Problems with Many Worlds Interpretation


by t_siva03
Tags: copenhagen, intepretation, quantum, universe, worlds
t_siva03
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#19
Aug21-11, 10:45 AM
P: 19
Brian Greene in his book the Hidden Reality claims that in fact MWI and CI may actually make different predictions, although I must admit I do not really understand his example or explanation.

Greene argues that in Many Worlds, the wave-function have multiple spikes, corresponding to different possible outcomes. He reasons that these "spiked waves" might interfere, causing an observable interference pattern, which would disagree with Copenhagen wave function collapse.
fleem
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#20
Aug21-11, 06:26 PM
P: 461
IMO non-local hidden variables explains it better than MW (which is weird) and Copenhagen (which requires some degree of "shut up and calculate"--hardly an explanation), because non-local hidden variables seems an inevitable side-effect of Mach's principle, for one thing. Why do the vast majority of scientists seem to consider Mach's principle only a quaint product of yesteryear rather than the inevitable (and profoundly fundamental) trait of reality that it is?
BruceW
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#21
Aug21-11, 06:44 PM
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Mach's principle doesn't mean information can travel faster than the speed of light, if that is what you're suggesting.
fleem
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#22
Aug22-11, 07:50 AM
P: 461
Quote Quote by BruceW View Post
Mach's principle doesn't mean information can travel faster than the speed of light, if that is what you're suggesting.
The same could be asked of someone defending the existence of entanglement. The problem, here, is that we keep applying concepts we learned solely from observing classical mechanics (the large-scale behavior of many QM events) to the behavior of QM events. Those concepts work to a point but eventually they must be modified or maybe dropped altogether.

Its like someone observing a swarm of bees from a distance and concluding that "all objects look like clouds, can change shape, and make a rushing sound". Then when that person gets the opportunity to walk closer to study a single bee, they insist that the bee is yet another type of cloudy thing that can change shape and makes a sort-of rushing sound.

Likewise we learned the details of the concepts of continuum (real numbers), dimension, time, space, speed, etc. solely from observing the "average" behavior of a lot of particles, but we continue to insist those concept will also be axioms in our understanding of particle-particle interactions. Now I'm not saying those concepts have no part in QM. Obviously they are very useful. but we must be leery of assuming they are axioms.

For all we know, everything is entangled, and that is Mach's principle. (And who is the observer that decohere's that entaglement?)
xts
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#23
Aug22-11, 08:33 AM
P: 882
Quote Quote by fleem View Post
Why do the vast majority of scientists seem to consider Mach's principle only a quaint product of yesteryear rather than the inevitable (and profoundly fundamental) trait of reality that it is?
[...] The same could be asked of someone defending the existence of entanglement.
[...] For all we know, everything is entangled, and that is Mach's principle.
Because vast majority of scientists are Occamian conservatists.
Bell+Aspect convinced most of us that non-locality is unevitable. That was painful (esp. for Einstein) but to stay honest we had to adopt that into our Weltanschauung. But we definitely prefer view with just two entangled particles over the spaghetti plate of whole Universe entangled.
K^2
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#24
Aug22-11, 12:41 PM
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MWI is the spaghetti view. Clearly, a big fraction, if not majority, do prefer to view the universe as fully entangled. Problem is that it doesn't help with GR vs Mach's Principle. It makes it worse, if anything.

We either need a linear theory that explains gravity or a non-linear theory that linearizes to RQFT. Former is preferable, but later is far more likely.
yoda jedi
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#25
Aug22-11, 05:10 PM
P: 381
Quote Quote by BruceW View Post
J I reckon experiments into wavefunction collapse (if indeed it does collapse) will probably bring about some new extension to QM theory that is neither CI nor many-worlds.
right and nonlinear quantum mechanics (unlike standard quantum mechanics, a linear one).


.
haael
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#26
Aug24-11, 07:01 AM
P: 397
MWI, among other things, is the only interpretation that precisely explains when the "measurement" happens. This might be used to differentiate between other interpretations. For instance, if we found an inanimate device that can perform "measurement" (wavefunction collapse), being still consistent with Copenhagen, it could rule out MWI, which openly says that only the living human consious being can "measure" the world.
Delta Kilo
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#27
Aug24-11, 10:11 AM
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Quote Quote by haael View Post
...MWI, which openly says that only the living human consious being can "measure" the world.
I don't think it does though. Some people might say that but a lot of other people (including me) would not agree.
IMHO dragging consciousness into the discussion does not provide any clear answers but brings a whole new can of worms, starting with the definition.
xts
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#28
Aug24-11, 10:29 AM
P: 882
Quote Quote by Delta Kilo View Post
IMHO dragging consciousness into the discussion does not provide any clear answers but brings a whole new can of worms, starting with the definition.
I don't agree. The worms spread out only if you take 'collapse' / 'world forking' / 'measurement' as something objectively real.
If you take them just as a mathematic trick, transforming (previously unknown) wavefunction into (known) experiment outcome, you introduce consciousness anyway, implicitely brought in by means of words 'known'/'unknown'.
But, if you take this as a trick only, you don't need to bother 'whos consciousness'. It depends on context and has no physical meaning. From Cat's perspective collapse/measurement/worldfork occured at the moment when gun fired or not, from Schrödinger's perspective it occured when he opened a cage, from my perspective - when I read his article. The outcome in all scenarios is the same.

Such approach is consistent even with solipsistic interpretation - it is only mine mind able to cause the collapse - you, cat and Schrödinger, are just more complicated apparata entangled on the way before the information reaches me.

Everett did not want to occur solipsist. So he invented more democratic version: every conscious creature causes world-fork. If he was a Buddhist, probably even mosquitoes would be able to cause collapses.

Fortunately it is not a wormbox - you may set a border wherever you like, just keeping your consciousness on one side, and interfering photons on other.
Delta Kilo
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#29
Aug24-11, 08:53 PM
P: 269
Quote Quote by xts View Post
From Cat's perspective collapse/measurement/worldfork occured at the moment when gun fired or not, from Schrödinger's perspective it occured when he opened a cage, from my perspective - when I read his article. The outcome in all scenarios is the same.
From camera film perspective - when the camera set on timer took a picture of the cat, from gun powder perspective - when it was ignited (or not) and from atom perspective - when it has decayed (or not). What's so special about consciousness, apart form inflated ego?

Quote Quote by xts View Post
Fortunately it is not a wormbox - you may set a border wherever you like, just keeping your consciousness on one side, and interfering photons on other.
We already have such a barrier between micro and macro and it is deeply unsatisfying. At least with micro vs. macro division one can attempt to approach it with physically meaningful terms like the number of degrees of freedom, entropy, information etc. Still leaves a lot of gray areas but it's a start.
On the other hand consciousness is so vague, there is simply nothing to go on. It's a dead end.
mitchell porter
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#30
Aug24-11, 09:28 PM
P: 748
There are several people in this thread saying "MWI gives the same predictions as Copenhagen Interpretation". This is a very questionable proposition. The reason is that if you get your MWI probabilities the obvious way, by counting the branches, you typically get the wrong predictions. To get the right predictions, you have to reproduce the Born rule, and that means that branches have to "count" in proportion to the square of their amplitude. But if all branches are equally real, the defining claim of MWI, why would some count for more than others?
Delta Kilo
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#31
Aug24-11, 10:26 PM
P: 269
Quote Quote by mitchell porter View Post
The reason is that if you get your MWI probabilities the obvious way, by counting the branches, you typically get the wrong predictions.
Because the 'obvious way' is obviously wrong. It's the same fallacy as saying the probability of winning a lottery is 50/50: either you do of you don't.

A cat can be alive in a lot of different ways and it can be dead in a lot of other different ways. To get the accurate probability you'd have to count them all very carefully. If you do I bet you would get Born rule at the end. And yes, the resulting branches (I hate this term) would have different 'thickness'.
mitchell porter
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#32
Aug24-11, 10:55 PM
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Quote Quote by Delta Kilo View Post
A cat can be alive in a lot of different ways and it can be dead in a lot of other different ways. To get the accurate probability you'd have to count them all very carefully. If you do I bet you would get Born rule at the end. And yes, the resulting branches (I hate this term) would have different 'thickness'.
So let's get this clear. You believe that there will be a derivation of correct, Born-rule probabilities within MWI, in which the probabilities come entirely from counting branches / worlds / (what term do you prefer?)? You won't have to attach unequal weights to the branches, and make some branches count for more than others, even though they are all supposed to be equally real?
Delta Kilo
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#33
Aug25-11, 01:09 AM
P: 269
Quote Quote by mitchell porter View Post
So let's get this clear. You believe that there will be a derivation of correct, Born-rule probabilities within MWI, in which the probabilities come entirely from counting branches / worlds / (what term do you prefer?)? You won't have to attach unequal weights to the branches, and make some branches count for more than others, even though they are all supposed to be equally real?
The 'branches' are emergent macroscopic phenomena. They are also arbitrary to a degree. You can choose to threat the entire section of the multiverse where the cat is dead as a single branch. Or you can split it in many branches where the cat is dead in many different ways. Of course the 'thickness' of these branches would be different. The trouble is as you go down the path of splitting branches they become less macroscopic and more fuzzy at the edges until they disappear entirely. So yes, counting them accurately is a challenge.

And to answer the question,yes, some branches will count more than the others even though all of them are real. I understand 'equally real' to mean that a particular branch does not become 'more real' simply because I just happen to be in it. For example, I might personally witness a quantum experiment to produce an outcome which is, according to all computations, extremely unlikely. This outcome is of course as real as it gets because I just saw it happening but it is still in some sense 'less real' than the other more likely outcome.

This is in contrast with a) objective collapse where the unobserved branches are constantly cut off and thrown away without a trace or b) Bohmian mechanics where the pilot wave completely describes the entire multiverse but particle trajectories select one true 'real' path through it or c)various consciousness-causes-collapse theories where everything I see is real and everything else is a figment of my imagination.
mitchell porter
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#34
Aug25-11, 01:19 AM
P: 748
Quote Quote by Delta Kilo View Post
The 'branches' are emergent macroscopic phenomena. They are also arbitrary to a degree. You can choose to threat the entire section of the multiverse where the cat is dead as a single branch. Or you can split it in many branches where the cat is dead in many different ways.
Well, presumably there is a non-"arbitrary" part of the multiverse which actually corresponds to me-here-now, having the specific experience I seem to be having?
Quote Quote by Delta Kilo View Post
And to answer the question,yes, some branches will count more than the others even though all of them are real. I understand 'equally real' to mean that a particular branch does not become 'more real' simply because I just happen to be in it. For example, I might personally witness a quantum experiment to produce an outcome which is, according to all computations, extremely unlikely. This outcome is of course as real as it gets because I just saw it happening but it is still in some sense 'less real' than the other more likely outcome.
In what sense is it less real? What you mean is that it is less probable. Which ought to mean that it is less frequent in the multiverse. And that is what we are trying to establish - whether MWI can show that branches which, empirically, ought to be more frequent, are actually more frequent in MWI, according to whatever recipe it provides for parsing the mathematical wavefunction of the universe as a physical multiverse of coexisting branches or worlds.
Delta Kilo
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#35
Aug25-11, 03:04 AM
P: 269
Quote Quote by mitchell porter View Post
Well, presumably there is a non-"arbitrary" part of the multiverse which actually corresponds to me-here-now, having the specific experience I seem to be having?
Well, yes and no. I'd say there is a whole bunch of you in the multiverse, having all sorts of experiences simultaneously. I would say that your experiences are macroscopic and the boundaries between them, between you-here-now and another-you-there-then are kind of fuzzy.

At every moment, all sorts of quantum superpositions get decohered around you one way or the other. Say if a photon just landed on your forehead, it won't matter that much to you whether it was horizontally or vertically polarized, your experiences will not be affected and you-here-now branch will include both alternatives. On the other hand if a stray cosmic ray hit a cell in a DRAM chip and crashed your computer, one of you would never read this message so you-here-now branch would split and diverge at that point. But between these two extremes there would be a gray area where it would be very hard to tell whether your experiences are sufficiently different to count it as a split.

Quote Quote by mitchell porter View Post
And that is what we are trying to establish - whether MWI can show that branches which, empirically, ought to be more frequent, are actually more frequent in MWI, according to whatever recipe it provides for parsing the mathematical wavefunction of the universe as a physical multiverse of coexisting branches or worlds.
Yes, I agree, this is a very good question to ask. I also admit that current answer is not entirely satisfactory: some people dismiss it by saying since it is the same old formalism it produces the same answers and doesn't require a separate proof, other people say they have proved it and yet other people say that all those proofs rely on circular arguments and are therefore invalid. I tried to follow these arguments and got seriously bogged down, so I don't have an opinion one way or the other but my gut feeling is that such proof should be possible.
mitchell porter
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#36
Aug25-11, 03:49 AM
P: 748
Quote Quote by mitchell porter View Post
Well, presumably there is a non-"arbitrary" part of the multiverse which actually corresponds to me-here-now, having the specific experience I seem to be having?
Quote Quote by Delta Kilo View Post
Well, yes and no. I'd say there is a whole bunch of you in the multiverse, having all sorts of experiences simultaneously. I would say that your experiences are macroscopic and the boundaries between them, between you-here-now and another-you-there-then are kind of fuzzy.
What you just said basically denies that there are any facts about what gets observed. The problem is not when you say there are duplicates or near-duplicates of me. The problem is when you say that the difference between one copy of me and another copy of me isn't absolute. I know some MWI fans are in love with the continuity of the wavefunction and consider it a virtue to talk about everything blending into everything else, but this is just incompatible with the specificity of observed reality.

Again, the problem is not when you say, you-here-now are observing one thing, but you-in-the-universe-next-door are observing something else; the problem is when you say that there is no objective difference between me-here-now and me-in-the-universe-next-door, that whether there is one person or two is a matter of convention, and that the facts about what happens to me here are not definite. This is a perfect example of a metaphysical belief (a "block multiverse" with no objective boundaries) overriding a basic fact about reality - the definiteness and particularity of anything that exists.

From experience :-) I find it extremely hard to get this point across to someone who has decided that they can think about themselves (or is it just about other people?) in this vague way. For example, sometimes there's a slippage between the incomplete and uncertain knowledge that one has of one's own conscious state, and the fundamental vagueness that is supposed to characterize the different branches of the wavefunction. That is, I might want to argue that you are definitely in a particular conscious state, and so, if this corresponds to a particular quantum state of your brain, then MWI must, with no ambiguity, say that that exact state is one of the substructures of the wavefunction which corresponds to a "world" or a "branch". But then I will be told that I don't know all the details of my conscious state, or that not all the physical details of my brain state matter for my conscious state, and this then provides the MWI advocate with an excuse for insisting that their theory doesn't have to have definite, exactly bounded branches, not even in principle.

So: what you are saying is ridiculous, because you are denying that there are definite facts at any level about what is happening to you. Everything blends into everything else, no quantum basis or state factorization is objectively preferred, and your theory (MWI) contains nothing that corresponds to specific realities.

Quote Quote by Delta Kilo View Post
At every moment, all sorts of quantum superpositions get decohered around you one way or the other. Say if a photon just landed on your forehead, it won't matter that much to you whether it was horizontally or vertically polarized, your experiences will not be affected and you-here-now branch will include both alternatives. On the other hand if a stray cosmic ray hit a cell in a DRAM chip and crashed your computer, one of you would never read this message so you-here-now branch would split and diverge at that point. But between these two extremes there would be a gray area where it would be very hard to tell whether your experiences are sufficiently different to count it as a split.
My point is that, whether or not it is "hard" to use, MWI must contain an objective criterion which (even if only in principle) tells exactly what the different "observer substructures" are in any given wavefunction, because that is the bottom line when it comes to relating reality to MWI. MWI isn't supposed to be just a holy dogma, it's supposed to be a theory of the physical world, and as such, the entities appearing in the theory have to have some relationship to the entities appearing in reality. You tell me that I can't take the appearances of external reality for granted, that this is just a brain state which is in a tensor product with a superposition of external states, some of which don't match what the brain state says? Fine. But then you tell me that MWI does not provide, not even in principle, a definite decomposition of the quantum state of my brain into basis states corresponding to distinct observer states? At that point, the last contact between reality and the ontology of the theory has been broken, and we are dealing with some sort of muddled dogma that doesn't even make comprehensible statements.

Hopefully I have made my point by now: FOR MWI TO WORK, THERE MUST AT SOME LEVEL BE AN EXACT AND OBJECTIVE WAY TO ANALYSE THE WAVEFUNCTION OF THE UNIVERSE INTO A PREFERRED SET OF SUBSTRUCTURES. And of course this is precisely what people who don't like the idea of splitting with respect to a preferred basis, etc, are trying to avoid. You don't have to have splitting - you can keep your transcendently unified wavefunction if you insist - but then you must specify definite substructures. I don't know what. Local maxima in configuration space. Some more abstract notion from fiber-bundle theory. They don't even have to be something whose details you can exactly specify in practice. It is often possible to prove that an equation has solutions, even if the exact solutions cannot be exhibited in detail. In the same way, all we need is something that is conceptually exact. You must be able to state precisely what sort of thing in the wavefunction corresponds to the specific realities which make up the whole of experience. Is it a tensor factor? Is it an infinite-dimensional wavelet? I don't know; this is your problem, not mine.

Quote Quote by Delta Kilo View Post
Yes, I agree, this is a very good question to ask. I also admit that current answer is not entirely satisfactory: some people dismiss it by saying since it is the same old formalism it produces the same answers and doesn't require a separate proof, other people say they have proved it and yet other people say that all those proofs rely on circular arguments and are therefore invalid. I tried to follow these arguments and got seriously bogged down, so I don't have an opinion one way or the other but my gut feeling is that such proof should be possible.
It's not the same formalism, since the Born rule has been removed. Obviously it's a cheat if MWI can only work by "postulating" the Born rule; if there are many worlds, there should be a natural way of counting them, or a natural measure on them, and the Born-rule probabilities should descend from that. But the insistence that it's OK to be vague about what a world or a branch is, insulates MWI from ever having to face this test: we can't count the branches, if there's no objective notion of what a branch is!


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