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I Schrodinger's cat, the multiverse and isolated systems

  1. Jul 26, 2015 #1
    I know the debate about Schrodinger's cat is usually about things like consciousness but I want to talk about what it might say about isolated systems.

    Does the wave function of isolated systems remain in a superposition of observable states no matter how large the system gets?

    Say you have the radioactive material isolated from the cat in the box and the box the cat is isolated from the outside environment, wouldn't a live/dead cat be two observable states of an isolated system until the box is open and the wave function collapses locally?

    This collapse isn't just because of the Scientist opening the box, but the state of the cat interacts with his lab, the car on the highway and the entire observable universe.

    So if an isolated system remains isolated do all probable states of that isolated system occur?

    If you extrapolate that out to the universe and the universe is an isolated system doesn't that mean that every probable universe exist because there's nothing external to the universe to collapse it's wave function.

    I was reading a paper where Max Tegmark said this was a postulate of Everett.

    All isolated systems evolve according to the Schrodinger's equation.

    So our universe could be an observable state of an isolated system and if this observable state is isolated then you can have probable states within probable states. This would occur maybe for trillions of years until two observable states of the isolated system interacted and maybe you get something like a big bang and everything reboots.

    I saw where Penrose talks of Objective Reduction where isolated systems self collapse after they reach a certain threshold due to quantum gravity but I haven't seen much evidence for that. I think Tegmark may have a point if the universe is an isolated system.
  2. jcsd
  3. Jul 26, 2015 #2
    If you use the definition of 'Universe' as meaning 'everything that exists', then an isolated system is what it is.

    Incidentally there is no underlying assumption in QM that the results of experiments have something to do with consciousness.
    Last edited: Jul 26, 2015
  4. Jul 27, 2015 #3


    Staff: Mentor

    Unless interacted with - of course.

    All an observation is is an interaction.

    As far as the universe is concerned by definition it cant interact with anything so you cant observe it.

  5. Jul 27, 2015 #4
    So the universe's state cannot collapse even in Copenhagen.
    Nevertheless, the state space can be factorized an almost infinite number of ways making it possible for an observer (one subsystem) to interact with another. Collapse is then not objective but is how a subsystem's state appears to behave when there is an observation.
    I believe the OP was talking about these improper collapses, to coin a phrase, :) under a factorization of the universe's state space, not about literally observing the universe as a whole.

    Nobody knows. But there is nothing in the standard formulations of QM to suggest anything otherwise.
    Every *possible* state. Yes.
    Not every interpretation even has wavefunction collapse, observer or no observer. But one should not think of this humungous superposition as being every *possible* universe existing alongside each other. Instead, each possible "world of observations" exists in the one superposition. Just as Schrodinger's Cat isn't two cats but one cat in two states. Which, for the purists, are of course entangled with those of the observer and the environment.
    I haven't read much Tegmark but I understand that he says there is no reason to assume that "our" universe has a monopoly on existence. The maths of physics, QM in this context, allows a vast space of possible universes and it it is perfectly possible that they all exist or don't exist to the same extent.

    I can't comment on the cosmological implications of Tegmark or of OR. The only time I heard of the universe rebooting was in Doctor Who :)
  6. Jul 27, 2015 #5


    Staff: Mentor

    No - and obviously so.

  7. Jul 29, 2015 #6
    If we think about the box isolating the cat from the rest of the environment and any "observer", then environmental decoherence would be prevented. Then we could think about the cat as being in both states at the same time but as far as I know, the only way that is being used to confirm superposition nowadays is by observing interference. In this case, I think as the cat and whatever else inside the box is macroscopic, you would not be able to see interference when some time in the future we make a measurement. Let's say that we don't see interference because the wavelength becomes too short and/or the phases disrupted by the interaction of the original microscopic quantum system with the immediate (inside the box) macroscopic surroundings. We could still think about a kind of superposition different form the typical quantum superposition in which we can observe interference. I think this could be described according to what some call an "improper mixture" and would correspond to a reduced density matrix where all the diagonal elements are still there.
    Of course it can be said that in practice it is impossible to make the box to prevent any leakage of information to the outside. But if we ignore that practical difficulty and imagine an ideal experiment in which the walls of the box achieve such insulating power, then it would be interesting to know what people adhering to different interpretations think about it. What do you think guys? (if there is a girl on this thread please forgive me for my sexist expression).
  8. Jul 29, 2015 #7


    Staff: Mentor

    That's incorrect. The cat cant be isolated from the environment and remain a cat eg it must breathe air.

    And it is entangled with the atomic source which means it is not in a superposition by the definition of entanglement - in fact its in a mixed state of dead and alive meaning it is either dead or alive - not in some weird combination of the two.

  9. Jul 30, 2015 #8
    Even without Alexepascual's isolating box, if one considers the system of [particle plus cat plus observer plus environment] then, under the assumed unitary evolution, the state of this system does evolve to a superposition of
    |everything that follows from the particle being emitted> or "|dead>" for short
    |everything that follows from the particle not being emitted> or "|alive>" for short
    which is just as wierd a combination as the cat itself being in a pure superposition.

    Would you not agree?
  10. Jul 30, 2015 #9


    Staff: Mentor

    This has been discussed many many times. I have even posted the math. For the umpteenth time, but from past experience it wont be the last, if you observe inside the system things do not remain in superposition - its a mixed state.

    You can do a google search and find where I went through the math.

    The solution to the whole thing in the ignorance ensemble interpretation is that the observation occurs just after decoherene. In Schroedinger's Cat the earliest point decoherence occurs is at the particle detector. The superposition of particle detected, not detected is converted into a mixed state and that mixed state is interpreted as a proper one so it no longer a superposition - but it is an actual state - not detected or detected.

    Now to someone observing it outside - everything continues on just the same - what they observe is also in a mixed state. The issue comes if they were to observe everything including the particle source - but that would be a totally different set-up.

    Last edited: Jul 30, 2015
  11. Jul 30, 2015 #10
    Well, it depends on what we mean by environment. I was using the word environment to refer to everything outside the box as once a signal leaks out of the box it is supposedly "uncontrollable" and we can't "erase" the information that has leaked out. According to Environmentally Induced Decoherence (at least as presented by Zurek) it is this uncontrollable leakage of information that produces decoherence. Now let's go back to the practical issue. I got a solution. We can put an oxygen tube inside the box and a small CO2 absorber. We can also put some toy for the cat to play so that he/she doesn't get bored. We would have to select a cat that is not claustrophobic. So Bhobba your statement is incorrect, the cat can continue alive for some time within the box.
    Last edited: Jul 30, 2015
  12. Jul 30, 2015 #11
    You make it sound as if you are getting tired of talking about the same things. But I suspect that on the other hand in some way you enjoy it.
    So let me see if I understand your position or interpretation. You are saying that as soon as the microscopic quantum system interacts with the macroscopic detector, the system-instrument-cat (and whatever else inside the box) are in a proper mixed state where the only reason we keep all values on the diagonal of the density matrix is because we don't "know" the actual value but one of those values has already been selected. In other words there is already definiteness, but on the other hand ignorance.
  13. Jul 30, 2015 #12
    Sorry to side track the discussion, but I'm hoping that this point helps me get a better handle on the general concept... which is the logical differentiation between "pure", "mixed" and "proper" quantum states. Please correct me where I'm wrong...

    In superposition, which is a "pure" state, the cat is dead AND alive (and I am simply accepting for the sake of discussion that the cat IS in superposition here).

    If we accept Bhobba's assertion that decoherence, even in the "isolated" system, prevents the cat from being in superposition (and I'm assuming that this does in fact cause "collapse" of the cat's quantum state), then the cat would be in a "proper", "mixed" state... meaning that the critter is dead OR alive, but we just don't know which.

    Is that correct?
    Last edited: Jul 30, 2015
  14. Jul 30, 2015 #13
    Let's see Derek what Bhobba thinks. I think that according to a more "many worlds" like interpretation, when the outside environment gets entangled with the cat, then a human observer also gets entangled with it. So you can say that the observer is in a state of superposition, but to each "version" of the observer, there is only one outcome and as Bohbba said the cat assumes a definite macroscopic state. The picture I proposed, was as the original poster did, to think of the box as isolating the inside of the box from the outside to prevent entanglement with the outside environment and with the observer. I was under the impression that in those circumstances, even other interpretations besides "many worlds" might see the cat as in a state of superposition. But this would be a superposition different from the one we are used to in quantum mechanics as it would not show any interference effects when measured. If we don't want to call it a superposition, we could always call it "indefiniteness". But for Bhbba, if I interpret him correctly, that's not the case and even if the inside of the box is isolated from the outside, as soon as the quantum system makes contact with the macroscopic instrument and we wait a little for the poison vial to break, the quantum system, the instrument and the cat assume a definite state. Well, I understand, the cat according to this view is always in a definite state because it is a macroscopic object. My view is different as you can see.
    Last edited: Jul 30, 2015
  15. Jul 30, 2015 #14
    Feeble, In my opinion your conclusion is correct if (as you said) we accept Bhobbas' assertion.
  16. Jul 30, 2015 #15


    Staff: Mentor

    I am saying the ignorance is in how an improper mixed state becomes a proper one. Here is the detail:

    Look up the ignorance interpretation in the above.

    The observation occurs at the particle detector. Everything is common-sense classical from that point on. Opening or closing the lid makes no difference to anything. Decoherence breaks the von-neumann chain by placing the cut at the earliest point decoherence occurs which in Schroedinger's cat is at the particle detector.

    Last edited: Jul 30, 2015
  17. Jul 30, 2015 #16


    Staff: Mentor

    You are talking about Zurecks Quantum Darwinism. I am talking about the ignorance ensemble interpretation which is much simpler. The mystery occurs at the earliest point decoerence happens - which is at the particle detector. Everything is classical from that point on.

  18. Jul 30, 2015 #17


    Staff: Mentor


    The trouble is the answer is technical and cant be explained in lay terms. You simply have to accept that this weird thing known as a superposition gets converted into this less weird thing called a mixed state by decoherence.

  19. Jul 31, 2015 #18
    Is my mistake in the "proper" designation after decoherence?
  20. Jul 31, 2015 #19


    Staff: Mentor

    Yes - its interpreted as a proper mixed state - its really a improper one.

  21. Jul 31, 2015 #20
    That's what I thought... and that's where my cognitive wheels grind to a halt. >_<

    I have a rough idea that it has something to do with diagonalizing through density matrices of potential quantum states... none of which I understand.

    But, my primary question is this... Does the "improper" mixed state imply that it's not arrived at by an ACTUAL quantum state "collapse" (assuming that such a thing really occurs)?
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