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Thread: uncertainty principal

  1. #1 uncertainty principal 
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    hey, can anyone clarify this principal for me? I understood it as we can only predict somethings position 95% of the time because we can't measure velocity and position at the same time. To me there is an obviousl flaw in this which makes me think I don't have all the facts. I think it's stupid to make a final conclusion unless you have all the facts so can you guys clarify for me?


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  3. #2  
    Forum Isotope Zelos's Avatar
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    what is there to explain?
    dp*dx ~ h/2pi
    dp is the uncertain in momentum and dx is the uncertainty in position


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    I can't explain it. I think it's a "particle" cop-out. Things like electrons aren't little billiard balls, and this is used to evade the question of what they really are.
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    Quote Originally Posted by Farsight
    I can't explain it. I think it's a "particle" cop-out. Things like electrons aren't little billiard balls, and this is used to evade the question of what they really are.
    they are wave/particle
    they are like a wave spread out in space like somekinda ameaba and you can never pin point where in the goo it is
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    To me what it sounds like is... WE can't predict the position. Maybe when we have better technology we could "pinpoint it in the goo"

    Zelos, what is h in that formula?
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    Quote Originally Posted by shawngoldw
    To me what it sounds like is... WE can't predict the position. Maybe when we have better technology we could "pinpoint it in the goo"

    Zelos, what is h in that formula?
    we can tell its somewhere in this cloud of probability but exact where we dont know

    h is plancks constant. a very very small unit
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  8. #7 hmmmmmmmmm 
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    Hmmmm.

    Have a go at this........

    Go into the control panel in your PC and select mouse. Now click on "Show pointer trails"

    Move your mouse around. You can see how fast the mouse is moving and the direction its heading in. But which one of those little arrows that you see is the "real" mouse pointer ?

    Now. stop moving your mouse around.

    Aha, there it is !!! (similar to collapsing wave function, it now appears to be a single particle) but which direction is it going to move in next ?

    Oh, you dont know.........well move it again and see.......now its moving again, which one is it ?

    lol
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    Good analogy Leo. Let's hope everyone remembers it is an analogy.
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    as far as i know this is where the uncertainty principle comes from:

    a particle is moving with particular velocity. In order to observe its position we will need to observe it, which requires a photon to be shot at it (ie light), this would however give the particle energy thus changing its velocity. Now i think the energy of the photon is related to its frequency, which is inversly proportional to its wavelength. The smaller the wavelength the more accurately you can locate the particle but because the photon has more energy, the particle's velocity will change more. So the more accurately you try to locate the particle the more its velocity will change. Its a sacrifice, thus you will not be able to know both velocity and location of a particle at the same time.

    This is heisenberg's original uncertainty principle, if i remember correctly.
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    I've tried for years to understand it and can't. I'm not a physicist but few of them would claim to 'get it' either. the analogies 'sort of' work but they always involve not knowing the numbers or variables rather than actual ultimate uncertainty. It's not a matter of filling in information but rather that the information doesn't exist to be filled in. (my head hurts).

    Like Relativity, the existence of unobservable dimensions and so on, the proof of the Uncertaintly Principle is in the experimentation. The theories work in explaining real phenomena. We are physical beings that are products of evolution and our ability for conception outside of our physical senses is limited. We can create numbers and formulae but they are more like words of explanation rather than physical concepts in our thinking. We have no means of really imagining what an atom is like, or the size of the sun up close. Our concepts are based on the limitation of our physical senses and comparisons in our limited experiences.
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    I've tried for years to understand it and can't. I'm not a physicist but few of them would claim to 'get it' either. the analogies 'sort of' work but they always involve not knowing the numbers or variables rather than actual ultimate uncertainty. It's not a matter of filling in information but rather that the information doesn't exist to be filled in. (my head hurts).

    Like Relativity, the existence of unobservable dimensions and so on, the proof of the Uncertaintly Principle is in the experimentation. The theories work in explaining real phenomena. We are physical beings that are products of evolution and our ability for conception outside of our physical senses is limited. We can create numbers and formulae but they are more like words of explanation rather than physical concepts in our thinking. We have no means of really imagining what an atom is like, or the size of the sun up close. Our concepts are based on the limitation of our physical senses and comparisons in our limited experiences.
    Hmmmm. Uncertainty principle is a very specific thing. I think you are confusing quantum mechanics with the uncertainty principle (which i have tried to explain above). although i made a mistake in one part, higher frequency does not mean higher change in velocity, but rather a higher range in possible changes in velocity. Otherwise, uncertainty principle is too do with the predictive ability of science. Classically it was sciences belief that if we have all information about everything in the world, and the laws of physics, then we can predict state of everything anytime into the future. Uncertainty principle states that this is impossible because we can never, as a rule, know everything about a certain particle because out knowledge of momentum and position are inversly proportional.
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    Quote Originally Posted by Sohy
    I've tried for years to understand it and can't. I'm not a physicist but few of them would claim to 'get it' either. the analogies 'sort of' work but they always involve not knowing the numbers or variables rather than actual ultimate uncertainty. It's not a matter of filling in information but rather that the information doesn't exist to be filled in. (my head hurts).

    Like Relativity, the existence of unobservable dimensions and so on, the proof of the Uncertaintly Principle is in the experimentation. The theories work in explaining real phenomena. We are physical beings that are products of evolution and our ability for conception outside of our physical senses is limited. We can create numbers and formulae but they are more like words of explanation rather than physical concepts in our thinking. We have no means of really imagining what an atom is like, or the size of the sun up close. Our concepts are based on the limitation of our physical senses and comparisons in our limited experiences.
    Hmmmm. Uncertainty principle is a very specific thing. I think you are confusing quantum mechanics with the uncertainty principle (which i have tried to explain above). although i made a mistake in one part, higher frequency does not mean higher change in velocity, but rather a higher range in possible changes in velocity. Otherwise, uncertainty principle is too do with the predictive ability of science. Classically it was sciences belief that if we have all information about everything in the world, and the laws of physics, then we can predict state of everything anytime into the future. Uncertainty principle states that this is impossible because we can never, as a rule, know everything about a certain particle because out knowledge of momentum and position are inversly proportional.
    Not really. One can't get therir head around the Uncertainty principle without a discussion of quantum mechanics. One naturally links the uncertainty principle with the probabilistic nature of quantum mechanics. The evolution of probabilities is modeled. The uncertainty principle comes from the wave structure of these probability densities modeled in quantum mechanics. Understanding without quamtum basics is two steps removed.
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  14. #13  
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    so in what way was my explanation of it incorrect?
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  15. #14  
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    Quote Originally Posted by Sohy
    a particle is moving with particular velocity. In order to observe its position we will need to observe it, which requires a photon to be shot at it (ie light), this would however give the particle energy thus changing its velocity.
    Well, as best as I remember (some time ago, from Chemistry courses, so I may be mis-remembering), it goes something like this.

    The Schroedinger wave equation assigns to some quantum entity (a "particle"), roughly speaking, its energy, given by the frequency of the wave. Schroedinger's equation, of course, resolves to a sine function, a "sine wave"

    It was Mach (I think) who pointed out that the square of this function should be interpreted as the probability density for the particle's location. Let's go with that.

    Now, if the square of Schroedinger's psi function is a probability density for location, it follows that, knowing the exact energy of a particle implies a mere probability for its location.

    Suppose now we know our particle's exact location. This is best described by a psi-squared wave that is zero almost everywhere, i.e. a flat line with a single peak = location with probability = 1. But the only way to generate such a curve is by "mixing together" (it's called superposition) of all possible energy waves for our particle in such a way that they destructively interfere at all but one point, the location.

    But, as we needed the superposition of all allowable energies to find the exact location, and that knowing the exact energy merely gives us a probability for location, it follows that we cannot simultaneously know both, to any degree of accuracy.

    Note that this has nothing to do with the precision of measuring instruments, nor photons hitting measurables, it is a matter of principle
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    Well, as best as I remember (some time ago, from Chemistry courses, so I may be mis-remembering), it goes something like this.

    The Schroedinger wave equation assigns to some quantum entity (a "particle"), roughly speaking, its energy, given by the frequency of the wave. Schroedinger's equation, of course, resolves to a sine function, a "sine wave"

    It was Mach (I think) who pointed out that the square of this function should be interpreted as the probability density for the particle's location. Let's go with that.

    Now, if the square of Schroedinger's psi function is a probability density for location, it follows that, knowing the exact energy of a particle implies a mere probability for its location.

    Suppose now we know our particle's exact location. This is best described by a psi-squared wave that is zero almost everywhere, i.e. a flat line with a single peak = location with probability = 1. But the only way to generate such a curve is by "mixing together" (it's called superposition) of all possible energy waves for our particle in such a way that they destructively interfere at all but one point, the location.

    But, as we needed the superposition of all allowable energies to find the exact location, and that knowing the exact energy merely gives us a probability for location, it follows that we cannot simultaneously know both, to any degree of accuracy.

    Note that this has nothing to do with the precision of measuring instruments, nor photons hitting measurables, it is a matter of principle
    i still dont see why all this is necessary. The heisenberg explanation, is a simple and beautiful explanation of the uncertainty principles. You dont need to resort to density functions.
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  17. #16  
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    Quote Originally Posted by Sohy
    i still dont see why all this is necessary. The heisenberg explanation, is a simple and beautiful explanation of the uncertainty principles. You dont need to resort to density functions.
    Yes, but it assumes the only way to know anything about a particle is to bounce photons off it (which is not obvious) and thus the particle has a defined momentum and position , we are just limited in measuring it because our methods are crude. This is a very weak form of the uncertainity principle! What guitarist posted is a lot stronger - the particle itself has no idea what its position or momentum is, it can only know one with more certainty by not knowing the other!
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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  18. #17  
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    Quote Originally Posted by Sohy

    i still dont see why all this is necessary. The heisenberg explanation, is a simple and beautiful explanation of the uncertainty principles. You dont need to resort to density functions.
    Two things, Sohy. Please don't use up unnecessary disc space by quoting whole posts, extracts (or none at all) will do.

    Second, as I understand it, what you call "Heisenberg's explanation" is merely an illustration as to why the uncertainty principle might be true. The explanation I offered gives the real meaning of the principle. As I tried to show, and, as river_rat emphasized, the uncertainty principle has nothing to do with our ability to measure things to arbitrary degrees a accuracy, it is a matter of principle that location and energy cannot simultaneously be known except probabilistically.

    If you don't believe me ask yourself this. Do you really think you can find the location of, say, an electron, by shining a torch on it? And even if you could, what would that tell you about it's momentum?

    Of course, if all you're really interested in are layman's explanations, then fine.
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  19. #18 Re: uncertainty principal 
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    Quote Originally Posted by shawngoldw
    hey, can anyone clarify this principal for me? I understood it as we can only predict somethings position 95% of the time because we can't measure velocity and position at the same time. To me there is an obviousl flaw in this which makes me think I don't have all the facts. I think it's stupid to make a final conclusion unless you have all the facts so can you guys clarify for me?
    it is quiet like a wave particle duality.
    it is a particle's basic properties been detected by bombarding with a wave or a photon. so initial momentum of the particle will be changed while it collapse. and also the position of the particle will be changed. so when we observe we are uncertain about the particles momentum and the position at that instant.
    that is what basically it is i suppose.
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