Schrödinger's cat

Discussion in 'Physics & Math' started by fess, Jan 30, 2020.

  1. arfa brane call me arf Valued Senior Member

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    You choose a certain amount of radioactive material, so you have the information that, given its half-life, there is a 0.5 probability of a decay event occurring in a known time interval. This knowledge in fact corresponds to a measurement because you have some classical information. It also tells you that after 1/10 of the time interval the probability of decay is 0.05; in fact you know the probability of decay over the entire interval of time. This measurement is usually called preparation of a quantum state.

    When the decay event occurs (or doesn't), the cat 'measures' the quantum state; actually the cat is measuring the state over the whole interval. When the box is opened at the end of the allotted time, a human observer sees the result of this measurement.
     
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  3. phyti Registered Senior Member

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    The wave function is a mathematical tool for calculation purposes, not a physical entity.
    In the same manner as the axis of simultaneity for Special Relativity.
    The problem in modern science is the inability to distinguish between mental concepts within the mind and the reality they represent outside the mind.
    In the coin example, while it's in the air,oscillating between H and T, you cannot know its state, since it is a dynamic system. You only know when it comes to rest, which 1 of the 2 it is. As has been mentioned, it is about your knowledge of something, which is only as accurate as your last observation. The star you see may not even be there NOW.
     
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  5. DaveC426913 Valued Senior Member

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    It is worse than this. It's not about what you can and cannot know - Bells' hidden variables theorem shows that it does not - and cannot - have a state at all while it's tumbling.
     
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  7. Halc Registered Senior Member

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    Different interpretations assert different things about the cat scenario. Some have the wave function collapse and others do not. Some have the measurement (observer?) play a role and some do not. QM itself only says the cat in the box is in superposition of being both dead and alive, which is a different state than say a coin flipped and still held, unknown, under one's hand.

    JamesR seems to give some good answers.
     
    Last edited: Feb 2, 2020
  8. arfa brane call me arf Valued Senior Member

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    You prepare what's known these days, as a cat state (and yes, there are kitten states), by essentially making what Schrodinger chose in his original thought experiment as a hidden observer of a quantum state, into some classical device that records the event, and gains some information so can store it in n bits or less.

    The box is also a device, in that context; closing then opening the lid of the box is an operation on this quantum state, because it's equivalent to a preparation--the state is physically hidden for a precise amount of time. An assumption is made about how "accurately" the sample will decay.

    The glass vial of poison gas is also a device, you don't really need a lab rat (or cat). In that paper I posted the classical devices are the players of a measurement game.
     
  9. phyti Registered Senior Member

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    Put a cat in a box, with no radioactive element or toxic substance, food or water, but with ventilation. From then on, you will still have to make an observation to know the cat’s state, dead or alive. There is no quantum connection, just a nonsensical misapplication, as noted by exchemist #4.
    Search techniques use the same idea, determining an area of location based on last known sightings. When a new sighting is made, their ‘wave function’ collapses, and is recalculated with a new origin.

    Quantum physics suffers the same symptoms as Relativity, over simplistic, absolute, ideal values.
    In Newtonian physics, you can’t determine position and momentum simultaneously.
    The first requires 1 measurement, the second 2 measurements.
    But who would’ve thunk it!
    This anonymous statement contains much truth.
    “Quantum physics isn’t as strange as human thinking.”
     
  10. phyti Registered Senior Member

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    If only we could know how the physical universe works at the quantum level.
     
  11. arfa brane call me arf Valued Senior Member

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    Here is a bumper-sticker version, an interpretation of QM and its logic (the one we all struggle with which implies a thing can be in two states).

    It seems to be the case that we need to remove a conscious, possibly intelligent (as a cat or as, a rat?) thinker, from the scene. We have to consider all observers as devices, able to store information in a finite space (or number of bits).

    So opening the box, an operation, equates to writing a finite string of bits in a memory. All the thinking (strange or otherwise) is not relevant to the information available; this is why Alice can cheat in a quantum game.
     
    Last edited: Feb 3, 2020
  12. wegs Matter & Pixie Dust Valued Senior Member

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    I've always come away a bit confused by this ''thought experiment.'' Is it suggesting (or directly stating) that when it comes to quantum physics, in every possible state in every possible location, at every potential moment in time, we can only observe one state?
     
  13. arfa brane call me arf Valued Senior Member

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    Well, you can think about it this way. If observers can only record events of any kind, so an observer is really just a classical device, you could argue that the universe of a single observer is equivalent to a quantum decay event (or some product of many events). But over continuous, "successive" intervals of time.
     
  14. James R Just this guy, you know? Staff Member

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    Every object or system has lots of different properties that can potentially have different values at different times. Looked at another way, there are lots of different kinds of observations we can potentially make on a system or object at any given time.

    For example, if we are looking at an electron, we might decide to determine its position, its speed, its spin, its charge, etc. Each one of those "observables" has a set of possible observational outcomes; for instance the spin can have only two states (sometimes called "up" and "down"), while the electron's position can have any of a theoretically infinite number of values. But whenever we make a measurement of one of those observables, we only ever see one value of the observable, out of all the possible values.

    The overall state of the system is described as a sort of combination of all the possible observations we might make on that system. Depending on what we choose to measure, the state can be affected in part by our measurements. That is, measuring any part of the state potentially disturbs the system, forcing that part of the total state that was measured into one of the possible observational outcomes.

    The only way to explain quantum behaviour, it turns out, is to allow a system to have more than one possible value of an observable at the same time until we measure that particular observable. An example would be an electron whose spin state (before measurement) is some kind of superposition (i.e. combination) of spin up and spin down until we measure it, at which time it becomes one or the other.

    Experiments and theory tell us that it is not possible to say that the electron was "really" in the spin up state, say, before we measured it or that any measurement of the spin was "always going to give a spin up result" because some kind of hidden variable that we don't have access to predetermined that outcome.
     
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  15. arfa brane call me arf Valued Senior Member

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    If in the cat in a box thought experiment we try to assign superposition of states to a classical observer, it gets ridiculous; that was Schrodinger's point, QM is ridiculous when you try to make quantum probability into a linear function, and assign such states to classical objects like cats or glass vials.

    You assume that time is linear, so, it is . . .
     
  16. James R Just this guy, you know? Staff Member

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    Seemingly so. The question then becomes: at what level of complexity do quantum superpositions become untenable, and why?

    Recall that nothing in quantum mechanics says that the theory does not apply to macroscopic objects like cats. So, assuming the theory does apply, something more interesting or subtle must be going on to explain why macroscopic superpositions are not observed in practice.
     
  17. QuarkHead Remedial Math Student Valued Senior Member

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    I confess I cannot see where superposition comes into it. As far as I know, the emission from a radioisotope is undetermined. (exchemist will correct me if I am wrong)
    If emission occurs, then the release of the gas is determined, and the life of the cat is likewise determined. If not, then not and then not.
    The fact that the "state" of the cat is not known until it is ovbseved is a trivial statement.
     
  18. wegs Matter & Pixie Dust Valued Senior Member

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    Who is the observer? Or set of observers? (Like when you say ''we,'' what do you mean?)
     
  19. arfa brane call me arf Valued Senior Member

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    Because of the preparation (an amount of radioctive matter is hidden for a period of time) a decay event can be said to be in a superposition of 0.5 occurring and 0.5 not occurring; the probabilities depend on the half-life.

    Likewise for a half-silvered mirror which reflects or transmits each photon with 0.5 probability. Superposition (in an experiment with observers who record information) exists precisely because it has been determined. Modern beam-splitters are precisely tuned waveguides, Schrodinger's cat is placed in the box for a precise amount of time.

    I also said that if the probability of a decay event is 0.5 then with the assumption that this is a linear relation, the probability is 0.05 over 1/10 the time (if time is linear). But that's a big fat assumption. All I can really say with certainty is that a given mass of radioactive matter has 0.5 probability of emitting a decay particle within a certain time.
     
    Last edited: Feb 4, 2020
  20. Halc Registered Senior Member

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    I understand the distinction you're talking about here. A flipped coin held under the hand is merely unknown, but not in superposition of heads and tails.
    The difference with the slits and the radioactive decay and such is that the particles, unmeasured, can still interfere with each other, and this cannot happen with a collapsed state such as the coin.

    That said, I know how superposition of say a photon taking one path or the other can be demonstrated. I don't know how they do it for radioactive decay, and hence I cannot precisely describe how the one situation is different than one that is merely unknown.
    They've put a macroscopic object (big enough to see without aid, but still not a cat) into superposition, but I don't know how they verified it being thus. The pop articles seem always to stop short of such details. Under certain interpretations of QM (including RQM, my preference), it is quite trivial to put a human into superposition of states. All you need is distance between the observer and the one observed. Under other interpretations, especially ones that hold to the principle of counterfactual definiteness like Bohmian mechanics, a cat cannot be in superposition, even in the box. None of these interpretations give any special consideration to consciousness. The Wigner interpretation does, but that one is so freaky that even Wigner himself backed away from it.
     
  21. James R Just this guy, you know? Staff Member

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    That's one of the big questions, actually.

    Some people argue that an actual, conscious being is required in order to "make an observation" in the relevant sense. Those people would say that our universe only exists as it does due to the existence of conscious beings.

    Other people - myself included - think that it is far more likely that the "environment" of a system somehow determines when an "observation" is made on a quantum system. That environment might include conscious beings, but it needn't do so. A typical example would be when a photon hits a piece of photographic film or an image sensor in a camera, thereby being "measured" to be in a specific location, despite possibly having been in a superposition of states before striking the film or sensor. On this view, there's no need for a human being, say, to look at the film in order for the photon wave function to have collapsed.

    Roger Penrose conjectured many years ago that maybe gravity has something to do with causing wavefunctions to collapse. I'm not aware of the current status of that idea.
     
  22. phyti Registered Senior Member

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    James R;
    #33
    [The part underlined would be a contradiction to the 'wave function' concept if it did occur.
    The word 'assuming', makes the remaining statements conditional on the existence of a 'wave function'. Is it about physical matter or about mental perception?
    I think a system has no state until it is observed. The 'wave function' is an attempt to describe the 'state of a system, in a formal manner rather than say "We don't know".
    The 'wave function' is about what we know, not about a system. If someone asked me, while composing this, if the Eiffel tower is still standing, my reply is "I don't know with 100% certainty" (based on last tv news, that did not mention anything to the contrary). I can't eliminate a terrorist attack, earthquake, etc. has occurred since then. I need an indirect observation via the news to verify it is/was there.]

    #38

    [The human observer can be replaced by a device, especially with a greater capability, like the Hubble telescope. The observations still require human assessment, since they only have meaning to human society.]
     
  23. gmilam Valued Senior Member

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    Which is putting the cart before the horse, so to speak. Since the universe is what brought about the existence of conscious beings.
     

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