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Shrodingers cat. Observer

  1. Jul 11, 2010 #1
    Please forgive my ignorance, I have no educational qualifications and am finding my self being sucked into the wonderful scientific world but one thing that troubles me is the Shrodingers cat experiment. As I beleive it to be the case that in order for the uranium atoms to decay they need an observer(is that the copenhagen interpritation?), so until some one looks inside the box the cat is both alive and dead(not bringing the MWI into it where the cat is alive and dead only in other universe) but is the cat not also an observer? if so the experiment is flawed as the observer is the cat in the case of the uranium atoms decaying or have I got this completley wrong?

    Please help, my head is hurting!
  2. jcsd
  3. Jul 11, 2010 #2


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    The point of this thought experiment is to show that QM says very clearly that if microscopic systems can be in superpositions, then so can macroscopic systems. However, the argument uses the Schrödinger equation, which only applies to systems that are isolated from their environments. In an actual experiment, an object as large and complicated as a cat will always have non-negligible interactions with its environment which will render all quantum effects unobservable. Essentially, all the "weirdness" is moved into the environment.

    Forget what you heard about opening the box and "looking". That's not how observations are done in QM. There's no such thing as "just looking" in QM.
  4. Jul 11, 2010 #3
    Thanks for that. I was watching a programme about hugh everett and got most of what I know from that, ref "looking inside" that was the catalyst(no pun intended) for me to suggest that the cat was also an observer, anyhoo thanks for explaining it to me, much appriciated
    I need to get back to school me thinks! ; )
  5. Jul 11, 2010 #4


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    Really? I've never understood why people think the box is important. It seems that that part is just due to misunderstanding what 'measurement' means. (The same misunderstanding that leads people to say that "QM says the moon is not there when you don't look at it", which the Bohmists seem to repeat a lot.)

    Say we shoot an electron at a screen, and the screen is divided into two parts A and B. There is an apparatus arranged such that if the electron hits part A, then a ball is pushed to the left. If it hits part B, then it is pushed to the right. Surely it means nothing to describe the ball as being in a superposition? This is why Bohr emphasized that in any well defined application of the quantum formalism, we must always make a distinction between the classical bodies (or experimental apparatus) and the quantum system.
    Last edited: Jul 11, 2010
  6. Jul 12, 2010 #5
  7. Jul 12, 2010 #6
    Hmmm. As a bona fide layman, very aware from having been following various threads on this forum recently that there are a fair number of contributors to this forum who really know what they are talking about, I have to be a little wary of presuming to explain things to you, gibbo76. But if I have understood you correctly, there is a purely technical aspect of this that you have not got quite correct. I think I am correct in suggesting to you that the decaying of the uranium atom does not require an observer. What requires an observer is to determine whether or not the uranium atom has decayed.

    If you have a lump of uranium large enough for you to see it as a lump of uranium, it will contain some incredibly large number of atoms. If you look at any individual atom in the lump, you cannot predict when that individual atom will decay. What you can say is that over the period that is the half life of uranium, exactly half of the atoms will have decayed, and half will not. Over the next similar period, half of the remainder will have decayed, and so on. You will thus notice that lost somewhere in that lump of uranium are some atoms that will never decay, because it is always half of the remainder that decay.

    So, observer or no observer, the uranium atom detonator in the box with the cat, may or may not have decayed in the period it was in the box with the cat. That we cannot know for certain unless we observe it. The Copenhagen Interpretation says that the atom is thus in a superposition of having decayed and not having decayed until we observe it to resolve it into one of the two circumstances. Schrödinger sought to highlight the absurdity of this notion by connecting the question of whether or not the atom had decayed to the survival of the cat, raising this notion of a cat that is in a superposition of being alive and dead at the same time. You should understand that Schrödinger came up with the thought experiment in the 1930s. Decoherence is a much more recent addition to the physicists armoury of understanding.
  8. Jul 12, 2010 #7


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    Just to clarify, I'm not saying classical bodies can't be in superpositions*. I just don't see how Schordiner's thought experiment shows this.

    (*That can be seen more clearly and directly: In the double slit experiment, the appearance of the interference pattern necessarily implies that the slit-screen is exactly coordinated with the spacetime frame, i.e. described classicaly If however we want to predict the direction in which the electron goes by measuring the momentum exchange with the screen, it must not be fixed, and has to be treated as part of the quantum system, and therefore it mist be affected by the Heisenberg uncertainty. This of course implies that it can be described as being in a superposition of moemntum/position states.)
  9. Jul 12, 2010 #8
    In decoherence isn't the wave packet is simply "bunching up" to a pointer like state at point of thermal interaction? (I'm noob, by the way.)

    Doesn't this mean that macro "superpositions" are still maintained to a certain extent? (Although, of course, these superpositions will be totally divorced from the type of superposition observed in highly controlled, isolating experiments).

    1 more thing, is decoherence fact? I.e. not just another interpretation, but definite? (I know dmitry said this, I think it was him).
    Last edited: Jul 12, 2010
  10. Jul 12, 2010 #9
    Living cat is an observer, dead cat isn't. So you can always say that the Schroedinger's cat is a superposition of an observer and something else lol :).

    I personally believe in not-so-many-MWI. That is: the observed collapse of wavefunction occurs when the observer himself undergoes quantum superposition. However, the spacetime is one and only for all of the universes, that means, spacetime curvature is not a subject to uncertainty.
  11. Jul 12, 2010 #10
    I'm not a physicist, but how is this picture coherent? Different strands of reality in MWI will have different assortments of matter within spacetime --> different curvatures.
  12. Jul 12, 2010 #11
    My picture isn't complete in any degree. This is just my idea of the physical world, I find it most consistent with current knowlegde and most cool.

    I don't know what is true. However, I can say what is false according to observations.

    1. Objective collapse theories are almost certainly false. We can see quantum superpositions of objects of arbitrary size, up to date. Maybe the Planck mass is the limit.
    2. Split states can interfere. So they belong to the same spacetime. There's no maths, that can describe interference of objects from different universes. I.e. when a photon gets split in double-slit experiment, MWI says that the universe split into two, with a photon passing one of the slits in each one. However, the photons interfere afterwards. So they couldn't have been travelling different spacetimes. They still were close to each other.
    3. In classical relativity, spacetime curvature depends both on position and momentum of particles. QM says, that position and momentum are Fourier transforms of each other. However, gravity does not treat them as such. The gravitational force of a particle of some momentum can not be expressed as a Fourier transform of particles of some position. That means, gravity is not linear, it does not depend on only one universe, but rather on the set of them.

    What I try to say: I believe in MWI, but different universes are not orthogonal, but they interact in nonlinear fasion. So they need to share a common "place", some hyper-universe so they can be added one to another. I don't think there is any hyper-universe bigger than our spacetime.
    Maybe a particle in one universe feels some force from particles in different universes, so it does not fly too far and can't bend spacetime in illegal way.

    There are all just thoughts. It will be cool to see the progress of science and see some of my claims proven of falsified. I think we're close to making experiments to test correctness of QM interpretations.
  13. Jul 12, 2010 #12
    This is called Quantum Suicide

  14. Jul 12, 2010 #13


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    I'm confused by the fact that you're putting this question and this statement together. They seem completely unrelated. (I agree about the box...and I think my post made that clear).

    Yes, really. Suppose you bet 1000 dollars that the result of a spin-1/2 measurement will be "up". If the composite system that includes you and the particle starts out in state |+>|:shy:>, it will evolve into |+>|:smile:>. If it starts out in state |->|:shy:>, it will evolve into |->|:frown:>. Now linearity implies that if the particle starts out in a superposition, the time evolution will be

    (a|+> + b|->)|:shy:> → a|+>|:smile:> + b|->|:frown:>

    The right-hand side is a macroscopic system in a superposition. Of course, this isn't what actually happens, even from my point of view, because I'm unable to prevent you from interacting with your environment. So the composite system will never be in this superposition, but the Schrödinger equation says that it would, if we could isolate you from the environment.

    This looks like a misunderstanding your part, but then I haven't actually read Mermin's article so I don't know what he was talking about. Think about it this way: Classical physics predicts that the Moon will have a particular orbit. This is a prediction about what we will find when we measure the position of the moon, but it's also a statement about what the Moon is "doing" when nobody looks. No one would argue that classical mechanics only describes reality at times immediately after a measurement. But in QM the situation is much more complicated. QM makes even better predictions about results of experiments, but it's not at all clear that the state vector can be thought of as a description of what the system is "doing" at times between state preparation and measurement. The question "is the moon there when nobody looks" is just a rhetorical question to make you think about how weird this is. It tries to explain how weird QM is by showing you how bizarre it would be to claim that classical mechanics doesn't (approximately) describe reality at all times.

    I think Mermin was trying to make a similar point, but as I said, I haven't read his article. I'm just assuming that he was saying something valid, and this seems to be the only valid point that this phrase can be used for.

    Of course it means something, and it's something very non-trivial, since superpositions are very different from "either this or that" situations.

    I think this is one of the most misunderstood things in physics. Bohr considered measurements to be a fundamental part of science, and was merely acknowledging the fact that we would never consider a measurement to have been completed until the measuring device can (for all practical purposes) be described as being in a well-defined classical state. (I'm not saying that you have misunderstood it, but I can't tell why you're saying "this is why Bohr...", so I don't know if you have or not).
  15. Jul 12, 2010 #14
    No, I don't believe in Penrose interpretation as I understand it.

    His interpretation is an objective collapse. He says, that two entangled states evolve independently, until the difference between them exceeds a certain limit. Then an objective collapse occurs.
    I don't like this approach, since it is so... discrete and sudden. I'd like physics to be more continuous.

    As Penrose said himself, his interpretation is in explicit contradiction with Liouville's theorem. Any objective collapse theory is. General relativity is also non-Liouville, in particular the no-hair theorem.
    MWI and similar are consistent with Liouville's theorem.
    The only question is - is Liouville's theorem really true. I believe so. I don't think the black hole thermodynamics can be done without it. That's why I believe in a variant of MWI.
  16. Jul 12, 2010 #15


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    Ok, here is my take on this.

    IMO, and in my "interpretation", the meaning of something beeing in superposition relative to a given observer, is pretty much a statement of the expected action of the observer. This is IMO the difference: The action of the observer, is different depending on wether something in it's environment is in a superposition or not.

    To take an example of gambling, the action of a gambler reflects ALL possible futures, as opposed to just one of them (at random).

    The real difference in the schrödinger cat experiment, is the behaviour or action of the outside observer, depending on how it's information is beeing updated.

    IMO, there is no objective meaning in a superposition as the state vector is observerdependent. The meaning of superposition is therfore equivalent to a statement of the expected action of the observer. The apparent objectivity of superposition of microscopic objects is because the environment is pretty much rigged or part of the preparation. Thus all the observer in the closest environment are in agreement, not because it's a forcing constraint, but because of it beeing so prepared or emerged.

  17. Jul 12, 2010 #16
    This is the problem with interpreting the formalism of QM; it doesn't require interpretation. The math works for the most part, and it is clearly not a final theory to describe nature. This is an amusing illustration of the microscopic using the macroscopic, and nothing more. This does not concern the average physicist using the Schrodinger equation; it is just an ontology with feet of clay.
  18. Jul 12, 2010 #17
    In my opinion, the next theory will be even more weird. We will long for the good old days, when the cat could be dead and alive.
  19. Jul 12, 2010 #18


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    In all this, you are assuming that the state vector can meaningfully be used in describing the classical bodies, like myself. That is what I am contesting. Another implicit assumption is that the 'state vector' has an interpretation as a 'state'. This is not at all clear.

    From my point of view (about distinguishing between classical bodies and quantum system in applying the formalism of quantum mechanics), this is superfluous. Everything an experimenter does must be described in ordinary language. There is no quantum description of experiments. If I go into a lab, and do some experiment, I will be able to explain exactly what I did to my mother, who knows no quantum terminology. So looking at a chair or looking at the moon or not looking at the moon is all ordinary unambiguous language, with clear meaning. QM says nothing more about that. Qm only comes in in descrining 'quantum systems', and this implies that this distinction between apparatus and quantum systems is necessary. Where exactly the line is drawn depends on the experimental situation.

    In principle, one never needs to speak about 'state vectors' in quantum mechanics. Only classical terminology suffices. This is a point that I think very few people understand. When you see a dot on the photographic plate, you say "the electron hit the screen". We can say things like "the number of electrons in the box is N", etc. Except that when we say these things, classically various other classical statements are immediately implied, but in quantum mechanics, different classical descriptions are complementary. A situation that permits the use of classical description A makes descriptions of type B meaningless.

    Me too.

    See above.
    Last edited: Jul 12, 2010
  20. Jul 12, 2010 #19


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    Sounds like we agree about most of this, but I don't think of it as a distinction between classical and quantum systems.

    Right. But now you're talking about measurement results (what we actually see). The comment about the Moon is only relevant when we're discussing what QM says about the system at times between measurements.

    But in QM, composite systems are represented by tensor products, and there's nothing in the theory that says that there's a size limit or anything like that. If you're thinking that decoherence is something like that, I would say that it isn't. It's a quantum effect that moves other quantum effects to places where they are hard to detect. Since QM isn't drawing a line between quantum and classical other than that measuring devices are defined as having no noticeable quantum effects, it's clear that QM is saying that it applies to large systems as well. It might be wrong, but that's another issue entirely.

    Not sure what you mean here. If you're saying that it isn't obvious that we can think of the state vector as representing all the properties of the system, or a description of what the system is "actually doing", that's what I've been saying myself.
  21. Jul 12, 2010 #20
    Weird or not, the fact that we're looking to a theory to unify or replace existing theories kind of argues against forming a vision of reality based on them. They are predictive and work, but worrying about the cat or Wigner's friend is, to me, pointless.
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