LIMITS OF THE MEASURABILITY OF THE LOCAL QUANTUM ELECTROMAGNETIC-FIELD AMPLITUDE

The precision with which the amplitude of the free electromagnetic fie ld can be measured locally in QED is evaluated by analyzing a well-kno wn gedanken experiment originally proposed by Bohr and Rosenfeld (BR). The analysis is performed by applying standard theoretical techniques familiar in quantum optics. The main result obtained for the precisio n is significantly different from the generally accepted Bohr-Rosenfel d result. This leads to questioning the widely accepted notion of the compensating field, fostered by these authors. A misconception at the origin of this notion is pointed out by a careful investigation of the self-force acting on the apparatus designed to measure the field. The correct expression for this self-force is found to be at variance wit h that proposed by Bohr and Rosenfeld and generally accepted. It is ar gued that, as a consequence of this new expression and in contrast wit h the generally accepted view, no compensating force of nonelectromagn etic nature is required in order to perform measurements of the quantu m field amplitude with any desired accuracy. It is shown that the only limitations to the precision of the measurement, in the BR gedanken e xperiment, arise from the time-energy uncertainty principle, as well a s from the finite dimensions of the measuring apparatus.[S1050-2947(98 )06903-0].