The end of universal collapse?
From collapse of modernism to collapse of collapse. Collapseption. Very meta, very post-.
I've talked about it a few times here... There's a growing momentum behind the subjective collapse interpretation of quantum mechanics in which quantum collapse is not universal: you might measure whether the cat is alive or dead, even tell me you have made such a measurement, but you'd remain in a superposition of having measured both live and dead cat to me until I made my own measurement (of the cat or your results).
There was an aspect of these recent Wigner's friend experiments that has been under dispute which is that they involve non-destructive quantum measurement, something that previously wasn't thought possible.
A new paper ( https://link.aps.org/doi/10.1103/PhysRevLett.126.253603 ) announces successful non-destructive measurement of a photon not once but twice. It's not directly pertinent, but probably tips the balance insofar as the dispute seems closer to being settled.
What does this mean? Some interpretations of quantum mechanics would be less compelling. The ontological Copenhagen interpretation is out. Many worlds is out, but not it's curious variant 'many minds' (in which there's no universal branching, rather the mind remains branched). Bohm and the epistemological Copenhagen interpretation look unaffected to me.
More broadly, can it really be that reality is subjective? It looks objective enough, but that's the classical limit at work. That said, Wigner and his friend aren't people in these experiments... Consciousness does not appear to be a prerequisite for having a unique external reality. Is it nothing more than localism, another relativism with another kind of reference frame?
And most of all, is this just a ruse to shoehorn in a third pomo-friendly thread to annoy ssu? Oh, more pluralism, more diversity, yes, very clever. (It's not, honest.)
I've talked about it a few times here... There's a growing momentum behind the subjective collapse interpretation of quantum mechanics in which quantum collapse is not universal: you might measure whether the cat is alive or dead, even tell me you have made such a measurement, but you'd remain in a superposition of having measured both live and dead cat to me until I made my own measurement (of the cat or your results).
There was an aspect of these recent Wigner's friend experiments that has been under dispute which is that they involve non-destructive quantum measurement, something that previously wasn't thought possible.
A new paper ( https://link.aps.org/doi/10.1103/PhysRevLett.126.253603 ) announces successful non-destructive measurement of a photon not once but twice. It's not directly pertinent, but probably tips the balance insofar as the dispute seems closer to being settled.
What does this mean? Some interpretations of quantum mechanics would be less compelling. The ontological Copenhagen interpretation is out. Many worlds is out, but not it's curious variant 'many minds' (in which there's no universal branching, rather the mind remains branched). Bohm and the epistemological Copenhagen interpretation look unaffected to me.
More broadly, can it really be that reality is subjective? It looks objective enough, but that's the classical limit at work. That said, Wigner and his friend aren't people in these experiments... Consciousness does not appear to be a prerequisite for having a unique external reality. Is it nothing more than localism, another relativism with another kind of reference frame?
And most of all, is this just a ruse to shoehorn in a third pomo-friendly thread to annoy ssu? Oh, more pluralism, more diversity, yes, very clever. (It's not, honest.)
Comments (123)
Another reason to stick with the MWI (as supported by Deutsch's work on quantum information/computing and generalization of "Wigner's Friend"). :wink:
Many Worlds is fully compatible with Wigner's Friend. It's just a situation where worlds not only can split but also merge again under the right conditions. That is, decoherence is a local and reversible phenomenon if the system in question is isolated (as it is from Wigner's vantage point).
Also I think Copenhagen has always been epistemological despite talk of collapse. Per Bohr,
Quoting Niels Bohr (as quoted by Aage Petersen)
Quoting Kenosha Kid
I've argued this before, but I think the usual Cartesian subjective/objective dichotomy is badly broken and not a useful way of thinking about the world. Instead I think it's sufficient to talk about systems with state, where state has an informational sense.
Quoting Kenosha Kid
That's how I think of it. What you are (for us, human beings) and where you stand can make a difference to what you measure as we find with Einstein's theory of relativity. In the Wigner's Friend scenario, what Wigner measures (interference) is different to what the friend measures (a definite result). That just is the reality from their perspective.
Everything we know - the act of knowing - has an ineradicably subjective pole which is never disclosed in experience, because the subject doesn’t see itself (that being the blind spot). And you can’t get outside of that structure to make any determination of what reality is in itself or without reference to the mind.
Modern scientific method excludes or ‘brackets out’ the subject, so as to arrive at ‘the view from nowhere’, i.e. an understanding of reality that is as devoid of all traces of subjectivity. But in so doing it then forgets or overlooks the fact that knowledge of anything whatever always requires the judgement of an observing subject.
How so?
Lets say scientists propose a model of phenomena X, they find, by experimental removal of subjective variables, a model of X which successfully predicts the behaviour of X for almost everyone who experiences X. In what way have they 'forgotten' that these people's experience of X is ultimately reliant on their mind's interpretation of the phenomena? All they've done is derive a working model of X that removes a lot of subjective variance, not a model which removes subjective experience of it.
When a scientist makes a claim that, say, F=ma, they're not suggesting that this doesn't involve any mind experiencing that fact, the issue doesn't even crop up most of the time, but when it does, the appropriate science (cognitive sciences, in this case) are acutely aware of the fact that the brain models external states, the values of which remain hidden. What they are suggesting is that this model holds true despite any variances in the minds experiencing it. Two people will both experience the relation that f=ma, regardless of their subjective mental states.
In fact, it's brands of woo like yours which 'forget' the importance of subjective judgement. Just because you feel humans are important and have a special purpose in the universe, does not then mean we all do. Your insight is filtered through your own subjective judgement, it's not a hotline to God.
Quoting Kenosha Kid
Is not the above exactly what Many Worlds says happens?
No, wait... Many Worlds says the first person remains in superposition until (so far as the second person can tell) the second person observes the first person, not the same thing the first person observed. Is that not what you meant? Surely, if Alice reports to Bob that she observed that the cat is alive, Bob is not seeing Alice as in a superposition of having both observed the cat alive and observed the cat dead; Alice is observably in one of those states. (Even if the actual fact of the matter, as I understand MW to say, is that Bob has merely decohered upon his observation of Alice, and Bob is now in a superposition of having observed Alice having observed the cat alive, and having observed Alice having observed the cat dead, etc).
That would be different to many worlds in itself. If you have to add a thing (merging criteria) that's a new theory.
Quoting Pfhorrest
No, many worlds is a universal branching. When Wigner's friend measured the cat, the universal wavefunction would split then universally. Wigner has to measure live cat in the branch in which his friend measured live cat and dead cat in which his friend measured dead cat. In Wigner's friend, Wigner's branching isn't determined until Wigner himself makes a measurement.
In MWI there is a single, real (ontic) wavefunction. The cat (third term) is in superposition and Wigner (first term) and his friend (second term) have made no measurement:
(A) | not measured > X | not measured > X ( | alive > + | dead > ) / root(2)
After the friend measures:
(B) | not measured > X ( | measured live > | alive > + | measured dead > | dead > )/root(2)
At this point Wigner and his friend are unentangled, however if they become entangled, it evolves to:
(C) ( | must measure alive > | measured live > | alive > + | must measure dead > | measured dead > | dead > )/root(2)
that is, once entangled, there can be no interference between the live and dead terms apparent to Wigner. What the recent experiments show is that, even after Wigner's entanglement, those interference effects persist, and Wigner remains as per (B). It is only when Wigner _knows_ his friend's measurement outcome that he himself branches, i.e. the wavefunction is epistemic, not ontic.
Quoting Pfhorrest
That's correct, hence:
Quoting Kenosha Kid
rather than "tell me what measurement you made".
Quoting Andrew M
Copenhagen was originally epistemological, yes. Iirc Bohr himself went the ontological route in the end (I didn't know this until someone here found a relevant quote, should be able to dig it out if need be). But anyway there's a bunch of ontological Copenhagenists out there.
Quoting Andrew M
Yeah me too but there's a lot of Cartesians here and I think they'd find it interesting.
I think I have said this to you before, but this is massively out-of-state thinking. Special relativity killed off the idea that there's a special frame of reference for an ideal observer (a god's eye view) and quantum theory made it abundantly clear that observing an experiment makes you part of the experiment (gonzo science if you will).
Quoting Wayfarer
Then how come
Quoting Wayfarer
The whole of the modern scientific method doesn't sound very 'dead' to me.
Also, if...
Quoting Kenosha Kid
...and you...
Quoting Wayfarer
...then how are you still claiming that science is looking for the view from nowhere? Is quantum theory not science, is special relativity a movement in literature?
Wait, like SR, the QM framework is objective – subject-invariant – but just not universal (or absolute), no?
The initial state of the universe before anything interacts with anything else has all wavefunctions unentangled with each other and so in superposition from each other's perspectives.
When a radioactive atom interacts with a detector, they become entangled with each other, and enter a superposition together from an outside perspective, while "collapsing" each other's superposition from each other's perspective.
When the detector triggers the release of toxic gas, that becomes entangled with the atom-detector system too, joining their superposed state as seen from outside, "collapsing" each other's wavefunctions from their inside perspective.
When the cat interacts with the gas, it does likewise; now there's a live state of the cat entangled with the undecayed atom (and the rest of the apparatus in between), a dead state entangled with the decayed atom (etc), and those entangled states are still superposed with each other from an outside perspective.
When Wigner's friend observes the cat, he becomes entangled with it all, and states of him having observed various outcomes are superposed upon each other from an outside perspective.
When Wigner observes his friend, or the cat, or at all interacts with that entangled system such in a way where the state of the system will affect the state of Wigner after the interaction, he becomes entangled with it, and different subsequent states of him are superposed upon each other from an outside perspective.
And so on, more and more of the universe becoming entangled as parts of it interact with each other, which from the perspective of anything outside of that web of interactions (anywhere the information of some quantum measurement has not reached yet) looks like all of that being in one big superposition together; so, from a hypothetical "outside the whole universe" perspective, the whole universe is in a superposition of every possible state it could be in given all the interactions that have happened in it, with each classical state in that universal superposition being a "world", of which there are thus many.
If that's not Many Worlds, what is that?
The recent experiments are being used to suggest that QM is observer-dependent:
https://www.mdpi.com/1099-4300/20/5/350
https://www.nature.com/articles/s41567-020-0990-x
https://www.nature.com/articles/s41567-020-0984-8
https://www.sciencemag.org/news/2020/08/quantum-paradox-points-shaky-foundations-reality
https://advances.sciencemag.org/content/5/9/eaaw9832.full
When Wigner's friend communicates to Wigner that they have made a measurement, if Wigner and his friend weren't already entangled (unlikely) they would be so at that point. Wigner hasn't made an observation of alive/dead, but is nonetheless entangled with the system that is in superposition. At this point, what Wigner observes in each should be determined: this is universal collapse in a nutshell, and branching in MWI replaces universal collapse. No interference between the two branches should be observed by Wigner after entanglement but prior to measurement.
This is why in MWI you don't need an additional branching to explain why Wigner's and his friend's observations always match up and no interference occurs: entanglement (not necessarily observation, just entanglement) ensure that Wigner always measures the same as his friend. You can construct the cross-terms, but these would be additional branches with a weight of 0 (since the probability is given by the integral over all space of two orthogonal terms, which is zero).
What we see instead is evidence of observer-dependent collapse: Wigner knows that collapse has occurred for the friend, but for Wigner the friend is still in superposition as evidenced by interference effects between the alive and dead terms (collapse has not occurred for Wigner).
This is inconsistent with MWI in which Wigner must either be in the branch in which his friend measured an alive cat or a branch in which his friend measured a dead cat.
As Andrew suggested, you'd have to modify MWI to allow Wigner to not only branch after entanglement but to observe evidence of the alive/dead cross-terms before Wigner's measurement is made (Andrew's merging), that Wigner is somehow sufficiently in communication with his friend to gain knowledge that his friend (EDIT) has made a measurement but not sufficiently entangled to collapse that superposition until he himself has made an alive/dead measurement.
As far as I see it, wavefunction realism as per MWI takes the onus away from physics and puts it back on consciousness if it's going to explain Wigner. To observe a superposition, Wigner would have to be in physical communication with something but remain effectively unentangled such that interference can be observed. Since consciousness isn't a factor in these experiments, I think that can be rejected.
My preliminary read interprets "observer-dependent" as experimental set-up (subsystem)-dependent: different subsystems observe (measure) different aspects of the universe – each a different one-universe of the multiverse (Deutsch) – analogous to separate observers reading clocks in different inertial reference-frames. Objective pluralism and not subjective relativism. Well, that's my read so far; more careful rereadings to come ...
Indeed, or even just reference frame dependent: Wigner occupies a different reference frame to his friend. Whatever the underlying reality, he and his friend have a different viewpoint and witness different aspects.
Quoting 180 Proof
I don't think subjectivism has anything to do with it either, but as I said above relativism a la SR (and QM is special-relativistic) looks very much on the table. However, and I've asked this of others in the past and not quite got an answer, what differentiation do you make between a maximally pluralistic objectivity (e.g. one objectivity per reference frame) and relativism (as in frame-dependence, not subjectivity, as would apply to an atom, a device, or a point in spacetime)?
P.S. I should add that observer-dependent facts is only one possible interpretation, even if it is the one that gets preferential treatment in paper titles.
The other options are non-locality (FTL communication) and super-determinism (backward or nonlocal causality). In a previous thread on determinism and quantum mechanics, I put my weight behind a kind of superdeterminism, in particular two-way causality, though I'm unsure how it applies to this seeming observer-dependence.
https://www.frontiersin.org/articles/10.3389/fphy.2020.00139/full
Einstein didn't accept the deprecation of what he regarded as the mind-independent nature of reality. That is why he posed the rhetorical question, 'doesn't the moon continue to exist when nobody's looking at it?' That is the key point as far as I can discern.
A good question with no easy answer but definitely dives headlong into the meaning of existence itself. How, may I ask, dear Albert Einstein, paragon of genius, did we ever come to the conclusion that there evere was a moon? Observation, another word for perception/detection with senses/instruments. If so, why is it difficult for you Albert to grasp that absent an observation, the moon's existence is a question mark, a very big question mark!
Wayfarer, kind person, can you help me out regarding a certain problem that I've been mulling over since last month.
The brain-No brain Thought Experiment
You know EEGs are used to detect so-called brain waves, right? So here's the scenario. Person A has an active brain (alive). A has an EEG reading. Person B has an inactive brain (dead). B has a flat EEG reading. "Person" C is a mannequin, has no brain. C has a flat EEG reading.
Clearly A is not the same as B or C, A has an EEG reading. However, B and C aren't the same. B has a brain but it's just no longer active. C has no brain. The EEg report though is identical for both B and C - no reading. Thus, it can be said that we couldn't distinguish between there being a brain (dead B/inactive brain) and there being no brain (C). An inactive brain = no brain. :lol: Reminds me of myself!
How does this relate to Einstein's question? Simply in this way: Insofar as our eyes are concerned, not seeing the moon is like the flat EEG. We can't know therefore whether the moon exists and it's not registering in our retinas (inactive brain B) or the moon doesn't exist (no brain C).
I think I went off on a tangent. Sorry if it was not worthwhile. G'day.
Minds are not required to bring things to a close: a molecule will do the job, and it's an act of epistemology to reduce the wavefunction to what fits with the molecule. This seems consistent with Bohr's meaning elsewhere, that collapse is an observer-independent decoherence that could be but is not necessarily caused by a conscious measurement.
For the record, I was with Bohr on this, insofar as it seemed unwarranted to assume that a photon actually traversed any space at all between creation and destruction (photons are clicks in photon detectors). The new paper linked in the OP might change my position on that: being able to detect a photon non-destructively suggests that it is a field, not just a delocalised event.
I think that is contestable. Registration, in that passage, is an 'act of measurement', not simply an interaction between any particles.
Quoting Kenosha Kid
To me, the philosophical point is that there is not some already-existing atom which is discovered by observation. Prior to it being observed it only exists as a distribution of probabilities; it has, if you like, only a tendency to exist. It is the registration on the plate which changes that state of affairs, which seems to bring it into existence. That is what the many worlds interpretation seeks to avoid.
So, I think implication is that photons etc have no intrinsic reality, that their reality is imputed. Remember 'phenomena' means 'what appears'. That's why physicists like Carlos Rovelli invoke N?g?rjuna, the Buddhist philosopher of emptiness. Emptiness means that 'phenomena lack own-being'.
[quote=Carlos Rovelli;https://www.ibsafoundation.org/en/blog/the-formula-that-makes-science-fascinating]N?g?rjuna's philosophy is centred on the idea that nothing exists in itself. Everything exists only because of something else, in relation to something else. The term used by N?g?rjuna to describe this lack of essence is ‘emptiness’: things are ‘empty’ in the sense that they do not have an independent existence, they exist thanks to, on account of, in relation to, from the perspective of, something else”.[/quote]
Which holes metaphysical realism under the waterline.
---------
Quoting Isaac
It was the classically modern scientific framework, I was referring to, with an observable and measurable object being the bearer of primary attributes, the presumed locus of whatever is objectively the case. That has indeed begun to change, but I'm talking about the strict Cartesian-Galilean picture of the world comprising inanimate elements governed by physical laws. In that picture, the role of the subject was never considered. If it is now, it's precisely because of being obliged to, by these very discoveries.
One of the contributors put it brilliantly and succintly in another thread:
Quoting sime
No. Many Worlds is a subject relative branching. It's simply part of the universal wavefunction.
No. This is described exactly in the introduction of Everett's "The Theory of the Universal Wave Function". Using the terms in the introduction, (A + S) is the object-system for observer B; in terms of Wigner's friend, B would be Wigner and A would be his friend. MWI is the proposal that S is not collapsed when A measures S.
Translating this to Schrodinger's cat (Wigner=Schrodinger, the cat=Wigner's friend), before Schrodinger opens the box, the state (A + S) is in a superposition of a dead cat in the box and a living cat in the box. The dead cat and the living cat are different worlds, but Schrodinger before opening the box sees both worlds in a superposition.
This "universal branching" notion you describe is not a thing.
ETA, this is precisely the alternative Everett entertains, quoted from the paper:
(underline mine; italics in the paper) ...just to show I'm not making this up. This is the fundamental assumption; the mechanics of branching are the mechanics of the wave function evolving via the Schrodinger equation ("Process 2"), not some new thing Everett came up with.
Why would that be relevant, given that...
Quoting Wayfarer
...?
But even with regards to...
Quoting Wayfarer
...Was it not this very enterprise that was responsible for said discoveries? It wasn't philosophers who discovered quantum theory. It was scientists, brought up in the exact modern scientific framework you criticised as it...
Quoting Wayfarer
... So how did it give rise to the discovery of innate subjectivity? How is it that the very method you critique derived the evidence you're using to substantiate that critique?
That's a contradiction right there.
Quoting InPitzotl
You're not making it up, but it doesn't say what you purport it to say, that branching is observer-dependent. That the observer is a physical system is not in dispute. In MWI, when system A is entangled with system B and system B branches, system A also branches. That is what a straight reading of the mathematics tells us.
Observer-dependence tells us something different, that branching may have occurred for B and not for A, even though A and B are entangled. That A might entangle with a superposed state and still see it as a superposed state. That is not compatible with MWI as is, at least not obviously or without modification.
Quoting Wayfarer
It's there in the quote you provided, unless you're a panpsychist and believe that silver bromide has mind.
Quoting Wayfarer
Indeed, and this is where the paper above differs. Effectively that registration reduces the number of possibilities to unity for you (or it), while staying distributed for me (or some other device). So something else is going on here.
Agreed, if 'reality' is left ambiguous between a unique realist objective ontology and many relativist subjective appearances. Philosophical disagreement and repeated failed attempts to discover some missing factor to make everything orthodox make all objectivist attempts suspect from the start.
But I don't see why objective approximations of the past would not be useful in approximate predictions of the future. Scientific approximations tend to improve in accuracy over time.
A consensus builds... I'd be interested in hearing both your thoughts on what kind of relativism this is. In addition to the special relativism of the Dirac equation and quantum field theory, there is an additional relativism of quantum mechanics put forward based on state.
The idea is that, since the laws of physics must apply in all inertial frames, there must be some transform between superposed states and eigenstates. For instance, an electron in the double slit experiment is in a superposition of positions in a non-negligible way (a wave) while the lab is not. However there must be an inertial frame for the electron itself, and in that frame the lab would be in a superposition of locations.
Since the observer-dependence of collapse in these Wigner's friend experiments is essentially a disagreement between observers in their own frames as to whether something is in superposition or not, something like this might be the answer.
https://www.nature.com/articles/s41467-018-08155-0
Quoting magritte
Oh definitely. You can Wang a satellite around the solar system and land it in a predetermined patch of ocean with Newtonian gravity alone.
You're severely confused here. You're certainly not addressing what I purport. Keep the terms to ensure you're not conflating things.
Quoting Kenosha Kid
S has the radioactive substance in it. That decays or doesn't decay. A is Wigner's friend the cat; A either survives or dies. Yes, when A measures S, and S branches, A also branches.
You have W1=a living cat (A1) with no decayed element (S1), and W2=a dead cat (A2) with a decayed element (S2).
Quoting Kenosha Kid
B is Schrodinger(/Wigner). Schrodinger need not be entangled with the substance or the cat; when not entangled with either, Schrodinger sees W1+W2. Since that's possible, worlds are not universal.
Okay, now I see you don't understand. No one is saying that Wigner has to be entangled. The point is that he can be entangled (as per the paper) and still observe the friend in superposition (also as per the paper). Not meaning to sound harsh, but you need to grasp what the paper is saying first, because everything you're countering with is irrelevant.
I'm not commenting on the paper; I'm commenting on the notion that branches are universal. I don't think I have any comments on the paper at this time.
ETA: Or perhaps I'm just misinterpreting what you mean when you say the branches in MWI are universal? What I mean to clarify is that "universe branching" is "subject relative"; what looks like different "universes" to the cat isn't necessarily different "universes" to Schrodinger.
Okay you're talking about subjectivities across branches, yes? That's not what's meant by observer-dependence here: it's not about different versions of you across different branches seeing different facts, rather that different observers will disagree within a branch/unbranched universe.
The equivalent in MWI if what's happening here is that friend measures cat, friend term in wavefunction branches, Wigner entangles with friend, but Wigner can still access both branches. This is a nonsense. We could modify the concept of entanglement to ensure that it can only be said to occur when Wigner makes an actual measurement, but then we're back in magical consciousness territory, contrary to the point you made earlier that observers are just examples of physical systems.
I agree that this is nonsense, but what you said that I responded to was this:
Quoting Kenosha Kid
...and there's certainly nothing universal happening in the sense that Wigner entangles with his friend when his friend entangles with the cat.
In MWI there is a single, universal, objectively real wavefunction. Any branching is universal: it is a branch in the universal wavefunction.
...but you apparently agree the branches are not universal in the sense that Wigner branches when Wigner's friend branches:
Quoting Kenosha Kid
Quoting Kenosha Kid
Right?
Does not every observation, as an interaction, cause entanglement? (Leaving room still for partial observation; see below).
Quoting Kenosha Kid
Is this while Wigner and his friend are already entangled? Because this sounds like the same relationship the friend has to the cat before opening the box: he knows there is a cat for whom the particle’s wavefunction has collapsed inside the box, but to him the cat is still in superposition of having experienced different kinds of particle collapse.
I’m getting the sense that this new evidence is of the possibility of SOME information from inside the box being communicated to the friend without it being enough of the right information to collapse the wavefunction; likewise the friend can communicate some info to Wigner without collapsing the wavefunction from Wigner’s perspective. Is that accurate? If so, would it also be accurate to say that the particle is not entangled with the friend or Wigner in those respective cases? Basically, Wigner can observe anything about his friend that can’t imply anything about the collapse of the particle’s wavefunction, and keep those aspects of the system (friend, cat, particle, etc) superposed from his perspective so long as he does so?
If that is what’s going on here, that’s as expected by my interpretation, which I’ve (perhaps erroneously) been calling MW.
The moon has a gravitational influence on Earth, such as with tides and keeping the Earth from wobbling too much, which keeps seasonal variation from being extreme. We can ask the same thing about the stars and galaxies out there, but inertia is based on all the mass in the universe resisting our change in acceleration.
Point is that there are all sorts of indirect observations being made even when we don't realize it. Nocturnal animals make use of the moon and starlight, even while we're asleep. The moon is part of the world we observe. Asking whether it exists when we're not looking is to ignore the larger context of the world itself the moon is part of. It can't just not exist and the world we observe remain the same. It's like how the ground holds us up even when we're not consciously aware of it.
That's the problem with asking whether things exist when we're not observing them. The things we do observe depend on the things we're not observing at the time.
What I'm critiquing is not science but scientism.
Indeed, scientists and philosophers have begun to see through it. The collapse of the purportedly observer-independent nature of fundamental physical particles is a big part of that.
Quoting Kenosha Kid
Doesn’t it suggest that perspective is fundamental? Intuitively, you would think that perspective is a product of the evolved brain, a recent layer of the pre-existing reality, but this suggests that perspective is actually a constituent.
[quote=Bryan Magee, Philosophy of Schopenhauer]Once, in Karl Popper's living-room, I asked him why he rejected [Kantian idealism], whereupon he banged his hand against the radiator by which we were standing and said: 'When I come downstairs in the morning I take it for granted that this radiator has been here all night'.[/quote]
But that's not MWI, that would be some pretty meaningless compromise. The whole point of MWI is a single, real, universal wavefunction.
In remember that quote exactly! Wonderful book. But man that was quite a silly thing for Popper to argue. It takes so much for granted, which is a common mistake or so it seems to me.
Nobody is saying it's MWI.
What I'm trying to unwind is what you could mean by this:
Quoting Kenosha Kid
Unwinding, (B) is this:
Quoting Kenosha Kid
...for clarity I've underlined Wigner's state and bolded Wigner's friend's states.
Given this, the above paragraph makes no sense:
To speak of Wigner being entangled (with the cat's life) without speaking of (C) has no meaning to me.
So what do you mean by it? It sounds like you're describing some state where Wigner's entangled with the cat('s life) yet still at (B).
I've discussed this many times on forums, and the consensus usually is, Johnson was right. (That said, I'm not a Berkeleyian idealist, but I won't divert the thread on that point.)
A bit surprising to see that even here. Which is why it is important to state what you (or anyone) take idealism to entail.
But no serious idealist would ever say that a rock is not solid or that I can move an apple by thinking about it or whatever craziness they may say.
In either case for QM, I know you think observation is crucial for the collapse. I personally think that despite what QM discovers, idealism, materialism, objectivism or anything else remain intact. Anyone who already believes in one of these views will simply accommodate the experiment to whatever metaphysical views they already have.
Probably not the best way to go about it, but I think it's true at this point in time.
Not only my opinion. The question I ask is, if the many worlds conjecture is a solution, what is the problem? What would its [s]fanatics[/s] advocates, like David Deutsche, be obliged to admit if it was shown it could not be real?
I know. I believe the view that human beings are specifically relevant for collapse is a minority one, though this by no means implies it is wrong at all.
I think they're trying to maintain the intuition of determinism. They don't like that current QM is probabilistic is my guess. I don't know what they'd do, but since there's no way to test it, arguments as to its implausibility is going to change minds. They know Many Worlds is pretty wild.
Unless we build a much larger collider, we seem to be stuck. And even that might not change the results.
The bigger the equipment, the bigger the questions.
Curious how one of the things 'the scientific view' does is, in the first place, to deprecate h. sapiens to being 'another species', 'chemical scum', in Hawking's charming phrase, and then on the other, to declare, nevertheless, that h. faber, scientific man, is the arbiter of reality.
It's a delicate balance imo. Not that reality isn't shaped to a very large extent by us, I agree with you on that point. But to think of ourselves as bigger than we are is a problem. Much misery came our way when we thought we were the center of the universe or that the sun went around us.
We've done remarkable things to discover all these new facts about the universe. But I think we ought to keep in mind our animal part, while appreciating our amazing intellectual capabilities. So no, I don't think we are "merely" another species, but I also don't think we should be too proud, as we burn our planet to ash.
'Size' has nothing to do with it. What we bring is perspective.
And knowledge, and explicitness and order, etc. ;)
For MWI, merging as well as splitting is entailed by the unitary dynamics. From the friend's perspective he has, for all practical purposes, performed an irreversible measurement - he measures spin-up in his world while his doppelganger measures spin-down in another world. But from Wigner's perspective, the friend (and his measurement) is simply in superposition (i.e., within Wigner's single world) and he can always apply a unitary transformation that reverses the friends' measurements, thus merging the friends' worlds back into one.
Quoting Kenosha Kid
I would be interested in the quote if you can locate it.
Quoting Kenosha Kid
Strictly-speaking, their position is more like an objective collapse theory (like GRW or the Penrose interpretation) which necessarily differs from the standard unitary formalism. In those particular objective collapse theories, superposition applies to microscopic objects only (which can include the microscopic Wigner's friend experiments that have been performed). Whereas the standard unitary formalism applies also to macroscopic objects.
Quoting magritte
This is where specific interpretations attempt to fill in the gaps. Those interpretations will remain with us until we have more definitive experiments.
Quoting Kenosha Kid
I think Rovelli's relational interpretation is helpful here. It provides a clean abstraction around the idea of reference frames that covers all the issues raised by Wigner's friend:
Quoting Relational quantum mechanics
As I see it, the main difference between MWI and RQM is that RQM is silent on what a superposition consists of (since it is a more abstract interpretation). Note that what is now called MWI was originally called the relative state formulation by Hugh Everett, and the two interpretations share that relational (or relative) aspect.
Quoting Kenosha Kid
I entirely agree. In fact, I see we had a brief discussion on this a year back!
Your initial objection was to my comment that MWI doesn't hold up any more if we accept the observer-dependence interpretation of the Wigner's friend experiments. If you agree that what you're proposing is not MWI, how is this progressing your counter-argument?
Quoting InPitzotl
prior to entanglement. But as I've told you, Wigner and his friend are not unentangled immediately prior to Wigner's measurement of his friend.
Of course, in practice, Wigner and his friend ought to have been entangled even at this stage, since their actions are correlated and entanglement _is_ correlation.
Quoting InPitzotl
The OP is asking for your views about the purported observer-dependence of facts (collapse/branching) in light of the recent strengthening of the validity of non-destructive measurement techniques.
What the Wigner's friend experiment show is that, after entanglement, after the friend has made his measurement, but before Wigner has made his measurement, when we would expect something like (C), Wigner is still seeing the world as per (B). Wigner is entangled with the lab insofar as communication and coordination about the experiment is ongoing between he and his friend, but Wigner still sees the lab in a superposition: the lab has branched for the friend, but both branches are evident to Wigner, contrary to MWI.
Proposing? What the heck are you talking about? I just described what's not a thing, and had thought you were agreeing it wasn't a thing. But given you think I'm proposing something that isn't MWI, I would say that clearly you're confused.
Quoting Kenosha Kid
This is a handwaved concept of entanglement. Wigner and Wigner's friend have lots of states that are entangled. What constitutes being "unentangled" to you?
Quoting Kenosha Kid
No. We would expect (B). Wigner's friend entangling with the cat's life does not instantaneously make Wigner entangled with the cat's life. If Wigner hasn't done any measurements, there's no reason for Wigner to be entangled.
Quoting Kenosha Kid
I've no idea what "entangled with the lab" means, but it sounds like a fuzzy red herring. Surely Joe, who does a simple double slit experiment (or quantum erasure experiment, if that's what's confusing you), is just as entangled with his lab as Wigner is with his.
Quoting Kenosha Kid
MWI has no objection whatsoever to the universal wavefunction being in a state like (B). Incidentally, (B) implies that Wigner's friend has branched. That live cat in (B) does not know what hydrocyanic acid smells like. But Wigner himself is not branched in (B). The branch is "universal" in the sense that it's a branch in the universal wavefunction (it's right there, in (B)). But it's not "universal" in the sense that Wigner's friend branching implies Wigner branched (he clearly hasn't). Having a person not branch with respect to a wavefunction that is in superposition is not a problem; how else will Joe see an interference pattern?
What you're describing art the start is two unentangled systems (B). That is not what the experiment is describing, in which Wigner and his friend are correlated (should be (C), but isn't).
As I said above, the alternative is to insist that entanglement doesn't occur just when two systems exchange information, but when an observer makes a measurement, which is not justified by the experimental setup.
As for merging, in this experiment Wigner is simply performing an interferometer experiment. Also since, in his frame, the lab is still in superposition, there's nothing to be undone. From the friend's point of view, there's nothing Wigner can do to find a different outcome: in the alive branch the cat is alive and this is what Wigner must find; in the dead branch the cat is dead and this is what Wigner must find.
Quoting Andrew M
Will do.
Quoting Andrew M
Yes, I think relational comes out strong.Quoting Andrew M
Well remembered!
Quoting InPitzotl
It's not a point of view. Can you at least look it up on Wikipedia or something?
Quoting InPitzotl
"after entanglement" here refers to Wigner's entanglement with the lab, not his friend's entanglement with the cat. Again you would understand it if you just bothered to read the paper containing the experiment rather than taking this course of adopting a strong position out of ignorance and me having to repeatedly point out where you're being irrelevant.
I imagine (because my math sucks!) the branches are like folds in an origami and the worlds of MWI are worldlines of particle-waves, that is, the many unfolded/enfolded shapes of the sheet. We 'inhabit' the edge of an unfolded shape (or worldline) that doesn't branch "into" (something else) but only branches away from other worldlines.
Quoting Kenosha Kid
I think of it this way: the latter concerns 'variable/configurable observer-observations of one (the only) worldline' (e.g. one elephant and the many blind men) whereas the former concerns 'any one-worldline measurement of N-possible worldlines' (e.g. locked inside a completely enclosed carnival ride and looking out at the carnival through a peephole). So tell me what's wrong with this pluralism/relativism conception or why it doesn't work for interpreting fundamental physics.
Quoting Kenosha Kid
They all probably work after a fashion (like epicycles & the aether) but I'll stick with the most parsimonious observer-invariant (my anti-idealist, anti-antirealist bias) interpretation of QM we've got which for me (as far as I know) is the MWI.
I'm not sure whether we're referring to the same experiment. I'm thinking of David Deutsch’s version of the Wigner’s friend experiment which Caslav Brukner describes in A no-go theorem for observer-independent facts (one of the papers you linked to earlier).
From that paper:
So the upshot is that the friend has made a definite measurement and reported that she has done so to Wigner, without telling him what the result was. At the same time the lab remains in superposition for Wigner, per your (B). Thus the fact about the friend's measurement result is reference-frame dependent (at least, that's the option I choose from Brukner's no-go theorem). That's prior to entanglement between Wigner and the lab, per your (C). At that point there will be no interference effects.
You clearly said that the scientific method as incapable of accommodating the subjective. The method, not the philosophy sometimes associated with it. You then went on to say that science derived directly from that method proved this.
You've basically disproven your point. Science is perfectly capable of exploring the intrinsic subjectivity of experience. There are dozens of theories about reality which include, or even hinge upon, the idea that we subjectively create aspects of it. It's probably the leading theory in neuroscience at the moment, for example.
If you're critiquing, not the method, but the philosophy - scientism, then the same critique should apply to all philosophies which attempt to acquire the 'view from nowhere'. Platonism, for example, which posits that there exist forms like numbers and essences which are true independent of the human minds which grasp them - the view from nowhere. Where is your critique for these approaches?
Do you recommend Daniel Dennett's approach to the subject?
To the extent that I'm aware of it, yes. I was in broad agreement with his article 'Quining Qualia' for example. I think it made some very good points about the subjectivity of experience. Again, it seem the opponents of Dennett's position who are wanting the 'view from nowhere', where 'the blue quale' is a thing that we can call into existence as a group of humans and talk about as if we all knew what it really means.
Accepting reality as being that which we subjectively model from hidden external states forces us to discard the objective reality of things like qualia, number, consciousness, purpose, God... All these things must only be models, better or worse for their utility.
In that case, nothing to discuss.
Ah yes, I forgot your aversion to talking with anyone who disagrees with you. My apologies. No doubt I just don't understand the issue. In fact, have you got a massive quote from someone to that effect you can paste in lieu of an actual response, by any chance?
That's due to the subject matter, which has no ultimate nature. Science went searching for the ultimate basis of matter, and this is what it found.
That's Wayfarer's m.o. for sure.
Quoting Isaac
Yep, that too.
Yep. Which it then had the decency to admit to, investigate and form models based on.
What happened when Plato, Buddha, Mohammed,...etc went in search of the really real essence of reality? A similar admission, or thousands of year's worth of woo?
Yeah, frustrating as hell. I can't get my head around why someone would come on to a public forum and then not do the one thing the forum is designed to do, I mean, it's not as if there aren't hundreds of topic-specific forums in which people can discuss the details of issues they're all in broad agreement on. This approach of launching into a topic with a very strong claim, knowing it will be contested, and then backing out almost as soon as it actually is contested is really intriguing. It's the main reason I keep engaging, I'm hoping one time I'll understand a little more about the thought processes here.
There are a number of people here that never respond to my posts. The reason it’s pointless to debate you, is because you have a fundamentally positivist attitude which is never going to be shifted by anything I have to say.
Why would you want to shift my attitude? Are you recruiting?
Notwithstanding, I've a couple of questions about this approach of yours...
Why 'attitude'? Are approaches for you clearly divided into 'attitudes' and conclusions (the former intractable, the latter the result of reasoned thought)? How do you distinguish an 'attitude' from a conclusion, such that you might find it worthwhile debating a wrong conclusion, but not a wrong 'attitude'? Surely persistence in the face of counter-argument can't be a measure alone as one would presumably persist no less with a well-reasoned and right, conclusion.
If someone disagrees with you and that's because you're actually wrong, then their position isn't going to shift is it, yours is. So by saying it's pointless to debate someone who position doesn't seem to be shifting no matter what you say is a self-immunised assumption that your position simply must be right. What if I'm not changing my position because all of your arguments are weak and unpersuasive? Does that bother you at all? What if I'm not changing my position because it's more resilient to counter-argument than yours? Is that something you just assume couldn't possibly be the case from the outset?
So the experiment has actually been done several times and, unless I made a mistake, one of the links was to one of those reports. So yes, same thing.
Quoting Andrew M
But at this point at the very latest Wigner and his friend should be entangled as they are exchanging information, i.e. they are not two independently evolving systems. This is how they'd appear, for instance, to an outside observer whose lab includes Wigner and Wigner's lab. In reality, Wigner and his friend were correlated well before. But then in reality we don't expect humans to be entangled at all (classical limit).
I was just asking what the difference is.
Not annoyed, but thank you for the thought. :smirk:
The difference as I understand it is this:
Relativism
"The cat is dead" is true for Wigner's friend but not for Wigner.
Plural realism
"The cat is dead is true for Wigner's friend" is true for everyone.
In one, the facts are observer-dependent; in the other, everyone has their own observer-independent facts. (An actual observer isn't necessarily required in either case, just a frame of reference.)
MWI doesn't help here because it branches on observations, not observers. When Wigner's friend makes a measurement of a superposed cat, the universal wavefunction branches into alive and dead branches. Wigner later makes his measurement but a version of him does so in each one of those branches. There is no observer-dependence here since, within a given branch, nothing is in superposition any more. In fact, MWI was formulated in part to explicitly exclude observer-dependence.
If I follow him rightly, @Andrew M is arguing for a conceptually modified MWI in which branching is observer-dependent, something along the lines of: before Wigner makes his measurement, his part of the wavefunction remains separable from the lab's. Within the lab there's a sort of miniverse that's cut off from the rest of the universe, within which the alive and dead branches evolve independently, but from the outside it's the whole that's evolving, including cross-terms | alive > | dead > and | dead > | alive > (which is where the interference effects come from). I'm not sure how that's going to work out, but could be the kind of pluralist realist thing you're looking for.
Quoting ssu
Out of interest, and before the real third and final part of my pomo triptych, what did they say? (Why am I anticipating the answer "They said as long as he doesn't do a third"? :rofl: )
But I'm not the brightest guy anyway...
It seems to me that in the MWI 'observers and observations' are identical.
Pluralism:
"The cat is dead" is true for Wigner's friend but not for Wigner.
is equivalent to
"The cat is dead [is true] for Wigner's friend"
Realism:
The cat is dead [is true for everyone].
In addition to pluralism, relativism whether ontological [dead] or logical [true] requires a second higher level functor.
Not mentioned yet is the even more radical alternative: the worlds that branch out are made of words. That is, at bottom, you actually see text in the wave function!
:cool:
No, not at all. Branching occurs when a system in superposition is "decohered" (not necessarily by a mind, just whatever is sufficient to cause collapse or branching). Basically as long as the superposition coheres, you can get interference effects (e.g. in the double slit experiment, there isn't branching as the electron goes through each slits, as each branch must evolve as if the other didn't exist, so you couldn't get that interference pattern), but once it decoheres, no interference can occur (such as on the back screen of the double slit experiment, where the prior possibility of finding the electron in one position cannot impact the probability of finding it in another).
In MWI, when the friend measures the cat, he can only measure one possible outcome, which is a sign of decoherence. In one branch he measures alive cat, in the other dead cat. In each of these branches, Wigner then comes along and measures his friend's results. In alive branch, Wigner must measure the alive result; in the dead branch he must measure the dead result. So one wouldn't expect in either branch for Wigner to be able to detect interference patterns (a sign of coherence) or, to put it another way, one wouldn't expect any observer-dependence: in each branch, the state of the cat is an objective fact.
Quoting magritte
Yes, I think this is what I meant. The first is relativism, the second pluralism, and they are equivalent. As I said, I encountered this first in a discussion on moral relativism versus objectivity, including pluralism, and I understood how the latter isn't just the former insisting it's the latter.
Quoting Kenosha Kid
We did this little dance a while back
https://thephilosophyforum.com/discussion/comment/433376
and it never crossed my mind until you just mentioned them in moral terms.
NB:
• relativism – subject-variant paths / values
• pluralism – X out of N-possible subject-invariant paths / landscape of values
The basic principle of MWI is: whatever the math says is what's happening. So you follow the math, and there ain't no observer-dependence in there.
Quoting 180 Proof
Ah it was YOU!!! What are the odds. Aye, that. Reminds me, I said I'd dig out a post for Andrew.
Rovelli doesn't agree with the MWI, not that that means I agree with him or not. His idea is that we should let the science say what the metaphysics is, not let our metaphysics guide our science. I take him mean that people who adhere to MWI have the intuition that the world must be deterministic, but the world isn't so.
How'd you think about this?
Quoting Kenosha Kid
https://en.wikipedia.org/wiki/Many-worlds_interpretation
ETA: Just to nip another misunderstanding in the bud, don't assume that my discussion style matches the typical "random internet person"; I'm not out to "win" an argument... my goal here isn't to "prove MWI" to you, or "prove you wrong" (i.e., I'm not "proposing" anything... except that MWI is as I understand it, as far as I understand it). My goal is to simply understand what you're saying. My issue is that your description of MWI makes no sense to me, and doesn't match my understanding of it.
Sorry Pfhorrest, I missed this. Yes, but not the other way around: not every entanglement is an observation. Entanglement occurs when two bodies or systems communicate or share a history. This doesn't have to involve a measurement. Indeed, something like the EPR paradox is about an entangled, unmeasured spin pair.
Quoting Pfhorrest
Should be, yes: Wigner and his friend are exchanging information. But yes the experiment shows that Wigner still observes interference effects, which suggests he is not entangled with his friend. Which is tricky because every branch his friend is communicating with him in should be completely independent.
Quoting Pfhorrest
Yes, and maybe this is what Andrew had in mind too. It's worth remembering that from the friend's point of view, his branch is an independent universe and he is in contact with Wigner before branching has occured for Wigner. His universe should go something like:
| Wigner before update > | Friend measured alive > | Alive cat >
to
| Wigner updated > | Friend measured alive > | Alive cat >
This is reality in his branch, and it's completely independent of the existence of any other branch. But Wigner can assuredly tell his friend that Wigner has detected the other branch:
| Wigner detects superposition > | Friend measured alive > | Alive cat >
This is weird, no? There is definitely evidence in the friend's branch that another branch exists.
It's not as simple as reversing these terms. Science is already a 'defeasible (hypothetical / statistical) metaphysics of reality' which must be accounted for, in order to be self-consistent, by a more general, indefeasible (categorical / modal) metaphysics of the real. Even 'fundamental sciences' can no more ground themselves than Rovelli (I'm a huge fan, btw) can rewrite his own/the past; physics is a 'mapmaking map' but is not the territory itself or the encompassing horizon.
Yeah, I agree. I don't necessarily follow Rovelli, particularly with his view that there isn't a metaphysical substrate or existence absent relations. I think physical stuff exists independent of us.
But I also appreciate someone trying to make sense of QM as is, which makes for an interesting thought process when taken seriously, as opposed to Chopra-style woo.
It's also not clear to me that we actually can suspend metaphysical inclinations: even the most avowed anti-metaphysician has a metaphysics, sometimes a variety of positivism or some kind of view related with sense data.
Wigner and his friend don't become entangled because the friend is sending exactly the same piece of information to Wigner from both branches, i.e., that a definite result has been obtained. So no which-way information is being sent. It's only when the actual result is measured by Wigner, thus distinguishing the branches, that entanglement occurs.
In other words, that single piece of information that is identical in both branches factorizes out and is what is measured by Wigner, leaving all other degrees of freedom (such as the alive/dead result itself) fully isolated.
Quoting Kenosha Kid
Yes (in this case, it's information communicated from the friend to Wigner).
The point of this discussion is precisely the ramifications of the lab _not_ being an isolated experiment by virtue of the fact that Wigner and his friend are exchanging information. On which...
Quoting Andrew M
If this was just a story about the friend telling Wigner that the measurement has been done by, say, sending a photon, ignoring everything else, even that the measurement was a quantum one, would you say that this process of sending a photon from one system to another didn't entangle the two systems? Because that doesn't appear right. This is a three-body problem and will be correlated. The point here is that this entanglement is occurring after branching, and what's going on in each of these branches is independent.
I think perhaps the correct MWI answer is that this entanglement has already branched the wavefunction. It's not that entanglement hadn't occurred, it's that any entanglement pertinent to the the uncertainty in the signal has already decohered, making Wigner effectively separable from the lab across branches because the signal is the same in all of them.
In reality, that kind of separability isn't realistic. Maybe for a macroscopic Wigner and friend, but not for the quantum observers of the experiment.
Amateur. I've got 4.6k...overnight. Looking forward to another few k today from my spontaneous fanbase.
Quoting Kenosha Kid
No. Both of your examples are simple pluralism. And second, saying 'true' of what ontologically 'is' is always redundant verbiage in your realism.
In addition to pluralism, relativism whether ontological [dead] or logical [true] requires a second higher level functor which establishes a dependency relation. Observation y depends on the value of x. It's two-dimensional, not a simple is/is-not here and there.
What do you mean by encode?
Simply that everything physical that happens is encoded in the wavefunction. There is no additional physical mechanism such as probabilistic collapse. In that sense, MWI is taking the physics first.
Quoting magritte
As in:
Quoting Kenosha Kid
Note the quotes: the statement is "The cat is dead". The truth value of that depends on who you ask.
that what exists are actually instructions, i.e. 'manifest electron here'. :wink:
My understanding is that Bohr would never have agreed with any of this. Talk about the state of anything when it is not being observed is empty words. We're just continually trying to create a stake in 'what is really there' but it is simply imaginative conjecture. It does not exist, but also does not not exist.
Sorry about that! I'm only trying to make a point to Kenosha Kid there.
I agree with you about QM. Exists is only meaningful to a philosopher and is an empty word for QM theory. For example, electrons can't philosophically exist because of their lack of 'substance', being only a bundle of instantaneously measurable ephemeral properties.
Why? Do you really want to elevate yourself to a condition of existence? Universalize self-dependence? :brow: Let's talk about Mars.
I'm not going to argue the science, but both the alive and dead cat evolve from the pre-experiment cat continuously. No matter is added to make another cat, for instance. So I think you need a more complete answer before you tell anyone they're confused ;)
Quoting Wayfarer
We can easily and meaningfully talk about Mars onlookers or not.
My intepretation is that scientific realism is a methodological step, not a philosophical principle, and that enormous confusion ensues from not seeing that. Scientific realism begins by assuming the independent reality of the world. But what this stance doesn't allow for is precisely 'the role of the observer', and it was the discovery of quantum mechanics that threw this into relief. The universe doesn't 'exist anyway', regardless of whether humans are in it, because the meaning of 'to exist' relies on the implicit layers of judgement and ordering which the mind brings to the picture (which is where Kant is crucial.) This doesn't mean that the observer literally creates the Universe, but that the mind provides the framework within which any judgement about what exists or doesn't exist is made. So the usual assumption that the Universe exists sans any observer is actually a kind of projection - the idea of the serene early universe that pre-existed the advent of h. sapiens is still mind-dependent, in that crucial sense. This is why Kant stresses that he is both empirical realist and transcendental idealist. It's not an either-or proposition. It's also why I said that reality absent any observer does not exist, but also does not not exist.
Put aside some time to look at this lecture by Michel Bitbol on 'Bohr's complementarity and Kant's epistemology'.
I would. For Wigner, the photon is separable from the superposed lab. From page 3 of Brukner's paper (where the above photon is system M):
Quoting A No-Go Theorem for Observer-Independent Facts - Caslav Brukner
Interestingly, Deutsch was originally using the Wigner's friend thought experiment to distinguish an objective collapse version of the Copenhagen Interpretation from Many Worlds. He says,
Quoting Quantum Theory as a Universal Physical Theory - David Deutsch, 1985
Also of interest, at the end of section 8 (p37) Deutsch uses the term "merge":
I'm sorry, but I just find this really creepy. And I still would like to know what Deutsch would be obliged to admit if it were shown it could not be true. I mean, what's he frightened of?
[quote=The Many Worlds of Hugh Everett;https://www.scientificamerican.com/article/hugh-everett-biography/ ]"Once we have granted that any physical theory is essentially only a model for the world of experience,” Everett concluded in the unedited version of his dissertation, “we must renounce all hope of finding anything like the correct theory ... simply because the totality of experience is never accessible to us.”[/quote]
I don't know what his motives are beyond thinking that the Everett interpretation is correct. In the above paper, Deutsch described a method for experimentally distinguishing between the Everett interpretation and a particular version of the Copenhagen interpretation. So he's making his own position subject to falsification. His thought experiment has been the basis for the recent experiments discussed in this thread, and has lead to progress in the area of quantum foundations.
I note the posit of a 'real, objectively existing world'. Presumably this is not regarded as an axiom? It would seem a philosophical pre-supposition, at least.
I will try and read this paper, but it's difficult without real ability to read mathematical physics. I suppose that's part of the territory.
Realism is a philosophical presupposition. Within that, the terms "intrinsic realism" and "participatory realism" have been proposed (see Table 1 in Interpretations of quantum theory: A map of madness) which differentiates Many Worlds from Copenhagen.
I'm not a fan of the "objectively existing" qualifier (with it's Cartesian implications). It seems enough to say that the task of physics is to seek a systematic understanding of the world. And we bring our philosophical presuppositions to that task.
I noticed Deutsch’s reference to the ‘lacuna’ in some of the standard interpretations in his opening remarks. Philip Ball is no fan of many worlds. He notes:
[quote=Phillip Ball, Too Many Worlds; https://aeon.co/essays/is-the-many-worlds-hypothesis-just-a-fantasy] As DeWitt put it: ‘every quantum transition taking place on every star, in every galaxy, in every remote corner of the universe is splitting our local world on earth into myriads of copies’. Recall that this profusion is deemed necessary only because we don’t yet understand wavefunction collapse. It’s a way of avoiding the mathematical ungainliness of that lacuna.[/quote]
Which is just how I see it. Again, for what it’s worth.
Good quote.
The formalism above necessarily neglects the fact that Wigner and his friend are entangled anyway. Previously an argument for this was the classical limit. However if we're in the regime of seeing macroscopic bodies in superposition (if!), we're not in the classical limit anymore and that approximation doesn't obviously follow.
What we should see in MWI is each branch evolving independently as if it were the whole universe. As I mentioned above to Pfhorrest, this result, if consistent with scale, leads to odd contradictions in which the friend can indirectly observe his other branches.
Yes, it's just a thought experiment that shows the implications of QM at a macroscopic level.
Quoting Kenosha Kid
But this is the point at issue. In the thought experiment as described in Deutsch's paper (and assumed in Brukner's paper), Wigner and his friend are not entangled and Wigner demonstrates this with an interference experiment. See below.
Quoting Kenosha Kid
Each friend's branch evolves independently. But Wigner, per MWI (and unitary QM), can in principle undo the friend's measurement and apply a Hadamard to the spin state (i.e., H((|spin up> + |spin down>)/sqrt(2)) = |spin up>), resulting in a single branch again. The friend would have no memory of what measurement she made [1] but she would have the message saying that she had made a definite measurement. That is, the final state would be:
|Wigner>|friend>|spin up>|I've observed a definite outcome>
The above is what MWI (and unitary QM) predicts. Whereas on an objective collapse version of the CI, the spin state after applying the Hadamard would be |spin up> with 50% probability only. That is, the final state [2] would be:
|Wigner>|friend>(|spin up> +/- |spin down>)/sqrt(2)|I've observed a definite outcome>
Hence the two interpretations (theories, really) are experimentally distinguishable.
--
[1] This is similar to a photon emerging from the second beam splitter of an MZI - it has no encoded information about which interferometer path it travelled along.
[2] For anyone wanting to check the math, if Wigner were correlated with the spin up state, then applying a Hadamard to that state puts it into superposition: H(|spin up>) = (|spin up> + |spin down>)/sqrt(2). Alternatively, if Wigner were correlated with the spin down state, then H(|spin down>) = (|spin up> - |spin down>)/sqrt(2). Either way, there's only a 50% probability of Wigner subsequently measuring the spin up state.
As someone whose job it was to calculate exact solutions to the many-body wave equation for simple, symmetric systems, I can tell you that is not true. Communication between two subsystems like this does not permit the factorisation of those subsystems. Wigner would have been communicating with an already branched friend so would be entangled within-branch. This is what MWI would predict too.
If there was truly zero entanglement prior to and throughout the experiment until Wigner made his own measurement, then he ought to see interference effects as Deutsche originally intended. However by communicating with his friend, e.g. by exchange of photons or electrons, directly or indirectly, after his friend had branched, he would see no such interference effects. It just isn't possible to separate Wigner out of the wavefunction the way you think we can.
So, on your view, Deutsch's thought experiment fails?
For reference, Deutsch's thought experiment is in section 8 (p32) with the interference experiment (distinguishing MWI from objective collapse CI) described on pp35-36.
Yes. Then, in Deutsch's thought experiment, Wigner performs a unitary operation that undoes the friend's measurement while preserving that record. The preserved record is essentially a single qubit of information that was flipped from |0> to |1> by the friend in both branches. Deutsch says:
Quoting Quantum Theory as a Universal Physical Theory - David Deutsch, p36
So the friend themselves (the observer in the above quote) can determine that the qubit is set to |1> which demonstrates for her, as well as for Wigner, that she had made a definite measurement.