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What's the difference between probability and probability density

  1. Feb 12, 2007 #1
    So the integral of [tex]|Psi|[/tex] squared represents the probability of finding a particle at a certain position at a certain time. Please correct me if this is wrong. SO what exactly does the "density" refer to?
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  3. Feb 12, 2007 #2
    The density of anything is the quantity itself per unit length/area/volume.

    If the [itex]|\psi|^2[/itex] is defined for a 3 dimensional system, then the probability density will be the probability divided by volume. And for 2 dimensional and 1 dimensional systems, we have area and length respectively.

    The best way to see this is that for 1 dimensional systems, for example, the probability of a particle being between points A and B is given by

    [tex]\int^B_A |\psi(x)|^2\ dx.[/tex]

    Here, we have multiplied the prob. density by a length, namely [itex]dx,[/itex] to get a probability. Therefore the density itself is a probability divided by a length. The extension to higher dimensions is easy.
  4. Feb 12, 2007 #3


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    In mathematics, probability density is the derivative of the cumulative probability. Specifically, let F(x)=Prob.(X<=x), where X is some real valued random variable. Then the density is F'(x).
  5. Feb 12, 2007 #4


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    Probability density : Probability :: (mass) density : mass
  6. Feb 13, 2007 #5


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    Nope, that's incorrect. It's the probability ascribed to a finite/infinite domain (in any # of dimensions, 1,2,3,...), not to a point of the domain.
  7. Feb 13, 2007 #6
    Thanks to everyone who answered :)

    NOW, I think understand why they call it probability density -- it is the probability of finding said particle in a given region (interval) of space.
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