  As is well known, the double slit experiment contains every key concepts of
quantum mechanics such as phase effect, probability wave, quantum interference,
quantum superposition. In this article, I will clarify the meaning of quantum
superposition in terms of phase effect between states. After applying standard
quantum theory, it leads to serious questions about the unitary process of an
isolated system. It implies that non collapsing interpretations including many
worlds may not be justified. This also could explain that there is no such
boundary between classical and quantum domains.
