That's So Second Millennium

In today's episode we discuss Stephen Barr's talk at the SCS conference on June 9. His topic was the observer question in quantum mechanics. The observer problem is closely tied to the issue of probability and wavefunctions. We spend quite a while discussing what this problem is and how the question arises in the context of experiments like the famous two-slit experiment. The example of "Schrodinger's Cat" is an attempt to make this problem more understandable to the non-quantum mechanic. The cat is in some uncertain state, neither alive nor dead, until the observer opens the box and "collapses the wavefunction" to either a live cat or a dead one. In a two-slit experiment, a particle exists in some distribution of possible positions until an observer collapses the wavefunction and "forces" it to one tight range of locations (and for that matter momenta...).

This is very weird. Barr cited a long list of quantum theorists (von Neumann, London, Bauer, Wigner, Peierls, and others) who considered the problem and whether mind as such is crucial to whatever it is that does the measuring and observing to collapse quantum systems. Wavefunctions, with their consequent probability distributions, evolve according to Schrodinger's [or Dirac's?...a question I've had in the back of my mind many times...] equation with no internal mechanism to cause this collapse. Clearly two very unlike things interact to form quantum mechanics as we know it, as von Neumann stated explicitly (calling the observer / collapse phenomenon "process 1" and the wavefunction evolution "process 2").

It is clear that we can shift our mathematical formalism to incorporate any physical measurment device into the "system" and thus recognize it to be in the realm of wavefunction behavior. There is the "Wigner's friend" thought problem where even a human observer of an experimental setup can be placed in the "box" from the point of view of another human observer.

When we consider the observer problem from the point of view of a descriptive science (geology, astronomy, zoology, etc.) there is the immediate and rather alarming philosophical question: What was happening to, say, this star or tectonic plate or ancestral population of invertebrates before there was an observer to collapse the wavefunctions? Someone raised this question with Dr. Barr in the question and answer session after the talk. There is a phenomenon called "decoherence" (warning: that link is informative in places but far from the clearest read) which occurs for systems that are very open, interacting with their surroundings. Broadly speaking, the observable in question can trade uncertainty with its surrounding and settles down into a tighter range of possible states, simulating to some extent the effect of an observer collapsing the wavefunction. However, the two phenomena are not the same.