A brood comb

….philosophical and other notes….

Physics vs. Physicalism

Posted by Tanas Gjorgoski on January 13, 2007

In this post I want to discuss how Quantum Mechanics presents a weak-spot for physicalism, opening possibilities for different types of accounts of mental-physical relation which are still compatible with physics. The argument is nothing new, but I hope to spell-out the relation more clearly, and I felt that I need to in part because of some things that I intend to argue in future posts.

The question of what is physicalism, is probably best approached through the notion of supervenience, so let’s start from there.
If there cannot be difference in set of properties A of two things, without there being difference in set of properties B for those two things, we say that – set of properties A supervenes on the set of properties B.

Applied to a usual case it goes like this: If there cannot be difference in mental properties of two things, without there being difference in physical properties of those two things, we say that mental supervenes on physical.
The statement that in fact this relation holds – that mental supervenes on physical, is usually taken as formulation of physicalism about mental properties (for detailed discussion of needed types of supervenience for physicalism see Pete’s post and paper over at Brain Hammer).

It should be noted here that in order for the claim – “There cannot be difference in mental properties, without difference in physical properties” to count as formulation for physicalism, the “cannot” should be taken as metaphysically necessary. That is so because even dualists can accept that claim if “cannot” is only nomologically necessary. They can hold that given the specific physical and psychophysical laws, it is true  that two things can’t differ in their mental properties without differing in their physical properties; but that metaphysically that it is not necessary. In such way dualist holds that zombies, beings that differ from us in mental properties (they lack conscious experience), but are same in their physical properties are metaphysically possible.

What I want to discuss here is however not specifically physicalism about mental, but physicalism in general, which would be characterized by the slogan: “there cannot be any difference without a physical difference”.

But how does this claim relates to the Quantum Mechanics (QM)?

Here is quick and dirty sum-up of QM (no, it is not your monitor, it is dirty mostly because of my limited understanding of the topic):

What we are interested in QM is a physical system. It is characterized by a)type, which defines the unchanging properties of the system and b)state of a system at time t – a complete specification of the properties of the system that do change with time, at time t.

Given a physical system, QM gives us two laws which tell us about the dynamics of the system (how it behaves though time):

D1:Given that we know the state of the system at time t1, and we know what kind of forces affect it, there is an equation called ‘Schrödinger’s equation’, which tells us what will be the state of that system at some other time t2.

D2:When a specific measurement is made of some variable of the system, there is only a statistical prediction based on the state of the system that tell us the probability that we will get certain value for that property.

So, we have two dynamical laws, one is deterministic and tells us about evolution of the unobserved system, and the other is probabilistic and gives us statistical prediction of what we would get if we measure specific variable of that system. It is open question of different interpretation of what constitutes a “collapse”, i.e. when does the rule D2 apply, and even if it ever applies, or if maybe the whole story could be told only by the Schrödinger’s equation.

There are different approaches to the problem of measurement (D2). Some interpretations of QM (like Everett’s interpretation) –  say that nothing but D1 is needed and it fully describes what is happening in the world, and that D2 can be interpreted (or reduced) to the experiences of the observers in that world (those observers also being fully determined by D1), or alternatively that D2 is not needed as there is Many Worlds each for any possible value of measured value. Other interpretations try to explain the probabilistic nature of D2 by hidden variables (For a overview of the discussions around the topic of possibility of hidden variables you can check the entries on Bell’s Theorem and Kochen-Specker Theorem on SEP).

by  Drift Words
Don’t look – you make things collapse

However here I will put my attention on the possibility of different approach to the problematic of measurement problem, in order to connect it to the issue of supervenience, and show how it leaves a weak-spot for physicalism.

It is a consequence of the D2 that – if we have two systems of same type, and which are in same state, and we do same measurement of both of them, it is not necessary that we will get the same results.
Hence, the result of the measurement doesn’t supervene on the state of the physical system, i.e. there can be difference in results of the measurements, without difference in physical properties.

Does this undermines physicalism?
I think it doesn’t have to from what is said so far, even we don’t accept any of the physicalist-friendly interpretations of the QM. The physicalist will just say that nothing in the story really leaves the realm of physical. After all the measurement and the quantity measured is measurement of physical quantity.

It does become problem though, if one buys into a variation of the Principle of Sufficient Reason, which we could state thus:there is sufficient reason for everything (event, thing, etc..) being as it is, and not otherwise.. In this case we get something which is not necessarily physical – namely the reason for the collapse being “as it is, and not different”, and we can say that the reason why the measurement gave such result doesn’t supervene on the physical state of the measured system.

So, QM gives us possibility to look for reasons for different behavior (as far as that behavior falls under collapsed properties for which D2 is needed) in something other than the physical state of the system. Most obviously, I guess, this opens possibility for interactionism (type of dualism, where the mental can affect physical), but as I’m not proponent of dualism, my interest in this possibility doesn’t lay there. More on this in some next post.


5 Responses to “Physics vs. Physicalism”

  1. It doesn’t seem possible to reconcile the Principle of Sufficient Reason with quantum indeterminacy, unless one postulates some form of hidden-variables theory. On a standard Copenhagen interpretation it seems that there is simply no sufficient reason for an electron in an equally weighted superposition of spin up and spin down states to be observed as being in one state rather than the other. Experiments have also ruled out a large class of hidden variable theories, and for reasons I do not understand, the remaining hidden-variable theories are not in favour with most physicists.

  2. Hi Ponder, thanks for your comment.

    As you say, one class of hidden variable theories seems to be ruled out by the Bell’s and Kochen-Specker Theorem, and the rest (like Bohm’s non-local hidden variables thory) are not favored. (There… Again I’m starting to talk about something which I have not much knowledge of, so I better stop).

    However, let’s say for the interactionist dualist, this opens possibility to argue that the world dynamics is determined by the Schrödinger’s equation, but that mental is a part of the reason of why the measurement yields this and not that value.

    Of course mere possibility doesn’t make something true, especially without a theory of how would mental provide the reason or part of the reason.

  3. Clark said

    One should ask why they aren’t favored – whether Bohmian mechanics or Kramer’s transaction model. There really aren’t massively compelling reasons. If anything it’s sort of a default position because so many physicists now tend to favor the MWI which makes things convenient but which (to me) has the massive problem of making no distinction between actual and possible. I think the Von Neumann idea of a collapse of the wave equation is just a problematic way of constructing things.

    But it’s somewhat moot until we get a working theory of quantum gravity. String theory seems to be dominating at the moment although there is definitely starting to be a backlash.

  4. Hi Clark,

    A question… does string theory provide (or intend to provide) sufficient reason of why the each measurement gives result as it does, and not other?

  5. Clark said

    There’s actually a book out on philosophy and quantum gravity (primarily string theory but also a bit broader) There’s not a lot of consensus. Lee Smolin of loop quantum gravity fame wrote a heavily discussed commentary last year that physicists need to embrace philosophy more if we want to make any progress. (Of course Smolin’s largely a Peircean, so that stance is somewhat understandable – although arguably Einstein’s progress was largely philosophical) In a way all of this is playing out.

    There issue is a tad complex. I plan on discussing it for that Science week where everyone is supposed to blog science starting Feb 4.

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