Many-Worlds Theory: Existence of Multiple Universes

[The Dagda]

You've lost sight of the question; the point is how best to understand quantum physics. At the time, CI was pretty much the only option, because the ramifications of relative states hadn't been discovered, indefiniteness hadn't really been treated seriously in a scientific context, and nobody had yet worked out any hidden-variable variations on the theory. But now that we have 90 years of experience, we can make a more informed decisions.

As a mathematician, I look at QT and see two parts: unitary evolution that happens most of the time, and collapse which happens occasionally. It's often a good idea to study things one piece at a time, so it's very natural to study the effects of unitary evolution (in other words, MWI). That's just a good way to learn and study things; it would apply to any theory, and it baffles me how vehemently people seem to reject that notion.

Anyways, while I may have been expecting to then add collapse into the picture¹, you run into decoherence -- the interesting phenomenon that the observational effects of collapse can be described as a by-product of unitary evolution. So again, just looking at the theory tells you that it's very natural to consider decoherence-based viewpoint. I'm baffled how many people won't even consider such a thing!

Sure, one can complain about all the non-observable things, but that's not a feature unique to MWI -- we've been doing it with physical theories for centuries. At its core, it's nothing more than the acknowledgment that many different mathematical states can represent indistinguishable physical realities. Again, I'm baffled that people would take such a stance to reject interpretations like MWI.

1: Yes: the CI posits that between collapses, the quantum world behaves exactly as described by MWI.

No I have not lost sight of the question, MWI just isn't science its arm waving away problems in a what if scenario. Tell me what exactly is the point of making something up that in all likelihood will and could never be tested. If its just to illustrate CI then fine, but it's still philosophy. I consider such a thing and I do not reject it, I fear though people as usual give too much weight to what is essential pure philosophy.

But, as the quote said, they're "magic". CI excludes, a priori, the possibility that such things can be described by quantum mechanics.

And so therefore it cannot possibly be correct? Is that your assumption and if so how do you reconcile it with the fact that experimentally it is true, by resorting to magic?

If all predictions of quantum mechanics can be computed (in principle) from the wavefunction, then that means the quantum wavefunctions really do refer to "real propagations in some quantum underworld".

So, I ask you, what predictions of quantum mechanics are not determined (even in principle) by a wave function?

The question is meaningless, since you have no idea what really happens, all it does is model what happens in experiment without knowing intrinsically anything about the wave. I agree with Bohr, there is nothing real about the wave function in as much as we know, and how people have the nerve to suggest such without actually knowing anything about it is beyond me and science. Claiming the wave function is real is all very well in principal but is it? Show me the money?

If QM is deterministic then how do you resolve the Bell inequalities, which show that it is not? This again is magical thinking, if I say it is, it must be because I say so.

If you ask me many worlds is just a neat way of avoiding hidden variables without actually tackling the issue at all or addressing gaps in our knowledge. In that sense it is no better than a God of the gaps theory, where CI is not, there I am by will of my imagination. Now as I say I have no problem with it as a hypothetical concern, but people take it as far, far more than that, and personally I don't think they have any reason to that is scientific at least.

Let's define what we mean by determinism, which is after all the same in science as it is in any other field.

Causal Determinism

Causal determinism is, roughly speaking, the idea that every event is necessitated by antecedent events and conditions together with the laws of nature. The idea is ancient, but first became subject to clarification and mathematical analysis in the eighteenth century.

http://plato.stanford.edu/entries/determinism-causal/

This article discusses the issues from classical to quantum mechanics without being too maths heavy.

4.4 Quantum mechanics

As indicated above, QM is widely thought to be a strongly non-deterministic theory. Popular belief (even among most physicists) holds that phenomena such as radioactive decay, photon emission and absorption, and many others are such that only a probabilistic description of them can be given. The theory does not say what happens in a given case, but only says what the probabilities of various results are. So, for example, according to QM the fullest description possible of a radium atom (or a chunk of radium, for that matter), does not suffice to determine when a given atom will decay, nor how many atoms in the chunk will have decayed at any given time. The theory gives only the probabilities for a decay (or a number of decays) to happen within a given span of time. Einstein and others perhaps thought that this was a defect of the theory that should eventually be removed, by a supplemental hidden variable theory[6] that restores determinism; but subsequent work showed that no such hidden variables account could exist. At the microscopic level the world is ultimately mysterious and chancy.

So goes the story; but like much popular wisdom, it is partly mistaken and/or misleading. Ironically, quantum mechanics is one of the best prospects for a genuinely deterministic theory in modern times! Even more than in the case of GTR and the hole argument, everything hinges on what interpretational and philosophical decisions one adopts. The fundamental law at the heart of non-relativistic QM is the Schrödinger equation. The evolution of a wavefunction describing a physical system under this equation is normally taken to be perfectly deterministic.[7] If one adopts an interpretation of QM according to which that's it — i.e., nothing ever interrupts Schrödinger evolution, and the wavefunctions governed by the equation tell the complete physical story — then quantum mechanics is a perfectly deterministic theory. There are several interpretations that physicists and philosophers have given of QM which go this way. (See the entry on quantum mechanics.)

More commonly — and this is part of the basis for the popular wisdom — physicists have resolved the quantum measurement problem by postulating that some process of “collapse of the wavefunction” occurs from time to time (particularly during measurements and observations) that interrupts Schrödinger evolution. The collapse process is usually postulated to be indeterministic, with probabilities for various outcomes, via Born's rule, calculable on the basis of a system's wavefunction. The once-standard, Copenhagen interpretation of QM posits such a collapse. It has the virtue of solving certain paradoxes such as the infamous Schrödinger's cat paradox, but few philosophers or physicists can take it very seriously unless they are either idealists or instrumentalists. The reason is simple: the collapse process is not physically well-defined, and feels too ad hoc to be a fundamental part of nature's laws.[8]

In 1952 David Bohm created an alternative interpretation of QM — perhaps better thought of as an alternative theory — that realizes Einstein's dream of a hidden variable theory, restoring determinism and definiteness to micro-reality. In Bohmian quantum mechanics, unlike other interpretations, it is postulated that all particles have, at all times, a definite position and velocity. In addition to the Schrödinger equation, Bohm posited a guidance equation that determines, on the basis of the system's wavefunction and particles' initial positions and velocities, what their future positions and velocities should be. As much as any classical theory of point particles moving under force fields, then, Bohm's theory is deterministic. Amazingly, he was also able to show that, as long as the statistical distribution of initial positions and velocities of particles are chosen so as to meet a “quantum equilibrium” condition, his theory is empirically equivalent to standard Copenhagen QM. In one sense this is a philosopher's nightmare: with genuine empirical equivalence as strong as Bohm obtained, it seems experimental evidence can never tell us which description of reality is correct. (Fortunately, we can safely assume that neither is perfectly correct, and hope that our Final Theory has no such empirically equivalent rivals.) In other senses, the Bohm theory is a philosopher's dream come true, eliminating much (but not all) of the weirdness of standard QM and restoring determinism to the physics of atoms and photons. The interested reader can find out more from the link above, and references therein.

This small survey of determinism's status in some prominent physical theories, as indicated above, does not really tell us anything about whether determinism is true of our world. Instead, it raises a couple of further disturbing possibilities for the time when we do have the Final Theory before us (if such time ever comes): first, we may have difficulty establishing whether the Final Theory is deterministic or not — depending on whether the theory comes loaded with unsolved interpretational or mathematical puzzles. Second, we may have reason to worry that the Final Theory, if indeterministic, has an empirically equivalent yet deterministic rival (as illustrated by Bohmian quantum mechanics.)

I know what you are saying when you say it is deterministic in the sense we can equate the maths to fit the facts by using abstract tricks like renormalisation and i, all I say is that is it reflective of the facts or are we just postulating an outcome and retrofitting the maths to that? If so how do we know it is fundamentally deterministic? And isn't the Schrödinger equation underivable?

 

[ThomasT]

... the point is how best to understand quantum physics.

Yes

At its core, it's (MWI) nothing more than the acknowledgment that many different mathematical states can represent indistinguishable physical realities.

Is MWI necessary for this?

... the CI posits that between collapses, the quantum world behaves exactly as described by MWI.

Does it? I've always thought of the CI as an instrumentalist view.

This is from the Stanford Encyclopedia of Philosophy discussion of the CI:

...
Bohr's more mature view, i.e., his view after the EPR paper, on complementarity and the interpretation of quantum mechanics may be summarized in the following points:

1. The interpretation of a physical theory has to rely on an experimental practice.
2. The experimental practice presupposes a certain pre-scientific practice of description, which establishes the norm for experimental measurement apparatus, and consequently what counts as scientific experience.
3. Our pre-scientific practice of understanding our environment is an adaptation to the sense experience of separation, orientation, identification and reidentification over time of physical objects.
4. This pre-scientific experience is grasped in terms of common categories like a thing's position and change of position, duration and change of duration, and the relation of cause and effect, terms and principles that are now parts of our common language.
5. These common categories yield the preconditions for objective knowledge, and any description of nature has to use these concepts to be objective.
6. The concepts of classical physics are merely exact specifications of the above categories.
7. The classical concepts—and not classical physics itself—are therefore necessary in any description of physical experience in order to understand what we are doing and to be able to communicate our results to others, in particular in the description of quantum phenomena as they present themselves in experiments;
8. Planck's empirical discovery of the quantization of action requires a revision of the foundation for the use of classical concepts, because they are not all applicable at the same time. Their use is well defined only if they apply to experimental interactions in which the quantization of action can be regarded as negligible.
9. In experimental cases where the quantization of action plays a significant role, the application of a classical concept does not refer to independent properties of the object; rather the ascription of either kinematic or dynamic properties to the object as it exists independently of a specific experimental interaction is ill-defined.
10. The quantization of action demands a limitation of the use of classical concepts so that these concepts apply only to a phenomenon, which Bohr understood as the macroscopic manifestation of a measurement on the object, i.e. the uncontrollable interaction between the object and the apparatus.
11. The quantum mechanical description of the object differs from the classical description of the measuring apparatus, and this requires that the object and the measuring device should be separated in the description, but the line of separation is not the one between macroscopic instruments and microscopic objects. It has been argued in detail (Howard 1994) that Bohr pointed out that parts of the measuring device may sometimes be treated as parts of the object in the quantum mechanical description.
12. The quantum mechanical formalism does not provide physicists with a ‘pictorial’ representation: the ψ-function does not, as Schrödinger had hoped, represent a new kind of reality. Instead, as Born suggested, the square of the absolute value of the ψ-function expresses a probability amplitude for the outcome of a measurement. Due to the fact that the wave equation involves an imaginary quantity this equation can have only a symbolic character, but the formalism may be used to predict the outcome of a measurement that establishes the conditions under which concepts like position, momentum, time and energy apply to the phenomena.
13. The ascription of these classical concepts to the phenomena of measurements rely on the experimental context of the phenomena, so that the entire setup provides us with the defining conditions for the application of kinematic and dynamic concepts in the domain of quantum physics.
Such phenomena are complementary in the sense that their manifestations depend on mutually exclusive measurements, but that the information gained through these various experiments exhausts all possible objective knowledge of the object.

Bohr thought of the atom as real. Atoms are neither heuristic nor logical constructions. A couple of times he emphasized this directly using arguments from experiments in a very similar way to Ian Hacking and Nancy Cartwright much later. What he did not believe was that the quantum mechanical formalism was true in the sense that it gave us a literal (‘pictorial’) rather than a symbolic representation of the quantum world. It makes much sense to characterize Bohr in modern terms as an entity realist who opposes theory realism (Folse 1987). It is because of the imaginary quantities in quantum mechanics (where the commutation rule for canonically conjugate variable, p and q, introduces Planck's constant into the formalism by pq − qp = ih/2π) that quantum mechanics does not give us a ‘pictorial’ representation of the world. Neither does the theory of relativity, Bohr argued, provide us with a literal representation, since the velocity of light is introduced with a factor of i in the definition of the fourth coordinate in a four-dimensional manifold (CC, p. 86 and p. 105). Instead these theories can only be used symbolically to predict observations under well-defined conditions. Thus Bohr was an antirealist or an instrumentalist when it comes to theories.
...

This is from the Wikepedia article on the CI:

...
The Copenhagen interpretation consists of attempts to explain the experiments and their mathematical formulations in ways that do not go beyond the evidence to suggest more (or less) than is actually there.

The Copenhagen interpretation was a composite statement about what could and could not be legitimately stated in common language to complement the statements and predictions that could be made in the language of instrument readings and mathematical operations. In other words, it attempted to answer the question, "What do these amazing experimental results really mean?"
...

I like the ensemble, or statistical, or probability interpretation.
From the Wikipedia article:

...
The Ensemble Interpretation, or Statistical Interpretation of quantum mechanics, is an interpretation that can be viewed as a minimalist interpretation; it is a quantum mechanical interpretation that claims to make the fewest assumptions associated with the standard mathematical formalization. At its heart, it takes the statistical interpretation of Max Born to the fullest extent. The interpretation states that the wave function does not apply to an individual system – or for example, a single particle – but is an abstract mathematical, statistical quantity that only applies to an ensemble of similar prepared systems or particles. Probably the most notable supporter of such an interpretation was Albert Einstein:

The attempt to conceive the quantum-theoretical description as the complete description of the individual systems leads to unnatural theoretical interpretations, which become immediately unnecessary if one accepts the interpretation that the description refers to ensembles of systems and not to individual systems.
—Albert Einstein
...

What's weird about observers, measurements, and our knowledge. All of these have objective physical referents.

But, as the quote said, they're "magic". CI excludes, a priori, the possibility that such things can be described by quantum mechanics.

Where does it say that? There's a quantum theory of measurement, isn't there? Is it necessarily at odds with the basic concerns of an instrumentalist approach?

If all predictions of quantum mechanics can be computed (in principle) from the wavefunction, then that means the quantum wavefunctions really do refer to "real propagations in some quantum underworld".

Yes, in some sense, because quantum correlations are, to a certain extent, predictable. But there's no way to know if it's revealing exactly what's happening in that underlying reality. The randomness of individual quantum phenomena suggest that it isn't. Wavefunctions, and various models of subatomic processes, are a synthesis of metaphysical heuristics and the record of objective instrumental behavior. MWI just goes too far in assuming that wavefunctions are in one-to-one correspondence with the 'quantum underworld' and so generates some physical absurdities. Assuming that wavefunctions describe the actual physical state of the underlying reality is an unwarranted stretch.

 

[jms5631]

BTW I am curious how Quantum Decoherence (QD) is interpreted in CI.
QD belongs to QM, it is independent of any interpretation

Quantum decoherence isn't explicitly referenced in CI, as it wasn't known about at the time of its formulation. However, its most salient charecteristic, that of the effective quantum- classical divide at a certain scale of largeness, is included in the copenhagen description. Effectively, this answers the same paradoxes as decoherence, as the copenhagen authors understood that macroscopic systems are normally classical. Schrodinger's cat couldn't be in a superposition according to the original formulation, because it has reached a certain level of complexity and obeyed classical mechanics. Decoherence offers the mechanism of environmental interaction as the explanation of this emergent classicality. I think decoherence, because of the effective similarites, is easily accommodated by Copenhagen. Consistent Histories really conceptually clarifies the role of decoherence and updates the copenhagen framework to incorporate it.

 

[Dmitry67]

I agree with you, there are no reasons why QD should not fit in the CI. CI must adopt it as QD is interpretation-independent.

But my question is what CI should do with the old, original wavefunction collapse? Before the discovery of QD measurements were explained by the collapse. But now...

Does CI use both QD and collapse, or collapse is replaced by QD in the modern CI, or something else?

(Personally I think CI is dead because of the discovery of QD as at the moment of discovery of QD Occams razor erased the wavefunction collapse)

 

[ThomasT]

... "our knowledge" is subjective, not objective.

Our collective or public knowledge is objective. This is the sort of knowledge that the CI is talking about. Materials, instruments, observational data -- that sort of thing.

But the main problem is that CI defines the behavior of the quantum particles based on 'measurement devices', while the measurement devices are themselves huge collections of particles. Such definition is deadly recursive and makes futile any potential attempt to axiomatize the QM (known as 6th Hilberts problem)

Axiomization would be nice. But it should be based on physical facts, shouldn't it? Shouldn't the historical and physical bases of the superposition principle, the projection postulate, and the Born rule be taken into account?

... there is only one cat in CI (and some other I) but *NOT* in MWI. It is a main point of the whole MWI: there are BOTH cats!

I see that as a problem with MWI.

Doesn't the MWI take the Schroedinger wave equation as the fundamental physical dynamic? If so, that seems to me to be on the right track -- because the fundamental dynamic governing the behavior of all physical systems in our universe would seem to be some sort of fundamental wave dynamic. But there is a problem with saying that quantum superpositions represent real physical states (eg., independent of observation, the cat is both dead and alive). Doesn't it make more sense to say that quantum superpositions represent no more and no less than the (probability weighted) possible experimental results?

I don't think that taking the Schroedinger wave equation or any wave behavior as the fundamental physical dynamic requires thinking of quantum states as real physical states. This goes back to the physical basis for quantum mechanics, which is the behavior of instruments, not of some 'reality' underlying that behavior.

Wasn't the motivation for Heisenberg's matrix mechanical representation to take the instrumental behavior as the point of departure for a theory of quantum phenomena? Isn't this more or less equivalent to the Schroedinger representation? If so, then what is the basic mathematical structure of quantum theory about? Is it (1) describing the actual physical behavior of a quantum underworld, or is it (2) relating the variety of our observations of instrumental behavior so as to produce the most accurate probabilistic predictions regarding future instrumental behavior? Certainly (2), but also, it seems, a bit of (1) insofar as it incorporates analogies from classical physics and our experience of macroscopic phenomena. But whereas (2) is complete in that everything that can be taken into account is taken into account, (1) has to be considered incomplete as long as the theory is unable to predict the behavior of individual quantum phenomena.

If the completeness of (1) is obviated by the quantum theory itself, then the MWI is obviated.

 

[Dmitry67]

ThomasT, I understand that it might be confusing as in MWI there are 2 realities. Max Tegmark had explicitlycalled them 'birds view' and 'frogs view'. Frogs view is an observer's view, in it reality is random. In the bird's view (God who observes all the realities at once) QM is deterministic and wavefunction is real.

The bird's view is used in GR, for example, when we talk about the 'baloon' analogy - looking at our world from the 'outside' or the universe.

So when you say 'is it real' you just need to specify if you are referring frog's or bird's view. In frog's view there is 'randomly' one cat, in bird's view there are (deterministically) both.

While in the birds view we talk about 'what had actually happened' (and on that level QM is not only deterministic, but also realistic. On the contrary, Frog's view, in the best Einstein traditions deals with only the observables, which are derived from the QM equations (Bird's view), but appear very different from them to the observers.

 

[Phrak]

There's some room for argument (as I find myself in a philosopy room, there's plenty more), but MWT is not quantum mechanics. It's missing a postulate.

 

[jms5631]

on a purely philosophical note, in addition to its problems with probability, I still can't fully appreciate the determinism offered by MWI. The branch *you* wind up in is entirely random, and impossible in principle to determine because there is no singular you, just many descendents on equal footing. From an individual perspective, that indeterminism is preserved and there is no escaping it- it seems forced to try to salvage determinism by extravagantly expanding reality and saying that the direct verification of deterministic phenomena is in principle beyond measurement, while maintaining the loathed indeterminism from any given single perspective.

 

[Phrak]

on a purely philosophical note, in addition to its problems with probability, I still can't fully appreciate the determinism offered by MWI. The branch *you* wind up in is entirely random, and impossible in principle to determine ...

Did you think science was the end-all, be-all of reason? Or is it limited in scope by the pivotal dependency on experimental evidence? Quantum mechanics already stresses the scientific formula. Qunatum mechanics is non-repeatable.

As the previously perceived requirement of experimental repeatability has reasonably undergone a metamorphosis to accommodate the statistical nature of quantum mechanical observation, how /or should unobservables, in principle, be reasonably accommodated?

If you figure it out, let me know

 

[The Dagda]

I agree with you, there are no reasons why QD should not fit in the CI. CI must adopt it as QD is interpretation-independent.

But my question is what CI should do with the old, original wavefunction collapse? Before the discovery of QD measurements were explained by the collapse. But now...

Does CI use both QD and collapse, or collapse is replaced by QD in the modern CI, or something else?

(Personally I think CI is dead because of the discovery of QD as at the moment of discovery of QD Occams razor erased the wavefunction collapse)

If its dead why then is it still the weapon of choice for the discerning physicist. Also what other interpretation has anything but the most tenuous evidence for its existence? None. This is still a money where your mouth is situation, and I hope you're right, sincerely, but hopes and dreams and what ifs are not science.

How anyone can use an axiom like Occam's razor to be the source of their conviction alone - and erroneously at that - All things not being equal, is beyond me.

 

[Dmitry67]

I still can't fully appreciate the determinism offered by MWI. The branch *you* wind up in is entirely random, and impossible in principle to determine because there is no singular you, just many descendents on equal footing.

I was also thinking about it when I read about MWI the very first time.
But there is no paradox: there are many *you*'s, so, in case of a cat, both *you* are saying: but look, why the cat is dead(alive), not vice versa? For *me* it is random! Why *my* consciousness has chosen that particular branch of reality?

And while for both *you* it is a total surprise, when you look at it from the bird's view, seeing all branches, the whole picture looks deterministic and perfectly symmetric

 

[Dmitry67]

If its dead why ...

You are answering my psycological comment, not my question itself.
And I know why. Because it is a BIG problem for CI.
Forget what I said about 'CI is dead'
Could you answer my question above?

 

[ThomasT]

So when you say 'is it real' you just need to specify if you are referring frog's or bird's view. In frog's view there is 'randomly' one cat, in bird's view there are (deterministically) both.

I often use the bird's eye view when speculating about things. The bird's eye view of MWI isn't supported by objective reality. The interpretation of the wavefunction that's the basic premise of MWI isn't supported by objective reality. It's a fiction which is obviated not only by our observations and by the quantum theory itself, but also by at least one interpretation which does basically the same thing as MWI, afaik, without all the fluff.

MWI might be an interesting exercise for some, but, imho, it's not very good natural philosophy.

 

[Dmitry67]

I often use the bird's eye view when speculating about things. The bird's eye view of MWI isn't supported by objective reality. The interpretation of the wavefunction that's the basic premise of MWI isn't supported by objective reality. It's a fiction which is obviated not only by our observations and by the quantum theory itself, but also by at least one interpretation which does basically the same thing as MWI, afaik, without all the fluff.

MWI might be an interesting exercise for some, but, imho, it's not very good natural philosophy.

Again, what objective reality?
Objective in frog's view or bird's view?
Note, we can not 'observe' the bird's view from insideof our world.

However, if you still insist, please provide additional details for:

The bird's eye view of MWI isn't supported by objective reality. The interpretation of the wavefunction that's the basic premise of MWI isn't supported by objective reality.

What contradictions do you see?

 

[ThomasT]

Again, what objective reality?
Objective in frog's view or bird's view?
Note, we can not 'observe' the bird's view from insideof our world.

However, if you still insist, please provide additional details for:

The bird's eye view of MWI isn't supported by objective reality. The interpretation of the wavefunction that's the basic premise of MWI isn't supported by objective reality.

What contradictions do you see?

Some bird's eye views are supported by objective reality. Some aren't. MWI's bird's eye view is one of those that isn't.

One reason that MWI isn't a good choice as a physical interpretation of quantum theory is because it leads to nonphysical results. But the main reason to reject it is because it starts from a premise that's contradicted by the theory itself.

Yes, CI says what quantum theory is -- essentially a theory of instrumental behavior. That's what an interpretation of the physical meaning of a theory is supposed to do. It's supposed to tell you what the theory is, not what it might be if it's fundamental dynamical equation were in one-to-one correspondence with the behavior of a reality underlying our objective apprehensions.

 

[The Dagda]

You are answering my psycological comment, not my question itself.
And I know why. Because it is a BIG problem for CI.
Forget what I said about 'CI is dead'
Could you answer my question above?

Yeah I can answer your question why is CI dead, when all there is to replace it is philosophy? Show me the money, or is just arm waving what you call science these days. Occam's razor is not a law of science, I've said that before and I'll say it again, nor is it applicable as no other interpretation has anything like the evidence CI has, it's not the mainstream because someone just thought they'd put up all the interpretations on a dart board and toss a dart over their shoulder. And making claims that CI is dead based on pure conjecture is an extraordinary leap of logic, or should I say non sequitur. I think the onus is on you to prove just how you came to that wildly speculative idea, n'est pas?

Amazing now your resorting to psychology, now you really have lost me, what makes you think your conceptions according to the planet you live on have anything to do with the way the Universe works at all, let alone make you an authority enough to judge what is correct on that basis?

Again, what objective reality?
Objective in frog's view or bird's view?
Note, we can not 'observe' the bird's view from insideof our world.

However, if you still insist, please provide additional details for:

The bird's eye view of MWI isn't supported by objective reality. The interpretation of the wavefunction that's the basic premise of MWI isn't supported by objective reality.

What contradictions do you see?

None, fancy talk doesn't pass for evidence and it never has. I'd say that's pretty much spot on. MWI has nothing but a philosophical basis.

And considering what it would take to prove it or even distinguish it from CI it probably always will.

 

[jms5631]

CI doesn't say objective reality does not exist(at the quantum level, remember its existence is not contested in the classical limit), its statement is rather :the nature of reality between measurements is unknowable and the only things that can be meaningfully discussed are the outcomes of measurements.

 

[Dmitry67]

Some bird's eye views are supported by objective reality. Some aren't. MWI's bird's eye view is one of those that isn't.

One reason that MWI isn't a good choice as a physical interpretation of quantum theory is because it leads to nonphysical results. But the main reason to reject it is because it starts from a premise that's contradicted by the theory itself.

You say, *some*, *nonphysical results* etc.
ANY EXAMPLES?
Please?

 

[Dmitry67]

Amazing now your resorting to psychology, now you really have lost me, what makes you think your conceptions according to the planet you live on have anything to do with the way the Universe works at all, let alone make you an authority enough to judge what is correct on that basis?

Listen, "CI is dead" was just a side note.
You use it to ignore my question.

The question was, now, after QD is discovered, how CI deals with it?
Do you have both old-style collapse plus QD in the modern edition of CI, or what?
You say, you don't like armwaving, this is good, answer a real question.

 

[jms5631]

Listen, "CI is dead" was just a side note.
You use it to ignore my question.

The question was, now, after QD is discovered, how CI deals with it?
Do you have both old-style collapse plus QD in the modern edition of CI, or what?
You say, you don't like armwaving, this is good, answer a real question.

Decoherence, as you know given that you use it in the context of an MWI framework, is not yet sufficient to solve the measurement problem on its own-with a few notable active areas of research that are the exceptions, namely Zurek's approaches. Decoherence provides the mechanism by which quantum interference effects are suppressed, but does not say why a pointer state is selected, nor what happens to the other elements of the wavefunction. There is no reason why collapse can't coexist with decoherence, and some versions of copenhagen have been revised to include it specifically. Personally, as I have said before, look to the consistent histories interpretation to provide a complete account of decoherence within a copenhagen style framework.

 

[The Dagda]

Listen, "CI is dead" was just a side note.
You use it to ignore my question.

The question was, now, after QD is discovered, how CI deals with it?
Do you have both old-style collapse plus QD in the modern edition of CI, or what?
You say, you don't like armwaving, this is good, answer a real question.

As was said above which I agree with. And also of course the standard explanation is the explanation that we find evidence for in both the double slit and Bell's inequalities. It may be a pain in the arse, and I hope something like the Bohmian model or MWI is in fact the correct model, but as yet it's too premature to throw the baby out with the bath water.

I've personally read a few papers of how say the Schrödinger Equation can be resolved from first principles or mathematically it is apt to consider the wave function real, but the problem is they rely on a priori assumptions, which as of yet have yet to be established; such as mentioned, if the wave function is indeed pictorially accurate, or there are indeed hidden variables or the wave function is resolved in other "worlds".

 

[Dmitry67]

1
Decoherence, as you know given that you use it in the context of an MWI framework, is not yet sufficient to solve the measurement problem on its own-with a few notable active areas of research that are the exceptions, namely Zurek's approaches. Decoherence provides the mechanism by which quantum interference effects are suppressed, but does not say why a pointer state is selected, nor what happens to the other elements of the wavefunction.

2
There is no reason why collapse can't coexist with decoherence, and some versions of copenhagen have been revised to include it specifically.

1 Correct, but do we need 2 things to fix 1 problem?
2 any links to the 'revised CI'?
I am really curious.

2 Dadga:
Ah, looks like I had misinterpreted your words.

 

[The Dagda]

1 Correct, but do we need 2 things to fix 1 problem?
2 any links to the 'revised CI'?
I am really curious.

2 Dadga:
Ah, looks like I had misinterpreted your words.

Having looked at the way I said it that is understandable, it's not an easy subject to clarify. I do like most, hope we are not hamstrung by CI, but I am not certain of anything and nor do I have any right to be being as I do not have the education to make overarching prepositions.

 

[ThomasT]

You say, *some*, *nonphysical results* etc.
ANY EXAMPLES?
Please?

What's all that branching stuff?
Hey, if anything I'm saying is wrong then let me know. I appreciate the discussions. They motivate me to look things up.

Are you a fan of MWI? If so, what questions do you think it answers? What problems do you think it solves?

I'm not against metaphysics, just metaphysics that seems unnecessary or not grounded in objective records.

For example, I think that there are some reasons to believe in multiverse ideas. But not the sort of multiverse produced by quantum wavefunction branching a la the MWI.

 

[Dmitry67]

1
What's all that branching stuff?

2
Are you a fan of MWI? If so, what questions do you think it answers? What problems do you think it solves?

3
For example, I think that there are some reasons to believe in multiverse ideas. But not the sort of multiverse produced by quantum wavefunction branching a la the MWI.

1
Let me repeat myself. MWI does not ADD anything new. It does not even postulate that these worlds exist. It doesnot posulate that ther world splits! MWI only DENIES the wavefunction collapse and (modern) MWI uses QD to show that QM without collapse is consistent with what we see.

So MWI does not need to justify the existence of alternative worlds and branching is a result of QD (which is interpretation-independent).

2
Yes, I am fan of MWI
MWI is very good for the axiomatization of physics
CI is in principle non-axiomatizable

3
Again, branching is not a part of MWI.
It is defined by QD and it is part of a standard, interpretation-independent QM

 

[jms5631]

1
Let me repeat myself. MWI does not ADD anything new. It does not even postulate that these worlds exist. It doesnot posulate that ther world splits! MWI only DENIES the wavefunction collapse and (modern) MWI uses QD to show that QM without collapse is consistent with what we see.

So MWI does not need to justify the existence of alternative worlds and branching is a result of QD (which is interpretation-independent).

Can you clarify your views on MWI somewhat further, I'm interested. I've never heard MWI without the definite positive statement that the other worlds need exist. I think without that postulate, you'd essentially have a Quantum Darwinistic, strict decoherence type approach.

 

[Dmitry67]

Can you clarify your views on MWI somewhat further, I'm interested. I've never heard MWI without the definite positive statement that the other worlds need exist. I think without that postulate, you'd essentially have a Quantum Darwinistic, strict decoherence type approach.

ok, so
1. We take a 'pure' QM, with deterministic evolution of a wavefunction (in terms of CI, 'between collapses)
2. Then we takeinto account Quantum Decoherence: http://en.wikipedia.org/wiki/Quantum_decoherence

So far it is interpretation-independent.
now the tricky part:

However, decoherence by itself may not give a complete solution of the measurement problem, since all components of the wave function still exist in a global superposition, which is explicitly acknowledged in the many-worlds interpretation. All decoherence explains, in this view, is why these coherences are no longer available for inspection by local observers. To present a solution to the measurement problem in most interpretations of quantum mechanics, decoherence must be supplied with some nontrivial interpretational considerations (as for example Wojciech Zurek tends to do in his Existential interpretation). However, according to Everett and DeWitt the many-worlds interpretation can be derived from the formalism alone, in which case no extra interpretational layer is required.

3. So, both cats exist (mathematically), they just can not interact. MWI just accepts it AS IS without adding any additional axioms. In that sense, it is 'shut up and calculate'.

CI claims that somehow (randomly) one cat becomes 'real' and another 'non-physical', claiming that nature had somehow made a choice between 2 paths. Another reality is nonphysical (and Stephen King's Langolieres have to eat all these 'abandoned' realities :) )

So in MWI alternative worlds exist not because MWI claims they exist, but because it does not contain an world-elimination axiom (collapse)! Oversimplifying, CI is MWI + 'langolieres' :)

 

[ThomasT]

(1)Let me repeat myself. MWI does not ADD anything new. It does not even postulate that these worlds exist. It doesnot posulate that ther world splits! MWI only DENIES the wavefunction collapse and (modern) MWI uses QD to show that QM without collapse is consistent with what we see.

So MWI does not need to justify the existence of alternative worlds and branching is a result of QD (which is interpretation-independent).

(2)Yes, I am fan of MWI
MWI is very good for the axiomatization of physics
CI is in principle non-axiomatizable

(3)Again, branching is not a part of MWI.
It is defined by QD and it is part of a standard, interpretation-independent QM

(1)Ok, then it comes down to the physical meaning of 'wavefunction collapse'. If wavefunction collapse objectively refers to the qualitative behavior of instruments, then there's no denying that of the various probability weighted, mutually exclusive possibilities described by the wavefunction, only one will correspond to the instrumental configuration for any single trial in an experimental run -- and that's the physical meaning of wavefunction collapse.
(The fact that only one possible result per trial is recorded is how we know that the possibilities represented in the formalism are, physically, mutually exclusive -- ie., that the mathematical representation doesn't mean that they all physically 'exist' simultaneously.)

The results of individual trials can't be predicted because the quantum theory of measurement is an incomplete description of the physical processes underlying the instrumental behavior. Is a complete description of the underlying processes even possible? The CI says no because the basic quantum hypothesis, the existence of a fundamental quantum of action, defines a relationship between canonically conjugate observables which limits what can be objectively demonstrated (ie., what can be known via instrumental behavior).

The MWI, however, starts by assuming that quantum wavefunctions and quantum superpositions are complete descriptions of the underlying processes (that quantum states are real physical states), even though we know that those wavefunctions and superpositions predict random results wrt individual trials. The result of this is that the MWI (even if, as you say, it doesn't generate a metaphysical picture that seems nonsensical) leaves us no better off, objectively, than simply not extending the minimalist interpretation of the formalism in the first place. This is why I think that the MWI is, and must be, an unproductive approach toward a better understanding of the interactions, etc., involved in quantum measurement processes.

(2)Are there any other approaches to axiomatization of quantum theory, besides MWI, that seem promising?

(3)A completely interpretation-independent qm (ie., if quantum wavefunctions were taken to be devoid of any physical meaning) wouldn't be of much use, would it?

 

[Hurkyl]

(The fact that only one possible result per trial is recorded is how we know that the possibilities represented in the formalism are, physically, mutually exclusive)

We can agree that "only one possible result per trial is recorded", but your next statement doesn't follow. For a 'realistic' measuring device governed by the Schrödinger equation, you would be left with a measuring device in a superposition of states, each one recording only one possible result.

 

[Phrak]

what does 'physical' mean, or 'physically' in this case?

I wouldn't call the many world theories valid physical theories, in the sense in which the word physical has commonly been used, anymore than quantum mechanics is a valid theory of physics verified by repeatable results --before this idea of repeatability was extended to include statistical repeatability of a new kind.

Hurkyl, I'm trying to think of a few examples, by comparison, of things that are taken as physical, but are canonically unmeasurable. I was thinking virtual particles would do, but there's the Cramer(sp?) force.

 

[Dmitry67]

(1)Ok, then it comes down to the physical meaning of 'wavefunction collapse'. If wavefunction collapse objectively refers to the qualitative behavior of instruments, then there's no denying that of the various probability weighted, mutually exclusive possibilities described by the wavefunction, only one will correspond to the instrumental configuration for any single trial in an experimental run -- and that's the physical meaning of wavefunction collapse.
(The fact that only one possible result per trial is recorded is how we know that the possibilities represented in the formalism are, physically, mutually exclusive -- ie., that the mathematical representation doesn't mean that they all physically 'exist' simultaneously.)

The results of individual trials can't be predicted because the quantum theory of measurement is an incomplete description of the physical processes underlying the instrumental behavior. Is a complete description of the underlying processes even possible? The CI says no because the basic quantum hypothesis, the existence of a fundamental quantum of action, defines a relationship between canonically conjugate observables which limits what can be objectively demonstrated (ie., what can be known via instrumental behavior).

The MWI, however, starts by assuming that quantum wavefunctions and quantum superpositions are complete descriptions of the underlying processes (that quantum states are real physical states), even though we know that those wavefunctions and superpositions predict random results wrt individual trials. The result of this is that the MWI (even if, as you say, it doesn't generate a metaphysical picture that seems nonsensical) leaves us no better off, objectively, than simply not extending the minimalist interpretation of the formalism in the first place. This is why I think that the MWI is, and must be, an unproductive approach toward a better understanding of the interactions, etc., involved in quantum measurement processes.

(2)Are there any other approaches to axiomatization of quantum theory, besides MWI, that seem promising?

(3)A completely interpretation-independent qm (ie., if quantum wavefunctions were taken to be devoid of any physical meaning) wouldn't be of much use, would it?

2. I don't know
personally I am am very conviced by the Max Tegmark's approach
http://en.wikipedia.org/wiki/Max_Tegmark
http://arxiv.org/abs/0704.0646

3. I agree

1. Form the Bird's view wavefunctions is a complete description of a system
But it does not give us a chance to predict the results of individual measurements, from our frog's perspective.

 

[Dmitry67]

Hurkyl, I'm trying to think of a few examples, by comparison, of things that are taken as physical, but are canonically unmeasurable. I was thinking virtual particles would do, but there's the Cramer(sp?) force.

Besides the interior of the balack holes, I am thinking about the statement that (if Omega is low enough or there is a repulsive gravity, but we are almost sure about it) spacetime of our Universe is 'open' and space in infinite.

While any statements about the local curvature are physical (falsifiable in the classical sense) the claim 'our universe in infinite' is non-falsifiable because you can not fly to the infinity to prove, that there is no 'edge of the universe', even in principle.

I think this is a very good example.

You can not prove experimantally that 'our universe is infinite'
But an opposite claim, that it is finite, is also inverifiable, but it is also weird and it introduces a strange object, 'an edge of the space'

In MWI, you can notprove experimentally, that other worlds exist.
However, an opposite claim, that they don't exist, is also unverifiable, but it introduces the strange process called wavefunction collapse (other-worlds-elimination)

 

[Hurkyl]

what does 'physical' mean, or 'physically' in this case?

Oh! I had completely glossed over that, just assuming ThomasT was referring to a definite outcome.

I feel silly, because I missed a chance to deliver one of my favorite arguments! It's clear that we can construct a 'real', 'physical' experiment to detect whether or not our measuring devices record only one result per trial... but what sort of experiment can detect whether or not the outcome is definite or indefinite?


If you're looking for analogies, in my mind it's very much like the notion of position (or velocity, or charge, or whatever): things that aren't preserved by physical symmetries, so they're not really 'physical' notions, but yet we keep them around because they make everything simpler, and simply deal with the ambiguity. (e.g. by measuring charge as a multiple of a fixed reference charge)

I find CI very much akin to doing Newtoniam mechanics with the restriction that the only coordinate charts you're ever allowed to use are ones where you're at the origin, and the x, y, and z axes are all forward, left, and up, respectively. You can do it, it eliminates some non-physicality, and it more closely resembles what you actually observe... but yet it's a silly restriction to adhere to.


(P.S. I am of the opinion that things like a CNOT gate are perfectly good toy models of a measuring device. And with a CNOT gate, you can experimentally demonstrate that you don't get definite outcomes)

 

[ThomasT]

Oh! I had completely glossed over that, just assuming ThomasT was referring to a definite outcome.

I thought I was referring to a definite outcome.

I feel silly, because I missed a chance to deliver one of my favorite arguments! It's clear that we can construct a 'real', 'physical' experiment to detect whether or not our measuring devices record only one result per trial... but what sort of experiment can detect whether or not the outcome is definite or indefinite?

Aren't all recorded results definite outcomes? Or is there a difference between results and outcomes?

(P.S. I am of the opinion that things like a CNOT gate are perfectly good toy models of a measuring device. And with a CNOT gate, you can experimentally demonstrate that you don't get definite outcomes)

What do you mean by 'definite outcomes'?

 

[ThomasT]

You can not prove experimantally that 'our universe is infinite'
But an opposite claim, that it is finite, is also inverifiable, but it is also weird and it introduces a strange object, 'an edge of the space'.

The boundary of a finite universe doesn't seem so strange if you think of that boundary as a wavefront expanding in a medium. Observations suggest that our universe is expanding, which suggests that it had a beginning, which implies that it's finite -- even if fapp it's infinite.

In MWI, you can not prove experimentally, that other worlds exist.
However, an opposite claim, that they don't exist, is also unverifiable, but it introduces the strange process called wavefunction collapse (other-worlds-elimination)

'Wavefunction collapse' refers to the registration of a detection, the appearance of a bit of data, the end of an interval that might define a trial where no detection is recorded, or the end of an experimental run. The term wasn't invented to eliminate "other worlds", at least afaik it wasn't.

"Other-worlds-elimination" isn't a problem for interpretations that don't produce them. The "other worlds" never appear. The simplest explanation for this is that they're interpretational fictions.