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Uncertain about uncertainty principle

  1. Oct 20, 2005 #1
    I have read the theory about uncertainty and cannot quite grasp it. Why is it that we cannot measure the speed of an electron and its location at the same time? As I understand it, the reason for probablility waves is because we cannot measure speed without disrupting where the electron is, or we can't observe where it is without disrupting its velocity? Something like that I think. Or does the electron literally dissapear and reappear in a different location? I'm quite confused. Any help would be awesome. Thanks!
  2. jcsd
  3. Oct 20, 2005 #2
    The interpretation of the uncertainty principal is still, I believe, up for grabs, but the principal itself is not so much a consequence of measuring either property as describing it. Once the wave-like nature of matter was discovered, the next logical step was to describe the wave. The de Broglie formula gives you the wavelength of a free particle with known momentum. However, this wave is a travelling wave extending infinitely in either direction of the particle's momentum. In short, this tells you next to nothing about where a particle is (although it can tell you where it isn't).

    To get a more particle-y picture, you can add more de Broglie waves to the existing wave to make wave packets. If you add enough waves, you end up with one wave packet, giving you the rough position of the particle. However, every time you add a wave, you add uncertainty in momentum: the momentum of the particle could be any one of the momenta for the de Broglie waves you added, or the original one.

    The conclusion is the uncertainty principal: the more you know about the position of the particle, the less you know about its momentum and vice versa.
  4. Oct 20, 2005 #3
    That definately helps, but I am still curious;
    Just because we cannot know what the exact velocity and exact position is at some time, doesn't mean that there isn't one. It seems that the probablilty wave needs to be created because we lack the capability of knowing both factors. Not that the electron doesn't posess both.
    So it's not that there is a probability of a particle being somewhere. The particle definately has a velocity and postion. The problem is that we just can't describe it because of an intrinsic problem with our perception/process of observation? Maybe? :uhh:
  5. Oct 20, 2005 #4


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    As expected, what you are aguing here is the principle of superpostion, which is a central tenet in QM. Please note that this isn't like throwing dice, and it isn't "either or". There IS a mixture of all the possible outcome in the system BEFORE a measurement. Why and how do we know this? Because a number of phenomena that we have observed can ONLY be explained using such a principle. Example: energy gap in hydrogen between bonding and antibonding band; the Schrodinger Cat-type measurements from the Stony Brook/Delft SQUID experiments.

    Superpostion IS real. The major problem for most people who do not study the formalism of QM is that these things have NO CONNECTION with the classical world that we are all familiar with. So these things appears as if they came out of nowhere and thus, appear strange and mystical. They aren't. The mathematical formalism of QM is very rigorious, and makes a number of predictions that agree with experiments with no classical counterpart.

    So the "strangeness" of wave-particle, uncertainty principle, and superposition are all CONNECTED, and without understanding the formalism, someone like me can only TELL you that this is what happened without any ability to connect with your present understanding.

    You may also want to do a search on PF on the uncertainty principle. There have been tons of threads and things written up about this that essentially repeat the things you are asking here.

  6. Oct 20, 2005 #5
    ZapperZ is right, there are many threads started by people with the same doubts about the HUP. I think unless you first hear about the HUP in class, which is unlikely, it is difficult to accept for a while. Part of this is because the HUP is very easy to understand in itself and so is taught early and appears in every other pop-sci book, while the reasons for the HUP are more complicated and diverse.

    It is good to be skeptical (about anything, so long as it doesn't spill over into dogma). If nothing else, when you do finally accept it you will have a good understanding of it, since you've approached it from every angle. But this isn't going to happen off one PF thread - you have to read up on it, surf the net, read about double-slit experiments and quantum tunnelling, that kind of thing.

    And don't think you're missing something obvious. Prominent scientists are still looking to demonstrate exactly what you are saying - that particles DO have exact position and momenta, but they are beyond our ability to measure. However, it's best to compare that with the rest of QM. Most people seem to find the alternative more compelling in the long run.
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