Friday, June 06, 2008

A Russell-Style Objection to Many-Worlds

In the standard formulation of Quantum Mechanics, there are two processes: in the absence of a measurement, a system described by a wave function evolves continuously and deterministically; when a measurement is made, the system instantaneously takes on a specific value according to the property being measured (often called a “collapse”). Everett’s Relative State formulation of Quantum Mechanics, the most famous version of which is called the Many-Worlds Interpretation, seeks to drop the collapse process from the theory. The interpretation then struggles to explain why we have the experience of measurement outcomes that we do (e.g. all outcomes are still happening but in many different worlds).

My common sense objection to the interpretation has always been based on the roots of the theory in scientific experimentation. QM was formulated to explain the phenomena we observe in the microscopic realm, including, obviously, the outcomes of measurements! It seems perverse to drop the process describing measurement.

Why then is the interpretation fairly popular? It is because the collapse process seems mysterious and “unphysical”, while the evolution of quantum systems in the absence of collapse is mathematically well behaved and, despite the many new complications and nuances, remains closer in spirit to the traditional dynamics of “matter in motion”.

I don’t know for certain whether Bertrand Russell ever wrote about Many-Worlds (it’s doubtful given the chronology – he died in 1970 and Everett’s 1957 work wasn’t widely discussed until the 70’s), but he made a careful philosophical study of physics (and all contemporary science) and I think his work helps clarify my objection to the interpretation.

Recall that Russell’s project was to analyze the data and methods of science and how they relate to human experience. He described how the realms of physical theory and experience can be brought closer together in an ontology of events and their causal relations.

Here are some quotes from Russell’s Human Knowledge (which I’m reading now, inspired by Carey Carlson’s references to it in his book):

Here’s the quote that first made me think of Many-Worlds: “Mathematical physics contains such a superstructure of theory that its basis in observation is obscured.” (p.41)

We need to remember: “…a datum for physics is something abstracted from a system of correlated psychological data.” (p.59)

And: “There is here a peculiarity: physics never mentions percepts [experiences] except when it speaks of empirical verification of laws; but if its laws are not concerned with percepts, how can percepts verify them?” (p.219)

The answer is that it doesn’t make sense to see the entities described in physics as something disjoint with the events of first-person experience. Science can be seen as describing a causal network which connects up in consistent ways with experiential events (think of how the human experiences of telescopic images provide the raw material and ongoing testing for astronomical and cosmological theories). Therefore, reality is best viewed as a web of events consisting of the directly experienced and the indirectly inferred, the latter of which are the usual target of physical theories.

In quantum physics, the wave function is derived as a description of reality which connects consistently with experiential observations. It has this in common with other theories of physics. Of course with quantum physics there is always a twist. The twist is that, unlike the entities of other physical theories, the wave function can itself only be viewed as a fully objective physical entity if we pretend that the measurement events don’t exist! Here we are unavoidably given a choice to elevate the reality of the directly experienced events (measurements) or the inferred physical entity (the wave function). The events must take priority since they are the part of reality which is not known through inference. In fact quantum measurement events are the best candidates to be the raw material for a consistent ontology of the concrete world.

Given all this, I think the best interpretation of quantum mechanics is a version of the relational or perspectivist approaches. In this interpretation, a relational network of measurement events (or interactions) constitutes the concrete world. All quantum systems interact (perform measurements upon each other) with no ontological distinction between the macroscopic and microscopic realms (or the human and non-human). The wave function in this interpretation describes a system’s propensities for interaction outcomes from the perspective of a particular measuring system.

Quantum Foundations Series (some highlights from posts with the Quantum Physics label):

The Duality at the Heart of Physics
I discuss how the two processes of QM are unavoidably interdependent.

Free Will All the Way Down
A paper describes the relationship between freedom at the human level and indeterminism at the micro-level.

Interpreting Quantum Probability
Is the probabilistic aspect of QM objectively real or epistemological?

The Limit of the Bayesian Interpretation
Why the Bayesian (subjective) interpretation of quantum probability is preferred but is still incomplete.

Wigner’s Friend and Perspectivist Quantum Theory
A superior interpretation of QM by Paul Merriam.

Merriam’s Quantum Relativity
How Merriam addresses the “preferred basis” problem for the perspectivist interpretation.


Blue Devil Knight said...

The fact that physicists still haven't killed Shroedinger's cat should be a scandal. Bohm's theory is probably my favorite even though it violates SR so is about as kooky as the rest!

I don't like theories that talk about experience. Experience has no place in physics! :)

Clark Goble said...

The transactional interpretation is my favorite at the moment.

The other view is a more holistic one where there are multiple possibilities (i.e. the universe as a wave function) and a single collapse but it appears like there's something mysterious going on because we don't like to think of a block universe. I suspect a block universe model will end up being necessary if we ever get a successful background independent theory of quantum gravity.

Steve said...

Hi Clark: I'm not sure a single collapse of the wave function for the entire universe makes sense unless we posit an outside entity which can causally interact simultaneously with everything. I think that the world is a constantly evolving network rather than a block. Emergence-style background independent quantum gravity programs fit with this concept (I think).