popper-logic-scientific-discovery

APPENDIX vi
Concerning a Non-Predictive
Procedure of Measuring (cf.
section 77)* 1
We suppose that a non-monochromatic beam of particles—for
instance a beam of light—moving along parallel paths in the
x-direction is subjected to selection of their momenta by the
* 1 Heisenberg—who speaks of measuring or observing rather than of selecting—presents the
situation, in form of a description of an imaginary experiment, as follows: if we wish to
observe the position of the electron, we must use high frequency light which will strongly
interact with it, and thus disturb its momentum. If we wish to observe its momentum,
then we must use low frequency light which does leave its momentum (practically)
unchanged, but cannot help us to determine its position. It is important that in this
discussion the uncertainty of the momentum is due to disturbance, while the uncertainty of the position is not
due to anything of the sort. Rather it is the result of avoiding any strong disturbance of the
system. (See appendix *xi, point 9.)
My old argument (which was based upon this observation) proceeded now as follows.
Since a determination of the momentum leaves the momentum unchanged because it
interacts weakly with the system, it must also leave its position unchanged, although it
fails to disclose this position. But the undisclosed position may later be disclosed by a
second measurement; and since the first measurement left the state of the electron
(practically) unchanged, we can calculate the past of the electron not only between the two
measurements, but also before the first measurement.
I do not see how Heisenberg can avoid this conclusion without essentially modifying