  The realization of Karl Popper's experiment by Shih and Kim (published 1999)
produced the result that Popper hoped for: no ``action at a distance'' on one
photon of an entangled pair when a measurement is made on the other photon.
This experimental result is interpretable in local realistic terms: each photon
has a definite position and transverse momentum most of the time; the position
measurement on one photon (localization in a slit) disturbs the transverse
momentum of that photon in a non-predictable way in accordance with the
uncertainty principle; however, there is no effect on the other photon (the
photon that is not in a slit) no action at a distance. The position measurement
(localization within a slit) of the one photon destroys the coherence
(entanglement) between the photons; i.e. decoherence occurs. This can be
understood physically as an electromagnetic interaction between the photon in
the slit and the electrons of the atoms in the surface of the solid that forms
the slit.
