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Basic understanding

  1. Aug 4, 2010 #1
    I lack the fundamental aspect of quantum entanglement and I am trying to fully understand it. The problem I have is that I can regurgitate all the information I have read however what I am saying does not make sense in my head.

    Schrödinger's cat for example: is entanglement completely dependent on the observer? Lets say I open the box and find the cat deceased. I then close the box and tell nobody of what I saw. I have not altered state of the cat by looking. I simply made its state known to myself. So does entanglement still exist for any other observer wanting to know the state of the cat? Expand that out to another entangled property like electron spin, how does observer B lose entanglement simply because observer A saw that his spin was up and inferred that observer B's spin must be down.
  2. jcsd
  3. Aug 5, 2010 #2
    Join the club elbeasto. And welcome to PF. Spanish? I'm guessing. Never mind. I noticed that nobody has replied to your question, so I'll give you my two cents.

    I'm not sure what you mean by lacking "the fundamental aspect of quantum entanglement", but I suspect that most everyone lacks this, and is still trying to fully understand it.

    Quantum entanglement has no physical meaning other than certain statistical correlations associated with certain experimental preparations described by certain mathematical formalisms. So, if you are, or become, familiar with the experimental preparations and formalisms of quantum entanglement, then you'll understand it as well as anyone in the world -- which is to say that you really won't understand it in a way that you will be able to explain it in ordinary language. Which is to say that it really has no particular physical meaning other than the stuff I mentioned -- the actual (underlying) physical meaning of which is an open question in physics.

    Well, how would you determine the presence of entanglement, or anything else for that matter, without observing something? But, Schrodinger's Cat isn't really about entanglement.

    Then you could confidently report that you observed a dead cat in the box. From which you could infer that the seal on the container of the poisonous gas in the box had been breached. From which you could infer that the mechanism that you had set up to break the seal on the container of poisonous gas had been set in motion. From which you could infer that the particle detector in the enclosure, designed to set in motion, upon registration of a detection, a sequence of events culminating in breaking the seal on the container of poisonous gas, had registered at least one detection. From which you could infer that at least one particle had been emitted from the material that you had placed in the box (with the sealed poisonous gas container and the cat).

    Then, it would be your secret.

    That's right. If the cat's dead, then it will stay dead.


    The observer isn't you. Nor is it the cat. Nor the seal on the container of poisonous gas. Nor the mechanism that you've used to amplify the registration of the particle detection by the particle detector. The observer is the particle detector itself. Once the particle detector registers a detection, then this is an irreversible fact of nature -- which can be observed by anyone, for as long as the data record persists, who chooses to look at the data outputted by the particle detector (especially if the 'data' is a dead cat). But this registration, by itself, has nothing to do with the usual conception of quantum entanglement. For that you would need at least two detectors, the combined, coincidental, outputs of which would yield statistics that would match the quantum theoretical prediction of entanglement.

    The Schrodinger's Cat situation is a parody of quantum superposition, not entanglement. It's an illustration of the absurdity of taking quantum superposition literally, ie., as corresponding to a description of reality.

    In certain experimental situations, if you know the result at A, then you can deduce the result at B, and vice versa. Now, wrt these sorts of situations where these sorts of deductions can be made, then it's just common sense, more or less, that if you detect, and therefore disturb (and alter the motional properties of), one of the presumably entangled (motionally related) particles, then you'll destroy the entanglement (ie., then you'll alter the motional relationship between the particles). There's really a lot of more or less common sense in quantum theory. You just have to recognize it as such.

    I sense that you really haven't gotten into quantum entanglement in depth yet. Things do get more problematic, and common sense conceptualizations of what's going on won't quite make it at some point in your explorations. When this happens, and if you choose to continue, then you'll be exploring the same 'territory' that professional physicists do. And then you'll begin to understand why it's so difficult to explain, or 'translate' to us common folk.

    Anyway, the progenitor of the concept of quantum entanglement was Schrodinger. I suggest you look up what he had to say about it, and proceed from there.
  4. Aug 5, 2010 #3
    I see. So, basically this is something I can't learn in a weekend. Got it.

    I am a technology major, more specifically in security. As you can imagine I have heard a lot about quantum computers and creating the unbreakable codes as well as quantum computing factoring large primes at ridiculous speeds for breaking codes. This is what initially peaked my interest.

    I appreciate your help and you actually made it much more clear to me what I was missing. Thanks again!
  5. Aug 6, 2010 #4
    The observer is whatever you want it to be.

    I'll keep the cat analogy going, but remember that it's an idealized situation. Let's say your partner opens the box and sees whether the cat is dead or not, but doesn't tell you (in some idealized way). Then your partner knows for sure whether the cat is dead or not. To you, he and the cat are now entangled: there is a certain probability of you seeing a dead cat if you look in the box, and there is a certain probability of him telling you that the cat is dead (we assume he's honest), but you have no idea which if you don't look at the box or find out from your partner. However, as soon as you look at the cat and see it's dead (or alive), you will know for sure that your partner will tell you it is dead (or not), and vice versa.

    Of course, as soon as you do this, the three of you are now entangled, from the point of view of anyone outside.

    (The idea that there is no preferred observer, and that all measurements are just entanglements between observer and observed, is the idea behind the (unfortunately-named) many-worlds interpretation of quantum mechanics, which I find to be the nicest interpretation.)
  6. Aug 6, 2010 #5
    I think its possible to discuss fundamental concepts of quantum entanglement conceptually using plain English for an admittedly screwy set of ideas!

    But if you think of things classically, consider the double pendulum problem. When you derive the equations of motion, you cannot reference the motion of pendulum 1 without referencing the motion of pendulum 2 in the sense that their associated differential equations are coupled. These couple diff eqs represent a physical situation where two pendulums are joined at one point and you cannot wiggle one without wiggling the other to an extent.

    Quantum entanglement is very similar conceptually because you can't reference the entangled property of a particle without referencing the entangled property of the other particle(s) to which it is entangled. It's like these things are joined at the hip or something and you know by now that this entanglement overcomes large physical distances.

    Even more interesting is the fact that there have been arguments over the nature of this process when two entangled particles are separated by a distance. Physicists want to know if the information the particles were transmitting occurred faster than light speed. However, the term 'information' is defined contentiously.

    See this link for plain English explanations of recent Chinese experiment http://news.discovery.com/tech/teleportation-quantum-mechanics.html
  7. Aug 6, 2010 #6
    The idea that there is no preferred observer, and that all measurements are just entanglements between observer and observed-adriank

    I like this idea on many levels.
  8. Aug 6, 2010 #7
    "To you, he and the cat are now entangled"

    Adriank - you just made the click sound in my head.
  9. Aug 6, 2010 #8
    I don't think that's unique to MWI, and IMO it has been explained much more clearly in other interpretations, e.g. relational quantum mechanics or David Mermin's Ithaca interpretation. This is a good paper about that:

  10. Aug 6, 2010 #9
    Consciousness is a different matter entirely; the feature I pointed out is really the only reason I preferred MWI over, say, the Copenhagen interpretation. I think that article by Mermin describes how I've really been trying to explain QM to myself all along. Thanks for the link.:)

    Of course, none of these conceptual issues matter when you're actually trying to solve problems, and as ugly as Copenhagen may be, it's very useful for calculations.
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