  We address here the question of interference of radio signals from
astronomical sources like distant quasars, in a very long baseline
interferometer (VLBI), where two (or more) distantly located radio telescopes
(apertures), receive simultaneous signal from the sky. In an equivalent optical
two-slit experiment, it is generally argued that for the photons involved in
the interference pattern on the screen, it is not possible, even in principle,
to know which of the two slits a particular photon went through and that any
procedure to ascertain this destroys the interference pattern. But in the case
of the modern radio VLBI, it is a routine matter to record the phase and
amplitude of the voltage outputs from the two radio antennas on a recording
media separately and then do the correlation between the two recorded signals
later in an offline manner. Does this not violate the quantum interference
principle? We provide a resolution of this problem here.
