Demystifier said:
So, would you agree with the following?
To make the DYNAMICAL equations of QM (which do not include the wave-function collapse) universally valid, both MWI and Bohmian mechanics (BM) introduce something ADDITIONAL that is not already present in the basic formalism of QM. Moreover, in both cases it is something ontologically CLASSICAL. In the case of BM, these are kinematically classical particle trajectories, while in MWI these are classically real subjective experiences. The main difference between MWI and BM is that the additional thing in BM is objective and formal, while in MWI the additional thing is subjective and not formal. In this sense, BM seems to be more in accordance with the usual practice in theoretical physics, which, of course, is by no means a proof that it is more likely to be correct. (Theoretical physicists may be wrong with their common belief that nature must be completely mathematical.)
Essentially, yes, I agree with what you write. I would, however, like to make two observations. The first one is, that even a classical formal theory, such as Bohmian mechanics, or Newtonian mechanics, is
still not giving any explanation of any subjective phenomena. That is, a theory/metaphysics/worldview/philosophy that wants to be universal, must account for the fact that certain parts of the formal ontology must be connected to what I'd call "the subjective experience". It is the mind/brain problem in a way, or the hard problem of consciousness in philosophy ; and also, related to this, the philosophical problem of heaceity (
http://en.wikipedia.org/wiki/Haecceity although it is not a very good article).
I mean by the above that, given a Newtonian (or Newtonian-like theory, such as Bohmian mechanics), you still cannot deduce what *I* will experience subjectively, unless you explicitly define that *I* corresponds to a
certain body, which is a subset (which one ?) of all the particles in your theory. You have to state explicitly, and
outside of your formal theory, that *I* will correspond to this set of (brain) particles, and that
these configurations will correspond to
these sensations.
However, it is true that in Newtonian-like theories, this step seems so intuitively clear that one doesn't even think about it. Nevertheless, it is part of any "theory of observation", even in Newtonian(-like) theories.
Now, in MWI, this step is also present, but becomes much less intuitively clear (and that's why many people don't like it, simply because they think that the problem *appears* in MWI, although it was also present in Newtonian(-like) theories, but could be overlooked). Here, the designation of what part of the "physical ontology" is going to correspond to the "I-experience" is a non-trivial and essential element in the "observational link" between formalism and perception. But it is based upon the same principle as in Newtonian-like theories: some part of the formalism must be postulated (outside of the theory itself) as to correspond to subjective experience. Now, the difference with Newtonian-like theories is that the part that corresponds to the "Ich-Erlebniss" is not a "material" part (a certain subset of particles), but rather a "state" part. This is counter-intuitive. We seem to be able to accept without problem that our "body" (= the part of the postulated ontology that generates our subjective perceptions) is a particular *material* part of the universe, but we seem to have intuitive difficulties accepting that it is a "state" part. Now, what is so terribly disturbing in MWI is that this definition of the "Ich-Erlebniss" - which is usually relegated to obscure philosophical discussions without any interest for practical people such as physicists - is now a crucial part of the entire phenomenon of observation. Because of its intuitively clear character in Newtonian-like theories, one can pretend to neglect this philosophical aspect, while one is pushed with one's nose into it when looking at something like MWI.
In other words, there is a philosophical problem hiding in *any* interpretation of a physical theory, which must ultimately link a formalism to actual (and hence subjective) perceptions, and which we could call the observational postulate. This problem is, deep down, just as present in a Newtonian-like theory as in MWI or relativity or whatever formal/mathematical physical theory. Only, we can "use our intuition" in these theories, and pretend somehow that the problem doesn't exist there.
As such, what is "attached" as a vague, subjective and informal element (namely, the observer-states which are subjectively perceived) in MWI is exactly that same philosophical problem, but now explicitly put forward, given that it is not intuitive anymore.
The second observation I would like to make is that the "gain" in formality in Bohmian mechanics (which I think is only apparent, because one substitutes the explicitly subjective part of MWI by a non-spoken part as in all Newtonian-like theories) is done away with by its non-compatibility with a spacetime manifold formulation. I agree that this is an observation of another order, as the spacetime manifold formulation of relativity could also be totally misguided, once we are doing all this.
) will increase that number.
