Correction: The car horn diaphram would of course be oscillating at a
frequency lower than the audible frequency when the car is approaching.

Jim Kelley wrote:


There is an interesting (hypothetical) effect - and maybe this is what
Cecil is talking about. Two systems; A and B, we're A. System B is
moving away from us at relativistic velocity. Sodium yellow light from
system B's street lights looks red from where we're standing in system
A. Lets say we can also measure the atomic transition frequency of the
sodium atoms in system B's street lights and discover that it resonates
at a lower frequency compared to our reference frame. [Note that if the
velocity between the two systems is indeed relativistic, then visible
light will be shifted down into the infrared. Also note that if we
could observe the diaphram of a car horn as it approached us, we would
see that its frequncy of oscillation visually would be higher than its
doppler shifted audible frequency.] If we assume that sodium behaves
the same way everywhere in the universe (which we usually do) and it
transitions at universally the same frequency everywhere, measured with
respect to its own reference frame, then there must be a difference in
the length of the unit time between the two reference frames in order to
explain the apparent observed frequency difference. We usually assume
the Doppler effect is linear with velocity, but temporal effects are
assumed to increase very non-linearly as the speed of light is
approached. So at modest velocities the apparent shift would be all
doppler, but at relativistic velocities the temporal aspect would become
more significant. I believe this is one explanation for the apparent
'acceleration' effect, where you look far enough out and things appear
to be accelerating away from us, not just moving away.

73, ac6xg