Born (1909) showed that the inner points of a Born rigid body in hyperbolic motion can only be in the region X / ( X 2 − − T 2 ) > 0 . Sommerfeld (1910) defined that the coordinates allowed for the transformation between inertial and hyperbolic coordinates must satisfy T < X . Kottler (1914) defined this region as X 2 − − T 2 > 0 , and pointed out the existence of a "border plane" (German : Grenzebene) c 2 / α α + x , beyond which no signal can reach the observer in hyperbolic motion. This was called the "horizon of the observer" (German : Horizont des Beobachters) by Bollert (1922). Rindler (1966) demonstrated the relation between such a horizon and the horizon in Kruskal coordinates.