Problems with Many Worlds Interpretation

[Samshorn]

Let P be the phase space of a universe-describing physical theory... Suppose we've solved the measurement problem for P... Now, construct a new physical theory whose phase space is PxP (i.e. a configuration of the new theory is a pair of configurations of the old theory)... The time evolution of (y,z) can be computed by evolving y and z individually according to the old theory, then pairing them back together...

What do you mean when you say "suppose we've solved the measurement problem for P"? The purpose of MWI was to solve the measurement problem. If you concede that it doesn't, I suspect there is no argument here. No one disputes that, given a satisfactory theory P of the universe, we could metaphysically postulate a theory consisting of the Cartesian product PxPx... of multiple independent universes. But it would be pointless. If we can already solve the measurement problem in P, what would be the point of postulating a Cartesian product of independent Ps?

 

[Demystifier]

Tegmark's argument is that the suicide experiment will convince the person who's trying to kill himself, because the MWI says that he/she is certain to survive. (That's his argument, not mine).

I criticize this in another (new) thread:
https://www.physicsforums.com/showthread.php?t=526213

 

[Hurkyl]

No one disputes that, given a satisfactory theory P of the universe, we could metaphysically postulate a theory consisting of the Cartesian product PxPx... of multiple independent universes. But it would be pointless.

The issue at the moment is mitchell porter's assertion that it's obvious from direct observation that reality has definite outcomes, versus my assertion that it is absolutely impossible to empirically distinguish between definite and the type of indefinite outcome I'm describing.

(and also my assertion that being able to understand that indistinguishability is a prerequisite for being able to understand a decoherence-based interpretation of QM)


If you already recognize PxP is empirically equivalent to P, then you have no need to pay attention to that post.

 

[Hurkyl]

On a different note, I worked out a thought experiment I wanted to come up with a couple days ago.

Scenario 1: I start with a qubit in a carefully chosen state and measure its spin along the Z axis. The state chosen so that my measurement will result in Z+ 60% of the time and Z- 40% of the time.

Scenario 2: I start with a qubit in a carefully chosen state and measure its spin along the Z axis. If I get spin down, I then measure its spin along the Y axis. The state was chosen so that my end result will be Z+ 20% of the time, Y+ 40% of the time, and Y- 40% of the time.

In both cases, I hide my results and pass the qubit along to you to experiment with however you want.


Fact: you get identical experimental results in both scenarios.

Or put differently, if we repeat this 1000 times and I use the same scenario each time and you are meant to figure out which one, you can do no better than guessing blindly.


From a collapse interpretation's point of view, I think this result is very surprising. In the first scenario, I'm sometimes giving you a Z- qubit, and never giving you a qubit in a definite Y spin state. In the second scenario, I'm never giving you a Z+ qubit, and sometimes giving you qubits in a definite Y spin state.

 

[mitchell porter]

The difference between the original theory and the new theory are completely imperceptible to anything or anyone described by the universe. If it is possible to talk about whether the state of the universe includes an experiment and what outcome occurred, then it is impossible to use this experiment to distinguish between the two theories described above.

And if no experiment can distinguish, then there is no scientific basis for insisting that one does a better job of describing reality than the other.

Neither theory contains an "indefinite outcome". Theory P has definite outcomes, and the new theory is just PxP - a doubling of definite outcomes.

you do notice that, in classical mechanics "probability distribution over N states" is essentially identical to "N worlds (with weights that sum to 1)" right?

If you mean "possible worlds" then I think I accept the equivalence.

Whether you want to presuppose something shouldn't affect your ability to acknowledge it equivalent or otherwise indistinguishable to something else.

"Actual" is different from "possible", and "essentially identical" is different from "indistinguishable".

 

[Hurkyl]

Neither theory contains an "indefinite outcome". Theory P has definite outcomes, and the new theory is just PxP - a doubling of definite outcomes.

Please clarify. The outcomes seem pretty clearly indefinite to me -- e.g. if H(x) and T(y) hold for theory P, then (x,y) is pretty clearly indefinite in PxP. (despite the fact that E(x,y) holds -- the coin flip definitely occurred)

In case it wasn't clear, remember that I am talking about experiments done internally to the universe.

 

[mitchell porter]

Please clarify. The outcomes seem pretty clearly indefinite to me -- e.g. if H(x) and T(y) hold for theory P, then (x,y) is pretty clearly indefinite in PxP. (despite the fact that E(x,y) holds -- the coin flip definitely occurred)

In case it wasn't clear, remember that I am talking about experiments done internally to the universe.

What do you mean by "(x,y)"?

 

[mitchell porter]

Actually, forget it. This whole discussion is nonsense. There is no such thing as an "indefinite outcome", it's tantamount to speaking of a "non-outcome outcome". Your attempts to provide examples of an indefinite outcome are just inappropriate uses of formalism, inappropriate because they don't have any meaning when you use them that way.

If, against all appearances, by an indefinite outcome you just mean an uncertain outcome, in the sense of experimental uncertainty: experimental uncertainty derives from a number of practical factors like low resolution of the pointer variable and unreliability of the process which correlates the pointer variable with the measured property. It would make no sense to reify uncertainty, and speak e.g. of a particle that is "objectively somewhere between 2 meters and 3 meters away, but also objectively not at any specific location between those two extremes". Concepts come with conditions of use attached, and if we really can't apply them according to those conditions, then we need new concepts. If it's wrong to speak of particles having exact positions and momenta, that means that they have some other properties, not that they have objectively inexact positions and momenta.

So, this is my last post in the thread. I'm going back to thinking about concrete physical questions, which include realistic explanations of QM, but which do not include logic-chopping debates about whether the concept of indefinite outcome makes sense or not. Hurkyl, though I have expressed my disdain for your philosophy of QM in this thread, obviously you can apply the machinery of QM sensibly enough. It's often like this with "philosophical" issues; you find people who, for philosophical/ideological reasons, will say bizarre things, but then it turns out that, yes, on a mundane level the bizarreness doesn't interfere with practical activities in the way you think it might, if it was taken literally. I think it's the same with you, you can use QM, you can get along in the world like any other intelligent practical person, but then you have this ideological superstructure in which you say nonsensical things because you believe them to be logically implied by QM - or for some other reason. Maybe I have you completely wrong, it doesn't matter now because I'm on my way out.

 

[Hurkyl]

If you don't want to think about the appearance of definite outcomes to an observer internal to the universe when the external view doesn't have them, that's fine. But don't tell other people not to think about it just because you don't want to. :tongue:

If it's wrong to speak of particles having exact positions and momenta, that means that they have some other properties, not that they have objectively inexact positions and momenta.

And one candidate for that "other property" is wavefunction-ness.

But the issue isn't about particles -- the issue is about quantum mechanics applying to systems "large" enough to include things that look like observers and measuring devices. If you are part of a quantum system, whether wave-functions-evolving-via-unitary-evolution can predict that you see a nice happy classical reality.

And on that last question, there were once lots of serious obstacles that have been overcome. I believe the only two remaining obstacles are:

• Develop quantum thermodynamics to a point where we can precisely formulate the question and test it
• Deal with people who refuse to consider the question

(I put "large" in quotes, because IMO all you need is 2 qubits and a CNOT gate)

 

[Hurkyl]

Actually, forget it. This whole discussion is nonsense. There is no such thing as an "indefinite outcome", it's tantamount to speaking of a "non-outcome outcome". Your attempts to provide examples of an indefinite outcome are just inappropriate uses of formalism, inappropriate because they don't have any meaning when you use them that way.

You don't like the phrase "indefinite outcome"? Fine, call them kumquats, I don't care. All I care is that there is no observational difference (for someone in the universe) between a classical universe with kumquats a reality that has a definite state, but we've assigned ignorance probabilities to a distribution of possibilities.

"P or not P" is a tautology. But viewed externally, even in classical logic, it does not follow that there are only two truth values e.g. the four-valued logic consisting of the four truth values

{(true, true), (true, false), (false, true), (false, false)}​

Note that, even in this logic, "P = true or P = false" is a tautology -- e.g. if (externally) we see P = (true, false), then when we compute internally to the logic:

"P = true or P = false" is "(true, false) or (false, true)" is "(true, true)"​

 

[fleem]

If consciousness never ceases, then the person repeatedly attempting and failing suicide could easily inductively prove to himself/herself that the probability of successfully committing suicide is less than what he might have estimated is the probability that there is life after death (in the classic sense of the soul living after the body dies). Yet that person also knows that others had "succeeded" in their suicides as per natural laws of physics. So anyone claiming that consciousness must always survive, is also claiming the probability of life after death is comparable to the probability that the natural laws we've observed are true.

 

[Ken G]

I'm sorry if I missed it in this long thread, but what I never understood about the "quantum suicide" scenario is why we can assume it is our own consciousness that will be the one that survives? It seems to me that in the Many Worlds view, there would need to be many consciousnesses associated with the me-like entity in lots of those other worlds, and I have no perception of them now, so why would I suddenly have a perception of a surviving one? I should no more care about that surviving consciousness than I should care about your consciousness when I attempt suicide. It can be argued that perhaps I shouldn't care any more about my own consciousness, on the grounds that any ownership there is illusory, but if I adopt that philosophy I don't need Many Worlds to tell me I can't succeed at suicide-- I can't as long as anyone else survives.

 

[Fredrik]

The discussion about quantum suicide continued in another thread. Demystifier linked to it in one of his posts above.

 

[Ken G]

OK, thanks. As for the interpretation of Many Worlds, there is never anything wrong with an interpretation, although one can explain one's reasons for the preference. I see "many worlds' as the ultimate tradeoff between sacrificing anything remotely resembling an empirically motivated ontology in favor of a rationalistically motivated ontology. But the empiricists can't really complain-- it gets all the predictions right. No one has suggested any different predictions, have they? (The one just above about failed suicide attempts is invalid, as we wouldn't have anyone in our world with multiple failed attempts, and none of us have done it. Have we?)

 

[Eqblaauw]

I think the poll of this David Raub fellow his highly misleading, the whole poll is unfindable (it's in a french magazine of an issue that's not online anymore), the outcomes are very weird
In no other poll more then half of the physics subscribe to the mwi, when they have the 'I don't know' choise. David Deutsch himself says 20% at best believe this theory. When you search for you David Raub you find out that he has send a letter to Everett, explaining how much he likes is theory. A poll by a fanboy isn't very reliable. David Raub hasn't got anymore hits then involving the many-world on google, that says something.

 

[Ken G]

Valid objections. What's more, I find it interesting that you used the phrase "believe this theory." Many-worlds isn't really a theory of anything, it is just an interpretation of a theory, and as such it is purely a matter of personal preference or pedagogical effectiveness. I'm not sure any pedagogical effectiveness has ever been attributed to the MWI (like, "I couldn't get the calculation done right until someone explained MWI to me"), so then it's all personal choice. But even more interesting is the choice of the word "believe." I was once asked if I "believe" in dark matter. The question kind of floored me-- is belief what counts in a scientific theory? Do I believe in Newton's laws? Did Newton? When scientists start to "believe" their theories, they have forgotten what science is all about. We don't believe in theories, we rely on them-- that's quite a bit different.

 

[fleem]

As I said earlier but feel compelled to iterate, we have classical minds and therefore the word "understand" to us means "have a classical analogy". But classical mechanics, although useful, is wrong.

 

[Ken G]

Yes, classical mechanics is wrong. Quantum mechanics is wrong, general relativity is wrong. Physics is always wrong, that's not the point. We're trying to get nature to fit in our heads, and we should expect some strain there! We always have to abstract away from our experience, and we always have to at some point come back to our experience. I really don't see quantum mechanics and classical mechanics as all that different on these points, but I agree that it is useful to try and unify them as much as possible.

 

[Hurkyl]

I'm not sure any pedagogical effectiveness has ever been attributed to the MWI (like, "I couldn't get the calculation done right until someone explained MWI to me"),

Delayed choice quantum eraser, that signals can't be sent with entangled photons, quantum computing, entanglement in general...

The issue isn't usually "I couldn't get it done right", but instead "I understood it fairly easily". I assert that most (all?) of the conceptual difficulties people have with these are directly attributable to collapse-based interpretation.

Decoherence-based interpretation is a little more complex, but it doesn't encourage a mode of thought that causes problems with the above examples.



As an aside, restricted to quantum systems, MWI is utterly normal. You wouldn't be able to tell the difference between someone thinking in that way versus someone who is not.

The "weird" stuff only comes into play when people start considering the topic of whether more macroscopic objects (like measuring devices and experimenters) can be treated as quantum systems. On a humorous note, I assert that things like Schrodinger's Cat and Wigner's Friend are examples of "calculations" that people can't get right until they understand MWI (or some other decoherence-based interpretation).

 

[Hurkyl]

As I said earlier but feel compelled to iterate, we have classical minds and therefore the word "understand" to us means "have a classical analogy". But classical mechanics, although useful, is wrong.

I have to disagree with this on general epistemological and pedagogical principles. "Having an analogy" is very much not understanding -- it is something you use when you don't understand something, but still need to reason about it anyways. At best, having an analogy is the start of one particular path to understanding.

 

[Eqblaauw]

I am a layman, and as David Deutsch said I think it's best for a layman it's best to describe to the prevailing theory, when a poll makes false claims that many-worlds is the prevailing theory and that poll becomes widely cited, there is a problem. Another problem (apart from the de unlikely outcomes, I've mentioned in my previous post) is that people like Gell-Mann, also are in the yes sector, while in my opinion (altough the maths may be a like), there is quite a difference between one universe and an infinity of universes.

 

[Ken G]

Delayed choice quantum eraser, that signals can't be sent with entangled photons, quantum computing, entanglement in general...

I don't have any particular problem with entanglement without invoking MWI, I simply view the wave function as a way to encode the holistic information required to predict and understand a system. There is still no need to take the wave function as literally as is done in MWI. Indeed, I'm generally suspicious about taking any physical ontology literally, as I feel history has clearly shown what a dubious proposition that is. Physics is epistemology, the ontology is just convenience (or fun!).

The issue isn't usually "I couldn't get it done right", but instead "I understood it fairly easily".

Right, that's my point-- what a person understands is quite personal, the only test of understanding is outcomes. That's why I asserted that the only demonstrable pedagogical advantage is one that actually assists in getting the answer right, and that can either be objective or subjective. AFAIK, MWI has not been shown to have objective pedagogical advantages in this sense, but it's a difficult experiment to control.

I assert that most (all?) of the conceptual difficulties people have with these are directly attributable to collapse-based interpretation.

I would argue that is only when they are using that interpretation improperly. Remember, it was not Bohr and Heisenberg who had the problem with entanglement, it was Einstein. So I would say the problem people have is with realism, not with collapse. MWI has collapse too-- the main difference is whether one attributes the fundamental reality to what exists prior to, or post, collapse, which is essentially the distinction between rationalism and empiricism. Rationalists like MWI, what more can be said?

Decoherence-based interpretation is a little more complex, but it doesn't encourage a mode of thought that causes problems with the above examples.

Even in Copenhagen-esque approaches, collapse should be interpreted in terms of decoherence. I don't think Bohr would have had any difficulty seeing the importance of decoherence, he might simply have said that decoherence explains why the wave function works in predicting the reality-- the reality would still be the collapse. It's all in the direction that one thinks that physics is working-- do we take what we observe, and reason backward to finding a description of it (empiricism), or do we take our theory literally, and imagine that it determines what we observe (rationalism). Both have problems-- the empiricist must answer why the theory can work to 12 decimal places, and the rationalist must answer why we don't think it will work to 25 decimal places.

As an aside, restricted to quantum systems, MWI is utterly normal. You wouldn't be able to tell the difference between someone thinking in that way versus someone who is not.

I would argue that, even if it is possible to restrict to quantum systems (Bohr always argued that isn't possible, as the whole idea of a "quantum system" predicates that our minds have cooked up the notion), there wouldn't be anything remotely resembling what we call quantum mechanics. Operators, eigenstates-- what does an electron care? If electrons could think, they might well have very different concerns than we do, ask very different questions as a result, and probably formulate a theory that is wholly unrecognizable to us. That's Wittgenstein's famous "if a lion could talk, we wouldn't understand him."

The "weird" stuff only comes into play when people start considering the topic of whether more macroscopic objects (like measuring devices and experimenters) can be treated as quantum systems.

Yes, or I might put it slightly differently: it comes in when we ask, is our experience valid testimony to what is really happening? And if so, why is our everyday intuition so inapplicable at the quantum level? My own view is that we should expect our experience, and also how we think as a result, to be quite limited based on our modes of perception, so we should never be surprised when we get surprised.

On a humorous note, I assert that things like Schrodinger's Cat and Wigner's Friend are examples of "calculations" that people can't get right until they understand MWI (or some other decoherence-based interpretation).

But of course you know that isn't fair-- those aren't calculations at all! The "right answer" to those queries already depends on one's interpretation, so the results are rigged.

 

[Hurkyl]

I simply view the wave function as a way to encode the holistic information required to predict and understand a system.

This sentence is a very good description of what it means for an interpretation to assert that the wave-function to correspond ontologically to a real-world system.

So, I find it quite odd that you suggest you are doing exactly the opposite...

(but, at least, you do put "to understand" in there -- many people who argue viewpoints like yours do not, or even explicitly decry the idea of using the wave-function to understand reality)

(aside: if you don't admit to using quantum mechanics to shape your views about reality and you're telling the truth, then what are you using?)

There is still no need to take the wave function as literally as is done in MWI.

Ignore the question about "taking something literally". MWI is merely the negative answer to the following two questions:

• Is "collapse" under time evolution part of the "holistic information required to predict and understand a system"?
• Is there any "holistic information required to predict and understand a system" that is not encoded in the wave-function?

I would argue that is only when they are using that interpretation improperly.

Of course. My point is that collapse-based interpretations encourage improper usage -- indeed the resolution to each of the issues I mentioned boils down to getting the student to stop applying the main method of the interpretation too early.

Remember, it was not Bohr and Heisenberg who had the problem with entanglement, it was Einstein.

As I recall, Einstein didn't have problems with entanglement -- he just didn't believe that it corresponded to reality.

MWI has collapse too--

No -- it collapse is a provably (essentially) impossible result of the unitary evolution of wave-functions.

MWI has decoherence, and the analytical method of separating a mixed state into a linear combination of states.

Even in Copenhagen-esque approaches, collapse should be interpreted in terms of decoherence.

If a wave-function decoheres and doesn't subsequently collapse, it's not a Copenhagen-esque approach.

I would argue that, even if it is possible to restrict to quantum systems

You misinterpreted me -- I meant "If there are two physicists analyzing a quantum system, you won't be able to tell which one is using MWI and which one is using CI". (except, of course, on the off chance one starts talking about the system actually collapsing)

Yes, or I might put it slightly differently:

Your rephrasing looks like an entirely different point. My point is "We have our experiences with how reality behaves. Can Quantum Mechanics reproduce that behavior?"

But of course you know that isn't fair-- those aren't calculations at all! The "right answer" to those queries already depends on one's interpretation, so the results are rigged.

It's at least partially a rather serious and important point -- some interpretations are wholly incompatible with the idea that quantum mechanics could apply in those domains, and leads people to scoff at (or even blatantly ridicule) attempts by others to consider the topic.

 

[Ken G]

This sentence is a very good description of what it means for an interpretation to assert that the wave-function to correspond ontologically to a real-world system.

So, I find it quite odd that you suggest you are doing exactly the opposite...

There's a key difference. By saying the wavefunction encodes the information to understand something, I have asserted an epistemological meaning to the wave function. MWI rests on an ontological meaning to the wave function, and that I pretty much view as folly, as it hasn't worked yet in the interpretation of any physical theory.

(but, at least, you do put "to understand" in there -- many people who argue viewpoints like yours do not, or even explicitly decry the idea of using the wave-function to understand reality)

Yes, I do think the "shut up and calculate" camp goes too far in making that assertion. None of them ever really do that anyway! The fact is, we don't just use physics to calculate, we use physics to understand also. But I'm saying, that understanding always stays in our heads-- it never graduates to being something that is actually going on somewhere else. The latter seems to be a key element of the MWI, at least the way it is often used.

(aside: if you don't admit to using quantum mechanics to shape your views about reality and you're telling the truth, then what are you using?)

Quantum mechanics can shape our views about reality without MWI. It certainly shaped Bohr's and Heisenberg's views of reality, did it not?

Ignore the question about "taking something literally". MWI is merely the negative answer to the following two questions:

• Is "collapse" under time evolution part of the "holistic information required to predict and understand a system"?
• Is there any "holistic information required to predict and understand a system" that is not encoded in the wave-function?

I agree that those two questions warrant negative answers, but I see all interpretations of quantum mechanics (other than deBB) as giving negative answers to those questions. (So I don't like deBB as anything but an exercise in demonstrating the fundamental limitations in either deterministic or "intrinsically random" ontologies. It should be used to show why ontology is impossible, not to argue for determinism.) I can't say exactly how Bohr would have answered them, but here's my guesses:
1) collapse is a concept required for us to connect our macroscopic perceptions of reality with a theory that works on the microworld. It is not part of the holistic information required to understand the system, because it deals with an aspect of the system that we never understand and are simply not equipped to understand.
2) there is no holistic information not included in the wave function, but we must recognize that "information" is on our side of the divide-- it is part of our epistemological task of understanding the system, it is not part of the system itself. So again, the wave function is all about our interface with that system, and the informatics required to achieve a successful interface.

Of course. My point is that collapse-based interpretations encourage improper usage -- indeed the resolution to each of the issues I mentioned boils down to getting the student to stop applying the main method of the interpretation too early.

But look at what MWI encouraged-- quantum suicide.

As I recall, Einstein didn't have problems with entanglement -- he just didn't believe that it corresponded to reality.

I would call that having a problem with it. But we agree-- Einstein felt QM could not be the fundamental truth, even though it worked. MWI says it is the fundamental truth. Bohr says there's no such thing as the fundamental truth, at least not anything we have access to.

No -- it collapse is a provably (essentially) impossible result of the unitary evolution of wave-functions.

And there's the problem-- it is "provable." Truth is never provable, what is provable is logical connection to a set of axioms. So yes, unitary evolution is logically connected to a set of axioms that are used to evolve a wave function. The connection to reality is what we are talking about-- the ontology of the axioms. You can only argue that because they work, the axioms must be the correct ontology. To that I would raise two objections:
1) there is no logical connection between a working axiom and a true ontology, and
2) the history of physics delivers a harsh assessment of the argument that good postulates are true axioms.

MWI has decoherence, and the analytical method of separating a mixed state into a linear combination of states.

True, but it has no idea how to connect those mixed states with what we actually observe in individual cases. So it must either invoke a concept of ontological randomness, which is bogus because randomness is an effective model not an ontology, or it must come to grips with what consciousness is, which we are presently ill-equipped to do (again I mention quantum suicide).

If a wave-function decoheres and doesn't subsequently collapse, it's not a Copenhagen-esque approach.

I guess I don't understand what you mean here-- collapse in Copenhagen is not the evolution of the wave function, it is the manual re-assessment of the wave function based on new information. Copenhagen sees a wave function as an expression of knowledge, with no ontological status at all, so it seems the wave function as evolving unitarily when there is no change in information status, but in other ways when we do have new information (like the outcome of an experiment). Thus, when I say Copenhagen is fine with decoherence, I mean that Copenhagen applies decoherence in the opposite direction as MWI-- MWI says the wavefunction collapse occurred after the decoherence (either by some random selection agent or some nonexistent model of how consciousness is being generated in the various branches), and Copenhagen says decoherence is what you get when you apply the new information that is being acquired in a gradual rather than sudden way (taking the perceptions of the consciousness as the fundamental reality and forcing all information to be consistent with it).

You misinterpreted me -- I meant "If there are two physicists analyzing a quantum system, you won't be able to tell which one is using MWI and which one is using CI". (except, of course, on the off chance one starts talking about the system actually collapsing)

I agree with your quoted remark, but I don't see how it adjudicates the options here. All interpretations are fine with that statement, it's essentially the reason these are interpretations and not theories.

Your rephrasing looks like an entirely different point. My point is "We have our experiences with how reality behaves. Can Quantum Mechanics reproduce that behavior?"

I was saying what I view as the source of "quantum weirdness," which is a bit different from what you see as its source. That may well be why we feel differently about MWI! (I don't really favor any interpretation, by the way, I see them all as inevitably flawed and would prefer to pick and choose from among them to make any particular point as needed. But I prefer the Bohr approach when it is used to foster skepticism about any ontological interpretation as being "the truth" of the matter.)

It is not weird that quantum mechanics can reproduce our experiences, what is weird is that the way it does it is to deviate so strongly from our common interpretation of our experiences. This should be a lesson to us about the flaws in all kinds of interpretations, rather than as evidence that a new one has replaced an old one! That's the key point I'm stressing-- the lesson of history should not be "the old was wrong, now we have it right", it should be "everybody always thought that, from Aristotle to Newton to Einstein to Weinberg-- isn't it time we got the real message, which is that all our theories and their interpretations have the ways in which they work for us, and the ways they fail us."

It's at least partially a rather serious and important point -- some interpretations are wholly incompatible with the idea that quantum mechanics could apply in those domains, and leads people to scoff at (or even blatantly ridicule) attempts by others to consider the topic.

I am sympathetic to your point here that the "cat paradox" and "Wigner's friend" are often misinterpreted, leading people to dismiss macroscopic extensions of quantum mechanics. I think the problem is simply that the important distinctions between epistemology and ontology are not enforced, but MWI makes that same mistake by suggesting an ontology when all it can really support is an epistemological understanding of the quantum domain. When interpretations are predicated with "to understand what this theory is telling us about our reality, although the theory never actually asserts this is true, you can imagine that...". This is along the lines of saying that you can understand what GR is telling us about the history of the universe by "imagining that space itself is expanding", without ever actually asserting that space itself actually is expanding, which is probably pretty close to nonsense. (The next theory might give some actual meaning to that claim, or it might expose it as nothing but a handy mnemonic, such is the history of physics.)

The bottom line is, physics theories are epistemological entities that generate ontological constructs along the way, but the theories are never judged by those constructs, they are judged by their epistemological successes. What's more, the ontological entities generated along the way are often not unique within a given theory (witness Newton's use of "forces", and Maupertuis's use of "action", as one common example). No one argues over whether there really are such things as forces when we teach all our undergraduates that there are-- this is simply because we are lazy about our distinctions between what is an epistemological success and what is an ontological claim on reality. That crucial imprecision is at the heart of all the debates about quantum mechanical interpretations, in my view.

 

[Samshorn]

I think the poll of this David Raub fellow his highly misleading...

The first time I saw any mention of that "poll" was in the sci.physics newsgroup back in Feb 1995, posted by a fellow named Michael Clive Price, an ardant proponent of the Many Worlds interpretation. He is still apparently active today, as I see he edits the Wikipedia article on many worlds... which includes the Raub "poll" results again! Even back in 1995 that "poll" was discounted by most people. Here is how Price presented it back then:

"Q30 Who believes in many-worlds?
"Political scientist" L David Raub reports a poll of 72 of the "leading cosmologists and other quantum field theorists" about the "Many-Worlds Interpretation" and gives the following breakdown.

1) "Yes, I think MWI is true" 58%
2) "No, I don't accept MWI" 18%
3) "Maybe it's true but I'm not yet convinced" 13%
4) "I have no opinion one way or the other" 11%

In response Ted Bunn wrote:

"The poll in question did not use a representative sample of physicists. The people polled were "cosmologists and quantum field theorists," a small and highly non-representative group, who are much more inclined to favor many-worlds than physicists at large are."

 

[BruceW]

I guess I don't understand what you mean here-- collapse in Copenhagen is not the evolution of the wave function, it is the manual re-assessment of the wave function based on new information. Copenhagen sees a wave function as an expression of knowledge, with no ontological status at all, so it seems the wave function as evolving unitarily when there is no change in information status, but in other ways when we do have new information (like the outcome of an experiment). Thus, when I say Copenhagen is fine with decoherence, I mean that Copenhagen applies decoherence in the opposite direction as MWI-- MWI says the wavefunction collapse occurred after the decoherence (either by some random selection agent or some nonexistent model of how consciousness is being generated in the various branches), and Copenhagen says decoherence is what you get when you apply the new information that is being acquired in a gradual rather than sudden way (taking the perceptions of the consciousness as the fundamental reality and forcing all information to be consistent with it).

Hurkyl is taking 'collapse' to mean the non-unitary evolution, while you are using it to mean something more general. I think I got confused with you on this term in another thread.

Also, decoherence means the same thing in MWI and CI.

 

[Fredrik]

Hurkyl, I still don't get what you're saying about the MWI or how QM describes reality. In the Schrödinger's cat experiment, does QM/MWI say that there exist at least one dead cat and at least one alive cat?

I'm not saying that there's no way to interpret QM as a description of a physical system that somehow contains several different cats. (I even have some ideas about how to do that). I'm just not buying that no additional assumptions are needed. At the very least, I think that we would have to make assumptions about what parts of the mathematics represent "worlds".

 

[xts]

does QM/MWI say that there exist at least one dead cat and at least one alive cat?

Exactly - that's MWI!
There exists multiple worlds, all of them being heirs, of our one, in some of them the cat is alive, in others - cat is dead. So if only the amplitude (probability) of any of those is non-zero, there exists alive cats in some wordld and dead cats in some other worlds

 

[Fredrik]

Exactly - that's MWI!
There exists multiple worlds, all of them being heirs, of our one, in some of them the cat is alive, in others - cat is dead. So if only the amplitude (probability) of any of those is non-zero, there exists alive cats in some wordld and dead cats in some other worlds

Right. That's the reason why "MWI" is spelled with an M and a W. What I'm trying to find out is if Hurkyl feels that this is implied by QM itself, or at least by the statements that he takes as the definition of the MWI.

The original MWI (Everett's) was an attempt to drop the Born rule from QM and to try to derive it by "branch counting". This idea was a miserable failure. What we're discussing now is the possibility that QM (which by definition includes the Born rule) describes a physical system ("describes" in the sense explained in my posts earlier in this thread). This could perhaps also be considered a MWI, but only if we make some assumptions that answer questions like: Where in the mathematics of QM do we find the worlds?

 

[BruceW]

In the most basic, essential MWI, it simply says that non-unitary collapse doesn't happen. So there is only one world. So the system would be represented as something like: lDead cat> + lLive cat>

 

[Fredrik]

In the most basic, essential MWI, it simply says that non-unitary collapse doesn't happen. So there is only one world. So the system would be represented as something like: lDead cat> + lLive cat>

It sounds like what you're describing is exactly QM plus the assumption that there's only one world. That would be a very strange way to define a MWI.

 

[Ken G]

I believe the core issue in MWI is the way to take the mathematical postulates of QM (especially the unitary evolution postulate) and use them to spawn an ontological description of reality. The result is that a universe in a pure state will always be in a pure state, but that pure state breaks down into non-interfering "worlds" via the natural result of decoherence. In my way of thinking of it (I won't attribute this to either Everett or Hurkyl), there simply isn't enough information in a pure state to populate all the possible coherences that appear when you start coupling hugely complex subsystems (like macro pointers), so most of the coherences don't get populated at all-- they separate into subsets that preserve their own relative coherences (like entangled particles), but maintain no coherence "between worlds." This is a bit like a block-diagonal matrix, where each "block" is a world (and here I don't really mean a matrix like a real-valued density matrix, this preserves correlations over entangled systems), and is more or less unaware and uncoupled to any of the other worlds, but they are all there. Then we say that somehow our experience is in some sense "trapped" in our own "block" of the matrix, and since we have no correlations to the other worlds, we cannot observe their effects.

 

[Ken G]

To expound a bit more, can we not say that the MWI is the natural outcome of the rationalist mindset that the universe "really does" obey the Rules of Quantum Mechanics? This mindset asserts that the way we get closest to reality is by thinking about it, not by experiencing it. Empiricists like Bohr take the opposite tack, of starting with the premise that reality is what we experience it to be, and the role of our minds is to try and make sense of reality, but the reality is never beholden to how much we can fit in our heads.

In my view, neither of those perspectives can be "correct", because our ability to perceive reality is limited, and our ability to conceptualize reality is limited. Indeed, those two propositions seem almost obvious to me! So empiricism will never be an exact rendition of reality, coming as it does through the filter of our experiences, and rationalism will never be an exact rendition of reality, coming as it does through the filter of our thoughts. Given this, the best we can do it try to combine them and see as far we can get, but a large grain of salt needs to be our constant companion in this exercise. Is this not the lesson of history?

 

[Dmitry67]

our ability to conceptualize reality is limited

Yes, this is why I do like Max Tegmark's MUH, because our ability to imagine crazy stuff is limited, but our math is not. BTW I think MWI is the only int. compatible with MUH.

 

[Hurkyl]

Hurkyl, I still don't get what you're saying about the MWI or how QM describes reality. In the Schrödinger's cat experiment, does QM/MWI say that there exist at least one dead cat and at least one alive cat?

Bird's view of the experimental setup: yes or no, depending upon what you mean by "there exist at least one". Yes, in the sense that we're going to write the state of the system as a linear combination of states (not kets) of terms of both types. No, in the sense that the coefficients on all of the "dead cat" terms will add up to less than 1, and the same for the "live cat" terms.


Frog's view: no. What it says is that "the cat is dead or alive" is a true statement, as is conditional statements such as "the cat is dead" conditioned on the hypothesis that "a dead cat was observed".


That last part, I think, is the key stumbling block for people who reject decoherence-based interpretation -- they don't want to condition future observations on past experiences. They want (exactly) one of "the cat is dead" and "the cat is alive" to be an unconditional truth.


(That above probably deserves some "approximately, but with error so small we have no hope of observing" as well as being conditioned on "the Schrödinger's cat experiment was performed")

I'm just not buying that no additional assumptions are needed.

The main assumption needed is that quantum thermodynamics behaves in a satisfactory manner. But that assumption is not there for MWI's sake -- it's there so that it even makes sense to consider the question of QM applying to such systems.

The original MWI (Everett's) was an attempt to drop the Born rule from QM and to try to derive it by "branch counting".

Do you have a citation for that? Skimming Wikipedia suggests otherwise.

As I understood it, Everett's key insight was to use relative states -- thus the "relative state formulation". Even if the state of the whole system is pure, the state of the subsystem corresponding to an observer can be mixed, and mixed states can be consistent with classical experiences. AFAIK this is the first time someone found a way around the no-go theorem, and it is the starting point for most attempts to consider how quantum mechanics might reproduce classical physics.