clocks, which, it is believed, when unconstrained, measure a small interval according to formula (2).
The identity of the two methods of exploring the geometry of the world is connected with a principle which must now be enunciated definitely. We have said that no experiments have been able to detect a difference between a gravitational field and an artificial field of force such as the centrifugal force. This is not quite the same thing as saying that it has been proved that there is no difference. It is well to be explicit when a positive generalisation is made from negative experimental evidence. The generalisation which it is proposed to adopt is known as the Principle of Equivalence.
A gravitational field of force is precisely equivalent to an artificial field of force, so that in any small region it is impossible by any conceivable experiment to distinguish between them.
In other words, force is purely relative.
