The phrase "collapse of the wave function" in many discussions of Orch- OR and the observer in QM rolls easily off virtual tongues, but I think it gets us off on the wrong foot. What's at the bottom is observables--like the pointers on measurement devices--and you need wave function collapse for observables, so that's how collapse of the wave function gets into the act. After all, quantum physics gives an unbroken description of an *unobservable* reality that superposes *possibilities*. Quantum reality has an ontology of possibility. If it weren't for the dang measurements that yield classical observables, everything would be hunky-dory for a consistent quantum physical description of reality.
Mind you, this is a reality that extends across scales from microwelt particles to mitwelt world objects (like pointers on measuring instruments). (Umezawa showed in quantum field terms how to extend the scale to mitwelt objects.) So quantum physics has great explanatory power, save for one thing: observables. Accordingly, on classical quantum physical accounts, you gotta have collapse unpredictably to one or another of the eigenstates of the wave function 'cause its the collapse that permits observables to appear.
Whether the collapse is thought of in terms of Heisenberg "actual events," Penrose's gravitational collapse, GRW theory or whatever, the notion retains an ad hoc character, superimposed on a pure quantum theory by the need to account for the observables that are the very epistemic basis of quantum physics itself.
Other people, however, have held the wave function never collapses. The best known of the no-collapse theories is Everett's "multiple-worlds" theory. (David Bohm's is also a no-collapse theory.) The wave function never collapses to a single world in a measurement but there are multiple worlds containing all possible results of measurement. (As in Jorge Luis Borges' "Garden of the Forking Paths.") Thus in Schroedinger's famous cat paradox, there is a world in which he observes a dead cat and a world in which he perceives a live one. Lots of people actually believe this wierdly over-stuffed story, so odious is ad hoc collapse. At least mathematical beauty prevails.
The great problem for no-collapse theories remains the original one: How do you get observables? If we could only get out a flow of observables without collapsing the wave function... Here's a way: You need neural tissue to do it. (Searle's right after all!) You need a brain *with quantum degrees of freedom*, biosubstrates that generate quantum fields which interact. These are emergent quantum fields hoisted by brain tissue. One quantum field re-presents the input order. Another quantum field realizes cognition. (Cognitive conditions of satisfaction = superposed possibilities.) This quantum cognition is conjugate to certain re-presentations of reality. When a quantum re-presentation of quantum reality interacts with quantum cognition and makes a match with its conjugate, then an observable appears.
The mitwelt is thus continually disclosed in quantum field interactions supported by neural tissue. Meanwhile outside the brain, all remains an unbroken unobservable quantum reality, just as quantum physics has always said. The mitwelt? the *world* (like pointers on measuring devices)? It's all in your head...and outside lies a quantum reality with no look at all. (Scary, ain't it? Enough to turn and embrace the ugly collapse of the wave function?)
One more extremity: "Inside the head"..."inside the brain," these observables, too, are generated by someone's brain. If I perceive my own brain through a window in my skull, my own brain is inside my own brain... What Borgesian vertigo! ("The Circular Ruins.") A handhold is the reminder that the "brain biosubstrates" which hoist quantum fields are not themselves observable. The biosubstrates we might actually perceive are not the ones I mean.
We can get away with thinking that the brain we might see through a window in someone's skull is what's hoisting quantum fields and supporting their interactions, because there is a definite relation between the "phenomenal" brain we perceive and the unperceivable real one, viz., the former conserves the latter's symmetries (under Yasue's principle of least neural action). It works out to say that our good old gray matter up there makes perception possible because that phenomenal stuff is symmetry-conserving with respect to what's actually doing the perception job.
So I say, be very wary of any "collapse of the wave function." I don't think it really occurs, and we don't need it to account for observables for this reason: The match between the brain's quantum re-presentation of reality and the superposed conjugate realities of cognition brings observables. Genug. No collapse needed. The "brain"--the real one, not the phenomenal one--is a quantum clearing where worlds are continually disclosed. The rest is quantum-night...
Gordon Globus
Globey@usa.pipeline.com
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