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What's the universe collapsing into?

  1. Jul 26, 2015 #1
    So as time goes on there are more and more interactions in the world. That means that there are more wavefunction collapses going on. Some think that there is a universal wavefunction that is guiding all the individual wavefunctions. So I have to wonder if all these smaller, individual, instantaneous wavefunction collapses are somehow a process describing a very slow collapsing of the universal wavefunction. If so, then can we tell what state the universal wavefunction is collapsing into?
     
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  3. Jul 26, 2015 #2

    mfb

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    Only in interpretations of quantum mechanics with collapses.
    Depends on the interpretation.
    No.
     
  4. Jul 26, 2015 #3
    We are certain of the past but not sure of the future. That means that as time goes on, more events enter the past. And there are more events we are certain of. So it seems we are getting more evidence to give us more certainty about... what? This sounds like a slow process of wave function collapse.
     
  5. Jul 26, 2015 #4

    mfb

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    That does not mean we would have more knowledge about the present. We do not.
     
  6. Jul 26, 2015 #5
    We perhaps don't have more knowledge about the present because the total number of events is increasing permanently (entropy) but we certainly accumulate experience on how the next and local future will look like. Leaving the darkness of ignorance is, at a fundamental level, the deep purpose of a scientific approach.
     
  7. Jul 26, 2015 #6

    mfb

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    Do we really know more about the quantum state in 10 minutes, than we knew yesterday about the quantum state 10 minutes in the future back then?

    I'm not counting scientific progress here, because the discussion is about a theoretical maximum knowledge.
     
  8. Jul 26, 2015 #7
    We can use the wave function to calculate the probabilities of various alternatives from some starting point to some ending point in the future. But if there are interactions along the way that we observe, then the original wave function must be updated to reflect the interaction we see. We learn more about the trajectory, and we can narrow the possible outcomes of the original alternatives. Is this not what is going on as we learn more about the past? Doesn't that tend to narrow the alternatives for the future so that the final state that it collapses to is more certain than if there were no interactions in the past of which we take account? So this tells me that history seems to be a process of a slow collapse of a wave function to the ultimate state. What does that tell us about the state to which we are headed?
     
  9. Jul 26, 2015 #8

    andrewkirk

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    This is an excellent example of why 'wavefunction collapse' is such a misleading term, which we are unfortunately stuck with for historical reasons. It would be much better to call it 'eigenket selection'.

    The reason it is misleading is that it implies getting smaller and more precise - that somehow there is more information than before. But there isn't. When we measure location, the range of possible locations 'collapses' to something very small, and the range of possible momenta correspondingly blows out, and vice versa. The system is as much in superposition as it was before, it's just that the widest superposition is now in a different basis - the conjugate one.

    PS: I love the title of this thread!. It is sweetly conjugate to that of all those threads in GR and Cosmology that ask what the universe is expanding into.
     
  10. Jul 26, 2015 #9
    Please elaborate
    (.
    develop or present (a theory, policy, or system) in further detail.
    )
     
    Last edited: Jul 26, 2015
  11. Jul 26, 2015 #10

    bhobba

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    Since collapse is only part of some interpretations the above is not correct.

    Thanks
    Bill
     
  12. Jul 27, 2015 #11

    bhobba

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    That does not follow eg see eternal inflation.

    Thanks
    Bill
     
  13. Jul 27, 2015 #12

    bhobba

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    That's not entropy.

    As I tried to allude in my previous post that the total number of events is increasing may not be true - even if it was of any relevance - which it isn't.

    Thanks
    Bill
     
  14. Jul 27, 2015 #13

    bhobba

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    You need to investigate many worlds. No collapse, no update - everything evolves deterministically.

    Thanks
    Bill
     
  15. Jul 27, 2015 #14
    I don't want to get hung up on the terminology. I'm referring to the math of quantum mechanics where a measurement or interaction results in only one of the possible eigenstates actually being realized in nature. Once an event/interaction/measurement occurs, it is part of the past which we know with certainty. Before it happens it is part of the future about which we can only calculate probabilities. Can we even separate the concepts of future/past from wave function prediction/measurement? Wave function prediction refer to measurements that haven't taken place yet. And once a measurement/interaction takes place, wave function predictions are no longer relevant for that measurement.
     
  16. Jul 27, 2015 #15
    Would the updated wave function have the same eigenvalues as the original wave function? Are interactions along the way telling us which of the original eigenvalues it's getting closer too? Or do interactions determine a whole new set of eigenvalues to be considered?
     
  17. Jul 27, 2015 #16
    Collapse isn't a permanent fixing of an observable, see the Stern-Gerlach experiment: two successive collapses of a type of observable don't add more information than a single one, and the past information on an observable can always be erased by a new collapse on a conjugate observable.
     
  18. Jul 27, 2015 #17

    atyy

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    There are two sorts of collapse.

    (1) The standard Copenhagen-style interpretation. Here neither the wave function nor the collapse of the wave function are necessarily real, and they are just tools to calculate the probabilities of measurement outcomes. It is not known whether the wave function of the universe has any meaning. Since the wave function and collapse are not necessarily real, and there is no known meaning to the wave function of the universe, the question is meaningless.

    (2) GRW or CSL. Here the wave function is real and collapse is real, and the wave function of the universe does make sense. These are not pure interpretations and eventually predict deviations from quantum mechanics. It is unknown at present whether these interpretations can explain the full range of quantum phenomena. However, there are discussions about testing these theories, eg. http://arxiv.org/abs/1410.0270. The wave function is in Hilbert space, and the collapse is in Hilbert space, but there is a link to the ordinary space we see. When the collapse occurs in Hilbert space, an event occurs in ordinary space.
     
  19. Jul 27, 2015 #18
    Actually, I'm not sure about the usability of a universal wave function. Wouldn't it have to interact with something else to measure its state. Yet there is noting outside the whole universe to interact with it. We could at best only calculate probabilities of future states, never know with certainty which state it collapses to. But if past events help at all in knowing its trajectory, maybe that helps us know where we are going.

    The other issue is what portion of the probabilities do past events represent? We don't know how many total events there are in the complete destiny of the universe. If we already know with probability 1 the past events, how can a probability of 1 enter a calculation for a new probability. More events entering the past does not change the probability of 1 for them. So I'm not sure these ideas go anywhere.

    Perhaps there are other constraints that would help with using these ideas. For example, maybe it's true that there is a particular total amount of information in the universe no matter what happens in the universe. Though, I'm not sure how that helps.
     
  20. Jul 27, 2015 #19
    That's why bhobba says you should look at MWi. In MWi there is no collapse so your statement that "a measurement or interaction results in only one of the possible eigenstates actually being realized in nature" is not true. What is true is that in a standard statement of QM, this probabalistic aspect of measurement is simply postulated. However MWi derives or claims to derive, all the important (observable) features of the postulate from unitary, deterministic, evolution. Obviously, since there is no actual collapse and no definite outcomes in this scheme, one of MWI's claims is that it can fully account for the appearence of collapse and the appearence of definiteness.
    Not necessarily. If we get entangled with the system - which is inevitable - then the system and ourselves enter a superposition of outcome-states. What is more, each entangled term comprises an observer state whose observation is 100% consistent with the associated system state. But all the outcome states for the observer-plus-system remain in superposition exactly as the system states were before the interaction.
    If you insist on wavefunction collapse then it is irreversible and there is a true past and a true future. But since MWi and possible some other interpretations of QM, manage perfectly well without collapse, the most charitable thing one can say about it is that it is added by hand to a theory that doesn't need it.

    Yes, without irreversibility there is a profound cosmological question - why does the universe appear to be time-asymmetrical? Someone here may be able to tell you whether there are any viable theories around that either manage without quantum collapse irreversibility or else rely on it.
    If you are referring to the wavefunction of the system being measured then it enters a mixed state. However the wave function you should, perhaps, be considering is that of the system and the observer together. See above.
     
    Last edited: Jul 27, 2015
  21. Jul 27, 2015 #20
    No. Just factorize the state space and see what goes on "inside" the universe rather than fretting about what it looks like from the outside.
    Everything will grind to an abrupt halt when the information runs out.
     
    Last edited: Jul 27, 2015
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