to be rotating uniformly with respect to
, say a plane disc rotating in its plane with constant angular velocity. An observer situated on the disc near its periphery will experience a force radially outwards, which is interpreted by an external observer at rest relatively to
as centrifugal force, due to the inertia of the rotating observer. But according to the principle of equivalence the rotating observer is justified in assuming himself to be at rest, i.e. the disc to be at rest. He regards the force acting on him as an effect of a particular sort of gravitational field (in which the field vanishes at the centre and increases as the distance from the centre outwards). This rotating observer, who considers himself at rest, now performs experiments with clocks and measuring-scales in order to be able to define time- and space-data with reference to
. It is easy to show that if, of two clocks which go at exactly the same rate when relatively at rest in the Galilean field
, one be placed at the centre of the rotating disc and one at the circumference, the latter will continually lose time as compared with the former.
Secondly, if an observer at rest in