ABSTRACT
In this chapter we shall come back to the issue of the undulatory
nature of matter. Interferometer experiments with a blocked path
show that this character cannot be taken in a classical sense.
Indeed, quantum probabilities are deeply affected by the presence
of features. The possibility of measurements without interaction
shows that such probabilities have in quantum mechanics an
ontological import. We then state a complementarity between local
events and non-local features. Delayed choice experiments sets
these aspects in new and enlightening terms.
