# Probability amplitude

(Redirected from Probability density)

In quantum mechanics, a probability amplitude is a complex number-valued function which describes an uncertain or unknown quantity. For example, each particle has a probability amplitude describing its position.

For a probability amplitude ψ, the associated probability density function is

ψ*ψ

which is equal to |ψ|2. This is sometimes called just probability density, and may be found used without normalisation (to have the total 1).

If |ψ|2 has a finite integral over the whole of three-dimensional space, then it is possible to choose a normalising constant, c, so that by replacing ψ by cψ the integral becomes 1. Then the probability that a particle is within a particular region V is the integral over V of |ψ|2.

The change over time of this probability (in our example, this corresponds to a description of how the particle moves) is expressed in terms of ψ itself, not just the probability function |ψ|2. See Schrödinger equation.

In order to describe the change over time of the probability density it is acceptable to define the probability flux (also called probability current). The probability flux j is defined as:

[itex] \mathbf{j} = {\hbar \over m} \cdot {1 \over {2 i}} \left( \psi ^{*} \nabla \psi - \psi \nabla \psi^{*} \right) = {\hbar \over m} Im \left( \psi ^{*} \nabla \psi \right) [itex]

and measured in units of (probability)/(area*time) = r-2t-1.

The probability flux satisfies a quantum continuity equation, i.e.:

[itex] \nabla \cdot \mathbf{j} = { \partial \over \partial t} P(x,t) [itex]

where P(x,t) is the probability density and measured in units of (probability)/(volume) = r-3. This equation is the mathematical equivalent of probability conservation law.

It is easy to show that for a plain wave function,

[itex] | \psi \rang = A \exp{\left( i k x - i \omega t \right)} [itex]

the probability flux is given by

[itex] j(x,t) = |A|^2 {k \hbar \over m} [itex]

The bi-linear form of the axiom has interesting consequences as well.

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