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Next: 5. Reconsidering Philosophical Foundations Up: 4. The Peculiarities of Previous: 4.3 Quantum Ontologies



4.4 The Problem of Measurements

The problem with measurement in quantum mechanics is that is essentially undefined. It is used to perform a `reduction of the wave packet' when the wave function splits up into alternatives that are perceptibly different, although it is never clear exactly when and why the reduction occurs.


Measurement by Observations

According to Bohr and his followers with the `Copenhagen Interpretation', measurement and reduction of the wave packet occurs whenever the scientist makes an observation of the quantum system.   ``No elementary phenomenon is a real phenomenon until it is an observed phenomenon,'' Wheeler proclaims.   Von Neumann [1932] showed that it doesn't matter whether you include the observing apparatus as part of the quantum system or as part of the observer. This theory never allows a splitting between the alternatives in the wave function to become apparent to the observer, because the very act of the observer `looking' is responsible for the reduction of the wave packet to a specific alternative.

Measurement by Consciousnesses

Difficulties with the Copenhagen Interpretation arise, Schrödinger and Wigner show, whenever living and/or conscious beings are included in the quantum formulation. For then these beings must be taken to be in a superposition of states, no matter how different these states may be, before the observer deigns to look at them.     Schrödinger [1935] shows, in his `Schrödinger's Cat' example, that a cat can easily imagined in a superposition of alternatives, where in one alternative it is alive, and in another is dead. We find this difficult to believe.     Wigner [1962] shows, similarly, when a conscious person (Wigner's friend) is included as part of the scientist's `apparatus', then the Cophenhagen Interpretation predicts that this person will be in superposition of distinct alternatives until the `real' observer interacts with him, by asking him a question, for example. We find this even more difficult to believe.

  Wigner resolves this difficulty by postulating that it is not merely the observing scientist which is responsible for the reduction of the wave packet, but any perceiving consciousness. This assumption at least has the advantage of not being anthropocentric with respect to the scientist. It is then presumably it is an empirical question whether Schrödinger's Cat has the necessary degree of consciousness to `reduce the wave packet' on its own, or whether it is sufficiently `unconscious' that it can be in a quantum superposition of distinct states of sensations.


Measurement by Actualising

In the face of the above (seemingly astonishing) proposals for the reduction of the wave packet, there have been proposals, from Heisenberg [1958] on, that the `reduction' occurs as a natural physical process 4.10, and is not dependent on the attention of any observing scientist. If quantum systems were conceived as some kind of potentialities, Heisenberg points out, then there ought to be some kind of `actualising' of these potentialities to some specific alternative. Thus the reduction of the wave packet could occur in part of the observing apparatus, and need not have anything to do with observers and/or their consciousness.   The way is then open for what Maxwell [1976] calls a `micro-realistic' theory of the quantum world. Some of these proposals for `objective actualising' will be examined in chapter 12. All these proposals aim to replace the category of `observation' in the Copenhagen Interpretation by some physical process that occurs quite often, and occurs regularly in the kinds of physical systems that we tend to use as observing apparatus.


No Special `Measurements'

As explained earlier in this chapter, the `many worlds interpretation' of quantum physics also aims to do away with the special category of `observation' or `measurement'. It does this by assuming that whenever the wave function splits into a superposition of alternatives, the whole (or part of) the world is split into a similar set of alternatives. Any observers (whether conscious, living or computer-based) will be automatically replicated into the several branches, and in each branch will record the appropriate observations for that branch! Again, this theory never allows a splitting between the alternatives in the wave function to become apparent to any observer, because the very act of any observer `looking' is responsible for the splitting of the wave function among the various alternatives.

One difficulty with this proposal is that it is not exactly clear what constitutes a `real split' of the wave function. Any quantum state can be analysed in terms of a momentum (or angular-momentum) distribution, for example, but is not clear whether that is a sufficient for there to be an actual splitting of the world (or world part). According to standard quantum mechanics, there is always some possibility (however remote it may be in practice) that two alternatives can be brought back together into some coherent combination. This means that in the many-worlds interpretation, we never really have many worlds, but really one `world set' which contains all the different `split worlds'. We therefore wonder whether all the branches should be regarded not as `equally actual', but as `equally potential' 4.11 , and still capable of remerging, in which case it becames much less clear what actually exists in the many-worlds interpretation. That intepretation becomes much less clear as a quantum ontology.  

What to do?

The discovery of quantum phenomena has given rise to an enormous number of interpretational difficulties. A corresponding range of rather astonishing proposals for quantum ontologies have been proposed, in the attempt to try to understand quantum mechanics realistically.

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Next: 5. Reconsidering Philosophical Foundations Up: 4. The Peculiarities of Previous: 4.3 Quantum Ontologies
Prof Ian Thompson


Author: I.J. Thompson (except as stated)