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Particles being in many places?

  1. Jun 24, 2010 #1
    The idea of quantum particles being in form when observed and then existing many places when not being observed makes little sense to me. Not just because its foreign or counter intuitive, but for two key reasons. Postulating that it is the case, then we must have a flaw in our observations to where we only see particles that are conveniently located in the same place they were last observed --- for example --- I look at my computer and then leave the room. One hour later someone from the other side of the world, whom I have never met, walks into the room. He would see the same computer as myself, scratches and all, right? (provided no observable changes or damages occurred in that hour) Is it a coincidence that we both agree on the black object being a computer and not particles existing everywhere? My second hurdle is, how self-centered is it to assume that the power of observation would dictate and give meaning to the universe? Please help, I want to understand :)

    Adam
     
  2. jcsd
  3. Jun 24, 2010 #2

    K^2

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    Particles are waves. Take a wave ~~~~~~~. Can you tell me exactly where that wave is? No. It occupies a certain amount of space, and any point in that space can be said to be where the wave is.

    A wave, in turn, can be represented as a point-particle that doesn't have a specific location. That's how Quantum Mechanics treats waves, because it is far more convenient mathematically.

    As far as a particle being "in one place" when you observe it, that's not entirely true. It depends on how you observe it. A particle, after observation, will fall into an eigan state of the operator associated with measurement. The measurement operator will be related to Hamiltonian of the interaction that took place for you to detect a particle. Because the whole thing is designed to detect location of the particle, they tend to localize them, but I'm not aware of any real physical way to measure particle's exact location.

    Best you can usually do is narrow the field to "somewhere over here". After such an observation, the particle really is "somewhere over there" and nowhere else. Exactly why that happens is a completely different discussion. In classical QM it's simply a postulate of collapse of the wave function, which is about as useful as saying, "God made it so." There are plenty of alternative explanations, from decoherence to Many Worlds Interpretation, and they are all way outside of scope of this discussion.
     
  4. Jun 24, 2010 #3
    Thanks! I have a much better understanding than I did an hour ago :)
     
  5. Jun 25, 2010 #4

    tom.stoer

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    The computer is both a bound state and a classical system.

    In a bound state the particles are in "many places", but only in a "narrow region". So even quantum mechanically a bound sate is localized.

    In addition as the computer is classical (i.e. it is a "large object") it strongly interacts with its environment (air molecules); so it gets constantly "measured" by the environment forcing it to stay classical. This Is described by an approach called decoherence trying to explain how the classical world emerges from an underlying quantum world.
     
  6. Jun 25, 2010 #5
    K^2:

    A particle is not a wave, since two interacting particles are described by a single sonlution to the wave equation subject to boundary and initial conditions, and the same holds for N particles. So it is inconsistent to say that waves are particles. A quantum system is described by a probability distribution that is a solution to Schrodinger's Equation. This wave equation describes the spatial probability distribution of an ensemble of similarly prepared systems.
     
  7. Jun 25, 2010 #6

    K^2

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    A single particle is an object indistinguishable from wave in R3. A pair of particles is an object indistinguishable from wave in R6. N-particles are an object indistinguishable from wave in R(3N).

    Where are you going with this?

    And your "probability" distribution is a wave, with Shroedinger Equation being wave equation with variable c.

    Even if I don't take you apart on Hilbert Space vs Fock Space and insist on Shroedinger's Equation being a mere classical limit, you're still not making a case.

    What I told skycastlefish is technically correct, even if ambiguous in many ways. It won't let him build a model, but it will help him visualize particle-wave duality a lot better. Which do you think is more important for this particular discussion?
     
  8. Sep 3, 2010 #7
    I agree with your opinion but what kind of waves? Is it classiacal waves propagating through some media or it's some oscillating field as in case of EM waves? No I don't think so... In QM they say that it's probability wave but such wave is just mathematical tool and not something real. Nevertheless they give it wavelength and frequency which are real things assosiated with particles momentum and energy so it must be some real wave but we don't know what kind of wave it is. Maybe it is something that oscillates in some other dimension?
     
  9. Sep 3, 2010 #8
    if it is a real oscillation of charge then it is a very strange oscillation indeed because it doesnt always result in the emission of light.

    if I were you I would start by studying hydrogen atoms and work from there.
     
  10. Sep 3, 2010 #9
    And what is charge? We don't know... But I'm not saying that it's oscillation of charge because it should emit light as you said. What I'm saing is that maybe it's something "real" that is oscillating (in some other dimension).

    K^2 says that particles are waves and I asked him what kind of ways.
     
    Last edited: Sep 3, 2010
  11. Sep 3, 2010 #10
    charge is the divergence of the electric field.
     
  12. Sep 3, 2010 #11
    The divergence of the electric field is a charge density divided by the electric constant. But it's not a definition of electric charge. Anyways, this has nothing to do with the qestion I asked...
     
  13. Sep 4, 2010 #12
    Seems to me that at a certain point in a person's study of these things, one gets lost and unable to seperate the difference between theory/model and reality - as if these theories/models are explicitly what creates reality. Dont get lost - the models/theories are simply tools to understand the behavior of reality, and they are not complete or devoid of "holes".

    In a lot of ways, science is a religion. If you believe that a your computer exists only because someone is looking at it, no one can argue with you. If that tree falls in the forest, what happens? If one is so compelled to find absolution, it will ultimately lay in the hands of belief.

    If it doesn't seem logical that your computer only exists as a computer when someone is observing it and that it exists in some other form otherwise, there is a simple test you can perform to figure it out. Write a program that performs a process over time - step away and dont let anyone look at the computer. Come back later and see if the process only operated when you were looking at the computer or if it was functioning while no one was observing it. :) See where I'm going? The idea of quantum particles having different form depending on if it is observed or not was not intended to create more confusion in understanding reality - it actually is a theory that tries to explain non-intuitive results from experiments which deal with things that are beyond the direct observation of human senses.

    Consider it like the particle-wave duality of light. How can one possibly form a seamless model of this behavior in their imagination? Ans: it isn't possible. In the macroscopic world we experience as people, there is nothing that possesses this type of contradictory characteristic. Logically speaking, the duality of light is a fallacy - this is the case simply because a wave and a particle are two independent entities. To embrace the theory of the duality of light and the explanations as to why this is the case is nothing more than faith - you are simply agreeing with someone elses belief that it is the way they describe it. The question you have right now ultimately depends on what you believe to be the case - is your computer only a computer when it is being observed by someone or is your computer no different than what you observe even when you aren't there? If you didn't have the word "computer" in your vocabulary to call it, would it be any different if you called it something else?
     
  14. Sep 4, 2010 #13

    Vanadium 50

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    It sounds to me that you are arguing "because I don't understand it, nobody understands it"
     
  15. Sep 4, 2010 #14
    Van:

    Being so thinly veiled gives the impression of having thick skin. Lets test the theory. ;-)

    The point I made was that the theories in QM that try to explain why something does what it does aren't intended to create greater confusion. In most cases, a reader will either improperly interpret the explanation or they will try to apply an explanation improperly to a given set of circumstances.

    As for the exact mechanisms that cause light to behave the way it does, who really knows? You and I can both agree that light has a particular set of characteristics but what underlying mechanism causes it to be this way is really what anyone believes it to be - there isn't any way to prove it is a one-armed monkey playing tricks or if it is the devil himself that makes them fotons do what they do.

    (BTW, I know it is spelled "photons".)

    Back to the OP's point:

    Is it self-centered to think that your observation dictates the state of the universe?
    No. If you observe something and you respond by doing something, your observation ultimately led to you changing the universe.

    Is the state of the universe dependent on your observation?
    No. It was here before you were and will be after you have gone.

    Does the universe exist even if you dont see it?
    Close your eyes - do your other senses tell you that it is still here?
     
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