  A physical experiment comprises along the time trajectory a start, a time
evolution (duration), and an end, which is the measurement. In non relativistic
quantum mechanics the start of the experiment is defined by the wave function
at time 0 taking into account the starting conditions, the evolution is
described by the wave function following the Schr\"odinger equation and the
measurement by the Born rule. While the Schr\"odinger equation is
deterministic, it is the Born rule that makes quantum mechanics statistical
with all its consequences. The nature of the Born rule is thereby unknown
albeit necessary since it produces the correct ensemble averaged measures of
the experiment. Here, it is demonstrated that the origin of the Born rule is
the projection from the quantum frame (i.e. wave description) to the classical
mechanics frame (i.e. particle description) described by a Ehrenfest
theorem-oriented Fourier transformation. The statistical averaging over many
measurements is necessary in order to eliminate the unknown initial and end
time coordinate of the experiment in reference to the beginning of the
universe.
