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

Shrodinger's cat is "invisible"

  1. Jun 5, 2015 #1
    Thought-Experiment :

    Let's just say we have Schrodinger's cat setup, with the radiation apparatus in place

    we place our cat inside a fully transparent box, so that we could see the cat in it's superposition without opening the box, then we could see if it's alive and dead right?

    But this is impossible, because if photons are being emitted by the cat, then that means they are absorbing photons as well, and thus the cat can't be in a superposition, due to this interaction. So if this cat were in a superposition, it would be invisible to us, even in a transparent box!

    Now invisibility may not exactly mean completely unseen. I assume that the cat would appear totally black, like the surface of a black hole.

    Any reason to think this is a faulty assumption?
     
    Last edited: Jun 5, 2015
  2. jcsd
  3. Jun 5, 2015 #2

    bhobba

    Staff: Mentor

    The cat is never in superposition. The observation occurs at the particle detector - everything is common-sense classical from that point on.

    Schroedinger's Cat is not what many popularisations make it out to be. No one ever took seriously the cat was in superposition. Its purpose was to show a defect in Copenhagen - namely it doesn't dictate where the quantum classical cut should be. With our modern understanding of decoherence its now a non-issue.

    Thanks
    Bill
     
  4. Jun 6, 2015 #3
    Before I even knew about quanthum mechanics and the superposition and all that other stuff, when I heard about this thought experiment, I thought it was about proving the point that if you're never gonna interact with something there's no point in talking about it, that is if some system X never interacts with system Z there's no point in X making a statement "Z is , or definitely must be like this or like that" .
     
  5. Jun 6, 2015 #4
    We actually don't know that observation (or to use another term, measurement) occurs at the particle detector. Otherwise the measurement problem is solved.
     
  6. Jun 6, 2015 #5

    bhobba

    Staff: Mentor

    That's one of the issues with Copenhagen. While its obvious in any actual set-up where you put the cut, and Schroedinger's Cat is no exception, the theory or Copenhagen doesn't demand it being put there. But with our modern understanding of decoherence it obvious that's where you put it, and in modern times that is where its put. Still the theory doesn't demand it - you need an additional interpretive assumption.

    This is one reason we have more modern interpretations like decoherent histories that avoids the issue entirely because it doesn't even have observations.

    Thanks
    Bill
     
  7. Jun 6, 2015 #6

    bhobba

    Staff: Mentor

    It was about highlighting an issue with the then dominant Copenhagen view:
    http://motls.blogspot.com.au/2011/05/copenhagen-interpretation-of-quantum.html

    Thanks
    Bill
     
    Last edited: Jun 6, 2015
  8. Jun 6, 2015 #7
    Okay, maybe I didn't word the experiment correctly.

    The question isn't really about whether a real cat/macroscopic object is able to be in a superposition. The question is that any object in a superposition is for all intensive purposes invisible. I just made it Schrodinger's cat to make the experiment more visual.
     
  9. Jun 6, 2015 #8

    bhobba

    Staff: Mentor

    An object in any pure state is always in superposition of many other states from the vector space structure of pure states. Your question is meaningless.

    QM is a theory about observations that appear here in an assumed classical world. The state, |u>, and the observable, O, determines the statistical outcome of those observations via the Born Rule, this being, for pure states, the expected outcome of the observation, E(O), is <u|O|u>.

    What's going on with a quantum system when not observed the theory is silent about.

    Thanks
    Bill
     
    Last edited: Jun 6, 2015
  10. Jun 6, 2015 #9
    I just don't see how the question is meaningless, no pun intended. I know invisibility sounds like something that carries a lot of pseudo-science baggage. But i'm trying to drive home a point about the physicality of quantum mechanics.

    Let me create a different thought experiment about why i don't believe the question is meaningless (if you don't like the cat, just replace it with a particle) :

    We have two transparent boxes, one box has a cat in superposition, the other has a blackhole with the same information content as the cat (basically a cat squished to the size of it's Schwarzchild radius)

    if left totally undisturbed, then there is no way determine the difference between the two transparent boxes, other then to disturb it with an interaction. Throw a photon in the first box, the cat decoheres and the re emitted photon provides information about the state of the cat at that instant in time. Throw a photon at the black hole, and nothing comes back, but grows bigger, whatever.

    But before we throw in the photons, what would the cat actually LOOK like. i'm proposing it would be totally invisible, no photons, no interaction, no definitive answer as to what the state of the cat is. The cat is still there in the box, but there exists no definitive information about it until it interacts with something. This lack of information is perceived physically as total darkness, absence of light, invisible; whatever you want to call it.

    Here is my analogy. Your grandfather asks you to get a book from his attic in his house. The attic is completely dark, with no windows, no lights, filled with old furniture. As you walk through this room, you bump into things as you search for this book. The first thing you bump into is a chair. The chair was completely invisible to you until you bumped into it. Once you've bumped into it, the chair fly's off in a ricochet, and is again totally invisible to you, until you bump into it again. Your grandfather checks out the attic to see where all the noise is coming from. He laughs and says that there are lights and flicks a switch. This attic was a special attic, where the walls are transparent and thick, and there are lights outside of it that shine onto the room. technically you should be able to see the objects in the room. But you can not. everything is still invisible or completely black, like the surface of a black hole.

    It's either that, or it's physically impossible to make a transparent box where you could see the contents inside without interacting with the objects within, which is the realistic answer. Even if the box wasn't transparent, the object in superposition inside should be invisible anyway is what i'm getting at.
     
  11. Jun 6, 2015 #10

    bhobba

    Staff: Mentor

    OK, before going any further can you define in your own words, not with a link somewhere else, or a cut and paste, but in your own words, what a superposition is?

    In a superposition of what?

    As a living macro object it cant be in a superposition of alive and dead.

    A cat squashed like that will not have the same information content of an ordinary cat - even assuming its a meaningful concept in such a situation - which it isn't. To see that precisely define the information content of a cat, bearing in mind a cat needs to interact with the environment to live.

    Thanks
    Bill
     
    Last edited: Jun 6, 2015
  12. Jun 6, 2015 #11
    Okay, just replace the word "cat" with the word "particle" and replace the words "alive and dead" with "Up spin and down spin" The cat being macroscopic is irrelevant here.

    And yes in my own words, superposition is being in a unique state of more then just one state, being in all possible states of up spin and down spin, which arises from non interaction with the environment. So far everything I've said has been straight from the noggin.
     
  13. Jun 6, 2015 #12

    bhobba

    Staff: Mentor

    That definition isn't correct - but its not too bad so we will leave it as is.

    Now you are defining a quantum system as spin up and spin down ie a qubit - it's represented by a Bloch Sphere:
    http://en.wikipedia.org/wiki/Bloch_sphere

    Can you explain to me how a black hole will have a qubit of information bearing in mind they are formed when really massive systems gravitationally collapse.

    Thanks
    Bill
     
  14. Jun 6, 2015 #13
    The black hole
    Well the black hole isn't in a superposition, even if it was, we wouldn't be able to know, but it's just a thought experiment about the similarity between how black holes are invisible in one box, and how the particle (the cat) is also invisible in the other box, which are 2 separate systems (I didn't specify that in the previous comment). The only way to tell if there was a black hole in the 2nd box would be to throw a photons into both boxes and see how it interacts with those systems, likewise with the superposition particle. I mean i could be wrong, but the idea is lended from the "black hole electron."

    natural black holes are created through gravitational collapse yes, but hypothetically they can be synthetically produced by squishing matter into concentrated regions. The extraction I'm using doesn't really require how the black hole got into the box :biggrin:. I think the details would be quite gory to squish a cat until it becomes a singularity. Really the black hole extraction isn't necessary, but i'm just using it to explain how the black hole horizon and superposition of a particle may have similar features. I've always thought about blackholes being macroscopic quantum objects, but i don't want to make that stretch until i know there's a definite similarity between the two.
     
    Last edited: Jun 6, 2015
  15. Jun 6, 2015 #14

    bhobba

    Staff: Mentor

    Ok then.

    What I can tell you is a single particle is described by a quantum field and is nothing like a black hole.

    All light will be sucked into a black hole - throw extra photons in a box containing a single photon and you wont notice much of anything at all because they are all excitations of the same underlying field and interact quite weakly by an indirect process:
    http://en.wikipedia.org/wiki/Two-photon_physics
    'From quantum electrodynamics it can be found that photons cannot couple directly to each other, since they carry no charge, but they can interact through higher-order processes. A photon can, within the bounds of the uncertainty principle, fluctuate into a charged fermion–antifermion pair, to either of which the other photon can couple. This fermion pair can be leptons or quarks. Thus, two-photon physics experiments can be used as ways to study the photon structure, or what is "inside" the photon.'

    Thanks
    Bill
     
  16. Jun 6, 2015 #15
    Thanks for showing me that wikiarticle, and that's cool and everything, but this is a little off-topic, because like i said, I don't want to go beyond what i understand and postulate something as crazy as macroscopic quantum objects. I glaze over the non-significance of the black hole in terms of how the photons interact with that system, but i know that the black hole gobbles up the photons, and returns no useful data, other then that its a black hole. The point was that it's the only way to tell the difference between the first box (the particle in superposition) and the 2nd box (the black hole) was to throw photons (interaction) into both of those systems. That's based on the assumption that the particle in superposition with no interaction is indistinguishable from the black hole because both systems are "black". I don't know enough to even speak further about it, but that's the only distinction i was trying to make with the 2nd thought experiment.

    and that's the question is whether the particle in superposition is indeed "black", with out without being in a transparent box.
     
    Last edited: Jun 6, 2015
  17. Jun 6, 2015 #16

    bhobba

    Staff: Mentor

    So what you are saying is in order to tell the difference you have to observe it.

    That's utterly trivial.

    Thanks
    Bill
     
  18. Jun 6, 2015 #17
    Yes, that is trivial, that's exactly why i said it's not that important ("I glaze over the non-significance of the black hole").

    It's what the superposition looks like while not observed (interacted with). which is the whole point of my original post. Is that if we could make a transparent box, we would see a black region, or invisible region. Of course none of this is possible because you cant make a transparent box without interacting with the system, but if you could that is what we would see, we would see absolutely nothing, or a totally black region in the box.

    That's whats up for debate, is whether we would actually see anything, like the tooth fairy flying around, 0's and 1's like the matrix, or absolutely nothing.
     
  19. Jun 6, 2015 #18

    bhobba

    Staff: Mentor

    In Schroedinger's Cat if you made a transparent box you would see an ordinary can live or die.

    If inside was a single photon it would not be that way for long with a transparent box and you would see nothing because as I pointed out before photons basically do not interact - which again is utterly trivial.

    Thanks
    Bill
     
  20. Jun 6, 2015 #19

    Nugatory

    User Avatar

    Staff: Mentor

    There is no such thing as a particle that is not in superposition. Consider three particles, one spin-up, one spin-down, and one in a particular superposition of spin-up and spin-down. It is tempting to say that the first two are not in superposition and the third is - but in fact this situation is just as accurately described by saying that the first two particles are in superpositions of spin-left and spin-right while the third particle is in the unsuperimposed spin-right state. These are two different ways of describing the same state in the same way that 2+2 and 3+1 are two different ways of describing the number four.
     
  21. Jun 6, 2015 #20
    So the consensus is that the cat, even though its been killed by the radiation machine, is still in a superposition after observation, at right angles? like half alive half dead, and half dead half alive?

    which means that even before and after being observed, its always been in a superposition, and always will be? just in a different superposition?

    Sorry if i worded that wrong but is that sort of what you mean?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Shrodinger's cat is "invisible"
Loading...