The
Doppler Effect is a distortion in what an observer sees or
hears caused either by the movement of the observer, by the
movement of what is being observed, or by the movement of both the
observer and the observed at the same time (in practice, everything in
the Universe is moving relative to everything else, to a greater or
lesser degree).
So far as photons are concerned, the
Doppler Effect alters their gravitational
colourshifting away from what
it would be if the emitting object and observer were
both stationary. The cause is the altering of the amount
of time spent within the
gravitysheath interface of either the emitter
or the observer by the movement. Consider these examples:
EXAMPLE A
At
first glance, these results run contrary to the conventional
wisdom which is that a receding object appears redshifted and that
an approaching object appears blueshifted - but they don't. In the
above examples, the emitter and the observer are both of the same mass.
In real life, when making observations from the surface of
the Earth, everything seen outside the Solar System is
substantially more massive than is our planet and so gravitational
colourshifting is the major factor in any such observation.
Having
said that, the colourshifting of a photon away from the wavelength
at which it first stabilised is a
multiprocess. In assessing the
reasons for the colourshift in an observed photon the following
factors, at the very least, need to be considered: