Help wanted: wave function realism

One often hears claims to the effect that the non-separability in quantum mechanics or hidden-variables interpretations of quantum mechanics such as Bohmian mechanics entail, or at least strongly suggest, some form of wave-function realism. (There’s no such entailment, to be sure, but a suggestion). It seems fairly clear how an argument like that would go, intuitively, but I have never seen one worked out carefully. Does anybody know of a helpful source which explicates and discusses this argument? Any “locus classicus” for such an argument?

I am also interested in how wave-function realism might relate to, or amount to, a form of modal realism. Any information, comments, suggestions for readings etc will be appreciated. Please either post as comment or send me an email ( Thanks a lot in advance!



Filed under My half-baked ideas, wuthrich

7 responses to “Help wanted: wave function realism

  1. Kelvin

    Hi there,

    After reading David Albert’s Elementary Quantum Metaphysics, in which he asserts wavefunction realism but does not really spell out an argument for it, I remember wanting to find such an argument, and so I searched for responses to his paper. The best I found was Peter Lewis’ “Life in Configuration Space”.

    For an interesting critique of wavefunction realism, see Wallace & Timpson…

    Also, as it happens I saw an interesting talk given last night, which attempted to reduce metaphysical modality to Everettian branches, hence defending a kind of Lewisian Modal Realism…

    Hope that was helpful,

    • Dear Kelvin

      Thanks, that’s great. Albert’s assertion is of course well known. Brad Monton has recently responded to both Albert and Lewis (at the PSA 2004):

      The Wallace and Timpson is also very helpful, it lays down the main pros and cons rather nicely.

      What seems to be missing in these discussions (although I only did a quick read and except, of course, for the talk you mention by Alastair Wilson) is the connection to modal realism. Of course, that’s not what most participants in the debate concerning the reality of the wave function care about. (For instance, there seems to be logical space for a position defending a wave function realism in an Everettian approach, insisting that the various branches are all actual and hence that no modal realism is implied.) Anyway, I am interested in attempts to cash out a modal realism from the resources of quantum mechanics.

      • Kelvin

        Hi Christian,

        I’m glad you liked the Wallace/Timpson, I also think it very instructive. The wavefunction evolves in a 3n dimensional space, but once one moves to a quantum field theory, where n is not well-defined, wavefunction realism seems inappropriate. The Albert/Lewis/Monton debates then appear to be gauging the ontology of a merely possible world (in which non-relativistic quantum mechanics is true) as opposed to the ontology of the actual world.

        I guess I’m not quite sure what your after regarding modal realism. If your looking to find truth makers for modal sentences simply from the resources of the true theory of the actual world (e.g. qm), then that would by definition be an actualist account of modality, as opposed to a modal realist account. David Lewis would (assuming he accepts qm) would supplement the wavefunction with concrete possible worlds.

        It’s true that many worlds interpretations do not entail modal realism. The Wilson paper is unique however, as it involves a very abstract attempt to “reduce” the space of metaphysical possibilities to the space of decoherent branches. I take it the idea is that if the two spaces are identified, then the problem of probability for Everett vanishes; having it vanish would solve the measurement problem, advancing science; a reduction that would advance science would be a good reduction; hence a rather indirect motivation for reducing the space of metaphysically possible worlds to the space of decoherent branches. Perhaps modal realism isn’t the best name for this theory – it’s just that the metaphysically possible worlds turn out to be concrete, and relatively isolated, just like Lewis’ metaphysically possible worlds.


  2. Thanks, Kelvin. This is very helpful. What I am interested in are connections between QM and modal realism quite in general. I figured that these connections are most likely found in an Everettian approach.

    You write that if I were to solely rely on the resources of QM (supposing it’s true of the actual world), I would not obtain a modal realist account of modality. But suppose I defend a configuration space realism. Suppose I am realist about the configuration space of a theory true of the actual world. Wouldn’t that give me a form of modal realism in that I would be committed to the existence of non-actual possible states of the physical system at stake?

  3. Hi Christian,

    This is obviously a topic close to my heart – glad to find people taking an interest. Kelvin’s done a good job of summarizing my proposal, but here are some more details.

    My claim is basically that the ontology delivered to us by Everett is structurally far more similar to a modal realist’s multiverse than it is to a modal actualist’s single actual world (at least as these options are normally conceived.) So there’s prima facie reason to think Everettians should be modal realists.

    In my doctoral thesis I try to shore up this prima facie reason by arguing that an Everettian modal realism solves the problem of probability in EQM, solves the measurement problem, gives a reductive account of metaphysical modality, explains the knowability and relevance of laws of nature, gives us a way of resisting Chalmers-style arguments for dualism, can specify truthmakers for counterfactuals, has powerful new resources for responding to the sceptic, and all without commitment to any ontology other than that which is derived from current best physics.

    The assumptions needed are the following: EQM is correct; models of a physical theory do not line up 1-1 with possible worlds; actuality is indexical (or at least: thisworldliness is indexical – the word ‘actual’ is disputed); contingency should be analysed in self-locating terms. I also prefer a ‘diverging’ rather than ‘overlapping’ interpretation of Everett – on this view, branches have no parts in common with one another.

    Some of these assumptions are controversial and I’d say that the position you say above there is ‘logical space’ for is the dominant view among authors on EQM (Greaves, Wallace, Albert, Maudlin, Lewis, Price, etc.) Simon Saunders is an exception.

    I wouldn’t suggest reading the paper of mine linked to by Kelvin – that was written while an undergraduate and is about 5 years out of date. A successor to it will hopefully be available soon. If you like, email me at and I’ll send you the slides from the talk Kelvin mentioned.

    All the best

  4. Kelvin

    Hi Christian,

    Sorry about the delayed reply – the ‘notify me of follow-up comments’ does not appear to be working, at least for me – and with much on my plate at the time I ended up forgetting…

    No worries if your thoughts are now on other things – but if not, always interested in your response!

    To answer your question…

    “Suppose I am realist about the configuration space of a theory true of the actual world. Wouldn’t that give me a form of modal realism in that I would be committed to the existence of non-actual possible states of the physical system at stake?”

    …I’m thinking that being a realist about configuration space is consistent with taking the actual world to consist in every member of configuration space.

    Here’s how: Imagine there exists just one particle P and that our configuration space assigns non-zero amplitude to members of configuration space that put P in positions X, Y and Z respectively. I’m thinking that it’s quite open to interpret this as being such that there actually exists one particle (P) and that one particle (P) exemplifies a single property: the property of being in a superposition of being at X, Y and Z. (This is similar to Monton’s view.)

    In this case there are no non-actual possible states of P (the physical system) at stake here. The merely possible states of P can be expressed by statements such as “P could have been in a superposition of A, B and C, but isn’t actually”.

    Another way of looking at it is this: assuming again our one particle P being assigned non-zero amplitude at positions X, Y and Z. We can interpret this in a more many-worlds style manner (the former being more congenial to GRW-style QM). That is, we can think of there being not really just one particle P, but three distinct particles P1, P2, P3, all existing in the actual world. The way to do this is to think of the actual world as consisting in branching mini-worlds. Thus, at t1, P may have been in an eigenstate of being at position Q, but was then at t2 triggered into being in a superposition of X, Y and Z. We interpret this as the mini-world at t1 where P is at Q, bifurcating, at t2, into three mini-worlds. One mini-world puts its P-descendent at X, another puts its P-descendent at Y, the other puts its P-descendent at Z. All of this mini-world branching is occuring within the actual world.

    In this case there are no non-actual possible states of P (the physical system) at stake here. The merely possible states of P can be expressed by statements such as: “The P-descendents could have been in mini-worlds where they were located at A, B and C respectively, but they aren’t actually”.

    Thus, it seems to me that nothing about the structure of configuration space entails anything about modal realism. In order to make the connection, I think, you would need to argue for that connection via much more indirect routes, such as the route taken by Alastair.

    I also wonder about the configuration space ontology… how does spin fit into it? If x-spin and y-spin are perfectly incompatible so that

    |up>x = c1(|up> + |down>)y

    then getting worlds out of the formalism is difficult, given that in the x-spin basis it looks like I’ve got one world with one particle that is spin-up, whereas in the y-spin basis it looks like I’ve got TWO worlds with a particle each, one is spin-up, the other is spin-down.

    Neither of the two bases look to be ontologically privileged over the other, so it looks like reading off worlds (or mini-worlds) from the formalism is problematic indeed: it involves basis-privileging.

    Now, realism about configuration space appears to be ontologically privileging the position basis. What does this sort of privileging entail about spin, and how does it solve (if at all) the above problem about reading off worlds?


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