Transcript discussion Wallace

July 19 12.00
How to think about ontology
Speaker: David Wallace
Commentator: Robert Geroch
Floor speakers (in order of appearance):
Valentini
Vaidman
Saunders
Hartle
Hemmo
Albert
Bacciagaluppi
Brown
Barbour
Lehner

4

Comment by Geroch

1. I don’t really have either the confidence nor the inclination to try to rebut these remarks, or even to argue them but I thought what I might do is just try to hit a few highlights to just give a few points that struck me. And I have two highlights I’d like to hit, at two and a half minutes per highlight. So the first one, it has to do with the word ‘is’. It seems to me the word is being used in two senses and I wanted to kind of push the idea of making a careful distinction between them. I’m sure there’s a philosophical word for all this but I’m not a philosopher. The first is as in the sentence ‘the cat is in a superposition of the dead state and the live state’. It seems to me that that’s a statement about the theory and so the question – there the statement is is the quantum description of the cat as a superposition? And the answer to that is yes. So that statement is true. A second kind of statement is ‘the cat is either dead or alive’ or, ‘the macro-variables have definite values’ or ‘the Everett branches are real’. Those statements, it seems to me, are not about the theory but they’re about the world. And we get our information about the world, not by osmosis but by doing experiments. So we might imagine that an experiment on the question that the cat is either in the dead state or the alive state. So the experiment might be to have another system, say a particle, turn on an interaction with the cat system through the projection operator onto the live cat state and then – so the result of the interaction is that the state of the particle here will move in one direction corresponding to liveness or deadness, and then we can have a second system that interacts with that and ask whether the wave packet ends up in the middle or ends up moved over to the live cat region or the dead cat region. And the result of that experiment will be that the last apparatus will indeed detect movement of the wave packet. In other words, if we conduct an experiment on the question ‘Is the cat either in a dead state or a live state?’ at least this particular experiment says that the answer is that the cat is indeed either in a dead state or an alive state. So from this point of view, keeping the distinction of the two meanings of the word ‘is’, there would seem to be no conflict between, on the one hand, the statement ‘The cat state is a superposition of live and dead’ and, on the other, the statement that the cat is either in a live state or a dead state, since they’re two different uses of the word ‘is’.

2. The second point I wanted to make is: where do these classical states come from? That’s an issue that we talk about and the answer is, of course, they come from the interactions. In this cat example its because the interaction is with the projection operator onto the live cat state that we get into a “live cat, dead cat” issue. More generally, when we turn on an interaction between a system that we’re interested in and some measuring system we do so through Hermitian operators on the two systems, we turn on the interaction, it gets in this superposition of states corresponding to values of this system and values for the measuring instrument and then we forget about this system and we look at the density operator and we look at the eigenvectors of the density operator and we indeed discover in the cat example that the eigenvectors of the density operator for the cat state that results from this interaction is indeed the live state eigenvector and the dead cat eigenvector. In other words, the classical states in this case, and in general, this is the decoherence argument, come from the interactions that are turned on. So we can now ask the question: how is it that the interactions seem to produce this classical occurrences, classical states, as coming out? So we can ask for example: is it true that all interactions in physics have the property that they single out in this manner the classical states? So if we take the example of a brick and a rabbit, we have a pretty good idea of what interactions take place between a brick and a rabbit and we can ask the question: is it true that brick-rabbit interactions are such that brick states, the density operator on a brick, ultimately has eigenvectors that correspond to classical brick states? And it seems to me that the answer is no. If you just take typical interactions from the physics we know, you do the calculation, you let the thing run for the age of the universe, it seems to me the brick will, quote, “not be in a classical state”. So from this point of view then we conclude that macro-systems would not be in classical states if we regard classical states as arising in this way from interactions.

3. So, we might ask, if macro-systems do not end up being in classical states where do the classical states come from at all and I presume that the place that they come from is people. That we believe - in the case of macro-systems interacting with each other, we pretty much know what the interactions are and we can compute whether the density operators are classical state density operators and the answer’s no. But the hope here is that people are so structured that the interactions that we have with macro-systems in fact are via operators that ultimately give a density operator on the system that indeed corresponds to the classical states. So the kind of mystery here is that our understanding of why systems are in classical states ultimately becomes trying to understand why people seem to manifest the interactions that they do.

Valentini
4. You talked about the quantum state and finding structure in the quantum state. When you say quantum state do you mean pure states?

Wallace
5. I don’t much mind. It seems to me, if I’m saying that the quantum state, we know very little about it other than structure then a corollary of that, I think, is that we don’t find it any more weird to understand the idea of a “mixed” state as a real and physical than a “pure” state. Whether the actual state of the universe is pure or mixed I haven’t a clue. It doesn’t seem to me to particularly matter here. Either way we’re looking for structure in that whole thing, although in practice, of course, we’re looking for structure inside sub-bits of that thing.

Valentini
6. The only reason I ask is that most models of decoherence, or many models of decoherence such as quantum Brownian motion, the sort of thing that Zurek writes about, they are…[noise]……the total state is mixed………..so that’s why……

Wallace
7. The position that says that mixed states are just as fine as pure states seems relatively unpopular amongst philosophers of physics for reasons I don’t understand, though they may be good reasons. My impression is that the use of mixed states in …the Zurek model is an inessential feature, that one could put rather complex stuff in instead but I could be wrong about that.

Vaidman
8. My question ……..you’re supposed to answer but really it’s also for Simon…….otherwise said. You say that we need this decoherence formalism to explain the separation of worlds. As I understand..very simple experiment of a photon, beam-splitter…..detector……Then many-worlds says there are two worlds and standard theory says there’s only one. The decoherence is obvious; there is not any interference whatsoever immediately, no preparation whatsoever you don’t have………..nothing. And if you consider other experiments like, you have a screen and you have diffraction then even in standard quantum mechanics one would start calculating when things are different or not exactly …..worlds?..are different. And when Simon brings this idea that…consistency condition, this is a kind of a consequence. When I have decoherence the probabilities work fine, why do………..these kinds of axioms…..and try to……in all normal cases it has to explain. It seems absolutely obvious; it’s necessary but it’s absolutely obvious.

Wallace
9. …multiple responses. Firstly, of course, what’s obvious to one person isn’t the same for another and not all of us share your genius Lev. Seriously, obviously it wasn’t that nobody had thought of decoherence before Zurek came along, I mean lots knew about decoherence. I think what decoherence brought us was a language and a sort of unified way of thinking, a way of asking questions like: why – sure, situation by situation it’s dead easy to model it, but what are the general features that lead to that sort of thing? And I that language is perspicuous to some, but I think the fact of the decoherence process, what’s driving it, is independent of whether one decides to call it decoherence and what is explained situation by situation. I find the language perspicuous and also very helpful as a way of getting at questions, or sort of seeing the structuralist aspect of this and getting at questions like: what about alternate ways of decomposing the wave function. Look, after all, as Tim Maudlin’s pointed out for instance, configuration space is not, or at least not guaranteed to be, a primary thing. Certainly not the classical configuration space that people talk about classically. So we need to ask something about why that particular decomposition is the thing to think about as special. I think there’s a sort of useful, unified way of thinking about it that makes the right answers to this, which are the answers to say I think, become a bit more obvious and a bit easier to talk about. So I don’t think it’s magic. It’s certainly not a new theory, it’s something that’s always in quantum mechanics.

Vaidman
10. …………………something which is not obvious, which is not obvious other ways? …..necessary to see. You say that the interference…you mentioned, it’s not complete?..there is some residual interference and then it’s a problem. Can you see something….

Wallace
11. I think there aren’t really examples and that’s kind of the point, it’s not a theory about – example by example one’s just doing the physics and if one wants to call it interference, go for it. I think the point is more to get a framework that reassures one in the fact that future examples will also work out that way. Look, the world’s not going to give us a once-and-for-all theorem that says, of course, all phenomenology of the classical world right up to Tony Blair’s resignation is indeed, yes, emergent from quantum mechanics. What one always finds in this situation is a sort of constellation of particular examples and thought experiments and computer simulations and bits of maths and actual experiments and theorems and folk-theorems and plausibility arguments and so on. That’s how we justify the emergence of zoology from molecular physics. That’s what’s going on here really. I think the claim that says one finds emergent quasi-classical structure in the quantum is very, very solidly supported through a range of things. Decoherence is a nice way of talking about a large chunk of that and furthermore a way of putting all of those arguments in place. If you want to be more general, we should expect this to be ubiquitous, here’s why.

Saunders
12. I’d just like to make a comment. Lev, what if some brilliant theorist comes along and finds a very alien……classical domain?..[background hum]….equations will seem to govern the independent autonomous behaviour of worlds, but worlds entirely different to our world. What would you say to that?

Vaidman
13. ..[continuing hum].. If the…worlds…physics is not local…interactions not as strong as here…no questions..Or maybe if the interactions are non-local..have non-local conscious?..objects

Saunders
14. It’s nothing to do with conscious beings, the point is is this or is this not interesting structure in the wave function? Clearly the answer is “yes”. Now, that would be an answer delivered through …direct?..analysis………..You, as an…….taking as….the sorts of experiences?.we have…. can identify very obvious………..decohering states of affairs…photon is registered here, registered there, clearly those are not going to interfere in….it’s obvious. But this alien set of entities, worlds, whatever, is at all obvious, because you don’t happen, as it were, to live in such a world……………

Wallace
15. Simon finds….world much more plausible than I do but-

Saunders
16. ..I agree. Jim I think you speculated on whether there …be……backwards in time relative to us

Hartle
17. Well, we speculated on all sorts of things……..Just to respond, I think what?..Lev is criticising in this approach is that he wants to restrict to?.a certain class of histories…….they’re described in quasiclassical terms, and he would agree that if we calculated…………….But we hope to start from some more general perspective in which all possible kinds of histories .. to be available to describe the world….which ones are more useful…..more reflective of this kind of goddamn?. structure. So, if we start from that perspective then the absence of interference…..necessary to get…probabilities. So it’s just a more general perspective, rather than jumping immediately to the idea that we already know what the top-down structure is – and if we confine ourselves to that we don’t have to worry about this problem?..because we can safely assume…but may be we should …here.

Vaidman
18. …we know the deconherence ..consistent history is zero. You ….be in your talk and say that many worlds [background hum starts again]..as it stands today. Because you spent……ten percent, twenty percent..time..so without this the many-worlds is deficient.

Saunders
19. I did want to remark on this, it’s a very remarkable thing. David Deutsch in his book ‘The Fabric of Reality’ mentions …….[background hum]…….You, Lev, in your Stanford Encyclopaedia article………………..So, it’s very that this way of explicating?..many-worlds………But I think to certain sorts of criticisms of many-worlds, for criticisms which say, look, I don’t even buy that the deterministic evolution can be taken as a given, where we know we’ll get the spin-up state and we measure that spin-up outcome….the criticism that says that is not consistent with unitary quantum mechanics as it stands, you can’t use that fact to leverage the interpretation. To?. that sort of line of criticism, the line of criticism that says: what tells me I’ve got a plurality at all, don’t put the plurality in by hand by talking about a preferred basis or something like that. What tells me there’s a plurality at all that decoheres……One needs……

[new question]

Hemmo
20. I just wanted to ask a quick question about the…..you mentioned of thinking about quantum mechanics……….of a single world………………..and I’m not saying it’s going to be easy to do that but one could see how it would go if we use decoherence as....fundamental?..way. Basically you just do whatever you guys are doing….Everett interpretation and just focus on one of these worlds as the actual one and I’m not saying ..[background hum again]…………..but the two problems you have…….and this is my question, don’t seem to be a problem for this particular……..If they are not a problem for …..[hum]……….

Wallace
21. I think the reasons why don’t think that’s viable are, if you like, quite locked up in some quite general philosophy of science. The reasons why I don’t think the one-world view works are because of two features of this approach which I wouldn’t claim originality for but of the approach I’m describing which work fine for the emerging structure story but not at all for the single world story; one of those is approximation and one of them is the top-down, in terms of higher-level stuff thing. I have nothing original to say as to why this is correct, but we do not think that it’s appropriate in our physics, in our micro-physics, to have fundamental axioms that make reference to all sorts of concepts which we have an irreducibly emergent description of, like structural claims or something like that. We’re fine with that if we can have a clean re-stated starting point but we’re not happy with the idea that you just put it in, if you like, by hand as an axiom. We could have solved the measurement problem like that a very long time ago. It wouldn’t have been, as you said, difficult. I don’t think it’s difficult at all. We could have done that long, long ago. We could have used higher-level terms like ‘conscious observer’ and that would’ve been just fine. We don’t really think that high-level way of talking ..[cross talk] ..it’s not deemed legitimate. For unoriginal reasons; I’ve got nothing new to add to it.

Saunders
22. I think that we can debate whether Copenhagen is acceptable or something but this isn’t what it’s?.about. We are interested in the question of whether Everett flies.

[new question]

Albert
23. This may be a little incoherent, I’m still trying to understand what you said and I think it’s really interesting. I want to point at what look like, at least superficially, fairly strong disanalogies with, say, the shadow matter case and so on and so forth. And I guess I just wanted to comment on these disanalogies. I don’t know if they’re ..[hum]..to the project or not but you didn’t seem to emphasise them in what you said. Look, take a simple branch post-measurement situation where I have branch A with a particle and a measuring device on it and I have branch B with a particle in a different state and maybe a measuring device in a different state on it. And I ask naively a question of the form: look at this quote ‘measuring device’ on branch A; does it have the functional and dispositional properties of the kind of thing we would usually call a measuring device? And somebody naively says, well, no, not at all, look, there’s a B particle right next to it and it’s not reacting to it at all. This seems to be a symptom of the following disanalogy between the examples you use and the kind of emergence that’s going on here. When we talk about the emergence of a tiger we still end up talking about the tiger as a collection of what the fundemental theory regards as fundamental ontological pieces of the world, particles or something like that. Something different seems to be going on here. That is, it looks like the right way to talk about emergence here is that the smallest unit that’s emerging is a world in its entirety rather that its pieces, okay.

24. Look at the shadow-matter case for example. In the shadow-matter case you can say there’s a measuring device here and there’s a shadow-matter measuring device right next to it and there’s a particle here and a shadow-matter particle right next to it. How can there be a measuring device which isn’t responding to the shadow particle in the way we expect measuring devices to react to particles? Answer: it’s not a particle, it’s a shadow-particle, it has quite different properties and there are different properties intrinsic to it which explain the dispositions of the measuring device to react or not to react to it. That’s apparently not going to be an available strategy in this case. In this case the overall structure of the wave function is determining which entire worlds emerge…and it doesn’t seem like we’re going to have the option to say about these worlds that emerge, oh, here’s how you make this world: you put in a tiger and you put in a lion and you put in a person and you put in a measuring device – something like that. If we just put those in we’re going to be missing, as it were, these world-membership relations among them that have to go along with it. So, that does seem like a disanalogy with the other cases of emergence. If we were committed to a claim of the form: worlds are the kinds of things that can be build up by putting in a lion and putting in a tiger and putting in a building and stuff like that then these aren’t going to count as worlds. Then we haven’t shown and emergence of worlds; this is a mysterious new kind of, on the emergent level, fundamental object. There’s a world, okay, and there’s some membership relation to a world that objects have or something like that. Rather than a world being a straightforward collection of these objects in the way that it is in these other emergence theories. I don’t have more to say than that and I don’t know how serious a worry this is, if at all, but it does seem disanalogous to the things that you were trying to draw an analogy to.

Wallace
25. Okay, great, there’s lots to respond to in that. The first point is, I don’t know how important it is, but I don’t find myself that the idea that fundamentally for?..things like cats and tigers the important thing is something like that they should fundamentally be thought of as a collection of stuff. I think the notion in which large things are collections of small things is derivative on the way in which they’re structured….small things rather than vice versa for reasons….

Albert
26. I just want to check one more thing, I’m sorry, but relevant to what you’re saying. Look at the simulation of a tiger on a computer. It’s easy to say why it’s not a tiger, it doesn’t have the functional properties of tigers, I can put a real mouse right in front of it and it won’t react at all.

Wallace
27. So, yes, I’m not sure how important it is. I think that sort of compositionality I’m not really happy with for reasons that James Ladyman’s done a lot more to spell out than I know what to say. I think that a lot of these analogies…..[hum]……these are not things in space. If we feel irreducibly committed to all of our relevant emergences being talked about in spatial terms then I think Everett has problems, I think that’s a profound mistake for general reasons I was trying to sketch out. What one’s got here, if you like, is a relation of dynamical distance such that if things are in this relation they can be dynamically isolated, which is not spatiotemporal..

Albert
28. ……It’s not like the shadow-matter

Wallace
29. ….So, in a sense it is interactional, that’s what the emergent interactions do, they don’t from this bit to that bit. Why is the particle detector on Earth wrong because it fails to detect a particle in the Andromeda galaxy? Answer: they’re spatially distant. Why is my detector bad because it doesn’t detect the particle which, assuming we can do spatial maps at all, is in a different world but in this spatial location? Answer: it’s distant in norm. The shadow matter thing…it doesn’t have that spatial thing and I that cashing that out in terms of intrinsic properties would quite surface. It would be dead easy to develop an example in a way that moves a bit away from the actual physics we have and say that essentially one has something at least relational?..here. We’re not generally inclined, at least I’m not generally inclined to say that the difference between positively and negatively charged things is that they have different intrinsic properties, one of positive charge and one of negative charge, it’s that they stand in the opposite-charge relation. So in the shadow matter case they stand in the different-in-shadowness relation and the reason I call the shadow ‘shadow’ is parochial.

30. And so, lastly, in terms of the tiger simulation, again, sure, you’re quite right, that’s one of the reasons we don’t call it a real tiger but that’s very much what I was calling….a ‘parochial’ problem with it. If we – okay, this is probably controversial and I don’t know if we have to accept it if we accept the Everett interpretation but I’ll run it anyway – If we lived in the Matrix then the tiger simulation, would we still call it a tiger? Our vocabulary might get terribly confused once we take the red pill and excape the Matrix but prior to that it seems the wrong way to think about the Matrix to say there are no cats, there are no tigers, there is this ….The right thing to say about the Matrix seems to be we’re wrong about the substratum which instantiates tigers, cats,….So it’s for parochial reasons there that we don’t call the computerised tiger a tiger, not fundamental reasons.

[new question]

Bacciagaluppi
31. [background hum again] …..Simon..explain a bit more what you were saying about differences between relative states and many worlds and, David, I didn’t understand what the…..problem some philosophers have

Saunders
32. I think the short answer is more or less what I said: use local projections. And consider dynamical structure that can be defined in those terms. I think it’s very clear that one can do that over small macroscopic mixtures. I don’t think you can do that in general over small microscopic dimensions. So this would be a different take on how to see structure in the universal state. One builds up from localised…..One can, as it were, build up a universe around us, here, which will be distinguished, that …value-definite over distances…within……macroscopic superpositions. But this is not in contradiction to the picture of worlds, we do have a superposition of worlds. Consider those worlds that coincide on this local state of affairs, and consider the superposition of those, and you regain the localised approach. And what that localised approach does, well, you can do this in spacetime terms, one looks at relations between events and soforth, the difference in Everett is that those relations don’t close in an equivalence relation. You don’t get equivalence relations, you get something much more structured and elaborately structured in the ……universal state. To be quick, but it’s roughly how I see this relational-structural approach.

Wallace
33. The philosophy of physics measurement problem essentially is the view that there is something wrong with quantum mechanics as a physical theory over and above the fact that it fails approximately to reproduce something structurally isomorphic to the pre-quantum world. So, for instance, that there’s a conceptual problem with the GRW theory, leaving aside the structure of the tails, even if the tails were bare?..there would be a problem with the GRW theory and further interpretative work to do with the GRW theory….. not in itself a solution to the measurement problem….It may be a straw man but I think it’s a relevant?.thing…[background hum]..

[new question]

Brown
34. I think it might be useful for some related discussion, David, to say a little bit more…………aside?..which is decoherence evolution……classical…can you say a little more about that.

Wallace
35. Okay, if you take a non-chaotic system without decoherence a coherent state does a pretty good job of tracking the classical dynamics on pretty long timescales. Take a chaotic system without decoherence it doesn’t do that. The reason it doesn’t do that is that what the classical dynamics wants to do is become highly fribrelated and the quantum system can’t cope with that when the fibrelation is thin compared to h-bar so you start getting a violation of quasi-classicality in a certain sense. Throw the environment in and what you find those differences lead effectively to wave-function collapse and the sense in which you map the classical dynamics is that you kind of – effectively what you’re reproducing is a coarse-grainedness of the classical dynamics which means branch by branch what you’ve got is something like a stochastic dynamics. You’ve still got a branch structure, the environment is still recording the full history, you haven’t got a sort of merging together of branches but it’s no longer the case that each branch does something classical because it’s not….in those details. All the sort of examples that get used in chaos theory popularisations are things that lead, if you like, to indeterminacy and to a branching phenomenology. So I don’t regard it as importantly different……..

Valentini
36. Does this mean that – you said something about in a non-chaotic case, in phase space you have different blobs each evolving along approximately classical trajectories

Wallace
37. although not configuration-space isolated blobs, necessarily

Valentini
38. So now does this mean that in the chaotic case you do not get blobs individually tracing out approximately classical trajectories?

Wallace
39. The blob language is confusing because it implies that something ……separating them

Valentini
40. ….assumption……..spreads very quickly

Wallace
41. Yeah, well if you look at that, looking at the density operator you’re seeing it smoothed, if you include the environment what you find is that for each – take the relative state, if you like, to any given little blob in the space what you’ll find is an incoherent superposition of lots of histories describing lots of different ways?..[background hum]..-the environment recording in it lots of different histories describing lots of different ways that got the system fixed up in that place?..It’s still a history structure but you’ve got a dynamics that’s stochastic.

[new question]

Barbour
42. I totally agree that structure’s what counts. I would like to question…[background hum]..justify that you should be looking only at the structure of the universal wave function. I can’t believe really that….completely satisfactory that way because the wave function must have some basis. I personally suspect that it is the configuration basis and I came to this conclusion from supposing that there were super-mathematicians who were looking at our universal wave function and saying: where is the information encoded that we’re in an expanding rather than a contracting universe? And I don’t see that that information can be encoded in the structure of the wave function, I think it must be encoded in the structure of configurations which are favoured by the wave function. And this makes me convinced in a way that quantum mechanics is a dual theory as we have it at the moment, it’s about a wave function and things on which it’s defined and that the interpretation has got to take that into account much more. I think everything you’ve said about decoherence is fine and makes a lot of sense but it’s not going to be the complete story.

Wallace
43. Okay, good. So it’s mistaken to think of the quantum state – and you’re not doing this – as some sort of point in a featureless very high-dimensional complex space, just as it’s wrong to think about classical mechanics as a point in a featureless very high-dimensional ….space. In all these cases that space has to be highly non-isotropic, highly structured so as to make the quantum state a structured thing. There are basically two ways one introduces that sort of structure, one does it by picking out certain operators as preferred, or one picks them out as regarding certain tensor-product decompositions as preferred and of course those are more or less interchangeable. How one does that comes under…the nature of those structure questions I was raising. Of course, if one’s prepared to use a sufficiently cooked-up set of operators one can put structure into anything, that’s an old problem with functionalism, I’ve got nothing new to say to it. If one isn’t going to do that I don’t think there’s an important difference between, say, representing the quantum gravity wave function and …..added structure in terms of the configuration space and thinking of it just in terms of say finding configuration and conjugate…as preferred operators or maybe some other set of operators that’s simply defined in terms of them. Plainly this sort of structural game is very standard. If one does do it in configuration space terms then I think it’s only a semantic distinction to talk about a difference between the wave function and the configuration space on which it’s peaked. I’d say……….that it’s peaked in a certain place this highly……space. What I think is dangerous is to help oneself uncritically to the idea that the right way to think about that configuration is as n particles or whatever because of course that’s not true. I don’t think your making that move….

[new question]

Lehner
44. As a historian, a thing I’d like to point out about Everett himself is that the way he framed his theory was in this very simplistic definition of an observer as a recording device, right. And it seems like that’s all very badly out of fashion, nobody wants to talk about that any more, and I have a suspicion people think that decoherence is the appropriate modern substitute for this model of the observer that Everett used. And what I just would like to point out is two things. One: the two don’t conflict with each other. Zurek’s claim that recording devices are decoherence-inducing devices, so Everett’s observers is of course in itself a decoherence machine. And secondly, from the societal point of view I do think that Everett’s model is actually a very useful and helpful to discuss a lot of these philosophical, metaphysical point that come up, especially for David Albert’s metaphysical complaints about the Everett interpretation. More helpful, I would claim, than the fog about approximate decoherence which tries to absolutely avoid speaking of …observers. We can speak about …observers and be perfectly functionalistic about it. I totally subscribe to your ringing endorsement of functionalism and I think that’s exactly what Everett is doing; he’s being functionalistic about consciousness in this sense and it perfectly matches with decoherence but it might be actually a very useful way of talking ….

Wallace
45.I think it has fallen out of fashion, I think it’s good that it’s fallen out of fashion, the reason it’s good that it’s fallen out of fashion is, firstly, it avoids having to get entagled by the mind-body problem and it lets us leave the job of understanding conscious observers to the people that ought to, i.e. psychologists. And secondly I think talking that way makes the Everett interpretation sound “even” more weird than it actually is because, to me, leaving aside issues about possibly?.branching into multiple people synchronically, ….the only thing about the Everett universe is that it is much, much bigger than I thought it was. But, if one talks about consciousness one doesn’t mean ……have an actual recovery of quasi-classicality, we might have a recover of the appearance of quasi-classicality but it wouldn’t necessarily be quasi-classicality. One starts talking in I think a very unhelpful way about the fact that there isn’t really any definiteness at all, just an illusion of definiteness. I think those are traps which one is walking on the edge of when one talks about observers too much.