  The famous gedanken experiments of quantum mechanics have played crucial
roles in developing the Copenhagen interpretation. They are studied here from
the perspective of standard quantum mechanics, with no ontological
interpretation involved. Bohr's investigation of these gedanken experiments,
based on the uncertainty relation with his interpretation, was the origin of
the Copenhagen interpretation and is still widely adopted, but is shown to be
not consistent with the quantum mechanical view. We point out that in most of
these gedanken experiments, entanglement plays a crucial role, while its
buildup does not change the uncertainty of the concerned quantity in the way
thought by Bohr. Especially, in the gamma ray microscope and recoiling
double-slit gedanken experiments, we expose the entanglement based on momentum
exchange. It is shown that even in such cases, the loss of interference is only
due to the entanglement with other degrees of freedom, while the uncertainty
relation argument, which has not been questioned up to now, is not right.
