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It is, for instance, impossible to choose a body of reference

such that, as judged from it, the gravitational field of the

earth (in its entirety) vanishes.

We can now appreciate why that argument is not convincing, which

we brought forward against the general principle of relativity at

the end of Section XVIII. It is certainly true that the observer

in the railway carriage experiences a jerk forwards as a result

of the application of the brake, and that he recognises, in this

the non-uniformity of motion (retardation) of the carriage. But

he is compelled by nobody to refer this jerk to a "real"

acceleration (retardation) of the carriage. He might also

interpret his experience thus: "My body of reference (the

carriage) remains permanently at rest. With reference to it,

however, there exists (during the period of application of the

brakes) a gravitational field which is directed forwards and

which is variable with respect to time. Under the influence of

this field, the embankment together with the earth moves

non-uniformly in such a manner that their original velocity in

the backwards direction is continuously reduced."