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If i stand still , you cant find me

  1. Sep 16, 2009 #1
    is someone so kind to explain the statement : quantum physics says : if i stand still you cant find me.

    i guess it have something to do with the " you can only know where its going and how fast" atom thingy..

    as you can hear, im not in to physics, im just really really interested in it.

    Thanks in advance!
    Noia


    typo
     
  2. jcsd
  3. Sep 16, 2009 #2

    haushofer

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    I guess you refer to the quantummechanical fact that momentum and position can't be determined at the same time. So the narrower you pinpoint the position of a particle, the more uncertainty you get where this particle is going to (momentum tells you where and how a particle is heading).

    Intuitively, you can understand this as follows: measuring the momentum of a particle requires radiation with a very small wavelength. Because, a small wavelength means a high frequency, which means a lot of waves in a certain time interval, and the more waves you have in a certain time interval the better you can detect changes in this wave due to the momentum of the particle you want to measure. But, the smaller the wavelength of the radiation, the more energetic the radiation becomes, and the more it will kick the particle of its position.

    So you decide to lower the frequency of the radiation in order to determine the position better, but then the uncertainty in the momentum becomes bigger again.
     
  4. Sep 16, 2009 #3

    DaveC426913

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    The problem with this explanation is that it appears to suggest that uncertainty is merely a measurement problem. It is not. Uncertainty really is a property of the system, not just the measurement technique.
     
  5. Sep 16, 2009 #4
    "If I stand still, you can't find me"

    In 1924 a physicist named Louis de Broglie suggested that since light was made up of photons (waves made up of particles), then perhaps particles were also wave-like in nature. People thought he was crazy at first, but experiments suggested that this was true: particles are waves.

    Now transfer your thoughts to a situation involving a rope tied to a wall. If you were to jerk the rope once, it would send a pulse across the length of the rope. You could see that single pulse and roughly describe where it is. It has a well defined position.

    Now let's say you jerk the rope several times to create a series of pulses, i.e. a wave. You could easily measure the frequency and wavelength of this wave you created (the frequency is the number of times your jerked the rope per second, the wavelength is the length between the top of two pulses.) If you multiply the wavelength and the frequency, you get the velocity of the wave. You can say this wave has a well defined speed.

    Now look at the first situation, the single pulse. You can say you know just about where that pulse is located, but you only have one pulse, so you don't know the wavelength (distance between two pulses). You can't quite describe the speed of this wave.

    In the second situation, you can tell someone that the waves are moving with some speed, determined by the wavelength and frequency. But what if you were asked where the wave was? The wave consists of a bunch of pulses, so it seems to be everywhere at once. This wave does not have a well defined position.

    What I'm trying to say is that for waves, you must give up some information in order to get an accurate measurement of some other aspect of the wave. You can either know the (approximate) position of the wave, or you can know the (approximate) speed of the wave, never both.

    Back to de Broglie's discovery: all particles are waves, this means electrons, protons, and neutrons (which make up sand, people, elephants, planets, etc) are waves. Now if you are a wave, and you are standing still, why can't I find you?
     
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