According to the Many Worlds Interpretation of quantum mechanics, the entire universe is in a massive superposition for all quantum states of every particle. — Marchesk

Also superposition of which particles exist in the first place.

A "world" is when a huge ensemble of entangled particles contains observers for whom the universe appears to be classical, at least until they devise experiments showing coherence.[/quote]No, a world is not a relation with an observer. Not sure where you get this. If you like, you can assign a world in relation to an event-state, but calling the system an observer seems to suggest a very different interpretation.

A potential issue arises here. What of all the entanglements that don't support observers?

Observers as such play no role. Think systems in a state, such as a classic rock at time T. Anything that rock has measured (a subset of what's in its past light cone) is part of the entangled state of that system.

Which means observers are fundamental for saying what counts as a world.

If you want to define 'world' that way, sure, but it's just a language definition then. The physics cares not if it is observed by say something you'd qualify with the word 'conscious', which seems to be what you're hinting as being an observer.

The universal wave equation makes no such distinctions. In fact, "observers" and "worlds" are classical concepts.

Yes.

Yes. — noAxioms

Problem is you have to square this with actual observations, which have classical results when a measurement is performed.

Observers as such play no role. Think systems in a state, such as a classic rock at time T. Anything that rock has measured (a subset of what's in its past light cone) is part of the entangled state of that system. — noAxioms

I'll refer you back to what Bohr had to say regarding experiments. Experiments have to be described in terms of the language of performing the experiment, not the mathematical formalism used to model what happens during the experiment. Rocks didn't come up with the Schrodinger equation or the Born rule. Physicists did after observing or learning about experimental results.

No, a world is not a relation with an observer. Not sure where you get this. If you like, you can assign a world in relation to an event-state, but calling the system an observer seems to suggest a very different interpretation. — noAxioms

If there's no observation, there's no world, since as we both agree, a world is a system that appears to be classical. Without an observer, you just have superpositions. Decoherence only matters in this context for explaining why observers don't notice the superpositions.

Problem is you have to square this with actual observations, which have classical results when a measurement is performed. — Marchesk

I don't see how classical observations in any way would have difficulty 'squaring' with that to which I answered 'Yes'.

I'll refer you back to what Bohr had to say regarding experiments. Experiments have to be described in terms of the language of performing the experiment, not the mathematical formalism used to model what happens during the experiment. Rocks didn't come up with the Schrodinger equation or the Born rule. Physicists did after observing or learning about experimental results.

Just so, but I'm not claiming rocks are the source of quantum theory. They only obey it, acting as a classic system as much as any human-body system (dead, alive, asleep, whatever), which is after all still just a classical physical system differing from the rock only in arrangement of material.

If there's no observation, there's no world, since as we both agree, a world is a system that appears to be classical.

I think you're going to need to define your use of the word 'observer' here, because I don't think we both agree with this given the common definition. I can think of only one obscure interpretation of quantum physics (Wigner) in which a living thing plays a special role, and even Wigner abandoned it after some time.
Yes, a classical rock takes measurements. If that makes it an observer, then fine. It doesn't need to know about Schrodinger's equation in order to measure a classical world. If you don't count that as an observation, then I completely disagree with your statement above. Use the word 'measurement' and not 'observation', or pick some other word that includes whatever the rock does.

As for the definition of 'world', that is a term added later on by DeWitt I think. Keep the following in mind:

What Everett does NOT postulate:
"At certain magic instances, the the world undergoes some sort of metaphysical “split” into two branches that subsequently never interact" — Tegmark

Everett doesn't control how the interpretation develops after him. Sean Carrol, a current proponent of MWI, talks of universes splitting. There's a Universe Splitter app: https://cheapuniverses.com/universesplitter/

Yes, a classical rock takes measurements. If that makes it an observer, then fine. It doesn't need to know about Schrodinger's equation in order to measure a classical world. If you don't count that as an observation, then I completely disagree with your statement above. — noAxioms

There aren't classical rocks or observations in MWI. And yet we make a classical observation, which some call the wave function collapse in other interpretations, every time a measurement is made. You can say the detector or a rock also makes the same observation. But the universal wave function doesn't make such a distinction. Every quantum state is still in superposition.

The point about human observers is we're the ones interpreting the mathematical formalism as meaning reality is this or that. Some physicists, mathematicians and philosophers say the wave function describes the universe. If it does, then the classical appearance of our world needs to be derivable from that equation.

Sean Carrol, a current proponent of MWI, talks of universes splitting. — Marchesk

I talk of universes splitting. It's part of the language of the subject.

There aren't classical rocks or observations in MWI. — Marchesk

Don't know what you mean by this. Certainly not that empirical evidence of rocks constitute a falsification of MWI. A rock is a system and a system is part of MWI. A rock, in a state, can be described by a wave function. It very probably is not a closed system.

Some physicists, mathematicians and philosophers say the wave function describes the universe. If it does, then the classical appearance of our world needs to be derivable from that equation. — Marchesk

Our classical appearance needs to be part of a valid solution to the universal wave function, and nothing says it is not.

Our classical appearance needs to be part of a valid solution to the universal wave function, and nothing says it is not. — noAxioms

Sabine Hossenfelder says it's not:

A rock, in a state, can be described by a wave function. It very probably is not a closed system. — noAxioms

A rock? Is it possible to do this? Over a short time interval my casual guess of 99.999% probability of its state is what a "wave function" might provide.

As an aside, I was listening to Sean Carol being interviewed, and he made it sound like the very low probability events didn't happen, at least not over the time period since the Big Bang (not nearly enough time had passed). However, they should be happening in the sense of being expressed as superpositions by the universal wave function in MWI.

So there's some states where the particles of the rock are located in other parts of the universe, under the understanding that a particle's position ranges over the entire universe, with most of the positions within the place we'd expect to measure them. But there still would be a few spread out everywhere else.

There should even be some human-like observers seeing a rock teleport some distance, or just vanish into being spread out all over the place, and all sorts of scenarios in between, even if it's a vanishingly small subset of observers.

:cool:

As an old math person my suspicion is that "superposition" and "collapse of wave function" is nothing more than experimenting to discover which of multiple solutions of the partial differential equations describing phenomena actually apply in a particular instance. Multi worlds I consider science fantasy.

So what do you make of David Deutsch's arguments in favor of the MWI?

(previously post here)

He has a very interesting idea on how to put MWI and wave-function collapse interpretations to the test. Assuming we can build a conscious AGI quantum computer.

The rest of what he says sounds similar to Sean Carrol's arguments for thinking MWI is likely correct. That it explains the interference patterns seen in experiments when a measurement isn't made, that there's no clear dividing line between the classical and the quantum, and entanglement means all the particles making up classical stuff should be quantum. And that any other interpretation would have be at least as complex as MWI, and probably more so.

That being said, my understanding is that the probabilities we use to calculate the likelihood of what to expect when a measurement is made still needs to be derived within the Schrodinger equation in a self-consistent manner without adding it in post hoc, since the wave function is supposed to describe the universe we live in, if MWI is true. So deriving the Born rule within MWI is an ongoing project.

Think systems in a state, such as a classic rock at time T. — noAxioms

By the principles of classic rock I would say there are many worlds for sure. Let time T be the time of John Lennon's "Imagine". Clearly there were many worlds at this time because the world "as one" is purely imaginary.

Sean Carrol, a current proponent of MWI, talks of universes splitting. — Marchesk

Not sure exactly what he suggests or how he words it, but there seems to be problems with two different universes (one with each measurement) existing. If there's all these universes/worlds and they exist, the more probable ones either have to 'exist more' than the lesser ones, or maybe there's just more of them. What does it even mean for one thing to exist harder than another?

Sabine Hossenfelder says it's not — Marchesk

Hossenfelder indeed seems to find issues the interpretation. This seems to be part of a series taking down each of the interpretations in turn, with a similar argument. Anyway, which comment in there (at what time) do you think counters my suggestion that a rock in a certain state is part of a valid solution to the universal wave function at some time in the past of the rock state?

There should even be some human-like observers seeing a rock teleport some distance — Marchesk

Yes, there should.

I didn't get where in the 2nd vid that Deutsch suggested some kind of empirical test that should yield different results from one interpretation to the next. I'm very skeptical of that.

As an old math person my suspicion is that "superposition" and "collapse of wave function" is nothing more than experimenting to discover which of multiple solutions of the partial differential equations describing phenomena actually apply in a particular instance. Multi worlds I consider science fantasy. — jgill

If, as is my understanding, there is no way to decide on a correct interpretation of QM empirically, it becomes not fantasy, but metaphysics. Or maybe just baloney.

What in your view is the problem for which MWI is a solution? In other words, what would proponents of MWI such as David Deutsch and Sean Carroll be obliged to acknowledge if it could be shown that this interpretation was untenable?

If, as is my understanding, there is no way to decide on a correct interpretation of QM empirically, it becomes not fantasy, but metaphysics. Or maybe just baloney. — T Clark

I think (for what it's worth, probably not much) that there are more and less credible interpretations. I rather like Chris Fuchs QBism, from which:

Q: You’ve written critically about the Many Worlds (or Everettian) Interpretation of quantum mechanics. What are its main shortcomings?

A: Its main shortcoming is simply this: The interpretation is completely contentless. I am not exaggerating or trying to be rhetorical. It is not that the interpretation is too hard to believe or too nonintuitive or too outlandish for physicists to handle the truth (remember the movie A Few Good Men?) It is just that the interpretation actually does not say anything whatsoever about reality. I say this despite all the fluff of the science-writing press and a few otherwise reputable physicists, like Sean Carroll, who seem to believe this vision of the world religiously.

For me, the most important point is that the interpretation depends upon no particular or actual detail of the mathematics of quantum theory. No detail that is, except possibly on an erroneous analysis of the meaning of “quantum measurement” introduced by John von Neumann in the 1930s, which is based on a reading of quantum states as if they are states of reality. Some interpretations of quantum theory, such as the one known as QBism, reject that analysis.

Q: So your position is that the Many Worlds Interpretation isn’t useful because it doesn’t constrain our theories of physics?

A: Allow me to get a bit technical to try to get the point across: Would Many Worlds work if quantum mechanics were based on real vector spaces instead of on complex ones? I would say yes. Would it also work if quantum mechanics used a different product structure than the tensor product? Yes. Would it work if quantum mechanics were nonunitary, i.e., didn’t obey the Schroedinger equation? Yes. And so it goes. One could even have a Many Worlds Interpretation of classical physics — as David Wallace, one of the most careful philosophers of the Many Worlds interpretation, once reluctantly admitted in a conference I attended.

The Many Worlds Interpretation just boils down to this: Whenever a coin is tossed (or any process occurs) the world splits. But who would know the difference if that were not true? What does this vision have to do with any of the details of physics? — Qanta

What in your view is the problem for which MWI is a solution? — Wayfarer

Don't know how to answer this. All interpretations are supposed to yield the same empirical results, so if there is an empirical problem to be solved (like getting a quantum computer to work), the problem is with quantum theory.
I didn't get a reply to my last question asking exactly what Deutsch thinks MWI can do that the others cannot. I didn't want to wade through a long video link to try to find it.

It is just that the interpretation actually does not say anything whatsoever about reality. — Quanta interview

MWI is and isn't a realist interpretation. It, like any almost all interpretations (QBism included), does not hold to the principle of counterfactual definiteness (that things really exist in the absence of measurement). Only under that principle is there 'spooky action at a distance", or faster-than-light cause/effect.
OK, so maybe I don't know what (presumably Fuchs?) is trying to say here. He indicates this von Neumann definition as the problem to be solved, which is seemingly an answer of the type which you are seeking about MWI. His example of Wallace admission is a good example of what is seen as the emptiness of MWI. The protest at the end seems empty:

Whenever a coin is tossed (or any process occurs) the world splits. But who would know the difference if that were not true? What does this vision have to do with any of the details of physics? — Qanta

Such a statement can be crafted of any interpretation.
I send a photon to the slits and it in fact goes through one slit or another before striking the plate. But who would know the difference if that were not true? What does this vision have to do with any of the details of physics?

A cool app, goes by the rather modest name umiverse splitter

Well go on then, split the universe! :party:

I think (for what it's worth, probably not much) that there are more and less credible interpretations. I rather like Chris Fuchs QBism, — Wayfarer

Seems like the text you quoted is consistent with my position. One part in particular - "Many Worlds Interpretation isn’t useful because it doesn’t constrain our theories of physics," is similar to what I wrote -

there is no way to decide on a correct interpretation of QM empirically — T Clark

Although I was speaking of all interpretations, not just the many worlds.

I hadn't head of QBism. I looked it up. Thanks for the new information.

Well go on then, split the universe! :party: — Agent Smith

Just did, nobody noticed. Or ever will.

What in your view is the problem for which MWI is a solution?
— Wayfarer

Don't know how to answer this. — noAxioms

Might I suggest that the motive for accepting the MWI interpretation is to avoid the philosophical conundrum of the 'collapse of the wave function'? That is at the root of the so-called 'observer problem' or 'measurement problem' in quantum physics, to wit 'The observer effect is the phenomenon in which the act of observation alters the behavior of the subject of observation. This is due to the ambigious nature of sub-atomic particles, which means that they can exist in multiple states simultaneously. When an observer measures a particular property of a particle, they are effectively collapsing the wave-function of that particle, causing it to assume a definite state.' The difficulty is how the act of observation can be considered 'causal' in this context. The approach of the MWI is to declare that the so-called wave-function collapse doesn't occur - but at the cost of there being many worlds.

Just did, nobody noticed. Or ever will. — Wayfarer

How does it work? Did you go into the mechanics of it? All I can recall from a video on the app is that a photon is split into two. What happens after that I haven't a clue. Google it?! :grin:

Might I suggest that the motive for accepting the MWI interpretation is to avoid the philosophical conundrum of the 'collapse of the wave function'? — Wayfarer

If one is seriously averse to wave function collapse, the list of interpretations on wiki (about 13) has only half of them supporting collapse. Point is, there are others to choose from besides MWI.
Interestingly, the list is sorted by date, and the ones without collapse lean more to the front of the list (older) and it is the more recent ones where collapse prevails. This suggests a trend and growing acceptance of collapse.

to wit 'The observer effect is the phenomenon in which the act of observation alters the behavior of the subject of observation'

OK, this is like a double slit setup with a which-slit detector behaves differently than a setup without one. That's not especially profound. If you get into the act of observing now changes something in the past, that's quite interpretation dependent.

This is due to the ambgious nature of sub-atomic particles, which means that they can exist in multiple states simultaneously.

Again an interpretation dependent statement. Not all interpretations suggest that a thing an exist in multiple states simultaneously. Bohmian mechanics for instance has but one state for anything. It is a hard realist interpretation where stuff is where it is. On the other hand, it necessitates backwards causation where decision not yet made can affect what a particle does now. I personally find that more offensive than collapse.

When an observer measures a particular property of a particle, they are effectively collapsing the wave-function of that particle, causing it to assume a definite state

Causing what to assume a definite state? The particle? Only some interpretations suggest this. With some (original Copenhagen for instance), the wave function is epistemological, describing only what one knows about a system. You take a measurement and your knowledge of the system changes, but the system is not affected by your acquisition of this knowledge.
RQM is another example, where a wave function of system X (Mars, 10 minutes ago) relative to system Y (Earth, now) changes upon some measurement. Say we observe a gamma emission. That changes the wave function of X relative to Y, but only because the new Y (post measurement) is now entangled with a particular outcome of a measurement. The ontology of the state of Mars relative to Y has changed (the gamma emission is now real instead of being in superposition), but nothing on Mars objectively changed, especially since there is no objective state under RQM.

So I guess I need to ask what you mean by an act of observation being considered 'causal'. Do you mean that shining a light on leaf to look at it will affect he leaf? Sure, that's pretty easy and is forward causality. I think you mean something deeper like the Mars example where the past ontology of some state changes relative to some present measurement.

The approach of the MWI is to declare that the so-called wave-function collapse doesn't occur - but at the cost of there being many worlds.

Something you apparently consider a substantial cost. I'm fine with that since I don't hold to the premise that there should be only one world, especially in the absence of evidence supporting that premise. My dismissal of MWI comes from other grounds.

MWI is simple. That might be the lead reason for its acceptance. I've heard critique (in linked things above) that it is simple to the point of emptiness.

So I guess I need to ask what you mean by an act of observation being considered 'causal'. — noAxioms

That prior to observation the particle doesn't exist in any specific place, that its possible properties are described by the wave-function, and that the act of measurement reduces all of the possibilities, except for the one in which it was measured, to zero. Cribbed from a website: 'Consider a photon shot from the laser gun. Soon enough, the photon is detected as a little dark dot on the photographic plate. Physicists would say that it has been “measured.” “Measured” really means that the photon has had an interaction with something in the physical universe. This interaction allows us to detect the photon. In this case, the photon is absorbed by an electron in the photographic plate, which creates a dark spot on the plate. Upon measurement, that is, this interaction, the probabilities calculated by the wave function instantaneously convert to a 100% probability for the specific dark spot and 0% everywhere else. The wave function has “collapsed.”

So that's how the act of observation is considered causal. Is that not correct?

I don't hold to the premise that there should be only one world, especially in the absence of evidence supporting that premise. — noAxioms

I would think that for this kind of hypothesis, the unity of the world is assumed and one would only require evidence for the contrary.

With some (original Copenhagen for instance), the wave function is epistemological, describing only what one knows about a system. You take a measurement and your knowledge of the system changes, but the system is not affected by your acquisition of this knowledge. — noAxioms

I don't recall reading anything like that about Bohr and Heisenberg's interpretation, it seems more like QBism which I mentioned above.

That prior to observation the particle doesn't exist in any specific place, that its possible properties are described by the wave-function, and that the act of measurement reduces all of the possibilities, except for the one in which it was measured, to zero. — Wayfarer

Don't get the last bit. It would seem that if you measured something's location, it is the location possibility which gets reduced to some much smaller deviation, and the others (momentum say) which are still just probabilities of what will be measured. The first bit talks about 'existing in a specific place' which is counterfactual terminology. Most interpretations do not hold to counterfactual definiteness, which means particles don't have actual positions (and other properties) in the absence of measurement. BM would say a photon exists en-route. Just pointing out the minefield of using terms like 'exists' which are defined differently from one interpretation to the next.
So far this isn't a whole lot different from classical physics. The championship game was played last week, but I don't know the outcome, so I have this function (something akin to a wavefunction) that describes the probabilities in detail. Then I pick up the paper and learn of the winner and of the spread and such, and the function collapses, reducing the possibilities to a much shorter list of unknowns.

So that's how the act of observation is considered causal. Is that not correct?

The measurement changed the wavefunction (relative to to the screen at least), so yea, that was caused by the interaction. Did the measurement change the photon? No, it's more like the photon caused the measurement. I'm trying to see the problem here.
Again, the wavefunction is different from one interpretation to the next. If it's epistemological (Bohm, Copenhagen), then the interaction just changes what we know. I see the dot and now know where it hits where before it would be a guess regardless of how much information I had on the system beforehand. If the wavefunction is metaphysically descriptive (RQM say), then the photon hit that spot on a screen with the dot. Much intuitive language doesn't work beyond that. It gets hard to describe. Maybe the example should be something like Objective Collapse interpretation(s), except I'm not very familiar with them. MWI could consider the wave function to be proscriptive, which means nothing changes upon the measurement, which takes place everywhere. Tegmark put out his mathematical universe hypothesis (MUH) and the draw of MWI to him is probably that it allows (does not demand) a proscriptive treatment of the wave function.

I don't recall reading anything like that about Bohr and Heisenberg's interpretation, it seems more like QBism which I mentioned above.

Don't know much about QBism, but it sounds a bit like all the idealism stops being pulled out. It defines existence in terms of beliefs and such, if I read it right.

Don't get the last bit. It would seem that if you measured something's location, it is the location possibility which gets reduced to some much smaller deviation, and the others (momentum say) which are still just probabilities of what will be measured. — noAxioms

But the point is, the object has no specific location until measured. You can't say 'the photon caused the measurement' because this assumes that it has some definite existence in some unknown location prior to being measured. There is no 'something' hiding in an unknown location until measured - the measurement makes it 'something'. Which is precisely the point at issue! That is why, indeed, 'exists' and 'real' have to be put in scare quotes in this context.

QBism is not really like idealism. Chris Fuchs has this to say about idealism:

Q: Does that mean that, as Arthur Eddington put it, the stuff of the world is mind stuff?

QBism would say, it’s not that the world is built up from stuff on “the outside” as the Greeks would have had it. Nor is it built up from stuff on “the inside” as the idealists, like George Berkeley and Eddington, would have it. Rather, the stuff of the world is in the character of what each of us encounters every living moment — stuff that is neither inside nor outside, but prior to the very notion of a cut between the two at all.

That's, I think, nearer to phenomenology than idealism.

Perhaps located in a grey zone where we are not sure which laws of physics, befween the large and thd very small pertain?

A potential issue arises here. What of all the entanglements that don't support observers? Those aren't considered worlds since there's no observers for things to appear classical. Which means observers are fundamental for saying what counts as a world. — Marchesk

Per MWI, worlds are persistent structures that emerge as a consequence of decoherence. In that sense, an MWI world doesn't depend on observers.

David Wallace on worlds:

“Worlds” are mutually dynamically isolated structures instantiated within the quantum state, which are structurally and dynamically “quasiclassical”. The existence of these “worlds” is established by decoherence theory. — Decoherence and Ontology - David Wallace

David Wallace on observers:

However, the structural approach is committed to an approach to the mind which ... denies observers some uniquely special status, but describes them as emergent as structures and patterns in lower-level physics (specifically, in lower-level classical physics, itself to emerge from unitary quantum physics via decoherence); — Everett and Structure - David Wallace

That being said, my understanding is that the probabilities we use to calculate the likelihood of what to expect when a measurement is made still needs to be derived within the Schrodinger equation in a self-consistent manner without adding it in post hoc, since the wave function is supposed to describe the universe we live in, if MWI is true. So deriving the Born rule within MWI is an ongoing project. — Marchesk

You might be interested in Sean Carroll's post on Why Probability in Quantum Mechanics is Given by the Wave Function Squared.

↪180 Proof He has a very interesting idea on how to put MWI and wave-function collapse interpretations to the test. Assuming we can build a conscious AGI quantum computer. — Marchesk

I didn't get where in the 2nd vid that Deutsch suggested some kind of empirical test that should yield different results from one interpretation to the next. I'm very skeptical of that. — noAxioms

Here's the relevant part of the conversation between physicists David Deutsch and Markus Arndt:

6:12 Deutsch: When we have quantum computers, we will be able to have very large, very complex entities existing in super positions. So, in principle, I suggested long ago before this was remotely on the cards experimentally, that if we had a quantum computer on which an artificial-intelligence program was running, say, with human level artificial-intelligence then this entity would be able to experience interference in its own consciousness.

6:46 Arndt: Well, some people would say that your consciousness would collapse your reality.

6:50 Deutsch: Yes, so if that happened that would refute the Everettian interpretation or, as I would say, it would refute quantum theory. That would be a very interesting problem and that's one of the reasons why scaling up both the size and the complexity and the mass of phenomena that I experimentally observe, but that can only be explained by quantum theory, is very important.

7:17 Arndt: I fully agree. We need to do that.

7:21 Deutsch: We just need to close the gap between that and the AI because the AI would not be having this conversation. Or, at least, the AI would not be able to make the argument that you just made. It would have to say I've only got evidence of many worlds on the scale of my mind, but not bigger so - and I guess that will always be true. — Are There Many Worlds? David Deutsch in conversation with Markus Arndt

Essentially, Deutsch's proposed experiment would implement the Wigner's Friend thought experiment.

In Deutsch's proposal, Wigner would be a human-level AI running on a quantum computer, with the friend (also a human-level AI) and the measured qubit being internal and isolated subsystems of Wigner.

By conducting an interference experiment on the joint friend/qubit subsystem, the Wigner AI would be able to determine whether physical collapse happened or not. If physical collapse were detected, then that would falsify standard quantum theory (and MWI along with it).