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

The Wave Function Of The Universe

  1. Dec 11, 2013 #1
    This may be a bizarre question, but if the entire universe has a wave function, and wave function collapse for real (unlike in Bohmian mechanics or the Many Worlds interpretation), then what caused the collapse of the wave function of the universe? It must have been collapsed before we came about, the universe didn't just exist as a quantum superposition. Wouldn't this presuppose there is some external classical system, collapsing the quantum system? If so, what collapsed that classical system?! This seems to be a problem for me, and I am having a hard time understanding it.

    What collapsed the wave function of the universe?
     
  2. jcsd
  3. Dec 11, 2013 #2

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    Well we know decoherence causes apparent collapse.

    The universe itself doesn't have to collapse - simply stuff inside it - and decoherence seems to handle that pretty well.

    But there is zero doubt MW is the most elegant interpretation as far as the wavefunction of the entire universe is concerned.

    Thanks
    Bill
     
  4. Dec 11, 2013 #3
    "Well we know decoherence causes apparent collapse."

    Yes, but by an outside system; what was outside the universe to collapse its wave-function? Also, if it is only "apparent", then this means that something like Bohmian mechanics, or the MW is true. If I am not mistaken, in standard (the Copenhagen interpretation) Quantum Mechanics; the wave-function does collapse. Can dechoherence account for real collapse, or just apparent collapse?

    "The universe itself doesn't have to collapse - simply stuff inside it - and decoherence seems to handle that pretty well."

    Well, if the universe has a wave-function; it must have collapsed. It didn't just exist as a superposition until the point when we came along.

    "But there is zero doubt MW is the most elegant interpretation as far as the wavefunction of the entire universe is concerned."

    How so? It seems to me that the MW interpretation comes with too much baggage.
     
  5. Dec 11, 2013 #4

    kith

    User Avatar
    Science Advisor

    Collapse -or its appearance- happens when a measurement is performed on a system. By definition, no measurement can be performed on the whole universe. So if it makes sense to assign a state to the universe as a whole, this state never collapses. It simply evolves according to Schrödinger's equation.

    Edit: The question is - does it make sense to assign a state to the whole universe? This is a matter of interpretation. The MWI and the Bohmian interpretation say yes, the Copenhagen interpretation says no.
     
    Last edited: Dec 11, 2013
  6. Dec 11, 2013 #5

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    Cant follow that at all. In MW the wavefunction simply evolves - no collapse - nothing. Other interpretations have a bit of an issue - but not MW.

    If you consider inflation it started with the wavefunction of the false vacuum that via quantum tunneling started our universe. It grew to our current universe - no observation of the entire universe as far as I can see was required.

    The issue here isn't the baggage MW comes with - its applying it to the whole universe. No collapse - no collapse issues at all with MW. Yes you can debate MW on other fronts - but the wavefunction of the entire universe trivial for it.

    Thanks
    Bill
     
  7. Dec 11, 2013 #6
    Are you saying that if standard Quantum Mechanics (The Copenhagen interpretation) is true, then the universe cannot have a wave-function?

    What about objective collapse theories, like GRW or Penrose?
     
  8. Dec 11, 2013 #7

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    That I agree with.

    Thanks
    Bill
     
  9. Dec 11, 2013 #8
    Ya, I'm talking about standard QM here; the Copenhagen interpretation.

    The universe tunelled into being? From what?! Also, if nothing collapsed the universe's wave-function, then it would still exist as a superposition.

    I'm talking about standard QM Though. If The Copenhagen interpretation is true, then something must have collapsed the universe's wave-function (unless the universe doesn't have a wave-function under standard QM, which I am not sure of).
     
  10. Dec 11, 2013 #9

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    I think he is talking about, in interpretations like the Ensemble interpretation, and Copenhagen, a state is associated with a state preparation procedure - which most of the time is really an observation in another guise.

    You will find a discussion of this interesting issue in Chapter 9 of Ballentine - QM - A Modern Development.

    Thanks
    Bill
     
  11. Dec 11, 2013 #10

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    Then that has an issue - a state is a state preparation procedure - what prepared the state of the entire universe?

    The false vacuum. In MW no collapse ever occurs - superposition's remain in superposition's. But that's not an issue - a superposition is simply another state - big deal.

    Copenhagen has issues. As far as I know those into applying QM to the entire universe use MW.

    Thanks
    Bill
     
  12. Dec 11, 2013 #11

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    This is getting a bit off-topic and has been addressed in many threads devoted to it so what I am going to briefly outline is not the whole story of this controversial issue.

    To understand apparent collapse you need to understand the concept of proper and improper mixed states:
    http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

    Now what decoherence does is transform a superposition into an improper mixed state Ʃpi |bi><bi|, the |bi><bi| being the possible outcomes of the observation.

    This has exactly the same form, and is observationally indistinguishable from, a proper mixed state. A proper mixed state is the state |bi><bi| randomly presented to be observed. If it was a proper mixed state measurement problem solved - no actual collapse occurred and what was there was revealed by measurement.

    The debate however is if that's good enough. We will not reach a conclusion on that in this thread. Best to read the link I gave above and form you own view.

    Thanks
    Bill
     
  13. Dec 12, 2013 #12
    Your questions might be at/beyond the edge of our knowledge, that said...

    The wave-function of the universe does not have to collapse. It can change shape, and form, continually.

    Wave-functions can break into smaller waves or join to form larger waves.

    Super-positions and entanglements continue to exist -- even when there is an apparent "collapse" of the wavefunction . They simply change form/partners.

    It's a question of semantics on what one considers as "collapse".

    The change in shape, form etc, of the wave-function, might/could have been triggered by the uncertainty/random fluctuations.

    As objects get bigger the effect of their overall wave-functions gets diminished.
     
    Last edited: Dec 12, 2013
  14. Dec 12, 2013 #13

    rubi

    User Avatar
    Science Advisor

    There's also the possibility in quantum theory that the universe isn't described by a wave-function (a pure state) at all, but rather just a density matrix (a mixed state) and the time-evolution isn't given by a Schrödinger equation, but rather by a Lindblad equation or even by a non-Markovian generalization of it. There needn't be a "surrounding" theory such that this density matrix comes from tracing over some degrees of freedom of a pure state (contrary to what happens in decoherence). (That's not just a crazy idea, but could actually happen in black hole evaporation for example as pointed out by Unruh and Wald.)
     
  15. Dec 12, 2013 #14

    atyy

    User Avatar
    Science Advisor

    In "standard QM", eg. Landau & Lifshitz, there cannot be a wave function of the universe, because a classical apparatus is needed to collapse the wave function. In L&L's view, QM is not a complete theory, because it requires classical measuring devices.

    I once asked a question about a similar topic, and had helpful answers from Demystifier and martinbn: https://www.physicsforums.com/showthread.php?t=667996 .
     
    Last edited: Dec 12, 2013
  16. Dec 12, 2013 #15

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    I don't get it (not that it's problematic - it certainly is - but cannot I think is a bit strong). There is no theory of the early universe I am aware of that requires the universe to be observed - so that it of course cant be observed is not an issue in that sense. As explained below its a bit strange considering what the wavefunction is in Copenhagen, but I don't think it's actually ruled out.

    I certainly agree applying the concept of state to the entire universe in the usual interpretations like Ensemble or Copenhagen is highly problematical because they associate a state with a preparation procedure.

    In Copenhagen the state encodes the results of observations if it was to be observed - and since it cant be observed one has to question how it can have meaning. However, logically that doesn't preclude it existing - you do not have to observe a quantum system for it to have a state. Still its a bit of a fine point in logic, and since a state in those interpretations require some kind of preparation its highly problematical for sure. But I don't think it's completely ruled out.

    MWI however handles it with ease, and most guys I have read that talk about the wavefunction of the universe seem to adhere to that.

    Thanks
    Bill
     
    Last edited: Dec 12, 2013
  17. Dec 12, 2013 #16

    Demystifier

    User Avatar
    Science Advisor

    Those who prefer some variant of the orthodox (Copenhagen) interpretation often believe that the enire universe does NOT have a wave function. According to such a view, wave function is only a tool useful for description of microscopic systems, and wave-function collapse is not real.
     
  18. Dec 12, 2013 #17

    kith

    User Avatar
    Science Advisor

    You seem to have a misconception about the concept of superposition. Being a superposition is not a property of the state. A particle which has a sharply localized position is at the same time in a superposition of momentum states. Whether you use a superposition description or not depends whether you are going to measure the position or the momentum.

    In QM, a state is a vector in Hilbert space. It is your choice of basis which decides whether you look at it as a superposition or not. This choice is arbitrary until something singles out a certain basis. This happens in measurements, where the final states need to be eigenstates of the physical quantity you are measuring.
     
  19. Dec 12, 2013 #18

    atyy

    User Avatar
    Science Advisor

    Perhaps "cannot" is indeed a bit strong. However, we do observe the universe and it doesn't seem to be in a superposition, but we do have a quantum theory which seems to match observations as to how cosmic microwave background fluctuations came about. So if we collapsed the wave function, then we would be excluded from the wave function as the classical apparatus.
     
  20. Dec 12, 2013 #19
    I think we should differentiate between standard QM and the standard interpretation of QM. Standard QM is simply QM as it is now. It seems standard QM has become a synonym of the phrase "standard interpretation of QM" in this thread.
     
  21. Dec 12, 2013 #20
    I thought wave function collapse was real under Copenhagen?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: The Wave Function Of The Universe
  1. Wave function (Replies: 2)

  2. +/- wave function (Replies: 5)

Loading...