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Which Interpretation of Quantum Mechanics do you prefer?

Discussion in 'Science & Technology' started by Leoreth, May 13, 2011.

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Which interpretation of Quantum Mechanics do you prefer?

  1. Copenhagen

    12 vote(s)
    36.4%
  2. Many Worlds

    7 vote(s)
    21.2%
  3. Hidden Variables

    5 vote(s)
    15.2%
  4. Don't Think, Calculate

    4 vote(s)
    12.1%
  5. Other (Please Specify)

    5 vote(s)
    15.2%
  1. r_rolo1

    r_rolo1 King of myself

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    I was exagerating a bit :p But the core issue remains the same IMHO: the Copenhagen interpretation has some issues with the definition of observer, that reeks on Deux ex machina.

    It might even be a accurate description of the world ( or atleast is the better one at the atomic scale ), but IMHO due to that issue it is pretty much in the same state of the theories I described above: in the same way as those theories required a external source to put the universe going ( the good ol'cosmic wathmaker ), if you try to apply the Copenhagen interpretation to our universe, you need a cosmic observer to colapse the wave ( since everything in this universe is nothing more than wave/particles interacting with eachother, including us )...
     
  2. uppi

    uppi Deity

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    If you postulate an outside observer, you already subscribe to Everett's idea of the universal wavefunction. From there it is just a small step to the many-worlds interpretation. In the Copenhagen interpretation the wavefunction is the knowledge an observer has about the system. So without an external observer, the universe has no wavefunction to collapse.
     
  3. Perfection

    Perfection The Great Head.

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    Uppi what is your opinion on the idea of a universal wavefunction?
     
  4. uppi

    uppi Deity

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    From a scientific point of view there is no value to an universal wavefunction*, as it describes the universe from a point of view that is inaccessible to us. We would not be able to remove ourselves from that wavefunction to experience any predictions made by it. And it would be needlessly complex because you would get a density matrix that has so many dimensions that the word infinite seems to small to describe it, but most of the off-diagonal elements would be zero anyway. So it has no predicitive value, because to make any prediction you would have to go to a much smaller subsystem, anyway.

    That means we can only discuss it form a philosophical/interpretational point of view. In the end it boils down to the question "what is real?" The standard Copenhagen answer would be that the wavefunction is not (necessarily) a real object, but reflects the (subjective) knowledge an observer has about a system. In that view the wavefunction is a complete description of the system, but not a real thing itself. In other words the inevitable cat is not dead and alive, but dead or alive - we just do not know until we look. Once we are in the classical regime, i.e. once decoherence has set in, this makes perfect sense: The wavefunction behaves just like a classical probability distribution. In the quantum regime, however, it gets more confusing as the probabilities begin to interfere and the question arises "is there anything real at this level". To which the current answer seems to be "maybe not?"

    The universal wavefunction takes the easy way out here, as it declares the wavefunction to be the one real physical entity. But that comes with new problems as we now need a mechanism to decide which branch of the wavefunction is actually realized. There you can either take the religious (or alien overlord or giant simulation) option and declare an external observer (which would theologians new options in the omniscience/free will debate, as in "God knows everything there is to know, but cannot predict how an individual will act). Or you take the complete package and say that every branch is realized. But this still leaves the question open, why we end up in universe A way more often than in universe B, so the Born rule that is the critical point of in the Copenhagen interpretation still haunts us here.
    Either way, if you go the way of the universal wave function you need to introduce more entities than we have proof for.

    Unless someone comes up with a way to make this accessible to science, this is a very philosophical debate that challenges our view of reality. While I see the appeal of the universal wavefunction, it is not free of philosophical troubles of its own (and without a good explanation of the Born rule, one could say that these are actually quite similar to the Copenhagen interpretation), and I do not think it is a definitive improvement over the Copenhagen interpretation.

    *For a consistent terminology I will use "wavefunction", although not everything can actually be described in such a way. "Universal quantum state" or something similar might be a better term.
     
  5. r_rolo1

    r_rolo1 King of myself

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    The universal wavefunction is a necessity that arises from the fact that the individual waves are prone to algebric sums ;) So if they can be added to each other, there has to be a universal wavefunction, period. I might not go Hawkins path and try to deduce it and solve it ( we would have to presume that we can actually find it first ,and then think that it might be solvable when even mono ionized dihidrogen is already analitically unsolvable as far as we know ), but there surely is a universal wavefunction ...

    And that is the thorn that makes me shun Copenhagen interpetation :p If the universe has no wavefunction to colapse, it does not exist for all proposes ... thus the Copenhagen intepretation either postulates a external observer for the universe or defines all particles in the universe as observers ( or apropriately/ironically actors/observers :D ) to permit the colapse of the function of every possible part of the universe to exist , that is a reductio ad absurdum because it colides with the premise of the observer being something diferent of the interacting particles.

    Atleast IMHO Copenhagen logically requires a entity that can observe every possible interaction in the universe but that is not part of the said universe ( in the same way that the infinite potential walls of the Schrodinger equation are not part of the system in study ... ).

    P.S Why do you say that Universal wavefunction->many worlds ? Can't see how , atleast deductively ...

    PS II massive x-post. Sorry. I see you adressed some of the points I have above. Will chew that and respond later
     
  6. uppi

    uppi Deity

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    You can only add wavefunctions if they are of indistinguishable particles. And the universe consists of different particles, which have different degrees of freedom, so their wavefunctions operate on different Hilbert spaces. You cannot add these.

    What you have to do is to expand the size of your Hilbert space for any distinguishable entity. Which you can do, but unless there are coherences between the old and new states, this is a rather pointless exercise.
     
  7. Souron

    Souron The Dark Lord

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    I like Many Words, though my ability to express phenomena in it's terms is limited, compared to the Copenhagen interpretation.
     
  8. Terrapin

    Terrapin Prince

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    The problem with Many Worlds is that it is simply unnecessary. In fact, many world violates the real nature of a quantum probability function. Why? Because probability waves represent the chance of this OR that happening. Many World asserts that this AND that happen, one in this universe and one in some other universe which supposedly popped into existence just to have a venue for that one alternative outcome. The simple analogy is to your local weatherman. If the weatherman says there is a 60% chance of rain tomorrow, then tomorrow it does, in fact, rain, then the chance of rain "collapses" to 100% and the chance of no rain "collapses" to 0%. It does not rain here and be sunny in some other universe. Get it?
     
  9. punkbass2000

    punkbass2000 Des An artiste

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    On what authority?
     
  10. Souron

    Souron The Dark Lord

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    The weatherman predicts a 60% chance of rain because he doesn't know all the hidden variables. So if your intuition is that the world has only one state, then look not at Copenhagen, but at people like de Broglie and Bohm. You'll have to live with the fact that causation is not local however: that the future can effect the past, and that every particle interacts with every other particle that ever was or will be.
     
  11. peter grimes

    peter grimes ... Moderator

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    I thought the Many Worlds interpretation was the result of trying to make sense of entanglement, and how two particles are able to communicate many orders of magnitude faster than the speed of light... and it's not so much that an entire new universe pops into existence when one of a pair of entangled particles is measured, so much that when the entanglement was established a 'split' in the universe occurred, one sheet holding one outcome, the other in the other.
     
  12. uppi

    uppi Deity

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    The universe cannot split when entanglement is established, because at that point the different quantum states can still interfere. And they could still recombine to give a deterministic result, so there is never any split. The split would have to happen after the entanglement has been destroyed, so the universes cannot influence each other anymore. And this happens after what a proponent of the Copenhagen interpretation would call a measurement. And so the measurement sort of creeps back in through the back door.

    But no matter when the universe splits, both branches of the wavefunction evolve independently. So after a while you end up with two branches that have not much in common but the origin. As both describe a whole universe, it is fair to say that these are different universes and that at one point there is a new universe popping up. At the beginning they only differ in one quantum state, but in the future they might be vastly different.

    Of course you can say that these are different "sheets" or whatever of a bigger entity, but that entity would not be a universe anymore, but some sort of "hyperuniverse".
     
  13. punkbass2000

    punkbass2000 Des An artiste

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    Why can't entanglement go between universes? Or is it a simple matter that if it can, then they are not separate universes by definition?
     
  14. GoodGame

    GoodGame Red, White, & Blue, baby!

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    http://en.wikipedia.org/wiki/Quantum_entanglement
    Like Uppi wrote right above you.

    Once observed, they can become unentangled and become separate (and under Many worlds, could go off to different universes).

    Really it's just the semantics of the theories that make it so. If you don't believe in Many Worlds, then they never split into different universes! :)

    However, entanglement is real and observable. The question is just what happens when you observe it, and if you collapse the wave function through observation. After that you have to consult with your favorite theory to guess what happens.
     
  15. punkbass2000

    punkbass2000 Des An artiste

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    Ok, but I still don't quite understand why they can't remain entangled and be in different universes. Is it just that we couldn't know that?
     
  16. punkbass2000

    punkbass2000 Des An artiste

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    I guess, trying to be more specific, my question is: "Why is this necessarily true?"
     
  17. warpus

    warpus In pork I trust

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    Many worlds is equivalent to copenhagen, right? In a way?

    I don't like either of them. In my opinion there is yet another interpretation that is equivalent to both, but we just haven't thought of it yet.
     
  18. uppi

    uppi Deity

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    Yes, they are equivalent as in they give the same predictions for every experiment. And they are not the only interpretations, there is a bunch of interpretations and there could certainly be even more that we have not discovered yet.


    Consider this experiment: We send a single photon on a beamsplitter that transmits 50% into mode A and reflects 50% into mode B. When we now put detectors in both modes, we will get a click in detector A with 50% probability and with 50% in detector B. According to the many worlds interpretation, after the measurement we have two universes, one where the photon was in A and one where the photon was in B.

    But what happens if we do not detect the photons immediately, but put a second beamsplitter behind the first, directs 50% from mode A into mode C, 50% into mode D and does the same for mode B. So if the universes had already split, we would expect 50% in C and 50% in D in both universes, so we would expect to measure the photon with 50% in C and with 50% in D.
    But this is not what actually happens in the experiment! The photon interferes with itself! If the optical path between the beamsplitters is exactly the same (and the experiment is perfect), we actually get 100% in C and 0% in D. So we get a deterministic result, just one branch of the wavefunction and just one universe. So the irreversible splitting of the universe cannot happen during the entanglement (in this case the split at the beamsplitter), but has to happen at a later time, as there is still the possibility that we get just one result and one universe.
     
  19. punkbass2000

    punkbass2000 Des An artiste

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    Wow. You're way better than wiki. :D

    Thank you.
     
  20. warpus

    warpus In pork I trust

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    What are some of the other ones? Anything with potential?
     

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