‘Interpretations’ of quantum mechanics

I’m going to alternate properly new posts with short extracts from my thesis. So here’s the first such extract, on why Everettian quantum mechanics is not just one ‘interpretation’ among many:

Often EQM is presented as an interpretation of quantum mechanics, for purposes of comparison with other ‘interpretations’; examples usually given are pilot-wave theory, the ‘Copenhagen interpretation’, and dynamical collapse theories such as that of Ghirardi, Rimini and Weber (Ghirardi, Rimini, and Weber 1986). I think that that this way of conceiving the foundational issues, where the equations of quantum mechanics are common to the different approaches and they are distinguished only by a quasi-metaphysical layer placed on top of the equations, is badly misleading.

The main reason for this is that that most ‘interpretations’ impose extra dynamical structure of their own onto the basic skeleton of the quantum mechanical formalism. For example, in addition to the unitary evolution of the quantum state, (non-relativistic) pilot-wave theory postulates point-like particles with definite trajectories, and a ‘guidance equation’ which tells the particles how to move based on the structure of the state.  As such, it is strictly speaking not an interpretation of quantum mechanics at all; it is an autonomous theory which happens to share much of the theoretical structure of quantum mechanics. (In recognition of this point, the name ‘Bohmian mechanics’ is often adopted by enthusiasts of pilot-wave theory.)

The Copenhagen interpretation, as preached by Bohr (Bohr 1934), (Bohr 1958), (Bohr 1963) is different from these explicit modifications of quantum mechanics; it takes the equations of quantum mechanics as they stand but accounts for their link with our experience of the macroscopic world in an non-standard way. In this respect it has much in common with EQM. The difference is that EQM is naturally thought of as a realist theory; it interprets the quantum state as a description of the physical properties of a system. The Copenhagen interpretation (at least as it is ordinarily understood[1]) is an instrumentalist theory; it tells us how to use quantum mechanics to predict behaviour in the macroscopic world (antecedently understood in terms of the theories and concepts of classical physics), but rejects any of talk of ‘correspondence’ between the quantum-mechanical description and any macroscopic reality. It is thus natural to group EQM and the Copenhagen interpretation together, treating the former as a form of scientific realism about quantum mechanics and the latter as a form of anti-realism[2] about quantum mechanics; we can then contrast them both with those theories which explicitly modify quantum mechanics.

The reasons for preferring EQM to Copenhagen are essentially instances of our more general reasons for preferring scientific realism to instrumentalism. EQM provides us with a picture of fundamental reality, albeit a strange one, while the Copenhagen interpretation rejects any such demands. According to the Copenhagen interpretation, questions about the fundamental nature of microscopic reality without reference to experimental context are simply misguided – there can be no informative answer to such questions.

EQM and the Copenhagen interpretation do not just differ in the metaphysical picture they give us. EQM can form of a coherent and unified theory of nature in a way in which the Copenhagen interpretation cannot. EQM is a theory of closed systems, and thus can be applied to the entire universe, unlike the Copenhagen interpretation which restricts quantum mechanics to describing the behaviour of quantum systems embedded in classical environments. This is reflected in the way that quantum cosmology invariably proceeds, implicitly or explicitly, with EQM as a background assumption.

[1] Bohr himself can be interpreted rather differently, as a realist who took classical mechanics to be fundamental. See (Saunders 2005).

[2] By ‘anti-realism’ here I mean something stronger than van Fraassen’s constructive empiricism (Van Fraassen 1980), which takes physical theories about unobservable entities literally but advises a restrained epistemic stance towards them. The kind of anti-realism about the microscopic exemplified by the Copenhagen interpretation is semantic rather than epistemic in nature.

‘Interpretations’ of quantum mechanics

Conditions for causal autonomy

My doctoral thesis is finally (almost) out of the way, and I’ve started a one-year post-doc at Monash University. Australia is awesome – and it’s been nice to get back to doing some more varied philosophy over the last few weeks, after a year working almost exclusively on Everettian QM. To make up for lost time, I’m going to try to update this blog at least weekly, usually on Mondays.

This entry relates to a really interesting talk by Peter Menzies I heard a few days ago at a great little workshop in Wollongong on causation and explanation at multiple levels. Peter told us about some joint work with Christian List, recently published in J.Phil., in which they respond to Kim’s exclusion argument against non-reductive physicalism by appealing to a difference-making counterfactual theory of causation, but without Lewis’s strong centering requirement. Over the course of Peter’s talk lots of stuff slotted into place for me – exactly what sort of challenge multiple realizability poses for the physicalist, how best to divide up the philosophical labour between two notions of causation (as difference-making and as production), and how to go about connecting my own conception of counterfactuals as level-relative with the Lewisian similarity-based account. It was also great to see the possible-worlds semantics for counterfactuals being put to serious work; it gets a lot of criticism, but here it really helped to illuminate the importance of a subtle issue in the logic of counterfactuals for an important and controversial debate in the metaphysics of mind.

The most startling claim made by Menzies and List is that whether Kim’s exclusion principle holds will vary on a case-by-case basis, and is a contingent and empirical matter. They show that under certain conditions the causal relation between some psychological event and some piece of behaviour will be realization-insensitive; roughly, that the behaviour would have occurred even if the microphysical event which actually realized the psychological event had not occurred. If the dependency is realization-insensitive, they argue, then the psychological event and not the microphysical event counts as the difference-maker for the behaviour in question. In that case, the exclusion principle holds, but we have a case of downwards exclusion; the psychological event’s being the cause excludes the microphysical event which realizes it from being the cause. Thus the causal powers do not ‘drain away’ to the microphysical level; the psychological level is completely watertight. This, argue Menzies and List, is sufficient to ground the required causal autonomy of the mental.

One response to this, which I imagine will be fairly common, would be to say that the Menzies/List sort of difference-making causal autonomy is insufficient to ground ‘genuine’ causal autonomy. For example, fans of a production account of causation will argue that the microphysical event is still doing the productive work, and so the psychological event lacks any productive-causal autonomy. This seems right to me, but it also seems like something that the non-reductive physicalist need not be worried by. The non-reductive physicalist claim isn’t that psychological events can produce things that microphysical events can’t (that would amount to magic); it is only that psychological events can explain things that microphysical events can’t. Difference-making causes seem to ground a perfectly adequate kind of causal explanation.

Menzies and List suggest that realization-insensitivity will be a very widespread phenomenon. In particular, they suggest that it will generally be present in cases of multiple realization: where there is a high-level event which can be multiply realized, any causal relations in which that event stands to other events will be realization-insensitive. The rationale behind this is that, in the closest worlds where the low-level event which realizes the high-level event is absent, a different low-level event will realize that high-level event. Their (real-life) example is a monkey’s brain, in which multiple distinct neural states N1, N2, Ni… correspond to the same intention I to reach for food (action A). Suppose in the actual world, some monkey is in state N1, hence has intention I, hence performs action A. Menzies and List argue, plausibly,that had the monkey not been in state N1, it would have been in one of the other neural states Ni instead, so still would have had intention I, so still would have performed action A. Then the causal relation between I and A is realization-insensitive, and we have grounds for saying I is causally autonomous.

Menzies and List mention one clear case where a causal relation will not be realization-insensitive – the case where there is only one low-level event which can realize the high-level event which stands in the causal relation to some other event. In this situation it would obviously be false to say that, had the low-level event not occured, the high-level event still would have done – since they are one and the same events under different descriptions. Thus they say (in footnote 3): ‘One might regard realization-sensitivity as a plausible criterion for identifying higher-level properties with their physical realizers.’ This seems to me rather too quick (at least if it is regarded as a necessary and sufficient criterion, and not merely as a necessary criterion). That’s because we can imagine multiply realizable events which still give rise to realization-sensitive causal relations.

An extremely general example of this sort of case arises from the symmetry between matter and anti-matter. It seems that every type of event above the atomic scale in our universe will be multiply realizable, if only because it could be realized by anti-matter rather than matter. Although it seems unlikely, cosmologists cannot rule out the possibility that some galaxies are actually made of anti-matter rather than of matter; and even if no actual galaxies are so constituted, anti-matter galaxies certainly seem to be physical possibilities. Imagine some bizarre high-level psychological event P which can be realized by only one very specific arrangement of atoms; still it will be multiply realizable, because the atoms could be matter atoms or anti-matter atoms. But this seems to be a case where multiple realization doesn’t lead to realization-insensitive causal relations. Suppose P is actually realized by matter atoms (event E), and that P causes some behaviour B. For P to be causally autonomous requires that, had E not happened, B would still have happened. Now there is of course a world where P is realized by anti-matter atoms (event A): in this world, B still happens. But it is quite plausible, if all the actual anti-matter is in some distant galaxy, that the closest not-E world isn’t an A-and-P world – it’s a not-P world. In this example, P is multiply realizable, but nonetheless the causal relation between P and B is realization-sensitive, and P is not causally autonomous. In such a case there is no exclusion in either direction – both P and A are causes of B.

Another example might be some mental event which in humans can be realized by only one very special neural event, but can also be realized by various states of a silicon chip. Supposing I were to instantiate that very special neural event, it might not be true to say that, had I not instantiated that neural event, I would still have instantiated the mental event – for that would have required me to be made of silicon.

[paragraph which follows edited to fix silly mistake]

These examples trade on multiple realizations of a high-level state that are very different in kind from one another. We can imagine other cases where the realizations are similar in kind, but where instead there are powerful factors suppressing the other realizers. Suppose we have a mental event which has three different realizations, but some tribe of people worships one of those realizers in particular, despising the other two; members of the the tribe set up mechanisms to ensure that, if any obtain, it would be the one the prefer. So they link precursor events of the despised realizers up to a number of independent mechanisms each of which is individually sufficient to destroy the system in question before the despised realizers occur. In this scenario, it might take what Lewis calls a ‘big miracle’ to get one of the despised realizers to obtain; in which cases the closest world where the preferred realizer does not obtain is a world in which none of them do.

These examples might all be seen as somewhat far-fetched, especially the third. (Some might see the third example as a reason to modify the Lewisian criteria for assessing counterfactuals rather than a reason to affirm realization-sensitivity for this causal relation). But I think the examples at least give an idea of how there can be realization-sensitive causal relations even in the presence of multiple realizability, and hence that multiple realizability is not a sufficient condition for high-level events to have autonomous causal powers. The additional condition they suggest is that all non-actual realizers of a high-level event must be sufficiently remote possibilities that worlds in which any of them realize the high-level event are less similar to the actual world than worlds in which the high-level event simply does not occur at all.

I don’t think Menzies and List need be too worried about this additional condition. Their aim, to rescue the causal autonomy of the mental, seems secured because the mental states of creatures like us are realizable in a colossal number of distinct ways, and not much of macroscopic importance will tend to hang on which realization is instantiated. Even if we count neural events rather than atomic events, and limit ourselves to some particular individual, there will still plausibly be millions if not billions of neurally distinct events which could realize a single psychological event. This number will reduce as we move to consider successively simpler creatures, or as we compare levels of description which are closer together – and that is exactly as it should be. Causal autonomy is something which we should expect to come in degrees.

The talk also triggered some interesting ideas about the context-sensitivity of causal ascriptions – but I’ll save those for another post.

Conditions for causal autonomy