Friday, February 23, 2007

A Matter for Science

My view is that the right notion of what goes on at the fundamental level of nature can help make sense of philosophical questions which arise at the human level. For instance, consider the mind/body problem. The proposal I endorse says that the elementary constituent of our concrete world is an actualization event which has an essential experiential aspect. The existence of these events is already known to us: they are the measurements (or interactions between quantum systems) in quantum physics. These events combine in a causal network which comprises the natural world. The simple experiential qualities of simple systems are *somehow* leveraged into the macro-level experiences with which we are familiar.

Now, the problem is, between the fundamental level and the human level there are many orders of magnitude and incredible amounts of complexity. The human brain/body system, in particular, is a complicated Rube Goldberg contraption. So that qualifier (“somehow”) looms large.

But the good news is that this sort of “combination problem” is primarily a scientific question. The normal course of scientific research will shed light on the philosophical proposal. Here’s an example. If I’m right, we will uncover facts which lead us to attribute (basic or “core”) conscious experience further and further down the evolutionary scale. This is because our own consciousness is continuously rooted in our evolutionary heritage. On this front, I’m interested in a paper like this one, which looks at the role of the brain stem in consciousness (HT: Brains).

Also, we should find evidence that non-trivial quantum effects are harnessed by biological systems, including our own. This is because our grounding in quantum mechanics is needed to make sense of life and mind in a way not possible in classical science. So, I want to keep a lookout for clues like this blog post at Neurophilosophy, which describes a proposed role for subatomic physics in olfaction.

Tuesday, February 20, 2007

Wigner's Friend and Perspectivist Quantum Theory

As discussed at the end of the last post in my series on quantum mechanics, I want to explore the ideas contained in Paul Merriam’s recent paper to see if his proposal for “quantum relativity” can address the challenges faced by the relational interpretation of quantum mechanics. But first I want to double back a bit by looking at Merriam’s previous (1997) paper on the philosophy of QM: called “On the Relativity of Quantum Superpositions,” it was included in an edition of an online journal called Metaphysical Review.

In this paper Merriam independently (to all appearances) arrived at the same general position as in the relational interpretation primarily identified with Carlo Rovelli. He also began to explore the idea of using relativity theory as a guide to further development of the relational view. To lay the groundwork, the paper begins by invoking the “Wigner’s friend” scenario and showing how it leads to the view he described as “Perspectivist Quantum Theory”.

Eugene Wigner, to highlight a certain aspect of quantum theory, presented an idealized thought experiment (a short account of this written by Henry Stapp can be found here). In the “Wigner’s friend” scenario, an experimenter (the friend) is performing a measurement on a quantum system that will result in one of two outcomes: the original example invokes an atomic state that will emit a visible photon either into the eye of the experimenter or elsewhere (Merriam’s paper imports Schrödinger’s’ cat into his version). A second experimenter (Wigner) is stationed outside the sealed laboratory where the measurement will take place. Inside the laboratory, from the perspective of Wigner’s friend, the experiment will collapse the quantum state into one of the two outcomes and the friend will observe the photon or not. To Wigner on the outside, the physical description of the state in the lab will be a superposition of the two scenarios, where the friend observes the photon and where she does not.

Wigner used the thought experiment to support his view that human consciousness was intimately bound up with quantum measurement: for him, it might make sense to view the two outcomes in the lab as being in superposition if the measurement was performed by an inanimate apparatus of some kind, but he believed that the consciousness of the human friend would have engendered collapse whether or not the he (the second observer) made contact with the situation in the lab. However, there is no part of quantum theory itself which makes any such special provision for human consciousness.

Merriam wants to take QM seriously as a description of nature, and notes the theory itself says nothing about applying only to microscopic or simple or non-conscious systems. QM doesn’t magically dissolve into classical reality at some objective threshold (there is no “Heisenberg cut”). The states of what’s happening inside the lab as known to Wigner and Wigner’s friend differ. This difference cannot be “swept under the rug”. What the Wigner’s friend example is telling us is that QM gives a true physical description relative to the observing/measuring system. “What prediction is made depends on which system is doing the predicting.” Quantum states are well defined only when relativized to a particular system. Quantum theory is an intransitive theory.

Next Merriam considers a comparison of this situation to that of relativity theory: “A quantum description of the state of the system is relative to the system doing the describing just as the description of a system in terms of space and time is relative to the motion (and gravity) of the describing system”. The question is whether we can build on this analogy in a useful way.

In considering the situation, note again that there is no reason to think QM is anything other than “democratic” in nature (my label). As Merriam says: “In QM, there is no ‘ontological’ difference between an experimenter and an electron…” From the point of view of the electron, the experimenter is in superposition between interactions. The next move is to postulate that Quantum Mechanics holds for the points of view of all quantum systems. We then note that while quantum states viewed from different points of view will differ, the perspectives of two systems must match up when they interact. The theory to be developed must be based ultimately on the interactions between systems. The interactions are the pivot to link things together. (This is a point stressed by Rovelli: the interactions are the fundamental entities; quantum states lack objective existence). So, we have a situation where we postulate that QM gives correct physical descriptions from a point of view, but there is a plurality of points of view. A theory of quantum relativity would note that “simultaneity” means two concurrent observations involving the same observer. Since systems’ perspective will match during an interaction, this may form the basis of extending the theory. For instance, the time interval between two succeeding interactions between two systems must match.

It is with this point that the paper ends. Merriam presses the idea of quantum relativity further in the next paper, which I will discuss in a further post.

Friday, February 09, 2007

Woolsey's Modal Realism

I recently discovered a paper on modal realism posted by Damon Woolsey, and I liked it very much. Here is a brief take on it.

Woolsey endorses modal realism, but has some problems with David Lewis’ version. He first considers the alternative of branching worlds as a way of reducing the extravagant number of non-overlapping causally distinct worlds in Lewis’ theory. This gives one a system of world-lines in a tree-like structure. A virtue of this may be that it is consistent with our intuition of indeterminism and an open future (for Lewis, the actual and possible worlds are deterministically fixed). On the other hand, the idea of a persistent individual persisting across different world-lines is problematic (vs. Lewis’ use of counterpart theory). Also, with branching worlds, we must accept that the “worlds” are constructions of a sort, rather than really unified worlds. Given this, we should bite the bullet and give upon fundamental possibilia as unified worlds and consider them instead as the set of possibilities considered from one’s particular point of view.

Without postulating all the details of a causal theory or a theory of mind, Woolsey asks us to consider that each of us comprises a causal nexus. We have direct acquaintance with the state of affairs which enter into our nexus. There could be various possibilities going on at points distant from us (patterns of sand in the Gobi Desert) which are consistent with our local facts. So, real possibilities are defined as those states of affairs which can mutually co-exist with a given causal nexus. Given this vision, a possible world is derived as a specific “maximally consistent” state of affairs. The paper has a good discussion fleshing out these notions.

To digress for a moment: one thing I like about this idea is how it coheres with the relational interpretation of quantum mechanics. Possibilia are real, but this reality is relative to a given quantum system=causal nexus (as opposed to a modal realism which features truly branching worlds, which would be consistent with the many worlds interpretation).

In Woolsey’s view, one can derive worlds also by tracing the chain of events back and looking at what other possible world-lines could have happened (he calls this the “causally constrained principle of recombination”). I like this, because it coheres with my discussion in prior posts about how we can know about metaphysical possibilities (see this post, including the comment thread, also this one on Alexander Pruss’ theory). The idea is that the real possibilia that ground our modal intuitions are those adjacent to us. Possible worlds and the unicorns and such which inhabit them are more distant entities which are extrapolated from this raw material. Our conceivability apparatus “rewinds the tape” from our current situation to infer the nature of more distant possibilities. It gets more difficult to imagine how this works when we go beyond nomologically possible worlds and individuals, but I trust our intuition that physical laws are contingent as well, and that our ability to conceive the full metaphysically possible landscape is basically sound. After all, on this account, the intuition is a form of knowledge grounded by contact with real adjacent possibilia. (One area where Woolsey differs is that he is reluctant to endorse possibilia which go beyond the ones consistent with our physical law and the initial state of our world).