' all the masses have equal and parallel accelerations; with respect to
' they behave just as if a gravitational field were present and
' were unaccelerated. Overlooking for the present the question as to the "cause" of such a gravitational field, which will occupy us later, there is nothing to prevent our conceiving this gravitational field as real, that is, the conception that
' is "at rest" and a gravitational field is present we may consider as equivalent to the conception that only
is an "allowable" system of co-ordinates and no gravitational field is present. The assumption of the complete physical equivalence of the systems of co-ordinates,