Thursday, April 30, 2009

Suárez on Quantum Propensities

Interpreting dispositional or power properties as propensities seems to me to be a very promising avenue for ontology. This is because theories employing powers (the ones I’ve seen) don’t get the modal structure of the world correct: by assuming that powers entail their manifestations, they fail to provide truthmakers for possibilities. Taking powers to be probabilistically manifested propensities solves this problem. It also bolsters a realist account of causality: actualizing propensities into specific outcomes gives causation some “real work” to do.

Finally, propensities can serve as a link between a philosopher’s ontology and the interpretation of quantum mechanics.* I was happy to find recently (via Philpapers) that propensities have a champion in Mauricio Suárez, a philosopher of science at Complutense University of Madrid. In several papers he has explored and advocated the propensity approach to understanding the properties of quantum systems. He also has pointed out the value of propensities to the dispositional/power property approach to ontology.

Popper and propensities

Propensity theory seems to be a relatively neglected topic these days. The work of Karl Popper may be one reason for this. Given Popper’s stature, the fact that his propensity interpretation of probability is widely regarded as a failure is discouraging. But Suárez makes a strong case that the problems with Popper’s theory are the result of his emphasis on interpreting quantum probabilities specifically, and also due to some particular assumptions which can be set aside or modified.

Popper wanted to interpret quantum probabilities using propensities, and he also thought propensities could be used to interpret probability in an objective manner generally. This effort has been roundly criticized. An important criticism is that known as Humphreys' paradox (after Paul Humphreys, see this paper). Humphrey pointed out that the asymmetric causal nature of propensities made them inconsistent with the symmetric character of conditional probability. But this paradox is only a problem for propensity interpretations of probability. When it comes to quantum theory, Suárez makes a wise point when he says that what we want to do is interpret quantum mechanics, not quantum probabilities. The probabilities observed in experiments would be explained by our account of quantum mechanics. This “clicked” for me: in my own reading of papers on interpreting quantum probability, I had found that the arguments tended to point toward subjective or Bayesian interpretations (see posts here and here), but this work didn’t seem to help one progress toward a satisfactory ontological interpretation of the physics. Perhaps it is better to interpret the ontology first.

With regard to some of Popper’s other assumptions about the nature of quantum propensities, Suárez explains in the paper “On Quantum Propensities: Two Arguments Revisited” how two other criticisms of Popper’s view may be avoided by a revised account of propensities – specifically Suárez’ ”selective propensities” proposal.

Selective Propensities

In addition to the paper mentioned above, Suárez has two other papers which discuss his selective propensities approach. In “Quantum Propensities”, he looks at some other historical attempts to employ propensities to interpret QM, and then contrasts his own proposal. 2004’s “Quantum Selections, Propensities, and the Problem of Measurement” develops the approach in the most detail, showing how it builds on Arthur Fine’s “selective interactions” solution to the quantum measurement problem.

Suárez’ approach is new to me and I’m still trying to understand it (I had not been exposed to Fine’s work before either). It seems that in the selective propensity interpretation, a quantum system possesses a number of dispositional properties coinciding with the observables we measure in experiments involving particles. These properties manifest themselves consistent with the probability distributions we observe in QM. We assert that in a measurement one interacts only with the property of the system selected. The interpretation then says that to explain the result, we can employ a mixed state of that property’s eigenstates to describe the initial preparation, rather than plugging in the full quantum state of the system. (The full quantum state encompasses all of the system’s properties.) We can still interpret the interference effects which result in some experimental setups as due to the interplay among the system’s various properties consistent with the full state superposition of the system.

This seems to imply that the description of the initial state of the system (setting up either a mixed state over one observable or the full state) is altered by how we set up the experiment. This seems strange at first glance, but I guess there’s always going to be something strange when you’re working with QM. I also wonder how to think about generalizing this scheme to understand how interactions work beyond the laboratory setting.

I’ll try follow up with more after re-reading and digesting this material further.
* I’ve often thought about the issues involved when philosophers try to make sure their metaphysical ideas comport with physical theories. On the one hand, philosophers very much want to avoid proposals which seem to conflict with science. On the other hand, since our physical theories are provisional (and likely to be replaced in time by improved theories), perhaps philosophers shouldn’t worry if well-motivated ideas imply revision to current scientific understanding. I’ve seen relativity theory invoked to criticize philosophical positions (e.g. presentism in the discussion of time – see an abstract of what looks like an interesting paper here), but many recent research programs in quantum gravity explore the idea that relativity is an effective (low-energy regime) theory rather than something fundamental. The search for a theory of quantum gravity implies relativity, quantum mechanics or both will need to be revised.

So, while I personally want my metaphysical theory to accommodate quantum mechanics (and worry less about conflicts with relativity), I realize that this is tricky territory. It seems best to just be explicit about one’s presumptions.

Monday, April 20, 2009

GPPC 2009 Public Issues Forum

This annual Greater Philadelphia Philosophy Consortium event is coming this Saturday afternoon April 25th, hosted at Drexel University. The topic this time is Just War Theory.

The three speakers:
Larry May (Washington University) on "Collective Responsibility in Warfare"
Robert D. Sloane (Boston University School of Law) on "The Cost of Conflation: The Dualism of Jus ad Bellum and Jus in Bello in the Contemporary Law of War"
Peter Tramel (U.S. Military Academy at West Point) on "Conscientious Objection and Volunteer Military Service"
Chair: Anil Kalhan (Earle Mack School of Law at Drexel)

April 25, 2009 1:00-5:00 pm Room 140 Earle Mack Law School, 3320 Market St., Drexel University, University City Campus, Philadelphia (Directions).
(Reception to follow).

Wednesday, April 15, 2009

Revisiting Actualism

Modal truths regarding possibilities and necessities need truthmakers, as much as anything does. If we conclude that our actual world cannot provide these truthmakers, this implies that reality must outstrip the actual: modal realism is the thesis that we must include possibilities in our reckoning of what exists. Actualism is the name for stances which would limit reality to the actual, and yet try to find adequate ground for modal truths.

This post is repetitive of previous ones (prior posts detailing my readings and evolving views on modal truthmaking are listed below), but I continue to think about this a lot, and recently read some new papers by philosophers on the topic. It struck me that there is something insightful about the insistence of some actualists on grounding modality in the features of the actual world.

Hardcore Actualism

These papers reminded me that in addition to the dispute between modal realists and actualists, another dividing line involves the use of worlds as truthmakers. Most often, modal realism is associated with the use of concrete possible worlds as truthmakers (as proposed by David Lewis). However, some actualist accounts make use of worlds as well, but propose that they still be considered part of the actual world, treating them as abstract or fictional, etc. (What makes this a bit confusing is that a model using abstract worlds could also be considered modal realism if the worlds truly transcended our own – existing platonically rather than as sets of propositions or “world-stories” or whatnot).

Actualism which rejects worlds of any sort is referred to as “hardcore” actualism in this paper by Gabriele Contessa. He favors this approach, as does Jonathan D. Jacobs in this paper. Jacobs explicitly endorses a power-property based approach to grounding modal truths.

A motivation for hard-core actualism is that our intuitions tell us that possibilities are grounded in the properties or causal powers of actual things. This is especially true if we consider the local, everyday, possibilities of life. I could have gone for a run this morning (although I didn’t). I know this is true because I have the natural capacities needed to do so, and in fact, it is something I often do. Why do I need to postulate something as extravagant and disconnected as a possible world (at which my counterpart did run) to provide a truthmaker for this homely fact?

Power-properties as truthmakers?

Jacobs makes reference to a number of philosophers who favor an approach which grounds modal truths in properties of actual things (one is Alexander Pruss, whose work I had commented on once here). There are two problems with these that I see. The first critique often raised is that these accounts seem ill-equipped to deal with more distant, but still intuitive possibilities. It seems possible that our universe might not have existed at all – what properties of the “actual world” could ground this possibility? I will leave this problem aside for now, because I think it might be addressable if we had an answer for a more pressing second problem. This is the fact that the power-property accounts I have seen to-date, while differing in their ontological set up, treat the powers (or dispositions) as entailing their manifestations. The entailment relation seems to assure (to my reading) that given two instances of precisely identical power-property complexes, the same manifestation would occur (I discussed this previously in the context of George Molnar’s work here).

So while the theory asserts that the powers could ground possibilities, in fact I don’t see that they really do the job. In my example, it appears my natural powers ground the possibility that I could have gone for a run, but the relation between power and manifestation in these theories doesn’t seem to explicitly account for the indeterminism involved here: if we somehow could “rewind the tape” and replay the events of this morning, what aspect of the ontology accounts for the fact that the manifested outcome REALLY could have been different?

Propensities at Work?

I have thought (with quantum mechanics in the back of my mind as usual) that instead of powers entailing their manifestations, the properties involved should be seen as propensities: causal powers with truly probabilistic rather than entailed outcomes. The set of unactualized possible outcomes, in this case, are real – they are rooted (and constrained) by prior actuality, but are not just theoretical entities. If this is right, then we’re not, strictly speaking, talking about actualism, anymore, of course, but a species of modal realism. However, at least the hard core actualist might be pleased that we’re not creating entire worlds out of whole cloth. Possibilities are manifestations -- I see them as events or event complexes -- they are not worlds.

What about possible worlds semantics?

Jacobs is so disenchanted with possible worlds as truthmakers, however their reality is conceived, that he wants to develop a replacement for the traditional modal semantics which utilizes worlds, and sets out on a path toward constructing an alternative. Fortunately, a good thing about my idea, I think, is that we can have a firmer basis for rationally constructing abstract worlds based on our acquaintance with these real local possibilities. Basically, we utilize the “replaying the tape” notion and utilize what we know about how probabilistic causation works in the actual world to construct alternatives for how the world could have been different. (This kind of approach was discussed by philosophy student Damon Woolsey in his modal realism papers – see here; also, Richard Chappell used the rewind/playback metaphor in a paper here). The further back you take this rewinding, the more distant possibilities you can envision. Note that counterparts are a bit different in this scheme – it seems either I’m in a world or I’m not given how they are constructed.

Can the set of these sorts of “worlds” ground the truth that our actual world might not have existed at all? Maybe – if you allow me to conceive of an initial probabilistic event which turned out to be the first seeding of our universe, but which was in no way guaranteed.

A Bloggy Exploration of Modal Truthmaking and Modal Realism
(in chronological order)

Whole Lotta Worlds
Notes on reading David Lewis’ On the Plurality of Worlds

Armstrong on Modality
About my inability to find modal truthmakers in D.M. Armstrong’s book Truth and Truthmakers

The World is Not Enough
On Actualism and Modal Fictionalism

Notes on Plantinga’s Modal Realism
A self-explanatory post title!

Local vs. Global Possibility and the Link to Causality
Alexander Pruss’ Aristotelian approach

Modal Realism, Modal Rationalism
Musing on how we know about modal truths

Modal Tenses
Parallels between modal and temporal metaphysics

Woolsey’s Modal Realism
An intriguing paper by Damon Woolsey found on the internets

Modal Problems with the Theory of Powers
Critique of powers as modal truthmakers in George Molnar’s theory

Actual as Indexical, After All?
Multiverses – Physical and Metaphysical
2 posts with more of my own musings