Reality of superposition principle and autocorrelation function for short pulses

We “see” light only when some material detectors (dipoles) respond to the incident EM field. EM fields do not operate on each other to make themselves visible to us. Superposition of multiple fields becomes manifest only when the intrinsic properties of these dipoles allow them to respond to all the superposed fields simultaneously and thereby summing the effects of all the fields. Accordingly, depending upon the different intrinsic properties of the detectors and the physical conditions of measurements (integration times, etc.) the manifestation of the “coherence” properties for the same set of superposed fields could be different. It is then prudent to represent the autocorrelation function for superposed fields in terms of the dipole undulation of the detectors rather than the fields themselves. Then the physics of the detectors and the measurement conditions automatically becomes an inherent part of the discussion on coherence. We illustrate our premise by presenting the analysis to understand the behavior of beam splitters, two-beam interferometers and an N-beam grating “interferometer” in terms of the autocorrelation functions due to a short pulse as would be experienced by the material dipoles of the beam splitters and detectors. Our approach reveals that superposition effects to become manifest the multiple fields must be physically superposed simultaneously on the detecting dipoles and hence the process is causal.