The Aharonov-Bohm effects show how a electro-magnetic field could affect a region of space in which the field had been shielded, although its vector potential did exist there and could interact with the wave function of say the electron. What practical application(s) (so far) can be derived from this principle? Also does Aharonov-Bohm effect prove that the wave function is objective in space favoring either Bohmian Mechanics or Objective Collapse or can it still be compatible with Copenhagen in which the wave function is just knowledge of the observer. In the latter case, the vector potential is also knowledge of the observer? As a consequence, electromagnetic field by nature is just knowledge of the observer and not really there in space at all?