Quantum theory - Nature Paper 18 Sept

[PeterDonis]

I am not sure how you get from knowing "the exact wave function of the entire system at one instant of time" to "you can predict the wave function at all future times exactly" after some series of measurements.

From the standpoint of the MWI, a "measurement" is just an interaction between different subsystems of the total system (which is the whole universe if we take things to their logical conclusion) that entangle the subsystems. These interactions are realized by unitary operators (basically $\exp i H t$, where $H$ is the full Hamiltonian including interaction terms), so the time evolution they induce is deterministic and reversible.

 

[DrChinese]

From the standpoint of the MWI, a "measurement" is just an interaction between different subsystems of the total system (which is the whole universe if we take things to their logical conclusion) that entangle the subsystems. These interactions are realized by unitary operators (basically $\exp i H t$, where $H$ is the full Hamiltonian including interaction terms), so the time evolution they induce is deterministic and reversible.

I can see now I have some studying to do. There are key elements of MWI I am clearly ignorant of.

 

[DarMM]

I can see now I have some studying to do. There are key elements of MWI I am clearly ignorant of.

I recommend Travis Norsen's chapter on it in his book "Foundations of Quantum Mechanics: An Exploration of the Physical Meaning of Quantum Theory"

 

[akvadrako]

I think this can be resolved much easier, directed at @akvadrako .

Firstly, do you accept Many-Worlds lacks OTS in its fundamental ontology, i.e. as a whole, not within a branch?

OTS is not part of it's ontology of course but it's ontology isn't operational, so I don't understand how it makes sense.

Just to clarify; when I say I'm using Everett's version of the theory, I don't mean his definition of world/branch, but just the objective part of the theory. I am not sure an exact definition is even possible without understanding the dynamics of an agent (or life), though the one used in the TSVF paper seems vaguely reasonable.

 

[akvadrako]

EDIT: Ignore this, I think I have a better approach below.

Let's say take the case with $x \in \mathbb{Z}_2$ and $y$ similar, classical probabilities for selection in $x$ preparation being $\frac{1}{3}, \frac{1}{2}$ in both cases. Easiest case probably being $x$ and $y$ involving preparation at different angles displaced by $\frac{\pi}{4}$.

I'll ignore the rest of the post until I see your other point, though I have one avenue to investigate still. I also want to mention that I don't know how to do my own analysis; I mostly just read papers and try to put the pieces of the puzzle together from what's already shown. For now, I find it higher priority to maintain a high-level overview of the literature. Maybe I'm slow but even on this rather specific topic that takes a fair chunk of time.

I might start a thread on Aravind-Mermin in Many-Worlds, as it seems confusing to me how each of outcomes at Alice and Bob's locations "know" that they are to pair up with worlds of the other observer in specific combinations without some nonlocality.

This would be interesting, though if someone did discover any nonlocal (hidden) effects in MWI, I would be very surprised.

 

[PeterDonis]

it seems confusing to me how each of outcomes at Alice and Bob's locations "know" that they are to pair up with worlds of the other observer in specific combinations without some nonlocality.

They don't have to "know" anything. All of the measurement interactions are local; there is nothing to "pair up". In terms of Bell's Theorem, the MWI violates one of the assumptions that underlies the theorem, namely, that measurements have single results. So the theorem does not apply to the MWI.

 

[DarMM]

They don't have to "know" anything. All of the measurement interactions are local; there is nothing to "pair up". In terms of Bell's Theorem, the MWI violates one of the assumptions that underlies the theorem, namely, that measurements have single results. So the theorem does not apply to the MWI.

I'm not sure about this, I think even in the field it is considered an open problem as to whether Many-Worlds is nonlocal in some sense, see Travis Norsen's book I mentioned above.

I know MWI violates one of the ontological framework axioms (and so escapes most results in quantum foundations) however that doesn't mean it automatically has the properties the no-go results typically prevent, e.g. just because you aren't covered by Bell's theorem doesn't necessarily mean you are local.

For example in the Aravind-Mermin case Alice's world splits locally into one of sixteen worlds, then Bob's world splits locally into sixteen worlds. This naively gives 256 possible worlds, however if Alice and Bob fly toward each other only worlds with common values for the shared vertex in the Aravind-Mermin Pentagram can meet/interact, i.e. there is only 128 worlds.

How during the initial local splittings do same vertex splittings emerge into the same world?

In some sense it wouldn't be too surprising if Many-Worlds was nonlocal as spacetime is only an illusion of a sort that exists in the quasi-classical branches that appears once an environment has emerged that a position basis can decohere against. The wavefunction naturally lives in a configuration space.

 

[DarMM]

I also want to mention that I don't know how to do my own analysis

Yeah the reason I switched is that I thought the original version would be way too long and detailed, in fact you'd probably have a paper at the end (i.e. a correction to Pusey and Leifer!)

This would be interesting, though if someone did discover any nonlocal (hidden) effects in MWI, I would be very surprised.

See the book by Travis Norsen I mentioned above to @PeterDonis . It's an open problem as to whether Many-Worlds is strictly local.

 

[DarMM]

OTS is not part of it's ontology of course but it's ontology isn't operational, so I don't understand how it makes sense.

That doesn't matter too much, remember the Pusey-Leifer theorem is about seeing if a theory has an ontological symmetry that directly reflects OTS, this is defined as Ontological Time Symmetry, see p.8

The only thing I'm asking is if you agree Many-Worlds lacks such an ontological symmetry. The definition makes sense to me when applied to Many-Worlds and I would say it violates it at a global level.

Leifer's reply to Tim Maudlin here might help:
https://arxiv.org/abs/1708.04364

The initial historical explanation part.

 

[akvadrako]

See the book by Travis Norsen I mentioned above to @PeterDonis . It's an open problem as to whether Many-Worlds is strictly local.

I will take a look. In general I agree it's an open question; the only proof of locality I'm aware of is Vindication of Quantum Locality (Deutsch, 2011), but I know it's not totally accepted.

 

[PeterDonis]

the Aravind-Mermin Pentagram

Is there a paper online that describes this scenario?

 

[akvadrako]

That doesn't matter too much, remember the Pusey-Leifer theorem is about seeing if a theory has an ontological symmetry that directly reflects OTS, this is defined as Ontological Time Symmetry, see p.8

The only thing I'm asking is if you agree Many-Worlds lacks such an ontological symmetry. The definition makes sense to me when applied to Many-Worlds and I would say it violates it at a global level.

First, to put it in my own words: for a theory to have Ontological TS (OnTS) it must be possible to swap the input/output while transforming the path, $(x,b,\lambda) \Leftrightarrow ( b, x, f(\lambda) )$, for every $(x, b, \lambda)$. This is based off the definition on page 8. The spirit seems to be that God “cannot tell the difference between a video played forwards and played in reverse”.

At least for finite systems, many worlds seems to have OnTS. The first reason is that due to being linear and the limited number of states available, every state $x$ which evolves to state $b$ will eventually return to $x$. Although that satisfies their definition of OnTS, it may not exactly fit the spirit because AFAIK the path from $x \rightarrow b$ may not be a mirror image of $b \rightarrow x$. However, there must be a point when it reaches a maximum # of branches and from then on they are more likely to merge, roughly mirroring the early forward evolution.

The second reason is that you can run your universal simulation on a quantum computer and invert all the gates. I’m not sure this is allowed, though it’s analogous to setting up an experiment backwards.

For infinite systems, it’s less clear and I need to guess a bit here. Wouldn’t it depend on the Hamiltonian? If the standard model is used, would the CPT symmetry suffice to satisfy OnTS, at least in spirit? If seems like for every $x \rightarrow b$, there is a mirror transform, $CP(b) \rightarrow CP(x)$, that’s indistinguishable in behavior.

If we are talking about our universe, then a big crunch situation would be similar to the finite dimensional version. Finally, if expansion continues forever then it seems like many worlds does violate OnTS, because any state has the tendency to spread out into new dimensions, even under a CPT reversal.

I'm not trying to make this overly complicated, but P&L's paper is full of aspects that don't apply in a straightforward manner to many worlds and they only devote a short paragraph to the issue. I think if they actually want to show that many worlds requires fine tuning, it would be a lot cleaner to just address that directly.

 

[DarMM]

Very good post, I'll need some time to think about it. These cosmological aspects of MWI are very interesting.

Just so you know where I'm going with this, Many Worlds in order to derive the existence of the classical worlds essentially needs to assume an environment/not-environment split with the environment having specific properties (see Zurek's work on Quantum Darwinism via einselection https://arxiv.org/abs/quant-ph/0105127).

Either this just happened to be in the initial conditions (fine-tuned) or it emerged (thermalisation). Ultimately Pusey and Leifer would be saying that restoring OTS in the branches is an additional thing this thermalisation needs to do.

However I see that this really needs a proper analysis.

Probably it's better to stick to the emergence of the "environment" early on in the universe's history as an example of where Many-Worlds would have different predictions.

 

[mfb]

Measurements happen inside the universe (deterministically, dependent upon preparations, parameter settings, and intentions of the experimenters), in a way not precisely specified by MWI. Hence its vagueness...

What is vague about "unitary evolution no matter what happens"?

 

[A. Neumaier]

What is vague about "unitary evolution no matter what happens"?

The unitary evolution itself is not vague. What is vague is the notion of measurement as event determined by the wave function - the only thing that is claimed to exist objectvely.

A criterion is missing that tells when and how the state of the universe indicates

• that, at any given place in the universe, a measurement is made,
• when the measurement result is deemed to be known, and
• how a particular observer's results are related to particular branches of the wave function at the appropriate moments.

Lacking this, all talk about measurement, observers, and results is just playing with buzzwords connected by vague, informative sounding statements.

 

[vanhees71]

Realistic is more clear in philosophy, and it means that there is some objective state. Realists believe there is something like that; anti-realists think everything is subjective. The MWI is realistic, because the wave function is real. In Bohmian QM, it's that plus a world-particle, to pick out the world you are in.

I don't think the word "realistic" means anything consistent to physicists.

Also, MWI is deterministic - it has only unitary evolution so that's obvious.

Well, I don't understand, what's clear with this definition since many philosophers (and maybe also a minority of physicists) consider QM as "non-realistic". On the other hand, it's clear that QM has a very clear definition of an objective state. It's even more explicit in defining what a state is than classical mechanics, where it is supposed to be implicitly clear from the formulation of the theory (the explicit statement on a fundamental level of classical mechanics, no matter whether Newtonian or relativistic, is that a state is represented by a point in phase space). In QM a state is represented by the statistical operator and operationally as an equivalence class of preparation procedures. That's an objective notion of state since a preparation procedure is clearly defined, and in my opinion it's utmost realistic, since this definition is in terms of real-world actions on the described system (e.g., at the LHC there's a preparation of protons with a pretty well determined momentum). The only difference between classical and quantum mechanics then is that the notion of the state in the latter is entirely probabilistic since a complete state determination (formally realized by the determination of the values of a complete set of compatible observables) doesn't imply the determination of the values of all possible observables on the system. That's also very "realistic" since this reflects our experience with testing QT for nearly 100 years with an amazing accuracy!

So, if you define, "realism" in this physicists's way, of course QT is realistic, even in the minimal statistical interpretation without additional esoterics a la MWI, where for me it is not clarified why we observe a unique outcome when measuring an undetermined observable; this is shared by all indeterministic interpretions; BM is different since it's a deterministic interpretation, where the probabilities are subjective, i.e., due to our incomplete knowledge of the particle's initial position as in classical statistical mechanics.

 

[stevendaryl]

In QM a state is represented by the statistical operator and operationally as an equivalence class of preparation procedures.

If you're using quantum mechanics to understand, say, the physics of cells in biology, or the physics of stars in astronomy, you're dealing with systems that were not "prepared" by anyone or anything. So do those systems not have a state?

 

[Lord Jestocost]

Well, I don't understand, what's clear with this definition since many philosophers (and maybe also a minority of physicists) consider QM as "non-realistic".

Regarding realists and anti-realists, Massimo Pigliucci describes the issue as follows (http://rationallyspeaking.blogspot.com/2012/08/surprise-naturalistic-metaphysics.html):

To put it very briefly, a realist is someone who thinks that scientific theories aim at describing the world as it is (of course, within the limits of human epistemic access to reality), while an anti-realist is someone who takes scientific theories to aim at empirical adequacy, not truth. So, for instance, for a realist there truly are electrons out there, while for an anti-realist “electrons” are a convenient theoretical construct to make sense of certain kinds of data from fundamental physics, but the term need not refer to actual “particles.” It goes without saying that most scientists are realists, but not all. Interestingly, some physicists working on quantum mechanics belong to what is informally known as the “shut up and calculate” school, which eschews “interpretations” of quantum mechanics in favor of a pragmatic deployment of the theory to solve computational problems.

 

[vanhees71]

If you're using quantum mechanics to understand, say, the physics of cells in biology, or the physics of stars in astronomy, you're dealing with systems that were not "prepared" by anyone or anything. So do those systems not have a state?

Then you have to figure out the state by measurements (on an ensemble of course ;-)).

 

[vanhees71]

Regarding realists and anti-realists, Massimo Pigliucci describes the issue as follows (http://rationallyspeaking.blogspot.com/2012/08/surprise-naturalistic-metaphysics.html):

To put it very briefly, a realist is someone who thinks that scientific theories aim at describing the world as it is (of course, within the limits of human epistemic access to reality), while an anti-realist is someone who takes scientific theories to aim at empirical adequacy, not truth. So, for instance, for a realist there truly are electrons out there, while for an anti-realist “electrons” are a convenient theoretical construct to make sense of certain kinds of data from fundamental physics, but the term need not refer to actual “particles.” It goes without saying that most scientists are realists, but not all. Interestingly, some physicists working on quantum mechanics belong to what is informally known as the “shut up and calculate” school, which eschews “interpretations” of quantum mechanics in favor of a pragmatic deployment of the theory to solve computational problems.

What am I then? Of course I believe in electrons as one thing existing in the "world as it is" and at the same time believe that physical theories are there to describe objectively observable facts about this "world as it is". That's the only criterion distinguishing scientific theories (scientific narratives if you wish) from fairy tales (including philosophical speculations of all kind). Also in a sense I think the right attitude is indeed the "shut up and calculate" attitude, but this has to taken with a grain of salt since of course you need also some heuristic intuition to be creative in finding new models to describe things, but these creations are only science if they are objectively testable by experiment and then either become a valid description of "the world as it is" or are put into the garbage can of failed trials to understand more about this "world as it is".

 

[stevendaryl]

Then you have to figure out the state by measurements (on an ensemble of course ;-)).

But the issue is: does it have a state before you measure it, or not? If not, then your view of "state" is not realistic, but is epistemological.

 

[akvadrako]

Well, I don't understand, what's clear with this definition since many philosophers (and maybe also a minority of physicists) consider QM as "non-realistic".

Do you think that:

1. There is an objective quantum state for the whole universe? (one we don't know, of course)
2. There is no non-unitary evolution (collapse) ?

That's the entirety of MWI. Those who say QM is non-realistic disagree with the first part.

Edit: If you consider the point of a scientific theory to make predictions for experiments, then MWI is not complete; that's A. Neumaier's objection. But the situation is better than string theory because we can use some weak additional assumptions to show the predictions are equal to standard QM, at least for practical experiments.

 

[A. Neumaier]

That's the entirety of MWI.

No.

All interpretative stuff - which is what makes up the MWI (I=interpretation!) - is missing in these two statements. Your two statements say nothing at all about how the state relates to reality in general and to measurement and probabilities in particular.

Though it may well be the only consensus among all variants of MWI.

 

[akvadrako]

No.

All interpretative stuff - which is what makes up the MWI (I=interpretation!) - is missing in these two statements. Your two statements say nothing at all about how the state relates to reality in general and to measurement and probabilities in particular.

Though it may well be the only consensus among all variants of MWI.

Then what would you call the theory (scientific or just mathematical) described by my two points? Would the term "unitary QM" be more clear?

 

[A. Neumaier]

Then what would you call the theory (scientific or just mathematical) described by my two points? Would the term "unitary QM" be more clear?

Yes, that's observer-free, unitary quantum mechanics.

By the way, it is the background upon which my (single world) thermal interpretation of quantum mechanics operates.

 

[vanhees71]

Do you think that:

1. There is an objective quantum state for the whole universe? (one we don't know, of course)
2. There is no non-unitary evolution (collapse) ?

That's the entirety of MWI. Those who say QM is non-realistic disagree with the first part.

Edit: If you consider the point of a scientific theory to make predictions for experiments, then MWI is not complete; that's A. Neumaier's objection. But the situation is better than string theory because we can use some weak additional assumptions to show the predictions are equal to standard QM, at least for practical experiments.

Ad 1. The notion "quantum state for the whole universe" is a non-physical fiction since there's no way to observe the "whole universe", not even in principle since only a tiny part of the whole universe is principally observable, i.e., within our horizon according to the standard model of cosmology. This holds for any theory, not only QT.

Ad 2. For closed systems (!) there's no non-unitary evolution. As soon as one measures something, the observed system isn't closed anymore, because it's interacting with the measurement device (and usually additionally to "the environment", i.e., anything else except the observed system and the measurement apparatus).

 

[stevendaryl]

Ad 1. The notion "quantum state for the whole universe" is a non-physical fiction since there's no way to observe the "whole universe"

That view is what I consider anti-realist. To believe that something exists only if it is possible to observe it is almost the opposite of realism.

 

[Stephen Tashi]

Do you think that:

In the MWI, what is meant by "you"? , or "I", or "me"?

As I understand MWI, "I" am something real at an instant in time, but in an instant later, that "I" has branched off into descendants of that "I" who are distinct things.

In fact, from the viewpoint of an instantaneous "I", what is the point of using a wave function to limit the possible branches that can "really" happen? Why not say that all imaginable futures happen? (I suppose that would be the many-many-worlds interpretation.)

The notion that a predictive theory has utility depends on the fact (or illusion) that "I" have a persistence in time and will experience the consequences of a prediction.

 

[A. Neumaier]

For closed systems (!) there's no non-unitary evolution.

But the only closed system is the universe, since any smaller system necessarily interacts with its environment. Thus small closed systems are a ''non-physical fiction'', to use your words.

there's no way to observe the "whole universe",

No. Whenever we observe part of the universe, we observe one of the properties of the whole universe.

This is completely analogous to observing the value of an observable of a tiny physical system, which gives us only some property of the tiny system.

Since you are willing to assign observability and hence a physical state to the tiny system because you can measure some of its properties, it would only be consistent if you also grant observability and hence a physical state to the whole universe.

 

[vanhees71]

That view is what I consider anti-realist. To believe that something exists only if it is possible to observe it is almost the opposite of realism.

Then I'm an anti-realist ;-).

 

[vanhees71]

But the only closed system is the universe, since any smaller system necessarily interacts with its environment. Thus small closed systems are a ''non-physical fiction'', to use your words.No. Whenever we observe part of the universe, we observe one of the properties of the whole universe.

This is completely analogous to observing the value of an observable of a tiny physical system, which gives us only some property of the tiny system.

Since you are willing to assign observability and hence a physical state to the tiny system because you can measure some of its properties, it would only be consistent if you also grant observability and hence a physical state to the whole universe.

We can NOT observe the properties of the whole universe, at least not if GR and the cosmological standard model are not completely wrong.

Of course you are right in saying that strictly speaking there are no exactly closed systems. However there are close enough approximations. If this weren't the case it's hardly conceivable that physics in its present form could ever work. E.g., ESA or NASA can fly to a comet with sufficient accuracy pre-calculating its about 10-year journey through the solar system, using all kinds of tricks like swing-by manoevres to reach that goal. That's only possible, because the solar system is at the accuracy sufficient to fulfill this non-trivial task with sufficient accuracy "closed", i.e., all there is to be taken into account to plan and successfully conduct this space travel of the probe.

 

[A. Neumaier]

We can NOT observe the properties of the whole universe,

We know certain observable properties of the whole universe, for example its approximate age, the approximate density with which its galaxies are distributed (at least sufficiently close to ours) or that it contains a solar system with a planet called Earth on which physicists perform measurements. Or would you claim that neither is a property of the whole universe?

But then would you claim that observing the age of a person, the color of a person's hair, or the genetic composition of one of the hairs is not observing something about the whole person?

strictly speaking there are no exactly closed systems. However there are close enough approximations. If this weren't the case it's hardly conceivable that physics in its present form could ever work. E.g., ESA or NASA can fly to a comet with sufficient accuracy pre-calculating its about 10-year journey through the solar system, using all kinds of tricks like swing-by manoevres to reach that goal. That's only possible, because the solar system is at the accuracy sufficient to fulfill this non-trivial task with sufficient accuracy "closed", i.e., all there is to be taken into account to plan and successfully conduct this space travel of the probe.

But we also observe the spacecraft during its flight, proving that a system that is effectively (but not truly) closed is still observable. But then you cannot argue the following:

As soon as one measures something, the observed system isn't closed anymore, because it's interacting with the measurement device

Moreover, realistic optical quantum systems are always lossy, i.e., have a nonunitary evolution, even without any measurement. This even holds for your favorable example of realistic systems, namely bunches of particles in an accelerator. Great care is needed to ensure that the losses there are small, but even then they are not negligible. And quantum systems in the kinetic (Kadanoff-Baym) or hydrodynamic (1PI) approximation used for most detailed calculations are dissipative, too, due to the collision terms. The underlying closed system is in the latter case a system extending to spatial infinity, i.e., (a local approximatin of) the whole universe!

 

[DarMM]

Well, I don't understand, what's clear with this definition since many philosophers (and maybe also a minority of physicists) consider QM as "non-realistic". On the other hand, it's clear that QM has a very clear definition of an objective state. It's even more explicit in defining what a state is than classical mechanics, where it is supposed to be implicitly clear from the formulation of the theory (the explicit statement on a fundamental level of classical mechanics, no matter whether Newtonian or relativistic, is that a state is represented by a point in phase space). In QM a state is represented by the statistical operator and operationally as an equivalence class of preparation procedures. That's an objective notion of state since a preparation procedure is clearly defined, and in my opinion it's utmost realistic, since this definition is in terms of real-world actions on the described system (e.g., at the LHC there's a preparation of protons with a pretty well determined momentum).

However this isn't what many of us grow up thinking of and it isn't how one thinks of things in classical mechanics. Thinking of a quantum mechanical object as literally being "those measurement statistics of observables on the system given an equivalence class of methods of preparation" is very far from how people think of say a tree. So far that it has earned the name "AntiRealist", because it is entirely about how it reacts to my devices not a "narrative" about what it is like in and of itself like one has in Classical Mechanics.

Whether it should be called AntiRealist is debatable (probably not as you are not saying it isn't real or something), but it's definitely unlike the normal conception of objects. Participatory Realist is the more modern term in Quantum Foundations, as you basically consider the quantum object only in terms of its participation in interactions with our macroscopic realm and consider discussions outside that, i.e. even the mere idea of what the universe is when not observable, as meaningless.

@stevendaryl points out the right sentence here:

The notion "quantum state for the whole universe" is a non-physical fiction since there's no way to observe the "whole universe"

This isn't the case in classical mechanics, e.g. there are things the theory says are real that I can't measure, e.g. the entire velocity profile of the Triangulum galaxy down to the centimeter level. Things are posited to exist even if observation on them isn't possible. So for example, "the number of black holes in the universe" is sensible, even if I can never know it.

Several interpretations of QM would say the same (e.g. Bohmian Mechanics, MWI, Type I $\psi$-epistemic interpretations), the universe has a state, it's just I can't learn it given physical constraints.

However to say it has no state because you can't observe it eliminates QM from discussing things as they are, independent of measurement. It becomes about the statistics of observations alone and not about the reality of what is going on with these systems, hence the historical name for it.

 

[vanhees71]

We know certain observable properties of the whole universe, for example its approximate age, the approximate density with which its galaxies are distributed (at least sufficiently close to ours) or that it contains a solar system with a planet called Earth on which physicists perform measurements. Or would you claim that neither is a property of the whole universe?

But then would you claim that observing the age of a person, the color of a person's hair, or the genetic composition of one of the hairs is not observing something about the whole person?But we also observe the spacecraft during its flight, proving that a system that is effectively (but not truly) closed is still observable. But then you cannot argue the following:

Moreover, realistic optical quantum systems are always lossy, i.e., have a nonunitary evolution, even without any measurement. This even holds for your favorable example of realistic systems, namely bunches of particles in an accelerator. Great care is needed to ensure that the losses there are small, but even then they are not negligible. And quantum systems in the kinetic (Kadanoff-Baym) or hydrodynamic (1PI) approximation used for most detailed calculations are dissipative, too, due to the collision terms. The underlying closed system is in the latter case a system extending to spatial infinity, i.e., (a local approximatin of) the whole universe!

Well, we extrapolate from our pretty local observations about the universe to the whole universe by assuming the cosmological principle. So far this works pretty well, but strictly speaking, we can't ever experimentally really test it.

Of course you are right in saying that all ”closed systems” are idealizations.

 

[akvadrako]

In the MWI, what is meant by "you"? , or "I", or "me"?

In modern Copenhagen and QBism, "I" is taken for granted and the hard part is figuring out what objective universe is compatible with multiple interacting "I"s.

In MWI, the objective universe is taken for granted and we try to figure out what "I" must be to approximate our experience. That's an important part of the program. A few ideas that try to derive standard QM at the subjective level are:

1. assuming "I" is a kind of rational agent and applying decision theory (Deutsch-Wallce)
2. assuming "I" is a roughly a point on the wavefunction (dBB / many interacting worlds)
3. assuming "I" is part of a stable classical history (entangled histories)
4. ...