Oh, more pluralism, more diversity, yes, very clever. — Kenosha Kid

Another reason to stick with the MWI (as supported by Deutsch's work on quantum information/computing and generalization of "Wigner's Friend"). :wink:

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. — Kenosha Kid

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,

There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature. — Niels Bohr (as quoted by Aage Petersen)

More broadly, can it really be that reality is subjective? It looks objective enough, but that's the classical limit at work. — 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.

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? — 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.

More broadly, can it really be that reality is subjective? — Kenosha Kid

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.

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. — Wayfarer

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.

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). — Kenosha Kid

Many worlds is out — 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).

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. — Andrew M

That would be different to many worlds in itself. If you have to add a thing (merging criteria) that's a new theory.

Is not the above exactly what Many Worlds says happens? — 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.

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. — Pfhorrest

That's correct, hence:

even tell me you have made such a measurement — Kenosha Kid

rather than "tell me what measurement you made".

Also I think Copenhagen has always been epistemological despite talk of collapse. Per Bohr, — 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.

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. — Andrew M

Yeah me too but there's a lot of Cartesians here and I think they'd find it interesting.

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. — Wayfarer

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).

I know that, but it’s still worth spelling out. And besides, it was the main issue in the decades long debate between Einstein and Bohr, so it’s hardly a settled matter.

The question as to whether the universe is probabilistic (Einstein vs Bohr) isn't a settled matter. But the dream of some objective measuring framework is long dead.

Yes, well I'm still quite interested in the philosophical implications.

I was hoping so :up:

the dream of some objective measuring framework is long dead. — Kenosha Kid

Yes — Wayfarer

Then how come

Modern scientific method excludes or ‘brackets out’ the subject, so as to arrive at ‘the view from nowhere’ — Wayfarer

The whole of the modern scientific method doesn't sound very 'dead' to me.

Also, if...

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). — Kenosha Kid

...and you...

know that — 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?

But the dream of some objective measuring framework is long dead. — Kenosha Kid

Wait, like SR, the QM framework is objective – subject-invariant – but just not universal (or absolute), no?

So now I'm confused if the interpretation that I've supported under the name "Many Worlds" all these years actually was Many Worlds or not. That interpretation that I've called thus has been:

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?

Wait, like SR, the QM framework is objective – subject-invariant – but just not universal (or absolute), no? — 180 Proof

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

There's a part of your post in which you briefly stopped speaking of entanglement and spoke of observation. That's the part I referred to in my previous reply to you.

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.

:up: Interesting. Thanks.

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 ...

My preliminary read interprets "observer-dependent" as experimental set-up (subsystem)-dependent: different subsystems observe (measure) different aspects of the universe — 180 Proof

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.

Objective pluralism and not subjective relativism — 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

This is what I had in mind. It's from Wheeler's essay, 'Law without Law'.



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.

doesn't the moon continue to exist when nobody's looking at it? — Wayfarer

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.

Ah okay, I wasn't aware of this. Note though:

A phenomenon is not a phenomenon until it has been brought to a close by an irreversible act of amplification such as the blackening of a grain of silver bromide emulsion or the triggering of a photodetector.

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.

Minds are not required to bring things to a close: a molecule will do the job — Kenosha Kid

I think that is contestable. Registration, in that passage, is an 'act of measurement', not simply an interaction between any particles.

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). — 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 Nāgārjuna, the Buddhist philosopher of emptiness. Emptiness means that 'phenomena lack own-being'.

Nāgārjuna'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 Nāgārjuna 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”. — Carlos Rovelli

Which holes metaphysical realism under the waterline.

---------

the dream of some objective measuring framework is long dead.
— Kenosha Kid

Yes
— Wayfarer

Then how come

Modern scientific method excludes or ‘brackets out’ the subject, so as to arrive at ‘the view from nowhere’
— Wayfarer

The whole of the modern scientific method doesn't sound very 'dead' to me. — 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:

Traditionally, the discipline of Physics charts only the primary qualities of objects, events and processes i.e. their mathematical interrelations, where the relationship of their primary qualities to their secondary qualities (i.e. qualia) is ignored and undetermined. The reason why the secondary qualities are classically ignored by physics is as a consequence of traditional physics treating it's subject matter to be independent of any particular observer, which is itself due partly to convenience and simplification, and due partly as a consequence of the objective of physics to model the causal relationships that hold between action and consequence irrespective of the contextual nuances and discrepancies of any given observer.

Strictly speaking, the propositions of physics are senseless, like an unexecuted computer program, until as and when the propositions are used by an agent and thereby become grounded in the agent's perceptual apparatus in a bespoke fashion, at which point Locke's secondary qualities become temporarily welded to the physical concepts.

Classical physical concepts are therefore by design irreducible to mental concepts; something has been a central feature of physics rather than a bug, at least up until the discovery of special relativity and quantum mechanics, both of which show that even the Lockean primary qualities of objects are relative to perspective. — sime

No, many worlds is a universal branching. — Kenosha Kid

No. Many Worlds is a subject relative branching. It's simply part of the universal wavefunction.

When Wigner's friend measured the cat, the universal wavefunction would split then universally.

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:

Alternative 5: To assume the universal validity of the quantum description, by the complete abandonment of Process 1. The general validity of pure wave mechanics, without any statistical assertions, is assumed for all physical systems, including observers and measuring apparata. Observation processes are to be described completely by the state function of the composite system which includes the observer and his object-system, and which at all times obeys the wave equation (Process 2).

(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.

It was the classically modern scientific framework, I was referring to — Wayfarer

Why would that be relevant, given that...

That has indeed begun to change — Wayfarer

...?

But even with regards to...

the classically modern scientific framework — 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...

excludes or ‘brackets out’ the subject, so as to arrive at ‘the view from nowhere’, — 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?

No. Many Worlds is a subject relative branching. It's simply part of the universal wavefunction. — InPitzotl

That's a contradiction right there.

Observation processes are to be described completely by the state function of the composite system which includes the observer and his object-system, and which at all times obeys the wave equation (Process 2).
(underline mine; italics in the paper) ...just to show I'm not making this up — 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.

I think that is contestable. Registration, in that passage, is an 'act of measurement', not simply an interaction between any particles. — Wayfarer

It's there in the quote you provided, unless you're a panpsychist and believe that silver bromide has mind.

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. — 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.

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? — 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. — Andrew M

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

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. — 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 not making it up, but it doesn't say what you purport it to say, — Kenosha Kid

You're severely confused here. You're certainly not addressing what I purport. Keep the terms to ensure you're not conflating things.

In MWI, when system A is entangled with system B system S and system B system S branches, system A also branches. — 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).

Observer-dependence tells us something different, that branching may have occurred for B and not for A A and not for B, even though A and B are entangled A and S are entangled. — 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.

Schrodinger need not be entangled with the substance or the cat; Schrodinger sees W1+W2. — InPitzotl

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.