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Parallel Universes

  1. Apr 29, 2007 #1
    Hi all,
    I'm trying to figure something out about parallel universes. I've read that according to quantum physics, Everett's interpretation can explain parallel universes, while according to M-theory, they can be explained by the 11th dimension.
    The question is: are this two kind of parallel universes the same? i.e. Everett's universes are included in m-theory?
     
  2. jcsd
  3. Apr 30, 2007 #2

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    No, they are not the same. These are two independent ideas, each may exist without the other. According to string/M theory, the universe we see may be only a small portion of the whole universe, in the same way the room in which you live is only a small portion of the planet Earth. The Everett parallel universes are something much more abstract, much more difficult to explain either by words or equations.
     
  4. Apr 30, 2007 #3
    there are proofs of Everett's parallel universes?
     
  5. Apr 30, 2007 #4

    f-h

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    no .
     
  6. Apr 30, 2007 #5
    I heard quantum computers can prove it, but...why?
     
  7. Apr 30, 2007 #6
    They can't. Quantum computers work on the same principles as everything else, so if the existence of the atoms, molecules, etc, does not prove Many Worlds, neither does Quantum Computing.

    The Quantum Computer argument is just that since the computations can be seen as handling a multitude of cases at once, that you can think of it as many classical computers in parallel universes all working together. This however ignores the fact that quantum computing is only probabilistic and that in the worst case scenario, the computation will actually never reach the right answer (though this is extremely unlikely).
     
  8. Apr 30, 2007 #7
    important papers arguing that the MWI and Unitary QM is the only tenable interpretation-

    http://xxx.lanl.gov/abs/quant-ph/0104033
    http://xxx.lanl.gov/PS_cache/quant-ph/pdf/0303/0303050v2.pdf
    http://arxiv.org/abs/quant-ph/0003146

    and- quantum computers are not considered proof of MWI yet- but David Deutsch has argued quite effectively that when they get big enough the MWI will be very hard to deny:

    "...there are indeed other, equally real, versions of you in other
    universes, who chose differently and are now enduring the consequences.
    Why do I believe this? Mainly because I believe quantum mechanics. Just write down
    the equation describing the motion of those fateful transmitter molecules, and their effect
    on you and on the environment. Notice that their ”randomness” consists in their doing
    two things at once: crossing that synapse and not crossing it; and that the effect on you
    was likewise that you did two things at once: buy my book and buy Penrose’s. Such effects
    spread out, making everything do many things at once, which is what we mean by saying
    that there are ”parallel universes.”
    Furthermore, the universes affect each other. Though the effects are minute, they are
    detectable in carefully designed experiments. There are projects underway - close to your
    heart, I know, as well as mine - to harness these effects to perform useful computations.
    When a quantum computer solves a problem by dividing it into more sub-problems than
    there are atoms in the universe, and then solving each sub-problem, it will prove to us
    that those sub-problems were solved somewhere - but not in our universe, for there isn’t
    enough room here. What more do you need to persuade you that other universes exist?
    "

    http://meche.mit.edu/documents/slloyd_deutsch_debate.pdf


    In your book, The Fabric of Reality, you are challenging the single universe conception of reality. In Chapter II, you clearly explain quantum theory which tells us about the behaviour of microscopic particles. You also explain the ‘single particle interference’ experiment and argue that there are intangible shadow particles, and then that there are parallel universes each of which is similar to the tangible one. This is a difficult step for many of us. Could you please clarify how you proceed from intangible particles to many universes (or multiverse as you call it)?

    Let’s start with the microscopic world, because it is only at the microscopic level that we have direct evidence of parallel universes. The first stage in the argument is to note that the behaviour of particles in the single slit experiment reveals there are processes going on that we do not see but which we can detect because of their interference effects on things that we do see. The second step is to note that the complexity of this unseen part of the microscopic world is much greater than that which we do see. And the strongest illustration of that is in quantum computation where we can tell that a moderate-sized quantum computer could perform computations of enormous complexity, greater complexity than the entire visible universe with all the atoms that we see, all taking place within a quantum computer consisting of just a few hundred atoms. So there is a lot more in reality than what we can see. What we can see is a tiny part of reality and the rest of it most of the time does not affect us. But in these special experiments some parts of it do affect us, and even those parts are far more complicated than the whole of what we see. The only remaining intermediate step is to see that quantum mechanics, as we already have it, describes these other parts of reality, the parts that we don’t see, just as much as the parts we do see. It also describes the interaction of the two, and when we analyse the structure of the unseen part we see that to a very good approximation, it consists of many copies of the part that we can see. It is not that there is a monolithic ‘other universe’ which is very complicated and has different rules or whatever. The unseen part behaves very like the seen part, except that there are many copies.
    It is rather like the discovery of other planets or other galaxies. Having previously known only the Milky Way, we did not just find that there are vast numbers of stars out there, far more than in the Milky Way. There are more galaxies out there than there are stars in the Milky Way. We also found that most of the stars outside the Milky Way are actually arranged in other little Milky Ways themselves. And that is exactly what happens with parallel universes. It is of course only an analogy but quite a good one; just like the stars and galaxies, the unseen parts of reality are arranged in groups that resemble the seen part. Within one of these groups, which we call a parallel universe, the particles all can interact with each other, even though they barely interact with particles in other universes. They interact in much the same way as the ones in our seen universe interact with each other. That is the justification for calling them universes. The justification for calling them parallel is that they hardly interact with each other, like parallel lines that do not cross. That is an approximation, because interference phenomena do make them interact slightly. So, that is the sequence of arguments that leads from the parallelism, which by the way is much less controversial at the microscopic level than the macroscopic level, right up to parallel universes. Philosophically, I would like to add to that that it simply does not make sense to say that there are parallel copies of all particles that participate in microscopic interactions, but that there are not parallel copies of macroscopic ones. It is like saying that someone is going to double the number of pennies in a bank account without doubling the number of Pounds.

    But couldn’t this interference phenomenon be due to a yet unknown law of physics within this universe?

    Well, there are very sweeping theorems that tell us that no single-universe explanation can account for quantum phenomena in the same way that the full quantum theory does. Quantum theory explains all these phenomena to the limits of present day experiment perfectly, and it is, according to some measures anyway, the best corroborated theory in the history of science. And there are no rival theories known except slight variants of quantum theory itself. We know that an alternative explanation could not be made along single-universe lines, unless perhaps it is a completely new kind of theory. So, the answer is ‘no’.

    A few years ago, BBC Horizon did a documentary on time travel in which you explained the parallel universes theory and suggested that there was ‘hard evidence’ for it. Well, it is a controversial theory and is accepted only by a minority of physicists, as you yourself acknowledge in your book. Why do you think there is such a strong reaction to this theory in the scientific community? And how do you reply to their criticism?

    I must confess that I am at a loss to understand this sociological phenomenon, the phenomenon of the slowness with which the many universes interpretation has been accepted over the years. I am aware of certain processes and events that have contributed to it. For instance Niels Bohr, who was the inventor of the Copenhagen interpretation, had a very profound influence over a generation of physicists and one must remember that physics was a much smaller field in those days. So, the influence of a single person, especially such a powerful personality as Niels Bohr, could make itself felt much more than it would be today. So that is one thing – that Niels Bohr’s influence educated two generations of physicists to make certain philosophical moves of the form "we must not ask such and such a question." Or, "a particle can be a wave and a wave can be a particle," became a sort of mantra and if one questioned it one was accused of not understanding the theory fully. Another thing is that quantum theory happened to arise in the heyday of the logical positivists. Many physicists – perplexed by the prevailing interpretations of quantum physics – realised that they could do their day-to-day job without ever addressing that issue, and then along came a philosophy which said that this day-to-day job was, as a matter of logic, all that there is in physics. This is a very dangerous and stultifying approach to science but many physicists took it and it is a very popular view within physics even to this day. Nobody will laugh at you if, in reply to the question "are there really parallel universes or not?", you answer "that is a meaningless question; all that matters is the shapes of the traces in the bubble chamber, that is all that actually exists." Whereas philosophers have slowly realised that that is absurd, physicists still adopt it as a way out. It is certainly no more than ten percent, or probably fewer, of physicists talking many universes language. But it is heartening that the ones who do tend to be the ones working in fields where that question is significant, which are quantum cosmology and quantum theory of computation. By no means all, even in those fields, but those are the strongholds of the many-worlds interpretation. Those also tend to be the physicists who have thought most about that issue. But why it has taken so long, why there is such resistance, and why people feel so strongly about this issue, I do not fully understand.






    some users on this forum try to dismiss Deutsch- but in 2005 Deutsch won the Edge of Computation prize- you can read the list of nominators and there comments and see for yourself how respected he is in the scientific community- http://www.edge.org/3rd_culture/prize05/prize05_index.html
     
    Last edited: Apr 30, 2007
  9. May 1, 2007 #8

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    Quantum computers could equally be used as a "proof" that the Bohmian interpretation of QM is correct.
    Similarly to MWI, in the Bohmian interpretation wave function does not really collapse, but all branches exist all the time. However, unlike MWI, the observed reality does not correspond to the wave function or some of its branches, but to something additional - the particle that moves along a single trajectory influenced by the wave function. Still, all branches of the wave function are here, not in parallel universes, but in our single Universe.
     
  10. May 1, 2007 #9

    jal

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    parallel universes??
    Time travel??
    Can I tie my machine to a well head and get oil forever?
    (It would come from the past or an other universe.)
    Can energy move from 3d to 11d?
    Let's keep it simple ... can energy move from 2d to 3d? Can it go back to 2d?
    Are we doing science fiction?
    jal
     
  11. May 2, 2007 #10
    No, according to the string theory, it can't. Every string matter is made of is locked in our 3d membrane and can't escape... I guess
     
  12. May 2, 2007 #11
    Yes; as you say, according to Bohm, all branches exist, but only one has particles in it. So why isn't Bohm a many-worlds interpretation? Why aren't the observers in all the particle-free branches conscious, like they are in the MWI? Do Bohmians have some sort of standard answer to this one? I don't think I've seen it.
     
  13. May 2, 2007 #12
    IANAST (I am not a string theorist), but from what I've heard, different braneworlds in the same 11D space can affect each other via gravity, so I'd expect the answer is yes. Not sure, though.
     
  14. May 2, 2007 #13

    jal

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    Okay!
    Let’s dismiss the 10^500 possible parallel universes and the different time frames. Nobody has stolen our oil, therefore, let’s put those possibilities out of reach … forever.
    Have you seriously considered what a dimension that contains only gravity would look like?
    Like! … man!! … It would be the mother of all the black holes. Now!, … how does this dimension know where to find 10^80 particles in our big universe so that it can endow each of them with a little bitsy bit of gravity which can then follow the inverse square law.
    Would you like to borrow my razor blade?
    jal
     
  15. May 3, 2007 #14

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    This is because the observers observe the particles, not the wave functions. Consequently, in empty branches there is nothing to observe, nothing to be aware of.
    That is the standard Bohmian answer.
     
  16. May 3, 2007 #15
    Would you say that Bohmian mechanics requires a nonstandard philosophy of mind? The way I see it, an observer observes something when that something engages in some sort of causal process that creates changes in his brain, like photons striking the eye which then sends further signals, etc. Do you agree that these processes happen in empty branches the same way they do in the branch with particles in it?
     
  17. May 3, 2007 #16

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    I do not agree. In empty branches, by definition, there are no photons (that could strike the eye). I am not sure what do you mean by "standard" philosophy of mind, but I would say that Bohmian mechanics requires a classical philosophy of mind.
    The only conceptual problem with Bohmian mechanics one might have is that the wave function is both "physical" and "unphysical", in the sense that it objectively exists but cannot be directly observed.
     
  18. May 3, 2007 #17
    OK, yes, in that sense, there are no photons in the empty branches. But the empty branches still have structures in them that look exactly like humans doing observations. How do you know those structures aren't us? After all, MWI is just Bohm without the particles (and some other stuff). If there's some reason why the observers in the empty waves in Bohmian mechanics aren't really doing observations, then that reason is also a direct disproof of MWI, because then the MWI picture of the universe has no observers in it. In MWI all the branches are empty.

    I'm not sure what I mean by a "standard" philosophy of mind either. Maybe what I mean is just "functionalism". I guess what I'm getting at is, a "standard" philosophy of mind would say that if something (like an empty wave) has the exact structure of a conscious observer (like a human brain), then that something is itself a conscious observer.
     
    Last edited: May 3, 2007
  19. May 3, 2007 #18

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    Ontoplankton, you fail to recognize the main difference between MWI and Bohmian interpretation. In MWI, wave function is a material object. In the Bohmian interpretation it is not. According to the Bohmian interpretation, a human cannot be made of wave functions, only of particles. Unfortunately, one cannot prove it independently of the assumption of the Bohmian interpretation, so MWI cannot be directly disproved. Still, there is an indirect argument that MWI cannot be correct:
    If MWI is correct, than any branch corresponds to a potentially physical world. In particular, a superposition of branches is also a branch, so superpositions should also be possible outcomes of measurements. This means that it would be possible to observe a particle located at two different points at the same time, or a cat that is both dead and alive. This, however, has never been observed. Of course, it is possible that it is never observed not because it is impossible in principle, but because it is only impossible in practice, due to decoherence that cannot be avoided in practice. Still, the fact that it is never observed suggests that it might be impossible even in principle, which would mean that MWI was wrong.

    Similarly, there is a problem if you say that a wave function with a brain structure is a brain itself. What would then be a superposition of two different brains? Unlike the wave function, a Bohmian particle cannot be in a superposition, so neither can brain made of particles.

    Bohmian mechanics can also be compared with a swimming pool. A swimming pool has the structure of the swimming pool even if there is no water in it. But can you swim in the swimming pool without the water? No! So, if you define the swimming pool as a pool in which you can swim, then an empty swimming pool is not really a swimming pool. Instead, the essential ingredient for swimming is water, while the role of the swimming pole is only auxiliary. Quantum mechanics is like water that can only be in a swimming pool, while classical mechanics is like water that can exist even without a swimming pole. You can swim in both.
     
    Last edited: May 3, 2007
  20. May 3, 2007 #19
    I don't think you (as an "interpretation designer") get to specify what's a material object and what isn't; it should follow from what you've already assumed. If Everett branches are material, then empty Bohm branches are also material for the same reason, no matter whether we call them material. To me, something is a material object if 1) it exists and 2) it behaves like a material object. In Bohmian mechanics, chairs and brains in empty branches exist (in the same sense that objects made out of particles exist, i.e., as a pattern), and they behave like material objects with respect to objects in the same empty branch. How are they not material objects? Anyway, even if only things made of particles are material, how do you know we're not "immaterial" (but existing) objects?

    For swimming pools, we understand what features of water allow us to swim in full pools but not empty pools. What features of Bohm particles allow structures with them to be conscious observers, while structures without them aren't?
     
    Last edited: May 3, 2007
  21. May 3, 2007 #20

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    Ontoplankton, your arguments are sound. Still, your arguments do not solve one of the main problems with MWI: why then we do not observe superpositions?

    Concerning your question on conscious observers, frankly, we have no idea how consciousness works, either with classical or quantum mechanics. Therefore it would be better to speak about brains, on which we do have some knowledge, according to which classical mechanics with particles (and without wave functions) explains them quite well.
     
    Last edited: May 3, 2007
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