  When the boundary condition of a quantum system changes, how fast will it
affect the state of the system? Here we show that if the response takes place
immediately, then it can allow superluminal signal transfer. Else if the
response propagates in space with a finite speed, then it could give a simple
explanation why our world shows classicality on the macroscopic scale.
Furthermore, determining the exact value of this speed can either clarify the
doubts on static experiments for testing Bell's inequality, or support the
pilot-wave interpretation of quantum mechanics. We propose an experimental
scheme for measuring this speed, which can be implemented with state-of-art
technology, e.g., single-electron biprism interference.
