  It is well known that Wheeler proposed several delayed choice experiments in
order to show the impossibility to speak of the way a quantum system behaves
before being detected. In a double-slit experiment, when do photons decide to
travel by one way or by two ways? Delayed choice experiments seem to indicate
that, strangely, it is possible to change the decision of the photons until the
very last moment before they are detected. This led Wheeler to his famous
sentence: No elementary quantum phenomenon is a phenomenon until it is a
registered phenomenon, brought to a close by an irreversible act of
amplification. Nevertheless some authors wrote that backward in time effects
were needed to explain these results. I will show that in delayed choice
experiments involving only one particle, a simple explanation is possible
without invoking any backward in time effect. Delayed choice experiments
involving entangled particles such as the so called quantum eraser can also be
explained without invoking any backward in time effect but I will argue that
these experiments cannot be accounted for so simply because they rise the whole
problem of knowing what a measurement and a collapse are. A previously
presented interpretation, Convivial Solipsism, is a natural framework for
giving a simple explanation of these delayed choice experiments with entangled
particles. In this paper, I show how Convivial Solipsism helps clarifying the
puzzling questions raised by the collapse of the wave function of entangled
systems.
