From: "Fred Chen"
> Can there be a transition region where
> both aspects are observable?

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It is difficult to observe a one-particle pattern
http://www.optica.tn.tudelft.nl/education/photons.asp
But if you are interested in things like whether there is
an experimental smooth, Yin-Yang type :-), transition between
the particle-like and the wave-like behaviour, try the links
below.
Greenberger and Yasin wrote P^2 + V^2 = 1, where P is the
*probability* for the particle taking one of the two possible
paths, and V the visibility of the fringes.
http://arxiv.org/abs/quant-ph/9908072
http://arxiv.org/abs/quant-ph/0311179
http://arxiv.org/abs/quant-ph/0201026
http://arxiv.org/abs/quant-ph/0404013
In other words, the Greenberger and Yasin relation states that
the "entity" has a double nature (wave-like,particle-like) and
that there is a "smooth" transition between one and the other
nature.
Following Greenberger and Yasin, we must restate the
complementarity principle as *coexistence* between particle-like
and wave-like properties, and not as reciprocal *exclusion*.
(Btw, it is well known that Heisenberg was against the
complementarity principle, since in matrix mechanics there
are no waves at all ... It is also well known that the
Bohr-Heisenberg debate, on this point, was very hard
indeed).