Measurability of Electromagnetic Field: Model and Path Integral Methods

The problem of the measurability of the electromagnetic field is investigated 1) in the framework of the abstract restricted-path-integral method, and 2) by explicitly accounting the action of the field onto the meter and its back reaction. The meaning of the previously obtained results as well as of the classical results of Bohr and Rosenfeld are made clear. The restricted-path-integral method with integration over field configurations is shown to give an estimation on the measurability of the field by any device not disturbing the measured field (in the process of measurement) more than by the measurement error. Such method of measurement is necessary for the control of the field in electronic devices.