Thursday, March 30, 2006

Top-Down or Bottom-Up Quantum World?

At Antonio’s suggestion in his comments on the last post, I reviewed more of Ulrich Mohrhoff’s work on interpreting quantum physics contained in this paper and on his website, thisquantumworld. I also plowed through some of the debate between Mohrhoff (posting as koantum) and Patrick Van Esch (vanesch) on this thread at physicsforums. This was extremely fascinating although often over my head.

From my reading so far, Mohrhoff is saying two things which I think have some merit:

1. He rejects the assumption that the wave function represents something real. Many physicists extrapolate from the elegance of the idea of the deterministically evolving wave function to an ontological interpretation of QM (many-worlds or many minds) which tries to elevate this side of the story and minimize the measurement process. There is no quantum theory without measurements!

I’m sympathetic here, since while I believe the both of the quantum processes are fundamental, I think the measurement events are the “more real”: they constitute our concrete world while wave functions are the abstract possibility space available to be actualized by measurements (they are “real”, too, but not in a concrete sense).

2. While rejecting the naïve assumption that the wave function represents something real, Mohrhoff does want to find an objective description of reality which doesn’t appeal to consciousness. QM gives the probability distribution for unperformed measurements. It is a mistake to see these as subjective probabilities. They are objective probabilities.

I agree with this to an extent: I don’t see full-blown human consciousness as the sole avenue to measurement, and think natural systems implement measurements ubiquitously. However, in my view, the phenomenon of first-person consciousness is rooted in an experiential quality which is part of all measurement events (in the spirit of panexperientialist proposals such as Whitehead's or Gregg Rosenberg's).

But what is Mohrhoff’s positive proposal? What is “this quantum world”?
Here are some notes I took from his writing with my editorial comments in italics.

1. The world is intrinsically non-local. (Don’t be confused by thinking about the existence of a space-time background of points and instants – we contribute that to the theory, its not intrinsic).

2. Identical particles cannot be distinguished from each other independent of their possession of properties which can be distinguished.

So, these two statements imply that the quantum world cannot be built from the bottom up.

So how is it built? From the top-down.

“What ultimately exists is one. Call it whatever you like. Matter and space both come into being when this enters into (more or less fuzzy) spatial relations with itself…””…the relations are self-relations,”

This sounds like the world possesses a power of self-measurement. This is certainly an interesting rearrangement of the mystery, but I don’t see how it increases our understanding of reality.

Nothing has a property until it is measured (including the property of existing in a space-time continuum). Measurements create their outcomes (it isn’t that these properties are ontologically carried by the wave function between measurements). No measurements/no world.

OK (although I would say the wave function carries properties in their form as possibilities, and they are not created but made concrete when measured).

The macroscopic world is real in a way the microscopic world is not, since the probability of finding macro-objects where classically they should not be is very low. “… we must be allowed to look upon the positions of macroscopic objects – macroscopic positions, for short—as instrinsic, as self-indicating, or as real per se.” “The ‘foundation’ is the macroworld… not the micro-world.” “As philosophers would say, the properties of the quantum domain supervene on the goings-on in the classical domain.”

But the existence of macroscopic objects is just a primitive in this interpretation. This top-down view is equivalent to saying we just can't explain macroscopic events or objects. I find this unsatisfactory. I want to see us build an improved bottom-up explanation of the world, where the raw material is a property dualism matching the properties embedded in the quantum probability space with an “ability to measure” property possessed by natural systems.


Clark Goble said...

I had the same feelings as you did in college. Although I have to confess reading Bohm convinced me of the problematic nature of the "collapse of the wave function" as key to much.

I slowly came around to the top down move rather than the bottom up view. It's interesting someone else is writing about this formally.

Steve said...

Thanks Clark. I'm not sure I understand well this top-down view (or did it justice in this post). I don't yet see it as something which improves our conception.

I think I understand your perspective on the wave function "collapse". But most everything I read leads me to conclude hidden-variable approaches didnt' work well and that measurements are indeed fundamental. (i'm not sure if Bohm's later views have anything in common with this top-down idea -I read Bohm & Hiley's "undivided universe" long ago and it's not fresh).

Clark Goble said...

It's honestly been too long since I last read Bohm to feel confident about expressing his position. As I recall though he sees the issue as one of holism. As such it bears some resemblance to some of the issues of say Davidson in language. Bohm thinks the quantum field is real and treats it in a manner more analogous to how we treated electric potentials in E&M. Then there are real objects that follow this in a method analogous to E&M. The difference is that we don't know how it is moving until we measure it for obvious reasons.

My big complaint with Bohm (once again going by memory) is that the ontology of these objects in the quantum field is anything but clear to me and seemed problematic. I also never could figure out how to reconcile vacuum fluctuations with his model.

I long ago read a bit on Kramer's transaction theory which is an other realist interpretation which supposedly fairs a bit better than Bohm. But I never made it far enough to be sure of his ontology. And its popularity seemed to wane.

I don't think that Bohm's view would be top down. Although perhaps some diagree. But his reliance on a real field arising from objects and independent objects seems a bottom up approach.

The top down approach tends to take the field, not the objects as fundamental and essentially holistic in nature. That is, that many quantum effects are due to not knowing the full system. This isn't that extreme a position and can be found in many early interpretations of QM. (Indeed I think the Bohr view partakes a lot of it) The controversial point is that there aren't objects independent of the field. Rather the field composes the objects.

I'll confess that one of the big reasons I like it is the parallel to GR. I have a sneaking suspicion that any unification of GR and QM will have to approach things in this fashion. Not that I'm qualified to make such claims. But it seems true on a gut level.

The interesting issue is attempts by some to approach SR in this fashion. They do so mainly for apologetic reasons: to preserve a strong sense of free will. But the neo-Lorentzian view which makes SR (and by extension GR) more an epistemological feature rather than a real description is interesting, even if I think ultimately problematic. But some (notably Bell) think that QM requires that one do just this to SR.

Steve said...

"the field composes the objects" vs. my view that micro-measurements comprise the world. Hmm. Your mention of GR leads to a thought: measurement events or interactions all take place relative to other systems and there is no preferred viewpoint. So you could think of this as a relational network whose nodes consist of interactions. This may not be too far from your view except that its sort of discrete for a "field". If you havent' seen it already , I might suggest you check out Carlo Rovelli's "relational interpretation of qm which is along these lines.

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Clark Goble said...

Relational theories have a long career. Indeed Leibniz tried to reformulation mechanics in a purely relational format. (Required by his monadology) Of course Leibniz was unsuccessful so Newtonian notions of absolute space dominated. However the quest for a fully relational model continued. Mach in particular tried to do it (ironically in an anti-realist ontology) This was also Einstein's aim although clearly he failed: GR still is a substance based ontology as I think Sklar presents fairly clearly. (His Space, Time, and SpaceTime is the seminal treatment of the issue)

In recent work on the ontology of quantum gravity the issue of a fully relational theory has been proposed as well. Of course some see working out the philosophical implications of a theory not yet developed as perhaps a bit premature. But as Einstein shows (and as Lee Smolin has recently argued) philosophical inquiry often leads to very fruitful physical theories. Something that perhaps quantum gravity needs more of. Lots of interesting appeals to the hole argument of course.

If any of you are interested, while its a tad dated in this fast moving field, Physics Meets Philosophy at the Planck Scale is definitely worth reading. Lots of interesting takes on the philosophical issues. (My favorite is the paper "Pre-Socratic Quantum Gravity")

Steve said...

Sorry - I was offline for a few days.

I second your recommendation, Clark. I recall there were a number of good papers in that volume.

Steve said...

I'm pleasantly surprised to see that there is a thread now in physicsforums where the relational interpretation of qm is being disussed and Mohrhoff (koantum) has offered up his opinion of it. I continue to prefer RQM to Mohrhoff's view: the fundamental entity as the quantum interaction rather than the "macroworld" as in Mohrhoff. Here's the post.

koantum said...
This comment has been removed by a blog administrator.
koantum said...

Hi Steve,

Here are some comments on your comments on my views on QM:

koantum matters: Top-down or bottom-up?

Steve said...

It was very good of you to take the time to review my comments. I will read the material you linked to and respond via a comment on your blog. (that is, if I think I have something semi-intelligent to say!)

Thanks, - Steve

Steve said...

One more follow up item: Rovelli has recently posted a new paper with Matteo Smerlak called "Relational EPR". Hat tip goes to this blog which was new to me: Reality Conditions.

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