  Any frequency selective device with an ongoing drift will cause observed
spectra to be variously and simultaneously scaled in proportion to their source
distances. The reason is that detectors after the drifting selection will
integrate instantaneous electric or magnetic field values from successive
sinusoids, and these sinusoids would differ in both frequency and phase. Phase
differences between frequencies are ordinarily irrelevant, and recalibration
procedures at most correct for frequency differences. With drifting selection,
however, each integrated field value comes from *the sinusoid of the
instantaneously selected frequency at its instantaneous received phase*, hence
the waveform constructed by the integration will follow the drifting selection
with a phase acceleration given by the drift rate times the slope of the
received phase spectrum. A phase acceleration is literally a frequency shift,
and the phase spectrum slope of a received waveform is an asymptotic measure of
the source distance, as the path delay presents phase offsets proportional to
frequency times the distance, and eventually exceeding all initial phase
differences. Tunable optics may soon be fast enough for realizing such shifts
by Fourier switching, and could lead to pocket X-ray devices; sources
continuously variable from RF to gamma rays; capacity multiplication with
jamming and noise immunity in both fibre and radio channels, passive ranging
from ground to deep space; etc.
