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Need Clarification on Uncertainty

  1. Apr 18, 2005 #1
    I'm reading "In Search of Shrodingers Cat" by Gribbin, and I've come to the part about the Uncertainty Principle. In describing it, he writes about a subatomic world in which it is impossible to know what particles (such as electrons) are doing when not observed.

    My question is, do we have at least some idea of what they are NOT doing? For example, we might not know their momentum, or what their position relative the nucleus might be, but can we at least be sure they aren't having tea parties or smoking dope?

    Gribbin makes it sound like they could be spelling out "Humans Suck" when we don't look (or that they might not exist at all), so I need some clarification on what the Principle really means this way.

    Thanks.

    [tex]\hbar[/tex]

    The Rev
     
  2. jcsd
  3. Apr 18, 2005 #2

    dextercioby

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    The HUP doesn't have anything to do with "it is impossible to know what particles (such as electrons) are doing when not observed".That is a consequence of von Neumann's 5-th postulate (the state vector collapse).


    Daniel.
     
  4. Apr 18, 2005 #3
    Just goes to show how little I understand what I'm reading. However, you really didn't answer my question. Do we have some idea what these particles are doing, or could it literally be ANYTHING?

    [tex]\oint[/tex]

    The Rev
     
  5. Apr 18, 2005 #4

    dextercioby

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    Literally,according to the 5+1 postulates in the Dirac/traditional/vectors and operators formulation,ANYTHING.

    Daniel.
     
  6. Apr 18, 2005 #5
    Damn!

    Thanks, I think...

    *Considering the number of subatomic particles in his body, The Rev fears the worst... (insert picture of 400 quadrillion particles smoking crack and watching Fear Factor here)

    [tex]\infty[/tex]

    The Rev
     
  7. Apr 18, 2005 #6

    dextercioby

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    There's no other logical interpretation of that 5-th postulate.It's part of the axiomatical structure.Refute it,and u refute the whole theory...

    Daniel.
     
  8. Apr 18, 2005 #7

    Galileo

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    The uncertainty means that, if we measure a physical quantity, we do not know for certain what the outcome will be. Suppose we setup an experiment to prepare an electron in a certain way and we measure its position (and note the result down). If we do the exact same experiment (under identical conditions) and measure the position the same way as before we might find something different. So we cannot say with certainty what the result of a measurement will be beforehand (there are some exceptions of a technical nature).
    But you are right, we have some idea of what they are NOT doing (and thus some idea of what they ARE doing) in the following sense: We cannot know exactly what the result of a measurement will be, but we CAN know what the probability will be of getting a certain outcome (if you throw a die, you know you won't get a 7, but you'll get 1,2,3,4,5 or 6 with some probability. This probability is of a different nature, but it illustrates the idea). So it's not like we cannot calculate anything anymore.

    This is uncertainty in QM in general. The uncertainty principle is a little different. Heisenberg's original uncertainty principle gives a relation (we can quantify uncertainty) between the uncertainty of a position of a particle and its momentum. As I said, we CAN calculate the probability we get a specific result. The uncertainty principle says that if we know that the probability of finding a particle in a certain small region of space is very high, almost one (the probability distribution for the particle's position is highly peaked at a certain point), then a measurement on the particle's momentum can give very large varying results (the probability distribution for the particles momentum has a wide spread). The converse is also true ofcourse. This is what Heisenberg's uncertainty principle says.

    In 'layman terms': "If we know where the particle is, we don't know it's momentum."
    The problem with that sentence is that here you're assuming the particle really HAS a position and a momentum before we measure it, but that's another story. QM makes no hypothesis on this matter, it just tells you how to calculate things and it always works out.
     
    Last edited: Apr 18, 2005
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