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Problems with Many Worlds Interpretation 
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#37
Aug2511, 06:27 AM

P: 882

I cannot however allow single atom to be an observer. Some experiments show that atoms evolve sometimes like they are in mixed states. So atoms must be on the quantum side of the border. So I must set a boundary somewhere in this large span of complexity: between my consciousness and single atoms. Everett chose anyone's consciousness as that level of complexity, which is safe. Personally I prefer solipsistic view  not because I am a solipsist, but as it reflects my treating the collapse as an information process rather than physical reality. 


#38
Aug2511, 07:11 AM

P: 424

Rather, out senses are special since they always measure position and time observables. We need special devices that convert other observables into our sensory signals. However, there might be some space aliens that live in a momentumenergy space and the concept of being in one point at a time is so wild for them as being in 2 different points for us. What I mean: it might be our construction that give us illusion of the world having collapsed wavefunction. 


#39
Aug2511, 07:37 AM

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P: 2,470

Doesn't matter. If the state you are measuring is not in the eigen state of the operator you are measuring, there will be a collapse or world splitting, depending on interpretation.



#40
Aug2511, 08:20 AM

P: 882




#41
Aug2511, 09:30 AM

P: 273

Besides you say your consciousness is special, do you have any reason at all to suspect that other people's consciousness is different in this respect? How is this not solipsism? 


#42
Aug2511, 09:58 AM

P: 461

I've no strong opinion on which is the most useful way to look at things (MWI, CI, or maybe a form of nonlocal hidden variables), and this post is not to refute anyone here, but I've a few comments/questions:
1. 'I think, therefore I am', is the only deductive proof, and therefore the only 100% proven thing. All other things are "proven" inductively and therefore not quite 100% proven. So solipsism (unfortunately) can't be discounted. However, IMO its extremely improbable. I think we're agreed on this. 2. I think one reason some embrace MW is that we are trying to make QM events consistent with classical probability theory so that we can "understand" QM events. But note that we learned classical probability theory by studying strictly classical phenomena. (like creating axioms to describe a swarm of bees and then trying to apply those axioms to a single bee) 3. Isn't CI just "shut up and calculate" and the word "superposition" without a rigorous definition? (This is an honest question). If so, then embracing that view will never allow us to answer the important philosophical question (IMO), "Is there true randomness (pure chaos) in the universe, or is there only pseudorandomness and no chaos?". Frankly I've vacillated between the two over the years. My latest guess is that the simplest reality is one in which all events are possible (chaos)but then perhaps only nonparadoxical events were able to evolve to higher order, like conservation of number, etc. So maybe the randomness we see in lowenergy events is a glimpse of that chaos. 


#43
Aug2511, 10:44 AM

P: 882




#44
Aug2511, 10:47 PM

P: 273

I guess the term MWI has evolved and means different things to different people. To me it means basically: a) The same quantum laws apply from micro all the way to the entire universe. b) The evolution is unitary, there is no collapse. c) The wavefunction describes the reality. (all parts of the wavefunction are equally real) As I said, I find the notions of 'splits' and 'branches' less than helpful. These were introduced to illustrate some concepts in a handwavy sort of way.So what is a branch, how does it come to be? Say you have a bunch of systems (observers) A,B,C interacting with common environment E. Say the current state is described as A>B>C>E>. Say at some point you introduce a particle P in a state of superposition P1>+P2>. Decoherence tells us that the system quickly evolves from (P1>+P2>)A>B>C>D>E> into something like (P1>A1>B1>C1> +P2>A2>B2>C2>)E>, where Ai,Bi,Ci, are new states of A,B,C after interacting with Pi in the preferred basis dictated by the the environment, the environment E has 'soaked up' all the offdiagonal terms, but it is so big that it appears unchanged for all practical purposes. Once this happens the two parts evolve independently and do not affect each other in any way. So we just say they represent two separate branches that he world around us has split into. Contrary to what is usually assumed, branches do not span the entire multiverse. No matter how many systems are split, there is always a bigger environment out there that 'does not care'. And the change is gradual, the branch just 'fades away' sort of, the information about the event that has caused a split becomes more and more thinly spread and less and less relevant. We never actually look at the exact details of it, we just assume it happens somewhere in the environment which is too big and complex to be analyzed in details. These kind if things (observations of particles in superposition by macroscopic observers) are happening all the time which causes the world to be constantly split every which way. The question "which branch does this point belongs to" does not make sense, one has to specify the event that has caused a split. Similarly, the notion of youherenow branch does not make sense because as a system, youherenow does not have welldefined boundaries. Presumably it includes your present state of mind including all the events that have influenced it up until now, which is quite a lot to ask about. You would have to define carefully which events and branches are relevant to the definition of youherenow and which are not. NEVERTHELESS, having said all that, if you ask the right questions you will most certainly get definite answers. If you ask about the outcome of a particular quantum measurement, as evidenced by the state of a particular macroscopic observer, you will find the boundaries between the branches to be pretty sharp and welldefined, and what's more, the branches defined by different observers will line up, that is the observers will generally agree on what they saw. Also they won't change if you move the boundary between the system and the environment back and forth. And if you perform a number of experiments you can then define intersections and unions of the branches etc. You can then use the set of perceived outcomes to label the branches, and the branch labeled with outcomes that you yourself perceived will be the branch containing that elusive youherenow. To sum up, branches are just noninterfering terms in a wavefunction (or chunks of reality described by those terms if you wish). They only make sense when the wavefunction can be written in a particular way with the understanding that they are statistical approximations of the real thing. They are just tools to be used when appropriate. Regarding the Born rule, as I understand, the derivation aims to show that: a) equally probable branches have the same magnitude and b) the sum of magnitudes squared after the split equals magnitude squared before the split. Posing that some branches are 'equally probable' is what usually raises questions, and the jury appears to be still out on whether or not they were answered satisfactory. 


#45
Aug2711, 04:26 AM

P: 751

For confused readers who might not follow what's happening this discussion, I want to emphasize that I am not just saying "I see one thing in one universe and another thing in another universe". That is not the "relativeness" that I am talking about. Delta is confirming that MWI does not offer a unique division of the multiverse into universes, or even a unique division of the 'local multiverse' into copies of me. You can chop things up however you like and they are all equally valid. You can take the quantum density matrix of the left hemisphere of my brain, treat it in terms of the position basis, and then you can still think about my right hemisphere as being in a superposition, or you can look at it in terms of the position basis, or the momentum basis, or any basis you like; and every one of these decompositions of the local part of the wavefunction of the universe is apparently equally valid. The consequence is that there is no answer to the question 'in this branch, what is my brain doing?', because there are multiple choices of quantum basis for different parts of my brain. Would it be a better world if there was a wider appreciation, among fans of physics, of just how absurd MWI is? It's hard to say, because the true absurdity depends on mildly technical details like those I mention above, and yet the standard understanding of what MWI says is just, 'there are parallel worlds', and that's not an intrinsically absurd notion. Somehow it needs to be conveyed that MWI is a nice idea, or at least a superficially valid idea, but the attempt to develop the details of that idea, within the actual context of QM, produces reams of nonsense. P.S. I want to add a remark, distinct from the debate about MWI, for readers who just want to know what QM says about reality. My advice is to start with the attitude that the wavefunction is not real, that it is just a calculating device, like a probability distribution. I see far too many discussions on the net in which people start out by assuming that wavefunctions are real, and then proceed to debate whether the wavefunction collapses  and if so, when, how, and why  or whether it doesn't collapse, leading to many worlds. In quantum mechanics, the things which can definitely exist are called "observables". For example, position of a particle, or energy density of a field. Wavefunctions offer a way to calculate probabilities for the possible values of those quantities. Quantum mechanics does not tell you which observables take values; this is why we can say the theory is incomplete. Now from here you can go in many directions. In my opinion, the path to discovering the truth about QM lies through the most advanced theories, like quantum field theory and quantum gravity, because those are the forms of quantum mechanics we are using to describe the real world. This doesn't tell you whether Bohm or Bohr or Everett or none of the above offers the best clue to the final truth; I just mean that extra knowledge about the details of advanced physics is far more important for your understanding than the usual "interpretational debates" that don't even take into account the novelties that come from QFT, such as the role of relativity. Of course, QFT is hard, and quantum gravity even more so, so this is not easy advice to follow. Unfortunately I don't already know "the answer" and can't tell it to you. But my best advice is this: you will not go wrong, in trying to understand QM and these more advanced theories, if you always make your starting point, and your fallback position, the view I described, i.e., the observables are real, the wavefunction is not. If you want to know what a particular superduperunified theory is about, try to find out what its observables are  those are what it is about. The wavefunctions for those observables will be highly abstract constructs living in infinitedimensional abstract spaces, dependent on particular "gauge fixings", and so on through many other details. But the observables are where reality is at in such a theory, and it's the behavior of the observables which an "interpretation" of QM, or a theory beyond QM, has to explain or reproduce. 


#46
Aug2811, 03:15 AM

P: 273

You seem to have this picturebook version of MWI where you have these parallel universes which are all cleanly separated and selfcontained and weherenow live in one of them. And when an atom decays (or not), an entire copy of the whole universe is made and the only difference between it and the original is that atom decays in one and stays put in the other. And then we (herenow) go along with one of the copies and the other is populated with our doppelgangers.
In this picture youherenow copy of you is sharply defined (as a single point of measure 0, as a perfect deltafunction in phase space) and distinct from all other points. The evolution in time is then represented by a tree (in graph theory sense) where the edges representing parallel universes are all sharply defined and have zero thickness. And there is one path through the graph that corresponds exactly to youherenow experiences. And you just coast along this path like a train along the tracks taking random turns at every junction. Well, this picture is wrong. It breaks down on micro level where the buckyball somehow goes through both slits and it doesn't work on a grand scale where we assume the evolution is unitary and superposition is maintained. It also has problems with assigning labels (coordinates) and probability measures to individual branches. But the biggest problem of all is that this picture is not what MWI is all about. I already told you what (as I understand it) MWI is all about: a) Introducing different laws at different scales has run into problems of actually finding the boundary and explaining what's so special about it. b) The only alternative is to modify Schroedinger equation to add explicit collapse. That's just plain ugly, not to mention a whole lot of other issues. Besides we already have decoherence which demonstrates how an appearance of wavefunction collapse happens to an observer coupled with its environment. c) Well, if not the wavefunction, then what? This is getting a bit metaphysical, but if we agree that wavefunction indeed describes the reality, then how could we single out just one term of this function and say well, this term is real and the rest are just figments of our imagination? Now, assuming for a moment the points a),b),c), where does it lead us? Again, we see that under certain conditions (more often than not) the wavefunction of a subsystem coupled with environment tend to evolve into a shape which can be approximated by a sum of independent terms where each term seems to describe particular state consistent with one of the outcomes. This is where all the splitting and branching come from. A few points I'd like to address (skipping obvious rant): 


#47
Aug2811, 03:48 AM

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P: 16,092

Now, it would be fair to suggest that reality is made up of things that can be observed in ways described by observables, in an effort to avoid trying to impose any additional preconceptions upon reality. But it turns out that if you pursue this route, you pretty much wind up right back at the existing quantum mechanical description of states. 


#48
Aug2811, 04:27 AM

P: 751

The question now is whether there is an alternative picture that makes any sense, or whether your three principles... This is a bad start, if the objective is for MWI to be plausible: This is another unresolved contradiction in MWI thought. After being told so many times that it's wrong to think in terms of a unique set of selfcontained parallel universes, one might have supposed that any local basis is as good as any other, ontologically. But no, we are only supposed to consider a basis in which the local density matrix is nearly diagonal, or as diagonal as possible? Please make up your mind. Why am I not allowed to think about my left hemisphere in the position basis and the right hemisphere in the momentum basis? Is there some maximum size for the offdiagonal elements, beyond which a particular basis must be rejected as not allowed? 


#49
Aug2811, 04:58 AM

P: 751

A wavefunction is not a probability distribution, but it has a lot in common with probability distributions. If we had a classically probabilistic fundamental theory, some of us would be involved in looking for a microscopic causal model of the probabilities  we wouldn't decide that "the probability function is reality itself". There are wellknown barriers to the construction of a locally deterministic theory (or even a locally stochastic theory) which reproduces QM; fine, then use a little imagination. The holographic mapping from boundary to bulk introduces a little nonlocality in the bulk; maybe QM can be derived from a local theory on the boundary (this is 't Hooft's idea). Maybe you can get spacelike correlations from closed timelike curves (this is Mark Hadley's idea, and also has something in common with John Cramer's transactional interpretation). Maybe you can have interactions that are local and fundamental objects that are 'multilocal'. And then we have the example of Bohmian mechanics, which, while unacceptable on account of the preferred reference frame, might be modified in one of these other directions. For that matter, I even think MWI's analysis of the wavefunction's structure could be a source of inspiration! But MWI as expounded in this thread, has neither predictive capability nor conceptual coherence. Possibly you are referring to eigenstates. Certainly there is a lot of casual slippage, in physics discourse, between "the electron being at point x0" and "the quantum state of the electron being in the position eigenstate with eigenvalue x0". But it's like the difference between "the cat sitting on the mat" and "the probability distribution which says that the cat is sitting on the mat with 100% probability". It's not hard to maintain the distinction. 


#50
Aug2811, 05:24 AM

P: 10

Hugh Everett was a dedicated hedonist; I honestly can't take his MWI theory seriously. However, I did write something last year in defense of MWI....
"Back in the 1950s, the orthodox explanation of quantum physics was called the 'Copenhagen Interpretation', which was propounded by Niels Bohr. And he was from Copenhagen, hence the name. The Copenhagen Interpretation posits that the observer is somehow separate, while the ManyWorlds Interpretation posits that the observer is not separate. In other words, prior to the 1950s, it was thought that reality could only exist if someone was around to observe it. But now it is thought that everything is being observed and the explanation for that is we continually split into parallel universes, since it is not possible to observe all possible outcomes in just one universe. The greatest problem with the Copenhagen Interpretation is that observers are real, which means they can only be real if they are observed, ad infinitum. It's called an 'infinite regression'." 


#51
Aug2811, 05:54 AM

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P: 9,255

That being said, I still think that the things I said in posts 4041 here, and the posts I linked to in there, are pretty good reasons to not think of QM as a description of reality. (It may not be an "effective" argument by your standards, but I think those standards may be unreasonably high, considering the unscientific nature of what we're talking about). Do you have an argument for the opposite position? 


#52
Aug2811, 09:19 AM

P: 47

The aspect of MWI that has always bothered me most is conservation of energy. Where is all the energy coming from to fuel all these new universes that keep popping into existence? There is enough of a problem trying to conceive where the energy came from in our universe without compounding the problem practically to infinity. Am I missing something in MWI which does attempt to account for this?



#53
Aug2811, 11:39 AM

HW Helper
P: 3,443

Some people interpret MWI differently. In the most wellaccepted version of MWI, no new universes are literally created. MWI in its most basic simply says that nonunitary collpase of the wavefunction does not happen.



#54
Aug2811, 11:54 AM

P: 172

Have you read the thread or did you just decide to jump in and make assumptions? This is one of the most detailed debates on MWI on this site to date, so unless you got some detailed rebutle for Mitchell Porter's posts, let's not mess the thread up by stating stuff without having read the thread 


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