Frequencies

"A tracer shell flashed out, drawing a line of fire through Svantovik's boat. Or, rather, through the space where shell and boat coexisted with different frequencies."
-Poul Anderson, "Hunters of the Sky Cave" IN Anderson, Sir Dominic Flandry: The Last Knight Of Terra (Riverdale, NY, 2012), pp. 149-301 AT XVI, p. 275.

Thus, entire universes might coexist not only in different dimensions but also at different frequencies. In DC Comics, a hero with the power of super speed, by changing the frequency of his bodily vibrations, transferred from Earth 1 to Earth 2, thus initiating twenty years of inter-cosmic crossovers until the Crisis in which most of the universes were destroyed while the surviving five were four-dimensionally merged into a revised timeline.