  In an effort to challenge the Copenhagen interpretation of quantum mechanics,
Karl Popper proposed an experiment involving spatially separated entangled
particles. In this experiment, one of the particles passes through a very
narrow slit, and thereby its position becomes well-defined. This particle
therefore diffracts into a large divergence angle; this effect can be
understood as a consequence of the Heisenberg uncertainty principle. Popper
further argued that its entangled partner would become comparably localized in
position, and that, according to his understanding of the Copenhagen
interpretation of quantum mechanics, the \qo{mere knowledge} of the position of
this particle would cause it also to diffract into a large divergence angle.
Popper recognized that such behaviour could violate the principle of causality
in that the slit could be removed and the partner particle would be expected to
respond instantaneously. Popper thus concluded that it was most likely the case
that in an actual experiment the partner photon would not undergo increased
diffractive spreading and thus that the Copenhagen interpretation is incorrect.
Here, we report and analyze the results of an implementation of Popper's
proposal. We find that the partner beam does not undergo increased diffractive
spreading. Our work resolves many of the open questions involving Popper's
proposal, and it provides further insight into the nature of entanglement and
its relation to the uncertainty principle of correlated particles.
