mechanical behaviour of the bodies relative to K´ is the same as experience would expect of them with reference to systems which we assume from habit as stationary; thus it explains why from the physical stand-point it can be assumed that the systems K and K´ can both with the same legitimacy be taken as at rest, that is, they will be equivalent as systems of reference for a description of physical phenomena.
From these discussions we see, that the working out of the general relativity theory must, at the same time, lead to a theory of gravitation; for we can "create" a gravitational field by a simple variation of the co-ordinate system. Also we see immediately that the principle of the constancy of light-velocity must be modified, for we recognise easily that the path of a ray of light with reference to K´ must be, in general, curved, when light travels with a definite and constant velocity in a straight line with reference to K.
§ 3. The time-space continuum. Requirements of the general Co-variance for the equations expressing the laws of Nature in general.
In the classical mechanics as well as in the special
relativity theory, the co-ordinates of time and space have
an immediate physical significance; when we say that
any arbitrary point has x_{1} as its X_{1} co-ordinate, it signifies
that the projection of the point-event on the X_{1}-axis
ascertained by means of a solid rod according to the rules
of Euclidean Geometry is reached when a definite measuring
rod, the unit rod, can be carried x_{1} times from the
origin of co-ordinates along the X_{1} axis. A point having
x_{4} = t_{1} as the X_{4} co-ordinate signifies that a unit clock
which is adjusted to be at rest relative to the system of
co-ordinates, and coinciding in its spatial position with the
