decoherence, we see only one of them. Some researchers feel compelled to add this as an extra (you might say ‘super-common-sensical’) axiom: they deﬁne reality as quantum theory plus uniqueness.
- All the same2, thanks to the theory of decoherence, we no longer have to make quantum measurement some magical and mysterious event that crystallises knowledge. We have a mathematical theory to explain how information gets out of the quantum system and into the macroscopic apparatus. We can use the theory to calculate how quickly that happens, and how robustly. We have, at long last, a theory of measurement. What’s more, it is a theory that confers no privileged status on the conscious observer3, stripping away the seemingly mystical veneer from quantum mechanics.
- There’s no longer any need for Bohr’s arbitrary division of the world into the microscopic, where quantum mechanics rules, and macroscopic, which is necessarily classical. Now we can see not only that they are a continuum, but also that classical physics is just a special case of quantum physics. Regarded this way, common sense is a direct and utterly sensible outgrowth of quantum sense.
- This quantum theory of measurement is a reversal of the usual way that science works. We normally take our human common sense and experience for granted, and work back from it to deduce more fundamental physical behaviours. Sure, what we discover that way might sometimes seem a long way from common sense – heliocentrism, Higgs bosons, black holes, etc. But we typically get to those points by taking it for granted that there is an uncomplicated relationship between what we measure and what is there.
- Decoherence theory doesn’t take that common-sense view of measurement for granted. It starts by accepting that the world is fundamentally governed by quantum rules, which seem at face value to run deeply counter to experience, and then it works upwards to see if it can recover common sense. Remarkably, it can.
- That is why the quantum theory of measurement can be thought of as nothing less than a ‘theory of common sense’. Decoherence theory explains where common sense comes from – namely, out of principles that seem very far from common-sensical. The challenge is then on all of us to reconcile our instinctive common sense with its quantum origins. But we no longer have to regard the two as being in conﬂict, since they are not only consistent but inextricably linked.
- We can seek solace in the knowledge that the conﬂict between classical and quantum is not in the physics. It’s just in our minds.
Footnote 2: The previous caveat seems to be left dangling.
- There’s much good stuff in this paper that deserves consideration, and I hope to consider it in due course.
Footnote 3: Excellent!
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