Strona 204 z 212

In the first place, we see from this expression that two clocks

of identical construction will go at different rates when

situated at different distances from the centre of the disc. This

result is also valid from the standpoint of an observer who is

rotating with the disc.

Now, as judged from the disc, the latter is in a gravitational

field of potential φ, hence the result we have obtained will hold

quite generally for gravitational fields. Furthermore, we can

regard an atom which is emitting spectral lines as a clock, so

that the following statement will hold:

_An atom absorbs or emits light of a frequency which is dependent

on the potential of the gravitational field in which it is

situated._

The frequency of an atom situated on the surface of a heavenly

body will be somewhat less than the frequency of an atom of the

same element which is situated in free space (or on the surface

of a smaller celestial body).