, the gravitational potential of which differs from that of a place
by the amount
, is connected with the time of oscillation there observed by the relation:
according to the principle of equivalence of Einstein's theory of gravitation.
This special case shows how the duration of an event is to be understood as being dependent upon the gravitational condition.
Moreover, one can regard every vibrating system (which emits a spectral line) as a clock, the motion of which, according to the investigation made just above, depends upon the gravitational potentials of the place where it is stationed. This same "clock" will have a different time of oscillation at another place in the field according to the gravitational potential, i.e. it will go at a different rate. Consequently, a particular line in the spectrum of the light which comes from the sun, e.g. an Fe-line (iron), must appear to be shifted in comparison with the corresponding line as produced by a source of light (arc-lamp) on the earth; the gravitational potential at the surface of the sun has, corresponding to the latter's great mass, a different value from that at the surface of the earth, and a definite time of oscillation (colour) is characterized in the spectrum by a definite position (Fraunhofer line). It has not yet been possible to observe this effect, which amounts to about 0.008