  By means of a thought-experiment, consisting of an interference experiment
with two interfering beams, it is shown that it can be demonstrated
experimentally that with one single particle a wave can be associated which
propagates in space and time as a physical reality, i.e. that it should not
merely be considered as a distribution of probabilities. The notion "physical
reality" should be understood such that, when this physical reality is
considered in a particular space at a particular time, it should be
experimentally possible to influence this reality in such a way that future
results of experiments show unambiguously that this reality has been causally
influenced by the experimental act in this space and at that time.
