Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Questions/Problems with Many-Worlds Theory.

  1. Sep 21, 2013 #1
    So I've been skimming about the Many-Worlds hypothesis and I just have a few questions as a curious student who is at the very beginning of learning physics. So pardon if my knowledge is limited compared to you guys:

    1. It talks about wave function not collapsing. But how is this so if wave function does collapse by the subjective observer? Wave function collapse can be observed all the time.

    2. Using Schroeder's cat example, what dictates whether an event observed will become branched off into a new reality? For instance, let's say I'm typing right now on my computer on this post. At what exact objective 'point' in time will my reality branch off? Can realities branch off even without performing a cause/effect decision of some sort? And if the branch-off is from causal decisions - what objectively 'senses' when such a decision has been made?

    If I fire a gun into someone's chest and that person dies, then at that split second down on the Planck scale there is automatically another reality branching off that we cannot observe where the individual survives the process? How is this even possible? At what measurement does the branch-off create another reality?

    3. So if there are a seemingly infinite number of realities existing then where are they? At what dimensional or operational plane do they exist at? If we can calculate a FINITE number of particles, atoms in the Observable Universe then are there also an infinite and never-ending number of atoms and planets and Earths ad nauseum being created? Huh?

    If Time is spherical and not linear in geometry then how does Time conflate or interact with reality branch-offs or wave function continuing ad nauseum with each observable action? How exactly does time play into wave functioning not collapsing.

    4. Falsifiability. Can we even falsify or empirically test using some sort of an experiment that wave oscillations intersect each other and cause 'ripples' but on the reality-level. I'm not talking about the Double Slit experiment. I'm talking about using an experiment to show that realities are objective-based 'lenses' of our observation and that they do, indeed, split. Can we show this happen in action?

    Sorry for the long post, I just want you to hear out on my questions and criticisms. Would love to hear you chew up my responses. I am willing to take criticism for my post as I am no where near as knowledgeable on this field as you so sorry if I made mistakes on some parts in my questions.
  2. jcsd
  3. Sep 21, 2013 #2
    It could appear subjectively to us that the system is in a definite state, but in fact according to physics it is still in superposition.

    That's subject to debate, and is called the measurement problem. When does an actuality arise, simply put.
  4. Sep 22, 2013 #3


    Staff: Mentor

    The MWI is one of those interpretations that makes use of what is known as decoherence.

    Unfortunately decoherence in the context of interpretations is rather controversial so I would rather not discuss my view of it because it can end up in heated debate. I will rather give you a link that discusses it in general terms:

    It explains its relation to the MWI and answers a number of your questions.

    Give it a read and post any further queries.

  5. Sep 25, 2013 #4
    That story about Schrodinger's cat is perhaps the main source of the many-world theories. The problem is that the story has been told incorrectly. If that cat really acted as sub-atomic particles do, you wouldn't just see a dead or alive cat. You would see a cat that had become both dead and alive, a versions that died immediately as well as versions that died just before you opened the box. And all of those versions would have had an opportunity to interact with each other. So when you finally opened the box, the result would have been a cat with a potentially very strange experience.

    Of course, cats don't act that way, but particles do. When the branch off into multiple lines, those lines don't create more universes, they all stay in this universe and interact with each other.

    Who knows, maybe there are other universes out there. But there's nothing about QM that would suggest this.

    Actually, there is another physics concept that really kills the multiverse theories. It's the notion of conservation of information. But that's another topic.
  6. Sep 26, 2013 #5


    User Avatar
    Gold Member

    I see the cat thing as an entangled cat and gun. It is a superposition of gun-fired/dead-cat and gun-not-fired/live-cat. If you observe either the gun or cat, you know the state of the other. Trying to relate to the cat or gun as independent things leads to illogical places because they don't exist independently. When you think about the cat, don't. You have to leave a hole there and accept the fact that the cat is just not a valid thing to think about (pre-observation). The confusion created by the cat as alive/dead concept is unfortunate. The system of cat/gun is in superposition. We can know nothing about cat or gun while that is true.
  7. Sep 26, 2013 #6
    Instead of wave function collapse, we speak of the observer becoming entangled with the system he is observing. Suppose I measure the spin of an electron, which is a superposition of spin up and spin down. After the measurement, the larger system of (me + electron) is in a superposition of the states "the electron is spin-up and I observed it to be spin-up" and "the electron is spin-down and I observed it to be spin-down." To the version of myself in each of these branches of the superposition, it looks like the wave function of the electron has "collapsed" into a definite state. From the broader point of view, though, no collapse has happened.

    Branching isn't an instantaneous process. Many natural events cause the region where the wave function is nonzero to gradually separate into two separate parts. Once there are two clearly separate regions, they are unlikely to interact and we can call them "worlds" and say that branching has occurred.

    If there's only one possible outcome of a process, then there's only one possible outcome and no branching can occur. If quantum mechanics allows multiple different outcomes for the same process, they all occur in different branches.

    The wave function lives in a space called "configuration space," which is just the set of all possible configurations of all particles in the Universe. We can call any small region of configuration space where the wave function is nonzero a "world." These worlds fission to form new worlds at a rapid but finite rate.

    I'm not at all clear what you are talking about here.

    What we can do is try to find evidence that wave functions actually do collapse. True collapse would have observable consequences. Namely, interference is only possible before wave function collapse. If a wave function is a superposition of two possibilities, then these two possibilities can interfere and you can do generalizations of the double-slit experiment and see interference patterns. But if the wave function collapses down to a single possibility, then no interference is possible.

    As I talked about above, what looks like wave function collapse to an observer can actually be described as entanglement between the observer and the system observed. So how can we tell whether collapse is *actually* happening? We can try to do an interference experiment on the larger system of (observer+observed). If the two branches of this larger wave function can interfere, then there was no collapse. If they cannot interfere, there was actually collapse.

    If there is no wave function collapse, you are stuck with something like many-worlds, because without wave function collapse the Schrodinger equation describes the world with a continually-branching wave function. How exactly you interpret this wave function is a matter of preference, but many-worlds is arguably the most straightforward way.
  8. Sep 26, 2013 #7


    User Avatar
    Science Advisor

    The branching of the universe (probably) doesn't happen at any point in time, but rather outside of time. Basically, there is an infinite set of universes which perhaps differ at some points in space-time. I see no reason for time to be singled out.
  9. Sep 26, 2013 #8
    There would also an infinite number of universes where your gun jams and doesn't fire at all. :bugeye: There's no way of knowing beforehand where would happen after a second down the Planck scale.
  10. Sep 26, 2013 #9


    Staff: Mentor

    There is no branching - simply a partitioning of the wave-function into 'sub worlds' from decoherence. Nothing happens 'outside time' - as far as I can tell anyway because I don't even know what that's supposed to mean.

    It's not hard. As a result of decoherence we have the improper mixed state Ʃpi |bi><bi|. That is from QM with no interpretation. Exactly what that MEANS is interpretation dependent. In the MWI each of the |bi><bi| is interpreted as a sub-world - its that easy. No collapse - everything simply keeps evolving and all is sweetness and light from that perspective. There are other issues, but collapse aren't one of them because there aren't no collapse.

    For more detail see the following paper on decoherence that examines all the interpretations that make use of decoherence - MW is just one of many:

  11. Sep 26, 2013 #10

    A paper published a few months ago suggests that if "wave function collapse" (i.e. branching) isn't instantaneous, then it is at odds with QM predictions. I will attempt to hunt down this paper, which was mentioned on this forum.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook