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Where a probability density function (on a measure space) is a real-valued function $\rho$ (satisfying certain conditions), a probability amplitude is a complex-valued function $\psi$ such that its pointwise absolute value squared
is a probability density.
Probability amplitudes appear as pure states of quantum mechanical systems in the form of wave functions $\psi$ on the phase space of a corresponding classical system?. The Born rule? of quantum physics says that the probability density $\rho = \psi^\ast \psi$ describes the probability to find the physical system in a given classical state (in a given region of its phase space).
The fact that probability amplitudes are complex-valued means that under addition (“superposition”) they exhibit quantum interference (the addition of their complex phases) in stark contrast to the addition of probability densities, for which this cannot happen.
For instance on some probability space $(X,\mu)$ there are the probability amplitudes $\exp(i \pi/2) \mu$ and $\exp(-i \pi/2)\mu$ whose associated probability densities are both just $\mu$ itself again. But the sum of these two probability amplitudes vanishes, in contrast to the sum of their associated probability densities. This is known as “destructive” quantum interference.
In perturbative quantum field theory the key probability amplitudes considered are scattering amplitudes, which encode the probability for a given configuration of field quanta to come in from the far past, interact with each other and hence “scatter off” each other, and then re-emerge as some other set of field quanta in the far future.
These amplitudes are collected in the scattering matrix of the perturbative quantum field theory.
Last revised on January 20, 2018 at 12:10:14. See the history of this page for a list of all contributions to it.