[nessus42]

Yes, I'm not sure why this is a Nature article. As far as I understand things, very few physicists who care about which interpretation of QM is correct would choose the Copenhagen Interpretation in this day and age. The Copenhagen Interpretation is not even a fully-fledged interpretation, because it uses undefined, unscientific terms like "measurement" to determine when a probability wave collapses.

As I understand things, most physicists don't give much thought to which interpretation is correct, since any experiments to distinguish between the various interpretations are virtually impossible to do. And most physicists don't care about distinctions for which there will be no experimental evidence.

Among the physicists who do care about the different QM interpretations, it is my understanding that most would go with the Everett (AKA "Many Worlds) Interpretation these days. All other interpretations that I know of are hugely problematic, but there are no significant problems at all with the Everett Interpretation. The only problem is that many people consider it to be "creepy". But not liking the best theory because it is "creepy" isn't very good science, if you ask me.

Regarding there being no single-world interpretation that is logically self-consistent, I'm not convinced about this: The Bohm Interpretation, for instance, is experimentally indistinguishable from the Everett Interpretation. I.e., no matter what incredible technology and powers of QM experimentation we might develop in the future, we will never be able to do an experiment, even in theory, that tells us which of these interpretations is the right one.

Consequently, it would seem that the Bohm Interpretation is logically self-consistent. The problem with the Bohm Interpretation is that it's very ad hoc and violates Occam's Razor. It only exists in order to calm our feelings about the universe being "creepy".

[ummonk]

Although I go with the Everett interpretation (which I like to think of as the "universal wave function" interpretation because it is really just a minimalist theory assuming that the laws of wavefunction evolution always apply), and the many worlds aspect is just a byproduct of the fact that a wave function left to its own devices would decohere into a bunch of practically non-interacting "worlds".

It should be noted, however, that the Everett interpretation does have one issue: it's not clear why probabilities should work the way they do. There are different approaches to deriving the laws of probabilities under Everettian physics, but things very easily get metaphysical once you try to go down that road.

As you point out, the Bohm Interpretation works as a single world interpretation, although it relies on reifying particles embedded in waves to essentially select a single world, which is rather ad-hoc. However, it does give us the probabilities for free, assuming any reasonable initial setup for the particles.

[nessus42]

> It should be noted, however, that the Everett interpretation does have one issue: it's not clear why probabilities should work the way they do.

Yes, back when I studied this topic seriously, this was an issue. E.g., if you toss a quantum coin that has a 1/3 chance of coming up heads and 2/3 chance of coming up tails, this seems to result in only two "worlds". And if there are two worlds, why are the observed probabilities then not .5/.5 rather than .3333/.6667?

I didn't mention this in my OP because (1) that would have been something of a deep-dive for a summary post, and (2) there were ideas being floated about to solve this problem back when I was studying this, but I don't know how these ideas ultimately panned out.

I'm surely curious as to what the current best ideas are about this issue.

[int_19h]

Why can't it result in three worlds, two of which are indistinguishable?

[nessus42]

Well, maybe it does. Only there really aren't separate worlds in the Everett Interpretation. It only seems that way to our superpositioned brains.

At some point this debate becomes a bit too confusing for me. All I can report is that the experts fretted over this.

[jostylr]

In a perfectly mathematically coherent version of Everett: https://arxiv.org/abs/0903.2211, the idea is to integrate out the wave function to get a mass density on 3 space. This mass density on 3-space is a mess at any particular moment, but one can witness its evolution over time to pick out particular correlated histories. There is no particular splitting into separate worlds. And, indeed, the whole picture of discrete spin measurements is misleading. It is always spatial measurement stuff ultimately going on and so plenty of smearing.

The relevant probabilities are not derived by number of "worlds". Pick some particular moment and correlated history, look backwards (what is recorded in the current "configuration") at experiments, and one should see the proper statistics appearing in the "vast majority" of experiences.

However, there will be plenty of experimenters who see wrong statistics. Everett predicts this with certainty. There is a "world", according to this, that just split from the moment I am writing this, in which all future experiments have spin up coming up 100% of the time from that moment on. Over time, we all end up correlated with this as the experimenters report their fantastical findings.

If they truly believe in Everett's theory, they would accept that they just happen to be in the branch where this happens. In Bohmian mechanics, they would say something else is going on. The odds of seeing something like that in Bohmian mechanics are so vastly, incomprehensibly small, that it is more likely to see cracked eggs reassembling themselves from random thermal motions. But in Everett, it happens with certainty to some universe.

This is the difference. Bohmian mechanics can be readily falsified based on statistical outcomes of experiments. Perhaps not with 100% certainty, but certainly with 100% practical certainty. Everett can never be falsified based on statistics. It could be falsified if something that was supposed to happen with a literal 100% certainty failed to happen, but with anything statistical, it simply can't because the theory says it does happen.

One could modify the theory to cut out the "outlier" worlds. This is, in some sense, what GRW with a mass density ontology does.

[nessus42]

I guess I am unclear on one of your points: Let's say we toss a fair quantum coin many, many times. In the Everett Interpretation, yes, there is a "world" in which that coin has always come up heads. But our chance of finding ourselves in that world is vanishingly small.

In the Bohm Interpretation, that coin could always come up heads too, but again with a vanishingly small probability.

So they seem equivalent experimentally to me. (And to the experts who have written entire books on the subject.)

Max Tegmark came up with a way to experimentally determine if the Everett Interpretation is correct. (I believe it was Tegmark who came up with this.) It has a high cost for the experimenter, though!

What you do, is rig a gun to a fair quantum coin, so when you pull the trigger, the gun fires 50% of the time. Now shoot yourself in the head with it many, many times. If you end up surviving many rounds of this, you can be pretty darn certain that the Everett Interpretation is correct.

Never mind the billions of other versions of yourself that you murdered to discover the truth!

[platz]

Right, with many worlds, if there is any probability of something quantum happening, with say a billion billion to one odds, it will happen with 100% certainty somewhere. And if you are that observer, how do you say it was unlucky/lucky, sine it must have happened

[beagle3]

It's no worse than a single world interpretation: These things happen all the time, the branch of math that deals with them is called Large Deviation Theory (and it's closely related to information theory).

One of the corollaries/interpretations of Sanov's theorem is that, generally speaking, when faced with an astonishingly improbable outcome (e.g. flipping 9,000 heads and 1.000 tails out of 10,000 independent coin flips), no statistical test can differentiate between "that improbable occurence with a fair coin" and "an unfair coin" - the fair coin, when it does something improbable, with have (with overwhelming probability) a specific tilted distribution that looks unfair.

[platz]

But in single interpretation only one outcome happens on a trial. The full distribution does not manifest on a single draw. In MWI all the possibilities occur, which is different

[Sharlin]

Somebody usually wins the lottery. That doesn't change the fact that it's really unlikely for any given player to win the jackpot.

[platz]

I guess you win. MWI is really not different from classical probability in any way.

[MrMontyBurns]

Any state is equally improbable. It's human judgement to tell something was special or not. If all states are all possible combination of numbers in a lottery, you just call 1 combination "I won" and all the others "I lost". Same thing applies to a cleaned up room, any arrangements of objects in it are equally probable, but the number of states you would call cleaned up are so much less than the messy ones, so you can say a cleanup up room is less likely than a cleaned up one (unless you do something about it :P) This is also the fundamental of entropy btw.

TL;DR By grouping states together (human choice), certain arrangements seem more probable than others.

[millstone]

> the Everett interpretation does have one issue: it's not clear why probabilities should work the way they do.

This is referencing the nature of the measure and the difficulty of deriving the Born rule. Why should the outcome of measurements be proportional to the square of the wave function? That's indeed a problem in MW.

But even if that's solved in some way, there's an even more foundational issue of how non-determinism can arise at all in a deterministic theory. The MW reply is that there is no non-determinism, but then has trouble explaining observed reality - which is not a good position for a theory to be in.

[westoncb]

> ... although it relies on reifying particles embedded in waves to essentially select a single world, ...

Is the mathematical formalism actually more unwieldy, or is it just this interpretation of what it's doing? The 'select a single world' sounds like taking some of the conceptual framework from Everett and trying to paste it on here, rather than giving the Bohm interpretation its own conceptual framework which could possibly be more elegant?

[jostylr]

The Bohm interpretation is quite simple. It does start with the idea that the world is made of particles and that we are satisfied if we can make correct predictions of where stuff is. In other words, I have a computer in front of me and a particle theory would say that is because there are particles making up the computer and they are located in front of me. So that is the correspondence with reality. For some, this is a reasonable starting point. For most others, it is blasphemy for some reason.

Once we settle on stuff with position, then we have to ask how that stuff changes. One option is that we specify accelerations; that's Newton's way.

Another option is to specify velocities. That is the Bohm way. Specifically, the velocity is derived from the wave function of quantum mechanics. It is basically the derivative of the wave function, normalized and made real. Done. You can see a simple derivation of the equations here: https://en.wikipedia.org/wiki/De_Broglie–Bohm_theory#Derivat... The first one derives Bohm's equation very quickly and simply from the starting wave relation equations of Einstein and de Broglie.

The complication is that the wave function is a function of the configurations of all the particles. Thus, to get the velocity, one technically needs the positions of all of the particles of the universe. Practically, one only needs the positions of entangled particles, but still. It is a non-trivial setup though the most basic, natural setup one could possibly have given particles and a wave function. Also, the particle positions do not influence the wave function evolution. This is rather unusual.

For the Dirac wave function, it is even simpler. That object directly gives the velocity of the particles; no derivative needed.

Contrast this with MW which says, I guess, that the fundamental thing that we are concerned with is the wave function. It does not seem that particles exist in any meaningful sense in that theory. It is as if we are machines able to track a singular aspect of the change of an abstract vector in Hilbert space. It is not clear why this vector is often represented as a function on configuration space when there is no configuration of stuff. The theory essentially says that actual reality is nothing at all like what we perceive. I would think an honest account of a theory which only has a wave function would be to formulate the theory on an abstract Hilbert space and derive configuration space and, thus, configurations from that. Not very likely, by the way.

Reality may be deceptive, but I certainly prefer to start with theories in which our experience is explained in a pretty simple and straightforward way: it looks like stuff is over there because there is stuff over there. We have singular experiences because a single experience is what actually happens.

Also note that in Bohmian mechanics, we specify the initial wave function and the initial particles and then evolve the system using differential equations. All of the operator stuff, collapse, etc., comes out of that evolution; we don't need to have any special considerations about them. The quantum formalism becomes analogous to thermodynamics, not a fundamental theory, but a useful practical one replacing the individual evolutions with some useful shorthand. In MW, there is this question of how to model an experimental situation. Where are the measurement operators coming from? What is a subsystem? In BM, these things arise essentially by conditioning on the configuration of the environment. In certain situations, this will give rise to roughly an isolated system evolving according to its own Bohmian dynamics. The measurement interaction is then represented by an operator, or its generalization, depending. But all of that emerges from the basic differential equations evolving the universe.

It is not clear to me how easy it is to do that kind of analysis in MW. After all, there is no singular experience to break it down to, there is no subsystem, there is no definitive experiment being done. It is not really clear how one would falsify a theory which, more or less, assumes everything happens.

[nessus42]

> Reality may be deceptive, but I certainly prefer to start with theories in which our experience is explained in a pretty simple and straightforward way: it looks like stuff is over there because there is stuff over there. We have singular experiences because a single experience is what actually happens.

This assertion does not make sense to me. The Everett Interpretation and the Bohm Interpretation are experimentally indistinguishable from each other, as I understand things. Consequently, there is no mystery at all with the Everett Interpretation as to why things appear to us the way that they do.

Since the Everett Interpretation is a significantly simpler theory than Bohm's, we should prefer it due to Occam's Razor. On the other hand, since they are experimentally indistinguishable from each other, we can never scientifically assert which of the two is correct, no matter how much evidence we have.

[jostylr]

Everett's theory categorically tells us that the results of experiments differ from the results in Bohmian mechanics. In Bohmian mechanics, a typical experiment will have one result. In Everett, experiments have all possible outcomes happening. These are not the same.

The "indistinguishable" part happens because, according to Everett, there is some version of the experimenter that will have the same experience as the single experimenter in the Bohmian world.

This is not simpler. I have no reason to believe that there are infinitely many copies of me out there. Everett's theory says that there are. Fine. I can't disprove it. I also can't disprove that every instant of my experience is being carefully orchestrated by a thousand angels. It is experimentally indistinguishable from any theory you care to posit.

But I prefer theories where my actual experience is supposed to be a reasonable reflection of reality. I experience a single me and therefore I would prefer a theory in which there is a single me. Bohmian mechanics provides that and in a completely natural and reasonable way.

Everett categorically disputes my experience as being reflective of reality. There are infinitely many copies of me and my experience of being singular is an illusion. I can't dissuade people from embracing that, but it certainly strikes me as peculiar.

Also, in terms of experiments, Everett has infinitely many copies of the universe where all of the statistics of the experiments come out wrong. There are infinitely many that come out right. Is that experimentally indistinguishable? I don't know. Kind of a strange question in the context of "most everything happens".

[int_19h]

This all hinges on a bunch of essentially random metaphysical choices that you have made. For example, what does it even mean for there to be a "single me"? I would argue that in the many worlds interpretation, once a fork happens, the other observers are not "me" anymore. So there's no contradiction between the experience of a "single me", and multiple copies - each copy has its own "single me" experience.

[ColanR]

To satisfy my own curiosity, do you have a source on that issue with the Everett interpretation? I'd love to read up on what you were talking about in that second paragraph.

[ummonk]

I don't have a summary offhand that would be better than anything you can Google up, but this post by Sean Carroll explains the issue and details one possible answer: http://www.preposterousuniverse.com/blog/2014/07/24/why-prob...

[amluto]

I don’t have a real source, but the basic problem is somewhat straightforward. Suppose I have a particle in the state |0> + |1>. (I’m ignoring overall normalization.). After the measurement, the state is (|0>|I measured 0>) + (|1>|I measured 1>). This is a pure (deterministic) state.

It would be nice to say that there’s a 50% chance that I measured 0, but how exactly do you get that in a rigorous way from the state vector above?

To make everything complicated, the answer should not treat the experimenter part of the universe specially.

[monktastic1]

Your second state is only pure if no information has leaked into the environment. The chance of a human-sized object measuring a state without any stray photons or air molecules interacting is basically zero.

As for why 50%, why not the Born rule? Or are you asking how we derive the Born rule?

[amluto]

> Your second state is only pure if no information has leaked into the environment.

Not true in a many-worlds model. The “I measured” part is intended to account for the environment, at least initially.

> Or are you asking how we derive the Born rule?

More or less. In a many-worlds interpretation, there is no Born rule per se. I’m saying it’s not entirely trivial to recover the statistics that the Born rule would give.

[abeppu]

> Among the physicists who do care about the different QM interpretations, it is my understanding that most would go with the Everett (AKA "Many Worlds) Interpretation these days.

In a small poll at a conference on the foundations of quantum mechanics in 2011, the Copenhagen interpretation was the most popular. This could differ from your definition of "these days" I suppose.

https://arxiv.org/pdf/1301.1069.pdf

Update: a 2016 survey by different authors found similar popularity for the Copenhagen interpretation.

[konschubert]

I can say that this weird acceptance of the logically undefined Copenhagen interpretation was a contributor to me wanting to leave the field ...

[AstralStorm]

Logically undefined in what sense?

Paradoxical traps or thought experiments filled with holes?

MW is even more undefined in terms that we cannot even construct a measurement/singular interaction... (it is always derivative of total system state)

[nessus42]

Well, I guess I can't argue with an unscientific poll, but it kind of boggles my mind that any scientist could take the Copenhagen Interpretation seriously.

I mean it builds the term "measurement", which is an undefined and unscientific concept, right into the laws of physics. Personally, I find this to be virtually nonsensical.

[pdobsan]

The 2016 survey: https://arxiv.org/pdf/1612.00676.pdf

Both surveys are fascinating reads. They clearly give a sense that despite of the spectacular success of QM how far we still are from the final word in that field.

[FooHentai]

>The only problem is that many people consider it to be "creepy".

That seems like an unfair characterization. The issue many have with the Everett Interpretation is that it relies on a fundamentally untestable existence of every possible universe in every possible state of being for the sake of conceptual tidiness. It's not unreasonable to apply Occam's razor and hold a measure of skepticism about this.

[nessus42]

In my opinion, Occam's Razor does not say that you should prefer the theory that has the simplest consequences, but rather that you should accept the simplest theory that explains the observations.

The Everett Interpretation is clearly the simplest theory because the deep mystery of what would cause probability waves to collapse is made completely irrelevant. There is no wave collapse to worry about or to explain.

[IIAOPSW]

>The Copenhagen Interpretation is not even a fully-fledged interpretation, because it uses undefined, unscientific terms like "measurement" to determine when a probability wave collapses.

No, measurement is just a poor choice of name for a type of interaction some matter can have with other matter. I'm a wave function, and sometimes I have a measurement interaction with other wave functions and that causes collapse. When wave functions of random environment particles do the measurement and cause (seemingly) spontaneous collapse, we call it decoherence and its a serious drag on building quantum computers.

I'm not advocating for Copenhagen, but it is not inconsistent for the reasons you stated. Outside the paper linked here, I'm unaware of any inconsistency in the Copenhagen interpretation.

[nessus42]

> No, measurement is just a poor choice of name for a type of interaction some matter can have with other matter.

That's not the Copenhagen Interpretation. Interpretations that claim this are called "Objective-collapse" theories. Back when I cared a lot about this, the most popular Objective-collapse theory was GRW:

https://en.wikipedia.org/wiki/Objective-collapse_theory

[kgwgk]

If "measurement" is an undefined and unscientific term, what is the scientific definition of "branching" in the multiple-words interpretation?

Edit: the most significant problem with the MWI, apart from the "creepy" metaphysical aspects, is the meaning and quantification of probabilities. In particular being able to derive Born's rule, which works so well in practice.

[nessus42]

> If "measurement" is an undefined and unscientific term, what is the scientific definition of "branching" in the multiple-words interpretation?

The "Many Worlds Interpretation" is actually something of a misnomer. This is why many people prefer to call it the Everett Interpretation.

It doesn't actually posit many worlds. It posits one very big complex world with very complex superpositions of state. But since your brain ends up in a superposition of states, different facets of this superposition of your brain state perceive this one big complex world, as smaller, simpler "worlds".

And the term for why different pieces of this superposition of states stop having an effect on each other is called "decoherence".

As for how the math works out in terms of probabilities, that is beyond me.

When I studied this, the discussion was usually simplified down to a quantum coin that when flipped would come up heads 1/3 of the time and tails 2/3 of the time.

This only results in two "worlds" though. A heads world and a tails world. So there was an issue that people debated at the time: Why do we perceive the .3333/.66667 probability for these two "worlds", rather than a .5/.5 probability?

I must admit that I am ignorant about the current state of this debate.

[kgwgk]

Is saying "measurement" really much more undefined and unscientific than saying "different facets of this superposition of your brain state perceive"?

[nessus42]

100% yes. You cannot give a scientific accounting of "measurement" since the term isn't even defined in the Copenhagen Interpretation.

There are various interpretations that attempt to define "measurement" in various ways, but those are not the Copenhagen Intepretation.

As for whether you can give a scientific account of how data is processed by a brain in a superposition of states, you most certainly can. (Ever heard of "quantum computing"?) It's just complicated.

[kgwgk]

“You most certainly can” is not a very satisfactory answer. I don’t say that the Copenhagen interpretation is very satisfactory but at least it predicts the probability of events. The term isn’t even defined in the Everett interpretation. In the best case, it’s incomplete and more metaphysics than physics.

Edit: I’ve never heard of any treatment of “quantum computing” which doesn’t include the concept of measurement, the Born rule and the projection postulate. Have you?

Edit2: it was maybe not fair to say that the probability of events is not defined in the Everett interpretation because many-world interpretations have addressed this issue since the original paper from Everett in 1957. But as far as I know they have not succeed. Measurement has also been addressed in countless papers and books for almost a century, for what it’s worth.

[Sileni]

Just wanted to say, as someone who's been following this on the fringe for years, I really appreciate you using words that were easily searched. Really let me step through what you wrote effectively even though I didn't recognize the interpretations by name.

[rocqua]

> Regarding there being no single-world interpretation that is logically self-consistent, I'm not convinced about this: The Bohm Interpretation

The paper on which this is based states in the abstract that this fails for Bohmiam Mechanics:

> This conclusion extends to deterministic hidden-variable theories, such as Bohmian mechanics, for they impose a single-world interpretation.

[konschubert]

> The Copenhagen Interpretation is not even a fully-fledged interpretation, because it uses undefined, unscientific terms like "measurement" to determine when a probability wave collapses.

Then why is it being taught everywhere? It's been driving me crazy since forever.

[chrischen]

I’d imagine because it is the easiest to explain.

[nessus42]

Also because it works 100% of the time, even if it doesn't ask or answer some questions that seem profoundly important.

[gpsx]

People have an adverse reaction to the "many worlds" theory. Maybe they don't understand it? Or maybe what I think is the many world interpretation is really something else. Back when I was in theoretical physics we weren't careful about defining terms like this because, as the original comment says, many worlds vs copenhagen was not really an issue of interest, other than in a philosophical sense for people like me. And I didn't take Copenhagen seriously.

If you take away any sensient beings, or whatever it is that is required to make one of these "measurements", then copoenhagen is the same as the many world interpretation. Wave functions go on evolving and there is no collapse. For example, an electron can be in spin up or spin down. It is not in both states. In one "world" it is spin up. In another "world" it is spin down. That is strange enough for all of us. But for some reason people have trouble extending this idea to people, so that a person can be in mulitiple states at the same time (in the different "worlds", in the same sense as the electron being in different "worlds".)

What made this strike home to me was when I was in graduate school and my advisor told me "There are no magic external observers. The observer is subject to quantum mechanics too. He is part of the experiment."

To describe the correspondance between copenhagen and "many worlds", suppose an observer measures if an electron is spin up or spin down. In "many worlds" case his memory of the outcome is correlated with the measured state of the electron. So in the "world" where the electron is spin up (that portion of the wave function) the observer also thinks the electron was measured as spin up. And in the "world" where the electron is spin down, the observer thinks the electron was measured as spin down.

In this "many worlds" case, The observer who measures the electron as spin up will not interact with the observer that measured it as spin down. For all intents in purposes, it is as if that other observer never existed. In the copenhagen case, that other observer does _not_ exist. In this interpretation, the wave function collapsed to only include the part with a single observer.

In effect the observed outcome of the two interpretations is the same. The difference being one of them, the copenhagen interpretation, postulates a magical change in the wave function of the universe.

(Aside - The fact that those observers will not interact is just in a practical sense, to my knowledge. I don't know if it is impossible for them to interact in theory. I don't think it is. Maybe someone else knows the answer to that.)

[monktastic1]

> For example, an electron can be in spin up or spin down. It is not in both states. In one "world" it is spin up. In another "world" it is spin down.

I don't think most MWIers would agree with this. Normally they consider worlds to have split only once (irreversible, or approximately irreversible) decoherence has set in. An electron in the coherent state |z+> + |z-> = |x+> wouldn't qualify.

In fact, this seems to be one of the biggest difficulties of the interpretation. Nobody knows whether there even is such a thing as in-principle irreversible decoherence, and if there's not, then the point at which it is "approximately irreversible" is arbitrary.

[gpsx]

Are you a many worlds person, Or is this your interpretation of what they believe? There is no need for an irreversible decoherence. Any real situation where there is a question of copenhagen versus many worlds is a pretty decoherent problem to begin with, since you are dealing with macroscopic beings.

Edit: Add the state change in the measurement

(|z+> + |z->)|obs> => |z+,obs+> + |z-,obs->

First there is an electron in one of two states, and the observer is uncorrelated. After the measurement, the observer becomes correlated with the electron.

[monktastic1]

The only explanation I've ever seen of when worlds split is when decoherence has become "effectively irreversible." That's not a well-defined physical event, and so it's hard to say that worlds "actually" split.

For your unentangled state on the left, Sean Carroll explicitly describes it as a state that doesn't have two worlds yet. I can find the post if you like, or maybe we already agree and I'm misunderstanding.

[gpsx]

If you have can find a post I'd like to see it. I don't quite follow.

In what I am describing, the two "worlds" don't really separate. It is possible that they can interact, theoretically. However, you can't construct an experiment to detect the different parts of the wavefunction interacting because of decoherence. You just can not make a coherent quantum system that invovles real people (to my knowledge). So in practice you can not do the experiment.In anything we observe, the two resulting observers (in a measurement with two choices) are effectively isolated.

In other words, decoherence is automatic. Also, it is inherently irreversible.

But, I am sure he (or whoever wrote that post) is saying something sensible so I would be interested in seeing it.

[monktastic1]

I just mean like this: http://www.preposterousuniverse.com/blog/2014/06/30/why-the-...

"We wouldn’t think of our pre-measurement state (1) as describing two different worlds; it’s just one world, in which the particle is in a superposition. But (2) has two worlds in it. The difference is that we can imagine undoing the superposition in (1) by carefully manipulating the particle, but in (2) the difference between the two branches has diffused into the environment and is lost there forever."

(State 1 is when the particle is in a superposition by itself and state 2 is when it's entangled with a macroscopic apparatus.)

My issue is that he uses words like "forever" and "impossible." These convey a sense of finality, but the decision of where to draw the boundary is subjective. The worlds can in principle (and under certain cosmological models, must) recohere.

See, for example: https://arxiv.org/abs/1105.3796

"Decoherence - the modern version of wave-function collapse - is subjective in that it depends on the choice of a set of unmonitored degrees of freedom, the "environment"."

See in particular Section 3.2 (Failure to irreversibly decohere: A limitation of finite systems)

(Edit: I should mention that I am not a physicist, by a long shot. Just a curious amateur.)

[kgwgk]

> the copenhagen interpretation, postulates a magical change in the wave function of the universe.

And the MWI postulates a magical branching into different "worlds", doesn't it?

[gpsx]

No. I may not have conveyed the idea very well in the comment. That is normal quantum mechanics. People just use the term "world" when it applies to these observers and not when it applies to something like an electron.

[kgwgk]

If this is normal quantum mechanics and the "worlds" are not real, separated physical objects -- and they are just mathematical constructions -- what problem does the MWI solve precisely?

If the universe is an isolated quantum system evolving unitarily, how does the MWI help to understand the laws of physics that we observe?

[nessus42]

> If the universe is an isolated quantum system evolving unitarily, how does the MWI help to understand the laws of physics that we observe?

It definitively answers the question of when wave collapse occurs and what causes it.

The answer given by the Everett Interpretation is simply that there is no wave collapse, and what we observe is the result of our brains being in a superposition of multiple states.

[kgwgk]

Consider this simple application of standard QM:

1) we have a one-particle system that has been prepared into a pure state by measuring the spin along the x-axis

2) we are going to measure the spin along the z-axis

3) the quantum state before is a superposition of the |up> and |down> states (in the basis corresponding to the Sz operator)

4) the theory predicts that we are going to find either |up> or |down> with equal probability

5) immediately after the measurement the quantum state will be either |up> or |down>, depending on the outcome

What is the answer given by the Everett Interpretation? What is the description of the initial conditions? What is the prediction of theory? What is description of the end state?

I hope the answer is not just handwaving and mumbling about "superposition".