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Question About Many Worlds Interpretation

  1. Sep 13, 2004 #1
    After having researched on many different Websites the Many Worlds Interpretation of Quantum Mechanics, it seems to me as if there are two positions defended by physicists:

    Position 1: All the outcomes of a system actually happen in reality. That is, every time a choice is made all possible choices are actually realised and these branch off into alternate Universes to create the multiverse.

    Position 2: All the outcomes of a system can happen in reality but only the most probable outcome (I think this is given by the sum-over-histories approach) happens in reality so that although it is possible that all these alternate Universes can be created the only existent Universe is the one we live in (i.e. there is no multiverse in reality).

    An article that helped me to come to this conclusion is one titled "Multiverses and Blackberries" by Martin Gardner. It can be viewed online here:

    I would appreciate any help and any clarification needed, as I find this MWI quite confusing!
  2. jcsd
  3. Sep 13, 2004 #2
    problem as i see it is who makes the decision of what is the most probable outcome? God? dice?

    i find it hard to belive that all other universes are destroyed except our one because the decisions made in this one are most probable. wat if a decision made in a branch was more probable does that mean our universe is destroyed while that one continues on without our concious or are we transfered over?

    and with no1
    everyone makes at least 100 choices each day and with about 6 billion people alive thats 600 billion new universes a day. i find that hard to belive

    then again im just using logic here :P someone will modify a mathematical equation to suite their needs just like they create new dimensions as they see fit :rofl:
  4. Sep 13, 2004 #3
    Just to explain the many-worlds-interpretation.

    Suppose we look at a bomb and we want to describe whether it explodes or not. In QM you would have a wavefunction of the bomb that is a superposition of these two possible outcomes. By looking at what will happen we are basically performing a measurement on this QM-state. The unitary-evolution of the system will be that once we measure this state our wavefunction will be entangled to that of the bomb; This means that we either "see" an explosion or we don't. Once we measured the superposition is broken due to the measurement.

    Since a wavefunction of the universe gives a complete description of the entire system, both possible outcomes (explosion or not) are equally real in this universe. Why is it then that when we do the measurement , the bomb either explodes or not ??? Well this paradox is solved by the entanglement of our wavefunction and the bomb-wavefunction, as stated above. We are also a part of the universe, right ... This is why the two possible outcomes do not occur at the same time in our universe.

    Now, you can see that one could also say that the bomb exploded in our universe and since the other possible outcome is also real, it must occur in some other universe to which we do not belong. This is why I said that we need to see ourselves as being part of this universe.

    Do these parallel universa really exist ???

    The best way to convince yourself is to look at the quantum eraser-experiment. This is an application of the famous which way experiment or Welcher Weg experiment involving mines. I am gonna assume you know this. If you don't, just let me know and i will explain.

    If there was only one universe , then the only way of knowing whether the mine can explode is to step on it and see what happens. If it did not explode then the mine was not "activated", if it did it was activated but ofcourse you lost the mine...that's logical.

    Now suppose that the bomb or mine wavefunction is entangled to some spin-measuring apparatus. This means that checking whether the mine is activated triggers a which way experiment yielding a spin up value when the mine did not go off and a spin down value when the mine exploded.

    In the multi-universa-interpretation, performing the which way experiment (this is checking whether the mine is activated or not) will lead to two universa. a priori we don't know this, but suppose we find a spin down-value and the mine DID NOT explode. In that case we directly know that there has been a which way measurement and that the mine exploded into the other universe. We found out about this explosion because of the entanglement of the electrons (the spins we measure) between the universa.

    Pay attention to the fact that we do not look at the mine directly. In stead we look at the electron spins in the which way experiment in order to gain info on the mine. If we would look at the mine, we would not have gained any info on the other universe because we would only se the explosion or not in OUR universe of which we are a part !!!

  5. Sep 13, 2004 #4
    Why must it occur in some other Universe? I mean, why can't it simply be a probability that it occurs but simply does not occur because the other probability (the explosion) is higher?

    Is this trying to proof the reality of existence of other worlds?

    OK thanks a lot for your point of view. So, as I see it, MWI really suggests that there are all of these infinite Universes popping into existence whenever there is more than one possible outcome. I'm sorry but it seems so hard to imagine this! Where do all these Universes go? Doesn't this violate the conservation of mass and energy? I mean, when new Universes are created new mass and energy should also be created no? I mean, it seems as such as an incredible idea!
    Last edited: Sep 13, 2004
  6. Sep 13, 2004 #5
    Keep in mind that the spin down value is one measured along the z-axis.

    The up and down values in the entaglement are along the X-axis.

    A spin up along the X-axis can be written as a superposition of the up and down spins along the Z-axis. This is the Welcher Weg part.

    Suppose we send a spin up bundle (along the z-axis) through some kind of apparatus. The the result of passing through the apparatus are two bundels of spins along the x-axis. (this is possible because of the above paragraphe).

    Now, we want two measure spins along the z-axis, but before you measure there are to beams of spins along the x-axis (spin up and down). Now we do not look in which of the two bundels an electron is at. In stead we join the two bundels back together and we measure the spins along the z-axis. Normally you would expect and spin up bundle along the z-axis, but in stead you will get also a spin down bundle along the z-axis.

    The solution to this "mystery" is the entaglement of the two x-bundles with detector that looks in which of the two x-spin bundels the electron is at.

    Outcome 0 corresponds to x-spin up and 1 is x-spin down.

  7. Sep 13, 2004 #6
    Keep in mind, Curious6 that these manyworlds are an interpretation of QM, just like the Standard Copenhagen-interpretation.

    It is just a way to look at the outcomes of QM.

    Feel free to disagree, many people do (just kidding)

  8. Sep 13, 2004 #7
  9. Sep 13, 2004 #8
    I am not really disagreeing, I'm trying to find out if it is really true that there are two positions defended by the ones who favour MWI, the one where parallel Universes are really created and exist somewhere or the ones who believe that these Universes can be created by the different outcomes but are just mathematical probabilities. Any views on this marlon (or anybody else)? Thanks for your help though!
  10. Sep 13, 2004 #9
    Also, I remember that in Gell-Mann's book he expresses that he believes it would be better to call it "many different alternate histories" interpretation, as if all these alternate histories could have happened but finally there has been only one history! That is what I mean by Position 1.
  11. Sep 13, 2004 #10


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    hello Marlon,

    Just a question: do you accept the following description as a typical quantum eraser experiment:



  12. Sep 13, 2004 #11


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    I have never heard of position 2) being refferred to as Many Worlds Interpretation. I'm not saying it never is called that, just that I've never heard it called that. If only one of the possible outcomes actually occurs, and here is only one actual universe, then I don't see how it could claim that title.

    Going to look at the link now, and see what it says.
  13. Sep 13, 2004 #12

    When reading the article i mainly focussed on the theory (sorry, guess this is a kind of handicap due to university :tongue2: )

    I completely agree with it and my answer to your question is YES. All the necessary ingredients are there : the decoherence, the restoration of coherence, the collapse of the wavefunction and thus the loss of information (you don't see the which way and the interference pattern in one measurement, when looking at one the other is lost), and ofcourse the entanglement.

    marlon, ps i find the multi-universa approach the most "natural" way to look at the outcomes of QM. Ofcourse this is a personal opinion and many others may agree or disagree for several reasons.
  14. Sep 13, 2004 #13
    I like many-worlds especially for the quantum-computing application.

    Viva the massive-quantum-parallellism and the genious of David Deutsch.

    One of the most fascinating "elementary" problems of this field would definetely be Deutsch's problem and the solution for it. How to evalute if a function is balanced or not by performing just 1 measurement.

  15. Sep 13, 2004 #14
    Hey Marlon
    you disapointed me on your last point :tongue2:
    Seriously, the many-world interpretation looks like star-streck to me. Who needs it ?
    Is not the Copenhagen-interpretation fine enough ?

    What would be great with the many-world, is if our conscience could pick the right world :surprised
  16. Sep 13, 2004 #15
    I am not saying that Copenhagen wouldn't be fine enough.
    I just think that manyworlds is a more "natural" approach to interprete QM. Especially when it comes to quantumcomputing. When looked at QM from the many-worlds point of view, the results of QM seem more natural to me. I always had difficulties with the concept of collapsing wave functions and the loss of info.

    I prefer the view that every term in the superposition DOES occur and not just one that we see. we are a intrinsic part of our universe. so we only see what we see, but to say that the other info would be lost seems unnatural. Why not imagine these other terms of the superposition correspond to events in another universe that we cannot see.

  17. Sep 13, 2004 #16


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    Well, until a few months ago I was also in favor of MWI. But I'm now of the opinion that MWI cannot generate the right probabilities without assuming the projection postulate or a logical equivalent. I've studied Deutsch's theorem in quite some detail, and then it occured to me that he assumes a logical equivalent of the projection postulate by requiring what Wallace calls "measurement equivalence" and "payoff equivalence". These assumptions sound *so evident* that you readily accept them, but they are the key to requiring probabilities to be given by the inproduct squared. If you require that probabilities be invariant under unitary transformations (even between hilbert spaces of different dimensions) and under more or less degeneracy of operators describing the measurement, then you have in fact stated that they have to be given by the Hilbert norm.
    After DeWitt's attempt to solve the quantitative probability problem in MWI, which is now recognized to have failed, I think Deutsch's attempt also fails. So to me, MWI is very nice, but doesn't reproduce necessarily the right probabilities.
    However, I might be wrong. I'd find that actually great if I were proved wrong, because conceptually, I prefer MWI over the Born rule, because it leads to all kinds of metaphysical considerations you don't have with MWI.

  18. Sep 13, 2004 #17
    OK, I have a few questions about the Many Worlds Interpretation though:

    (a) what corresponds to a measurement in the Copenhagen Interpretation is supposed to set the moment when worlds branch off no? However, doesn't that mean for example, that in a basketball match, where hundreds of decisions and "measurements" are made that in a single match there are about hundreds of thousands of new universes each branching off? Where do they all go? Why don't they interact with ours again?

    (b) Doesn't the splitting off of new Universes violate the conservation of mass as constantly new mass and energy are being created when Universes branch off?

    (c) Why do people talk about the conceptual problems of wavefunction collapse in the Copenhagen Interpretation and don't talk about the conceptual problems of an infinite amount of different Universes?

    Just some points!
  19. Sep 13, 2004 #18


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    Well, that "branching" is just the "wave function of the universe" which is written in more and more terms which will never interfere again, because essentially orthogonal. This can happen in a high-dimensional (Hilbert) space, because there are so many dimensions. The "universes" are just these "independent" terms of the wave function.

    The fact that these terms will not interfere again (once correlations with a thermal bath have been established) is indicated by what is called decoherence.

    No, it is just quantum mechanical superposition. In fact the term Many Worlds is misleading if you first hear it, and then know the theory. Many Worlds is essentially the unitary evolution part of quantum mechanics, without the projection postulate. (and my personal opinion is that they don't manage and have to sneak in somehow the projection postulate)

    Well, the Measurement Problem in QM is not about some bizarre situation (like many worlds), it is about the apparent conflict between what is "normal physics" (the unitary evolution with a hamiltonian and so on) and what is "an observation" (the projection). Why should the latter be a _different_ process from the former ? And if you extrapolate this, you end up by saying that consciousness is the origin of the "measurement process".
    The MWI is an attempt to do away with the projection postulate, and say that to an observer in one of the branches everything will happen _as if_ the projection postulate were true.

  20. Sep 14, 2004 #19
    Does this mean that whenever a measurement is made and a Universe branches off it does this in a higher-dimensional space? Does this mean that if more than our 3 space and 1 time dimensions are shown not to exist would it falsify this interpretation?

    I'm sorry but I've just finished secondary education and I don't really understand it when you say Many Worlds is "essentially the unitary evolution part of quantum mechanics, without the projection postulate". Could you please explain a bit more, or put it in simpler terms? Also, what do you mean by saying that the term Many Worlds is misleading?

    Fro my (basic) understanding of Quantum Mechanics, I was under the impression that this is what happened (I'll use a double-slit experiment): once a photon is released from an emitter it obeys a wave function until it reaches the detectors by the two slits. Once a measurement is made as to where the photon passes through it "collapses" the wave function because the photon has been localised and so acts as a particle. Why is this so hard to imagine? I mean, I thought that the wave function just represented the probability of the photon for being at different positions (but that obviously it is at just one position even though there is probabilities of it being at other positions) so even though it is measured to go through one slit what is so wrong with that? I'm sorry if it sounds confusing but I've tried to explain my question as well as I could! Thanks for the help!!!
  21. Sep 14, 2004 #20
    I have a remark on these multiple universes. I read things like would they have higher dimensions, or what about the conservation of energy, what about the generated mass in those universes.

    These are indeed very good and critical questions, though i think we don't have to look at these universes like this.

    Keep in mind that WE are an intrinsic part of THIS ("our") universe. So based upon our physical laws that describe our universe we cannot make statements about these other universe. The only we to know something about the other universes would be via indirect "communication" via entangled wavefunctions. I gave such an example in my post on the quantum-eraser and the which-way-experiment.

    I don't think we need to look at those universes "too practically". I mean by that the attempt to describe them with OUR physical laws. You could say that they have there own physicits that are describing their universe, NOT OURS.

    Just a thought :smile:

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