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About probability densities

  1. Jun 16, 2005 #1
    It is well known that the wave function for a free particle in a one-dimentional infinite potential well is just a sinusodial wave with each end being a node.

    Let me suppose that the infinite square well is v=infinity at x=0,L and the coresponding wave function is sqrt(2/L)*sin(2*Pi/L).

    From the wave function, the probability of finding the particle on the left side (x=0~L/2) and the right side (x=L/2~L) are exactly the same.

    The question is, if at x=L/2 (midpoint) the probability density is 0, how can the particle go from the left to the right?
  2. jcsd
  3. Jun 16, 2005 #2
    Firstly the particle doesn't actually move; to see it moving you put it into a position eigenstate, and furthermore, you want to integrate the probability density function from one value of x to another to get the probability of the particle being in that region.
  4. Jun 17, 2005 #3
    Maybe you can put the 'particle' like picture away for a minute and think of it in terms of wave. Then amplitude of wave could be zero at one point but follows continous thereafter.
  5. Jun 17, 2005 #4


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    The wavefunction's incorrectly written.Notice that the only relevant physical quantity is the probability.

  6. Jun 17, 2005 #5
    Quantim uncertianty and spontainous particle generation

    Here is an interesting thought experiment.
    An astronomical observation of the redshift of very distant objects reveals that their velocity approaches the speed of light. If Hubbels interpretation is correct regarding the expanding universe the implication is that there is a relativistic horizon where Vr (Velocity of recession) is equal to c. Nothing can be observed beyond this distance and it is probably meaningless to refer to anything "outside" this horizon.
    Now here is the question. Given the uncertianty principal the more accurately we know the momentum of an object the less accurately we know its location. If we "know" the recession velocity of an object to be vanishingly close to c then we know "nothing about its location." The object, once observed, can be anywhere in the universe! Could this explain the apearant spontanious production of particles in intergalactic space? If so could this be an alternative explanation to the expansion of the universe?
  7. Jun 17, 2005 #6


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    HUP does not apply at macroscopic scales.
  8. Jun 17, 2005 #7
    Following from dextercioby's comments, consider a helical wavefunction...
  9. Jun 18, 2005 #8
    Try to think on what you really measure in order to avoid such wrong deductions from the HUP.

    HUP allows you to know with a fantastic precision (with a 0 error at the limit) some values, such as the position and velocity of macroscopic objects.

    What do you mean?

  10. Jun 18, 2005 #9
    It looks like he was referring to ProfChuck just to say that our ability to measure momentum of a macroscopic object is so inexact that it removes the possibility of a significant HUP effect.
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