While these investigations were concerned with flat spacetime, Wolfgang Rindler (1960) analyzed hyperbolic motion in curved spacetime, and showed (1966) the analogy between the hyperbolic coordinates (2d, 2e) in flat spacetime with Kruskal coordinates in Schwarzschild space. This influenced subsequent writers in their formulation of Unruh radiation measured by an observer in hyperbolic motion, which is similar to the description of Hawking radiation of black holes.