These conclusions
imply that the explanation of the Stern–Gerlach experiment
typically found in textbooks is too simplistic. The Stern–
Gerlach device is not an example of an ideal measurement.
The measurement affects the state of the system studied through
a T2 relaxation process. This is just another example of the
principle effect that the measuring device affects the state of a
microscopic quantum system. However, there is a special twist
to the Stern–Gerlach measurement. A system initially polarized
along the z-direction passes through the device unaffected, but
since the same spin state is reached by relaxation also for other
initial conditions there is no way to tell with certainty a posteriori
whether or not the relaxation process has occurred.