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

Schrodinger's schrodinger's cat

  1. Mar 31, 2007 #1


    User Avatar
    Homework Helper

    I'm sure this has been thought of before, since it's a very natural extension of the original thought experiement, but I'm too lazy to look for it, so I'll start a new topic.

    Say you carry out the schrodinger's cat experiment. The gist is there is a cat in a box, and the box contains something that will kill the cat (I won't go into the details here) with a 50% chance, depending on some microscopic event, like the radioactive decay of a nucleus.

    Quantum theory tells us that the nucleus is in a wavefunction that is a superposition of two states: one where it decays and one where it doesn't. When we observe it, this wavefunction collapses, and the nucleus has either decayed or it hasn't, not both. We can precisely calculate the odds of these two events, but no additional amount of information can help us predict with certainty which we will observe.

    The point of the cat is that we have now linked a macroscopic object to this microscopic event, and exposed the weirdness to a thing closer to our intuition. We now have to say that the cat is in a superposition of a dead state and an alive state, and until we observe it there's no way of knowing which will appear.

    Some use this to suggest the wavefunction collapse is brought about when a microscopic system interacts with a macroscopic device, like a cat or a photon counter. I don't know the logic behind this, but it would seem such a suggestion is within our current ability to verify or falsify, and since it isn't widely accepted I'll assume it lays on a tenuous argument. But if someone knows better, please let me know.

    Another idea is that the wavefunction collapses when observed by a conscious being. Maybe cats are conscious, maybe they aren't, but surely the experimenter is, and when he observes the cat, there's no superposition in his mind of a dead cat and a live one.

    But now say this entire setup is in an enclosed area, and another experimenter is waiting outside. To him, the entire room is in a superposition. Even after the experimenter observes the cat at 12:00, the room is in a superposition of one state with a living cat and a relieved experimenter and one with a dead one and an experimenter with some regret. But of course, the 1st experimenter at 12:01 is in no such superposition, it turned out exactly one way for him.

    So is the 2nd experimenter wrong? Despite him having all the physical information and not being able to make a prediction with certainty, is there really an answer lying somewhere? A hidden variable in the 1st experimenter's consciousness?
    Last edited: Mar 31, 2007
  2. jcsd
  3. Mar 31, 2007 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Here's my take on it. I believe the second experimenter is not wrong. Let's start from the point of the 1st guy entering the room. As he enters the room, and makes his observataion, he becomes part of the experiment. So, to the 2nd guy outside, the room is in a superpostition of states, one with a dead cat and 1st guy, the other with an alive cat and 1st guy. The fact that the 1st guy knows what has happened, does not mean that the second guy can find out until he observes it. Of course, when the 2nd guy goes in and looks, then he knows what has happened, and becomes part of the experiment.

    The point I am trying to make (probably badly) is that we need an external observer-- i.e. one who is not part of the system being observed.
  4. Mar 31, 2007 #3


    User Avatar
    Homework Helper

    But we're assuming the second experimenter has all the relevant physical information. Then, according to quantum mechanics, it is still entirely random what he will see when he opens the door, and no amount of extra information can help him. But at 12:01, the first experimenter is experiencing something, not two things. The wavefunction of the cat has collapsed for him, but not for the second experimenter. When it does collapse for the second experimenter, it must do so in the same way as for the first. So it wasn't random.
  5. Apr 1, 2007 #4
    This problem is referred to as 'Wigner's Friend'

  6. Apr 1, 2007 #5


    User Avatar
    Homework Helper

    Ok, so what is the resolution? It seems Wigner concluded minds aren't material, but that doesn't seem helpful. The other "many-minds" hypothesis seems far fetched.

    If consiousness is a hidden variable, then wouldn't bell's theorem imply it means quantum theory is non-local? I'm not looking for a definite answer, because I know no one has it, but how does this narrow the possibilities?
  7. Apr 1, 2007 #6


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Hi statusx,
    I see Wikipedia states that the Wigner's friend thought experiment would indicate that consciousness is not material, but that really doesn't make much sense to me. The thought experiment doesn't get away from the fact that there must be a material substrate on which consciousness can be manifest, it really only says that QM indicates reality is relative to the observer (material substrate) that perceives anything.

    Wigner seems to want to tie consciousness to reality. From Henry Stapp:
    Ref: http://www-physics.lbl.gov/~stapp/WignersFriendStapp.doc
    Wigner says this because the conscious observer perceives some material in a state of supposition. Thus, there is an implicit assumption that the unconscious material is ALSO in a state of superposition with respect to itself. Note that this assumption goes unmentioned.

    Perhaps there's an alternate interpretation. I think one could go a step farther and suggest that although unconscious material might be considered to be in a superposition of states from the perspective of any conscious observer (assuming the material's proper isolation), one could also suggest that the material itself is no different from Wigner's friend, in that the unconscious material is not in a superposition relative to itself. I don't see any distinction being made between conscious and unconscious material, only that all material must be in a superposition relative to other material.

    In the case of the unconscious material, it can't perceive anything so it isn't able to make this distinction. Were the S's cat or Wigner's friend replaced with a p-zombie computer, is there any reason to believe it would not be able to tell you exactly what happened while it was in this magic box that isolated the p-zombie from the rest of reality? I don't think so. I think the p-zombie would tell you how many times it had to change batteries before you opened the box, and nothing odd or out of the ordinary happened while it sat there twiddling its fingers. The WF experiment tells us only that material is in a superposition relative to other material. But in the end the perspectives of two systems must 'match up' when they interact.

    Take for example, the C60 Buckyball experiment by Dr. Marcus Arndt in 1999. The C60 ball went through a diffraction grating and was found to produce an interference pattern, even when only one ball was fired at a time. If you took a ride on such a ball, you would have no way of knowing which path you took to the target, you would simply have waited for the few microseconds it took to reach the receiver and you would have reported that everything was perfectly normal during your ride. However, Marcus would have then told you that you must have taken both paths to arrive at your destination, and you'd reply, "No I didn't! I only took one path, I can be sure of that. I never stopped scanning the horizon during the flight!" And Marcus might respond, "Well didn't you see where you were, and where you were going?" But of course you DID look around, but you saw nothing because there was no light interacting with your C60 buckyball. If there was, your location would have collapsed and you'd only have taken one or the other path, not both.

    The question is, would this observer relative interpretation of QM disagree with the conventional view in any way? There's an interesting remark from Wikipedia:
    From that I assume there is a mathematical point of contention regarding this interpretation. I wonder if there's any one here that might know what that point of contention is, or if in fact it even exists. Wiki isn't the best resource and I'd always take what they say with a pinch of salt.
  8. Apr 2, 2007 #7


    User Avatar
    Homework Helper

    Let's ignore consiousness for a second. Can a brain be in a superposition of states?

    If not, what would be special about it? It's macroscopic size? Maybe, I don't know the details of this interpretation of collapse. The fact that it processes information? I don't think so, because this would suggest quantum computation is impossible.

    So let's assume there is nothing special, and a brain can be in a superposition of states. What does this imply for consciousness? It seems logically impossible that an experience could be in a superposition of states, so this would mean a departure from the one-to-one identification between consciousness and brain processes. It would seem to imply the many-minds hypothesis, that there become two experiences and, I guess, two experiencers. Then nothing really collapses, your consciousness just picks one of the brain eigenstates to live in. How does it choose which one? I don't know. I'm just rambling now.
  9. Apr 2, 2007 #8


    User Avatar
    Gold Member

    As far as I know experience has two components: stimulus and response. The stimulus can come from the same source as the response (the brain) and it can come from other sources. Response is limited to the neuronal activity of the brain. But experience requires these two elements to take place. Especially in the experience of self I would think there may be a superposition taking place where the self is the stimulus as well as the response and this would be a case of two different states simultanieously taking place in the same position.

    We seem to have the ability to choose between being a stimulus and being in response to stimulus. However we often end up existing in and exhibiting both states. I don't know if this pertains to your thread. Its just a response stimulated by your thread.:rolleyes:
  10. Apr 18, 2007 #9
    Going back to the OP:
    I think that as the 1st experimenter also observes himself at all times, then there's no superposition of states for the 2nd experimenter to collapse.
    I'd go further, and suggest that the 2nd "experimenter" is actually no such thing, just a stressed physicist in need of coffee :biggrin:
  11. Apr 23, 2007 #10
    If you'll forgive the pun, conciousness is probably immaterial. Even observation isn't necessarilly required for wave function collapse. The copenhagen view, though still used in contemporary quantum theory, has been revised since the 1930s.

    The relational model (see Wikopedia) is an updated version and appears to hold up quite well. Here, superposition can exist within a closed system and mere interaction with another system is sufficient to collapse the wave function.

    If true, this evolution of the theory has implications. Note: from the point of view of someone outside a closed system, events inside can remain in a state of superposition regardless of how many interactions or observers there are within that system. A collapsing wave function only describes the interaction between the two systems

    So from the point of view of the 1st experimenter (and the cat) inside a sufficiently sealed room, there is no superposition; there is instead one state of reality. From the point of view of the 2nd experimenter, the room remains in a state of superposition. The cat's status is not only unknown but not determined. However, this wave function only exists relative to those outside the room, not inside, and collapses the moment the door is opened.

    Enter the 'many worlds' interpretation. According to this, while the room remains sealed, there are two seperate 'mini-universes' objectively co-existing inside the room - one with a dead cat, the other with a live one. Both are real. What remains a matter of chance is simply which of the two mini-universes will be encountered by the 2nd experimenter when the room is unsealed. At that point, as soon as the door is opened, the 2nd experimenter now becomes two versions of himself in seperate universes - one that discovered a dead cat, the other that discovered a live one. Thus, the collapse of a wave function describes the process in which two possible states continue to exist in separate universes, though only one state is observed. Superposition, in so far as it is sometimes manifested (as with twin slit interference) describes a representation of two states that have not yet divided from a particular observer's point of view.

    Last year I posted my 'revisionist' version of Shrodinger's Cat.


Share this great discussion with others via Reddit, Google+, Twitter, or Facebook